PARAGENETIC STUDY OF THE ORES AT SANTA

BARBARA, CHIHUAHUA, MEXICO

by

JIMMY L. GALEY, B.S.

A THESIS

IN

GEOLOGY

Submitted to the Graduate Faculty of Texas Tech University in

Partial Fulfillment of the Requirements for

the Degree of

MASTER OF SCIENCE

Approved

Director

Accepted

/

Decernber, 1971

SOS-

1971 /Mo.23/ do p. 2

ACKNOWLEDGE.MENTS

The author wishes to express his appréciation to Dr.

Rae L. Harris, Jr. for his guidance in the directing and

writing of this thesis. The author is grateful to George

Percival, Unit Superintendent at Santa Barbara, and Ameri­

can Sinelting and Refining Company for perm.ission to con-

duct a minéralogie investigation at Santa Barbara. Spécial

thanks go to Kelly Spilsbury, Mine Superintendent, for the

collection of many samples and for serving as interpréter.

11

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ii

LIST OF TABLES V

LIST OF ILLUSTl^TIONS vi

I. INTRODUCTION 1

A. Physiographic Location . . . 1

B. History and Production 1

C. Previous Géologie Work 4

D. Purpose of Investigation 4

E. Sample Collection 5

F. Préparation of Polished Sections 6

II- GENERJiL GEOLOGY 7

A. Géomorphologie 7

B. Stratigraphy 7

C. Regio-nal Structure 11

D. Structure of the Veins 13

E. Post-minerai Faults 19

F. Géologie History 20

III. RESULTS OF EXSOLUTION 22

A. Theory of Exsolution 22

B. Previous Work 23

C. Observed Textures 24

D. Interprétation of Textures 34

1 1 1

IV. RESULTS AND INTERPRETATION OF DATA 39

A. Description of Ore Shoots 39

B. Mineralogy 41

1. Hypogene Minerais 41

2. Supergene Minerais 45

3. Oxidation Minerais 4 8

4. Gangue Minerais 50

C. Paragenesis 52

D. Zonation 57

E. Interprétation of Paragenesis and Zonation . 65

F. Origin 67

G. Température of Formation 69

H. Summary and Conclusions 72

BIBLIOGRAPHY 75

IV

LIST OF TABLES

Table Page

1. Production figures of January, 1971 3

2. Nomenclature of vein Systems 15

3. Classification and disposition of the major

veins in the Santa Barbara area 16

4. SumiTiary of textures and their properties . . . . 33

5. Minerais occurring in the Santa Barbara area

and their relative abundance in each zone . . . . 51

6. Maximum températures of minerai assemblages . . . 70

7. Classification of ores 71

v

LIST OF ILLUSTRATIONS

Figure Page

1. Parral .-lining District 2

2. Santa Farbara Mining area pocket

3. Projected trace of the axial plane in the

Santa Barbara area 12

4. Déformation scheme of fault Systems . 14

5. Hanging wall veins and fooi.v/all veins of horses and wedges 17

6. Grains and blebs in emulsion texture 28

7. Grains, blebs, and wedges in oriented emulsion . 29

8. Stringer formation 29

9. Stringers and rim formation 30

10. Rim texture 30

11. Veins 31

12. Lamellae 31

13. Swarms 32

14. Polyhedrons 32

15. Proposed order of exsolution 38

16. Occurrence of bornite and chalcocite in

the Coyote-Los Hilos Veins 47

17. Proposed order of déposition 54

18. Galena replacing sphalerite 56

19. Areas of decrease in sphalerite 59

20. Areas of decrease in galena 60

VI

n 21. Areas of increase in chalcopyrite 61

22. Occurrence of arsenopyrite 62

23. Occurrence of hedenbergite 63

vil

CHAPTER I

INTRODUCTION

Physiographic Location

The Parral Mining District is in the southern part of

the State of Chihuahua, Mexico (Fig. 1). Hidalgo del Parral,

San Francisco del Oro, and Santa Barbara are the three major

cities within the Parral District. The Santa Barbara Mining

area is in the southern part of the Parral District and

covers approximately twenty-seven square kilometers (Fig. 2).

The Santa Barbara Mining area is a distinct unit inside the

larger Parral Mining District. The city of Santa Barbara is

geographically situated in the center of the mining area and

is twenty-five kilometers, by paved road, southwest of the

principal city of the district. Hidalgo del Parral.

History and Production

Before the arrivai of the Spaniards, the mines of Santa

Barbara were mined on a small scale by the Indians for sil-

ver and gold, which was used mainly for décorative purposes.

In 1563 Rodrigo del Rio led a Spanish expédition into the

area and discovered the vein, "Veta Mina del Agua." Santa

Barbara was founded in 1567 and the area was being actively

mined by 1575. The mines were first worked for the gold and

silver in the oxidized zone, and after the gold had been

2

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pig^ 1^—Parral Mining District (Taken from Koch, 1956 and West, 1949)

depleted, the number of active opérations declined al'tliough

some veins continued to be worked for their poor silver and

lead content.

By 1600 Santa Barbara was the most important city in

the old Spanish Province of Nueva Viscaya which inclucled

the présent states of Chihuahua, Durango, Sonora, Senaloa,

part of Coahuila, Texas, New Mexico, and Arizona (Valverde,

1968). Santa Barbara became increasingly important as a

military garrison, and by 1631 had declined as a mining

center due to large silver strikes at other areas in the

Parral District.

The area was mined intermittently until 1913 V7hen

American Smelting and Refining Company started production.

The Company presently produces approximately 55,000 tons of

ore per month. Table 1 shows the breakdown of métal produc­

tion figures for the month of January, 1971. Thèse values

can be used to approximate the average yearly production

figures.

Table 1. Production Figures of January, 1971

Ore Amounts per ton

Zinc 5.5%

Lead 3.9%

Copper 0.69%

Silver 1.52 grams

Gold 0.4 8 grams

Previous Géologie Work

American Smelting and Refining Company initiated géo­

logie work soon after they began opérations in the area.

The first geologists to visit the area were J. E. Spurr,

Basil Prescott, J. G. Barry, W. M. Davy, and Harrison

Schmitt. In 1937, W. P. Hewitt and F. W. Farwell, under

the direction of J. P. Clendenin, started a formai program

of surface and subsurface mapping. Major published reports

on the Santa Barbara area consist of: Harrison Schmitt's

(1928) discussion of gênerai geology and mineralogy, fol-

lowed by M. D. Kierans (1956) , and J. B. Scott (1958)

writing on tlie structure of the veins and their application

to the déposition of ore. G. S. Koch (1956) studied the

fracture System of the San Francisco del Oro area, which

lies to the northwest of the Santa Barbara area. A compre-

hensive study of the ore minerais using reflective micros-

copy has never been published. Such an investigation was

a major objective of this thesis.

Purpose of Investigation

The ore of the Santa Barbara Mining area occurs as

fissure veins in a séries of faults that range in strike

generally northwest to northeast and hâve steep dips both

eastward and to the west. The purpose of this investiga­

tion is to détermine mineralogy and paragenesis. Exsolution

5

textures are of particular interest due to the présent con-

flict in the récognition of replacement versus exsolution

textures. The conflict is caused by the observation that

some criteria used for assuming replacement are the same

as those used for exsolution. It is possible, therefore,

that exsolution textures may hâve been identified as re­

placement textures in past studies and thus produced an

erroneous paragenetic séquence.

Sample Collection

Two, one week trips were made to Santa Barbara,

Chihuahua, Mexico, for the purpose of collecting samples and

for observing the géologie relationships of the veins and

the minerai assemblages. The samples were collected so as

to represent a horizontal and vertical distribution of the

ore minerais. Control for location was obtained through

the use of company mine maps and cross sections. The sam­

ples were selectively chosen in order to obtain représenta­

tive samples of the ore. Approximately three hundred sam­

ples were collected; such a large number of samples was

necessary so that the ore, as seen in polished sections,

would be evenly spaced throughout the area and would be

représentative of the ore at any immédiate point.

Préparation of Polished Sections

The original samples were first eut into slabs nearly

fivc millimeters thick, and an area of approximately two

square centimeters v/as selected and chipped out with a

chisel-headed rock hammer. The sample v/as hand ground with

a coarse, fixed abrasive to rerriove saw marks, and was then

mounted with epoxy resin and hardener in a bakélite ring

twenty-five millimeters in diameter. After setting, the

mount was hand-ground with a coarse and 300 grit, fixed

abrasive to remove the epoxy from the face of the sample.

Fine-grinding and polishing were accomplished with 240, 400,

600, 1200, and 3000 grit, sliding-abrasives on rotating

laps. Final polishing \vas accomplished witli a 0.3 micron

alumina powder.

Grinding and polishing were carried out with ample

water to reduce heat, and at no time did the sample feel

warm to the touch. Excess heat is undesirable as it could

possibly disturb the relationship that is attained when one

minerai is exsolved from another.

One hundred and five polished sections were made for

the investigation, and a binocular Leitz reflecting micro­

scope was used in their observation. Spécifie références

used in minerai identification were Schouten (1962) and

Uytenbogaardt (1968).

CHAPTER II

GENERAL GEOLOGY

Geomorphology

The Santa Barbara area is situated in the transition

zone of the Sierra Madré Occidental and the Sierra Madré

Oriental. The area is typical of a foothill topography with

the Sierra de Santa Barbara located ten kilometers to the

south. Some mesas with basait flows as cappings are scat-

tered outside of the Santa Barbara area.

The average rainfall is eighteen inches per year v/ith

végétation consisting of scrub oak, scrub Juniper, mesquite,

and steppe grasslands. Most of the trees are restricted to

the valleys. The original végétation is believed to hâve

been stripped from the area and used as fuel and timbering

by the early miners (West, 1949).

