Geochemistry and structure of tertiary volcanic rocksin the southwestern Monte Cristo Range, Nevada

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Authors Hambrick, Dixie Ann

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GEOCHEMISTRY AND STRUCTURE OF TERTIARY VOLCANIC ROCKS IN

THE SOUTHWESTERN MONTE CRISTO RANGE, NEVADA

by

Dixie Ann Hambrick

A Thesis Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE '

In the Graduate College

THE UNIVERSITY OF ARIZONA

:i 9 8 4

STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg­ment the proposed use of the material is in the interests of scholar­ship. In all other instances, however, permission must be obtained from the author.

SIGNED

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

Datbs Jr "

P. E. DamonProfessor of Geosciences

ACKNOWLEDGMENTS

The development and completion of this thesis was realized

through the help and guidance of many people. I would like to thank

all members of the Tucson office of U.S. Borax for their support during

the course of this study. Particular gratitude is extended to Barry

Watson, who helped formulate this thesis topic and provided guidance,

assistance and much pertinent information during this project. The

financial assistance provided by this office helped make the chemical,

isotopic and radiometric work done in this study possible.

My gratitude also goes to Dr. P. E. Damon whose knowledge of

geochemistry and volcanic systems helped improve this project. Funding

for the analyses performed in this study was partially provided by the

Laboratory of Isotope Geochemistry. Appreciation goes to all members

of this laboratory for their assistance. A special thanks goes to

Drs. M. Shafiquallah and D. Lynch, who taught me the tedious labora­

tory procedures of isotope work.

I would like to extend my appreciation to Dr. J. Stewart of the

U.S. Geological Survey for his interest in this study, and for sharing

with me his knowledge of the geology of the Monte Cristo Range.

iii

TABLE OF CONTENTS

LIST OF ILLUSTRATIONS . . . . . . . . . .

LIST OF TABLES ................. .. . . .

ABSTRACT o @ o o o @ e o @ p o @ @ o @ @

1o INTRODUCTION . . . . . . . . . . . . .

Purpose and Location of Study . . Methods of Study ..............

20 REGIONAL GEOLOGY AND TECTONIC SETTING

e o o o o Vllo e o o e I X

Page

30 ROCK UNITS

Pre-Tertiary Basement . . . . . . . . . .Tertiary Volcanic and Sedimentary Rocks

Castle Peak Volcanic Sequence . . . Castle Peak Tuff1 o T Cp o o o e o o e e e o e2 o T Cp o o o o o o o o o o o3 o ■ T Cp o e o o o o o o o o o 4,o T Cp 0 o o o o o o o o o o oBanded Rhyolite Intrusion p » .1O Tri O O O O O O O O O O O O

Coaldale Volcanic Sequence 0 . . . .. Sediments and Tuffs e . . »1O Tstl o O O 0 0 - 0 9 o o o o2 o Tstu O O O O O O O O O O OHornblende Andesites 0 » » o »1o That O O O O O O O O O O O 2 o Thai O O O O O O O O O O O 3o Thai O O O O O O O O O O O. 4 O Tha O O O O O O O O O O O OFine-Grained Andesites 0 » » o1o Tfai o o o o o o o o o o oPyroxene Andesites 0 » o « » »1 o Tp 3-t O O O O O O O O O O O2 o Tpai o o o o o o o o o o o3 o Tpa O O O O O O O O O O O O

O O O O o o o

O O O O O o o

o o o o o O o

111212121414161921212122222223232628313434343536 39

iv

V

TABLE OF CONTENTS— Continued

Page

Intrusive Rhyolite . . . . . . . . .1 Ti-*-e J.O. o o o e o o o o o o e e . o o o

Blair Junction Volcanic Sequence ... .. . .1 T c-*-e J.o-/ 0 0 0 0 0 0 0 0 0 0 6 0 0 0 0

2 o T C a b o o o o e 0 0 o o o o 0 o 9

3 o T e a 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

a T-fa• o. 1 J- CL 0 0 0 g 0 0 0 0 o o 0.0 o 0 , 0

Younger Sedimentary and Basaltic Sequence1 Tp .J.O A w Q 0 o O O O O O O 0 O O O O O

2 Th— ' o ls p o 0 0 0 o e o o .o 0 0 0 0 0

Quaternary Deposits . . . . . . . . . . . . .lo Q . . . • e e o e e o o o o o o o2 e QSp 0 0 . e . . . . a . . .

40404242424445464647 47 47 47

4. AGE AND CORRELATION OF TERTIARY UNITS

Castle Peak Volcanic Sequence . . . . . . . . . . . . . . 50Coaldale Volcanic Sequence . . . . . . . . . . . . . . . 54

' Blair Junction Volcanic Sequence . . . . . . . . . . . . 57Younger Sedimentary and Basaltic Sequence . . . . . . . . 57

5. GEOCHEMISTRY . . . . . . 59

Whole Rock Chemistry . . . . . . . . . . . . . . . . . . 59Isotope Geochemistry . . . . . . . . . . . . .......... 71

6. STRUCTURAL GEOLOGY . . . . . . . . . . . . . . . . .......... 74

F o l d S e o ft e e e e o e 0 . . .Faults . . . . . . . . . . . .Hornblende and Pyroxene Dikes . Discussion o’. o e e . o e o ft

74

7. ECONOMIC GEOLOGY . . . . . . . . . . . . . . . . . . . . . . 86

Formation of Borate Deposits . . . » . . . . . . . . . . 87Distribution of B in the Southwestern Monte Cristo Range . . . . . . . . . . . . . . . . . . . 90Cause of B Anomalies in the SouthwesternMonte Cristo Range . . . . . . . . . . . . . . . . . . . 104

Original Magmatic Composition . . . . . . . . . . . . 105Secondary Hydrothermal Alteration . . . . . . . . . . 106

Age of Alteration in the Southwestern Monte Cristo Range . . . . . . . . . . . . . . . . . . . 109

viTABLE OF CONTENTS— Continued

8. TERTIARY GEOLOGIC HISTORY . . . . . . .

9. CONCLUSIONS . . . . . . . ............

APPENDIX A: SAMPLE DESCRIPTIONS . . . . . .

Page

112115

119

REFERENCES 136

LIST OF ILLUSTRATIONS

Figure Page

1 e Index Map of a Part of Southwestern. Nevada . „ » , * .

CM

20 Geology of the Southwestern Monte CristoRange 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 in pocket

30 Structural Features within the Walker Laneof Nevada and California 0 0 0 0 0 0 0 0 0 . 0 0 0

00

4. Texture in Unwelded Member of the CastleP eak Tuff © o . © © © © . © © © © © © © © © © © © © © © 15

5© Multiple Cooling Units of the Castle Peak Tuff in the Northwestern Portion of the Study Area © e © © © © © © © © © © © . © © © © © © © 00

6© Exotic Block of Oxidized Feldspar-Rich Rhyolite in Unwelded to Partially Welded CastleP©ak Tuff © © o o o o e o o o o o o o o e o o o o

OCM

7© Series of Photographs that Depict Textures inthe Hornblende Andesite Tuff Breccia © © © © © © 0 © o o 25

8© Silicified Lower Portion of the HornblendeAndesite Lahar near Blue Mountain © © © © © © © * CM

9 © Blue Mountain Intrusion © © © © © © © © © © © © © © © © . . . 30

10© West Flank of Coaldale Ridge © © © ,© © © © © © © © © © „ . . 32

11© Pyroxene Andesite Tuff Breccia nearBlue Mountain © o © © © © © © © © © © © © © © © © . . . 37

12© Small Pink Rhyolite Plug in the SoutheasternPortion of the Study Area © © © © © © © © © © © © o o . 41

13© Blair Junction Volcanic Sequence © © © © © © © © © © © . © . 43

14© Structural and Geochemical Overlay © © © © © © © © © © in pocket

15© Geologic Sections Across the SouthwesternMonte Cristo Range © © © © © , © © © © © © © © © © ©

vii

in pocket

viii

LIST OF ILLUSTRATIONS— Continued

Figure Page

17. IUGS Classification of Chemical Analyses,Based on Normative Mineralogy ................. ' . . . . 64

18. Variation Diagrams of Chemical Analyses,Differentiated by Volcanic Sequence . . . . . . . . . . 65

19. Triangular Variation Diagrams of Chemical Analyses,Differentiated by Volcanic Sequence . . . . . . . . . . 66

20. Oxide-Silica Variation Diagrams of Coaldale andBlair Junction Chemical Analyses, Differen­tiated by Rock Type . . . . . . . . . . . . . . . . . . 68

21. Oxide-DI Variation Diagrams of Coaldale and BlairJunction Chemical Analyses, Differentiatedby Rock Type . . . . . . . . . . . . . . . . . . . . . 69

22. Equal Area Stereonet Plots of Strike Data . . . . . . . . . 75

23. Equal Area Stereonet Plots and Frequency Diagramsof Fault Data . .. . . . . . . . . . . . . . @ .. . . 78

24. Equal Area Stereonet .Plot of Trend and Plunge ofFault Slickensides . . . . . . . . . . . . . . . . . . 7 9

25. Equal Area Stereonet Plots and Frequency Diagramsof Dike Data . . . . . . . . o . . . . . . . . . . . . 83

26. Caved Shafts on the Western Flank of CoaldaleRidge o e e o . ' e . o o o t t o o o o o o o o e . e e o-o 88

27. B-Sr Variation Diagram of Average Values . . . . . . . . . . 100

28. B-Li Variation Diagram of Average Values . . . . . . . . . . 101

29. B-Sr-Li Triangular Variation Diagram of AverageValues . . . . . . . . . . . . . . o o . 0 . 0 0 . . . 102

30. Sericitized Hornblende Andesite Flow(?) on theWestern Flank of Coaldale Ridge . . . . . . . . . . . . 108

LIST OF TABLES

Table Page

lo Average Thin Section Modal Percents ofTertiary Units o o o o o . o o . o o o o o o o e o o o o o 13

20 Analytical Data for Age Determinations ofTertiary Rocks „ <> o <, o « » o » «> <> «»«<,«, o <, e « «, » 51

30 Chemical Analyses (in weight percent) ofTertiary Units , . o , „ . . „ „ „ . „ , „ » . . . * 60

4o Barth Normative Mineralogy (in weight percent)of Tertiary Units „ . . „ * „ , „ , * , 61

5» IUGS Classification of Chemical Analyses ............... .. . 62

60 Analytical Data for Initial Sr Isotopes ofTertiary Rocks @ o o o o o @ e o @ o @ o @ o o o e » @ 72

7e Sx9 and Li Geochemistry of Rock Chip andSoil/Fluff Samples o o o o o o o o o o o o o o o o o o 91

8o Average B5 Sx9 and Li Geochemistry......................... 97

9o Comparison of Average Fresh Rock B5 Sr5 and Li Geochemistry to the Average Composition of Similar Rock Types and Clarke Values ofTurekian (1972) » o'© © © © © © © © © © © © © © © © © © 103

ix

ABSTRACT

The chronological, geochemical and structural history of

Oligocene to Miocene rhyolites, latites, trachytes and sediments in

the southwestern.Monte Cristo Range exemplifies the complexity of a

small, discrete volcanic center. Geologic, isotopic and geochemical

constraints deny a genetic relationship between the four volcanic

sequences in the area. Prominent north-trending, high-angle faults with

minor right-lateral displacements were caused by east-west Basin and

Range extension. Secondary conjugate shear stress formed moderate displacements on east-trending, left-lateral faults. These deformations

are compatible with the regional Walker Lane and Warm Springs discon­

tinuities.

Widespread boron anomalies in the area were caused by a

Pliocene(?) hydrothermal event. A sericitized alteration center is

surrounded by clay alteration, an opaline vein system and weak B

enrichment in the permeable volcanic rocks. Mobilization and/or

leaching have caused localized, B-rich surficial accumulations.

x

CHAPTER 1

INTRODUCTION

Purpose and Location of Study

The southwestern portion of the Monte Cristo Range, Esmeralda

County, Nevada, is composed of Tertiary volcanic rocks and lacustrine

sedimentSo Initial reconnaissance sampling by UoS* Borax has shown that the volcanic as well as the sedimentary units are at least super­

ficially enriched in B, Sr and Li (Bo Watson, pers, conmio, 1982), This

anomalous condition is unusual in an interstratified volcanic sequence;

B enrichment in lacustrine sediments is not uncommon and many major

borate deposits are known to occur in similar sequences (Bates, 1960)«

The structural and depositional history of the Esmeralda Formation

sediments in the Blair Junction area of the Southern Monte Cristo Range

has been described by Moore (1981), although no detailed study of the

adjacent volcanic rocks has been done. This study was undertaken to

provide the basic geologic and geochemical evidence necessary to attempt

to explain the cause of the anomalous B enrichment of the volcanic

sequence in the southwestern Monte Cristo Range.

The project area is located on the southwestern flank of the

Monte Cristo Range approximately 60 miles west of Tonopah in south­

western Nevada. Figure 1 is an index map of this part of Nevada and

shows the geologic map area (Figure 2). The project area lies,within

the Coaldale and Blair Junction 7.5 minute quadrangles. Two predominant1

118*15' 118*00' 117*45 ’ 117*15'

Figure 1. Index Map of a Part of Southwestern Nevada.Geologic map (Figure 2) and localities referred to in the text shown.

38*30'

r 38*15'

- 38*00'

-37*45 '

3

volcanic landforms9 Coaldale Ridge and Blue Mountain, are informally

named to aid in description of localities mentioned in the text and

are shown on Figure 2.

The topography and geology of the study area are conspicuously

dominated by a large, central pre-Tertiary basement high flanked on ,

either side by thick volcanic accumulations (Figure 2). As the volcanic

rocks are the major emphasis, the pre-Tertiary basement is only

evaluated in this study for ways in which it could have affected the

volcanic or Tertiary history of the area. To distinguish the primary

or secondary nature of the B anomalies in the area the aims .of this

study are threefold: first, to determine the chronological history;

second, to define the structural history; and third, to examine the

geochemistry of the Tertiary rocks in the southwestern Monte Gristo

Range.

Methods of Study

The methods used to accomplish these aims involved both field

and laboratory studies. The majority of the field work was performed

during the period from August through October, 1982. Minor follow-up

work took place in the latter part of April and early May, 1983. The

bulk of the laboratory work was accomplished from November, 1982 to

March, 1983, although several analyses were not completed until November,

1983.

Detailed geologic mapping at a 1:12,000 scale of approximately

10 square miles covering the Tertiary exposures was done using air

photographs as a base. Regional reconnaissance of volcanic rocks in

4

the nearby Silver Peak Range and Candelaria Hills was performed to com­

pare compositions and gross structural style of similar units. More

detailed reconnaissance of the remainder of the Monte Cristo Range was

completed, partially in conjunction with J. Stewart of the U.S. Geo­

logical Survey, to evaluate the geology and correlative possibilities

in the immediate surroundings of the project area.

Several laboratory analyses were also necessary to further

refine the field work and the chronological and geochemical history of

the project area. Thin section petrography of 41 samples was used to

improve descriptions and clarify differences between the field units.

Detailed sampling of all units throughout the project area was done

during the latter part of the field work. A total of 75. samples was

collected; 70 unduplicated samples were analyzed for their B, Sr and Li

contents to establish the amount, stratigraphic preference and geographic

distribution of the geochemical anomalies in the study area. Nine of

these samples were also analyzed for their whole rock geochemistry to

determine the normative composition and variation of the major volcanic

units. Three of these geochemistry samples were further isotopically

analyzed. Four K-Ar mineral age dates were performed on these three

samples to define the duration of volcanic activity and to give minimum

age constraints to the structural deformation in the study area., Finally, 8 7 g r

initial ---- ratios were determined for these three samples to identify86Sr

possible magmatic source materials for the volcanic sequences. The

relative freshness of the geochemical and isotopic samples was estab­

lished prior to their analysis by thin section inspection.

5

The B, Sr and Li analyses were performed by atomic absorption

in the U.S. Borax Research Laboratory located in Anaheim, California.

The whole rock geochemistry determinations and isotopic analyses were

done in the Analytical Research Laboratory and the Laboratory of Isotope

Geochemistry, respectively, at the University of Arizona in Tucson.

Initial sample preparations for both these analyses were done by the

author. These preparations included: crushing, homogenization and

cleaning; mineral separation for K-Ar determinations, utilizing standard

vibration, magnetic and heavy liquid techniques; K analyses by atomic

absorption; Sr determinations by X-ray fluorescence; and Sr extractions

by calibrated ion exchange resin columns. R. Butcher performed the Ar

isotope analyses and D. Lynch the Sr isotope analyses on 6" sector

Nier-type mass spectrometers. Normative and variation calculations of

the whole rock geochemistry were performed through a computer program

by D. Lynch.

CHAPTER 2

REGIONAL GEOLOGY AND TECTONIC SETTING

The Monte Gristo Range is characterized by complex geology in

both its pre-Tertiary and Tertiary unitso It was originally mapped by

Ferguson and others in 1953 at a regional scale of-1:125,000. The

dominant pre-Tertiary unit in the Monte Cristo Range, the Ordovician

Palmetto Formation, is considered allocthonous and forms the leading

edge in the Roberts Mountain thrust of the Antler Orogeny (Stewart,

1980)o Although this orogeny is believed to have caused most of the

complex pre-Tertiary structure seen, isolated Triassic Excelsior

Formation exposures are part of the Golcanda thrust and evidence the

effects of the Sonoma Orogeny6 These major thrusting episodes have

given the Monte Cristo Range a structurally complex and brittle base­

ment framework for Tertiary deformations.

The Tertiary volcanic rocks in the Monte Cristo Range have been

grouped into three units: basalts, the Gilbert Andesite, and a lower

rhyolitic breccia unit in the Esmeralda Formation (Ferguson et al.,

1953; Albers and Stewart, 1972). A recent study by Moore (1981) in the

Blair Junction area has informally named the latter rhyolite the Castle

Peak Tuff, and has firmly dissociated it from the overlying sedimentary

Esmeralda Formation for its obvious ash-flow features. The entire Monte

Cristo Range is currently being studied in detail by J. Stewart and

others as part of the U.S. Geological Survey Tonopah 2° sheet project.

6

7

Recently published geologic quadrangle maps nearby the project

area include areas in the Silver Peak Range by Robinson and others

(1976) and the Miller Mountain region by Stewart (1979). Detailed work

by Moore (1981) in the Blair Junction area and by Speed and Cogbill

(1979a-c) in the Candelaria Hills provide a basis for comparison with

some of the units and structures in the project area.

More regionally, the project area lies along the western margin

of the Basin arid Range Province, a region dominated by north-south

mountain ranges and intervening linear valleys (Eaton, 1979). Steep

normal faults control this physiography arid formed as a rigid crustal

response to regional extensional stress operative in this region from

approximately 17 m.y. ago until the present time (McKee, 1974; Stewart,

1978, 1980). The study area occurs in an area affected by, and near the

intersection of, two regional structural features in this Province, the

Walker Lane and the Warm Springs lineament.

Although originally defined as a physiographic lineament by .

Locke and others (1940), many workers have shown that the Walker Lane

can also be considered a structural province (Shawe, 1965; Albers, 1967;

Stewart, 1967; Ekren et al., 1976; Speed and Cogbill, 1979a). This broad

zone, shown in Figure 3, is believed by.most to be a large-scale deforma-

tional feature characterized by right-lateral displacements accommodated

by both strike-slip faults and pervasive folds (Stewart, 1980). The

timing and amount of offset in this zone is still a matter of great

debate, but minimum generalizations can be made.

i.Reference

Bell and Slemrnons, 1980Offset 10 mi

AgeMiocene -*■ Recent2. Bonham, 1967 20 mi Miocene(?)3. Stewart, 1980 I 10 mi Miocene Recent4. Ekren et al., 1979 Z 20-30 mi Early Miocene5. Albers, 1967 12 mi Miocene (?)6. Gilbert and Reynolds, 1973 ? Oligocene -*• Recent7. Speed and Cogbill, 1979a ? Oligocene -*■ Recent8. Moore, 1981 Z <2 mi Miocene -» Recent9. Stewart, 1980 60-80 mi Miocene10. Albers, 1967 10-12 mi Miocene

11. Stewart, 1968 25-40 mi Late Miocene

Figure 3. Structural Features within the Walker Lane of Nevada and California. East- northeast lineaments (Ekren et al., 1976): YR = Yerrington-Rawhide; PR = Pancake Range; WS = Warm Springs.

9

South of Tonopah (Figure 3) the Walker Lane is dominated by

large right-lateral strike-slip faults. These have at least 50 to 80

miles of offset and tend to die northwards into large arcuate mountain

ranges, or 'oroflexes' as defined by Albers (1967). The northern

section of the Walker Lane is characterized by en echelon sets of

smaller right-lateral strike-slip faults which have offsets that range

from less than one mile to greater than 20 miles (Bonham, 1969; Ekren

et al., 1980; Stewart, 1980). This change in deformational style in

the Walker Lane occurs over a narrow zone that includes the current

project area. Although a Jurassic age for some of the deformation in

this zone is advocated (Speed, 1978), most workers believe that the

offsets are of Miocene to Recent age. Many of the northern faults have

Quaternary and historic displacements whereas most of the southern ones

may be older (Bell and Slemmons, 1980).

Recent work in the northern Walker Lane has identified three

east-northeast zones of left-lateral strike-slip faults (Ekren et al.,

1976). Each has a strong geomagnetic expression and is characterized

by short en echelon strike-slip faults, similar to the style in the

northern Walker Lane (Stewart et al., 1977). Offsets along individual

faults are usually small but total displacements across fault zones up

to 10 miles have been documented (Stewart, 1980; Speed and Cogbill, 1979a;

Ekren et al., 1979). The east-northeast Warm Springs lineament of Ekren

and others (1976) projects into the current project area. The age of the

deformation in these zones is believed equivalent to that of the Walker

Lane deformation.

10Tectonically* many have likened the Walker Lane to a

continental version of the San Andreas fault (see Stewart* 1980)» The

east-northeast lineaments are equally analogous to the Garlock fault»

Both these features are thought to be the continental result of late Tertiary to Recent regional conjugate shear stress across the western

Great Basin.

I

CHAPTER 3

ROCK UNITS

Pre-Tertiary Basement

The pre-Tertiary (pT) rocks in the study are composed

dominantly of the Ordovician Palmetto Formation. They consist of

interbedded shales, siltstones, tan argillaceous limestones and cherts

Interlayered limestone and bedded black chert units predominate in

the central portion of the project area whereas shale and siltstone

units predominate in the northeast (Figure 2). The color of the

shales and siltstone is varied and includes greenish gray, brown,

reddish purple, gray and black. Alternation of dark and light units

is common. Tan weathering argillaceous limestones are medium gray■' - -

when fresh. Limestones" usually occur interlayered with bedded cherts

but also exist as thin interbeds within the shale and siltstone units.

Minor exposures of the Triassic Excelsior Formation have also

been mapped in and near the study area (Albers and Stewart, 1972).

Consisting of greenstone and greenstone breccias, this unit occurs in

the northernmost portion of the project area. It has also been

suggested that Devonian strata may exist as structural interleaves

within the Palmetto Formation in the Monte Cristo Range, as they do

northward near Miller Mountain (Stanley et al., 1977; Stewart, 1980).

Neither of these age strata were noted within the study area during

this project.

i i ■

12Tertiary Volcanic and Sedimentary Rocks

Four sequences of Tertiary volcanic and sedimentary rocks have

been identified in the project area* These are informally named (from

bottom to top): the Castle Peak volcanic sequence5 the Coaldale

volcanic sequence, the Blair Junction volcanic sequence, and the

younger sedimentary and basaltic sequence* Each sequence has distinct

mappable units. The following discussion describes the distribution,

contact relations, lithology and petrography of the Tertiary map units

in the study area. Table 1 summarizes the average thin section compo­

sition of the volcanic units.

