University of Nevada Reno the Geology and Mineralization of The

Home , Gabbs Formation, Sunrise Formation

University of Nevada

Reno

The Geology and Mineralization of the

Challc Mountain and Westgate Mining Districts

Churchill County, Nevada

.A thesis submitted in partial fulfillment of the

requirements for the degree of Master of Science

in Geology

Dennis Paul Bryan

December 19/2 The thesis of Dennis Paul Bryan is approved:

0- C~. lltL/ Department chairman

-/ Dean, Graduate School

University of Nevada Reno

December 1972 ACKNOWLEDGEMENTS

The writer is grateful for the assistance of many individuals during the course of this study. Dr. Anthony L. Payne originally suggested Chalk Mountain for possible investigation. Dr. Arthur Baker

III, the major thesis advisor, suggested combining both Chalk Mountain and Westgate as a topic and subsequently offered encouragement and assistance throughout the scope of the project. Drs, E. R. Larson,

Anthony L. Payne and D. B. Slemmons provided helpful suggestions.

Messrs. E. J. Murphy and M. P. Allen of the Nevada Bureau of Mines and

Geology provided assistance with the microscopic and analytical aspects of the study. Special thanks go to James Green and Robert Barnes for permission and assistance in mapping on the Chalk Mountain property.

G. E. Schrock assisted with the underground mapping, Wayne Kemp critically read the report and provided helpful comments. ii

ABSTRACT

The Chalk Mountain and Westgate districts were mined for lead and silver during the early part of this century with the Chalk Mountain

Mine the major producer from the area.

Sedimentary rocks at Chalk Mountain consist entirely of dolomite of Mesozoic age and those at Westgate consist of dolomite, limestones, shales, and calcareous sandstones, also of Mesozoic age. A Late

Paleozoic, to Early Mesozoic volcanic unit occurs as the upper plate of a thrust fault in both districts. Igneous rocks consisting of quartz porphyry, quartz monzonite, and granite porphyry were intruded during the Mesozoic while an extensive biorite dike is Cenozoic in age.

S k a m zones developed in the dolomite adjacent to the quartz monzonite and quartz porphyry intrusives at Chalk Mountain.

Thrusting and deformation of the Mesozoic sediments probably occurred during the Cretaceous. Predominantly normal faulting of

Tertiary to Quaternary age produced the physiographic features of the districts and continues to the present day.

The ore deposits of the districts are oxidized to known depths of 500 feet ana consist mostly of secondary lead minerals, limonites, and occasional primary sulfides. The ore deposits xn some cases are related to s k a m formation and appear both spacialiy and genetically associated with the Mesozoic intrusives. iii

CONTENTS

Page

ACKNOWLEDGEMENTS ...... i

ABSTRACT...... ii

ILLUSTRATIONS...... v

INTRODUCTION ...... 1

Location...... 1

Scope of the Study...... 1

Previous W o r k ...... 3

GENERAL GEOLOGY, ...... 4

STRATIGRAPHY...... 4

Late Paleozoic to Early Mesozoic Volcanics ...... 4

Mesozoic Sediments ...... 7

Gabbs Formation ...... 8

Sunrise Formation ...... 9

Westgate Formation...... 10

INTRUSIVE IGNEOUS ROCKS ...... 12

Granite Porphyry...... 12

Quartz Porphyry Dikes...... 13

Syenite Porphyry S i l l s ...... 14

Porphyritic Quartz Monzonite ...... 15

Quartz Porphyry...... 18

Diorite Porphyry ...... 20

STRUCTURE...... 24

Pre-Tertiary Structures...... 24

Chalk Mountain...... 24 iv

Page

Westgate...... 27

Thrust Faulting...... 28

Tertiary and Quartemary Structures...... 29

METAMORPHISM...... 34

Dynamic and Regional Metarcorphism ...... 34

Contact Metamorphism...... 35

Chalk. Mountain...... 36

S k a m s ...... 38

Westgate ...... 42

GEOLOGIC HISTORY...... 44

ORE DEPOSITS...... 46

History of the Districts ...... 46

Chalk Mountain ...... 47

Chalk Mountain Mine ...... 51

Westgate ...... 56

Oxidation of the Ore Deposits...... 59

Ore Associated with Skarns...... 64

DISCUSSION OF THE ORE DEPOSITS...... 69

APPENDIX...... '...... 73

REFERENCES CITED...... 76 V

ILLUSTRATIONS

Plate Page

1 Geologic Map of the Chalk Mountain and

Westgate Mining Districts ...... In pocket

2 Geologic Map of the Chalk Mountain M i n e ...... In pocket

Figure

1. Location map cf the Chalk Mountain and Westgate

Mining Districts...... 2

2. Photograph of Chalk Mountain...... 17

3. Photomicrograph of quartz monzonite ...... 17

4. Photomicrograph of quartz porphyry...... 22

5. Photograph of diorite d i k e ...... 22

6. Photograph of the Westgate district...... 25

7. Photograph of south end of Chalk Mountain...... 25

8. Photograph of thrust fault at Westgate...... 30

9. Photograph of scarp on Chalk Mountain ...... 30

10. Photograph of silica associated with faulting ...... 32

11. Photograph of 1954 fault scarp...... 32

12. Photomicrograph of phiogopite and chlorite...... 37

13. Photomicrograph of serpentine ...... 40

14. Photomicrograph of olivine pseudomcrphs ...... 40

15. Photomicrograph of endoscam...... 43

16. Photomicrograph of chiastolite...... 43

17. Photograph of oxide vein at Chalk Mountain...... 48

18. Photograph of Chalk Mountain Mine ...... 48 vi

Figure Page

19. Strike-frequency diagram of oxide veins at

Chalk Mountain ...... 49

20. Photomicrograph of dolomite breccia...... 55

21. Photomicrograph of oxidized galena ...... 61

22. Photomicrograph of boytroidal limonite ...... 61

23. Geologic Map of the Northern Thrust Area at

Chalk Mountain . . 66 1

INTRODUCTION

Location

Hie Chalk Mountain and Westgate mining districts are situated near the eastern edge of Fairview Valley some 38 air miles east-southeast of

Fallon, Nevada in Churchill County (Figure 1). The Westgate district lies at the southern end of the Clan Alpine Mountains, Chalk Mountain lies in the valley 2 miles to the west of Westgate and is approximately

2 miles long and a mile wide, rising to a height of about 1,000 feet above the surrounding terrain. Both districts are midway between the

Fairview and Wonder mining districts and are sometimes considered to be a part of the former which lies S miles to the south. Both areas are easily accessible from U. S. Highway 50 whicn is along their southern edges.

Scope of the Study

Approximately 9 square miles of bedrock x^ere mapped at a scale of

1:12,000. Nearly a mile of underground workings were mapped at a scale, of 1 inch to 50 feet. Emphasis was placed on relating the geology of Chalk Mountain to that of Westgate and investigating the mineralization at Chalk Mountain, the more important mining district. Field work was conducted mostly during the summer of 1971 with total days in the field amounting to approximately 35. Fifty-four thin sections and

9 polished sections were investigated and 51 vein samples were analyzed for up to 5 elements each. 2

Figure !

Location Hap of the Chalk Mountain end Weslgate Mining D is tric ts

JP 3

Previous Work

The first documentation of the ora deposits at Chalk Mountain and

Westgate was by Schrader (1947) in a U. S. Geological Survey open file report on the Carson Sink area. Schrader included a map of Chalk

Mountain on a scale of 1 inch to 3,500 feet. Speed and Wilden investigated the geology and mineral deposits of Churchill County, including

Chalk Mountain and Westgate, in a preliminary report by the U. S.

Geological Survey in 1968 and included a 1:24,000 scale map of

Westgate.

The geology and biostratigraphy of the Westgate area was described in detail by J. I. Corvalan in 1962 in an unpublished Ph.D. thesis from

Stanford University. Corvalan's map was at a scale of 1:12,000. D. C.

Thorstsnson (1968) investigated the geology of Chalk Mountain in an unpublished report from Northwestern University, Thorstenson’s map was at a scale of 1 inch to 3,000 feet. Neither of tnese papers dealt with the ore deposits of the districts. 4

GENERAL GSOLOCY

The rocks of the Chalk Mountain and Westgate districts range in age from Permian (?) to Recent. Fossils are abundant in the Mesozoic strata at Westgate but are absent at Chalk Mountain where the age of the sediments has been inferred indirectly.

The sedimentary units consist of dolomites, limestones, shales, and calcareous sandstones of Mesozoic age, and Quaternary valley fill.

A Late Paleozoic to Early Mesozoic unit occurs as the upper plate of a thrust fault.

Igneous recks include granite porphyry, quartz monzoaite, quartz porphyry, and minor felsic dikes and sills of Mesozoic age, and dio-

rite porphyry of Tertiary age. Skarn zones and recrystallization are present along the contacts of some of the felsic rocks where they have

intruded the Mesozoic, sediments.

Tertiarv volcanic rocks consisting of rhyolitic and latitic tuffs

and flows, as described by Corvalan (1962, p. 98), border the Westgate

district to the north and east but were not studied for the purpose of

this report.

STRATIGRAPHY

Late Paleozoic to Early Mesozoic Volcanics

Volcanic rocks of probable pre-Late Triassic age (Corvalan, 1962,

p. 36), which would make them the oldest rocks exposed in the area,

occur as the upper plate of a thrust fault at both Chalk Mountain and

Westgate. 5

At Chalk Mountain these volcanics are found in a narrow strip approximately 3,000 feet long, along the southern base of the mountain and also in a smaller exposure less than 1,000 feet in length again at the base of the mountain to the northeast. In each instance the volcanics are in fault contact with the dolomite that composes most of the mountain. The volcanics of the northern, exposure lie in thrust contact with the underlying dolomite. The thrust here dips east at an intermediate angle into the alluvium of the valley where it is probably truncated by Tertiary normal faulting. The southern exposure of the volcanics is in fault contact with the underlying dolomite but it is doubtful if the fault is a thrust as the fault dips approximately 50 degrees to the southwest, and is believed to be a normal fault. Ap- proximately 1,200 feet south of where this exposure dips into the alluvium of the valley, there is a small inaccessible shaft with volcanics of the same composition cn its dump.