Stratigraphy

Shale: The Parral Formation (Valverde, 196 8) is the

oldest formation in the Santa Barbara and surrounding area.

The Parral Formation consists of highly indurated, nonporous,

imperméable shale. Koch (1956) believes that the formation

is actually an argillite. Fresh surfaces range from gray

to black with bluish tints, and the rock varies in composi­

tion from noncalcareous, pyritiferous shale to almost pure

8

limestone. Weathered outcrops grade from light yellowish-

brown, brown, to reddish-brown. Thèse colors are attributed

to oxidation of the pyrite. Beds range in thickness from

one centimeter to twenty-five centimeters. Support in

stopes and drifts cutting this formation is seldom required

except in the vicinity of faults and shears.

Only a few isolated fossils hâve been found in the

shale. Dwarfed ammonites, which were discovered in 1906

near the Palmillas Mine, three miles northwest of Parral,

were identified as characteristic of the deep water faciès

of the Gault-Vraconian (Albian) in central Mexico (Burck-

hardt, 1930) . However, the circumstances of the discovery

were uncertain and the aye regarded as tentative. Bose

(19 0 6) discovered some Aptychus, but no ammonites, and con-

cluded the shale was Upper Jurassic. However, the fossils

were poorly preserved and the âge was considered doubtful.

In 19 65 a fossil was found in an ore basket used to carry

ore to the mill from the San Francisco del Oro mines and was

identified as Acanthohoplites aff. aschiltaensis (Anthula)

which corresponds to Upper Aptian-Lower Albian. Using the

above information Valverde (19 68) correlated the Parral

Formation as équivalent to the Texas Fredericksburg and the

Washita Croups. Interviews conducted with mining personnel

showed the majority believe that the Parral Formation is

Upper Jurassic. Thus, the âge of the shale is définitely

questionable.

The massive shale does not contain marker horizons, so

répétition of beds is either unknown or unrecognized. The

shale has been estimated to be over 930 meters thick. The

deepest shaft in the area is the San Diego shaft, and the

lowest point of the shaft remains in the shale at an éléva­

tion of 1,216 meters.

Andésite: Andésite flows, possibly Oligocene-Miocene

in âge, unconformably overlie the shale and are in the

southern part of the area, Fresh surfaces vary from dark

gray to dark brown, and weathered surfaces are light brown

to white. At the shale-andesite contact, the andésite is

fine-grained but becomes porphyritic away from the contact.

Rhyolite: Rhyolite dikes are younger than the andé­

site and eut the older shale, andésite, and fissure veins.

The rhyolite varies from pink to light brownish-white and

contains five percent quartz phenocrysts which sometimes

reach five centimeters in length. Scott (1956) applied the

term aplite to the intrusive. The dikes vary in width with

depth, and the majority of the dikes strike north. Some

dikes hâve a strike of N 70° E which is not parallel to any

mineralized vein but is parallel to one of the post-minerai

fault groups. The largest dike in the Santa Barbara area

has a width of two-hundred meters at the surface and narrows

to twenty meters at a depth of five-hundred meters below the

surface. The dike intersects the Tecolotes and the Hidalgo

10

veins and makes a prominent topographical knob which is

calied the "bufa." Because the dikes are résistant to éro­

sion, most are easily distinguishable in the field and

détectable in aerial photos.

Conglomerate: A conglomerate of possible Pliocène âge

and cemented by a calcareous cernent and sa.nd, is betv/een the

shale and overlying basait and is possibly Pleistocene in agc

The conglomerate ranges in thickness throughout the area

with a maximum of 3 meters, and in many places is absent.

Diabase and Basait: Fine-grained pyritiferous diabase

dikes intrude post-ore faults and intersect the mineralized

veins. The dikes are believed to be feeders to the overly­

ing basait.

Basait flows of possible Pliocène and Pleistocene âge

unconformably overlie the shale. The basait is dark gray

to black, hard and dense, and breaks with a conchoidal frac­

ture where it does not hâve a high content of vesicles.

Koch (1956) found that the basait in the San Francisco del

Oro area contains 85% feldspar and pyroxene in a ratio of

3 to 1, 10% divine phenocrysts, and 5% vesicles which are

partly filled with calcite. The basait usually occupies the

topographical highs of the area and caps the mesas. The

basait and shale hâve undergone extensive érosion since the

extrusion of the basait. Schmitt (1928) estimated that the

basait originally had a thickness of over 915 meters.

11

Alluvium: Unconsolidated alluvium is found along the

streams and arroyos. The particles range considerably in

size and vary in composition. The larger fragments consist

of shale, limestone, rhyolite, and welded tuff, with some

gold bearing quartz from vein outcrops.

Régional Structure

The structure of the area is discussed with référence

to the shale because the shale is the dominant ore-bearing

formation of the area. Koch (19 56) examined the structure

of the San Francisco del Oro area and found an assymetrical

anticlinorium trending N 28° W and plunging 12° N. The

Southwest limb has an average dip of 30° W and the northeast

limb of 8° E. Clendenin (1971) conducted a study of the

shale in the Santa Barbara area but did not report any défi­

ni te structural feature. Scott (1958) concluded the

following:

At the south end of the structure at Santa Barbara, no definite structure interprétation could be made except that a very complex anticlinorium is présent and has a N 30° W trend. The detailed structure is very complex because the individual shale beds are thrown into complicated drag folds and are broken by numerous small faults. However, underground map­ping in a crosscut 2.7 km long, that extends from the Segovedad shaft to the Cobriza shaft area, gave a very definite picture of an anticlinorium. The west limb seems to dip more steeply than does the east limb.

The trace of the axial plane was projected from the San

Francisco del Oro area into the Santa Barbara area and is

shown in Fig. 3.

12

YX Basait

^ Rhyolite

Major Vein Systems

Trace of Axial Planes

0

Kilometers

Fig. 3.--Projected trace of the axial plane in the Santa Barbara area.

Valverde (19 68) reported that in the area to the south-

east of Santa Barbara, the strike of the shale ranges from

north-south to N 15° W and dips 35° W. The area to the

south of Santa Barbara has a north-south strike and dips to

the west. The structure at Parral consists of a broad, flat

dôme with a stock exposed in the center of the dôme. The

shale away from the dôme has a north-south strike and dips

to the west. The area is believed to be the western limb

of a régional antic.line. The compressional forces are

possibly the resuit of upward movement of the Sierra Madré

13

Occidental with a faster rate of uplile during tije Hioalgoan

orogeny (Laramide orogeny) which would contribut'. nortîjeasc

to east compressional vectors.

Valverde (1968) suggested that the anticlinorir.-.i in Uie

Santa Barbara and the San Francisco del Oro areas ir.ay Le

the resuit of a.n intrusion at depth similar to chat in L'.'e

Parral area. The stock may be similar in genetic origin to

that at Parral but not necessarily tho same stoclc.

Structure of the Veins

Koch (1956) conducted a structural study of the vain

System at San Francisco del Oro and made the follo'.vi.ng

conclusions :

1. A plot of the vein pôles onto a Schmidt equal-

area net showed the veins fall into four distinct systeLis.

2. The four Systems consist of two groups of conjugate

shears (Fig. 4) .

3. The two groups were formed from two distinct periods

of déformation.

4. The delta and gamma Systems served as sites for ore

déposition and also reopened the earlier alpha and beta

Systems.

The two Systems of veins présent in the San Francisco

del Oro area are also présent in the Santa Barbara area.

Table 2 gives the nomenclature used for the vein Systems

14

Fig. 4.--Déformation scheme of fault Systems. The top diagram shows the planes of fracture of the first déforma­tion and the lower diagram shows the planes of fracture of the second déformation. Pn and P2 are directions of the greatest principal stress; R-^ and R2 are directions of the least principal stress. (Taken from Koch, 1956)

15

m the two areas and the average strike and dip of each

System. Table 3 shows the average strike and dip of the

major veins in the Santa Barbara and the vein System to

which each vein belongs.

Table 2. Nomenclature of Vein Systems

San Francisco Santa Barbara Average Strike del Oro and Dip

Frisco Alpha N 2° E, 75° E

Transvaal Beta N-S, 70° W

Footwall Delta N 23° E, 51° W

Cobriza Gamma N 31° W, 58° W

With a tensional stress directed horizontally in an

east-west direction and the principal compressive stress

directed vertically, the resuit is the formation of high

angle normal faults. The alpha and beta Systems hâve mainly

dip-slip movement and minor strike-slip movement. The dips

of the alpha and beta veins are very erratic. The strikes

of the alpha veins vary little, but the strikes of the beta

veins are slightly erratic. Mining opérations reveal the

two Systems intersect at depth in the eastern part of the

area, and intersect the delta and gamma Systems in the west­

ern portion of the area.

The delta and gamma conjugate shears were formed by

tensional stress directed east-west and plunging slightly

16

east, with the principal compressional stress directed north-

south and plunging slightly to the north. The delta and

ganmia Systems are mainly strike-slip with small dip-slip

movement. The delta and gaimma veins are erratic in striJce

and are not as abundant as the alpha and beta veins. The

delta and gaiiïïua Systems offset the alpha and the beta Sys­

tems, and therefore, are younger.

