San Juan Volcanic Field - Mapping Study

Nested Caldera Study: San Juan and Silverton Caldera Complex Analysis

June 2015 – Volcanism Focus

Johanna Vaughan

The focus of this study encompasses the San Juan Volcanic Field (SJVF) analyzing the

volcanology, hydrothermal geology, and ore deposit geology of the San Juan Mountains. These

mountains are composed of eroded mid-Cenozoic intermediate-felsic deposits associated with the SJVF.

This area represents a structurally complex and intensely altered region of several remnant volcanic

eruptive centers with unique geographical distribution. The SJVF is associated with Tertiary volcanism of

the Cenozoic Era (spanning the course of approximately ~5 Mya.), unique from other types of worldwide

arc volcanism as large scale subduction occurs far west of the SJVF. The mechanisms and modes of

deposition in association with volcanic activity encompass the deposition of several volcanic and

subvolcanic intrusive rocks, involving a range of unique deposits within a concentrated area. This study

is focused on the nested caldera complex of the Silverton caldera forming within the older San Juan

Caldera near Silverton, Colorado. Filed mapping has allowed for the production of a geologic map

(spanning 2,630 meters in length) displaying the various deposited, as well as the location of major fault

clusters. The geologic map is rich in orientation measurements, incorporated into quantitative stereonet

analysis in support of interpretation. Qualitative data, such as field observances from outcrop stops over

the course of this study support the cross sectional figure showing the projection of various units to

depth, giving insight into spatial distribution.

This study focuses on the unique ignimbrite flare up during the Tertiary, producing high volumes

of intermediate and felsic volcanic rocks (Figure 1 A and B.). The total volume of erupted material has

been approximated at 40,000 km^3, however erosional effects over geologic time have shaped the

landscape through mechanisms such as mass wasting. The tectonic control of the deep mantle source is

still a topic of controversy, but the deposition of unique and closely spaced units allows for

interpretation of recent explosively related volcanic activity. The Silverton caldera is interpreted as

experiencing a major collapse episode with eruption occurring during periods of caldera filling. This

sequence has resulted in “trap-door” regional subsidence to the southeast section of the caldera. The

caldera spans a vast region, and the study area is focused around the western portion of the complex

where significant faulting and various deposits occur. The larger scale “trap- door” subsidence is not

encompassed within the study area, and as such mechanics of faulting as described as “piecemeal” due

to interpretation based on observations.

This study area is important as there is the occurrence of intensive mineralization and

hydrothermal alteration (Figure 2.) occurring as a separate and later phase of deformation post-

volcanism. Mineralization throughout the area involves the recurrent intrusion and extrusion of magma

along areas of weakness such as ring fracture zones and accommodational faulting zones. The high

magnitude of normally faulted planes with down drop towards the southeast (Figure 3.) is associated

with the geometry of the larger scale Silverton caldera complex. Subsidence and eruption of this

complex has produced a heavily faulted area related to the western rim of the Silverton caldera. The

caldera structure has provided fractured and faulted areas that have guided the emplacement of

igneous intrusions following the deposition of volcanic rock units (Figure 1 A and B.). This progression

has produced the rich concentrations of ore deposits (Silver, Lead, Zinc, Copper, and Gold) located

within fractures of the heavily fractured zones located in association with the main rim faults (Figure 3.).

Here we examine the intricate pattern of fracture related structures, the wide distribution of volcanic

deposits, and the post volcanism alteration associated with the nested caldera complex surrounding

Silverton, Colorado.