Castle Peak Volcanic Sequence

The Castle Peak volcanic sequence has been subdivided into two

related rock types. The first, the Castle Peak Tuff (Tcp), occurs

throughout the project area whereas the second, a banded rhyolite intru­

sion (Tri), occurs only in the south central portion (Figure 2).

Castle Peak Tuff. The Castle Peak Tuff was first named and

identified by S. Moore (1981) in the Blair Junction area of the southern

Monte Cristo Range east of, and adjacent to, the current study area. He

divided the Castle Peak Tuff into three informal members comprising

different degrees of welding in an ash-flow (Smith, 1960a,b). These

members have been extrapolated westward into the area of this report as:

Tcpj, the unwelded member; Tcp2, the partially welded member; and Tcp^,

the densely welded member. In addition to these members, however, the

tuff in the southwestern Monte Cristo Range contains large mappable

if

TABLE 1

Average Thin Section Modal Percents of Tertiary Units Southwestern Monte Cristo Range

Sample Number of Thin Sections Name

Phenocryst Composition (%) Groundmass Composition (*)

Quartz Sanidine Plagioclase(series) Hornblende Augite Hypersthene Total Microlitic

Plagioclase Opaques Pyroxene Alt.

Tb 1 Bas. Andes. _ - - - - 5 95 65 2 10 -Ti 2 Rhyo. Int. 20 10 <1 (ab) 2% biotite - 68 cryptofelsic textureTpa 3 Px Andes. (tr in one

thin sect.) - 25 (olig) - 5-7 - -65 10 5 10 -Tpai 1 Px. Int. - . - 30 tr 10 - 60 < 5 5 2 -

1 Px. Dike - - 20 - 5 - 75 20 2 5 -Tpat 1 Px. TXiff bx. - (tr?) 35 - 5 - 60 50% clasts FeOxTfai S Fg Andes. Int. - - 8 (olig) 8 8 (10 in one

thin sect.) 76 32 5 2 ChiTha 3 Hb Andes - - 10 (olig) 10 2-5 -75 35 3 tr -Thai 2 Hb Int. - - 20 18 - - 62 tr-20 2 - -

1 Hb Dike - - 20 (olig) 7 - - 73 10 2 Chl/CarboThat 2 Hb Tuff Bx. - 5 25 5-10 - - -65 30% to 50% clasts FeOxThai 2 Hb lahar - - 25 20 - - 55 tr 3 - -Tstu 1 Lithic tuff 20 25 tr 5% biotite - 50 55% clasts;

unwelded texture Chl/FeOxTstu 1 Air-fall tuff - 5 5 tr biotite - 90 extremely fine-grained -Tea 2 Cg Andes. (tr?) - 15 (olig) 10 5 - 70 25 3 - ChiTcab 1 Cg Andes. Bx. - - 25 10 tr - 65 50% clasts FeOxTfa 2 Fg Andes. - - 7 5 3 - 85 35 3 - ChiTri 2 Banded Rhyo. Int. 2/tr(?) 5/7 5/25 - - - 88/58 alternating light/dark bands FeOxTcp S Castle Peak Tuff 8 13 10 2% biotite - 67 5% clasts; eutaxitic texture FeOx

TcPe 3 Exotic Blocks - variable. see text - —

14blocks of exotic material, Tcpe, not noted by Moore in the Blair

Junction area0

1„ Tcpj o The thickest and most extensive subdivision of the

Castle Peak Tuff is the unwelded member* Its thickness is variable and .

ranges from less than 251 in the central south to greater than 300? in

the northeast. Where observable, the unwelded portion of the Castle

Peak Tuff overlies the Palmetto Formation with angular conformity. An

irregular erosional surface is indicated by the undulatory contact

between these units in the southwestern exterior of the Monte Cristo

Range. A linear exposure of the tuff's lower contact in the north

central portion of the study area suggests structural control on its

deposition may have also existed.

Lithologically, the unwelded member is distinctive in its gray

to white color. Little to no flattening of pumice occurs although

devitrification and vapor phase alteration of the groundmass are common

(Figure 4). It usually contains up to 5% lithic clasts but up to 10%

is not uncommon near its base. The basal contact zone differs only in

that it is a thin (6n) pink to orange chilled margin; only one occur­

rence of a true basal vitrophyre occurs in the study area. • Distinctly

bedded lapilli tuff beds in the unwelded member seen in the Blair

Junction area (Moore, 1981) are not observed in the current study area.

2. Tcp2 o The partially welded member of the Castle. Peak Tuff

is distinguished by its light pink to purple color, moderate eutaxitic

texture and substantial vapor phase alteration of both its phenocrysts

15

- W - .

. * ■

. ■• V ;V

"

Figure 4. Texture in Unwelded Member of the Castle Peak Tuff.

■■■■

16

and groundmasso Flattening ratios (long axis to thickness) of

pumices range from 2:1 to 10:10 This more resistant unit commonly

forms the slopes and ledges above the lower unwelded member whereas

more gradational upper contacts are seen with the overlying welded

membere This member contains slightly fewer lithic clasts, 3% to

5%, throughout its variable thickness0

3e Tcpg0 The upper densely welded member of the Castle Peak

Tuff is dark reddish purple to brown and exhibits a- strong eutaxitic

texture, with flattening ratios reaching a maximum of 30:1* Extremely

resistant, this member commonly caps ridges in the west and central

portions of the study area*. The densely welded member is very sili-

cified and completely devitrified but does not show the vapor phase

alteration prevalent in the partially welded tuff. Maximum welding to

a black vitrophyre with a pronounced eutaxitic texture is exposed only

in the south central portion of the study area* The lower contact of

the densely welded member is variable and usually abrupt, with the"

upper member frequently forming ledges or cliffs 10* to 50* .thick..

Lithologically, this member is similar in composition to the partially

welded member.

Thin section inspection reveals that all three members of the

Castle Peak Tuff are compositionally similar. Quite crystal-rich, the

phenocrysts in the tuff compose approximately 35% of the rock and con­

sist of quartz, sanidine, plagioclase and minor biotite (Table 1). The

quartz and sanidine phenocrysts are very clean, neither embayed nor

altered, and average 0.5 mm to 1 mm in size. Fine-grained biotite

■ 17is also fresh and has only thin magnetite resorbtion rims. Plagioclase

crystals exhibit only minor clay alteration in the unwelded and densely

welded members; maximum alteration of these grains occurs in the

partially welded member with intense vapor phase alteration. Lithic

clasts are composed of Paleozoic rock types and minor andesite.

Although locally more abundant, they average 5% of the total composition.

Lunate and cuspate glass shards dominate the groundmass, commonly very

Fe-stained in the welded subunits. Large pumices, usually 1 mm to 6 mm

in length, and interstitial opaques are also ubiquitously present.

Devitrification of the shards and pumices to axiolitic intergrowths

of microcrystalline quartz and feldspar increases with welding in the

tuff. Microscopic'alignment and flattening of shards, pumices, biotite. • . ' .. ■ . ■

and plagioclase laths evidence the increase in welding in the tuff.

At least two cooling units in the Castle Peak Tuff have been

identified in the northwest portion of the study area. Here, two

densely welded zones are separated by a partially welded unit that con­

tains a distinctive 10' thick baked rubble zone. Lithic clasts in this

zone are up to 2s in size and composed of a rhyolitic(?) vitrophyre and

pre-Tertiary sediments (Figure 5a). This rubble zone marks the bottom

of the upper cooling unit (Smith, 1960a,b). No compositional differ­

ences occur between the partially welded tuffs on either side of the

rubble zone. The upper cooling unit has a much thicker (201 to 50*) and

more resistant densely welded zone than the lower (81 to 10*). The

upper is more siliceous and the lower has slightly higher flattening

ratios. Both make ridge-forming dip slopes (Figure 5b), with the rubble

Figure 5. Multiple Cooling Units of the Castle Peak Tuff in the Northwestern Portion of the Study Area.

a. Rubble zone between the two cooling units of the Castle Peak Tuff shown in Figure 5b.Clasts are composed of welded and vitrophyric tuff (red and black), as well as Paleozoic limestones (light gray).

b. Multiple cooling units of the Castle PeakTuff are indicated by two densely welded zones that form dip-slope ridges (center of photo­graph) . Age date sample CVD 7 was taken in the upper cooling unit on the ridge to the left. Looking north, Columbus Marsh occurs to the far left and the Candelaria Hills in the distance. _

18

Figure 5a.

Figure 5b.

19

bed near the center of the intervening valley. The majority of the

Castle Peak Tuff exposures in the remainder of the project area only

indicate one cooling unit.

4. Tcpe. The unwelded member of the Castle Peak Tuff contains

large exotic blocks of different compositions. These blocks have been

identified elsewhere in the Monte Cristo-Range (Stewart, pers. comm.,

1982) and indicate proximity to the source area of the tuff. Apparently

unrooted, the blocks are oxidized, Fe-stained and commonly have slicken-

sided, silicified rinds. The tuff surrounding them is unaltered and

has only a slight darkening of color. Three compositionally distinct

blocks have been identified in the study area. The largest, at least

200* x 400* x 200* large, occurs in the northeast and is composed of a

partially vesiculated, feldsparr-rich rhyolite (Figure 6). Heavily

Fe-stained, banding and pumice alignment in the block is northerly and

almost perpendicular to the attitude of the surrounding tuff (Figure

2). Two other large blocks have been identified in the southern portion

of the study area; each is compositionally distinct and heavily Fe-

stained. The one in the south central portion is composed of a lithic-

and plagioclase-rich andesite tuff and is clearly surrounded by the

Castle Peak Tuff. The others in the west central portion do not exhibit

as clear contact relations as the ones discussed above and consist of a

crystal-poor, lithic-rich rhyolitic tuff. Each block has attitudes

highly discordant to the orientation of the surrounding Castle Peak

Tuff.

20

Figure 6. Exotic Block of Oxidized Feldspar-RichRhyolite in Unwelded to Partially Welded Castle Peak Tuff. Exposed in the north­eastern portion of the study area, the smaller blocks to the right are 101 to 20' wide.

21Banded Rhyolite Intrusion*1. Tri. The banded rhyolite intrusion crops out only in the

central exterior of the southwestern Monte Cristo Range (Figure 2)„

Characterized by its alternating light gray and reddish purple bands

Oo5 mm to 2 cm wide, the rhyolite forms three large, east-west trending

dikes that intrude the Castle Peak Tuffe Nearly vertical, the rhyo­

litic mass is fault-bounded on its northern edge but has a thin

vesiculated chilled zone at its southern margin. No pumice or eutaxitic

textures are noted in the interior of the dikes or at the other Castle

Peak Tuff contacts. The tuff at these contacts, however, is highly. 1 .altered and consists almost totally of clay.

In thin section, the alternation of the light and dark bands

is very pronounced. The dark bands are composed dominantly of a very

Fe-stained aphanitic groundmass and have only 5% to 1% resorbed and

altered plagioclase phenocrysts. The light bands are much more

plagioclase-rich and contain minor quartz; these unaltered phenocrysts

occur in a slightly devitrified, aphanitic groundmass (Table 1). In

thin section the bands have irregular contacts, occasionally pinched and

feathered, although they are sharp and continuous in hand sample. Rare

inclusions of the Fe-rich material also occur in the light bands.

Coaldale Volcanic Sequence

This volcanic sequence dominates the Tertiary exposures in the

study area. It is informally named and grouped for its local exposure

and lack of continuity throughout the rest of the Monte Cristo Range

and other nearby volcanic centers. The Coaldale volcanic sequence is

22composed of five major subdivisions, three of which can be broken down

further into, mappable units. In increasing age, these subdivisions are:

sediments and tuffs (Tstl and Tstu), hornblende andesites (That, Thai,

Thai, and Tha), fine-grained andesites (Tfai), pyroxene andesites

(Tpat, Tpai, and Tpa), and an intrusive rhyolite (Tij.

Sediments and Tuffs. The sediments and tuffs at the base of

this volcanic sequence can be divided into upper and lower members.

The majority of the exposures of both members is in' the southern

portion of the study area. The unit is exposed best just east of, and

on the east flank of, Coaldale Ridge (Figure 2).

1. Tstl. The lower member of this unit is composed of poorly

resistant, highly weathered tuffs with clasts of a feldspar-rich

andesite up to 21 in size. These more resistant clasts form a

pronounced "bpuldery" weathering characteristic in the lower member:

unaltered clasts weather out of, and sit atop, a clay-altered tuf-

faceous matrix. The matrix has white phenocrysts in a greenish gray

groundmass; both are now totally altered to clays. This member is at

least 50* thick and its lower contact is never observed.

2. Tstu. The upper member of this unit is more variable in

composition. Upsection, it consists of a lithic- and pumich-rich pink

ash-floW tuff, thin-bedded air-fall tuffs, reworked tuffs as white

paper shales, and thin quartz-rich rhyolitic tuffs. The pinkish brown

lithic tuff.is quite distinct from the Castle Peak Tuff; unwelded and

friable, it is composed of up to 50% clasts of variable volcanic

23

compositions in an Fe-stained quartz- and plagioclase-rich matrix. No

pre-Tertiary clasts, common in the Castle Peak Tuff, were noted in this

tuff. It is only observed directly east of Coaldale Ridge; nothing

similar was seen further east or west although the air-fall tuffs and

quartz-rich tuffs also seen in this area are seemingly continuous.

These latter units are relatively unweathered and thin-bedded. The

entire sequence is of variable thickness, and at least 150* thick where

both upper and lower contacts are observable. In.general, the upper

contact is with Quaternary alluvium and the lower contact is grada­

tional but usually covered with the lower member of the sediment and

tuff unit.

Hornblende Andesites. The hornblende andesites are the most

common volcanic group in the Coaldale volcanic sequence. Except for

the northeastern and north central portions, these andesites occur .

throughout the study area (Figure 2). They consist of four mappable

units: a hornblende andesite tuff breccia (That), a hornblende

andesite lahar (Thai), hornblende andesite intrusives (Thai, including

dikes, sills and plugs), and hornblende andesite flows (Tha).

1. That. This unit is the oldest of the widespread hornblende

andesites and occurs throughout the southern portion of the project

area (Figure 2). Like the sediment and tuff unit, it is best exposed

on the east flank of Coaldale Ridge. This unit is of variable thick­

ness; over 300' thick in the Coaldale Ridge area, the tuff thins to

approximately 25' thick in the southeast, the tuff breccia conform­

ably overlies the sediment and tuff unit described above. Its upper

24

contact is usually quite sharp with the overlying hornblende andesite

flows.

The hornblende andesite tuff is composed of andesite clasts,

comprising up to 50% of the unit, in a hornblende- and plagioclase-rich

tuffaceous matrix (Figures 7a and 7b). The predominance of clasts in

this unit predicates the term "tuff breccia" as its descriptor. Clasts

in the lower portion of this unit are totally composed of a feldspar

porphyry andesite. Upsection the clasts include a hornblende-rich

andesite, sometimes exclusively, as well as the feldspar porphyry. The

matrix also becomes slightly more hornblende-rich upsection. The color

of the tuff breccia varies from reddish purple to bluish gray in

fresher samples. In thin section the matrix hornblendes are present

in a 5% abundance and average 3 mm in length. These grains are rela­

tively clean, only slightly fractured and have only thin magnetite

resorption rims. Plagioclase phenocrysts dominate the matrix, average

0.5 mm in size and are very altered to clays. Sanidine is present in

minor amounts. The phenocrysts occur in a heavily Fe-stained, glassy

groundmass (Table 1). The clasts range in size from less than 0.5"

to greater than 2* large. In thin section these clasts are all rimmed

by extremely heavy Fe-staining and opaques.

On the east flank of Coaldale Ridge the vertical succession in

this unit includes a distinct 10* to 20' thick flow unit; its clasts

are monolithologic and composed of vitrophyric hornblende andesite.

This marker unit is not seen elsewhere in the study area. Other flow

units are delineated by the presence of several 6" yellowish gray ash

Figure 7. Series of Photographs that Depict Textures inthe Hornblende Andesite Tuff Breccia.

a. Hornblende-rich clasts enclosed in a bleached tuffaceous matrix.

b. Embayed contact between two flow units of the hornblende andesite tuff breccia.

c. „ Multiple flow units in the hornblendeandesite tuff breccia. Looking west- southwest, pyroxene andesite flows and Columbus Marsh occur.in the back­ground.

(Photographs a and b are textures exhibited near the contact shown in c.)

25

Figure 7a. Figure 7b.

Figure 7c.

26layers that are interspersed throughout its thickness. Multiple flow

units are also inducated by chilled contacts between flows in the

northwest (Figure 7c); the embayed, deformed and bleached upper contact

of the lower flow in this figure suggests little time elapsed between

flow eruptions.

2. Thai. The hornblende andesite lahar occurs only on the

flanks of Blue Mountain (Figure 2). Its true thickness in indeter­

minate, but exposures over 2501 thick occur on the southwestern flank

of Blue Mountain. Its upper and lower contacts with the hornblende

andesite tuff and intrusion are usually obscured by float.

This unit consists of a monolithologic hornblende andesite

breccia; the clast composition is indistinguishable from the matrix

composition except for degree of silicification. The matrix is

softer and tuffaceous whereas the clasts are very silicified and quite

hard. As Blue Mountain is approached, the matrix of this unit becomes

more and more silicified until only weathering accents the clast

outline (Figure 8). Multiple flow units are indicated by irregular

bedding in the lahar; beds 10' to 50' thick can be identified only in

its upper, more tuffaceous portions. Where basal contacts are chilled,,

ledgy exposures result. A true "laharic" origin of this unit is

doubtful; the term is used here to distinguish it from the hornblende

tuff breccia, its slightly tuffaceous matrix and bedded nature suggest

a pyroclastic origin on the flanks of a major andesite volcanic center

and are representative of laharic features.

27

Figure 8. Silicified Lower Portion of the Hornblende Andesite Lahar near Blue Mountain.

28Compositionally this unit is much more hornblende-rich than

the tuff breccia. Clasts and matrix in the lahar contain up to 20%

altered oxyhornblende phenocrysts with thick magnetite resorption rims.

Plagioclase phenocrysts are fine-grained and gradational with micro- litic plagioclase in the groundmass. These grains are only moderately

altered to clays. Light gray to bluish gray, the groundmass is very

vitrophyric and unaltered (Table 1).

3. Thai. The hornblende andesite intrusions occur as dikes

(+H+M+H-), widespread throughout the southern and western portion of the

project area, sills (Thais), localized in the south central portion, and

large plugs (Thai), prominent in the west central portion (Figure 2).

The dikes range from 2' to IS' wide, but an extremely wide, multiple

dike intrusion occurs near the largest plug. Only two sills have been

identified in the area, each at least 25* thick. The largest intrusive

plug occurs at Blue Mountain and another, only slightly smaller, to

the west. Two smaller plugs have been tentatively identified in the

extreme northwest.

The dikes and sills are of similar appearance and composition

(excluding the large dike complexes near Blue Mountain). With a

greenish gray matrix, both are very porphyritic with hornblende

phenocrysts that average 8 mm in length but can reach 2 cm in size.

These grains have thick, well-defined magnetite resorption.rims and

are very fractured. Abundant plagioclase phenocrysts are much finer-

grained, with 1 mm as an average size, and are gradational to microlitic

groundmass plagioclase. The crystals are very altered to clays and

29calcite in their cores and on their rims. The groundmass is very dirty

and usually altered to a fine-grained mixture of chlorite, calcite and

clays. Calcite also commonly fills the fracture openings in the horn­

blende phenocrysts (Table 1).

The dikes and sills have well-defined chilled margins 6" to 3’

thick when the contacts are exposed. Usually instrusive into the

unwelded Castle Peak Tuff, the tuff shows only minor alteration by a

color change from white to purple within 10' of the contact and chilled

zones less than 2' wide. When intrusive into other volcanic types, the

chilled margins are much thinner and no contact alteration is evident

in the intruded units.

The large intrusive plugs in the study area occur close to the

contact between pre-Tertiary and young volcanic rocks. The Blue

Mountain plug is the largest of the intrusions and has a conical shape.

Intrusive into the hornblende lahar, it has steep slopes that are

parallel to the joints and flow foliation in the andesite (Figures 9a

and 9b). Very large dike complexes trend northwest-southeast on either

side of this intrusion. These dikes do not exhibit the chloritic

alteration common in the smaller dikes throughout the area; rather, they

are much more similar in composition to the Blue Mountain intrusion.

Semi-parallel chilled rinds occur on either side of the central plane

in a few of the dikes present in. this complex; mirror image repetition

of the chilled margins indicates a multiple intrusive history in these

rocks. ,

Figure 9. Blue Mountain Intrusion,

a. Looking northeast, Light unit on the flanks of the plug is the hornblende ; andesite lahar; dark units in the foreground are hornblende and pyroxene andesite tuff breccias intruded by dikes,

b. Looking northwest, Ledgy units to the left are hornblende andesite lahar flows(?).

Figure 9b.

31The large intrusion to the northwest of Blue Mountain is

elongate rather than conical in shape (Figure 2). Flow foliations are

variable in this body but generally dip to the south. A small, dike­

like plug with characteristic chloritic alteration continues to the

south of this intrusion.

Two additional, but much smaller, plugs occur in the north­

western portion of the area. Contact relationships in these units are

not as clear-cut as in the intrusions discussed above. These smaller

plugs and the large intrusion northwest of Blue Mountain are in contact

with the Castle Peak Tuff rather than the hornblende lahar.

The intrusions and large dikes are much more hornblende-rich

than the related dikes and sills. The former contains 15% to 20%

unaltered aligned hornblende phenocrysts which average 3 mm in length

and have only thin magnetite resorption rims. Abundant plagioclase

phenocrysts are fine-grained, average 0.5 mm in size and are generally

unaltered to clays. Aligned microlitic plagioclase is absent in the

intrusive plugs but well-defined in the large dike complexes. The

plugs* groundmasses are extremely glassy and aphanitic with no evidence

of alteration; the large dikes show only slight chloritic alteration of

a less glassy groundmass. Both contain up to 2% unoxidized interstitial

opaques (Table 1).

4. Tha. Hornblende andesite flows are the most pronounced

unit in this andesitic series. Occurring throughout the southern

portion of the study area, this resistant unit commonly caps ridges

(Figure 10). The thickest and best exposures of the hornblende flows

32

Figure 10. West Flank of Coaldale Ridge. Most ledges and ridge caps in the background are horn­blende andesite flows. Lower reddish brown ledge is a pyroxene andesite flow; CVA 29 was taken in the outcrop to the left. Small greenish gray outcrop on the lower left is a sericitized hornblende andesite flow(?) where CVA 25 was taken (see Figure 30). All unresistant units are the hornblende andesite tuff breccia with thick salt accumulations just beneath the surface.

33are on. the west flank of Coaldale Ridge (Figure 2), Two major flows,

each greater than 50? thick, occur in the south central portion of the

study area whereas only one flow is evident near the Blue Mountain

plug. The two flows in the south are very siliceous horizons separ­

ated by a thin, less siliceous unit of similar composition. Multiple

flow units near Blue Mountain are suggested by the thickness of the

unit in this area, 100• to 300*, but are not defined because of their

proximity to the intrusive source. Hornblende andesite flows usually

conformably overlie the tuff breccia.