Table 3. Classification and Disposition of the Major Veins in the Santa Barbara A-ea

Alpha

Clarines Cobriza Los Hilos-Coyote Pilares San Carlos San Rafaël

N 8° E, 70° E N-S, 76° E N 7° E, 75° E N-S, 80° E N-S, 70° E N 6° W, 70° E

Beta

Cabrestante Los Angeles Predilecta San Albino-Tecolotes San Diego Santiago

N 5° E, 85° W N-S, 54° W N-S, 70° W N 3° E, 80° W N 7° W, 70° W N-S , 6 5° W

Hidalgo Mina del Agua-

Palo Blanco San Martin

Delta

N 27° E, 42° W

N 21° E, 55° W N 20° E, 55° W

Primavera Seca-Coyote

Gamma N 30° W, 60° W N 28° W, 57° W

17

The four sets of fractures hâve been complicated by

wedges, horses, and intersections of the différent Systems

In the discussion of horses and wedges, the hanging v/all

vein refers to the vein above the horse or wedge, and the

footwall vein refers to the lower vein (Fig. 5). The San

Rafael-Clarines and the Cobriza-Pilares are examples of

wedges. The hanging wall vein is usually wider than the

footwall vein. This discrepancy is attributed to gravity.

Fig. 5.—Footwall veins and hanging wall veins of horses and wedges

The hanging wall vein is not always wider than the

footwall vein. The footwall vein may be wider than the

hanging wall vein at one end of the wedge, or horse, but

with the reverse true at the opposite end. Horizontal dis

placement is believed to hâve partially rotated the wedge,

or horse, to produce this effect. Also, a combination of

18

horizontal and vertical movement, the shape of the wedge,

and the dip of the fault could resuit locally in a wider

footwall vein.

An excellent example of a horse is in the Seca vein

with the top of the horse at the fifth and seventh levels,

and the bottom at the tenth level (Scott, 1958) . Horses

and wedges are présent in almost every major vein in the

area,

Vein intersections complicate the structure and are

very common. Three good examples are the intersection of

the Seca-Coyote with the San Albino and the Tecolotes veins,

the San Martin vein, and the Seca-Coyote with the Hidalgo

veins (Fig. 2). The San Martin vein does not comform in

strike and dip to any of the four sets of fractures;

nevertheless, it is believed to be part of the delta System

that intersected the beta System. Présent mining opérations

show that the Seca-Coyote (gamma) and the Hidalgo (delta)

veins possibly intersect at depth to form one continuous

vein.

A fifth set of mineralized veins which hâve an east-

west trend is to the southeast of Santa Barbara, but is

not in the study area. This set may belong to a post-

minerai fault group representing a second introduction of

suifides.

19

Post-minerai Faults

A séries of faults and joints intersects and displaces

the mineralized veins. The faults and joints fall within

two major trend groups. One group, which contains most of

the major faults, strikes N 80° E, and the other group,

which contains both faults and joints, strikes N 45° W.

Scott (1958) reported that strike-slip movement averages

approximately ten meters, with a maximum of sixty meters,

in a fault which intersects the Coyote vein. Also, dip-slip

movement is moderato with a maximum of ninety meters on a

fault which displaces the Calrines vein.

The faults hâve local mineralized zones of quartz or

calcite, with some faults contaiiiing only fault gouge. The

mineralized and the gouge zones range in thickness from 0.1

to 1.0 meter, with a crushed zone, ranging from 1 to 12

meters, usually surrounding the inner zone. Some of the

gouge-filled faults hâve diabase dikes in the central zone.

The post minerai faults are believed to hâve occurred

over a long period of time. They often displace young dia­

base dikes, sills, and the older mineralized veins. Also,

a récent fault-line escarpment is found four kilometers east

of the southern part of the area.

The N 80° E group in the Santa Barbara area is believed

to be part of an east-v/est trending group further to the

southeast. If they do belong to the same fault System, this

20

may be évidence that the supposed post-minerai faulting

started before the end of mineralization with either mineral-

ization or faulting concentrated to the southeast. However,

this assumption is based on the promise that mineralization

was continuous. The mineralized, east-west trending veins

may represent a second introduction of suifides. The area

to the southeast has been investigated by mining companies,

but to the author's knowledge the data has not been released.

Géologie History

The shales were deposited in Late Jurassic or Early

Cretaceous, and are believed to be a younger formation be-

longing to a séries of limestones and shales which are

found outside of the Parral district. The shales were

folded during the Hidalgoan orogeny (approximately corréla­

tive to the Laramide orogeny) in Early Eocene (Guzman and

DeCserna, 1963). Andésite flows erupted in Oligocene-Miocene

time and covered the eroded shale. The exact period of the

ore fractures is unknown, hov/ever, they hâve been found to

intersect the andésite flows. A second stage of fracturing

reopened a former fracture System and allowed mineralized

fluids to enter the faults. Faulting continued after ore

mineralization, with quartz and calcite déposition and rhyo­

lite intrusion in some of the post-ore faults. Gravel de­

posited in Pliocène time was partially eroded; next basait

21

flows covered the irregular terrain and were su.ly3oqucrt.i /

reshaped to give the présent erosionaJ surface.

uafusscaaiBiLKs DDCI^IVBMH»*

CHAPTER III

RESULTS OF EXSOLUTION

Theory of Exsolution

Exsolution is the séparation of a solid solution into

distinct phases, usually by a décline in température. At

high températures the atomic structure of the host phase

is distorted, and impurities can be accommodated in the

atomic structure. A lowering of the température results

in a more rigid structure and the impurities are forced out

or exsolved, from the host and form a distinct minerai phase;

i.e., the greater the température, the greater the amount

of solid solution and substitution possible. The exsolved

phase may be arranged in accordance with the crystallo-

graphic structure of the host; then as exsoiution increases,

the smaller grains coalesce to give non-orientation of

grains, and with extensive exsolution, development of veins.

The veins usually are not oriented and often outline crys-

tals or homogeneous areas of the host phase.

Exsolution is controlled by many factors. Température

is of prime importance because the maximum température of a

solid solution régulâtes the amount of dissolved material

which the host can accommodate. The cooling rate régulâtes

the amount of impurities which can be exsolved. Slow cool­

ing rates allow the migration of ions in attaining

22

23

equilibrium and the solid solution becomes saturated, thereby

increasing exsolution; i.e., the greater the amount of super­

saturation, the less the amount of exsolution.

Tho diffusion rate of the dissolved phase is controlled

by the température, the concentration gradient, and the dif­

fusion coefficient. The concentration gradient is partially

dépendent on the solubility and concentration of the exsolved

phase. Diffusion is also a function of the physical and

Chemical properties of the exsolved and host phases. Other

variables include the coefficient of expansion, the size of

the host, and the stresses exerted on the host.

Therefore, exsolution is a very complicated process

that involves m.any variables which are dépendent upon one

another. A phase that shows limited exsolution may be the

resuit of low concentrations of the exsolved phase, or could

be the resuit of rapid cooling or supersaturation. Also,

extensive exsolution may be due to slow cooling from a high

température, or annealing at a low température.

Previous Work

Exsolution has long been recognized to occur in sul-

fides, and the textures hâve been studied extensively

(Schwartz, 1931; Bastin, 1950; Ramdohr, 1950; Edwards, 1954;

Oelsner, 1961) .

24

Brett (19 64) summarized some of the important expéri­

mentai work on the Cu-Fe-S System and, combined with his

own experiments, reached the following conclusions:

1. Larnellae form from rapid and slow cooling, with

the géologie environment more compatible to slow cooling.

2. Lamellae can form from a low initial concentration

of the exsolved phase if the cooling rate is slow enough so

that the degree of supersaturation is never high.

3. An early and late stage of lamellae formation

resuit with neither stage prédominant over the other.

4. Certain minerais persistently exsolve from other

minerais despite the initial ionic concentration.

5. Pressure and trace éléments do not affect the

préservation of lamellae but they may affect the diffusion

rate.

6. Veins may form from exsolution as well as replace­

ment and the introduction of a new phase into a fissure.

7. Mutual boundaries and textures suggesting replace­

ment may be the resuit of exsolution.

Observed Textures

Exsolution of chalcopyrite from sphalerite occurs ex­

tensively in the ores from the Santa Barbara area. Upon

examination of exsolution textures, the author found that

a method of nomenclature was necessary in describing the

25

many différent textures that were observed. Most of the

observed textures were found in photographs in the liter-

ature; however, they were sometimes described only as

"exsolution bodies" or "ségrégations." Therefore, if the

textures were not described and named in the literature,

the author used a descriptive name in the identification.

The following list describes the exsolution textures

which occur in the Santa Barbara ores. Only the sphalerite-

chalcopyrite System was observed. Where dimensions of the

exsolution bodies are given, thèse dimensions are approxi­

mate values that do not hâve strict maximum and minimum

limitations.

Grains: The grains are small, irregular, spherical

bodies, frequently with convex boundaries. They range in

size from one to seven microns and are usually equidimen-

sional (Figs. 6, 7, and 13).

Blebs: The blebs range in size from seven to thirty

microns. They hâve convex and concave boundaries, but most

of the boundaries are convex. They are either equidimen-

sional, oval, or irregular in shape, with none of the shapes

dominant (Figs. 6 and 7).

Lenses: The lenses vary considerably in size but usu­

ally range from seven to fifty microns. They hâve convex

boundaries and are from two to four times longer than they

are wide. They resemble a cross section of an optical

convex lens.

26

Wedges: The wedges are very irregular in shape but

hâve a minimum average diameter of thirty microns. They

exhibit convex and concave boundaries (Fig. 7).

Emulsion: yAn emulsion texture is composed of grains,

blebs, lenses, and wedges which are randomly distributed

throughout the sphalerite (Fig. 6). An emulsion texture

refers to the exsolution bodies which show no orientation

within the host phase. In the case of exsolved chalcopyrite

from sphalerite, Edwards (1954) concluded that the chalcopy­

rite is distributed in the (100) and the (111) directions

of the sphalerite. When grains, blebs, lenses, and wedges

are oriented, the texture is calied "Oriented emulsion"

(Fig. 7) .