At Westgate the exposure of these volcanics is much greater, comprising approximately 50 percent of the mapped outcrop area. Here they reach an exposed thickness of 1,950 feet as recorded by Corvalan (1962, p. 18). The underlying Mesozoic sedimentary rocks of the lower plate are exposed in three erosional windows, the largest of which encompass- es slightly less than one square mile in the northern portion of the

Westgate district. To the north and east this old volcanic unit is in normal fault contact with Tertiary volcanics which were not investigated for this report. Locally, in the southern half of the area, the Late

Paleozoic to Early Mesozoic volcanic sequence is intruded by a Late

Mesozoic granite porphyry', which was also included in the thrusting. 6

Corvalan (1962, p. 17) calls this volcanic sequence at Westgate a metavolcanic unit and states that it has been uniformly altered by low- grade regional metamorphism because biotite, hornblende, and augite have been partially altered to sericite, chlorite, epidote, and calcite.

This writer would prefer to call this alteration the result of deuteric or later solutions producing propylitization as no distinctive metamorphic minerals or textures were observed in either thin section or outcrop.

Corvalan describes this volcanic sequence as medium to dark green, poorly to fairly-vell stratified, thickheaded breccias, tuff breccias, and lavas of andesitic to rhyodacitic composition. He notes that in one horizon of the sequence epiclasts of andesite, granite, diorite, quartzite, and limestone are abundant which do not correspond to any formations present in the Westgate or surrounding areas.

He has tentatively correlated this volcanic sequence with the

Permian Pablo Formation of the Toiyabe Range 50 miles to the southeast as described by Ferguson and Cathcart in 1954 and Silberling in i957, based on compositional and textural similarities of the unit as well as the similarity and abundances of the epiclasts. The Pablo Formation is part of the widespread Diablo Sequence of clastic and volcanic rocks of Middle Permian to Earliest Triassic age covering parts of Mineral,

Nye, and Esmeralda counties as described by Silberling and Roberts

(1962, p. 25). Corvalan (1962, p. 36) has placed the age of these volcanics as pre-Late Triassic, thus dating it as older than the oldest sedimentary unit exposed in the area. No evidence was found in the present study to either confirm or refute Corvalan*s dating and 7

henceforth this unit will be referred to as the Late Paleozoic to Early

Mesozoic volcanics.

The volcanic rocks of Chalk Mountain have been correlated with the

Late Paleozoic to Early Mesozoic volcanics at Westgate by Thorstenson

(1968, p. 1.9) based on lithologic and textural similarities as well as epiclast content. This writer concurs with this correlation.

Mesozoic Sediments

The lower plate Mesozoic sedimentary rocks of Chalk Mountain and

Westgate, the host for mineralization in the districts, consist of dolomites, limestones, shales and calcareous sandstones ranging in age from Upper Triassic to Middle Jurassic according to Corvalan. Silber- ling and Roberts (1962, p. 38) described this sequence at Westgate as being the northernmost known exposure of what they called the Tuning

Sequence which they believed w~as a continuous succession of generally shallow water marine sedimentary rocks deposited in central western

Nevada during the first half of the Mesozoic. Recently Stanley (1971, p. 455) has noted the existence of similar lithologies that are more widespread in west-central Nevada and believes that Early Jurassic sedimentary conditions typical of the miogeosynclinal region of the eastern Cordillera extended uninterrupted across Nevada. These units in west-central Nevada are equivalent in time and probably grade into the Early Jurassic calc-alkaline volcanic suites in, western Nevada and eastern California which may reflect ancient island-arc assemblages within the Ccrdilleran eugeosyncline. Stanley also concludes that the presence of Navajo-like sandstone in western Nevada suggests that central Nevada was not a positive area as earlier writers had thought. 8

Gabbs Formation

Tne Gabbs Formation has been described by Corvaian at Westgate,

and what is inferred to be the Gabbs has been described by Thorstenson

at Chalk Mountain. It is the lowermost unit of Mesozoic sediments

exposed in the districts.

At Westgate the Gabbs Formation is found in the southern window

of the thrust fault on both sides of U. S. Highway 50. Corvaian

(1962, p. 37) describes it as of uniform lithology with 480 feet of exposure, consisting of thin bedded light to dark gray recrystallized

limestone with intercalations of shale in Its upper part. A few poorly

preserved fossils were found by Corvaian in the upper part of the unit

and on the basis of lithology and this fossil content he correlated it with the Triassic Gabbs Formation as described by Muller and Ferguson

(1939, p. 1603). Both Thorstenson (1968, p. 18) and this writer found

that the Gabbs Formation at Westgate is actually dolomite rather than

limestone.

At. Chalk Mountain the Gabbs (?) Formation is the only Mesozoic

sediment present and is much thicker and more widespread than at West-

gate. It composes most of the southern two-thirds of the mountain and

is also found in a narrow outcrop approximately 1,000 feet long at the northern tip of the mountain.

The Gabbs (?) Formation of Chalk Mountain is a partially to

totally recrystallized fine-grained dolomite containing a little detri-

tal material. Its color ranges from dark gray in the least altered

outcrops to pure white where recrystallization has been most pervasive.

The dark dolomite shows a characteristic "elephant-skin" texture on 9

weathered surfaces. Bedding attitudes are difficult to discern in out-

crop partially due to the effects of contact met amor phi sin and the fact

that locally the unit is highly folded, fractured, and faulted. Thor-

stenson (1968, p. 20) has estimated an exposed stratigraphic thickness

for the dolomite of Chalk Mountain of from 2,000 to 3.000 feet which is

reasonable in light of the fact that bedding is obscure and faulting may

be complex.

On the basis of similar lithology, color, and secondary micaceous

minerals, as well as spatial proximity, the dolomite of Chalk Mountain

is tentatively correlated xvith the Gabbs Formation at Wesrgate. Thor-

stenson (1968, p. 18) also states that the orientation of bedding,

cleavage, and small fold axes are similar to those of the Gabbs at

Westgate, An exact correlation is not possible at present because no

identifiable fossils have been found at Chalk Mountain, t’neretora the

unit will simply be referred to henceforth as the dolomite of Chalk

Mountain.

Sunrise Formation

The Sunrise. Formation is exposed in all three of the windows at

Westgate and has been described in detail by Corvalan (1962). It con-

sists of a sequence of shales, slaty shales, slates, sandy mudstones,

and silty and sandy limestones that conformably overlies the Gabbs

Formation. Its thickest exposed section, 1,170 feet, is found in the

northernmost window at Westgate where it is the limb of a north-south

trending overturned syncline. The lowermost member of the Sunrise,

which is a limestone exposed in sections 18 and 19 (T17N, R35E), 10

locally contains skarn minerals which are spatially related to some of

the mineralization found at Westgate.

Corvalan (.1962, p. 72) correlated this unit, on the basis of sim-

ilar lithology and fossil content, with the Jurassic Sunrise Formation

at its type locality in the Gabbs Valley Range 60 miles to the south

as described by Muller and Ferguson (1939 , p. 1609), The. Sunrise

Formation at Westgate is very fossiliferous vTith the ammonites, espe-

cially, providing an excellent basis for correlation.

Westgate Formation

The Westgate Formation, a name given by Corvalan (1962, p. 84),

is exposed only in the northern window at Westgate where it is part of

the overturned syncline. It conformably overlies the Sunrise Formation

and consists of a sequence of thick-bedded calcareous sandstones, sandy

limestones, and limestones reaching a maximum exposed thickness of 560

feet.

Corvalan concluded, from fossil and lithologic content, that this

unit does not correlate with any other Mesozoic unit so far reported

from Nevada. The only other sedimentary unit that may be chronologic-

ally equivalent to the Westgate Formation is the Jurassic Dunlap Forma-

tion which is the youngest formation of the Luning Sequence. The Dun-

lap Formation is predominantly a clastic-volcanic sequence and has been

interpreted as being deposited concurrently with folding, faulting, and

destruction of a Mesozoic marine depositional basin (Muller and Ferguson,

1939, p. 1619). From this Corvalan (.1962, p. 141) concluded that the

mechanism which destroyed the Mesozoic marine depositional basin was not 11

initiated at the same time in the Westgate area and believes these younger strata, the Westgate Formation, represent a continuation of marine conditions at Westgate that persisted for a more prolonged period of time than in the areas to the south.

Stanley (1971, p. 475), however, has shown that Early Jurassic miogeosynclinal sedimentary conditions extended uninterrupted across

Nevada and there apparently was no depositional basin. He believes the Dunlap Formation was locally derived and represents a local structural event in west-central Nevada during late Early Jurassic time that included folding, erosion, sedimentation and volcanism. Stanley

(1971, p, 467) also found the quartz sandstone in the Westgate Forma- tion and at other localities to be mineralogically and texturally akin to the Navajo Sandstone in Utah and he suggests that Navajo sand was dispersed across much of what is now western Utah and eastern Nevada. 12

INTRUSIVE IGNEOUS ROCKS

Granite Porphyry

Granite porphyry, described by Corvalan (1962, p. 108) as an aplite, is found at Westgate as several small intrusions into the Late

Paleozoic to Early Mesozoic volcanics of the upper plate of the thrust.

The color ranges from white to light orange and it weathers to brown in most instances.

The rock is a porphyry consisting of phenocrysts, up to 4mm in diameter, of quartz, orthoclase, and lesser amounts of plagioclase in a fine-grained, allotriomorphic granular grcundmass of mostly quartz and orthoclase. The phenocryst content, in part glomeroporphyritic. ranges from about 25 percent to 50 percent of the total rock. Partially re- sorbed anhedral to suhhedrai quartz makes up from 30 percent to 40 percent of the total phenocryst content. Subhedral orthoclase and plagioclase (An30) make up the rest of the phenocryst content with orthoclase greatly outnumbering plagioclase. These feldspars are partially to totally altered to clay and lesser amounts of sericite with alteration of the plagioclase being more complete. There are very minor amounts of bictita which have partially altered to chlorite, and traces of

zircon present.