Minor Paleozoic sedimentary rocks and Castle Peak Tuff clasts

occur as lithic inclusions at the very base of those flows farthest

from the intrusions. Vertical cooling joints and gently dipping flow

foliation parallel joints are pronounced in the siliceous center(s)

of the flow(s). Less siliceous, more weathered, hornblende andesite

also occurs as interlayers between flows and as individual units to the

southeast of Blue Mountain. Although highly fractured, these do not

exhibit the pronounced foliation parallel joints seen in the siliceous

portions of the flows.

The hornblende andesite flows differ from the other hornblende

units by containing 2% to 5% fine-grained augite(7) phenocrysts.

Averaging 0.5 mm in size, these crystals are fractured but unaltered

although thin rims of pigeonitef?) are common. The flows also differ

by having finer-grained, 2 mm long, aligned oxyhornblende phenocrysts

in less abundance. Thick magnetite resorption rims surround the

oxyhornblende grains. Fine-grained plagioclase phenocrysts, 10% of

34the mineralogy, have clay-altered cores and rims, Microlitic

plagioclase shows a well-defined flow foliation and is generally

unaltered. The groundmass is very glassy and has only a slight

chloritic alteration. Pyroxene exists as interstitial granules within

the groundmass. The less siliceous equivalents of this unit have

totally altered oxyhornblende phenocrysts and a heavily Fe-stained

groundmass (Table 1)..

Fine-Grained Andesites.,

1. Tfai. Only one phase of the fine-grained andesite has been

identified in the study area. The majority of the exposures of this

intrusive unit occurs in the south central project area, although minor

outcrops in and near the northern edge of Coaldale also exist (Figure

2). Always intrusive into the hornblende lithic tuff, the fine-grained

andesite exhibits only thinly chilled margins. Minor baking of the tuff

along the contact is displayed by a slight darkening of its color.

Closely spaced joints, 0.5" to 2" apart, parallel to the flow foliation

are prominent in all exposures of the fine-grained andesite. Typically

a deep reddish brown on weathered surfaces, the andesite is gray when

fresh. No younger volcanic rocks were seen to intrude or overlie this

unit.

Compositionally the fine-grained andesite is related to both

the hornblende and pyroxene andesites and contains equal proportions of

hornblende, pyroxene and plagioclase fine-grained phenocrysts. The

hornblende crystals average 0.6 mm in length and have thick magnetite

resorption rims with only moderate flow alignment. Augite(?)

35phenocrysts are less than 0.4 mm in size and are unaltered although

commonly rimmed by pigeonitef?). Plagioclase phenocrysts are slightly

coarser-grained and. average 0.8 mm in size. The majority of the

grains have cores altered to clays. Microlitic plagioclase in the

groundmass is unaltered and exhibits a discontinuous grain alignment.

The groundmass is glassy, with interstitial pyroxene, and typically

altered to clays, chlorite and carbonate(?). In only one occurrence in

the south central portion of the study area has hypersthene been

recognized in this unit.

Pyroxene Andesites. This volcanic group overlies and intrudes

the older hornblende andesites and is prominently exposed along the

western exterior of the Monte Cristo Range (Figure 2). Like the horn­

blende andesites these andesites exist as both intrusive and extrusive

phases and can be subdivided into three mappable units: a pyroxene

andesite tuff breccia (Tpat), pyroxene andesite intrusions (Tpai,

including dikes, sills and plugs), and pyroxene andesite flows (Tpa). 1

1. Tpat. This unit is analogous to the hornblende tuff breccia

and, except for a pyroxene composition, is very similar to it in

character. Although the best exposures are on the southern flank of

Blue Mountain, the pyroxene tuff occurs throughout the northwestern

portion of the study area. The tuff is not as widespread or as thick

as its corresponding hornblende unit. It irregularly overlies the

hornblende tuff breccia and is overlain or intruded by the pyroxene

flows and sills. Although its true thickness is unknown, exposures of

the tuff vary between 10' and 80' in thickness (Figure 2).

36

Commonly reddish purple on weathered surfaces, fresh pyroxene

tuff breccias are pinkish gray or green, Compositionally it is almost

monolithologic, with coarse-grained siliceous pyroxene clasts in a

tuffaceous pyroxene-rich matrix. Clasts contribute up to 50% of the

total rock, again predicating the use of the term "tuff breccia." The

clasts average 4" in size although blocks greater than 21 are not

uncommon (Figure 11)„ Only 10% of the clasts are composed of a

feldspar-rich porphyry rather than the pyroxene andesite. The matrix

typically consists of 5% augite(?) and 35% plagioclase phenocrysts,

Augite(?) grains are highly fractured, sometimes with clays filling

the interstices, and commonly occur as large cummulates, The plagio­

clase grains are highly altered to clays. The groundmass is heavily

Fe-stained and has minor chloritic alteration. Fine-grained pyroxene

and opaque granules are interstitial in a slightly devitrified

groundmass (Table 1), The pyroxene clasts are similar in composition

with a more glassy and less devitrified groundmass, As in the

hornblende tuff breccia, the clasts are always rimmed by heavy Fe-

staining and numerous opaques,

2, Tpai, The pyroxene intrusions are represented in three

phases. Dikes (—» ■>-) are the most abundant and widespread of the

intrusions and occur throughout the northwestern portion of the study

area. Several dike-fed sills (Tpais) have also been tentatively

identified near Blue Mountain, Intrusive plugs are much less numerous

and much smaller than their hornblende analogs and occur intermittently

in the northwestern arid central portions of the project area (Figure 2),

t

37

Figure 11. Pyroxene Andesite Tuff Breccia near Blue Mountain. Monolithologic siliceous pyroxene-rich clasts occur in a tuffaceous matrix. Most clasts are less than 4”, although blocks up to 2' are not uncommon (top left).

38

All phases of the intrusions weather a dark reddish brown but are

greenish gray when fresh.

Pyroxene dikes are concentrated on the southern flank of Blue

Mountain and the northwestern edge of Coaldale Ridge. Intruding the

hornblende tuff breccia in the latter area, they have strongly altered

the surrounding tuff and produced a widespread Fe-staining. In the

Blue Mountain area they intrude the pyroxene tuff breccia as well as

the hornblende unit and are again associated with a widespread

Fe-staining<, The unresistant dikes are 5 1 to 158 wide and sometimes

trap more resistant wedges of hornblende andesite flows between

closely spaced intrusions. In thin section the dikes have a con­

spicuous, very glassy groundmass with only minor chloritic alteration.

Phenocrysts of augite(?) and plagioclase are less abundant than in the

other pyroxene phases. Plagioclase crystals exhibit excellent flow

foliation by alignment of laths (Table 1).

A few of the dikes in the Blue Mountain area can be followed

upsection to overlying pyroxene sills or flows (Figure 2). The intru­

sive or extrusive nature of the latter units cannot be determined as

they lack overlying exposures. They are typically very coarse­

grained and exhibit only thinly chilled lower contacts; on this basis,

and for petrographic dissimilarities (see below), they have been

tentatively identified as sills rather than flows.

The other intrusive phase of this unit is very small pyroxene

plugs, none of which have areas greater than 300 square feet. In this

phase augite(?) phenocrysts are more abundant than in the other phases

39

and have a distinct bimodal size distribution. Most are fine-grained

and average 0.5 mm to 1 mm in size, but at least 3% are coarse-grained

and average 3 mm to 8 mm in size. Minor cummulate grains up to 1.5 cm

large also occur. Abundant plagioclase phenocrysts, 1 mm in size, have

only moderate clay-altered cores. Similar to the other pyroxene

phases, the groundmass includes interstitial pyroxene and opaque

granules. However, the groundmass is slightly coarser-grained and

shows greater chloritic alteration than in the other phases (Table 1).

3. Tpa. The pyroxene flows are best exposed at the extreme

western edge of the study area and range in thickness from 101 to 50'.

Like the hornblende flows, these resistant units commonly form dip

slopes and ridge caps. Their lower contacts have moderately chilled

margins usually 2" to.5" wide. The pyroxene flows usually overlie

unaltered hornblende or pyroxene tuff breccias, and occasionally,

hornblende andesite flows. At least one angular unconformity occurs

between this unit and underlying volcanic rocks on the east flank of

Coaldale Ridge (Figure 2). .

These flows are composed of augite(?) and lath plagioclase

phenocrysts which average 2 mm and 1 mm, respectively. The augite(?)

grains are unaltered but fractured with groundmass and clays filling

the interstices. Plagioclase laths exhibit fairly good alignment and

have clay-altered cores. Quartz was tentatively identified in trace

amounts in one thin section. In addition to these phenocrysts, an

altered equigranular mineral is ubiquitously present and contributes-

up to 15% of the total mineralogy. Fine-grained, usually 0.5 mm in

40

in size, it has a strong, nonpleochroic reddish brown color. It has

been tentatively identified as hydrobiotite, an original alteration

product of a fine-grained, unstable pyroxene„ This mineral is not seen

at all in the pyroxene sills discussed above, supporting an intrusive

rather than extrusive origin for these questionable outcrops near Blue

Mountain. The groundmass of the pyroxene flows consists of aligned

microlitic plagioclase and interstitial opaques in a chloritically

altered glass (Table 1).

Intrusive Rhyolite.

1. Ti. Only one exposure of the intrusive rhyolite has been

identified and it occurs in the southeastern portion of the study area.

Quite small, this plug does not intrude or contact any other rock type

and seems localized along a major fault (Figures 2 and 12). It is a

late volcanic phase and tentatively identified with the Coaldale

volcanic sequence.

This rhyolite is remarkably fresh when compared to the Castle

Peak Tuff and related banded rhyolite dikes. Very light pink, it has

a dense finely crystalline texture in its interior. Toward its

exterior, however, lithic clasts composed of feldspar- and hornblende-

rich andesites and vesiculated pumice become prominent in a light

brown chilled margin. The pumices show a variable attitude around the

plug's exterior and circumscribe its contact. This unit is crystal-rich

with abundant unaltered quartz, sanidine and minor biotite phenocrysts.

Glass shards are prevalent near its outer margin but become more indis­

tinct in the fine-grained felsitic groundmass in its interior.

41

Figure 12. Small Pink Rhyolite Plug in the Southeastern Portion of the Study Area. Age date sample CVD 2 was taken on the far side of the plug (center of photograph). Prominent hills just behind the plug are fine-grained andesite intrusions. To the right of the rhyolite plug are Esmeralda Formation sedi­ments (CVA 8 sample locality), as are the light rocks in the distance.

42

Blair Junction Volcanic Sequence

This volcanic sequence only occurs in the northeastern and

north central portions of the study area (Figure 2). The units are

represented throughout the remainder of the Monte Cristo Range and are

especially prominent in the Blair Junction area; they are herein

informally named for this exposure. This sequence overlies the Castle

Peak Tuff in all cases and can be subdivided into four mappable units:

sediments (Ts), a coarse-grained andesite breccia (Tcab)/ coarse-grained

andesite flows (Tea), and fine-grained andesites (Tfa),

1, Ts, These sediments overlie the Castle Peak Tuff and are

preserved under an andesite cap along the northeast ridge in the north­

eastern portion of the project area. They are usually 25r to 508 thick

and a distinct bluish gray color. Very tuffaceous and volcaniclastic,

they consist of interbedded mudstones, siltstones and coarse-grained

pebbly sandstones. The fine-grained units are laminated and thin-

bedded, 2n to 6f? thick, whereas the coarse-grained units are more

moderately bedded, 6?f to 8fT thick. Occasional massive, unsorted

pebble sandstones also occur (Figure 13a). Clasts in the sandstones

are composed of pumice and tuff chips.

2. Tcab. The coarse-grained andesite breccia invariably under­

lies the coarse-grained andesite flows. Of variable thickness, usually

ranging from 5 8 to 50*, it weathers to a reddish purple color but is

gray when fresh. This basal breccia is monolithologic and consists"

of 50% to 70% angular clasts of coarse-grained andesite which range

Figure 13. Blair Junction Volcanic Sequence.

a. Volcaniclastic. sediments underlying the coarse-grained andesite flows in the northeastern portion of the study area.

b. A typical volcanic section in the northeastern portion of the study area. Greenish gray ridge caps are coarse­grained andesite flows. CVC 12 was taken on the far ridge. These flows overlie a reddish purple monolithologic breccia. Basal unit is the unwelded member of the Castle Peak Tuff.

43

Figure 13a.

Figure 13b.

44in size from less than 0.5" to greater than 2 *„ These siliceous clasts

are encased in a matrix of comminuted andesite (Figure 13b).

Thin section inspection shows the clasts contain more, coarser-

grained, and less altered oxyhornblende phenocrysts than the surrounding

matrix. Plagioclase crystals are present in each in equal amounts and

have prominent clay-altered cores. The groundmass in the matrix is

less dense and less Fe-stained than in the clasts. As in the other

breccias in the project area, each clast is rimmed by an extremely

Fe-stained and opaque-rich groundmass. Only less than 5% of the clasts

are of a slightly different composition: these resemble crystal-rich

cummulates with less than 20% evident groundmass and equal proportions

of oxyhornblende arid plagioclase laths (Table 1).

3. Tea. The coarse-grained andesite caps the prominent ridge

in the northeastern portion of the study area (Figures 2 and 13b).

It abruptly overlies the breccia described above and varies in thick­

ness from 25* to greater than 500* north of the current study area. The

color of the coarse-grained andesite ranges from light gray to, more

typically, a grayish green. Discrete exposures of this andesite alorig

the northeast ridge have pronounced rosette-type columnar jointing.

Marked with an asterisk on Figure 2, these are tentatively identified

as source vents for the local exposures of this widespread unit.

This unit is characterized by fresh, very coarse-grained laths

of plagioclase that average 2 mm in size but commonly reach up to 1 cm

large. Oxyhornblende is the only other dominant phenocryst and

exhibits a bimodal size distribution. Most average only 1 mm in size

45and are totally altered to Fe-oxides and chlorite. At least 2%,

however, average 5 mm to 8 mm.in length and are less altered. The

finer-grained oxyhornblende grains have thin, very distinctive ghost

outlines of fine-grained clays. Minor augite(7) also is present as

fine-grained phenocrysts and are 0.5 mm to 1 mm in size. Although,

fractured, these grains are unaltered. Biotite and quartz (?) occur

only in trace amounts. The groundmass consists of oriented plagioclase

microlites and interstitial opaques in a chloritically altered glass

(Table 1).

Tfa. The fine-grained andesite is compositionally similar to,

but texturally different from, the coarse-grained andesite. It occurs

as discontinuous dikes and small plug in the northeastern and north

central portions of the project area (Figure 2). In all cases it

intrudes the Castle Peak Tuff without large chill margins or major

alteration in the surrounding tuff. It does not overlie or intrude

the characteristic breccia associated with the coarse-grained flows.

The fine-grained andesite lacks the large plagioclase laths

characteristic of the coarse-grained andesite; rather, it contains

slightly less abundant plagioclase phenocrysts. that average 0.5 mm to

1 mm in size. Fine-grained oxyhornblende and pyroxene phenocrysts are

present in minor abundance and are both equal in size to the plagioclase

grains. Like the finer-grained oxyhornblende grains in the coarse­

grained flows, these crystals in the fine-grained andesite have thin

ghost outlines of clays and are totally altered to Fe-Oxides and chlorite. The groundmass is aphanitic and chloritically altered; it

46

consists of aligned plagioclase microlites and interstitial opaques in a glassy matrix (Table 1) «,

Younger Sedimentary and Basaltic Sequence

The younger units in the study are Esmeralda Formation sediments

(Te) and basalt (Tb)» These units have limited exposures in the study

area although both are regionally distributed.

1. Te. The Miocene age Esmeralda Formation is only represented

in the southeastern portion of the project area (Figure 2). One

definite outcrop, and two questionable occurrences of this formation

exist although it is extensively exposed in the adjacent Blair Junction

area to the east. vSe Moore concentrated his work on this sedimentary

sequence and the reader is referred to his analysis (Moore, 1981) for

a complete description of the Esmeralda Formation in the southern Monte

Cristo Range.

The one definite exposure of this formation is composed of

interbedded tuffaceous mudstones, siltstones, reworked air-fall tuffs

as paper shales, and lignitic shales. This occurrence represents less

than 300? of Moore’s lowest member of the Esmeralda Formation. Mud­

stones and reworked tuffs dominate the observable section, each

present in intervals greater than 50’ thick. The other units are

thinner and interbedded with mudstones and reworked tuffs. This outcrop

is in fault contact at its southern extent with hornblende andesites

and the lower member of the sediment and tuff unit. Its northern

contact is sloughed and of an ambiguous nature. The lack of fault

47

indicators in this zone suggests that the sediments may be in deposi-

tional contact with the underlying lower member of the sediment and

tuff unit. A depositional contact between these units is also

suggested by the questionable outcrops further south (Figure 2).

2. Tb. The basalts occur as small isolated outcrops in the

northwestern portion of the study area (Figure 2)„ These are presum­

ably thin remnants of the voluminous basalt flows seen in the northern

Monte Cristo Range.. The basalt overlies the Castle Peak Tuff; contact

exposures are poor and cannot refute an intrusive origin for this unit.

The dark gray to black basalt is very aphanitic. The only

phenocrysts present are hypersthenef?) which average 1 mm in size and

suggest a basaltic andesite composition. These crystals occur in a

pilotaxitic groundmass consisting of extremely abundant microlitic

plagioclase with interstitial opaques, pyroxene granules and glass

(Table 1).

Quaternary Deposits

1. Q-. Quaternary alluvium occurs throughout the project area

and has not been subdivided into older and younger units.

2. Qsp. Two spring deposits with geochemical significance are

given special designation in this study. One modern spring occurs in

the northeastern portion of the project area (Figure 2). Currently

water-producing, this spring is surrounded by calcite crusts and thick

white salt- and gypsum-rich accumulations in the nearby soils.

48

The other spring deposit is one tentatively identified in the

northwestern portion of the study area (Figure 2). Here, thin-bedded

sediments overlie the Castle Peak Tuff and hornblende tuff breccia in

angular unconformity. These gently dipping sediments consist of a v

basal chert conglomerate and upper thin-bedded sandy limestones.

Although the majority of these sediments resemble older alluvium or

even the Esmeralda Formation, distinct local limestone breccia beds

with layers of open-space filling chalcedony and travertine occur within

the sequence. The layered silica and calcite suggest a spring may have

functioned during their deposition.

CHAPTER 4

AGE AND CORRELATION OF TERTIARY UNITS

Many correlations of the rock units in the Monte Cristo

Range have been made without radiometric age control. The variety

of andesites alone in this large mountain range makes correlation

based only on petrographic similarities difficult. When originally

mapped by Ferguson and others in 1953, all andesites in the Monte

Cristo Range were grouped and collectively called the "Gilbert Andesite."

K-Ar ages obtained from the "Gilbert Andesite" have been reported at

15.5 ±0.5 m.y. (Silberman et al., 1975) and 15.5±0.6 m.y. (Albers

and Stewart, 1972). Subsequent correlations did not account for the

number and variety of andesites included in the "Gilbert Andesite,"

and thus many porphyritic andesites in the Monte Cristo Range have

been assumed to be 15 m.y. old. The age relationships between the

many tuffs, andesites and sediments in the Monte Cristo Range are

currently being studied by J. Stewart and others of the U.S. Geological

Survey. His detailed mapping has revealed many subdivisions of the

"Gilbert Andesite" and many age relationships not previously identified.

Although contact relationships can. be used for relative age deductions

within a local area, radiometric age data are needed for more regional

correlations— even within the same mountain range if it has had a

complex volcanic history.

49

50

Radiometric K-Ar age determinations for three key units in the

southwestern Monte Cristo Range are presented in this report. They

include two mineral dates on the Castle Peak Tuff, and one mineral date

each on the Coaldale rhyolite and hornblende andesite intrusions.Table 2 summarizes the age data obtained and descriptions for all

samples are given in Appendix A.. Figure 14 is an overlay to the

geologic map and shows the sample localities in the study area. Four

geologic cross sections across the study area that illustrate most of

the contact relationships between the units are given in Figure 15.

With these age controls and crosscutting relationships a complete,

correlatable stratigraphic sequence for the study area can be devised.

Regional correlations with the new data are tentative but, in light of

Stewart’s current work, more accurate than previously possible.

Figure 16 presents a correlation chart that references thel *Tertiary units in the project area to those in the nearby Candelaria

Hills and Silver Peak Range. The regional Tertiary section by Gilbert

and Reynolds (1973) for the western margin of the Basin and Range Pro­

vince is given as an overall comparison. Asterisks mark positions of

currently available age dates, while those in parentheses are units

that have age dating in progress. The time scale used in this chart

is that being used by the U.S. Geological Survey in its Decade of North

American Geology project.

Castle Peak Volcanic Sequence

The Castle Peak Tuff in the study area is correlative to many

late Oligocene and early Miocene rhyolite ash-flows in the Basin and

TABLE 2Analytical Data for Age Determinations of Tertiary Rocks

Southwestern Monte Cristo Range

SampleNo.

LithologicUnit

MineralDated IK v°

(xl0~12 moles/gm) %AratmAge m.y.

(± one std. dev.) Location

CVD 2 Ti sanidine 5.882 190.7 3.8 18.6 ± 0.4small plug in 38*02.51' N

SE

CVD 5 Thai hornblende 0.769 29.85 25.4 22.2 ±0.5

117*50.36' W

SE flank Blue 38*04.35' N

Mtn.

CVD 7 TcP3 sanidine 6.252 260.9 4.2 23.9 ±0.6

117*51.40' W

Upper cooling unit

CVD 7 Tcp3 biotite 7.195 306.8 12.8 24.4 ± 0.6

in NW38*05.24' N

117*52.54' W

Constants used: BX

40K/K

4.963 * 10'10 yr"1 0.581 xlO"10 yr"1 5.544 x10~10 yr"1 1.167x10 atom/atom

All analyses done at the University of Arizona Isotope Geochemistry Laboratory. Ar determina­tions done with a Nier-type 60° sector 6" mass spectrometer. K determinations by atomic absorption.

Figure 16. Regional Correlation of Oligocene to Pleistocene Rocks in Selected Areas of Southwestern Nevada. Vertical lines indicate strata absent. Local unconformities are not shown. Asterisks mark available age dates, while those in parentheses indicate age dating in progress. • In southern Monte Cristo Range (column 4): T e = Esmeralda Fm.; Tai= intrusive andesite;Tri = intrusive rhyolite. See text for correlation of undated units in this column with age dated volcanic rocks of this report (column 6). Definition and correlation of units throughout the Monte Cristo Range (column 5) are still in progress.

®*y• b*p•

Epoch

Early

10-

18-

Early

Ollgocene

Early

Generalized Western Margin of the Basin and Range Province(Gilbert I Reynolds,

1973)

Morgan Ranch Fn.

Basaltflows

CoalValley Fn.

Aldrich Station Fn.

RhyoliteIntr.

Andesite flows I dikes

Older Andesite

RhyoliteIgninbrltes

CandelariaMills

(Speed | Cogblll, 1979b)

PlioceneBasalt

MioceneSedlnentary

Rocks

]]]]]]]]:MioceneAndesite

Upper Ollgocene ■Hiffs

North Fish Lake Valley

Stiver Peak Range (Robinson I Chowder,

1973)

JJJ/

ITltlTlTSilver Peak Volcanics

Esneralda Pn. •

Basaltic Andesite

TIT)"oznrJJJJ Tel 1U^>er Trl

Andesite

Hornblende Andesite

Rhyoliteniffs

Southern Monte Crlsto Range

(Moore, 1981)

Esneralda Fo nation

OHIO ICO :Coarse-grained Andes.

icon icon:Fine-grained Andes.

Castle Peak Tuff

Monte CrlstoRange

(Stewart,pers. conn., 1983)

_ _ I"'Basalt jsed. Rocks

|Rhyo. Intr.*

Esneralda Fo nation

icon icon:Gilbert Andesite

McLeans Fn. (diaton.)