Drained texture: The area surrounding large wedges

and veins often exhibit a "drained" texture (Fig. 7) . The

grains and blebs that surround the wedges and veins get

continually smaller as the wedges are approached. The area

immediately adjacent to the wedges or veins may be com-

pletely void of exsolution bodies.

Lamellae: The lamellae hâve a variety of shapes and

sizes which are dépendent upon the exsolved phase and the

host phase. In the case of chalcopyrite lam.ellae in sphal­

erite, lamellae are rod shaped with the long axis twenty to

thirty times greater than the short axis. They range in

thickness from two to seven microns with the long axis

27

usually greater than fifty microns (Fig. 12). Some lamellae

hâve a long axis of one hundred-fifty microns; moreover, the

length of the lamellae does not appear to be proportional to

the width; i.e., the longer lamellae do not necessarily hâve

the greatest width. Apparently, the width is controlled by

the crystallographic structure of the sphalerite v/ith the

length being controlled to a smaller extent.

Stringers, veins, and rim texture: The stringers range

in width from five to fifteen microns with non-parallel,

irregular sides, and they are often very sinuous (Fig. 9).

They often do not exhibit orientation. A "rim" texture is

formed by stringers that connect to form irregular rings

(Fig. 10), and should not be confused with a "rimming" tex­

ture which commonly dénotes replacement. Veins are wider

than stringers, hâve non-parallel to parallel sides, and

are not as sinuous as stringers (Fig. 11).

Complex exsolution: The individual textures seldom

appear separately. In an area of one square millimeter, it

is more uncommon to find only an isolated texture than to

find a combination of the différent textures. In gênerai,

the greater the amount of exsolution, the greater the numtber

of différent textures (Figs. 9 and 11).

Some textures were observed which the author found

uncommon, and a search of the literature revealed that they

hâve not been described or named.

28

Swarm texture: A "swarm" texture is the accumulation

of grains into masses resembling swarms, The swarms havo

definite boundaries and are separated from other swarms by

areas void of exsolution bodies (Fig. 13).

Polyhedrons: Polyhedrons hâve from four to six sides

which are straight and show little irregularity. Of the

few samples that were observed, polyhedrons usually were

oriented by the sphalerite structure (Fig. 14).

Fig. 6.—Grains and blebs in emulsion texture

29

Fig. 7.--Grains, blebs, and wedges in oriented emulsion

Fig. 8.--Stringer formation

30

Fig. 9.--Stringers and rim formation

i

SRS"' If ,.is

Fig. 10.—Rim texture

31

Fig. 11.--Veins

Fig. 12.--Lamellae

32

Fig . 13.--Swarm t ex tu re

F ig . 14.—Polyhedrons

Table 4. Summary of Textures and Their Properties

33

Texture

Grains

Blebs

Lens

Wedges

Emulsion, oriented emulsion

Drained

Boundary Shape

Convex

Convex, concave

Convex

Convex, concave

Size (Micro

1-7

7-30

7-50

30

Remarks

Equidimensional

Irregular in sliape

Lens shaped

Irregular in shape

Composed of grains, blebs, lens, and wedges

Grains decrease in size as wedge or vein is approached

Lamellae

Stringers

Rim

Smooth and parallel

Non-parallel , irregular

Non-parallel, irregular

2-7 X 50-150

Sides-5-15

Sides-5-15

Rod shaped

Sinuous

Form irreg

Veins

Complex exsolution

Swarm

Non- Commonly parallel 15 to parallel

Not as sinuous as stringers

Any combination or mix­ture of textures

Uncommon; masses of grains with masses having dis­tinct boundaries

Polyhe-dron

Commonly straight

5-15 Uncommon; usually hâve 4-6 sides

34

Interprétation of Textures

Before textures were analyzed, several factors had to

be considered. Brett (19 64) recommended caution in treating

exsolution textures quantitatively due to the number of

variables involved in exsolution and their dependence upon

one another. Also, exsolution is not an entirelv stoD-

wise process; exsolution is governed by properties of the

host and exsolved phase. The orientation of the sample has

to be considered, as a cross section and longitudinal sec­

tion of lamellae give différent shapes. Schwartz (1931)

and Edwards (19 54) concluded that most exsolution textures

are also criteria used in replacement.

Grains are commonly found in the textures observed.

The author suggests that grains are the first phase of ex­

solution. In some samples few exsolution bodies are présent,

or the sample exhibits "limited" exsolution. Grains are

prédominant where limited exsolution is présent. Most of

the samples from the Cobriza Mine exhibit limited exsolution

with grains being the major exsolved phase. At high tempér­

atures the sphalerite structure is less rigid than at lower

températures; thus, rounded grains possibly reflect the

situation that no dominant factor governs the shape of the

exsolved body at high températures, but the sphalerite does

control the orientation of the grains to a small extent.

35

Blebs are very common in the textures observed. They

form from the coalescing of grains (Fig. 7) and possibly

represent the continuai exsolution of grains so that the

grains increase in size to form blebs. As grains become

larger the sphalerite lattice has to be deformed or replacée-

in order to accommodate the grov/ing chalcopyrite v/ith the

sphalerite influencing the shape of the blebs. (As blebs

and V'v'edges become larger, they increase in concavity . )

This may be one factor in the great variation in the shapec

of the blebs. The coalescing of grains and the orientation

of the sphalerite also give a great variation in the shape

of the blebs.

Coalescing of blebs and grains and increased exsolution,

or the growing of the blebs, produce the larger wedges. The

prédominance of concave sides possibly shov/s that the sphal­

erite is exerting a marked influence on the shape of the

wedges.

Lens development is typical of some minerais as an ex­

solution texture. Lenses are not abundant in the sphalerite

so that they may be the accidentai shape of blebs and v/edges.

Therefore, the author suggests that lens development is not

characteristic of chalcopyrite exsolving from sphalerite.

In past studies the emulsion texture has been attributed

to rapid cooling with the chalcopyrite being unable to dif­

fuse to grain boundaries. The problem may also be an

36

orientation problem because emulsion textures hâve been

found adjacent to oriented emulsion and complex emulsion

textures.

As exsolution increases, the grains and blebs coalesce

to form either sinuous or straight stringers (Figs. 9 and

14) . The straight stringers obviously are cont.rolled by

the crystallographic structure of the sphalerite, sinuous

stringers may show the coalescing of g.rains and blebs across

the crystallographic structure of continued exsolution (Fig.

8) . They also could be a longitudinal view of a stringer

controlled by the sphalerite structure or exsolution along

the sphalerite grain boundaries.

Edwards (19 54) concluded that rim textures reflect

exsolution along grain boundaries. The author suggests that

a rim texture may also form by the linking of stringers,

and the sphalerite enclosed by the rim may not necessarily

represent a distinct grain (Fig. 8).

Veins possibly form by continuous exsolution of string­

ers; therefore, it is possible that veins may form by ex­

solution as well as by replacement and fracturing with the

introduction of younger phases.

In many exsolution bodies lamellae are the initial form

of exsolution. This may not be true in the exsolution of

chalcopyrite from sphalerite. Lamellae are seldom présent

in the Cobriza Mine where limited exsolution was dominant.

37

Also, lamellae are not présent in other areas where exsolu­

tion is not extensive; i.e., grains are not cross sections

of lamellae because lamellae are never observed in limited

exsolution. Therefore, it is not simply an orientation

problem.

Lamellae are only observed where extensive exsolution

is présent. Some lamellae are présent where an emulsion or

extensive bleb and wedge development is observed. Extensive

lamellae occur frequently where stringers and rim textures

are présent. Therefore, the author suggests the lamellae

represent a late stage of exsolution where the structure of

the sphalerite is rigid and the lamellae are elongated by

the crystallographic structure of the sphalerite.

Grains are usually absent where lamellae are présent;

thus, the grains appear to hâve a lower limit of formation.

Lamellae apparently coalesce along their sides (Fig.

12) and not along their long axis. Lamellae exsolve only

in one direction and never intersect each other; thus, the

lamellae are oriented by the rigid structure of the sphaler­

ite, and stringer formation results from lamellae as well

as from grains and blebs.

In summary, the first stage of exsolution is the forma­

tion of small grains. As the grains coalesce and grow, the

sphalerite exerts more control on the chalcopyrite to give

irregularly shaped blebs. Continued exsolution gives the

38

development of stringers, rims, and veins. Grains forni

continuously until the sphalerite forces the exsolution t •

occur as lamellae. Final exsolution textures are the co­

alescing of lamellae to give highly irregular stringers

Stringers, veins, and rims form by tho coalescing of

lamellae and blebs. Blebs and wedges decrease as larne.ri.ae

start to form. Fig. 15 shov/s the proposed relative orcer of

exsolution. The length of the line is not intended to rep­

resent the actual length of the time of exsolution.

Grains

Blebs

Wedges

Stringers

Veins

Lamellae

Fig. 15.—Proposed order of exsolution

Polyhedrons possibly reflect the situation where sphal­

erite exerts a maximum influence on the exsolving chalcopy­

rite. The author cannot explain the formation of a swarm

texture. Sugaki (1955) discussed a similar texture in the

exsolution of chalcopyrite from bornite but did not explain

the texture. The texture was produced by heating the sample

at 320° C for one hour.

CHAPTER IV

RESULTS OF INTERPRETATION OF D7\TA

Description of the Ore Shoots

The ores of Santa Barbara are excellent examples of

fissure veins. Some minor amounts of disse-minated pyrite,

sphalerite, and chalcopyrite are found occasionally in the

calcareous shale, but the disseminated ore does not reach

ore grade. The shale is highly silicified an average of

one meter on both sides of the veins.