The orthoclase of this rock shows no discernible exsolution and

the groundmass in part shows a trachytic texture which suggests that

the intrusion was emplaced at a very shallow depth. Locally, along tne

contact of the granite porphyry with the host volcanics, a minor amount of alteration is present. 13

Gorvalan (1962, p. 110) puts the age of this rock, the granite porphyry, after the deposition of the Mesozoic sediments and before the extrusion of the Tertiary volcanics which are found on the out- skirts of the West.gate district. At Westgate, however, the granite porphyry is part of the overriding thrust plate and is in thrust contact with the Mesozoic sediments which would date it as pre-thrust faulting. It is believed that this rock is therefore either Triassic or Jurassic.

Quartz Porphyry Dikes

A quartz porphyry dike, up to 5 feet across, striking northwest and dipping approximately 60 degrees to the southwest cuts the Sunrise

Formation at Westgate in the central window in section 29. In the southern portion of section 13, in the northern window, is similar dike rock reaching a maximum of 20 feet in thickness and striking north 50 degrees east and dipping very steeply to the southeast. - At this location bull quartz veins within the dike reach a foot across and have the same attitude as the host quartz porphyry dike. Both of the above quartz porphyry dikes are altered and bleached to a dirty white with occasional limonite pseudomorphs after pyrite present.

Microscopically the rock is composed of 10 percent phanocrysts, consisting of quartz, orthoclase, and minor amounts of plagioclase in

a groundmass of the same composition. Orthoclase is the dominant mineral. The plagioclase is sodium rich and may be albite. The quartz phenocrysts arc anhadral showing a great deal of resorption. Sericite

is very pervasive as an alteration mineral and composes up to 60 14

percent of the total rock in places. Feldspar phenocrysts have been

altered to both sericite and clay.

Syenite Porphyry Sills

Dirty white to yellow syenite porphyry sills are found, for the

most part, in one horizon of the Sunrise Formation in the western limb

of the syncline in the northern thrust window at Westgate. Along most

of its extent the intrusive is in the form of two sills within 20 feet

of one another but occasionally cuts across bedding and branches into

three distinct sills. Tire sills range from 2 to 25 feet in thickness

and have been mapped as one sill on the accompanying geologic map

(Plate 1). The sills have not noticeably altered the host rock.

In thin section the rock is composed mostly of feldspar. Ten per-

cent of the rock is composed of subhedral phenocrysts of plagioelase

(An20) and K-feldspar with plagioelase being more prominent in a fine-

grained groundmass of mostly K-feldspar. Quartz composes less than 5

percent of the groundmass. There has been a small amount of alteration

of the feldspars to both clay and sericite.

The quartz porphyry dikes and syenite porphyry sills described

above, are small in extent and are considered to be Late Jurassic or

Cretaceous as they cut the Mesozoic sediments but not the Late Paleozoic

to Early Mesozoic volcanics of the upper thrust plate. On the basis of

composition Corvalan (1962, p, 110) has described these two rock types

as being apophyses of the granite porphyry found in much larger intru-

sive masses in the Late Paleozoic to Early Mesozoic volcanics. This writer has found that the sills of syenite porphyry are not of the same 15

compositions as the granite porphyry and it has been previously shown

that the granite porphyry is part of the thrust slice and was thrust

into place. This writer would also like to separate the above des-

cribed dikes of quartz porphyry from being equivalent to the granite porphyry based on the above and the fact that there are no observable

spatial connections between the two and the nature of the alteration in

the quartz porphyry is distinctive.

Porphyritic Quartz Monzonite

Porphyritic quartz monzonite of probable Cretaceous age occurs at

Chalk Mountain as three separate intrusions outcropping along the west-

ern edge of section 23 and the southern edge of section 14 T17N, R34E

(Figure 2).

The rock is gray in color and weathers to a greenish brown. In

fresh hand sample the rock is phaneritic consisting of occasional large

phenocrysts, up to an inch across, of pink orthoclase in a finer-

grained groundmass of K-feldspar, white plagioclase, and lesser amounts

of biocite and hornblende. Grain size and composition varies locally

with finer-grained textures closer to the margins of the intrusive.

Locally light green inclusions of a fine-grained igneous rock become

very abundant, making up more than 50 percent of the outcrop in some

instances. The inclusions range from inches to many feet across and

appear to be the same composition as the host except that the mafic

mineral content is slightly higher.

Microscopically the rock consists of from 10 percent to 30 percent

anhedral quartz grains, 30 percent to 35 percent subhedral to anhedral 16

K-feldspar (both orthoclase and lesser amounts of microcline), 40 percent to 55 percent euhedral to subhedral piagioclase (An40), and up to

3 percent biotite with lesser amounts of hornblende and a trace of opaque minerals (Figure 3).

There are two distinct phenocryst sizes present, the larger being the grossly euhedral pink orthoclase. These phenocrysts are partially zoned, perthitic, and many show overgrowth rims of piagioclase forming a rapikivi texture. The smaller phenocrysts, averaging about 5mm across* consist mostly of euhedral to subhedral piagioclase and are occasionally glomeroporphyritic. Many of the piagioclase grains are zoned in part and show albitization rims. Quartz also occurs as phenocrysts in this size range and is commonly embayed. The groundmass size averages from 1mm to less than ,1mm in diameter and consists mostly of orthoclase and quartz with lesser amounts of piagioclase and mafic minerals.

Dark green metamorphic skarn zones are prevalent along the quartz monzonite - dolomite contacts with alteration within the quartz mcnzon- ite becoming more pronounced closer to the contact zones. These alteration zones may extend many feet into the intrusive and here the feldspars have been almost totally altered to sericite and clay minerals while the mafic minerals have been altered to chlorite.

Thorstanson (1963, p. 4) and earlier workers have called this intrusive rock a granodiorite but this writer has found it to be a quartz monzonite as evidenced from the data cited above.

Ferguson and Muller (1949, p. 7) found that in the region to the south the development of the pre-Tertiary structural features was 17

Figure 2. West side of Chalk Mountain showing dark quartz monzonite intrusive.

Figure 3. Porphyritic quartz monzonite showing plagioclase overgrowths along rim of large orthoclase phenocryst on the right. Polars crossed, X34. 18

essentially complete before the intrusion of the granitic rocks. It has been therefore interpreted that the quartz monzonite is Cretaceous in age as no evidence to the contrary exists and this correlates with the age of many similar intrusives throughout west central Nevada.

Quartz Porphyry

Quartz porphyry of possible Cretaceous age occurs as a large in-

trusive mass comprising the northern third of Chalk Mountain. Three small apophyses of this intrusive, ranging from 5 to 200 feet across in exposure intrude the dolomite just south of the quartz porphyry -

dolomite contact in the northeast corner of section 23 (T17N, R34E).

The quartz porphyry ranges in color from light pink to yellow to

dirty white on fresh surfaces and weathers to various shades of light

red, yellow, and brown. From a distance the intrusive shows a distinct

color banding in its upper part, ranging from dark pink at the top to

yellow and then into unbanded mixtures of yellows, pinks, and browns.

Microscopically the rock consists of phenocrysts of quartz, K-

feldspar, and altered plagioclase in a microcrystalline groundmass of

the same (Figure 4), The intrusive shows both a compositional and

textural variation from north to south. The southern part of the in-

trusive contains phenocrysts composing 70 percent of the total rock and

consisting of quartz (20 percent of the phenocrysts), K-feldspar (60

percent), and plagioclase (20 percent). The phenocrysts are glcmero-

porphvritic in part and partially embayed. The groundmass consists o£

fine-grained quartz, K-feldspar, and minor plagioclase. Bxotxte and

epidote make uo less than 1 percent of the total rock. As one goes 19

north and east the. phenocryst composition changes to dominantly quartz

and h feldspar in approximately a 2 to 3 ratio. Plagioclase pheno-

crysts are rare and all phenocrysts are less embayed than those to the

southwest. The groundmass is very fine-grained quartz and K-feldspar with only traces of biotite. Total phenocrysts content here is approx-

imately 40 percent of the total rock. Throughout the intrusive the quartz phenocrysts are euhedral to subhedral, up to 3mm in diameter,

and in part rhomb-shaped. The K-feldspar consists of euhedral to subhedral crystals of perthitic orthoclase and minor amounts of micro-

cline ranging to 5mm in diameter. The plagicclase (An40), when present, is sometimes zoned and partially to almost totally altered to sericite and clay minerals. Oxidized pyrite is found consistently throughout the intrusive.

At least one small inhomogeneity exists within this intrusive at the northern base of the mountain where it extends furthest east.

Late stage quartz stringers up to 10mm across demonstrate internal flow characteristics and cut a very fine-grained feldspathic matrix composed mainly of plagioclase (A.n30) and quartz. This zone is approximately 20 feet across and grades into the surrounding quartz porphyry.

The intrusive's eastern and part of its southern edge is entirely bounded by recent faulting but part of the southern edge and northeast edge show intrusive relationships to the dolomite of Chalk Mountain.

Here s k a m zones up to 20 feet wide have formed at the contact between the dolomite and the intruded quartz porphyry. The quartz porphyry does not change composition as the s k a m zones are approached nor is it altered. 20

It was found that the contact between the quartz porphyry and the

quartz monzonite to the west was very difficult to discern. For the

most, part the contact between the two appears to be gradational with

phenocryst content in the quartz porphyry decreasing going away from

the quartz monzonite. The quartz porphyry is therefore believed to be

a late stage differentiation product of the parent magma that solidi-

j.ied to rorra die quartz monzonite and this would put the age of the

quartz porphyry as slightly younger than the quartz monzonite.

Thorstenson (1968, p. 24) has placed the age of the quartz por-

phyry as possibly Cretaceous, and dates it as older than the quartz

monzonite. He bases his conclusion on the fact that he interpreted two

sroaj.1 intrusions found cutting the quartz porphyry as being eouivalent

to the quartz monzonite, when in fact they are small diorite dikes of

Tertiary age. As has been stated earlier the boundary between the

quartz porpnyry and quartz monzonite is gradational and no crosscutting

relationships have been obse.rved between the two. Based on this evi-

dence i_his writer suggests a similar Cretaceous age for both the quartz

monzonite and quartz porphyry of Chalk Mountain.