§ Upper Mb. ^ 1 Coarsegrain.andes.

Junct;

latior | \55 Lower Mb.

1Fines

1 1• - sedlhehts “.i i u n sitrain.andes.

JULLCastle Peak Tuff

Southwestern Monte Crlsto Range

(this report)

Esneralda Fo nation

Rliyo. Intr.

PyroxeneAndesiteOne-grainHornblendeAndesite^

sedsjtuff

Banded r l iy o ^ /

Castle Peak Tuff

LnK)

53

Range (Figure 1.6). It has been dated at 23.9 ±0.6 and 24.4 ±0.6 m.y.

in the northwestern portion of the project area (Table 2; Figure 14).

This unit clearly corresponds to the late Oligocene tuffs in the

Candelaria Hills (Speed and CogbilL, 1979a) and rhyolite tuffs at the

base of the Silver Peak volcanic sequence (Robinson et al., 1968).

Current work by Stewart (pers. comm., 1983) in the Monte Cristo

Range suggests the Castle Peak Tuff, mapped by Moore and myself as one

unit in the south, may be composed of two tuff units. He places the

Castle Peak Tuff (proper) below the informally named tuff of Blair

Junction (Figure 16). Stewart feels that the age date reported here

may be on the upper ash-flow. This would make the "true" Castle Peak

Tuff of Stewart equivalent to the older tuffs of Speed and Cogbill

(1979a). Supporting this hypothesis is an older and regionally exten­

sive series of ash-flow tuffs in the Gabbs Valley and the Gillis Ranges,

northwest of the Monte Cristo Range (Ekren et al., 1980). These range

in age from 24 to, 28 m.y.

As mentioned above, however, my work did not find any appreci­

able petrographic differences in the Castle Peak Tuff as mapped in the

current study area. Thin section inspection of the upper and lower

cooling units present where the age date sample was taken (Figure 14)

showed no irregular phenocryst or lithic assemblages. On this basis,

and supported by the similarity of andesites as discussed below, the

Castle Peak Tuff in the current study area is taken to be one unit.

Although two tuffs may be present elsewhere in the Monte Cristo Range,

only one is believed present on its southwestern flank. Further

54

radiometric age control and a detailed study of the tuffs should

clarify the possibility of multiple ash-flows in this region.

The flow banded rhyolite dikes clearly intrude the Castle Peak

Tuff in the central portion of the study are (C-C, Figure 15). Their

compositional similarity to the Castle Peak Tuff suggests an age only

slightly younger than the 24.2 m.y. old tuff.

Coaldale Volcanic Sequence

The sediment and tuff unit of this sequence clearly underlies

all andesitic units and overlies the Castle Peak Tuff; it is the

oldest of the Coaldale volcanic sequence (Figure 14). This unit has

no known correlatives outside the Monte Cristo Range (Figure 16).

Intruded and overlain by the pyroxene and fine-grained andesites,

the hornblende-rich units are the oldest andesitic volcanic rocks in

the Coaldale sequence.• All the hornblende units are related closely in

age; the Blue Mountain plug has been K-Ar dated at 22.2 ± 0.5 m.y. (Table

2). Within this series the flows are the youngest and overlie the tuff

breccia. Although the tuff breccia and lahar may be time equivalent,

one near source and the other more distal, their contact exposure

suggests the tuff breccia, at least in part, overlies the lahar (B-B',

Figure 15). The relative age of the plugs is more difficult to ascer­

tain. Although the Blue Mountain plug intrudes the lahar, it could

have been partially time equivalent to lahar and flow eruptions.

Studies of modern andesitic centers (MacDonald, 1972) suggest intrusion

occurs throughout the volcanic activity, with a larger pulse towards its

end. Such a scenario is implied in the Blue Mountain area. The

55chloritically altered dikes and sills of the hornblende andesite intrude

almost all units in the study area. Their intrusion presumably climaxed

during the maximum phase of hornblende andesite activity but definitely

continued through most of the Coaldale volcanism (C-C, Figure 15).

The pyroxene andesites show variable contact relationships.

They unconformably overlie and intrude both the hornblende-rich units

and the sediment and tuff unit (Figures 2 and 15).. This suggests at

least a moderate angular deformational and/or erosional period occurred

prior to their extrusion. Like the hornblende-rich units, the pyroxene

andesites are all closely related in age and probably just slightly

younger than 22.2 m.y. old Blue Mountain plug. The pyroxene tuff

breccia is clearly 'the oldest unit, and the flows, dikes and sills the

youngest. The small, plugs probably did not span as great a length of

intrusion as the hornblende plugs, and are most likely the youngest in

this series.

Although the fine-grained andesite is younger than the horn­

blende units (C-C, Figure 15), its upper age limit is not constrained.

As it is compositionally intermediate between the widespread hornblende

and pyroxene volcanic rocks, the age of the fine-grained andesites is

believed to be restricted between these two events. Without good age

control, however, a younger age cannot be refuted.

The youngest unit of the Coaldale volcanic sequence is the

small rhyolite plug in the southeastern portion of the study area. With

its lack of contacts with any of the other units, the age of this plug

was unknown. It has been K-Ar dated at 16.8 ± 0.4 m.y. (Table 2),

56slightly younger than the andesitic units that dominate the south­western Monte Cristo Range,

Although difficult to correlate regionally, the Coaldale

volcanic sequence is easy to equate with other volcanic rocks within

the Monte Cristo Range, Regionally, this sequence may correspond with

the undated andesitic volcanic rocks in the Silver Peak Range (Figure

16). The lower hornblende unit is especially similar in composition

and texture to a hornblende andesite in the Silver Peak Range. Within

the Monte Cristo.Range, the Coaldale sequence is considered equivalent

to the Blair Junction Formation of Stewart (Figure 16). The lower

member of the Blair Junction Formation is fine-grained and hornblende-

rich and the upper member coarse-grained and plagioclase- and

pyroxene-rich, strikingly similar to the Coaldale units. Again, no

intervening ash-flow tuff occurs between the andesites in the study

area as in Stewart's section.

The Coaldale sequence corresponds to the coarse- and fine­

grained andesites of Moore (1981) in the Blair Junction area. Without

age control, Moore considered these equivalent to the 15 m.y. old

"Gilbert Andesite" because of their petrographic similarity. Recent

work by Stewart (pers. comm., 1983) suggests the dated "Gilbert

Andesite" is equivalent to Moore's younger basaltic andesite (Figure

16). The young rhyolite plug in this study is the same as Moore's

Tri unit, and much older than he suspected. His Tai unit is the same

as the chloritically altered, coarse-grained hornblende dikes in the

current project area. Moore assigned a younger age to both these units

57

(Figure 16) because he thought they intruded the Esmeralda Formation.

A re-evaluation of these contacts makes this interpretation suspect.

The interior joint planes, and when observable the exterior, of the

dikes are silicified and slickensided, suggesting a fault-controlled

contact of the two units.

Blair Junction Volcanic Sequence

The Blair Junction andesites were not radiometrically dated in

this study; they are compositiona11 y and stratigraphically similar to

the Coaldale volcanic sequence. On this basis, they are assigned an

early Miocene age and correlate to the coarse- and fine-grained units

of Moore (1981) and the Blair Junction Formation of Stewart (pers.

comm., 1983) as shown in Figure 16.

The relative ages of the units in this sequence are quite clear.

The sediments overlying the Castle Peak Tuff are the oldest and roughly

correlate to the sediment and tuff unit at the base of the Coaldale

sequence to the southwest. The volcanic breccia and andesite flows are

probably time equivalent, with the basal breccia being formed during the

transport of the flows. The fine-grained andesites cannot be assigned

a definitive time bracket, but most likely postdate the sediments and

are equivalent to the flow units.' - • ’ , :

Younger Sedimentary and Basaltic Sequence

The Esmeralda Formation sediments in the study area are just a

small part of this regionally widespread unit (Figure 16). They are a

portion of the lowest member of the Blair Junction sequence of the

58

Esmeralda Formation of middle Miocene age (Moore, 1981). Excellent

exposures of the Coaldale sequence of this formation are seen just

south of Coaldale at the northern end of the Silver Peak Range

(Robinson et al„, 1968).

The local outcrops of the basalt in the northwestern portion of

the study area are believed the youngest volcanic rocks in the south­

western Monte Cristo Range„ Extremely fine-grained and hypersthene-rich,

they are similar to the voluminous undated basalts in the northwestern

Monte Cristo Range. Young basalts of Pliocene age occur throughout

this region (Figure 16). Although the regional basalts are olivine-

rich, a tentative correlation can be drawn with the basalts of the

Candelaria Hills, with isotopic ages of 3.0 ±0.1 and 4.0 ± 0.4 m.y.

(Marvin et al., 1977), and in the Silver Peak Range, with an isotopic

age of 4.9 ±0.6 m.y. (Robinson et al., 1968). As the basalts in the

study area lack age data, they could possibly correlate to Moore's

basaltic andesite and Stewart's "Gilbert Andesite" although they lack

the porphyritic texture distinctive in both.

CHAPTER 5

GEOCHEMISTRY

Whole Rock Chemistry

Whole rock geochemistry was done on nine representative

volcanic units in the study area. In addition to the three samples

radiometrically dated, the Castle Peak Tuff, the hornblende intrusion

and rhyolite plug, the six units chemically analyzed were: the fine­

grained andesite, the chloritically altered hornblende sill, the

banded rhyolite dikes, and flow units of the hornblende, pyroxene,

and coarse-grained andesites. The chemical analyses and normative

mineralogy are presented in Tables 3 and 4, respectively. Sample

descriptions are included as Appendix A and sample locations are on an

overlay to the geologic map (Figure 14). The analysis of the hornblende

sill sample falls well below a 100% oxide composition; although not

acceptable data, its oxide composition is similar to the other horn­

blende units, and it is included in the following discussions.

The terms applied to the units thus far in this report were

field descriptions; with the chemical data one can apply a stricter

rock classification to the units. The IUGS classification (Streckeisen,

1979), based on the normative mineralogy calculated from the chemical

analyses, is used here and presented in Table 5.

According to this classification, all the Coaldale andesitic

units should be considered quartz latites and the coarse-grained Blair

59

TABLE 3

Chemical Analysis (in weight percent) of Tertiary Units Southwestern Monte Cristo Range

Oxides CVC 3 CVC 4 CVD 2 CVC 6 CVC 8 CVD 5 CVC 10 CVD 7 CVC 12Tfai Thais Ti Tri Tha Thai Tpa TcP3 Tea

sio2 58.40 58.60 77.90 67.20 . 57.90 58.30 59.00 70.90 61.10Ti02 1.04 0.97 0.34 0.74 0.86 0.79 1.26 0.37 0.88Al2*3 18.50 16.60 11.80 16.30 18.10 17.40 17.00 14.10 16.30

1.97 2.03 0.29 0.89 2.04 2.09 2.51 0.54 - 1.62FeO 3.61 3.71 0.63 1.95 3.79 3.89 4.53 1.15 3.13MnO 0.11 0.07 0.03 0.44 0.10 0.11 0.09 0.02 0.04MgO 2.08 2.43 0.15 0.30 2.14 2.67 2.48 0.53 1.86CaO 5.07 4.72 0.22 1.30 5.84 4.77 5.86 1.86 4.05Na20 4.36 4.12 1.87 4.22 4.57 4.54 3.89 3.54 4.09K2° 1.96 2.09 4.70 4.53 2.21 2.23 1.90 3.96 2.91P2°5 •— — -- -- — . — ■ — —CO' 0.04 0.16 0.02 0.07 0.03 0.04 0,11 0.03 0.19

H2° 0.51 — 0.67 1,17 0.68 1.37 1.23 0.94 1.59Total 98.05 95.92 98.69 98.92 98.69 98.63 100.37 98.06 98.11Sum of the Alkalies 6.32 6.21 6.57 . . 8.75 . 6.78 ... ..5.76 . . 5.79 7.50 7.00

All analyses done at the University of Arizona Analytical Research Laboratory.

OnO

TABLE 4

Barth Normative Mineralogy (in weight percent) of Tertiary Units Southwestern Monte Cristo Range

Normative CVC 3 CVC 4 CVD 2 CVC 6 CVC 8 CVD 5 CVC 10 CVD 7 CVC 12Mineralogy Tfai Thais Ti Tri Tha Thai Tpa TcP3 TeaSialic Minerals 82.28 81.05 98.14 94.29 81.15 80.42 78,79 94.47 84.63

Quartz 12.13 13.55 46.92 21.01 8.01 9.32 13.38 29.75 15.18Orthoclase 11.90 12.94 29.13 27.48 13.32 13.54 11.46 24.31 17.89Albite 40.22 38.76 17.61 38.90 41.87 41.88 35.65 33.03 38.21Anorthite 12.84 11.94 0.53 3.17 14.72 12.08 14.62 4.73 10.07Corundum 5.19 3.87 3.95 3.73 3.24 3.60 3.68 2.64 3.27

Femic Minerals 17.72 18.95 1.86 5.71 18.85 19.58 21.21 5.53 15.37Enstatite 5.90 7.03 0.43 0.85 6.03 7.57 6.99 1.52 5.34Wollastdnite 5.14 4.78 0.21 1.27 5.89 4.83 5.85 1.89 4.03Ferrosilicate 3.02 3.24 0.37 1.47 3.48 3.74 3.72 0.95 2.65Magnetite 2.11 2.23 0.32 0.95 2.18 2.24 2.63 0.58 1.77Ilmen it e . 1.49 1.42 0.50 1.06 1.22 1.13 1.79 0.54 1.28Calcite 0.06 0.26 0.03 0.11 0.05 0.06 0.18 0.05 0.31

Plagioclase oligoclase oligoclase albite albite oligoclase oligoclase oligoclase oligoclase oligoclaseComposition Ab76-An24 Ab76-An24 Ab97-An3 Ab92~An8 Ab74~An26 Ab78™An22 Ab7r An29 Ab87-An13 Ab79~An21

Differentiation Index 64.25 65.24 93.66 87.39 63.20 64.74 60.49 87.10 71.29

os

62

TABLE 5

IUGS Classification of Chemical Analyses Southwestern Monte Cristo Range

Sample Lithologic Unit Classification

CVC 3 Tfai Quartz Latite

CVC 4 Thaig Quartz Latite

CVD 2 . Ti Alkali (feldspar) Rhyolite

CVC 6 Tri Alkali (feldspar) Rhyolite

CVC 8 Tha Quartz Latite

CVD 5 Thai Quartz Latite

CVC 10 Tpa Quartz Latite

.CVD 5 Tcp Rhyolite

CVC 12 Tea Quartz Trachyte

Classification based on normative mineralogy, from Streckeisen (1979).

63

Junction andesite a quartz trachyte. As most of the "andesites" in

these sequences lacked modal quartz (Table 1), a latitic and trachytic

IUGS classification can be applied. Both intrusive rhyolites in the

study area are classified as alkali (feldspar) rhyolites by their

chemical and modal compositions. The Castle Peak Tuff is of rhyolitic

composition. Figure 17 shows the placement of these units in the IUGS

classification scheme.

Oxide variation diagrams have been used in many volcanic centers

to help determine the genetic relationship of variable rock types

(Krauskopf, 1979; Carmichael et al., 1974; Cox et al., 1979; Keith,

1977). The weight percent oxide compositions are commonly plotted

against percent SiO^. Also useful are plots of percent oxides versus

a differentiation index parameter, defined as the sum of normative

quartz + orthoclase + albite + nepheline + leucite + kalsite (Thorton

and Tuttle,.1960). Both these variables serve to measure the change

in a magma's composition with increasing differentiation. Triangular

plots, usually of alkali, iron and another variable, are also useful

and show the simultaneous variation of three components.

Figure 18 is a plot of the sum of the alkalies, Na20 and K^Q,

versus percent SiC^ for all the analyses, and a plot of percent SiO^

versus the differentiation index (DI). Two triangular plots of the

analyses are given in Figure 19 and show the variation of alkalies-

iron-magnesium.(AFM) and sodium-potassium-calcium (Na-K-Ca) oxides.

The calc-alkalic— tholeiitic compositional division is plotted on the

alkali versus SiC^ plot and the general calc-alkaline differentiation

64

Q

alkali (-feldspar) rhyolite

rh yo li te dacite

quartz-alkali(-feldspar)

trachyte

quartzlatite

alkali( fe ld s p a r )

trachytebasaltquartz

trachyteandesite

latitetrachyte andesite

Figure 17. IUGS Classification of Chemical Analyses, Basedon Normative Mineralogy. Q = quartz; P = anorthitic plagioclase; A = alkali feldspar.

Figure 18* Variation Diagrams of Chemical Analyses, Differen­tiated by Volcanic Sequencee Calc-alkalic— tholeiitic division on upper plot from Carmichael and others (1974)0 Differentiation index (DI) in lower plot defined as the.sum of normative quartz + orthoclase + albite + nepheline + leucite + kalsite (Thorton and Tuttle, 1960)e

O • Castle Peak Sequence

0 Blair Junction Sequence

O Coaldale Sequence

WT.

% S

i02

Sum

of

Alk

alie

s65

10-i

&#5 4 th o le i l t ic /

ca lc - alkaline

H h 60 70 80 90

WT. % S i 0 2

80-|

70- ©©

60-• v

Q

50-

H h60 70 80 90 100

D I

Figure 18

Figure 19. Triangular Variation Diagrams of Chemical Analyses, Differentiated by Volcanic Sequence. Coordinates of lower plot defined as: A = Na20 + K2 O; F =FeO-j.; M = MgO. Calc-alkaline trend shown from Carmichael and others (1974).

© Castle Peak Sequence

E3 Blair Junction Sequence

O Coaldale Sequence

66

k2o

Na20

F

ca lc -a lka l ine trend

Figure 19.

67

trend on the AFM diagram. Both these reference lines are from

Carmichael and others (1974). In these figures each volcanic sequence is differentiated by symbol.

As readily apparent from these figures, the analyses straddle

the calc-alkalic^-fholeiitic composition line but also conform well

with a calc-alkalic differentiation suite. These apparent relation­

ships, particularly emphasized by the linear trend on the SiO^ versus differentiation index plot (Figure 18), are misleading. There are no

geologic indicators that these units have a common magmatic parent;

certainly no caldera features (other than the exotic blocks) were noted

in the study area that suggested an immediate source of the Castle Peak

Tuff, nor does the 'relative position of the Blair Junction trachyte

indicate a common magmatic source with the Coaldale latites. What

these plots do suggest by their linear trends is that the individual

magmas may have had similar compositions with similar differentiation

paths.

Gradational petrographic compositions and the local distribution

of the older hornblende-rich to pyroxene-rich Coaldale volcanic rocks

suggest a single zoned magmatic system was succesively tapped to ever

deeper levels through time (Figure 2; Table 1). The variation within

one system can be tested by using only the Coaldale latites in oxide

variation plots. Figure 20 presents plots of the weight percent oxides

versus SiC^ composition, and Figure 21 gives the oxides versus the

differentiation index for the Coaldale latites. The Blair Junction

trachyte is included for comparison. Most of these diagrams do hot

CaO

A

I2O

3 N

a20

08

2.0-

0.5-

6 A

6 ©

4' 0

56 ASIO,

Coaldale L a tltea

• hornblende O fine-grained A pyroxene

B lair Junction Trachyte

B coarse-grained

Figure 20 Oxide-Silica Variation Diagrams of Coaldale and Blair Junction Chemical Analyses, Differentiated by Rock Type.

69

6.0-1

18-1

nOCM18-

17- A

©

ie-J--r66 60 65B

70D I

7-nA

K*'O£ 6-

□5 5 6 0 6 5 7o

D I

Coaldale Latltea

• hornblende © fine-grained A pyroxene

Blair Junction Trachyte

□ coarae-grained

Figure 21. Oxide-DI Variation Diagrams of Coaldale and Blair Junction Chemical Analyses, Differentiated by Rock Type.

70

show pronounced patterns because the units do not have greatly varying

compositions. The Coaldale rhyolite plug is not included in these

plots as a consequence of its extremely SiC^-rich composition and the

lack of intermediate dacitic or rhyodacitic units in the Coaldale

sequence. This lack makes the assumption of a common magmatic source

for the Coaldale latites and rhyolite plug uncertain.

A few of the oxide plots in Figures 20 and 21 do show trends

within the Coaldale latites. Both the flow units are considerably

more CaO-rich and SiO^-poor than the intrusive units, a function of

the predominance of microlitic plagioclase in each (Table 1). The

pyroxene flow is much more Fe-rich and slightly less K^O- and Na20-rich

than the hornblende units. A more complete analysis of the Coaldale

units is not possible; the tuffaceous and other intrusive units of

this sequence (Tpai, Tpat, That, Thai, and Tst) are generally too

weathered to yield reliable chemical results'.

The inclusion of the Blair Junction trachyte in these plots

allows a comparison of the relative position of magmatic differentiation

at the time of the andesitic eruptions. The high SiC^ and K^O and low

CaO, MgO, and FeO content of the Blair Junction trachyte suggests a

more differentiated parent magma than the Coaldale latites (Figures 18

and 19). In fact, the overall trends of the oxide versus differentia­

tion index plots (Figure 21) show a surprising linearity considering

they are of different source magmas, and further supports a similar

differentiation path in each.

71

Isotope Geochemistry87Sr

Initial ---- isotope ratios were determined on the three age86Sr

dated samples, the Castle Peak Tuff, the hornblende intrusion of Blue

Mountain, and the small rhyolite plug in the southeast portion of the

study area. As mentioned above, all sample locations are plotted on

an overlay to the geologic map (Figure 14), and all sample descriptions

are included as Appendix A„ Initial Sr isotopes have long been used as

geologic indicators of commonality of parental magmas and the type

and/or contamination of the source material from which these magmas

were derived. That is, magmas derived from sources with low Rb/Sr87Sr

ratios (e,g„ the mantle) should have low initial — ratios, whereas ■ , 8bSrthose derived from, or contaminated with, sources with high Rb/Sr

ratios (e.g, the upper continental crust) should have higher initial 87cr— — ratios (Cox et al„, 1979; Faure and Powell, 1972; Faure, 1977b),8 6 SrA summary of the isotope data is presented in Table 6,

Both the rhyolitic and latitic initial ratios are in the

range of other determinations of equivalent rock types. At 0,7086 and

0,7089 the rhyolitic values are slightly more radiogenic than the

0,7054 average of ash-flow tuffs in the San Juan caldera complex

(Lipman et al„, 1978)* In contrast, they are less radiogenic than other

rhyolites in southwestern Nevada which have ratio values that range from

0.710 to 0.715 (Noble and Hedge, 1969). The hornblende latite initial

ratio of 0.7054 is also slightly more radiogenic than the average 0.704

value of andesites in continental margins (Carmichael et al., 1974).

TABLE 6Analytical Data for Initial Sr Isotopes of Tertiary Rocks

Southwestern Monte Cristo Range

SampleNo. LithologicUnit K-ArAge to(PPm)

Sr(PPm) 87to/86Sr ■ Measured87Sr/86Sr Initial87Sr/86Sr Location

CVD 2 Ti 18.6 212 26.8 22.92 0.71474 ±0.00007 0.7086 small plug in SE 38°02.51' N 117°50.36' W

CVD 5 Thai 22.2 70 1398 0.145 0.70542 ±0.00006 0.7054 SE flank of Blue Mtn.38°04.35' N 117°51.40' W

CVD 7 Tcpj 24.2 155 450 0.997 0.70921 ±0.00007 0.7089 Upper cooling unit in NW 38°05.24' N 117°52.54' W

decay constant: X = 1.42 ICT1 yr~* All analyses done at the University of ArizonaIsotope Geochemistry Laboratory. All isotopeuncertainty: 87Sr/86Sr ± one std. deviation determinations done with a Nier-type 60° sectorD. q meaf" ine 6” mass spectrometer. Elemental determinationsmeas. -5 t0 1U'6 done with X-ray fluorescence.