The high grades of ore are found in ore shoots which

range in thickness from one to fifteen meters, with an aver­

age of two to three meters. Along the strike of the vein,

most of the ore shoots are wide at the top and taper with

depth. Over half of the ore shoots in the area hâve a

vertical or near vertical rake. Scott (1958) constructed

thickness diagrams of the ore shoots and found two trends

of maximum thickness. One trend was parallel to the rake

(vertical), and the second was almost perpendicular to the

rake. He attributed the two trends to a séries of joints

and fractures which controlled the warps in the faults pro-

ducing wider areas for ore déposition. The wide areas in

the faults were also attributed to movement along warped

faults with wider areas developing on the steeper portions

39

40

of normal faults, the flatter portions of reverse faults,

and changes in tlie strike of a strike-slip fault.

The veins away from the ore shoots hâve an average

thickness of one meter. Often a pronounced decrease in ore

minerais and an increase in gangue is found, possibly sug­

gesting a différence in the depositional environment.

Banding of ore minerais and gangue is common with in­

dividual coatings traceable for long distances; one quartz

band was followed for over ten meters. Quartz often ex­

hibits a comb structure, although much of the ore is massive

with no évidence of banding. Breccia fragments are found in

the veins and become concentrated at the ends of the ore

shoots. The breccia fragments also are concentrated on the

footwall of the veins. Cockade structure is found through­

out the veins. Vugs are common and range in size from a

few millimeters in length to over sixty meters. The largest

known vug is located between the 800 and 1000 levels of the

Cobriza vein.

Vein outcrops commonly produce pronounced topographical

ridges, some of which can be traced for several kilometers.

The résistant veins are attributed to quartz in the veins

and silicification of the adjacent shales. A talus of box

work is concentrated around the vein outcrops.

41

Mineralogy

Hypogene Minerais

Sphalerite: Sphalerite is the most important and abun­

dant ore minerai in the Santa Barbara area. The sphalerite

varies in color from dark brown to black reflecting the high

iron content, with black sphalerite denoting the iron rich

variety, marmatite.

The sphalerite is commonly présent as large crystalline

masses. Some vugs contain a cube and tetrahedron crystal

form of sphalerite but the occurrence of sphalerite crystals

is rare.

Using crossed niçois in reflected light, some crystals

are translucent and vary in color from light brown, to red,

to dark brov/n. The light brov/n color dénotes a low iron

content. Some crystals change from opaque to translucent

with no distinct boundary, and some crystals are entirely

translucent. Thèse crystals are always concentrated along

the edges of isotropic masses or adjacent to intersecting

quartz veins. This change represents either a decrease in

température that limits the amount of iron that can be sub-

stituted into the lattice or a graduai decrease in the amount

of iron in the depositing solution. In some instances, the

brown sphalerite is concentrated in fractures in the opaque

sphalerite and is not associated with quartz or any other

minerai. The author suggests the possibility that in this

42

case the fractures are shrinkage fractures resulting from

cooling, and brown sphalerite represents a loss of iron

substitution caused by a decrease in température.

The translucent sphalerite may also be the resuit of

small amounts of chalcopyrite in solid solution with the

sphalerite. Buerger (193 4) heated a sample containinç;

chalcopyrite exsolved from sphalerite and found at increased

tempéraitures, the sphalerite, in areas away from the ex­

solved chalcopyrite, became opaque as unmixing of chalcopy­

rite progressed. Therefore, at lov/er températures the

amount of chalcopyrite in solid solution with sphalerite is

decreased and the sphalerite becomes translucent. However,

this also may be the resuit of the iron disseminated within

the sphalerite.

Galena and silver: Galena is the second most impor­

tant ore minerai at Santa Barbara. The galena is associated

with sphalerite and chalcopyrite. Most of the silver is

associated with the galena and high silver content coming

from a fine grained galena which is usually associated with

quartz. The author identified a few small blebs of tetra-

hedrite and tennantite associated with the galena. The

tennantite is located in the northern and central portions

of the area and the tetrahedrite is located in the central

and southern portions of the area. Argentite has been re­

ported in the area (Schmitt, 1928, and Percival and

43

Spilsbury, 1971) but was not observed by the author. How­

ever, the majority of the silver is believed to be carried

as an impurity in the galena.

Chalcopyrite: Chalcopyrite occurs in two forms : 1S

massive chalcopyrite anC- as exsolution from sphalerite.

Chalcopyrite exsolves extensively from the iron rich, opaque

sphalerite but chalcopyrite is not présent in the iron dé­

ficient, translucent sphalerite. This suggests that the

translucent sphalerite is formed at lower températures be­

cause low températures limit the amount of solid solution

between sphalerite and chalcopyrite. However, v/ith a low

iron content in the hydrothermal solution, excess iron is

not available for the form.ation of chalcopyrite. Therefore,

the absence of chalcopyrite from the translucent sphalerite

neither proves nor disproves that the translucent sphalerite

is a resuit of low iron content in the hydrothermal solu­

tions or a lowering of the température.

Pyrite: The pyrite in the Santa Barbara occurs in

four forms: vein pyrite, metasomatic pyrite, sedimentary

pyrite and pyrite crystallized from rhyolite.

The vein pyrite commonly occurs as euhedral and sub-

hedral cubic crystals with individual crystals seldom over

a few millimeters in length. In reflected light only a

few samples were completely isotropic. Most of the pyrite

exhibits weak to moderate anisotrophism with colors

44

alternating from a dark steel blue to a dark rusty brown.

Metasomatic pyrite is found in the shale close to the veins

and commonly exhibits subhedral crystals. The metasomatic

pyrite shows weak to moderate anisotrophism having the same

colors as the pyrite deposited in the veins.

Sam.ples of outcrops of shale contain sm all amounts of

pyrite. The grains are anhedral and conform to the bound­

aries of the minerais in the shale, thus, possibly denoting

a sedimenta.ry origin. The pyrite varies from weak to moder­

ate anisotrophism v/ith no distinct colors.

The pyrite contained in the rhyolite forms euhedral

cubic crystals and was isotropic.

Evaluation of the m.ode of occurrence, crystal form,

and anisotrophism of the pyrite shows it to hâve three dis­

tinct sources: 1) the pyrite associated with hydrothermal

solutions, 2) sedimentary pyrite, and 3) pyrite which crys­

tallized from a magma.

The différence in the anisotrophism of the pyrite

should be noted because pyrite commonly exhibits isotrophism,

Each source of pyrite has the same isotrophism or anisotro­

phism. In this case isotrophism is used as a criterion to

show the différence of origin. Polishing has been suggested

to account for the isotrophism and anisotrophism of pyrite.

Because the samples were polished by the same method, the

polishing technique employed by the author possibly did not

affect the isotrophism of pyrite.

4 5

Arsenopyrite: Arsenopyrite occurs as eunedrai. to 3e'. -

hedral rhombic crystals with cross-sections of Lh e rhoTj:.oid.=:

producing tabular forms. The arsenopyri i.e is clorely associ­

ated with pyrite and chalcopyrite.

Other hypogene minerais include magnetite, specular.l ce,

and marcasite, but thèse minerais are rare in the SanLa

Barbara area. The magnetite and specularite are in the

northern and central portions of the arefi and piaicasitc,

which is associated with pyrite, is in Vue cejvtrf.i ane south­

ern parts of the area. Fine grained native go le eppea." ": to

be concentrated in localized areas with no apparent zonation.

However, the oxidized zone was mined mainly for its gold

content (West, 19 49).

Supergene Minerais

A thin supergene zone is at an average elevetion of

2,0 60 meters. The supergene zone represents the original

water table before it was lowered by pumping of the lower

levels. The supergene zone was not investigated because it

lies in the abandoned portions of the mines. However, micro-

scopic examination revealed small amounts of supergene min­

erais in ail levels of the mines. The supergene minerais

include bornite, chalcocite, and covellite. The présence of

thèse minerais below the supergene zone is attributed to the

percolation of ground water through vugs and faults that

intersects the veins.

46

Bornite selectively replaces chalcopyrite along grain

boundaries, which produces a rimming texture, and along

fractures. The rinmiing bornite often contains shrinkage

fractures which are the resuit of a réduction in volume

from the bornite replacing the chalcopyrite. The bornite is

located in the intermediate and lower levels of the m.:ines

with no bornite occurring in the upper levels. However, the

supergene zone does hâve bornite replacing chalcopyrite

(Schmitt, 1928).

Chalcocite replaces chalcopyrite, galena, bornite, and

to a limited extent, sphalerite. Chalcocite selectively

replaces chalcopyrite in some samples, galena in some, and

bornite in others. In some samples chalcocite shows no dis­

crimination between chalcopyrite, galena, and bornite. With

extensive replacement chalcocite often replaces sphalerite.

The chalcocite is restricted to the intermediate and upper

levels of the mines.

The occurrences of chalcocite and bornite were plotted

on cross sections of the veins in an attempt to détermine if

a relaxation exists between the limitations which govern the

occurrence of the two minerais. Fig. 16 represents a cross

section of the Coyote-Los Hilos veins. The diagram shows

that the bornite and chalcocite boundaries do not parallel

each other and therefore, they do not hâve a common relation

governing the limits of the two minerais. Investigation of

47

the overlap of the two areas reveals that in the présence

of born.ite and ciialcopyrite, the chalcocite selectively

replaces the bornite.