Biorite Porphyry

Diorite porphyry of Tertiary age occurs as an almost continuous axke at Westgate and what is believed to be the equivalent of this unit is found on Chalk Mountain as irregular dike-like intrusions.

At Westgate the dike's thickness reaches 250 feet and it can be traced for nearly three miles. It cuts the Mesozoic units of the lower plate, the volcanics of the upper plate, and the lowTer part of the 21

Tertiary volcanics surrounding the district. Hie dike strikes approximately north 80 degrees west with a steep dip to the northeast. Corva- lan (1962, p. 113) beli eves the dike is controlled by, and follows, an older preexisting fault.

At Chalk Mountain the diorite porphyry is found as three small dike-like intrusions up to 1,000 feet in length and 150 feet thick near the southern edge of section 14. The diorite here intrudes both the quartz monzonite and quartz porphyry. The same rock was also found on the dump of a small pit in the alluvium a short distance from the eastern base of the mountain.

These occurrences of diorite porphyry at Chalk Mountain are in direct line with the extension of the strike of the diorite porphyry dike at Westgate, the western exposed end of that dike being a mile and a half to the east across the alluvium of the valley (Figure 5). For this reason, along with close similarities in composition and texture cited below, these two rocks are considered to be synchronous. The diorite porphyry is considered hypabyssal as is evidenced from its textures and relationships to the surrounding rocks. It shows very little variation in composition or texture throughout its extent, and introduces no observable contact metamorphism in the host rocks.

Megastopically the diorite porphyry is of uniform greenish gray color and weathers to a dark brown. Phenocrysts make up between 30 percent and 40 percent of the total.rock and consist of subhearal plagioclase with lesser amounts of mafic minerals and quartz. Pheno- cryst percentage is slightly less in the diorite porphyry of Chalk

Mountain. A few dark green, fine-grained inclusions up to 7cm across are found in the diorite at Chalk Mountain while Corvalan (1962, p. 2

Figure 4. Quartz porphyry showing quartz and altered orthoclase phenocrysts in groundmass. Polars crossed, X34.

Figure 5. Diorite porphyry dike at Westgate striking toward Chalk Mountain in background. 23

113) reports a £e,;:' li3ht gray, fine-grained inclusions not larger than

4cm in the diorite at Westgate .

Hic.rosc--pically the rock was seen to be as Corvalan ri1:sc~ib~d it at Westgate with phenocrysts up to 6mm long in a fine-grained, allo- triow.orphlc gra..,ulc1r groundmass. They consist of zoned plagioclase

(An 40) making up approximately 20 percent of the total ruck, biotite

(5 percent) that has comple t ely altered to chlorite , m1hedral ewba.y ed quartz (3 percent), and less than 1 percent hornblende that has also altered t o '.:hlorite . The groundmass consists mostly of plagioclase and minor amounts of quartz less than • lmm in diameter. It was found that the diorite porphyry at Chalk Motmtain may contain up to 5 percent fine-grained secondary sericite while that at Wes tgate contains less than 1 percent. The sericitic alteration is most prevalent in the plagi.cclase phc:r,0crysts but is also fo1.u1d disseminat=d throughout t !1e groundm~ss . Other. alteration minerals include trace s of epidote, calcite, and clay minerals . 24

STRUCTURE

The structural features of the Chalk Mountain and Westgate districts are here divided into two discernible epochs of tectonic activity. Following the deposition of the Early Mesozoic sediments tectonic activity deformed these shelf sediments and consisted of folding, faulting, thrusting, and igneous intrusion which is referred to as the

Jurassic and Cretaceous Orogeny by Silberling and Roberts (1962, p.

39). During Tertiary time this area, along with most of the rest of

Nevada, was the site of extensive, predominantly normal faulting that produced the Basin and Range Province. In the area concerned with this report this normal faulting has persisted to the present day (Figure

6).

Pre-Tertiary Structures

Chalk Mountain

The main structural features of the Mesozoic sediments at Chalk

Mountain and Westgate are similar in that the major structural trends strike north-south. On a broad scale, the dolomite composing most of

Chalk Mountain appears to be in the form of an antiform plunging gently to the northwest, the eastern limb of which is the only part exposed with dips approximately 45 to 75 degrees to the east (Figure 7).

The western limb and most of the nose of the antiforra is faulted, eroded, or Intruded off.

The more detailed structure of the dolomite of Chalk Mountain was impossible to map with any certainty because of the uniform character 25

Figure 6. The southern Clan Alpine Range looking southeast toward Westgate. Note the recent fault scarp at the base of the range.

Figure 7. South end of Chalk Mountain showing the dolomite dipping steeply to the east.. 26

0l- the unit and tne .Lack of good bedding criteria. The structural re-

gime of the unit as a whole is best seen from a distance. In the

northeast corner of section 23 both large and small scale folds were

most notable ranging from open to isoclinal. In general the observ-

able colds are asymmetrical and concentric. Small scale structures

were most easily discernible within the Chalk Mountain Mine, the best

example being a 3cm thick dark siliceous bed which is tightly folded,

contorted, and in some cases completely sheared, found on the 500 foot

level of the mine.

Faulting, fracturing, and brecciaticn are pervasive in the dolo-

mite of Chalk Mountain and in most cases it was impossible to determine

displacements but in general, except for the Tertiary and Quaternary

faults, it has been considered small and probably most on the order of

a few feet. It is not possible to distinguish between Mesozoic and

Tertiary faulting in many instances. Probably the majority of the

faulting is Mesozoic as the mineralization was found to occupy fault,

fracture, and breccia zones and the Tertiary and Quaternary faults tend

to form bold scarps with much longer areal extent. It seems highly probable, however, that some or the Tertiary faulting was simply renewed movement along older established Mesozoic faults.

The unbleached weathered surface of the dolomite generally shows a well developed cleavage striking northerly. Thorstenson (1968, p. 15) believes there may be a general north-northeast trend in the cleavage strikes for the dolomite as a whole and he also states that the cleavage dips steeply and is not parallel to bedding. He concluded that the attitude of cleavage with respect to the few observable beds along the 27

east flank of the mountain may indicate an anti form to the west, or

southwest.

Following and perhaps during the orogenic activity permissive em-

placement of the Mesozoic intrusives took place. The southern mass of

quartz monzonite at Chalk Mountain (Figure 2) may in part have accentu-

ated the anticlinal form of the dolomite present. Here the quartz mon-

zonite occupies the axial plane of the fold, which would be a region of weakness, and the eastern contact of the intrusive is in part concord-

ant with the overlying sediments while the western contact is in part

faulted against the sediments. The intrusives of the northern part of

the mountain show only discordant and fault contacts with the .

sediments.

Westgate

The structure of the Mesozoic sediments at Westgate is dominated by a large north-south trending syncline which may represent the eastern complementary structure to the Chalk Mountain antiform but higher in the section. The east limb of the sync.line is overturned to the west and dips between 65 and 80 degrees east. The western limb of the syncline dips approximately 25 degrees east with gradual shallowing toward the axis. The syncline plunges gently to the north as seen in the two northern windows where it is best exposed.

Corvalan (1962, p, 121) believes the folds found in the Mesozoic

formations were at least partially developed prior to the epoch of

thrusting, but he believes the overturning of the syncline was the result of the more intense compressional deformation which followed the 28

initial folding and which was accompanied by thrusting.

Faulting at Westgate is both Mesozoic and Tertiary but as at Chalk

Mountain the ages of many of the faults found in the Mesozoic windows

cannot be named with certainty. The large northern window of Mesozoic

sediments is the site of numerous, possibly Mesozoic, faults as mapped

by Corvalan, He refrains from putting a definite age on the faults but

describes them as steeply dipping normal faults downthrown to the west.

These faults do not cut the volcanics of the upper plate. Their ver-

tical displacements vary from 20 feet to 80 feet except for the large

east-west fault near the northern edge of section 19 and 20 which is

downthrown 350 feet on the north.

Faulting, fracturing, and other deformational characteristics at

Westgate were found to be much less pervasive than at Chalk Mountain

arm mucn easier to discern* Lhis could be due to such factors as ? the

units being less competent with resultant less fracturing; no large

intrusive masses evident; and the fact that most cf the sediments at

Westgate are higher in the section than those at Chalk Mountain resulting in less compressional forces.

Thrust Faulting

A thrust fault occurs both at Westgate, where it outcrops exten- sively, (Figure S) and in at least one outcrop at Chalk Mountain.

Late Paleozoic to Early Mesozoic volcanics and Triassic or Jurassic granite porphyry have been thrust over later Mesozoic sediments in both cases.

Locally, along the thrust, brecciation and recrystallization are 29

found in the Gabbs Formation at both Westgate and Chalk Mountain. The

brecciation in the Gabbs is usually just a few inches thick while the

recrystallization may be many feet thick.

Corvalan (1962, p. 130) has described the thrust fault at Westgate

in detail. He found limestone blocks of Triassic age along the thrust

surface in tne central window at Westgate and believes these have been

moved as much as two miles from the southeast where a similar lithology

is found, indicating that the overthrust sheet moved in a northwesterly

directibn. Where the thrust cuts the Sunrise Formation, Corvalan found

conspicuous breccia zones reaching 10 feet in thickness, composed of

rounded Sunrise Formation, granite porphyry, and volcanic fragments.

Tertiary and Quaternary Structures

Basin and Range normal faulting of Tertiary and Quaternary age is very evident and is responsible for the physiography of both the Chalk

Mountain and Westgate districts. These high angle faults cross all structures in the districts and have displacements of from inches during recent movements to perhaps thousands of feet in total movement. Most of the faults shown on the accompanying geologic map (Plate 1) are of this type.

At Chalk Mountain the major trend of Tertiary faulting is north- northeast to northeast and downthrown to the east. At Westgate the pre- dominant trend is to the northwest and downthrown to the west. Many of the faults on the accompanying map show definite signs of movement including scarps, slickensided scarps (Figure 9), and breccia zones reaching 20 feet across. 30

Figure 8. Gabbs Formation in southern window at Westgate showing thrust and dark upper place volcanics.