73

Several implications of commonality and type of magma sources

of the volcanic rocks in the study area are apparent in these initial 8 7 S r— — determinations. The hornblende intrusion, with a low value, ti6Srclearly was derived from a less contaminated or less radiogenic source

material than the rhyolites. The higher values of the Castle Peak Tuff

and Coaldale rhyolite plug indicate derivation from similar radiogenic

sources. The discrepancy between the initial ratios of the Coaldale

rhyolite plug and latite intrusion deny the possibility of a common

magmatic source, unless the magma was contaminated by crustal material

after the andesitic phase. Although the Castle Peak Tuff and the

Coaldale rhyolitic plug were probably derived from magmas with similar

source materials, a single parental magma is not supported by their

age difference and absence of geologic indicators (see above). When

taken in concert with the implications of the oxide variation diagrams,87g

the difference in initial ---- ratios suggests that the Castle Peak86Sr

Tuff, Coaldale rhyolite and Coaldale latite each had discretely

different magmatic sources that followed similar differentiation paths.

Further, the latitic magmas had less crustal contamination than the

rhyolitic magmas.

CHAPTER 6

STRUCTURAL GEOLOGY

The project area lies near the intersection of two highly-

faulted, large structural discontinuities, the northwest-southeast

Walker Lane shear zone and the east-west Warm Springs lineament

(Figure 3; Albers, 1967; Albers and Stewart, 1972; Ekren et al., 1976;

Speed and Cogbill, 1979b)„ It is not surprising, then, that its

dominant structural feature is a series of steeply dipping faults that

trend east-west and northeast to northwest. These faults crosscut a

broadly arched volcanic homocline that dips to the south and flanks a

complexly deformed pre-Tertiary basement. Four structural domains

have been defined to aid in identifying changes in structural style

throughout the project area. The northeast (NE), southeast (SE),

central (C), and northwest (NW) domains, with the fault traces in each

are located on an overlay to the geologic map (Figure 14). Figure 15

contains four geologic cross-sections that illustrate the structural

and stratigraphic relationships within each domain.

Folds

The attitudes of the volcanic units within the study area vary

only slightly; their average attitude, from a contoured plot of the

poles to bedding or foliation, strikes 32°W and gently dips021- S

(Figure 22). Evaluation of the attitudes in the NW, C, and SE domains

75

N

All Domains n* 169

avg. 32°W/21eS

NE Domain n>39

avg. 80oE/22°S

SE DomainC Domainn*55

Figure 22. Equal Area Stereonet Plots of Strike Data. All plots with 5%-10%-(15%-20?o) contour intervals.

76

individually» however, shows a gradual change in strike and dip across

the area that defines a broadly arched, south dipping homocline (Figure

22)„ The average attitude in the NE domain is similar to the east-

northeast trend in the SE domain but cannot be considered part of the

large fold structure because of the wide intervening expanse of highly

deformed pre-Tertiary rocks in the central portion of the study area

(Figure 2; B-B’, Figure 15).

The large arched homocline is ill-defined but is the only

recognizable fold structure in the volcanic rocks of the study area.

Unconformable contacts between the pyroxene latites and other strati-

graphically lower units (C domain. Figure 2) imply that periods of

uplift and/or erosion occurred during volcanic activity. The broad

arch that dominates the southwestern portion of the project area may be

controlled in part by the original depositional attitudes of the

volcanic units, but has also been accented by the tilted uplift of the

Monte Cfisto Range during Basin and Range faulting.

Faults

A pronounced faulted structural fabric dominates the project

area (Figure 2). Most faults are readily apparent on both aerial

photographs and in the field. Slickensided surfaces, silicified

breccias, clay gouge zones and minor silica or calcite veining are the

most common features associated with the faults and are very pronounced

in the wider fault zones. The high competency of these units makes

local drag structures along fault planes rare. Although not studied

77quantitatively, most faults have 1' to 25’ wide envelopes of closely

spaced joints 0.5" to 2 ’ apart.

The study area has two discrete sets of steeply dipping faults

that trend east-west and northeast to northwest. The orientations of

the faults in the area are included in Figure 23a. This includes a plot

of all trends on a polar coordinate frequency diagram and two equal area

stereOnet plots, one of the poles to the fault planes and another with

5% and 10% contoured intervals of the same data. Dips of 75° were

assumed for faults without observable data; measured dips range from

60° to 90° and average close to 75°. Dip direction was chosen by the

observed displacement of units. As can be seen in these diagrams,

fault trends vary almost continuously throughout the study area. When

the frequency distributions are examined by domain (Figure 23b), a

gradational change in fault orientations occurs across the southwestern

portion of the study area. The prominent trends vary from strongly

northeast-southwest directions in the SB domain to more variable north-

south orientations in the C domain to, finally, northwest-southeast

trends in the NW domain. Both the NW and C domains are also character­

ized by an east-west fault trend (Figure 23b); this secondary trend is

almost as strong in the NW domain as the predominant northwest trend.

Measurable normal separation along the majority of these faults

is rare because they lack known marker horizons and the thickness

variations of the units offset. Using regular thickness changes in

the units, the normal separations shown on the geologic cross-sections

(Figure 15) are approximate but reveal offsets from 10’ to 300’. The

Figure 23, Equal Area Stereonet Plots and Frequency Diagrams of Fault Data,

a. Fault data of all domains. Equal area stereonet plots of poles to fault planes and a contoured plot of the same data (5%-10% interval), Frequency diagram of all fault traces in polar coordinates*

b. Frequency diagrams of fault traces in polar coordinates, differentiated by domain.

X NW Domain

O C Domain

A SE Domain

□ NE Domain

°o t>n

N

Figure 23a.

NI

S X 1 O 1n

NW Domain

N

T i

C Domain

N

i i iNE Domain

I

* 1 1 0 1 2 3n

SE Domain

Figure 23b

79

N

n = 10

□ — NE Domain Q - C Domain X ” NW Domain

Figure 24. Equal Area Stereonet Plot of Trend and Plunge of Fault Slickensides.

80

west flank of Coaldale Ridge (D-D', Figure 15) has a 500' separation

of the base of the hornblende flow units across four major faults that

strike north and dip steeply west. Lateral separations of the units

along the faults are more easily documented. Most north-northwest to

north-northeast trending faults show right-lateral separations of 20'' /

to 100*, whereas most east-west trending faults show separations of

507 to greater than 5007 (Figure 2)0

True slip directions were determined on 25% of the faults in

the project area* The trend and plunge of the measured slickensides

are shown in Figure 24* These lineations show both steep and shallow 'plunges, the latter with northwest to southeast orientations. Com­

bining these directional indicators with known separations along the

faults indicates that although both sets are oblique-slip faults, the

north-trending ones are dominantly dip-slip faults with only minor

right-lateral components and the east-west faults primarily left-

lateral strike-slip with minor normal components. This can be

generalized to categorize most of the normal and left separation faults

throughout the project area.

Although most of the faults offset all volcanic units, there

are several indications -of fault activity concurrent with volcanism.

Localization of the small rhyolite plug (CVD 2, Table 2) along one of

the faults in the SE domain (Figure 2), suggests that faulting occurred

as early as 18 m.y. B.P. Unconformable volcanic contacts indicate

periods of uplife and/or erosion during volcanic activity; active

81

faulting in modern volcanic complexes is common (MacDonald, 1972) and implies faulting helped control volcanic deposition..

As the prominent north and east trending faults in the project

area displace all Tertiary volcanic rocks, most of the fault deformation

is younger than 18 m.y. B.P. Mutually crosscutting fault relationships

in the study area indicate overlapping movement histories. On Coaldale

Ridge the east-west faults offset the north trending faults, but in the

NW domain at least one northwest trending fault offsets an east-west

fault (Figure .2), Variable displacements along several faults suggests

multiple movement histories and reactivation by segments. The post

18 m.y. faulting may have reactivated some of the earlier faults active

during vplcanism.

One pre-Tertiary fault that controlled the deposition of the

Castle Peak Tuff in the interior of the central Paleozoic high in the

study area deserves special mention (Figure 2). Breccias, slickensides

and complex drag folding occur on either side of this structure.

Although only three segments could be documented, it is believed to

be a single (or at least en echelon) fault structure that strikes north­

west and dips moderately to steeply northeast. Nowhere was the tuff

offset by this structure, rather, the base of the tuff was occasionally

underlain by severely baked soil horizons. This northwest structure is

not only older than 24 m.y. but it also had a strong topographic

expression that caused the ponding of the Castle Peak Tuff. Several

other isolated outcrops of the tuff in the Paleozoic high also occur and

are most likely expressions of older fault-controlled topography (Figure

2).

82

Hornblende and Pyroxene Dikes

These dikes occur throughout the southwestern portion of the

study area in the NW, C and SE domains. Dikes are tensional features

and their orientations commonly indicate the minimum stress direction

that operated during volcanic activity, unless emplaced in a strongly

oriented prevolcahic basement fabric.

The hornblende and pyroxene dikes are largely very linear

features with easily measured attitudes; only in a few cases are the

dikes more arcuate than straight and contact attitudes or flow folia­

tions unobservable (Figure 2). The orientation of the dikes throughout

the project area is shown in Figure 25a. As with the fault data, this

figure includes a frequency diagram as well as two equal area stereonet

plots of their poles and 5% and 10% contour intervals of the data. The

plots,show a strong northwest trend with steep, but variable, dips for

the overall area. A secondary northeast trend with southward dips also

occurs. A northwest trend is apparent in each domain (Figure 25b), but

accented by the strong alignment and concentration of the dikes in the

C domain. The northwest trending dikes in this domain occur on the

southern flank of Blue Mountain and dip toward this intrusion (D-D',

Figure 15); The dikes form a half-arc, or partially "concentric,"

intrusive pattern around their source, as is common in other andesitic

volcanic centers (MacDonald, 1972). The northeast trending dikes occur

outboard of Blue Mountain and form a perpendicular to radial pattern

away from this plug. Like the northwest dikes, the northeast trending

dikes are found in all domains (Figure 25b).

Figure 25« Equal Area Stereonet Plots and Frequency Diagrams of Dike Data,

a. Dike data of all domains. Equal area stereonet plot of poles to dike orientations

and a contoured plot of the same data (5%- 10% interval),

b. Frequency diagrams of dike orientations in polar coordinates, differentiated by domain.

X NW Domain

O C Domain

A NE Domain

S3

N N

All Domains

n»49

i 2 a 5 ! e 7 el r I iAll Domains

Figure 25a.

N NI I

s 2 r 1 2 3

SE DomainNW Domain

C Domain

e ; ; i i $ j

Figure 25b

84

Excellent flow foliation, lack of xenolithic material and

only thinly chilled margins along the dike contacts indicate non-

obstructed dike emplacement. Most dikes do not show much^ if any,

relative movement of their walls.

Discussion

The folds, faults and dike's are all structural elements that

can be used to interpret the forces and differential stresses that

caused the observed deformation in the study area. The broad arching

of the volcanic rocks indicates uplift in the central domain, perhaps

near the Blue Mountain intrusion, and a relative downdropping of the

surrounding volcanic pile especially in the southeast. Localization

of the small rhyolite plug along a northeast trending fault in the SE

domain (Figure 2) suggests that tensile stress was operative during

volcanism, and may have been a determining factor in the eruptive

history of the area. The strong northwest orientation of the dikes on

the flank of Blue Mountain implies a N40-50°E directed tensional stress

near their intrusive source. A more northeast and northwest rectilinear

dike pattern elsewhere in the area suggests that a much less directed

stress regime also operated during the volcanic activity. Their

patterns may also be controlled in part by an older fracture pattern

in the pre-Tertiary basement.

The post-volcanic fault system has two elements. First, the

northeast to northwest trending, dominantly dip-slip faults indicate a

general east-west extensional stress. Its orientation grades from more

northwest-southeast in the SE domain to northeast-southwest in the NW

85

domain„ These largely dip-slip faults belong to the north trending,

high-angle Basin and Range style normal faults predominant in this:,

physiographic province. Their right-lateral oblique movements are

consistent with the right-handed shear direction documented in the

Walker Lane deformational zone (Albers, 1967; Albers and Stewart, 1972;

Stewart, 1980). The east-west trending, dominantly left-lateral strike-

slip faults create a secondary fabric across the study area. These

irregularly present faults form a gross conjugate shear system with

the northwest to northeast trending right-lateral dip-slip faults.

This conjugate system is similar to fault trends in adjacent

areas. Left-lateral movements along the eastrwest faults in the

Candelaria Hills are well-documented by Speed and Cogbill (1979), and

similar structural trends are suggested north of the Volcanic Hills

(Stewart, 1979) and in the northern Silver Peak Range (Robinson et al.,

1976). Although oblique-siip, the east-west faults in the study area

probably formed in response to the same regional forces that caused

the fault deformation in adjacent areas and indicate conjugate shear

as well as extensional stress existed. The secondary east-west faults

are analogous to the left-lateral oblique-slip faults documented

throughout the Warm Springs lineament suggested by Ekren and others

(Figure 3; 1976).

CHAPTER 7

ECONOMIC GEOLOGY

Previous exploration and mining in and near the Monte Christo

Range has included coal, industrial minerals, and both base and precious

metals. Lignitic coal was discovered in the Esmeralda Formation south

of the study area near Coaldale, and much of the Monte Cristo Range

was included in a Nevada, coal withdrawal area in the early 1900's.

The Gilbert and South Gilbert districts, northeast of the study area

in the center of the Monte Cristo Range, have produced gold, silver,

antimony and mercury. Molybdenum, with minor gold and antimony, was

mined just east of Gilbert in the mid-1900's (Albers and Stewart, 1972).

Precious metal exploration continues sporadically throughout the Monte

Cristo Range and recent drillsites, collared in jasper- and barite-rich

pre-Tertiary rocks, were found in the southern portion of the project

area.

Extensive borate exploration and production has occurred in

this portion of Esmeralda and Mineral Counties. Total borate production,

occurring in the late 1800's, may have reached $900,000.00. Most of

this production was from cottonball ulexite (NaCaB^Og ° SH^O) mined from

marsh, or playa, deposits. The earliest production was from Columbus

Marsh just west of the study area. Fish Lake Valley (west of the Silver

Peak Range), Teels Marsh (west of the Candelaria Hills), and Rhodes

Marsh (west of the Pilot Range) are other mined ulexite deposits. Minor86

87

borate exploration and production occurred in 1939 and concentrated

in young Tertiary lacustrine sediments in the Silver Peak Range (Albers and Stewart, 1972).

Fallen timbers mark two caved shafts on the western flank of

Coaldale Ridge in the study area (Figure 26). Surrounded by extremely

B-rich soils over the volcanic tuffs in this area, these structures

indicate that active borate exploration also took place in the current

project area, probably in the late 1800?s.

Formation of Borate Deposits

The two major types of borate deposits are the playa concen­

trations, such as Columbus Marsh, and the bedded borates in young

lacustrine sequences, such as the Kramer deposit near Boron, California.

In both, the borates are associated with anomalous Li and (sometimes)

Sr. The major borate minerals found in the lacustrine deposits are

borax (Na^B^Oy "lO^O), kernite (Na^B^Oy ° 42^9), ulexite (NaCaB^Og- SH^O)

and colemanite (Ca^B^O^^ <> SH^O). Ulexite and borax are the major

borate minerals in playa deposits (Kistler and Smith, 1975; Muessig,

1959).

Although these deposit-types have somewhat different origins,

both result from one determining geochemical property of B, Its. extreme

solubility. As large ion lithophile (LIL) elements, B and Li are

preferentially enriched in late stage magmatic, and metamorphic fluids

(Foldvair-Volg, 1978; Krauskopf, 1979). The cause of the association

of Sr in some of these deposits is less clear. Sr is a dispersed rare

element that geochemically follows Ca. Only rarely does it form

88

Figure 26. Caved Shafts on the Western Flank of Coaldale Ridge. The timbers are surrounded by fluff- rich soils overlying the upper member of the sediment and tuff unit. Sample CVA 26 was taken adjacent to the timbers to the right.

89independent carbonates and sulfates, usually found in hydrothermal

deposits (Faure and Powell, 1972; Faure, 1977a)«

The solubility of B allows its concentration in the fluids

responsible for the formation of borate deposits. The bedded borate

deposits in lacustrine sequences are believed the result of borate

precipitation from B-rich brines in a lacustrine basin (Bates, 1960;

Kistler and Smith, 1975). Searles Lake in California can be considered

a Recent analog of the Tertiary deposits. The downward migration of

B-rich saline brines has caused the deposition of borax in the late

Quaternary substratum of the lake (Freidman et al., 1982; Smith, 1979).

All major bedded borate deposits have many indicators of

contemporaneous biitiodal voleanism. Air-fall tuffs and/or basalts are

present in or near the deposits. This association has led most workers

to believe that the source of B in the brines is by volcanic exhala­

tions (Bates, 1960) of B-rich primary magmatic fluids.

The playa deposits are also a result of the high B solubility.

These form from the continuing concentration of B in saline evaporative

waters. Migration and evaporation of such waters finally leads to

solution saturation, and the deposition and growth of the high hydrate

borates in the playa muds (Muessig, 1959). The B in these waters is

believed caused by the leaching of the surrounding units, which are

likely to be of sedimentary origin.

Evapo-transpiration, or the migration and evaporation of ground

water, operates in arid climates and may cause B enrichment in soils.

Concentration of B in eluvium is the result of highly sorptive minerals.

90

such as kaolinite, in weathering profiles (Shcherbov, 1982). These

clays provide sites for the solution transported B. This process of

continuing concentration leads to the surface accumulations of B that

commonly occur as white salt crusts, coatings or "fluffs." Fluffs are

thick carbonate- and sulfate-rich concentrations just above soil

profiles that sometimes contain observable cottonball ulexite.

Bedded borate deposits almost always have such surface expressions,

although surface accumulations do riot necessarily indicate a borate

deposit at depth. Springs in B-rich environments, or that carry B-rich

waters, are commonly surrounded by these crusts, fluffs, or even

carbonate sinters.

Distribution of B in the Southwestern Monte Cristo Range

To evaluate the extent and distribution of strong B anomalies

in volcanic and volcaniclastic rocks, as indicated in previous recon­

naissance sampling by U.S. Borax (B. Watson, pers. comm., 1982),

sixty-three rock chip and seven soil and/or fluff samples were

collected throughout the study area. These were analyzed by atomic

absorption for their B, Sr and Li contents. The assay results are

given in Table 7; asterisks indicate mostly to wholly fluff and/or soil

samples, whereas asterisks in parentheses mark samples that include

only minor fluff and/or soil. Sample locations are plotted on an over­

lay to the geologic map (Figure 14) and complete sample descriptions

are included in Appendix A.

TABLE 7

Southwestern Monte Cristo Range

B, Sr, and Li Geochemistry of Rock Chipand Soil/Fluff Samples

SampleNo.

AlternateDesignation Lithology Domain B Sr

(PPm)Li

CVA 1 Tha SE 10 523 12CVA 2 That * SE 1300 346 67CVA 3 That SE 21 317 14CVA 4 CVC 3 Tfai SE 4 312 14CVA 6 CVC 4 Thais SE 4 245 82CVA 7 That (*) SE 165 307 53CVA 8 Te (*) SE 98 211 34CVA 9 CVD 2 Ti SE 4 14 14CVA 10 Tstl * SE 186 115 24CVA 11 Tstu (*) SE 299 154 24CVA 12 Tfai SE 7 322 10CVA 13 Tstl (*) SE 286 379 43CVA 14 -H+H- SE 7 289 . 43CVA 16 TcPe SE 28 53 14CVA 16A CVC 6 Tri C 31 38 10CVA 17 Tcpj (*) C 37 509 14

TABLE 7— continued

Southwestern Monte Cristo Range

B, Sr, and Li Geochemistry of Rock Chipand Soil/Fluff Samples

SampleNo.

AlternateDesignation Lithology Domain ‘ B Sr

(ppm)LI .

CVA 18 Tcpi (*) C 38. 29 <5CVA 19 Thais C 12 293 14CVA 20 • Tha C . 5 226 19CVA 21 Tpai C 4 302 24CVA 22 Tpa * C 879 274 67CVA 23 Tstu * C 265 , . 192 19CVA 24 Tpa C 29. 442 19CVA 25 Tha * C 19 605 19CVA 26 Tstu (*) c 791 235 34CVA 28 Tha c 15 398 10CVA 29 That 0) c 70 451 10CVA 30 CVC 8 Tha C 12 389 10CVA 31 Tstl (*) c 196 302 34CVA 32 That c 58 235 34CVA 33 opaline veins c 33 182 <5CVA 34 c 12 293 10

TABLE 7— continued

Southwestern Monte Cristo Range

B, Sr, and Li Geochemistry of Rock Chipand Soil/Fluff Samples

SampleNo.

AlternateDesignation Lithology Domain , B Sr

(ppm)Li

CVA 35 That C 15 278 14CVA 36 Tstu C 26 240 19CVA 37 Tpat C 16 374 24CVA 38 C 58 394 29CVA 39 Thai C 11 250 24CVA 40 CVD 5 Thai C 9 379 ■ 10CVA 41 TCP! C 8 264 10CVA 42 H+Hf C 10 408 24CVA 43 That C 35 221 31CVA 44 Tpat C 9 415 26CVA 45 TcP2/3 C 7 <10 14CVA 46 Thai C 6 206 10CVA 47 Tcpj C 150 . 149 10CVA 48 ■ Tfai C 9 293 24CVA 49 Tfai C 7 187 10CVA 51 That C 21 158 <5

TABLE 7— continued

Southwestern Monte Cristo Range

B, Sr, and Li Geochemistry of Rock Chipand Soil/Fluff Samples

SampleNo.

AlternateDesignation Lithology Domain B Sr

Cppni)Li

CVA.52 Tha C 18 302 10CVA 53 CVC 10 Tpa NW 11 326 10CVA 54 That '(*) NW 7 206 14CVA 55 Tstu * NW 268 235 24CVA 56 Thai NW 20 192 48CVA 57 PT NW 14 115 <5CVA 58 Tcp3 NW 13 72 14CVA 59 That NW 34 67 38CVA 61 Tstu (*) C 106 43 29CVA 62 Tcp2 c 20 240 10CVA 63 CVD 7 Tcp3 NW 9 110 10CVA 64 Tb NW 16 456 19CVA 65 Qsp (OA?) NW 12 235 13CVA 66 Tcpe NE 11 34 10CVA 67 Tfa NE 8 322 14CVA 68 Qsp * NE 349 293 58

TABLE 7— continued

Southwestern Monte Cristo Range

B, Sr, and Li Geochemistry of Rock Chipand Soil/Fluff Samples

Sample No.

AlternateDesignation Lithology Domain B Sr

(ppm)Li

CVA 69 - Ts NE 66 326 34CVA 70 CVC 12 Tea NE 6 1.54 53CVA 71 Tcab NE 4 125 10CVA 72 Tcpi NE 71 394 10CVA 73 PT • NE 11 38 29CVA 74 Te SE 10 240 14

* highly weathered, contains significant fluff f * 1v weathered, contains minor fluff

96

Assay values range from 4 to 1300 ppm B, and average 76 ppm*

Cottonball ulexit e was observed only in two fluff-.rich samples, CVA 2

and CVA 22. Sr and Li values range from <10 to 605 ppm and from <5 to.