Elévation 19 00 f

1800

+ 1000 00 -1000

1700

1600

1500

1400

• B L ï- ' >Û(B;W^- -iMJ-M

/T^s

y mSfsa,-JȉSt.-T-GH

\

/

•^jmtaamÊamd

•Bornite boundary Chalcocite boundary

Fig. 16.—Occurrence of bornite and chalcocite in the Coyote-Los Hilos veins

Covellite occurs at ail levels of the mines and does

not exhibit sélective replacement. It replaces bornite,

chalcocite, chalcopyrite, galena and to a limited extent,

sphalerite. In the présence of bornite or chalcocite,

covellite preferentially, but not selectively, replaces

bornite and chalcocite as opposed to galena and chalcopyrite

Covellite frequently replaces the bornite and chalcocite

along the grain boundaries of the minerai that the bornite

and chalcocite are replacing. Covellite replaces

48

chalcopyrite, galena, and sphalerite along grain boundaries

and fracture-^ and often replaces galena along cleavage

planes.

Oxidation Minerais

The zone of oxidation v/as examined only at the outcrops

because the oxidized zone lies in the abandoned upper levels,

and the old shafts and pits made by the Spaniards are unsafe.

Therefore, the mineralogy of the oxidation mdnerals may be

incom.plete.

The zone of oxidation varies in thickness throughout

the area and reaches a depth of one hundred meters at the

Hidalgo Mine. The outcrops were investigated at three loca­

tions. The outcrops of the veins are extensively oxidized

witli forms of limonite composing the majority of the oxidized

minerais. They vary from dark brown to varions shades of

red and yellow, and primarily occur as coatings on silicified

shale and on quartz. Small prospect holes reveal large ac­

cumulations of limonite at depth.

Minor malachite and small amounts of azurite could be

found only near prospect holes where the original outcrop

had been disturbed. Malachite occurs as thin coatings on

quartz grains and as concentrations in small pockets. Azur­

ite was found only by breaking large boulders. Small amounts

of smithsonite and anglesite were identified at one prospect

49

hole. The smithsonite was a thin coating on the residual

boxwork. One sample of anglesite, which was surrounding a

small sphère of galena five millimeters in diameter, v7as

found. This sample of anglesite was found after many at-

tempts of breaking large, non-porous quartz boulders.

Trace amounts of native copper v/ere found in the lower

levels of the Coyote vein where bornite, covellite, and

chalcocite extensively replaced the hypogene minerais.

Schmitt (1928) investigated the mineralogy of the oxida­

tion zone. The follov^ing are the minerais that Schmitt re­

ported and the author did not observe:

Cerussite: Cerrusite is the most abundant and impor­

tant oxidation ore m.i.neral. It occurs in tabular and .bi-

pyramidal forms.

Plumbojarosite: Plumbojarosite occurs as yellow

micaceous material with platy, hexagonal crystals.

Mimetite: Mimetite is found in small amounts of yellow

crusts or crystals. The crystals are often shaped like

minute doughnuts.

Pyromorphite: Pyromorphite occurs as yellow and brown

crusts and as small, hexagonal crystals.

Hisingerite and jarosite: Thèse minerais occur in

small amounts.

Hemimorphite: Hemimorphite occurs as radiating sheaf-

like groups of tabular and prismatic, white crystals.

SCI agi

50

Plattnerite: Plattnerite occurs as small, black, globu-

lar masses associated with hemimorphite.

Gaiigue Minerais

The gangue minerais were not extensively studied in

this investigation. The gangue minerais which were identi­

fied in this investigation are quartz, calcite, hedenbergite,

garnet, datolite, epidote, and orthoclase.

Quartz and calcite compose the bulk of the gangue min­

erais. The quartz is usually massive and commonly white,

gray, and colorless, with some having shades of light green.

Amethyst is found in localized areas. Vugs contain quartz

crystals which vary from white to colorless. Calcite is mil

white to colorless with some samples being violet and brown, ^ «n

and is commonly massive with vugs containing small crystals. •«! tiiil

Hedenbergite occurs as fiberous, radiating crystals *"

varying from light green to dark green. Thin sections were

made for examining the angles of extinctions and revealed

the minerai ranges from ferrosalite to hedenbergite.

Epidote occurs in small veins inside the shale and

along the shale and ore contacts. Datolite is found in the

Mina del Agua mine and occurs in small fractures in massive

quartz. Orthoclase appears to be widespread throughout the

area. Other gangue minerais présent are fluorite, which is

located in the northern part of the area, garnet (isotropic

I-

and biréfringent) , pyroxene (clino-enstatite) , zoesite, end

the questionable présence of herderite and vesuvianiée.

Table 5 is a list of the minerais found in the Santa

Barbara area.

Table 5. Minerais Occurring in the Santa Bsrba: Area and Their Relative AV'Undance

in Each Zone

Minerai

Sphalerite Galena Chalcopyrite Pyrite Arsenopyrite Native gold Argentite Magnetite Specularite Marcasite Tennantite Tetrahedrite

Bornite Chalcocite Covellite

Cerussite Limonite Malachite Azurite Plumbojarosite Mimetite Hemimorphite Smithsonite Anglesite Pyromorphite Hisingerite Plattnerite Jarosite Native Copper * A-abundant,

tionable

For.'-iula

Hypogene M i n e r a i s

Abun ' ance

C-

ZnS PbS CuFeS2 FeS2 FeAsS Au Ag2S Fe304 Fe203 FeS2 (Cu,Fe)i2-^S4Si3 (Cu,Fe)i2Sb4Si3

Supergene Minerais Cu5FeS4 CU2S CuS

Oxidation Minerais PbC03 FeO(OH)•nH20 CU2CO3(OH)2 CU3(003)2(OH)2 PbFe6(S04)4(OH)i2 Pb5Cl(As04)3 Zn4Si207(OH)2'H20 ZnC03 PbS04 Pb5(PO.,As04)3Cl Variable Pb02 KFe3(OH)6(^04)2 Cu

common, S-small, tr-trace,

A A A C C S s tr

tr tr tr

C C S

A A A A C C S S S S S S tr tr

?-ques-

" X „^

5

Table 5.—Continued

Quartz Calcite Hedenbergite Fluorite Pyroxene Orthoclase Epidote Garnet Datolite Zoisite Herderite Vesuvianite

Gangue Minerais Si02 CaC03 (Ca,Fe)Si03 CaF2 MgSi03 KAlSi308 Ca2Al30(Si04) (SinOn) (OH) A3B2(Si04)3 Ca2B2(Si04)2(0H) Variable CaBe(P04)F Caio(mg,Fe)2AI4(Si04)5

(Si207)2(0H)4

T.

A C c c

L •'

s S S S

?

Paragenes-ts

The depositional séquence v:as established by reflected

light microscopy and analysis of hand spécimens. The pol­

ished sections v/ere exam.ined for textural relatiorj.shi-s er.e

then the corresponding spécimen v/as examined for eollabora-

tion. The banding of some hand spécimens gave additiona]

évidence that the analysis of the textures that v/ere observed

in the polished sections v\7as correct.

Several criteria were used in establishing the order

of déposition. Relie structures, pseudomorphs, and guidée

pénétration are some of the criteria used for replacement.

Vein minerais which intersect pre-existing minerais are évi­

dence to show that the intersected minerais are older than

the intersecting minerais. The intersecting veins were very

distinguishable as they often showed distinct boundaries

53

between the younger minerai and the older, intersected rain­

erai. The vein sides were either parallel or non-para.tlel.

Gangue minerais comiTionly gave parallel vein sides. If the

gangue was accompanied by intermittent ore minerais v>::i;i

replaced the host, the vein sides at the site of rep.lacenient

were non-parallel. Veins consisting of only oj:e min€::uôl;

seldom had parallel sides.

Crystal forms were used in establishing the order o.C

déposition in several v/ays. Banded ores commonJy hâve, e -ye-

tals extending outward into the younger x^hase. Eutj.ed:; al

crystals which are completely enclosed by another mireica.l.

may possibly be older than the enclosing minerai. This c:ri-

terion was used with extrême caution because of the property

that certain minerais commonly form crystals in the material

in which they grow or replace. However, when the crye'ais

compose the majority of the ore, the minerai which fills the

voids was regarded as being younger than the minerai that

forms the crystals.

The exsolved phase in an exsolution texture is defi-

nitely younger than the host. Older, brecciated minerais

surrounded by younger, matrix minerais gives a reliable

séquence of déposition.

Fig. 17 shows tlie order of déposition which is common

throughout the area, where the length of the Une does not

represent the actual length of the time of déposition.

54

Fig. 17 represents an idéal order of déposition and many

exceptions to the séquence are common. The limits imposed

on the déposition of a minerai are definitely not rigid.

Pyrite

Arsenopyrite

Chalcopyrite

Sphalerite

Galena

Quartz

Fig. 17.--Proposed order of déposition

Pyrite and arsenopyrite show some overlap of déposition

and often exhibit mutual boundaries with pyrite replacing

the arsenopyrite. Arsenopyrite crystals are commonly ter-

minated by the pyrite and the arsenopyrite appears to re­

place the pyrite to a very limited extent. Brecciated

pyrite has been found to be enclosed by arsenopyrite. Mas­

sive arsenopyrite and pyrite are associated with a massive

quartz containing orthoclase.

Chalcopyrite and quartz often fill the area between the

crystals of pyrite and arsenopyrite. The chalcopyrite re­

places the pyrite to a greater degree than the arsenopyrite.

The boundaries of chalcopyrite and pyrite are frequently

irregular whereas the arsenopyrite commonly retains its

straight crystal boundaries.

55

Sphalerite shows successive déposition after tiie quartz

and chalcopyrite and seldom fills the voids of the arsenopy­

rite crystals. However, the sphalerite is found n.ore ccii:-

monly with pyrite and in banded samples, v/hile the pyrite is

commonly found adjacent to, and enclosed by, the spha.terite.

Thus, pyrite and sphalerite show overlapping déposition.