Figure 9. Fault scarp at Chalk Mountain showing recent movement, during the earthquake of 1954. 31

A notable feature of many of the scarps in the dolomite along the southern and southeast flank of Chalk Mountain is the presence of small scale pervasive fractures filled with secondary silica. On weathered surfaces these silica filled fractures, averaging less than 1mm across, stand out in a boxwork-like pattern and give a dark brown color to the rock which contrasts sharply with the white or gray color of the non- silicified dolomite (Figure 10). These brown fracture zones are here considered to delineate fault zones and they reach thicknesses of 5

feet in many instances.

The Dixie Valley - Fairview Peak earthquakes of 1954 resulted in surface faulting along the eastern side of Chalk Mountain and the western side of the Westgate district. Stingaree Valley, separating the two districts, dropped graben fashion relative to the two mountain fronts. This recent faulting is separately identified in Plate 1.

At Chalk Mountain this recent scarp height is 5 feet in the south with lesser amounts of right lateral displacement. The longest recent scarp is continuous for almost a mile and bounds the entire southeastern margin of the quartz porphyry (Figure 11).

A triangulation station on the west side of Chalk Mountain was

found to have moved 4.1 feet north 10 degrees west as a result of this

recent faulting (Whitten, 1957, p. 321). On analysis of the horizon-

tal shifts, Whitten believes that the blocks on the opposite sides of

the faults are not only shearing but are also pulling apart, creating

a void that explains the lowering of the valley between the faults.

The three en echelon faults trending northeast along the eastern base

of Chalk Mountain may be similar in nature to generally smaller scale 32

Figure 10. Silicification along fault at Chalk Mountain. Note scarp dipping to the left.

Figure 11. 1954 fault scarp cutting bedrock at Chalk Mountain. The scarp resembles a trail going up left side of canyon. Dolomite on left, yellow quartz porphyry on right. 33

tension gashes and reflect this deeper seated north-trending right lat-

eral component of movement.

At Westgate the maximum recent fault displacements a):e 4 feet with

2 feet of right lateral displacement as reported by Slsmmons (1957,

p. 353). The faulting extends the entire length of the district and is

downthrown to the west.

In places at both Westgate and Cnalk Mountain the faults leave the

alluvium-bedrock contacts and cross into only bedrock as exemnlified in

Figure 11. Slemmons (1957, p. 353) states that this may be a result of

recent directional changes in the tectonic forces which cause increased

strain and result in failure near the irregular bends of the main

faults.

This recent faulting, in most cases, represents renewed movement

along older preexisting faults. Much of the faulting is at the base of

the mountain fronts near the bedrock-alluvium contacts, being generally

vertical where it cuts through the hanging alluvium to the surface and

exhibiting lower dips where it cuts only bedrock.

Basin and Range normal faulting produced the mountain fronts at

Chalk Mountain and Westgate and subsequent erosion highly dissected

them and formed the broad alluvial filled valley between the districts.

The depth of the alluvium in the valley is uncertain but the Nevada

Bureau of Mines (1964, p. 261) indicated a depth of 266 feet to water

in an abandoned well, exact location unknown, just south of Chalk

Mountain near U. S. Highway 50. Therefore the alluvium of the valley is at least this deep and probably goes much deeper. 34

METAMORPHISM

Metamorphism, or the mineralogical arid textural changes exclusive

of diagenesis and weathering that have affected the rocks of the Chalk

Mountain and Westgate districts, has been of two types, dynamic and

contact. Low-grade regional me tamo rph i s m may be responsible for some

of the mineralogy and textures observed at certain localities but can-

not be distinguished from the effaces of contact metamorphism.

Dynamic and Regional. Metamorphism

Dynamic metamorphism comprises changes due to the influence of

stress accompanied by little or no thermal effects. It has produced

slaty cleavage locally in the argillaceous units of the Mesozoic sedi-

ments at Westgate and is directly related to compressive forces pro-

duced during folding. It may have also produced slight recrystalliz-

ation of the Mesozoic carbonate units in the areas of more intense de-

formation, but if so, its effects are partially masked by later contact me t amo rph ism.

Regional metamerphism is that which is developed over large areas.

Corvalan (1962, p. 17) has stated that the alteration assemblage of the

Late Paleozoic to Early Mesozoic volcanic upper plate at Westgate Is indicative of low-grade regional metamorphism. This writer, as previously described, calls this alteration the result of deuteric or later stage solutions producing propylitization as no distinctive metamorphic minerals or textures were observed. The possibility that some of this alteration is the result of low-grade regional metamorphism cannot be discounted however, as a detailed study of this unit was not undertaken 35

for this report. The Pablo Formation, described by Silberllng (1959, p. 7) in the Shoshone Mountains 30 miles to the east, with which this volcanic sequence is correlated, has undergone a low degree of regional metamorphism in part. Silberling states that the interbedded andesitic flows are altered to greenstone. He also states (p. 9), though, that

the effects of local hydrothermal alteration superimposed on a low- grade regional metamorphism have changed the original mineralogy.

The Gabbs Formation of both Chalk Mountain and Westgate contains up to seven percent secondary micaceous minerals consisting of phlogopite and chlorite with orientation parallel to bedding planes in places.

This may be the result of low-grade regional metamorphism but this writer believes it is mostly the result of contact metamorphic effects and hydrothermal activity because these secondary minerals are more abundant in areas of obvious hydrothermal activity or in the recrystallized hales produced by igneous intrusions. In a review of part of the literature concerning the Gabbs Formation no similar minerals were described as being the result of regional metamorphism.

Contact Metamorphism

Contact metamorphism comprises changes effected in the rocks by igneous intrusions and associated fluids and occurs in restricted zones adjacent to bodies of plutonic rocks. It includes both endomorphism, or internal alteration of the intrusive rock itself, and exomorphism, or alteration of the host, rocks at or near the intrusive contact.

Contact metamorphism has affected the dolomite of Chalk Mountain to a large degree, creating broad halos of recrystallization and 36

bleaching in the dolomite and skaro mineralization along the dolomite- intrusive contacts. The effects of contact metamorphism are found to a much lesser degree at Westgate.

Chalk Mountain

The dolomite of Chalk Mountain shows the effects of contact metamorphism throughout its entire outcrop area in both the bleached and unbleached exposures. The unbleached dolomite is crystalline in part with the crystals averaging less than .1mm across. Throughout the unbleached dolomite there are distines recrystallized discontinuous veinlets with dolomite crystals up to ,5mm across which are also interpreted to be due to contact metamorphism.

The bleached dolomite is completely recrystallized with grains from .2mm to .5mm across. This 'white, bleached dolomite forms halos around both the quartz mensonits and quartz porphyry intrusives at

Chalk Mountain (Plate 1). Isolated remnants of unbleached dark dolomite are often found within these broad bleached and recrystallized halos in which bleaching and recrystallization follow fractures and bedding planes and give a banded appearance to the dolomite in places.

The white bleaching is believed .to be due to the expulsion of carbon- aceous matter during recrystallization.

Secondary minerals consisting of phlogopite and lesser amounts of colorless chlorite, probably the Kg-rich Pennine variety, occur throughout the unit, but are. more abundant in the bleached zones and cJ.ose to where mineralization is evident (Figure 12). They occur as platy to fibrous masses up to 4mm in length, and in some cases show a parallel

orientation. Chrysotile is sometimes found as central zonal replace- ments in the phlogopite. These minerals were found to be most preva-

lent on the southeast flank of the mountain, where most of the mineralization is found, and they compose 5 percent of the dolomite in some cases. Limonite pseudomorphs after pyrite are occasionally found within the bleached dolomite and are also more prevalent in areas of evident mineralization.

Within the Chalk Mountain Mine phlogopite and chlorite are present

along with tremolite and minor brucite. Tremolita occurs in the lower

levels ef the mine as elongate fibrous crystals up to 1cm in length and

composing 3 percent of the dolomite. What is believed to be brucite

occurs very sparingly in the lowest level of the mine.

Ska ms , S kam mineralization is found along the contacts of both

the. quartz monzcmite and quartz porphyry where they intrude the dolomite

of Chalk Mountain (see Plate 1). Generally these s k a m zones are bounded by sharp contacts, are light to very dark green, and average between 5 and 10 feet in thickness but may be greater than 20 feet in some instances. The s k a m zones are absent where the dolomite and in-

trusive are in fault contact.

The skarns associated with the quartz porphyry intrusive are not

as extensive nor as complex as those associated with the quartz monzonite. The quartz porphyry skarns consist mostly of serpentine with

small amounts of intermixed magnetite and calcite. The only endomor-

phic effects found within the quartz porphyry were a few small skam

veinlets near the southern margin of the large intrusive in the northernmost part of section 23 (T17N, R34E). These consisted of 39

magnetite, ilmenite, epidote, and chlorite with the epidote replacing the quartz porphyry host rock up to 1cm away from the veinlets.

The skams associated with the quartz monzonite are more prevalent and much more complex mineralogically, consisting of serpentine, calcite, magnetite, epidote, talc, diopside, K-feldspar, chlorite, and grossularite. The most abundant skarn mineral present is serpentine with the fibrolamellar anrigorite variety being much more prevalent than the fibrous chrysotile variety (Figure 13). From thin section studies some of the serpentine was found to be an alteration product of what was probably forsterite (Figure 14), as relict outlines remain and in some instances fine-grained magnetite is present which is a common by-product of this replacement. Magnetite is in all cases closely associated with the serpentine and sometimes is found as veins within the serpentine. Massive magnetite occurs occasionally, the largest mass seen being approximately 10 feet in extent and found in the skarn zone along the southeastern edge of the southernmost quartz monzonite body in section 23.

Anisotropic, zoned euhedral grossularite crystals, up to 7mm across in a calcite matrix, occur in the skarn zone along the margin of

the quartz monzonite near the boundary between sections 23 and 14.

Minor amounts of garnet were also found in the skarn along the western edge of the quartz monzonite body in section 23 along with more prevalent intimately mixed diopside. epidote, talc, and lesser calcite.

The quartz monzonite intrusive contains much more endogenic skarn mineralization than the quartz porphyry and in some cases it approaches

a dark green color, Near the southern margin of the intrusive body in 40

Figure 13. Serpentine and magnetite in s kam zone at Chalk Mountain. Polars crossed, X34.