82 ppm, respectively. Sr analyses average 224 ppm and Li 21 ppm.

Differing ,?whole earth1’ models cause a range in the clarke of B from

8 to 12 ppm (Foldvair-Vogl, 1978); using an average 10 ppm for this

useful parameter, the average B concentration in the study area is over

seven times more enriched than the clarke.

To determine the geochemical distribution of B, Sr and Li

concentrations within the Tertiary section, their averages were

calculated by rock type and differentiated by freshness of the sample., / • ' •Table 8 presents the average concentrations of each rock type; "altered"

samples are those that showed chloritic or sericitic alteration, or

most commonly, extreme- weathering. Asterisks indicate averages from

fluff- and/or soil-rich samples, whereas those in parentheses mark

averages from weathered samples with minor fluff and/or soil contents.

. Clearly apparent in this table are that the altered/weathered samples

are much more enriched in B than the fresh samples. The only exceptions

to this are the chloritically altered hornblende and pyroxene dikes

which have concentrations more like the fresh volcanic units. The

altered/weathered samples average close to 260 ppm B when the dike

concentrations are excluded, or 26 times the clarke concentration of .

B. The fresher -units are much less B-rich, only slightly greater than

the clarke, and dominate the samples taken in the study area. Largely

representative, the fresh samples indicate the average volcanic and

TABLE 8

Average B, Sr, and Li Geochemistry Southwestern Monte Cristo Range

LithologicUnits

Number of Samples

B

Fresh(PPm)

Altered (* with fluff) (ppm)

Fresh Altered Sr Li B Sr Li

Opaline 1 33 182 5VeinsQsp 1 1 12 235 13 349 293 58 *

Tb 1 — — 16 456 19 -- — —Te 1 1 10 240 14 98 211 34 (*)

Ti 1 -- 4 14 14 — --Tpa 2 1 20 . 384 15 879 274 67 *Tpai 1 -- 4 302 24 — --

— - . 2 -- 35 344 20 'Tpat 2 -- 13 395 25 — --

■--

Tfai . 4 — 7 288 15 -- --Tha 5 12 . 368 12 mm mm mm mm mm

Thai*-1-1 5 10 263 33^) — mm — mmmm

-H-H-f- — 2 —- -- 9 346 34

TABLE 8— continued

LithologicUnits

Number of Samples

Fresh(ppm)

Altered (* with fluff) (ppm)

Fresh Altered B Sr Li B Sr Li

That 6 4 26 236 18 389 427 37 (*)Thai 2 -- 23 159 31 — —Tstu 1 5 26 240 19 128 172 26 (*)Tstl — 3 — — 223 265 34 (*)

Tcu 1 — 6 154 53 — — - - -Tcab 1 -- 4 125 10 — —Tfa 1 -- 8 322 14 — : —Ts 1 66 326 34 — - — -

Tri 1 31 38 10 -- — — --Tcp ' 7 2 17 176 11 110 272 10 *Tcpg 2 — 19 44 12 -- — --

pT 2 -- 13 77 17 — --

Avg. 49 21 17 244 13 211(3) 289 27

Includes analyses from Thais„ f 21 •'20 ppm without one very anomalpus sample CVA 6i ^^256 ppm without chloritically altered dikes.

99

sedimentary rocks in the study area are only slightly anomalous with

respect to B.

B versus Sr and B versus Li variation diagrams for the average

fresh and altered values of each rock type are shown in Figures 27 and

28, respectively. Included in each plot are the modern spring crusts

and pre-Tertiary rock compositions for comparison. Both plots indi­

cate low B concentrations for fresh samples and higher values for the

altered samples. Tie lines connect fresh and altered average concentra­

tions of equivalent samples. The Sr versus B plot (Figure 27) shows a

wide range in Sr values in fresh samples and both Sr enrichment as well

as Sr depletion in the altered/weathered samples (excluding the dike

compositions). The Li versus B plot (Figure 28) shows that most of

the fresh rocks cluster between 10 and 20 ppm Li, with a few anomalies

near 50 ppm. Most samples show a strong increase in Li in the altered/

weathered samples. A triangular B-Sr-Li variation diagram (Figure 29)

illustrates the relative dominance of Sr in the fresh rock types and

the higher relative B concentrations in the altered/weathered samples.

In all the plots, the altered/weathered sample averages fall well below

the B content of modern spring crusts in the area and above the pre-

Tertiary average.

Table 9 compares the average concentrations of B, Sr and Li in

the fresh samples of the study, area by major rock type to the reported

averages of equivalent rock types and the clarke values of Turekian

(1972). It indicates that the B contents of the volcanic rocks in the

study area are slightly, but consistently, greater than the volcanic

B (p

pm)

4 0 0 - i

• - fresh

300-

* - pTor Qsp

200-

100-

Sr (ppm)

Figure 27. B-Sr Variation Diagram of Average Values. Tie lines connect fresh and altered samples of the same rock type. Qsp and pT values given as reference.

100

102

fresh

altered

pT or Qsp

Figure 29. B-Sr-Li Triangular Variation Diagram of AverageValues. Fresh and altered samples differentiated; Qsp and pT values given as reference.

TABLE 9

Comparison of Average Fresh Rock B, Srp and Li Geochemistry to the Average Composition of Similar Rock Types and

Clarke Values of Turekian (1972)

LithologicUnit

This Study TurekianType B Sr Li B Sr Li

Basalt Tb 16 456 19 5 465 17AndesitesW All 13 314 22

Blair Junction Tcap Tcab, Tfa 6 200 26 9 440 24Coaldale Tpa, Tpai, Tpat, Tfai

Tha, Thai, That, Thai 15 329 23

RhyolitesC2) All 17 101 11 •tuffs Tcp 17 176 11 10 100 40intrusives Tri, Ti 18 26 12

Sediments All 25 180 20

PT PT 13 77 17 52 310 29^T Ts, Te. 38 283 24

Clarke(3) All 17 224 18 8 375 24

^High Ca rocks from Turekian (1972) e ^ L o w Ca rocks from Turekian (1972) „

Average from model A and model B from Turekian (1972)0 Includes sandstonesP shales and carbonates.

103

104

rock averages and over twice the clarke values. Sr and Li concentra­

tions in the fresh units are generally much closer to the norms,

although the Sr values show a large variation. The sedimentary rock

averages are much lower than the Turekian values. Subdivisions of the

andesitic, rhyolitic and sedimentary analyses are given in Table 9 to

show the variation within one rock type. Rhyolitic rocks vary only

in their Sr contents whereas both andesitic and sedimentary rocks

differ in all three values.

The assay results allow some basic generalizations about the

relative B anomalies in the study area. First, the fresh volcanic and

sedimentary samples are representative of the overall area and are only

slightly anomalous in B, with generally low Li and variable Sr. Second,

weathered units are consistently much more enriched in B and Li than

the fresh samples. Third, fluff and/or soil over the weathered units

can contain very anomalous amounts of B and Li, Fourth and last,

modern springs in the area are surrounded by B-rich crusts and fluff

and are themselves presumably anomalous in B,

Cause of B Anomalies in the Southwestern Monte Cristo Range

Some of the geochemical variations present in the fresh samples

(Table 9) are easily explained, while others are more ambiguous. The

general depletion of B, Sr and Li in the pre-Tertiary rocks, for

example, is probably caused by dehydration during metamorphism. On

the other hand, the Tertiary sediments have a more average composition,

although they are also low in B. This may be caused by the inclusion

105of marine shales which have high B contents (100 ppm) in Turekian‘s

sedimentary rock average. The Tertiary sandstones and lacustrine

mudstones are more equivalent to the Turekian sandstone average of

35 ppm B, 20 ppm Sr and 15 ppm Li■(Turekian, 1972). The high Sr con­

centrations in the Tertiary sediments in this comparison are caused by

the large calcic-rich volcanic component in these tuffaceous sediments.

The cause of the overall B and Li enrichment, although slight,

and the wide Sr variations in the fresh samples (Table 9) is less

clear and needs further consideration. The most probable causes of

the enrichment and variations are (1) original magmatic compositions

and (2) secondary hydrothermal alteration. The first can be examined

in light of the whole rock geochemistry performed on all major units,

and the latter in respect to the physical properties of the units and

the distribution of the anomalies in the study area.

Original Magmatic Compositions

The variable Sr contents in the fresh volcanic units is caused

by original magmatic compositions. Sr is well known to follow Ca in

magmatic differentiation (Faure, 1972), and the lower CaO concentration

in the Blair Junction trachytes (Figure 20) explains their lower Sr

content. In thin section, this is seen by a greater plagioclase

abundance in the Coaldale latites (Table 1).

The greater B and Li values in the Coaldale latites, however,

are not so strongly suggestive of a primary magmatic origin. As both

are LIL elements, they should have at least slightly greater concen­

trations in the more differentiated Blair Junction trachytes (Figure

106

21). The higher B and Li contents in the Coaldale units require their

magmas to be originally more B- and Li-rich than the Blair Junction

magmas. Although this possibility cannot be refuted, several other

lines of evidence lead to an epigenetic hydrothermal cause of the

enrichment of B and Li in the study area. •

Secondary Hydrothermal Alteration

The enrichment of B and Li in the fresh volcanic rock samples

by a superimposed hydrothermal event is more difficult to document than

the cause of the Sr variations. Comparing the geochemistry of competent

and relatively impermeable hornblende flow and intrusion samples in

Table 8 to the less competent, more permeable hornblende tuff and

lahar samples reveals strikingly different averages. The competent

units have an average of.11.3 ppm B, very close to the expected

Turekian value. The more permeable units, however, have an average

of 24.3 ppm and are definitely enriched with respect to B. The perme­

able units also show higher Li values, if one extremely Li-rich sample

(CVA 6) is excluded. If one anomalous pyroxene sample is also

ignored (CVA 24, see below), the difference between competent and

permeable units is further substantiated in the Coaldale latites.

The direct relationship between enriched B and Li and permeable,

fresh volcanic units strongly suggests an epigenetic enrichment of

these elements in the study area. Hydrothermal alteration by a late

B-rich fluid can be inferred by three relationships: first, the pre­

sence of small, but significant, B anomalies in fresh units and large

anomalies in weathered units near secondary, crosscutting opaline veins;

107

second, the extreme weathering and thick fluff accumulations near

such veins and present around a sericitic alteration center; and third,

the presence of modern B-rich springs in the area.

Three instances of crosscutting opaline veins occur in the study

area. On the east flank of Coaldale Ridge, several north trending, 1"

opaline and chalcedonic veins crosscut an unweathered hornblende tuff

breccia (Figures 2 and 14). Separate analyses of the vein material

and surrounding tuff reveal small B anomalies in each material. These

assayed 33 and 58 ppm B, respectively (Table 7, CVA 32 and 33).

Another opaline vein system occurs on the southwestern exterior of the

Blue Mountain complex (Figures 2 and 14, CVA 47). Here the veins are

locally surrounded by extremely weathered and fluff-rich soils, but

also crosscut less altered Castle Peak Tuff. A sample of the latter

contains 110 ppm B (Table 7).

The third locality of the secondary opaline veins is in aysericitically altered outcrop of the hornblende latite (flow?) on the

western flank of Coaldale Ridge (Figures 2, 14, and 30). This occur­

rence is perhaps the most convincing evidence of a hydrothermal

enrichment of B and Li in the study area. The sericitized latite is

not itself very anomalous in B (19 ppm, CVA 25), but it is surrounded

by an extensive area of extremely weathered units, commonly completely

altered to clays, and very thick (6" to 12") surface fluff accumula­

tions. A fluff-contaminated sample of the clay-altered upper sediment

and tuff unit surrounding this locality assays 791 ppm B (CVA 26).

Also adjacent to this sericitized outcrop are hornblende tuffs and

/ ■ .

Figure 30. Sericitized Hornblende Andesite Flow(?) on the Western Flank of Coaldale Ridge. Irregular "ribbed” weathering is caused by thin resistant opaline veins. This outcrop is the location of sample CVA 25. The photograph is a detail of the area shown in Figure 10 and just east of that shown in Figure 26.

109

pyroxene latites that contain 70 and 29 ppm B, respectively (CVA 29 and

24). It was this area that was explored for its borate potential in

the late 1800's.

Some fracture and fault control of B in this area is suggested

by salt coatings oh fractured, fresh hornblende flows adjacent to the

fault east of the sericitic outcrop (Figure 2). An early sample of

this flow assayed 75 ppm B and helped to provoke the original interest

in this area (B. Watson, pers. comm., 1982). An equivalent sample was

taken 200' east of this fault where no fracture coatings occur (Figures

2 and 14, CVA 28). It assayed only 15 ppm B and suggests at least

partial fault and/or fracture control of the geochemistry in the area.

Other fresh samples in faulted or fractured areas, outside of this

alteration center (CVA 4, 34 and 35, for example) are not significantly

enriched in B.

Age of Alteration in the Southwestern Monte Cristo Range

The timing of the hydrothermal event responsible for the B

and Li enrichment in the project area is uncertain. Definitely later

than the volcanism of the Blue Mountain, the presence of B-rich springs

suggests that it was fairly recent. Bimodal volcanism, commonly

associated with bedded borate deposits (Bates, 1960; Kistler and Smith,

1975), occurred in the Monte Cristo Range as early as 7 m.y. ago and

perhaps lasted a considerable length of time (Albers and Stewart, 1972;

Stewart, pers. comm., 1983). Even more recent bimodal volcanism is

indicated in the Silver Peak Range and Candelaria Hills where both

110

Pliocene and Pleistocene volcanic rocks occur (Figure 16; Robinson

and Chowder, 1973; Speed and Cogbill, 1979b), The presence of young,

very B-rich sediments in Fish Lake Valley also supports a young altera­

tion age. Robinson (1972) has been able to constrain the age of these

lacustrine sediments between 4.8 and 6.0 m.y. B.P. All these factors

support a latest Miocene to Pleistocene age for the alteration in the

project area.

The localized surface accumulations of B-rich fluffs in the

area are only indirectly related to the main alteration event. Evapo-

transpiration of B during weathering resulted in the formation of

locally significant, B-rich surface concentrations. The widespread

distribution of these concentrations— some quite distant from the

sericitized center or opaline veins--deserves further consideration.

It can be argued that the occurrence of B-rich fluffs through­

out the project area was caused by a downward leaching of an overlying,

B-rich sedimentary sequence. At the beginning of this study, the

Esmeralda Formation was believed to be a likely candidate for this

hypothesis. Despite the strong surficial B enrichment in these

lacustrine sediments in the Blair Junction area, drilling has shown

that the anomalies do not continue to depth (B. Watson, pers. comm.,

1982). Thus, the only other support for this hypothesis is _if a

younger sedimentary sequence, contemporaneous with alteration, was once

present in the study area. The obvious analog for these would be the

young B-rich beds in Fish Lake Valley. Perhaps the Qsp (Oa?) sediments

)

Ill

in the northwestern portion of the area are an erosional remnant of

such a (hypothetical) sedimentary sequence.

A more straightforward cause of the widespread fluff accumula­

tions exists. Modern and/or paleo-groundwater mobilization of the

earlier concentrated hydrothermal anomalies would afford the widespread

distribution of the B-rich fluff localities, as well as the weak

anomalies in all the permeable units in the study area. The presence

of modern B-rich springs suggests this recycling continues unto the

present time.

CHAPTER 8

TERTIARY GEOLOGIC HISTORY

The earliest volcanism throughout western Esmeralda County was

the extrusion of rhyolitic ash-flow tuffs. In the Monte Cristo Range

this episode involved the 24.2 m.y. old Castle Peak Tuff which was

deposited on an irregular pre-Tertiary erosional surface. Presumably

of caldera origin, the source region for this tuff was to the northeast

in the center of the Monte Cristo Range. The Castle Peak Tuff was

accompanied in the project area by local rhyolitic diking; both were

derived from magmas with substantial upper crustal contamination.

In early Miocene time, independent andesitic volcanic centers

formed in the southwestern Monte Cristo Range. These centers produced

the petrologically and geochemically distinct latitic Coaldale and

trachytic Blair Junction volcanic sequences, each derived from magmas

with only minor crustal contamination* The Blair Junction trachytes

were erupted throughout the northern Monte Cristo Range and formed in

part from localized vents in the northeastern portion of the study

area. The Coaldale units were localized in the southwestern exterior

of the Monte Cristo Range. Volcanism in both areas began after the

deposition of a volcaniclastic sedimentary package that included thin

air-fall tuffs and ash-flows in the southwest.

More varied than the Blair Junction sequence, the Coaldale

latites were erupted from the Blue Mountain intrusion, a discrete

1 1 2

113

volcanic center surrounded by a half-arc of "concentric" dikes and

thick volcanic lahars. This 22.2 m.y. old sequence includes three

distinct latites, two of which have extrusive tuff breccias and flow

units as well as intrusive expressions. Discrete yet gradational

petrographic signatures of each latite suggest a single zoned magmatic

system was tapped to ever deepening levels through time. Intermittent

faulting during this period of volcanism caused periods of uplift

and/or erosion to occur, and controlled the deposition of units to

form a broadly arched, south dipping volcanic homodine. N40-50°E

extension produced northwest orientations of both the large intrusions

and smaller dikes. Volcanism in the Coaldale region continued until

18.6 m.y. B.P., when a rhyolitic plug intruded the southeast portion

of the study area, perhaps localized by early extensional stress.

Geochemically distinct, this unit was derived from magmas with a major

upper crustal component, apparently unrelated to earlier andesitic

Coaldale volcanism.

Quiescence dominated in at least the southeastern portion of

the study area and in the adjacent Blair Junction area during the

middle Miocene. Deposition of the tuffaceous Esmeralda Formation

13 to 6 m.y. ago was accompanied by concurrent volcanism in the region,

although no active volcanism took place in the nearby Blue Mountain

center.

Strong east-west directed extension formed north trending,

primarily dip-slip faults in the latest Miocene to Pliocene. Secondary

conjugate shear caused right-lateral displacements on the north-south.

114

dominantly dip-slip faults and left-lateral displacements on east-west,

dominantly strike-slip faults. Multiple movement and reactivation of

faults occurred during this lengthy period of extensiohal and shear stress.

Late-stage B-rich hydrothermal fluids altered the Coaldale

volcanic rocks and caused localized, minor B and Li enrichment in the

permeable units of the study area. Major faults and highly fractured

areas partially served as conduits for these fluids. Sericitizing the

latites in the immediate source area, the fluids also produced more

widespread opaline stringer veins and minor clay alteration of the

permeable volcanic units throughout the area. This secondary event

may have coincided'with Pliocene bimodal volcanism in the Monte Cristo

Range and adjacent areas, although the latter is only locally repre­

sented by basalts in the northwest portion of the study area. Modern

B-rich springs in the area attest to the fairly young age of this

alteration event. Modern and/or paleo-groundwater systems, and/or

the downward leaching of young lacustrine sediments contemporaneous

with alteration, produced the widespread B enrichment in the soils and

fluffs overlying the volcanic and sedimentary rocks in the southwestern

Monte Cristo Range.

CHAPTER 9

CONCLUSIONS

The volcanic, geochemical and structural record in the south­

western Monte Cristo Range exemplifies the complexity of even a small,

discrete volcanic center and allows-comparison of its units to more

widespread, regional volcanic sequences. Of the four major Tertiary

rock types in the area, only the isolated, distinctive Coaldale

volcanic sequence has local distribution and is easily associated with

a specific source vent. The others, including the Castle Peak volcanic

sequence, the Blair Junction volcanic sequence, and the younger sedimen­

tary and basaltic sequence, are composed of units with regional extent.

In the Monte Cristo Range, the rhyolitic Castle Peak Tuff is

24.4 m.y. old and correlates well with many other late Oligocene ash-

flows in adjacent areas. The younger sedimentary and volcanic rocks

are composed of middle to late Miocene lacustrine sediments of the

Esmeralda Formation and Pliocene (?) basalts. Both are widespread and

correlate to equivalent units throughout this region. The Blair

Junction volcanic rocks are present through'much of the Monte Cristo

Range and consist of a thick, homogeneous sequence of trachytic flows

overlying a basal monolithic breccia. The 22.2 m.y. old Coaldale latites

are probably correlative with this widespread- sequence, although

neither of these units have definite correlatives outside the Monte

115

116

Cristo Range. Both volcanic units overlie a volcaniclastic sedimentary

sequence that includes air-fall tuffs and ash-flows in the southwest.

The petrographically and geochemically distinct Coaldale

volcanic sequence is quite variable and consists of three latitic

compositions: older hornblende-rich tuff breccias, lahars, flows and

intrusions; intermediate age(?) fine-grained hornblende- and pyroxene-

rich intrusions; and younger pyroxene-rich tuff breccias, flows and

intrusions. These discrete units are petrographically gradational

suggesting a single magmatic source was successively tapped to

increasing depths through time. Thick volcanic lahars and a half^arc

of "concentric" dikes, as well as thickness variations of the volcanic

rocks, identify the Blue Mountain intrusion as the source vent for

this varied sequence.S .7 srLow initial —--- ratios in the Coaldale latites and oxide8 6 Sr

variations in both of the volcanic sequences imply that each were

derived from magmas with little upper crustal contamination which

followed similar differentiation paths. The 18.6 m.y. old rhyolitic

plug in the southeast portion of the project area, considered part of

the Coaldale volcanic sequence, is geochemically similar to the 24.287Srm.y. old Castle Peak Tuff and rhyolitic dikes. High initial — —86Sr

ratios in both suggest derivation from magmas with a significant

crustal component. The lack of intermediate volcanic compositions and

a geochemical dissimilarity deny the genetic relationship between the

Coaldale latites and this rhyolite plug.

Most of the structural complexity in the project area is related

to Basin and Range extensional tectonics, although minor deformation

during volcanism is also indicated. Prominent northeast to northwest

orientations of faults with largely dip-slip movements formed in

response to west-northwest directed extension in the southeast to east-

northeast directed extension in the northwest. Secondary conjugate

shear stress, with orientations compatible with northwest trending,

right-lateral Walker Lane shear zone and the east trending left-

lateral Warm Springs lineament, caused the right oblique-slip on

north-south, dominantly normal-slip faults and the left oblique-slip

on east-west, primarily strike-slip faults. Simultaneous or over­

lapping extensional" and shear Stress deformations and reactivation

of faults are indicated by mutual crosscutting relationships and

variable offsets.

The widespread B anomalies in the Coaldale volcanic sequence

were caused by a late-stage hydrothermal event. Anomalous in B and Li,

the fluids produced localized enrichment in the permeable units of the

study area. Presumably controlled by temperature, pressure and composi­

tional restraints, the fluids severely sericitized a local source area

without any major B deposition. Outward from this center, however, more

widespread opaline stringer veins and clay alteration were associated

with weak B enrichment. Faults and highly fractured zones sometimes

served as conduits for these fluids. Modern B-rich springs indicate a

young age of this alteration, perhaps coincident with Pliocene bimo.dal

volcanism elsewhere in the Monte Cristo Range and adjacent areas. The

117

118occurrence of 4.8 to 6.0 m.y. old, B-rich lacustrine sediments in Fish

Lake Valley strongly suggests an equivalent age for the alteration in

the proj ect area.

It is doubtful that an economic borate deposit ever formed as a

result of this hydrothermal event. Modern or paleo-groundwater mobiliza­

tion of the original, weak and localized anomalies, and/or the downward

leaching of a more B-rich sedimentary sequence contemporaneous with

alteration, have caused the locally significant surface B enrichment

in modern soils and fluffs in the study area.