Sphalerite exhibits overlapping déposition with the

later chalcopyrite and galena. The overlap v/ith chalcopyrite

is definitely shown by exsolution and by massive chalcopyrite

to a smaller extent. Hand spécimens reveal that most of the

massive chalcopyrite was deposited after the sphaler.-i.te.

Microscopic examination of the contacts show irregular bound­

aries with large masses of chalcopyrite in the sphaJerite.

Chalcopyrite is also found in cross-cutting veins in the

sphalerite. The galena is commonly enclosed by the sphal­

erite and has irregular boundaries.

Galena replaces sphalerite in many instances, and

Fig. 18 shows the texture that is frequently présent between

galena and sphalerite. As the sphalerite is replaced, the

chalcopyrite blebs near the contact become smaller. At the

contact the chalcopyrite is recrystallized to form large

masses which hâve a rounded, convex portion extendrng into

the galena, and an irregular boundary paralleling the sphal­

erite contact. The recrystallized chalcopyrite bodies are

seldom found isolated in the galena. Therefore, the galena

56

replaces the chalcopyrite but at a slower rate than the :

placement of the sphalerite. Oelsner (1961) describo.î a

similar texture of galena replacing the sphalerite.

Fig. 18.--Galena replacing sphalerite

Quartz is found throughout the depositional séquence,

with an increase toward the end of the chalcopyrite and

galena déposition. Calcite is the last hydrothermal minerai

to be deposited. It intersects ail other minerais and is

found in vugs covering quartz.

Banding of the ore is very comraon and the séquence con­

tinually repeats itself. In one polished section, three

distinct periods of the sphalerite, chalcopyrite, and galena

séquence were identified.

mi

57

Numerous exceptions are found to the proposed order

of déposition: pyrite in some instances appears to be con-

tinuously deposited throughout the entire séquence. Some

banded areas show only successive déposition between sphal­

erite and galena with no overlap of déposition, and chal­

copyrite is présent only as an exsolution phase. Arsenopy­

rite has been found to overlap into the déposition of

sphalerite. Galena sometimes is deposited v/ith the first

chalcopyrite phase. The pyrite and arsenopyrite are some­

times absent and sphalerite is the first minerai deposited.

Hedenbergite is more common than previous investiga­

tions indicate. Microscopic identification reveals that it

contains small amounts of pyrite which hâve been replaced

by hématite. The hématite to pyrite ratio increases as

hedenbergite becomes altered. The hedenbergite is closely

associated with sphalerite and massive quartz. Banding com­

monly shows hedenbergite radiating from sphalerite, with

massive quartz deposited after the hedenbergite. Some chal­

copyrite is associated with the hedenbergite; therefore, the

hedenbergite was deposited after déposition of some of the

sphalerite.

Zonation

The spécimens were selectively collected so as to com­

prise a représentative sample of the immédiate area. Also,

-'6

polished sections from individual sa.iples were carefuiiy

chosen to include the complète jirineral aeseiiJ lage. .: ;. th _

microscopic study, percentages of the minerai identified

were recorded, and the percenta.ges were plotted on veJ.n

cross-sections in an attempt to establish zonaticrs of the

ore minerais. The results shov/ed the distributi.orj o.t per­

centages to be very erratic, and contouring of the i/ercent-

ages was not possible. However, definite areas of increa.: e

and decrease of the ore minerais, arsenopyrite, and pyrit.^

became apparent. Thèse areas of increase and decrease v.?e-re

plotted on three-dimensional, block diagranis with planes

representing the major fault Systems (Figs. 19, 20, 21, 22,

and 23) . The diagrams show approximate élévations ar.â

should not be used to show exact locations of increase ard

decrease in ore minerais.

Figs. 19 and 20 show the areas of decrease of sphaler­

ite and galena, respectively, and Fig. 21 shows the areas

of increase in chalcopyrite. The Coyote-Los Hilos veins

hâve an increase of sphalerite at depth and toward the north

and a slight increase toward the upper levels in the south.

Galena has a marked decrease with depth which slopes slightl:

southward. Chalcopyrite has a graduai increase to the north

and downward. Therefore, as the chalcopyrite increases

downward and to the north, the galena decreases.

59

COYOTE SECA——

-1000

1000 2000

3000

2100

2000

COYOTE-LOS HILOS

LA PAZ-MINA DEL AGUA

HIDALGO

Fig. 19-.--Areas of decrease in sphalerite

60

COYOTE SECA

COYOTE-LOS HILOS

LA PAZ-MINA DEL AGUA

HIDALGO

Fig . 20.- -Areas of decrease in galena

61

COYOTE SECA

COYOTE-LOS HILOS

LA PAZ-MINA DEL AGUA

HIDALGO

Fig. 21.--Areas of increase in chalcopyrite

62

COYOTE SECA

COYOTE-LOS HILOS

LA PAZ-MINA DEL AGUA

HIDALGO

Fig. 22.--Occurrence of arsenopyrite

63

COYOTE S E C A - -

1400 1350

COYOTE-LOS HILOS

LA PAZ-MINA DEL AGUA

HIDALGO

Fig. 23.—Occurrence of hedenbergite

The La Paz-Mina del Agua System has two areas of low

sphalerite, one in the northern lower levels and the other

in the southern upper levels. The location of the decreased

areas suggests a slight slope to the south. The galena in­

creases toward the intermediate and upper levels with a

gentle slope to the south. The chalcopyrite has a definite

increase at depth corresponding partially to the sphalerite

decrease and slightly below the low galena concentration.

The chalcopyrite concentration also slopes slightly to the

south.

Comparing the La Paz-Mina del Agua vein to the Coyote-

Los Hilos System .shows that the areas of increased galena

and chalcopyrite are higher in élévation in the La Paz-Mina

del Agua System. No definite conclusion can be deducted

from the sphalerite destribution; however, the decrease in

both vein Systems is sloping to the south.

The Hidalgo-Cobriza System shov>/s sphalerite and galena

increase in the lower and upper levels of the southern part

and an increase of galena in the upper levels of the north­

ern part. The chalcopyrite has an increase which corresponds

closely to the decrease in galena and sphalerite. A gentle

slope to the south is indicated.

Results from plots of pyrite and arsenopyrite were very

erratic but increases at depth and in the upper levels were

indicated. Figs. 22 and 23 are plots of the occurrence of

65

arsenopyrite and hedenbergite. No arsenopyrite is found in

the Cobriza Mine and only minor amounts in the northern La

Paz-Mina del Agua System. The arsenopyrite forms an arc

with the center being to the north of the area. Also, the

hedenbergite is not found in the northern part of the La

Paz-Mina del Agua System and is absent in the upper portions

of the veins.

Some zonation of the ore minerais at Santa Barbara ap­

pears to be présent. Sphalerite seems to increase at the

lower and upper levels of the mines with galena decreasing

in the lower levels. Chalcopyrite has a gênerai increase

which lies slightly below the sphalerite increase and cor­

responds closely v/ith the decrease in galena. Each vein

System differs slightly from the others in the occurrence

and overlapping of thèse zones. However, there is a definite

sloping of the ore body to the south and southeast.

Interprétation of Paragenesis and Zonation

The banded ores suggests that the hydrothermal solutions

were continually changing chemically and thermally. The

répétition of the sphalerite, chalcopyrite, and galena sé­

quence possibly shows new surges of hydrothermal solutions.

The great variety in the order of the depositional séquence

suggests that such local conditions as pressure, température,

chemistry of the fluid, and the times of vein reopening

varied considerably throughout the entire area.

The zonation of chalcopyrite, sphalerite, and galena

resemble a telescoping effect with the chalcopyrite concen-

trated at depth and the ore body sloping to the south and

souther.st. However, the increased sphalerite toward the

upper levels indicates the telescoping overlaps itself.

The high température silicates suggest a rise in tem­

pérature of the hydrothermal solutions resulting in the

sphalerite and galena being deposited at higher élévations.

This température change is reflected in the rise of sphal­

erite, arsenopyrite, pyrite, and some chalcopyrite in the

upper levels.

Jones (1934) demonstrated that even though an igneous

66

bor'^ 'f ay continually cools, the area around the intrusion has a

rise in température before cooling begins. Therefore, the

introduction of heat has a maximum value in the country rock

which does not correspond to the initial introduction of

heat. The présence of the rhyolite, basait, andésite, and

the rise in température of the hydrothermal solutions are

taken as évidence of an intrusion at depth which may contain

the ore ions or serve to concentrate the ions.

The initial déposition was interrupted with an increase

in température so that the high température silicates, which

are associated with abundant quartz and some chalcopyrite,

were deposited in the intermediate levels. Sphalerite and

galena were deposited at the higher élévations. Cooling v/as

67

slow enough so that the sphalerite and galena zones v,ere

gradually depressed to the lower élévations witi: no int.ir­

ruptions in déposition. The author does not su-eest tarée

separate stages, but overlapping of the telescoeed zor.e,

as a resuit of a rise and décline of teieperature i/ith con­

tinued déposition. The overlap of the différent rainer.-^

assemblages is partially responsible for the lack of auv

definite zonation.

Numerous exceptions to the proposed overlap ef depo. i-

tion theory can be found throughout the area. 'l'hese ex­

ceptions would be expected. Two previous ly s ta ted rea. een.s

for the exceptions are the continued surge of liydrotherjùa 1

fluids, and the rise in température to yivc; o e.i:lap of the

telescoped zone. A third explanation is the movement a.long

faults. Microscopic and field évidence shov/ that movei.vrnt

along the veins and intersecting faults was continuous

throughout the period of déposition. Therefore, the hyclro-

thermal fluids did not always hâve ready accessibility to

the veins which v/ould cause local interruptions in déposi­

tion of the ore.

Origin

Several criteria are available to help establish origin

and movement of the hydrothermal fluids.