Figure 14. Same picture as above, polars not crossed. Relict outlines of what may have been forsterite can be seen along with fine-grained magnetite. X34. 41

section 23 epidote occurs intimately mixed with K-feldspar with lesser amounts of chlorite and calcite (Figure 15). Within the quartz monzonite near the skarn zones the feldspars are almost totally altered to sericite and clay minerals while the mafic minerals have altered to chlorite,

In most cases the serpentine and magnetite are segregated from the other skarn minerals recognized and appear to occur as axoskams, that is as replacement deposits within the host dolomite. The other minerals present occur closer to the intrusive body or sometimes within it as is the case with the epidote and K-feldspar cited above.

Later secondary calcite is often found disseminated and filling narrow veinlets within the skarn zones. Late quartz and gypsum are also found sparingly. Oxidized sulfides are often associated with the skarn and will be discussed in more detail later.

At places, most notably in the northeast part of section 23, nar-

row dark to light green banded veinlets, averaging less than an inch across, occur within the bleached dolomite at some distance from any intrusive. They consist of serpentine, magnetite, chlorite, minor garner, and pyrite, and later calcite. At one locality on the wTest side of the mountain radiating green prismatic crystals of actinolite 2cm long were found filling fractures in the dolomite. Skarn mineralization was also found associated with many of the oxidized ore veins of

Chalk Mountain and will be discussed in more detail in the section on ore deposits.

Tine skarn zones originated during the emplacement of the intrusions at Chalk Mountain and resulted from the interaction between the 42

components of the igneous intrusion and those of the reactive dolomite.

The skarn mineralization can be best placed in the albite-epidote— homfels facies of contact metamorphism as described by Winkler (1967, p. 64) , with seme overlap into the higher grade hcmb.lende-hornfels facies with the appearance, of grossularite, diopside, and possibly forsterite, The facies boundary between the two, as given by Winkler

(p< 72), would be between 520° and 540°G at a fluid pressure between

500 and 2000 bars.

Westgate

The Gabbs Formation at Westgate shows metamorphic effects similar to the dolomite of Chalk Mountain. As described earlier, the unit here is generally recrystallized with secondary phlogopite and colorless chlorite locally composing nearly 7 percent of the unit.

The lower members of the Sunrise Formation in sections 18 and 19

(T17N, R35E) also show the effects of contact metamorphism, indicating a possible intrusive body at shallow depth. Recrystallized bleached limestone is found with calcite crystals reaching 1cm across. A few oxidized veins are found within this bleached zone and limonite pseudomorphs after pyrite are scattered throughout the bleached areas.

Euhedral garnet composing 15 percent of the rock in places and minor amounts of brucite were reported by Corvalan (1962, p. 54) from this locality. Chiastolite, a variety of andalusite, composing 20 percent of the rock (Figure 16) and what is believed to be very fine-grained epidote is also found near here in a matrix of recrystallized calcite. 43

Figure 15. Epidote and K-feldspar in endoskam zone within the quartz monzonite intrusive at Chalk Mountain. Polars crossed X34.

Figure 16. Chiastolite from Westgate. This is a variety of andalusite containing geometrically arranged graphite inclusions. Polars crossed, X34. 44

GEOLOGIC HISTORY

Although, the. sequence of events that outline, the geologic history of the Chalk Mountain and Westgate districts is fairly well established, an accurate dating of all the events cannot, be certain. The Meso-

zoic rocks have been dated on paleontological evidence and correlation, but the. sequence of events after their deposition is partially uncertain. Major folding and uplift of these Mesozoic sediments was initi-

ated sometime during the Late Jurassic or Cretaceous. Thrusting took place after the. major folding and before the intrusion of the quartz monzonite and quartz porphyry bodies. Ore deposition came after the emplacement of the intrusives. Volcanism dominated deposition during

the Tertiary with related intrusion of the diorite porphyry. Later in the Tertiary Basin and Range normal faulting was initiated, producing

the present physiography, and it continues to the present.

Summary of the Geologic History of the Districts

Triassic

Deposition of volcanies of upper plate

Deposition of Gabbs Formation

Jurassic

Deposition of Sunrise Formation

Deposition of Westgate Formation

Cretaceous (?)

Folding, faulting, uplift, and erosion of the Mesozoic sediments

Intrusion of minor dikes and sills (?) 45

Thrusting of the Late Paleozoic to Early Mesozoic volcanics and

the Triassic or Jurassic granite porphyry over the Mesozoic

sediments (?)

Intrusion of quartz porphyry and quartz monzonite and concurrent

contact metamorphism

Ore deposition

Tertiary

Deposition of surrounding volcanics

Intrusion of diorite porphyry

Basin and Range normal faulting producing the present physiography 46

ORE DEPOSITS

History of the Districts

Schrader (1947, p. 116) reports that ore was known to occur at

Chalk Mountain during the Comstock days, but until 1921 there was only sporadic production, At this time the Chalk Mountain Silver-Lead

Mines Co, was formed and more extensive ore deposits were found through its efforts, A mine was developed by two shafts, with lateral workings •3 on six levels. This company remained the chief producer in the district during the principal production period which was between 1923 and

1929.

Vanderburg (1940, p. 18) reported production of $135,000 from

2,300 tons of ore, or $58.70 per ton, up to June, 1927. This produc-

tion consisted of lead and silver acid a minor amount of gold. Couch and Carpenter (1943, p. 24) reported a production through 1928 of

$120,268 from 2,528 tons of ore, or $48.10 per ton. The discrepancy between these two sets of figures may be due to the fact that Chalk

Mountain was at times considered to be a part of the Fairview district to the south and some of its production may have been included with that from Fairview. Vanderburg (1940, p. 18) reports that some ere was also shipped in 1928 and 1929.

In 1929 a 50-ton mill was erected but was unsuccessful metailur- gicallv. In 1930 the company suspended operations and since then the property has been operated sporadically by lessees with occasional small shipments. 47

The Westgate district is somewhat similar to the Chalk Mountain

district, but there was little production (Schrader, 1947, p, 118).

Trie only record of production is by Lincoln (1923, p. 13) who states

that ore was produced in 1915, with values in silver, lead, and gold.

In 1939 a custom mill was erected at Westgate that treated ores from other districts within a 50 mile radius.

Chalk Mountain

Tiie mineralization at Chalk Mountain consists of oxidized lead- silver ores occurring as vein and replacement deposits in the Gabbs (?) dolomite. Shafts, adits, and smaller prospects are numerous throughout the dolomite and in almost all cases follow oxidized veins (Figure 17).

The largest.mine in the district, the Chalk Mountain Mine, which ac- counted for almost all of the production from the district, has the only accessible shaft (Figure 18).

The majority of the surface mineralization at Chalk Mountain is in a zone trending NIOC along the eastern base of the mountain with oxidized veins found sporadically- in a belt up to 2,000 feet across.

the mountain.

entaticn of strikes from N10W to N4GW, which may partially reflect bedding control on mineralization because along the zone of mineralization on the eastern flank of the mountain both the strike and dip Figure 17. Part of a 5 foot wide oxidized vein along the eastern base of Chalk Mountain.

Figure 18. The Chalk Mountain Mine showing the headframe for the Dawes Shaft. Fairview Peak is in the background.

of the veins are generally similar to that of bedding where bedding can be discerned. Many veins parallel bedding while others are along obvious fault and fracture zones crosscutting bedding. The mineralization obviously followed avenues of weakness within the dolomite.

The veins range from about an inch up to four feet, across and are generally discontinuous. In all cases they consist of brown to red to yellow earthy limonitic material with occasional inclusions of altered host rock. Rare, small unoxidized remnants of galena were the only primary mineral observed.

A total of 37 oxidized vein samples .from Chalk Mountain were analyzed for lead, silver, zinc, and copper by atomic absorption spectro- photometry. In addition, silver and gold fire assays were made on 17 of these, samples. The analyses show a random distribution of metal contents with the values found being partially, at least, a function of oxidation processes. The analytical results of samples are listed in the appendix and the sample locations are shown on Plate 2.

Lead values in the minor oxidized veins of Chalk Mountain range from less than 1 percent to 33 percent where primary galena is recognizable. Most lead values along the. mineralized belt on the eastern flank of the mountain are greater than 1 percent. The higher silver values, up to 7 oz. per ton, and zinc values, up tc 6 percent, are con- centrated in the mineralized belt along the eastern flank of the moun-

tain also, and generally are associated with the higher lead values.

Both silver to lead ratios and zinc to lead ratios increase as lead values decrease and silver to zinc ratios increase as zinc values decrease. This would indicate that sphalerite was at one time abundant 51

locally and that silver was associated with this sphalerite. At Chalk

Mountain total zinc concentrations were greater than total lead concentrations for values of lead less than 0.6 percent. The silver to zinc ratios increase as zinc, values decrease reflecting the greater mobility of zinc than silver in the secondary environment.

The distribution of copper values shows no definite pattern except that they tend to be higher in the skarn related veins, some of which have values near 0.2 percent Cu. Zinc and copper sulfides nay have been present in small amounts, but oxidation and subsequent leaching has completely removed them. Gold is sparingly present but randomly distributed with the highest vaJ.ue found being 0.1 oz. per ton and this on the east flank of the mountain (Sample no. 17).

Chalk Mountain Mine

The Chalk Mountain Mins is on the eastern flank of the mountain in the southeast corner of section 23. The mine was mapped at a scale of

1 inch to 50 feet using brunton compass and tape (Plate 2). The mine was originally developed on six levels with two shafts but presently the mine is accessible only through the Dawes Shaft which extends ver- tically to the lowest level. Another shaft extended to the 110 foot level but is completely caved. The total extent of lateral workings mapped is 4,470 feet,

The dominant structural trends in the mine are fault and shear zones trending north to northeast and dipping between 50 and 75 degrees southeast. The ore bodies occur as irregular vein and tabular replacement bodies from 1 to 12 feet in width along certain of these 52

structures. Bedding in the dolomite is generally indistinguishable and where recognizable is highly contorted.