APPENDIX A

SAMPLE DESCRIPTIONS

The appendix below describes the location and petrology of all samples analyzed in this study. Sample locations are given by struc­tural domain, and are plotted on an overlay to the geologic map (Figure 14). Sample designations are as follows: (1) CVD— Coaldalevolcanic rocks date, (2) CVC— Coaldale volcanic rocks chemistry, (3) CVA— Coaldale volcanic rocks assay, and (4) Coaldale volcanic rocks thin section. UAKA and UARS are University of Arizona Laboratory of Isotope Geochemistry identifications.

Isotope Analyses

Sample no.: CVD 2 ,Alternate designations: CVC 5, CVA 9, CV 1, UAKA 83-06, UARS 83-06.Location: SE; lat. 38°02.51' N, long. 117*50.26' WDescription: Ti; light pink to brown, partially vesiculated crystal-

lithic rhyolitic intrusion. Lithics (5%) are composed of andesite and Paleozoic chert, but are not included in the isotopically or chemically analyzed sample. Quartz (20%) is the major phenocryst and sanadine grains are minor (5%) but very fresh. The groundmass is very glassy, with well-defined shards, and only partially devitrified. Vesiculated outer edge of outcrop is not included in analyses.

Sample no.: CVD 5Alternate designations: CVC 9, CVA 40, CV 12, UAKA 83-07, UARS 83-07Location: C; southeast flank of Blue Mountain; lat. 38*04.35* N,

long. 117*51.40* WDescription: Thai; light blue-gray hornblende andesite intrusion.

Hornblende (20%) and plagioclase (10%) phenocrysts are extremely fresh in a slightly devitrified, glassy groundmass. Less than 2 % of the hornblende crystals have oxidized cores and rims.Only slight chloritic alteration of the glassy groundmass is observed in thin section.

Sample no.: CVD 7Alternate designations: CVC 11, CVA 63, CV 30, UAKA 83-08, UARS 83-08Location: NW; lat. 38*05.24 N, long. 117*52.54* WDescription: Tcp?/^; light tan to gray, crystal-lithic, partially and

densely welded tuff. The sample was taken in the upper flow119

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unit of the tuff. Sanadine (10%) and tiiotite (2%) phenocrysts are very fresh and unfractured. Eutaxitic pumices (5%) and portions of the. very glassy groundmass (75%) show vapor-phase alteration and minor devitrification. Paleozoic chert and limestone lithics (<3%) are not included in the analyzed sample.

Chemical Analyses

Sample no.: CVC 3Alternate designations: CVA 4, CV 38, CV 11Location: SEDescription: Tfai; greenish-gray fine-grained andesite intrusion. The

hornblende phenocrysts (1 0 %) are altered to chlorite and clays, with magnetite resorption rims prominent. Augite phenocrysts (10%) are unaltered. The plagioclase crystals (7%) and cumulate grains (<2%) are altered to clays. The groundmass is composed of unaltered plagioclase microlites, pyroxene granules and glass. The sample was taken toward the center of the intrusion with closely spaced (0.5" to 2") flow fractures.

Sample no.: CVC 4Alternate designations: CVA 6 , CV 14Location: SE xDescription: Thais; slightly weathered, green-gray hornblende andesite

sill. Hornblende grains (20%) are moderately fractured, with only a few ( 5%) that have cores altered to chlorite and clays. Plagioclase crystals (20%) are generally more weathered; all large phenocrysts, both individual as well as cumulate grains, are converted to clays and calcite. Approximately 10% of the groundmass consists of clays, chlorite and carbonate. Although weathered, this represents the freshest sample of the hornblende dikes or sills.

Sample no.: CVC 6

Alternate designations: CVA 16A, CV 21Location: CDescription: Tri; reddish purple banded rhyolite intrusive dike. The

sample includes interior banded rhyolite (90%) as well as vitrophyric outcrop edge (10%) Interior of dikes consists of alternating reddish purple and gray bands, 0.5" to 1" wide.The dark bands are 90% to 95% hematite-stained, very fine­grained groundmass with minor phenocrysts of unaltered plagio­clase (5%). The light bands are plagioclase-rich (35%), in a fresher groundmass composed of glass shards and plagioclase.The vitrophyric phase of the sample shows minor vesiculated pumice and much less pronounced banding.

121Sample no.: CVC 8 ,Alternate designations: CVA 30, CV 10Location: C; southern end of Coaldale RidgeDescription: Tha; bluish gray hornblende andesite flow. The hornblende

phenocrysts (1 0 %) are rimmed by magnetite, but relatively fresh. The augite grains (5%) are much finer-grained and unaltered. Large plagioclase crystals (10%) almost always have cores altered to clays. The groundmass is composed of unaltered plagioclase microlites (30%) and glass (45%). This sample is very representative of its rock type.

Sample no.: CVC 10Alternate designations: CVA 40, CV 4Location: C; north of Coaldale RidgeDescription: Tpa; greenish gray pyroxene andesite flow. Augite

phenocrysts (7%) are relatively fresh with minor magnetite inclusions. Hydrobiotite(?) is present (10%) as pseudomorphs of an extremely fine-grained pyroxene. Plagioclase (30%) occurs as unaltered, medium- to fine-grained phenocrysts (0.5 mm). The groundmass (50%) is an intergrowth of pyroxene granules, chlorite and glass.

Sample no.: CVC 12Alternate designations: CVA 70, CV 23Location: NE -Description: Tea; greenish gray coarse-grained andesite flow. Horn-

belnde phenocrysts (1 0 %) are very altered; 2 % retain hornblende cores within a magnetite rim, whereas 8 % are almost completely replaced by magnetite and clay. All have ghost outlines of the crystal laths composed of clays. The augite phenocrysts (5%) are much finer-grained and only altered to clays along frac­tures. Biotite crystals (2%) are fairly fresh. Plagioclase phenocrysts (15%) are altered to carbonate and clays along their rims. The groundmass (60%) is composed of fine-grained opaques and unaltered plagioclase microlites in a chloritically altered glass.

Assay Analyses

Sample no.: CVA 1Alternate designations: noneLocation: SEDescription: Tha; bluish gray hornblende andesite flow. The hornblende

grains are slightly altered to chlorite and clays, whereas the plagioclase crystals are moderately altered to clays. The sample locality has a low fracture density (6 " to 1 '), and occurs 50* below the top of 1251 thick flow.

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Sample no.: CVA 2Alternate designations: noneLocation: SBDescription: That; weathered hornblende andesite tuff breccia. The

sample consists of mixed gypsum-rich fluff (50%) and fresher grayish green weathered tuff (50%). The fluff occurs as a 6 " thick layer under a crusty soil. The tuff breccia is completely weathered to clays with only relict clast textures preserved.

Sample no.: CVA 3Alternate designations: honeLocation: SBDescription: That; reddish brown hornblende andesite tuff breccia.

The hornblende grains are totally altered to biotite, and the plagioclase crystals to clays. Clasts of a feldspar porphyry occur in a tuffaceous feldspar- and hornblende-rich matrix.The sample is slightly weathered but representative of this rock type. The sample locality occurs 20' below capping fine­grained intrusion. -

Sample no.: CVA 7Alternate designations: noneLocation: SBDescription: That; light reddish brown hornblende andesite tuff

breccia. Fine-grained hornblende- and feldspar-rich porphyry clasts (35%) occur in a tuffaceous matrix of similar composi­tion. The sample is slightly weathered to clays, but no fluff is included in it.

Sample no.: CVA 8

Alternate designations: noneLocation: SBDescription: Te; Esmeralda Formation lacustrine sediments. The

sample represents a 2 0 ' section of interbedded tuffaceous gray clay, a gray pumice-rich sandstone, reworked air-fall tuff paper shales, lignitic shale with plant debris, and grayish green tuffaceous shales. Only minor gypsum-rich fluffs are present, although salts along fractures are common. Overall, the sample is only moderately fresh, but representative of this unit.

Sample no.: CVA 10Alternate designations: noneLocation: SBDescription: Tstl; weathered lower member of sediments and tuffs.

The sample is from a tuff unit with large boulders of a hornblende- and pyroxene-rich andesite weathering out of a tuffaceous matrix of similar composition, each of which is equally represented. Remnant clay clasts are apparent in the soil, and gypsum-rich fluff (1 " thick) occurs just beneath

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the surface and was included in the sample (30%)„ The sample -. locality is less than 1 0 0 * away from a major fault trace.

Sample no.: CVA 11Alternate designations: noneLocation: SEDescription: Tstu; weathered upper member of interbedded sediments and

tuffs. . The sample consists of interbedded fine-grained quartz- rich tuffs and gray, biotite-rich claystones. Gypsum-rich fluff (3" thick) occurs in the soil above the latter, and minor contamination of the sample by this fluff is probable.

Sample no.: CVA 12Alternate designations: CV 37Location: SEDescription: Tfai; greenish gray fine-grained andesite intrusion.

Hornblende, pyroxene and plagioclase phenocrysts are relatively fresh. Hornblende crystals (7%) have thin magnetite resorption rims and have a thin ghost outline of clay and chlorite. The pyroxene grains (7%) have only minor clay alteration in their cores. Both rims and cores of the plagioclase phenocrysts (10%) are altered to clays. The groundmass consists of an intergrowth of glass, pyroxene granules and slightly altered plagioclase microlites. In general, the sample is very fresh and representative of its rock type. The sample locality has closely spaced (0.5" to 3") flow fractures..

Sample no.: CVA 13Alternate designations: noneLocation: SEDescription: Tstlj weathered lower jnember of the sediment and tuff

unit. Clay-altered pyroxene- and hornblende-rich andesite clasts occur in a tuffaceous hornblende-rich andesite matrix. Both phases are equally represented in the sample. The matrix is highly weathered, greenish gray in color and almost totally altered to clays. Thick fluff occurs just beneath the surface and minor contamination is probable.

Sample no.: CVA 14Alternate designations: noneLocation: SEDescription: -h +h -; greenish gray hornblende andesite dike. Very

coarse-grained hornblende phenocrysts occur in a chloritically altered groundmass. Minor carbonate is also present. The sample dike has moderately spaced (6 " to 1 ') flow fractures, which increase in density toward the dike's contacts.

Sample no.: CVA 16Alternate designations: CV 33Location: SE

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Description: Tcpe; light purple exotic block in Castle Peak Tuff.The sample consists of a fine-grained, plagioclase-rich porphyritic tuff with discordant attitudes and well-defined contacts with the Tcp. The plagioclase is altered to clays but the groundmass is relatively fresh. The block has moderately spaced fractures (6 H to 1 *) which increase at its contacts.

Sample no.: CVA 17Alternate designations: noneLocation: CDescription: Tcp^; light pinkish, unwelded Castle Peak Tuff. Quartz-

and sanadine-rich, the tuff occurs between two large Tri dikes. The sample includes clay-altered pink tuff and siliceous light purple tuff at the dike contacts, as well as fresher white tuff in the interior of the exposure. Minor fluff occurs above the clay-altered tuff but was not included in the sample.

Sample no.: CVA 18Alternate designations: CV 29Location: CDescription: Tcp^light purple, unwelded Castle Peak Tuff. Baked

and siliceous, the tuff is at the contact between a large Tri dike and the Tcp. This baked zone is the largest in the study area, and is 50* to 70' wide. The tuff is generally unfractured and fresh. Sanadine phenocrysts (10%) have thin clay-altered rims. Quartz (5-7%) crystals are unembayed and unfractured. Plagioclase grains (7-10%) are more altered to clays. The groundmass is dense and glassy, and only partly devitrified.

Sample no.: CVA 19Alternate Designations: noneLocation: CDescription: Thais; grayish green hornblende andesite sill. The

hornblende phenocrysts are fine-grained and relatively fresh. Plagioclase grains are largely altered to clays. The sample includes material from a 3’ thick chilled margin above the Tcp. The outcrop has moderately to closely spaced (3" to 1') fractures.

Sample no.: CVA 20Alternate Designations: noneLocation: CDescription: Tha; light bluish purple hornblende andesite flow. The

hornblende phenocrysts are completely altered to chlorite and biotite, and the plagioclase to clays. The groundmass is unsiliceous and altered to clays. The sample was taken from in-place float, as no outcrop of this unsiliceous phase of the Tha.

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Sample no.: CVA 21Alternate designations: CV 6

Location: CDescription: Tpai; light green pyroxene andesite plug. Augite

phenocrysts (10%) are unaltered and unfractured. Unlike the Tpa, this sample has no fine-grained hydrobiotite phenocrysts. The plagioclase grains (30%) have clay-altered cores. The groundmass (60%) is an intergrowth of glass, chlorite and clays, with interstitial pyroxene and magnetite grains. This sample is fresh and representative of the rock type.

Sample no.: CVA 22Alternate designations: noneLocation: CDescription: Tpa; soil and fluff sample above the basal contact of a

pyroxene andesite flow. The base of this flow is very weathered and consists of red-brown (above) to green-gray (below) clay with few remnant Tpa clasts preserved. The soil is fluff- rich and has gypsum and cottonball ulexite as distinct layers and crosscutting veins.

Sample no.: CVA 23Alternate designations: noneLocation: C; eastern flank of Coaldale Ridge.Description: Tstu; weathered upper member of the sediment and tuff

unit. The sample consists of equal proportions of fluff, soil and weathered tuff. The tuff is a white, thinly bedded, quartz-rich rhyolite with most of its feldspars and groundmass altered to clays. This exposure has large clasts of a feldspar porphyry weathering out of a clay-altered matrix at its base, and is capped by the rhyolite tuff. Thick fluff (6 " to 8 ") occurs in the soil above the former unit. The sampled interval is 50' thick.

Sample no.: CVA 24Alternate designations: noneLocation: C; western flank of Coaldale RidgeDescription: Tpa; light reddish purple pyroxene andesite flow. The

augite grains are unaltered, but the plagioclase crystals all show pronounced clay alteration. The groundmass is porous and unsiliceous. The sample was taken 3* above the base of a flow, very near major fault traces, and immediately adjacent to the sericitized alteration center (CVA 25).

Sample no.: CVA 25Alternate designations: CV 41Location: C; western flank of Coaldale RidgeDescription: Tha; light tan sericitized hornblende andesite flow(?).

Almost all the plagioclase crystals (30%) are altered to

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sericite with minor calcite. Most of the large hornblende grains (1 2 %) are altered to chlorite and have thick magnetite resorption rims. The groundmass is a fine-grained intergrowth of calcite, sericite and minor chlorite. The weathered surface of the outcrop has irregular ribs of resistant opaline veins. The sample locality forms, a small alteration center surrounded by clay-altered and fluff-rich tuffs, and occurs near a large ridge-bounding fault (Figure 30).

Sample no.: CVA 26Alternate designations: noneLocation: C; western flank of Coaldale RidgeDescription: Tstu; altered/weathered upper member of the sediment and

tuff unit. The sample consists of clay-altered, highly frac­tured (0.5".chips) light gray quartz-rich tuff. The chips have clay pseudomorphs of feldspar crystals and salt coatings on most fracture surfaces. Gypsum-rich fluff (6 " thick) occurs just beneath the surface above the tuff, and is included in the sample (10%). This locality is just west of the sericitized alteration center (CVA 25), and within 50‘ of the westernmost ridge-bounding fault.

Sample no.: CVA 28Alternate designations: CV 9Location: C; western flank of Coaldale RidgeDescription: Tha; light bluish gray hornblende andesite flow. The

hornblende phenocrysts (5%) ubiquitously show strong magnetite resorption, with over half of these having cores altered to clays and chlorite. Augite crystals (5%) are unaltered and fine-grained. The plagioclase grains (25%) are largely altered to clays on their rims and cores. The groundmass consists of plagioclase microlites, glass and minor magnetite. The sample was taken 2 0 0 ' further east of a similar rock chip sample taken by B. Watson. The latter sample was anomalous in B (75 to 100 ppm) and near a large fault trace. CVA 28 is less frac­tured and further away from this fault than the earlier sample.

Sample no.: CVA 29Alternate designations: noneLocation: C; western flank of Coaldale Ridge . .Designation: That; light bluish gray hornblende andesite tuff breccia.

Spheroidally weathering clasts of hornblende andesite occur in a tuffaceous hornblende-rich matrix. The hornblende grains in each phase are totally altered to chlorite and clays as are the plagioclase crystals. The groundmass is slightly altered to clays. The matrix material has salt crusts beneath the weathered surfaces. The sample includes both phases, and overall, is fairly fresh for this rock type.

127\

Sample no.: CVA 31Alternate designations: noneLocation: C; southeastern end of Coaldale RidgeDescription: Tstl; weathered, greenish gray lower member of the

sediment and tuff unit. The sample was taken primarily from the weathered matrix of a clast-rich tuff. The piagioclase crystals are altered to clays. Gypsum-rich fluff under the soil surface and salt coatings on fractures slightly contami­nate the sample..

Sample no.: CVA 32Alternate designations: CV 16Location: C; eastern flank of Coaldale RidgeDescription: That; gray hornblende andesite tuff breccia. The sample

consists of hornblende-rich clasts that occur in a tuffaceous matrix of similar composition. It also includes material from several 3” to 6 " thick ash layers that form thin interbeds in the tuff. The sample area is crosscut by several opaline and chalcedonic veins, 0.5" to 2" wide. The sample is rela­tively fresh, with no salt crusts or fluffs, in the immediate area; The plagioclase crystals are only slightly altered to clays, and the hornblende to chlorite and bibtite. The groundmass, however, has plagioclase microlites almost com­pletely altered to clays, and pumices and glass shards strongly altered to clays, chlorite and carbonate.

Sample no.: CVA 33Alternate designations: noneLocation: C; the same as CVA 32Description: Opaline and chalcedonic veins, 0.5" to 2" Wide, that

crosscut the tuff described in CVA S2. The veins are white to gray, banded opal and chalcedony, with open-space filling textures.

Sample no.: CVA 34Alternate designations: CV 7Location: C; eastern flank of Coaldale RidgeDescription: > > ■>-> ; green pyroxene andesite dike, the sample

consists of a chloritically altered pyroxene dike that cross­cuts the hornblende andesite tuff breccia. Augite phenocrysts (5%) are fresh and the glassy groundmass is chloritically altered. Plagioclase crystals (20%) are only slightly altered to clays. The dike is highly fractured, especially near its contacts.

Sample no.: CVA 35Alternate designations: noneLocation: C; the same as CVA 34Description: That; dark purple to green hornblende andesite tuff

breccia. Baked tuff breccia at its contact with the pyroxene

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dike described in CVA 34. The clasts are a very dark hornblende-rich feldspar porphyry that occur in a light green clay-altered tuffaceous matrix. The sample includes both matrix and clasts in the altered tuff.

Sample no.: CVA 36Alternate designations: CV 18, CV 19Location: C; west of Coaldale RidgeDescription: Tstu; fresh upper member of the sediment and tuff unit.

The sample is of the best exposure of this member. It includes an upper air-fall tuff (5% unaltered plagioclase phenocrysts, 95% glass shards), thin-bedded reworked tuff paper shales, and a lower tan quartz-rich ash-flow tuff. The lower tuff is relatively fresh, with quartz (20%) and biotite (5%) pheno­crysts unaltered. Plagioclase (20%) and sanidine (20%) phenocrysts have only thin rims of chlorite and clay. The groundmass is slightly devitrified.

Sample no.: CVA 37Alternate designations: CV 8

Location: C; southern flank of Blue MountainDescription: Tpat;, light purplish gray pyroxene andesite tuff breccia.

The tuff consists of dark pyroxene-rich, siliceous clasts (50%) in a light purplish gray tuffaceous matrix of similar composition. The plagioclase crystals are clay-altered. White salt crusts occur on the ground surface nearby the sample locality, but are not included in the sample.

Sample no.: CVA 38Alternate designations: noneLocation: C; southern flank of Blue MountainDescription: >■> > > ; greenish gray pyroxene andesite dike. A highly

fractured, but relatively fresh chip sample of this rock type. The augite phenocrysts are fractured but unaltered, whereas the plagioclase grains are mostly altered to clays. It is more fractured than any other Tpa sample.

Sample no.: CVA 39Alternate designations: CV 17Location: C; southern flank of Blue MountainDescription: Thai; light bluish gray hornblende andesite lahar. The

sample consists of both clasts and matrix of this rock type.The clasts (40%) are very siliceous and the matrix less siliceous, slightly more weathered hornblende andesite. The hornblende phenocrysts (2 0 %) are very fresh, with only thin magnetite resorption rims. The plagioclase grains (25%) are only slightly altered to clays in the clasts, and moderately altered in the matrix. The groundmass is very glassy, with little to no chioritic alteration.

129

Sample no.: CVA 41Alternate designations: CV 35Location: C; southwestern flank of Blue MountainDescription: Tcpj; white to gray unwelded Castle Peak Tuff. A fresh

sample of this quartz-rich unit. Lithie clasts in the tuff consist of Paleozoic chert and limestone. The sample is from relatively fresh tuff near its contact with Thai.

Sample no.: CVA 42Alternate designations:, noneLocation: C; southeastern flank of Blue MountainDescription: w h -; greenish gray hornblende andesite dike. The horn­

blende phenocrysts are fresh, but the plagioclase grains and groundmass are weathered to clays, chlorite and carbonate. Moderately spaced fractures (lu to 6 ") are parallel to flow foliation and increase in density toward the dike's outer contacts.

Sample no.: CVA 43Alternate designations: none ■Location: C; southwest of Blue MountainDescription: That;, grayish purple hornblende andesite tuff breccia.

The sample includes both the clasts (35%) and matrix (65%) of the breccia. The clasts are composed of a light red hornblende- and feldspar-rich porphyry, and the matrix is a tuffaceous material of the same composition. The hornblende grains are slightly altered to chlorite and clays. The plagioclase phenocrysts and groundmass are both altered to clays. The sample was taken 10' above the Tcp-That contact. Minor silicification of the That occurs at this contact.

Sample no.: CVA 44Alternate designations: noneLocation: C; southwest of Blue MountainDescription: Tpat; purple pyroxene andesite tuff breccia. Moderately

weathered, the sample includes both siliceous clasts (45%) and tuffaceous matrix (55%) of the tuff breccia. Both are composed of augite and plagioclase phenocrysts in a glassy, chloritically altered groundmass. The sample was taken at the contact of this unit with a pyroxene dike.

Sample no.: CVA 45Alternate designations: noneLocation: C; northeast of Coaldale RidgeDescription: TCP2 /3 ; pink to orange, partially to densely welded

Castle Peak Tuff. Unweathered, the sample is fresh and unaltered. Quartz and sanidine crystals show no alteration.The groundmass is devitrified and slightly clay-altered.Lithic clasts in the tuff consist of Paleozoic chert and a coarse-grained feldspar porphyry.

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Sample no.: CVA 46Alternate designations: noneLocation: C; southwest of Blue MountainDescription: Thai; light bluish gray hornblende andesite intrusion.

Very fresh sample of a large plug of a hornblende andesite. Hornblende and plagioclase phenocrysts are only slightly altered in a glassy groundmass.

Sample no.: CVA 47Alternate designations: noneLocation: C; southwest of Blue MountainDescription: Tcp^; light purple to white, unwelded Castle Peak Tuff.

The sample locality is 10* away from a fluff-rich soil above extremely clay-altered tuff. The sample area is crosscut by opaline veins as in CVA 32. The chip sample taken is relatively fresh, with only slight clay alteration of the groundmass and feldspars. No fluff is included.

Sample no.: CVA 48Alternate designations: noneLocation: C; southwest of Blue MountainDescription: Tfai; light green to purple fine-grained andesite

intrusion. This fresh sample of andesite was taken 2 1 above its contact with That, The plagioclase crystals and groundmass are only slightly altered to clays, with minor chloritic alteration occurring at the contact. The hornblende pheno­crysts are relatively fresh. The sample locality is character­ized by closely spaced (0.5" to 2") flow foliation fractures.