The identification of deposit units, and significant structures within the study areas has been

performed through qualitative and quantitative analysis encompassing field mapping done through

group effort. There are cumulatively six identified units deposited within the study area (Table 1.) which

have been analyzed to interpret the characteristics of the Silverton caldera volcanic activity of the

Tertiary. From youngest to oldest there is the occurrence of; Quaternary Deposits, Porphyritic

Rhyodacite Intrusions, Welded Vitric Lapilli Tuff, Weakly Welded Propylized Tuff, Heavily Altered

Andesitic Tuff, and Andesitic Lava Flow. The characteristics of each of these deposits has been

incorporated into Table 1., and the distribution of units can be seen clearly through the geologic map

construction (Figure 1. A, B.) All structural measurements giving insight into the tectonic history

associated with volcanism have been incorporated into stereonents (Figure 4. A, B, C, D, E, F, G), and

give insight into bedding and fault orientation. The alteration and mineralization observed throughout

the study area (Figure 2.) gives support for ore deposit occurrences.

The interpretation of this study places geologic sequence in the following order; large scale trap-

door subsidence and collapse of regional nested caldera complex associated with the deposit of several

volcanic units (Welded Vitric Lapilli Tuff, Softly Welded Propylized Tuff, Pervasively Altered Andesitic

Tuff, and Andesitic Lava Flows). Sub volcanic intrusive rocks (Porphyritic Rhyodacite Intrusions) are seen

to encompass two separate stages within the study area, with the older intrusion occurring

superimposed above the younger (which displays abundant and larger phenocryst content). There is the

unique occurrence of sequential faulting units displaying an overall oblique sense of movement

incorporating both a normal component with down drop to the east, and a strike slip motion in

association with accommodational fault mechanics. The most intensely faulted region of the map

(Figure 3. – where there is the occurrence of two fault groups within close proximity forming an oblique

angle between dips) is interpreted as being associated with rim faulting mechanics. The blue set of faults

(Figure 1 B. and Figure 3.) are interpreted as accommodation in association with regional collapse

mechanics. The faulting throughout the region has helped to facilitate the intrusive subvolcanic deposits

(Porphyritic Rhyodacite Intrusions) which occur after the explosive eruption cycle associated with the

nested caldera collapse. These intrusions are associated with uplift mechanics following subsidence as

compensation of isostatic balance within the area. The intrusions and associated volatile components

have helped to form the rich mineralization ore deposit regions throughout the region, now exposed

and once widely mined for ore content. Hydrothermal alteration occurs later in geologic time, effectively

altering the volcanic and subvolcanic intrusions mainly through fault and fracture pathways. The most

intense alteration of units is interpreted to occur in association with Tuff deposits, with the oldest

(Green heavily altered Andesitic Tuff – Figure 3.) unit experiencing pervasive alteration in association

with weak welding and high porosity of character of the matrix.

Stereonet interpretation fits well with the larger scale tectonic picture (Figure 3.), as the major

fault clusters (Figure 4 C.) are seen to trend generally north/northeast (Figure 4 D.). This interpretation

fits well with the geographic position of the study area, on the western edge of the Silverton caldera rim.

There are three main fault clusters (Figure 4 C.) roughly identifiable through stereonet analysis, and all

of the faults measured correlate well with north-northeast trending strikes. (Figure 4 D.) Although

faulting is very prevalent throughout the study area (Figure 4 B.), there is no evidence for significant

folding, as the poles to bedding planes show a circular trend concentrated towards the center of the

stereonet projection. This occurrence supports the lack of folding within the area, and works well with

the horizontal deposition of layers, remaining largely intact (following the counters of topography)

despite immense erosional mass wasting. The poles to joints (Figure 4 F.) display a wide array of

orientations and support the observance of intense fracturing and stress related deformation along

major fault zones. Slickenline measurements are sparse within the area (Figure 4 G.), but support the

regional oblique faulting trends observed. Strike slip motion is supported through Slickenline stereonet

analysis, and the occurrence of normally faulted areas in the western region of the map (Figure 1 A.)

together support the oblique sense of displacement the study area has experienced.