Breccia zones up to 20 feet wide and lesser fractures are common, attesting to the highly broken-up nature of the dolomite. The entire length of the ’workings on the 110 foot level is in an intensely brecci- ated and weakly mineralized zone with superimposed minor faulting.

Post mineral faulting is common within the mine and produces slicken- sides, offsets veins, and in some instances pulverizes the oxidized ore.

The ore in the stopes on the 110 foot level of the mine is in the footwall of a fault zone trending between N15E and k25E and dipping 60 to 75 degrees southeast. Most stopes on this level are inaccessible but the vain appears to widen as one goes up. A major fault zone at the surface, up to 50 feet wide, is probably the up-dip projection of this vein but contains only spotty occurrences cf mineralization. The ore vein that is present on the 110 foot level extends downward to the

23.0 foot level as recorded by Schrader (1947) on his cross-section of

the upper levels of the mine. The stopes along this vein below the 110

foot level were not accessible during this writer’s inspection of tne mine.

The other accessible stopes arc found on the 3d5 and 510 foot

levels. Evidence is present of a continuous stope from the 510 to

above the 335 foot level along a fault zone trending near north 50

degrees east with a 50 degree dip to the southeast. The vein shifts to

a more northerly direction on the 510 foot level with a dip greater

than 60 degrees to the southeast. Schrader (1947, p. 116) reported the 53

vein on the 335 foot level to be 12 feet wide, decreasing to 6 feet on the 510 foot level. At the tine of this writer’s inspection of the mine most of the ore had been removed by mining and it was not possible to tell how wide the vein had been. The ore appears to be in the form of a tabular replacement deposit along the fault cone and is probably localized by a roll or change in attitude of the fault zone.

The vein on the 110 foot level is not an extension of the ore body found on the 335 and 510 foot levels. As evidenced by the cross section on Plate 2, the ore veins are curved fault zones dipping east and generally concave upward.

Minor veins within the mine are usually only inches wide and follow minor zones of weakness such as bedding planes (?) or fractures within the dolomite.

The ore is largely oxidized to the lowest levels of the mine and

the only primary sulfides found were galena and pyrata. From produc-

tion records it is obvious that some of the slopes contained higher

grade ore than was evident during this writer’s inspection of the mine.

The highest grade ore found by this writer is where primary galena oc-

curs and probably, therefore, the high grade stopes reported by Schra-

der (1947, p. 116) contained some galena.

The results of 10 analytical samples (see appendix and Plate 2)

from within stopes in the mine show no evident zoning of mineraliza-

tion with lead, zinc, and silver values randomly distributed. Lead

values approach 80 percent where primary galena was found (sample nos.

31 and 36), and here also is where the highest silver values are found,

65 oz. per ton. Silver to lead ratios are. highest, 1:370, where this 54

primary galena is found and generally decrease until there is no silver present when lead values were less than 0,8 percent. This will be discussed in more detail in the section on oxidation of the ore deposits.

Zinc values are randomly distributed, ranging from 0.4 percent to

8 percent (sample no, 32) in a minor vein on the 335 foot level, sug- gesting that sphalerite may have been present in limited amounts as a primary mineral. Zinc to lead ratios are very irregular and show no discernible pattern. The highest gold value, 0.14 oz, per ton, is in the minor stope ori the 335 foot level also (sample no. 32). Copper is found in trace amounts throughout the mine.

Alteration of the host dolomite, associated with ore deposition, is believed to be minor and perhaps masked by later oxidation. Sili- cification is occasionally present along sene of the fault zones. A good example of this silicificatlon is in the eastern branch of the north-trending drift on the 510 foot level where, recrystallized, bleached dolomite breccia fragments are in a brownish matrix material consisting mostly of crystalline dolomite with intermixed quartz. The quartz is also found in small veinlets cutting both the matrix and breccia fragments (Figure 20). Recrystallization of the dolomite evi- dently is directly related to the silicification for in the unsilici- fied portions of the breccia zona the fragments are of dark unrecrys- tallized dolomite in a soft claylike matrix. Oxidized pyrite is disseminated throughout the entire silicified rock but there is no sign of other sulfides present. Silicified fault zones are also found outcropping at Chalk Mountain as previously described.

Roth bleached and dark dolomite are found within the mine but show

no definite patterns in relation to mineralization as the veins are

found in both. The lower two levels of the mine best exemplify the relationship between the two and hint at the possibility that the bleaching at this depth follows original, rather obscure bedding hori- zons within the dolomite. The attitudes of the apparent contact between bleached and unbleached dolomite follow a general northerly trend which coincides with the known attitudes of the dolomite bedding where it outcrops at the surface. Some minor bleaching has definitely accompanied ore deposition within the mine but it is believed that the major pervasive bleaching, caused by recrystallization, preceded ore deposition in time.

Some of the dolomite adjacent to mineralization in the lower two levels of the mine is light brown and recrystallized. This rock is completely made up of dolomite with no calcite, which contrasts with the small amount of calcite usually found in the dolomite of Chalk

Mountain, Ibis could possibly be the result of dolomitization being superimposed on the dolomite by a solution with a very low Ca/Mg ratio as described by Holland (1S&7, p. 413), but the evidence is too incon- clusive to be certain. The. brown, color may be due to the proximity of the rock to the oxidizing sulfides.

Westgate

The mineralization at Westgate is minor with no production records available. Spotty occurrences of primary sulfides are present but as at Chalk Mountain most of the surrounding occurrences are oxidized. 57

Gome evidence of ir.ineralizati.cn is present in all three of the

exposed windows of the thrust at Westgate. The southernmost window

(Figure 9) is composed of partially bleached Gabbs dolomite and con-

tains a number of small workings but very little mineralization. A few

narrow silicified zones with related weak oxidized veins are present

and follow bedding attitudes which strike northwest and dip northeast.

Secondary copper staining is associated with the veins, and indigenous limonite after pyrite is sometimes found disseminated through the

dolomite.

The middle window at Westgate, centered in section 29 (T17N,

R35E) , exposes shale, and limestone members of the Sunrise Formation.

A small group of workings, labeled !,A" workings on Plate 1, near the southern edge of the window, contains a vein reaching a foot across, striking northeast and dipping southeast. The vein contains minor amounts of urtoxidissed galena, a picked sample of which assayed 43 percent lead and 36.4 oz, per ton of silver (sample no. 45),

The northernmost and largest window at Westgate contains spotty occurrences of mineralization along its western edge near the base of the mountain range. Here the lower members of the Sunrise Formation are partially retrystallized with some skarn alteration as previously described. Breccia and fault zones are common and are host to weak veins approaching a foot across.

In an adit approximately 1,000 feet: northwest of where the diorite dike cuts across the syenite porphyry sills, labeled "B" workings, small amounts of galena, sphalerite, arsenopyrite, and pyrite are found along minute veinlets and as disseminations in altered limestone, 58

Arsenopyrite and pyrite are the earliest sulfides with galena and

gangue replacing them. Pyrite is the most abundant sulfide and is dis-

seminated as euhedral to subhedral crystals up to 1cm across throughout

the limestone. Galena and sphalerite appear contemporaneous as islands

of each appear in the other. All the sulfides contain numerous blebs

of, and appear to be replaced by gangue minerals. Galena and sphalerite are replaced to a greater extent than the t*vrite which may explain

the greater amounts of pyrite observed. The sphalerite shows orange to

red internal reflections in polished section, indicating the presence of iron and also contains a few small blebs of exsolved chalcopyrite»

The gangue or alteration minerals associated with these sulfides include quartz, zoisite, and what is believed to be secondary K-feldspar.

The zoisite occurs more frequently nearer the sulfide veinlets.

A small inclined shaft, labeled "C" workings, near the intersec- tion of the diorite dike and syenite sills contains primary galena on its dump but none was found underground. Salting of dumps was found to be common in the West.gate district.

Silver values at Westgate (see appendix and Plate 1) taken from 11 vein samples average well below 1 oz. per ton. Lead values range from a trace to 43 percent, while silver to lead ratios vary considerably with no evident pattern.

Zinc, values at Test gate are similar to those at Chalk Mountain with values ranging from a trace to 6 percent, and as at Chalk Mountain zinc to lead ratios increase as lead values decrease. Zinc values were found to be higher than lead values for over half of the sample.s, which were those samples with a total lead analyses of less than 4 percent. 59

Primary sphalerite is sparse but present at Westgate, but no secondary

zinc minerals were found.

ihe Late Paleozoic to Early Mesozoic volcanic upper plate at West-

gate has minor amounts of veining and hydrothermal alteration related

to the mineralization, but not nearly as extensive as that shown in the

Mesozoic sediments. The volcanic unit was probably much less reactive

to the hydrothermal solutions than the carbonates.

Oxidation of the Ore Deposits

Ine mineralization in the Chalk. Mountain Mine, as well as the manor veins on Chalx Mountain, are largely oxidized lead ore consisting of secondary lead minerals and limoni tes. Slopes on the 110 and 335 foot levels of the Chalk Mountain Mine contain scattered masses of galena along narrow, mostly oxidized veins. The average grain size of the galena is up to lent in diameter on the 110 foot level, where the vein averages about one foot wide (sample no. 36), while on the 335 foot level the galena averages 1mm in diameter in a small veinlet about three inches wide, (sample no. 31).

The common oxidation sequence for galena is:

galena.— anglesite~~ cerussi te— plumb ojarosite

For the most part this is well exemplified in polished sections from the Chalk Mountain Mine (Figure 21). Galena is being replaced along cleavages, fractures, and grain, boundaries by anglesite. The anglesite forms dark rythmic encrustations on the galena less than .5nun thick and often contains small islands of unoxidized galena.

The anglesite in turn i.s oxidized to banded cerussita which is 60

much wider than the anglesite layer, containing no galena remnants.

Outside this layer some of the cerussite probably goes to plumbo- jarosite, but positive identification of this mineral could not be made.

There is also some wulfenite in this zone as observed in hand specimen,

Limonite is found irj abundance outside the cerussite zone,

Both the above galena samples contain silver values near 65 oz. per ton and the silver probably occurs as substitutions within the galena lattice as no admixed silver minerals were discernible in polished section.