The vague zonation of the ore minerais shows chalcopy­

rite at depth and increased sphalerite upward, with galena

68

becoming abundant in the intermediate levels. Pyrite and

arsenopyrite are also found at depth. Tne zonation patt^rn

resembles a warped plane which slopes to the so..L aad

southeast. The zonation and the south s.lope .-iiggest that

the hydrothermal fluids came from the nc^ th or the rortev:eet

at depth.

Another line of évidence suggesting an origi-i for ti,

hydrotliermal solutions is the outline of tl e oc., .-rence of

arsenopyrite and hedenbergite. ïhe arc .ahape of their dis­

tribution allows either a heat source to the north or a

heat source to the south. A heat source to the north is

supported by several observations. Arsenopyrite is found

in the northern part of the area, v. hereas, :io a.rsc noy yrite

is observed in the Hidalgo mine. Also, the limited exsoiu­

tion of the chalcopyrite in the Hidalgo raine could possibly

indicate low températures and relatively fast cooling. The

sphalerite in the Hidalgo mine is commonly tran^vlucent,

which suggests limited iron or chalcopyrite content, because,

low températures allow small amounts of substitution into

the sphalerite lattice (Kullerud, 1953).

The hedenbergite occurrence is arcuate in shape and re-

flects the same source as the arsenopyrite.

Other minerai assemblages suggest a heat source from a

northernly direction. Specularite is observed only in the

northern part of the area, and marcasite is found only in

69

the soutl.ern part of the area. Tetrahedrite is found in the

south and tennantite in the north.

The zonation of the ore and the mineralogy suggests

that the ore fluids came from depth and slightly to the

north.

The intersections of the fault system.s were plotted

on a Schmidt equal-area net. The intersections of veins

would give paths by which the hydrothermal solutions could

more easily migrate. However, the intersecting Unes were

randomly oriented and therefore, did not confirm or support

the proposed source location.

If the hydrothermal solutions are from depth and

slightly north, the alpha, beta, delta, and possibly gamma

fractures would allow the hydrothermal solutions to migrate

to the south. The présence of the anticlinorium would also

influence the migration of the hydrothermal fluids.

Température of Fomiation

The température of formation was not an original part

of this investigation but some definite conclusions can be

made from the minerai assemblages présent. Table 6 lists

the maximum températures of several minerai assemblages.

The arsenopyrite-pyrite assemblage has a maximum sta-

bility température of 491 °C at low pressures. At elevated

70

températures pyrrohtite and liquid sulfide form. The ab­

sence of pyrrhotite suggests the maximum température would

be slightly over 500 °C, at low to moderate pressures.

Pyrite and marcasite can exist at températures below 430 °C

at low pressures. Pressure increases further lower the sta-

bil i ty température of the pyrite-marcasite invariant point.

The présence of some marcasite indicates the température may

hâve been slightly less than 430 °C at low pressures. Garnet

becomes biréfringent at températures greater than 860 °C.

The présence of biréfringent garnets shows that the tempér­

ature of introduction of the silicates was greater than

860 °C.

Minerais

Table 6. Maximum Températures of Minerai Assemblages

Maximum Temp-

Arsenopyrite 491 + 12 °C and pyrite

Marcasite 430 °C and pyrite

Garnet 860 °C

Date

Invariant point

Invariant point

Isotro­phism

Source

Barton and Skinner (1967)

Kullerud (in press)

Allen and Fahey (1957)

If the maximum thickness of the basait was 915 meters,

the pressure was possibly low to moderate during déposition

of the ores as the rock strength would maintain free space,

and pressure in the veins would be less than the weight of

the rock column.

71

Assuming the values in Table 6 are correct, the environ­

ment Of déposition was one of low to moderate pressures with

a maximum température slightly over 500 °C for ore déposition

and a rise in température of approximately 860 "C for déposi­

tion of the hedenbergite and biréfringent garnet (Allen and

Fahey, 1957). Table 7 lists the classifications in which the

ores of Santa Barbara hâve been classified, and the character-

istics of each deposit.

1

Classification

Pyrometaso-matic

Hypotherm.al

Leptothermal

Xenothermal

•aDie /. c.

Pressure

Skarns

High to very high

Moderate

Low to moderate

Lassification of (

Temp. Depth "C (Feet)

300- 10,000 D U U D U , \JK)KJ

125- 3,000 250 10,000

300- 1,000 500 4,000

Dres

Source

Allen and Fahey (1957)

Scott (1958)

Valverde (1968)

Koch (1956)

The deposits at Santa Barbara were deposited at tempér­

atures and pressures that correspond to the upper boundary

of the xenothermal classification. Xenothermal deposits are

associated with volcanic rocks of comparatively récent âge;

telescoping is commonly présent, and fissure veins are the

major ore bodies. Santa Barbara has trace amounts of magne­

tite and specularite which are found in xenothermal deposits.

However, it does lack other high température minerais

commonly found in xenothermal deposits such as cassiterite'

wolframite and molybdenite. Despite this discrepancy, the

ores should be classified as xenothermal.

BHlˣy_. lJ; .d Conclusions

Microscopic examination of the ores at Santa Barbara

shows a preferred depositional sequenee of arsenopyrite and

pyrite, chalcopyrite, sphalerite, with galena and chalcopy­

rite deposited last. Quartz was deposited throughout the

séquence which varies considerably throughout the area.

Banding is common, with répétitions of the séquence possibly

reflecting the spasmodic introduction of the hydrothermal

fluids. The spasmodic injections constantly changed the

chemistry, température, distribution, and pressure of the

hydrothermal fluid so that the depositional séquence con­

tinually varied.

The telescoped ores commonly hâve chalcopyrite at depth,

with sphalerite concentrated slightly above areas of high

chalcopyrite content. Sphalerite increases to a small ex­

tent in the upper levels of the mines. Galena increases in

the intermediate levels and pyrite and arsenopyrite are con­

centrated at depth and in the upper levels of the mines.

Pyrite exhibits less of a pattern than the arsenopyrite.

The ore body slopes gently to the south and southwest.

Arsenopyrite and hedenbergite hâve an arcuate occurrence,

with lim.ited arsenopyrite found in the northern part of the

73

La Paz-Segovedad mines. Trace amounts of the high tenvoera-

ture minerais specularite and magnetite are concentrated

in the northern part of the area. Therefore, évidence sup­

ports the theory that the hydrothermal solutions caïae fror;

depth and slightly north or northeast of Santa Barbara, wiih

the fluids guided by faults and possibly by an anticlinal

structure.

A rise in température, which possibly represents a heat

lag from an intrusion located at depth, introduced the high

température silicates consisting mainly of hedenbergite,

pyroxene, and som.e garnet. The déposition of the oie miner­

ais was concentrated at slightly higher élévations but v/itli

limited déposition of the ore and continued déposition of

massive quartz.

As the area cooled, the zone of déposition of the ore-

was lowered. The rise and décline of température gave over­

lapping of the telescoped déposition which concealed any

definite zonation pattern. Also, movement along faults re­

stricted the hydrothermal fluids in certain areas, so that

thèse two factors, combined with spasmodic introduction of

hydrothermal fluids, give many exceptions to the depositional

séquence and the proposed telescoping of the ores.

The deposit is classified as xenothermal but is on the

upper température and pressure limits of the xenothermal

classification.

7 4

Examination of the exsoiution textures shcvas that

chalcopyrite exsolves from sphalerite producing grains,

blebs, veins, and laiTiellae. Small grain.3 ferr.i in the initial

stage of exsolution with later blebs coalescing to form veinai

Lamellae are believed to form in the more advance.l stagt.s of

exsolution.

Veins commonly hâve been used as criteria for replace­

ment, but they also form from exsolution. Therefore, cau­

tion should be exercised in the interprétation of texturea

so that a false, paragenetic séquence is not produced.

——•••~^««'«*fWHm

BIBLIOGRAPHY

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Barton, P. B. and Skinner, G. J., 1967, Sulfide Minerai Stabilities, Table 1 in Ceochemistry of Hydrothe.-mal Ore Deposits, Barnes, H. L. éd., Holt, Rinehart, and" Winston, Inc., New York, p. 271.

Bastin, S., 1950, Interprétation of Ore Textures: The Geological Society of America Memoir 45, TÔ'l p.

Bose, E., 1906, Excursions dans les Environs de Monterrey et Saltillo: International Geological Congress, lOth, Mexica, Guide Excursion 29.

Brett, R. , 1964, Expérimental Data from the Cu-J'e-S and their Bearing on Exsoiution Textures in Ores: Economie Geology, v. 59, pp. 1241-1269.

Buerger, N. W. , 1934, The Unmixing of Chalcopyrite from Sphalerite: The American Mineralogist, v. 19, pp. 525-530.

Burckhardt, W. H., 1930, Etude sur le Mesozoique Mexicain: Soc. Paleont. Suisse Mens., 49-50, p. 280.

Clendenin, T. P., 1971, interview at El Paso, Texas, March 27

Edwards, A. B., 1954, Textures of Ore Minerais, Australasian Institute of Mining and Metallurgy, Inc., Melbourne, 242 p.

Guzman, E. J. and DeCserna, Z., 1963, Tectonic History of Mexico, in Backbone of the Americas: Tectonic History from Pôle to Pôle, American Association of Petroleum Geologist Memoir 2, Childs, 0. E. and Beebe, B. W. eds., pp. 113-129.

Jones, R. H. B., 1934, Température Relations to Ore Déposi­tion: Economie Geology, v. 29, pp. /n z -

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76

Koch, G. S., 1956, The Frisco Mine, Chihuahua, Mexico-Econom.ic Geology, v. 51, pp. 1-40. Mexico.

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