Interpretation of units support the regional “trap-door” subsidence model, as intrusive outcrops

are exposed within the northwest section of the map (Figure 1 A. and Figure 3.) where erosional effects

have exposed the intrusive units that were emplaced higher when the southeast section of the Silverton

caldera collapsed. However, the study area shows support of piecemeal subsidence mechanics in

conjunction with the repetitive sequences of normal faulting observed through the three faulting units

(Red, Yellow, and Blue Fault lines – Figure 1. A). As a result of faulting and down drop towards the

southeast, there is the observed repetition of units across cross sectional view (Figure 3.), with Andesitic

Lava Flow units representing the thickest sections (~15 meters). There is an observed increase in the

thickness of units as we move further east, towards the central Silverton caldera.

The conclusion of this study shows the unit distribution and structural mechanics associated

with a typical nested caldera complex, with the occurrence of deposition of differing units in association

with subsidence and uplift during Tertiary volcanism of the SJVF. The volcanic collapse mechanism is

piecemeal in nature, characterized by an oblique sense of offset created through rim faulting

manifestation. Snow cover during the course of field work prevented the first person observation of

potentially helpful outcrops. The occurrence of quaternary deposits and displacement of large boulders

related to glaciation of the area are the manifestation of erosional effect in present geologic time. The

volcanic character of the study area is concluded as explosive and significant (accounting for ~40,000

km^2 of deposited materials) in deposition volume. Quantitative analysis through stereonet use

correlates well with qualitative in field data and quantitative orientation measurements. The

mechanisms fueling volcanic activity at depth are uncertain, as well as the depth to which certain units

are orientated. Further study of the area could give insight into the nature of explosive eruptive volcanic

complexes, and the larger scale SJVF occurrence.

*Figures one through three consist of handmade maps and figures and have not been included in

this online submission

Description of Study Area Units

Quaternary Deposits

Description

There are various types of quaternary deposits located throughout the study area including:

Fluvial Sediments Deposits are well sorted with normal grading and rounded cklasts (sand-boulder sized). These deposits are associated with transport and deposition through water related mechanics

Colluvium This deposit type is observed as poorly sorted in nature, consisting of unconsolidated sub-angular to angular material mostly composed of andesitic lava flow fragments.

Talus Slope Deposits

Deposits are large and widespread in distribution in association with structural history and erosional mechanics. Deposits are angular in character.

Man-Made Human developed infrastructure and associated mining activity scattered throughout the study area.

Landslide Material There is the occurrence of large clasts suspended within finer grained matrix in association with high energy eruption mechanics.

Porphyritic RhyoDacite Intrusion

Description

Fresh Rich grey/greenish grey

Weathered Light pink to grey weathered surface

Appearance There are two observed closely situated intrusions of differing ages.

Magmatic Event A Densely distributed phenocrysts displaying euhedral grains and litics (1 mm to 4 mm). There is the prominent occurrence of plagioclase and k-feldspar (~0.35-2 cm)

Magmatic Event B There is the occurrence of abundant, and very large (0.5-2.5 cm) plagioclase phenocrysts content (~20-30% of the unit composition). There is the occurrence of alteration from plagioclase to epidote appearing green in alteration zones.

Quartz content is vitreous (~0.5 cm) and unfragmented in nature. Pyrite grains are seen, but are small in size (~0.5-1 mm).

Welded Vitric Lapilli Tuff

Description

Fresh Light grey/Greenish grey

Weathered Rusty brown/grey

Appearance There is the occurrence of vitreous quart, fragmental in nature, and abundant throughout the matrix.

Unlike softer tuffs of the study area, this deposit is moderately to well welded and majorly clast dominated.

There is the occurrence of grey bombs (~30 cm in diameter with polymict nature.

There is a minimal percentage of mafic component contained within phenoclast content.

Softly Welded Propylized Tuff

Description

Fresh Light green

Weathered Dark grey

Appearance This unit is weakly welded making way for pervasive hydrothermal alteration through the ash dominated matrix.

Flow banding is observed in this unit (8-10 cm in thickness). This interpretation observes the indication of deposition as pyroclastic flow.