Sample no.: CVA 49Alternate designations: CV 40Location: C; northern end of Coaldale RidgeDescription: Tfai; dark grayish green fine-grained andesite intrusion.

The sample was taken in a 150* wide fault zone that has three 2" to 6 " wide quartz-cemented fault breccias. The sample includes only the relatively fresh andesite adjacent to each of the breccias, but not the fault breccias. The plagioclase phenocrysts are only slightly weathered to clays, and the augite and (minor) hornblende grains are very fresh. The andesite is moderately to highly fractured, with the joints 1 " to 6 " apart.

Sample no.: CVA 51Alternate designations: noneLocation: C; north of Coaldale RidgeDescription: That; light pinkish purple hornblende andesite tuff

breccia. The sample consists of both the clasts (35%) 'and matrix (65%) of this relatively fresh tuff. Of similar compo­sition, the matrix is more tuffaceous and slightly more

131

hornblende-rich than the siliceous clasts. Only slight clay alteration of the plagioclase grains and groundmass is apparent„

Sample no.: CVA 52Alternate designations: noneLocation: C; northern end of Coaldale RidgeDescription: Tha; bluish gray hornblende andesite flow. The horn­

blende phenocrysts are very fresh, whereas the plagioclase grains are moderately altered to clays. The groundmass is very glassy, with only slight chioritic alteration. Moderately spaced (6 " to 1 ') flow foliation fractures are pronounced in the sample locality.

Sample no.: CVA 54Alternate designations: noneLocation: . NWDescription: That; gray hornblende andesite tuff breccia. The sample

includes bleached tuff breccia at the intersection of pyroxene and hornblende andesite dikes, and was taken within 2 ' of both contacts. The hornblende and plagioclase phenocrysts are slightly clay-altered. The tuffaceous matrix is only slightly more weathered/altered, and shows minor chloritic alteration. Closely spaced fractures (0.5" to 3") are parallel to the weathered surface and dike contacts at this locality.

Sample no.: CVA 55Alternate description: noneLocation: NWDescription: Tstu; fluff sample in soil above the upper member of the

sediment and tuff unit. Thick gypsum^rich fluff (6 " to 8 ") occurs above an extremely clay-altered quartz-rich tuff. The sample does not include any fresh material, nor identifiable cottoriball ulexite.

Sample no.: CVA 56Alternate designations: noneLocation: NWDescription: Thai; bluish gray hornblende andesite intrusion. The .

hornblende phenocrysts are altered to chlorite, and the plagio­clase to clays. The groundmass is an intergrowth of clays, chlorite and carbonate. In the sample locality, a rectilinear pattern of calcite veins (0.5" to 1") crosscut highly jointed andesite. The intrusion is a large dike-like plug that occurs near the contact between the young volcanic rocks and the Paleozoic limestones. The sample was taken less than 500* away from this contact.

132

Sample no.: CVA 57Alternate designations: noneLocation: NWDescription: pT; Palmetto Formation (?) sediments . This sample

represents the geochemistry of the basement to the Tertiary volcanic rocks. It includes dark gray, laminated argillaceous limestones and thin-bedded phyllitic shales. The sample was. taken less than 500’ northwest of the contact between the pre- Tertiary rocks and volcanic plug described above (CVA 56).

Sample no.: CVA 58Alternate designations: noneLocation: NWDescription: Tcp^; reddish brown densely welded Castle Peak Tuff. The

sample was taken at the contact of the tuff with a Thai plug. Highly fractured and baked, the tuff has feldspar crystals altered to clays, and biotite laths converted the chlorite.The groundmass is extremely rich in Fe-oxides and is slightly clay-altered.

Sample no.: CVA 59Alternate designations: noneLocation: NWDescription: That; hornblende andesite tuff breccia. The sample

consists of both clasts (40%) and matrix (60%) of this unit. Bluish gray to pink, the clasts have relatively fresh hornblende laths and clay-altered plagioclase crystals. Although most of the clasts are of the same composition as the matrix, minor clasts of a baked, quartz-rich tuff (Tcp?) also occur. The less siliceous, light gray matrix has fresh hornblende crystals and altered plagioclase. The sample was taken 5 * below the baked base of an overlying flow unit of the same tuff breccia. The lower, sampled flow unit of the tuff breccia shows soft- sediment deformational features; the overlying tuff has an embayed contact with the lower tuff (Figure 7).

Sample no.: CVA 61Alternate designations: noneLocation: C; west of Coaldale RidgeDescription: Tstu; weathered upper member of the sediment and tuff

unit. The sample consists of a highly fractured (1" chips), quartz- and biotite-rich clay-altered tuff. The groundmass is completely altered to a deep red clay, and the feldspar pheno- crysts to a white clay. The biotite and hornblende (?) laths are altered to clays and chlorite. Minor fluff occurs in the soil above the tuff and a salt coating on the chip's fracture surfaces is common. The fluff was not included in the sample, but minor contamination is probable.

133

Sample no.: CVA 62Alternate designations: noneLocation; NW-Description: Tcpg; greenish gray partially welded Castle Peak Tuff.

The sample was taken at the southern extent of two northwest trending faults. In the 20 square foot sample area, three small-scale left separation faults occur in a highly fractured, silicified and chloritically altered tuff. Silicification decreases as clay alteration increases with distance away from each small fault. The Castle Peak Tuff in this locality is very biotite-rich (now chlorite) and is part of the upper cooling unit defined in this area.

Sample no.: CVA 64Alternate designation: CV 2Location: NWDescription: Tb; black basalt flow(?). This sample was taken from one

of the few Tb outcrops in the area. It consists of fresh hypersthene crystals (15%) in an extremely fine-grained pilotaxitic groundmass. The lack of olivine indicates this rock type may be better classified as a "basaltic andesite."

Sample no.: CVA 65Alternate designation: noneLocation: NWDescription: Qsp(?); bedded spring deposits in alluvium(?). The

sample includes a basal limy chert conglomerate, and upper alternating beds of chalcedony, limestone breccia, opal and travertine. Open-space filling textures are common in these upper bands. These gently dipping sediments unconformably overlie both the Tcp and That. They are most likely older alluvium, but could be a remnant Tertiary sequence or even Te.

Sample no.: CVA 6 6

Alternate designations: CV 34Location: NEDescription: Tcpe; exotic rhyolite block in Castle Peak Tuff. The

sample is of the largest exotic block within the Tcp in the study area. The block is heavily Fe-stained and oxidized. It is partially vesiculated, and has pronounced eutaxitic pumices in portions of the outcrop. The attitudes do not vary in this exposure and are almost perpendicular to the surrounding Tcp. The pumice, feldspars and groundmass are altered to clays, but quartz grains are fresh. Biotite laths are altered to chlorite. Pronounced silicification of the groundmass also occurs.

134

Sample no.: CVA 67Alternate designations: CV 25Location: NEDescription: Tfa; gray fine-grained andesite intrusion(?). The

hornblende grains (5%) are altered to chlorite and most are totally resorbed by magnetite with ghost envelopes of clays and carbonate. The augite phenocrysts (2%) are fresh. The plagioclase phenocrysts (8 %) are only slightly altered.to clays. The groundmass consists of microlitic plagioclase, glass and pyroxene granules. Overall, this sample is very fresh and representative of this rock type.

Sample no.: CVA 6 8

Alternate designations: noneLocation: NEDescription: Qsp; fluffs and crusts surrounding a modern spring. The

sample includes white gypsum-rich fluff (6 " to 8 ") that occurs under a crusty, limy soil surrounding a modern spring. The area affected by the spring is at least 500 square feet.

Sample no.: CVA 69Alternate designations: noneLocation:/ NEDescription: Ts; gray thin-bedded tuffaceous sediments. The sample

consists of alternating beds of sandy siltstones, silty mud­stones and pebble conglomerates. All units are extremely volcaniclastic, with clasts of pumice quite pronounced in the coarser units. The fine-grained sediments are very biotite- rich and tuffaceous. The sample is fresh, with no evidence of salts or fluff anywhere nearby.

Sample no.: CVA 71 ’Alternate designations: CV 26Location: NEDescription: Tcab; reddish purple coarse-grained andesite breccia.

The sample contains both clasts (50%) and matrix (50%) of the breccia. Dark red clasts are very hornblende-rich (25%) with minor plagioclase phenocrysts (5%). The groundmass of the clasts is composed of glass and microlitic plagioclase and is heavily Fe-stained. The matrix has less and finer-grained hornblende phenocrysts (1 0 %) in a plagioclase-rich glassy groundmass with little to no Fe-staining. In both the clasts and matrix, the hornblende grains are resorbed by magnetite. The plagioclase crystals are slightly altered to clays. In general, the sample is fresh for this rock type.

Sample no.: CVA 72Alternate descriptions: noneLocation: NE

i

135

Description: Tcp^; white unwelded Castle Peak Tuff. Very fresh, thetuff in this locality is lithic-rich with clasts of a feldspar andesite and Paleozoic chert, limestone and phyllitic shales. The groundmass and sanidine grains are only slightly altered to clays.

Sample no.: CVA 73Alternate designations: noneLocation: NEDescription: pT; Palmetto Formation (?). The sample represents the

pT basement geochemistry in the northeast. It consists of light tan to gray phyllitic shales and minor thin-bedded lime­stones.

Sample no.: CVA 74Alternate designations: noneLocation: SEDescription: Te(?); Esmeralda Formation lacustrine sediments. This

sample is representative of fresh, fairly coarse-grained Esmeralda Formation sediments. The sample consists of an upper gray sandstone, minor tuffaceous siltstones, and lower limy pebble conglomerates. The sample locality includes 25* of section.

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Bates, R. L., 1960, Sedimentary Minerals and Sulfur, in Geology ofthe Industrial Rocks and Minerals: Harper Brothers, publisher,pp. 393-400.

Bell, E. J., and Slemmons, D., 1980, Neotectonic analysis of the Walker Lane, western Nevada and northern California: Geol. Soc. Am.Abs. with Prog., v. 14, p. 148.

Y -

Bonham, H. F., 1969, Geology and mineral deposits of Washoe andStorey Counties, Nevada: Nev. Bureau of Mines and Geol. Bull.70, 140pp.

Carmichael, I. S. E., Turner, F. J. and Verhoogen, J., 1974, Igneous Petrology: McGraw-Hill Inc., publisher, 739pp.

Cox, K. G., Bell, J. D. and Pankhurst, R. J., 1979, The Interpretation of Igneous Rocks: George Allen and Unwin LTD, publisher, 450pp.

Eaton, G. P., 1979, Regional geophysics, Cenozoic tectonics, andgeologic resources of the Basin and Range Province and adjoining regions, in Newman, G. W„, and Goode, H. D., eds.. Basin and Range Symposium and Great Basin Field Conference: Rocky Mtn.Assoc, of Geol. and Ut. Geol. Assoc., pp. 11-39.

Ekren, E. B., Bucknam, R. C.', Carr, W. J., Dixon, G. L. and Quinlivah,W. D., 1976, East-trending structural lineaments in Central Nevada: U.S. Geologic Sur. Prof. Paper 986, 16pp.

Ekren, E. B., Byers, F. M., Hardyman, R. F., Marvin, R. F. andSilberman, M. L., 1980, Stratigraphy, preliminary petrology, and some structural features of Tertiary volcanic rocks in the Gabbs Valley and Gillis Ranges, Mineral County, Nevada:U.S. Geol. Sur. Bull. 1464, 54pp.

Faure, G., 1977a, The Rb-Sr method of dating, in Principles of Isotope Geology: John Wiley and Sons, Inc., pp. 75-95.

136

137

Faure, G., 1977b, Isotope Geology of Strontium, in Principles of Isotope Geology: John Wiley and .Sons, inc„, pp. 75-95.

Faure, G., and Powell, J. L., 1972, Strontium Isotope Geology: Springer-Verlag, Publisher, 188pp.

Ferguson, H. G., Muller, S. W. and Cathart, S. H., 1953, Geology of the Coaldale Quadrangle, Nevada: U.S. Geol. Sur. Geologic Quad­rangle Map GQ-23, Scale 1:125,000.

Foldvair-Vogl, M., 1978, Theory and Practice of Regional Geochemical Exploration: Akademiai Kiado, publisher, 272pp.

Preidman, I., Smith, G. I. and Matsuo, S., 1982, Economic implicationsof the deuterium anomaly in the brine and salts in Searles Lake, California: Econ. Geol., v. 77, pp. 694-703.

Gilbert, C. M., and Reynolds, M. W., 1973, Character and chronology of basin development, western margin of the Basin and Range Province: Geol. Soc. Am. Bull., v. 84, pp. 2489-2509.

Keith, W. J., 1977, Geology of the Red Mountain mining district,Esmeralda County, Nevada: U.S. Geol. Sur. Bull. 1423, 45pp.

Kistler, R. B., and Smith, W. C., 1975, Boron and borates, inLefrond, S. J., ed.. Industrial Rocks and Minerals., 4th edition: American Inst, of Mining, Metallurgical and Petroleum Engineers, publisher, pp. 473-496,

"Krauskopf, K. B., 1979, Introduction to Geochemistry: McGraw-Hill,Inc,, publisher, 617pp.

Lipman, P. W., Doe, B. R., Hedge, C. E. and Steven, T. A,, 1979, Petrologic evolution of the San Juan volcanic field, southwestern Colorado: Pb and Sr isotope evidence: Geol. Soc.Am. Bull., v. 89, pp. 59-82.

Locke, A., Billingsley, F. R. and Mayo, E. B., 1940, Sierra Nevadatectonic patterns: Geol. Soc. Am. Bull., v. 51, pp. 513-540.

MacDonald, G. A., 1972, Volcanoes: Prentice-Hall, Inc., publisher,510pp.

Marvin, R. F., Mahnert, H. H., Speed, R. C. and Cogbill, A, H„, 1977,K-Ar ages of Tertiary igneous and sedimentary rocks of the Mina-Candelaria region: Isochron/west, no. 18, pp. 9-12.

McKee, E. H., 1974, Timing of late Cenozoic crustal extension in the western United States: Geol. Soc. Am. Abst. with Prog.,Cordilleran Section, p. 218.

138

Moore, S. W., 1981, Geology of a part of the southern Monte CristoRange, Esmeralda County, Nevada: U.S„ Geol. Survey Open-FileReport 81-710, 160pp.

Muessig, S,, 1959,. Primary borates in playa deposits: minerals of highhydration: Boon..Geol., v. 54, pp. 495-501.

Noble, D. C., and Hedge, C. E., 1969, Sr87/Sr86 variations within individual ash-flow sheets, in Geological Survey Research 1969: U.S. Geol. Survey Prof. Paper 650-C, pp. C133-C139.

Robinson, P. T., 1972, Petrology of the potassic Silver Peak volcaniccenter, western Nevada: Geol. Soc. Am. Bull., v. 83, pp. 1693-1708.

Robinson, P. T., and Crowder, D. F., 1973, Geologic map of the Davis Mountain Quadrangle, Esmeralda and Mineral Counties, Nevada, and Mono County, California: U.S. Geol. Survey GeologicQuadrangle Map GQ-1078, scale 1:62,500.

Robinson, P. T., McKee, E. H. and Moiola, R. J., 1968, Cenozoicvolcanism and sedimentation. Silver Peak region, western Nevada and adjacent California, in Coats, R. R., Hay, R. L. and Anderson, C. A., eds., Studies in Volcanology: Geol. Soc. Am.Memoir 116, pp. 577-611.

Robinson, P. T., Stewart, J. H., Moiola, R. J. and Albers, J= P., 1976, Geologic map of the Rhyolite Ridge Quadrangle, Esmeralda County, Nevada: U.S. Geol. Survey Geologic Quadrangle Map GQ-1325,scale 1:62,500.

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139

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140

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;

LEGEND:FAULT, BAR AND BALL INDICATES DOWNTHROWN SIDE

c v a COALDALE VOLCANIC ASSAY SAMPLE

cvc COALDALE VOLCANIC CHEMISTRY SAMPLE

c v d COALDALE VOLCANIC DATE SAMPLE

c v a 23 SAMPLE DESIGNATION 12 BORON VALUE IN PPM

---------DOMAIN BOUNDARIES

STRUCTURAL AND GEOCHEMICAL OVERLAYSOUTHWESTERN MONTE CRISTO RANGE

X

FIGURE 14

DIXIE A. HAMBRICK, MS THESIS, DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, 1984

/' ~ V Z * ' z

3/ ^ ^ '

/ z x. \

Geologic Sections Across The Southwestern Monte Cristo RangeEsmeralda County, Nevada

6000'n r 6000Tcp3 Tcp3

Thai Cp Tcp2 Tcpg TcP2x | / f ! / / / \ _

5 0 0 0 - -5 0 0 0

400 0 4000

7000'n r 7000TcabTcab A Tcab

BLUEMTN

Tcp _ —

6000 '- -6 0 0 0Thai (?) Tcp

5000 - -5 0 0 0

4000 ' 4000

LITHOLOGIC UNITS

r6 0 0 0ROAD 7 TCP (?)Tfai /That TCP That

5000 - 5000

Tfai I '

4 0 0 0 4 0 0 0

6000 ' -i COALDALERIDGE r 6000

Tstl

5 0 0 0 - -5 0 0 0Tha HTpoi

TstuTha That VlTstl _ -n / Tstu-V- — - Tstu 4

y// Tcp4000 4 0 0 0

QUATERNARY DEPOSITS

YOUNGER SEDIMENTARY AND BASALTIC SEQUENCE

COALDALE VOLCANIC SEQUENCE

|Tpa| |Tpdis|

| Tpat|

|fpai X

IHIH M 1 Thai 1

| Thai |

| Tstu|

| That|

lTs"l

BLAIR JUNCTION VOLCANIC SEQUENCE

1 Tcal

|Tcob|

PH

SCALEl: 12,000

2QQQ __ 1000 Q_________1000 2000______3000

(F E E T )

CASTLE PEAK VOLCANIC SEQUENCE

500t—

(M E T E R S )

500 1000 — |

C A STLE PEAK T U F F

|Tcpe| | Tcp11 |Tcp2|

NO VERTICAL EXAGGERATIONPRE-TERTIARY BASEMENT

FIGURE 15

DIXIE A. HAMBRICK, MS THESIS, DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, 1984

w

Geology of the Southwestern Monte Cristo RangeEsmeralda County, Nevada

EXPLANATION OF MAP UNITS

QUATERNARY DEPOSITS

QUATERNARY ALLUVIUM Qsp QUATERNARY (?) SPRING DEPOSITS- S A L T AND C A R B O N A T E - RICH CRUSTS

AND F L U F F S ; BEDDED C H AL C ED O N IC AND T R A V E R T IN E DEPOSITS

YOUNGER SEDIMENTARY AND BASALTIC SEQUENCE

BASALT FLOWS (?)

ESMERALDA FORMATIONS- T A N TO GREEN TUFFACEOUS M UDSTO NES,

S IL T S T O N E AND SANDSTONES

COALDALE VOLCANIC SEQUENCE

INTRUSIVE RHYOLITE- A L K A L I (F E L D S P A R ) R H Y O L ITE

PYROXENE ANDESITE FLOWS

- L A T I T E 1-

Tpais PYROXENE ANDESITE SILLS Tpoi PYROXENE

ANDESITE INTRUSIONS / PYROXENE ANDESITE DIKES

Tpat

Tfoi

PYROXENE ANDESITE TUFF BRECCIA

FINE-GRAINED ANDESITE INTRUSIONS- L A T I T E f

HORNBLENDE ANDESITE FLOWS

Thai, HORNBLENDE ANDESITE SILLS Thai

- L A T I T E 1

Thai

- L A T I T E 1-

HORNBLENDE ANDESITE LAHAR

HORNBLENDE ANDESITE INTRUSIONS

- L A T I T E */ HORNBLENDE

ANDESITE DIKES

That HORNBLENDE ANDESITE TUFF BRECCIA

Tstu

Tstl

UPPER MEMBER OF SEDIMENT AND TUFF UNIT- I N T E R B E D D E D A I R - F A L L T U F F S , PAPER S H A L E S

AND Q U A R T Z - R I C H ASH FLOWS

LOWER MEMBER OF SEDIMENT AND TUFF UNIT- H I G H L Y W E A T H E R E D , L IT H IC - RICH T U F F S

BLAIR JUNCTION VOLCANIC SEQUENCE

Tea* COARSE-GRAINED ANDESITE FLOWS Tfa( A S T E R IK S MARK POSSIBLE V E N T AREAS)

FINE-GRAINED ANDESITE INTRUSIONS

- T R A C H Y T E t

Tcab COARSE-GRAINED ANDESITE BRECCIA

SEDIMENTARY ROCKS- G R A Y V O L C A N IC L A S T IC PEBBLY CONGLOMERATES,

SANDSTO NES AND S ILTSTONES

CASTLE PEAK VOLCANIC SEQUENCE

h-CL

fiT^-u ' 1 Q Tri BANDED RHYOLITE INTRUSIONS - A L K A L I (F E L D S P A R ) R H Y O L IT E 1-

CASTLE PEAK TUFF- R H Y O L I T E 1"

( 4 members )

EXOTICBLOCK MEMBER

Tcp, DENSELY WELDED MEMBER

Tcpg PARTIALLY WELDED MEMBER

TCP3 UNWELDEDMEMBER

PRE-TERTIARY BASEMENT

PRE-TERTIARY ROCKS- M O S T LY T H E ORDOVICIAN P A L M E T T O FO R M A T IO N , ' IN T E R B E D D E D

L I M E S T O N E S , B L A C K C H E R T S , AND P H Y L L IT IC S L A T E S

t IUGS CLASSIFICATION

CORRELATION OF MAP UNITS

QUATERNARY

4

IOOO

FAULTS

(M E T E R S )

CONTOUR INTERVAL - 20 FEETBASE FROM US GEOLOGICAL SURVEY

COALDALE AND BLAIR JUNCTION QUADRANGLES

MAP SYMBOLSS T R IK E AND DIP OF BEDS - DASHED WHERE A P P R O X IM A T E

S T R IK E AND DIP OF FLOW BANDS OR OTHER P L A N A R FLO W FE A T U R E S - DASHED WHERE A P P R O X IM A TE

S T R IK E AND DIP OF O P A L IN E V E IN

D E P O S IT IO N A L OR IN T R U S IV E CONTACT, SHOWING DIP W HERE KNOWN - DASHED WHERE A P P R O X IM A T E , D O TT E D WHERE IN FE R R E D

82 R I G H T - L A T E R A L FA U L T , SHOWING DIP WHERE AND PLUNGE 0 F A n Y L IN E A T IO N St 5*35 d a s h e d W HERE A P P R O X IM A TE . BAR AND B A L L ON DOWNTHROWN SIDE

PLIOCENE

UPPERANDMIDDLEMIOCENE

UNCONFORMITY

IB.6 m. y.

Tpa Tpoi Tpais < < <% T p o t ^ ^

Tfoi Tea

2 2 . 2 m.y. Thai' T h a i s

III H + + - ,

Tha — —

hat Thai Tcab

Tstu"Tstl"

LOWERMIOCENE

UNCONFORMITY

2 4 .2 m.y. Tcpe

UPPEROLIGOCENE

TERTIARY

t35vT9

DIKES

kfsTHicLAi£6L« rrowKsSoLri.r"85" ANVHORNBLENDE AN DESITE D I K E , SHOWING DIP W HERE KNOWN .

PYROXENE A N D E S ITE D IK E , SHOWING DIP WHERE KNOWN

UNCONFORMITYORDOVICIAN PRE-TERTIARY

FIGURE 2

DIXIE A. HAMBRICK, MS THESIS, DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, 1984

\4 9 f %

\ \