Calcite content is observable as phenocrysts ranging in size from 0.5 to 1 cm in diameter. This is associated with hydrothermal activity in relation to Propylitic and Phyllic (quartz - sericite - pyrite) alteration.

Phenocrysts are fragmental in habitat indicative of explosive eruption style.

There is the observance of heavy faulting and jointing with the association of an important fault zone present associated with heavy Phyllic alteration.

Pervasively Altered Andesitic Tuff

Description

Fresh Light green/yellow-tan

Weathered Dark grey - brown

Appearance This united is in association with prominent pyrite content (~0.25 mm - 1 cm) seen to weather slightly through local alteration zones within the unit. There seems to be the presence of chlorite and epidote content within the matrix having a light green-blue color.

Alteration within this unit is seen to occur abundantly in association with fractures and joints associated with insitu units.

There is the occurrence of sericite, produced from the weathering of plagioclase.

Andesitic Lava Flow

Description

Fresh Dark and blue/grey in color

Weathered Dark brown

Appearance Massive and resistant to erosional effects often associated with topographic highs

There is the presence of dense phenocryst content. Phenocryst content is observed as quartz, k-feldspar, and plagioclase rich with euhedral habit

Locally non-pervasive alteration zones occur in association with weaknesses such as columnar jointing and fracturing as a result of regional faulting mechanics and deformation

Upper 5 m of unit outcrop displays phyric texture

Devitrification in association with quartz and chert is seen through lithophyse content

Fracture zones within the rock are consistent with alteration zones (Figure 3.). The distribution of heavy alteration ranges from 20 cm - 2 m

Mafic component of lava flow is rich in pyroxene mineral content; appearing dark and vitreous with a tabular habit

Significant occurrence of quartz content

Table 1. Volcanic and associated subvolcanic intrusive unit descriptions. Each of the units have been observed throughout the nested caldera region of the San Juan and Silverton caldera complexes.

Figure 1.

A. Finalized field map interpretation of the study area with structural and geology related

significances indicated through color and symbolism use. The key for symbolism and color

used has been incorporated into the group study submission.

B. Field map as drawn and formed through field work surrounding the nested caldera study

region. This figure represents the earliest form of the finalized geologic map (Figure 1 A.),

and has also been incorporated into the group study submission.

Figure 2.

Hydrothermal alteration zoning displayed through the use of vellum paper to clearly see the

zones of alteration in association with the geologic map. This figure has been incorporated into

the group study submission.

Figure 3.

Geologic cross section from A-A’ spanning the length of the study area with key incorporation for

interpretation of color and symbolism. The present day erosional surface constitutes the

topographic profile and no vertical or horizontal exaggeration has been used. This figure has

been incorporated into the group study submission.

Figure 4.

All of the following stereonet figures have been derived from the correlation of structural

measurements taken in the field. These stereonents all display the North trending azimuth

(000,360) at the top portion of the net. Through quantitative analysis these measurements are

used to support the geologic distribution observed over the course of this study. The number of

data points used for each stereonet analysis is displayed:

(Stereonet data appears on the following pages)

A.

Figure 4 A. Poles plotted to bedding measurements taken over the course of this filed study from several location spanning the map region.

B.

Figure 4 B. Fault bedding planes plotted from various outcrop locations from throughout the study area.

C.

Figure 4 C. Poles plotted to fault bedding planes from Figure 4 B. The distribution of poles is seen to form loose clusters. There are three distinct clusters.

D.

Figure 4 D. Fault azimuth orientations plotted through the use of stereonet techniques. There is a clear trend of major faults within the N-E quadrant. There is an encompassing trend seen with the direction of faulting relative to the mapping area.

Figure 4 E. Bedding planes from joint measurements measured throughout the course of this field study.

Figure 4 F. Poles to joints plotted through the use of stereonet analysis.

Figure 4 G. Slickenline poles plotted through the use of stereonet analysis.