Sections of oxidized ore from stopes in other sections of the mine

revealed, for the most part, undifferentiated limonite. Pseudomorphs

after pyrite, or indigenous limonites, were also observed. Here the

term "limonite" can best be described as Blanchard does (1968, p. 10):

"... a collective term designating all of the reddish, yellowish, brownish, and blackish-brown supergene ferric oxide hydrate precipitates derived from decomposing iron—yielding substances in nature which have not been more specifically identified."

Most of the limonite appears to be exotic as defined by Blanchard

(1968, p. 12). The limonites contain many voids and occasional un-

identifiable boxworks. At one location on the east side of Chalk

Mountain cellular boxworks of limonite after galena were recognized.

A small adit: near the Dawes Shaft shows characteristic textures

of crystallized gels with limonitic crystal fibers arranged perpendic-

ular to the. surface of boytroidal rythmical concentric shells of

limonites (Figure 22) ,

In addition to the above there are many other minerals associated

with the oxidized ores in the Chalk Mountain Mine. The gangue 61

Figure 21. Galena^ oxidizing to anglesite which in turn is oxidizing to cerrusite (darker pocky-looking material at lower left). Reflected light, X66.

figure 22. Exotic limonite forming boyt.roidal masses in voids along an oxidized vein at Chalk Mountain. Reflected light, X52. minerals besides iimonite include calcite, dolomite, chalcedony,

quartz, and gypsum, A combination of transportation downward during

oxidation and post mineral faulting has produced many open cavities in

some of the stores in the mine and they have been subsequently filled with these, and possibly other unidentified minerals. Cerargerita,

vanadite, and copper carbonates were reported to be present by

Schrader (1947, p. 117) but were not found during the course of this

study. In a large scope on the 510 foot level of the mine a 3 foot

diameter vug. was found completely lined with small green pyromorphite

crystals on a background of orange descloizite. Both were identified by X-ray fluorescence.

Oxidation of the ores of the Chalk Mountain Mine is probably a

function of the abundance of pyrite, which is known to hasten the solu-

tion of other sulfides, and fracturing of the dolomite with resulting

easy accessibility of downward percolating groundwater, A large per-

centage. of the limonitic area is composed of voids indicating that dur-

ing and following oxidation this groundwater leached and dissolved some

of the oxidation products, carrying them away.

The original primary sulfides within the mine were probably py-

rite, galena, sphalerite, and chalcopyrite (?) in that order of abun-

dance. Pyrite was present in quantity as evidenced by the abundance cf

Iimonite. Sphalerite was probably present in limited amounts, as 0,4

percent to 3 percent zinc was found, but it oxidizes readily, yielding

an oxidation product much more soluble than that of galena. Because of

the absence of secondary zinc minerals and the abundance of Iimonite,

nowever it. is not believed that appreciable amounts of sphalerite were 63

originally present. There may have been a small amount of chalcopyrite

associated with sphalerite.

Emmons (1917, p. 374) described, in part, the oxidation of pyrite-

galena-sphalerite ore bodies in a carbonate host. The lead and much of

the silver, originally in the argentiferous galena, remain essentially

in place during oxidation for the oxidation products of galena, angle-

site and cerussrte, are only slightly soluble while the oxidation prod-

ucts of sphalerite, the zinc sulfide, are easily leached. It is be-

lieved that the lead values found at Chalk Mountain generally reflect

the original concentrations cf galena. A limited amount of leaching

and migration of the secondary' lead minerals may have taken place but

it is believed not to have been significant.

According to Boyle (1968, p. 191) silver mobility' during oxidation is generally greater than lead., depending on various factors. If it is assumed that the silver was mostly in the form of argentiferous galena then as lead values decrease silver values should decrease to a greater extent. This is exemplified by the fact that silver-lead ratios in the

Chalk Mountain Mine do decrease as lead values decrease. In an environment of a rapid neutralizer, however, such as dolomite, the acid solutions would co. travel far before they7 would neutralize and deposit their load. The silver, however, would be able to travel further than the lead and become more disseminated hence giving lower silver values.

This would also be a function of the nature of the plumbing or per- me ability and the time involved and may be the case with the steeply' dipping veins in the Chalk Mountain Mine. Here the nature of the veins is such that during oxidation the percolating ground water may take 64

longer to react with the host: dolomite and deposit its load. In other

words, the larger the ore body the more neutralizing wall rock needed

to react with the acid solutions and therefore migration may extend

further than in a smaller ore body.

The oxidation of the ore occurrences at Westgate is somewhat sim-

ilar to that at Chalk Mountain. Primary sulfides are more abundant at

Westgate than at Chalk Mountain even though the mineralization at West-

gate is much weaker than at Chalk Mountain. This may be due to the

more competent and less permeable nature of the units at Westgate.

Galena, sphalerite, arsenopyrite, and pyrite were found disseminated in

the limestone ("B" workings, Plate 1) and partially oxidized galena was

found within an adit in the middle window ("A" workings, Plate 1). The

oxidized minerals in the veins, like at Chalk Mountain, consist mostly

of limonites, secondary lead minerals, and minor copper oxides. It

seems reasonable to assume, because of the presence of primary sphal-

erite, that minor amounts of smithsonite and/or hemimorphite must be

present in the areas of oxidation even though none was found during

this writer's investigation of the district.

Ore Associated with Skaras

Some of the skarn zones along the intrusive contacts at Chalk Moun-

tain have minor amounts of iron oxide vein material associated with

them. In all cases the vein material cuts the skarn arid is therefore

later. The veins generally are smaller and not as extensive as the veins in the dolomite. Secondary copper minerals are commonly associ-

ated with these veins within the skarn and account for the higher cop-

per concentrations found here. 65

As has been previously mentioned, magnetite comprises a large per-

centage of the. s h a m in some cases but is also found, along with other

typical lime-silicate minerals, associated with vein material at some

distance from the intrusive. Near the northwest corner of section 24

there is massive magnetite and minor s k a m alteration in the dolomite along the thrust contact (Figure 23). Iron oxide veins up to 3 feet wide are found along the thrust contact and within the altered dolomite parallel to it in a small adit cutting through the thrust, proving ore deposition is post thrusting. The massive magnetite has an average grain size of about ,2mm with lixaonite alteration along fractures, grain boundaries, and as boytroidal masses filling voids. The volcanic upper plate of the thrust has been partially altered along its contact with the underlying dolomite with epidote making up to 30 percent of the rock. Tn turn this rock has been subsequently fractured and these minute fractures filled with zoisite.

The massive magnetite described above and other smaller bodies of magnetite associated with the s k a m alteration on Chalk Mountain may be considered as "companion ores" to s k a m formation in limy rocks as described by Zharikov (1970, p. 760). He believes the magnetite deposition is a facies of the slcarn-fcrming metasomatism with replacement of s k a m minerals by magnetite triggered as the result of increasing acidity of the solutions. The distribution of these magnetite ores in the. skamal zones is most probably controlled by their preferential associ- ation with exoskarns, or skarris within the dolomite. Zharikov (p. 767) also states that as the acidity of the solutions increase skamization is succeeded by the deposition of magnetites then by the deposition of Rgb

L-mas^ive magnetite / / 7 -v / etite and skarn skarn R gb

thrust contact

Oxidized zone 3 wide

45< Thrust contact‘-~-‘-i- s lig h tly mineralized

Volcanic upper plate

Assumed elevation of sta. I = 4 6 0 0 '

P-Rvs thrust contact

GEOLOGIC MAP OF THE NORTHERN THRUST AREA

R g b AT CHALK MOUNTAIN

mapped by

Dennis P. Bryan Rgb Triassic Gabbs (?) Formation bleached only

Ram Triassic Gabbs (?) Formation showing magnetite alteration For symbol explanation see plates 1 a 2 contour interval 10 P-RVS Paleozoic to Triassic volcamcs 0 40 scale I" = 40' Mineralization, strong to weak 67

su^-‘:i6e cues, chaiacterizxng the latest racies of the skarn— forming

process. This may be the case with the base metal veins associated

with the skarn zones at Chalk Mountain,

One vein, approximately 1,000 feet north of the Dawes Shaft, has a

narrow, light colored, fine-grained skarny zone in the adjacent dolo-

mite with the mineralogy complex and in part uncertain. Calcite com-

poses approximately 40 percent of the zone while euhedral to subhedral

anisotropic grossularite, up to 2mm across, composes 30 percent of the

zone, with respectively lesser amounts of magnetite, zoisite (?), talc, wollastonite (?), and sphene. The paraganesis could not be unraveled.

The presence of zoisite would indicate that the unit was nonmagnesian at this point. At least seme of the zoisite (?) appears to be an al-

teration product of the- grossularite. In t u m some of the zoisite (?) is altered to talc and some of the talc is altered to chlorite. A sma.L.L amount of spidote is found as an alteration of the grossularite.

What is believed to be earlier wollastonite occurs as minute laths disseminated throughout the rock. Very minor amounts of a hexagonal mineral were observed and may be quartz and a small amount of pyrite and secondary copper staining are also present.

Other occurrences of skarn minerals associated with veins were present at Chalk Mountain but a more detailed study of their relationships and mineralogy were not undertaken for this report.

The skarn mineralization at Westgate, as previously described, is minor but is spatially related to some of the mineralization in the district, Small weak iron oxide veins are found in the limestone unit of the Sunrise Formation which contains disseminated garnet, chiastolite, brucite, and possibly epidofce. The relationship between skarn 68

and mineralization at Westgate is uncertain but may be the same as that at Chalk Mountain. 69

DISCUSSION OF THE ORE DEPOSITS

Ore deposition at Chalk Mountain and Westgate took place some time

in >-he ratter part of the Cretaceous following erogenic activity and

intrusion of granitic rocks in the area. Hydrothermal ore bearing so-

lutions deposited their dissolved load along avenues of weakness in the

Mesozoic sediments and these ore deposits were subsequently oxidized

during the later part of the Tertiary.

Hie ore deposits of Chalk Mountain appear to be both specially and

genetically related to the Mesozoic intrusives in the district. Veins are found throughout the exposed dolomite of Chalk Mountain and all occur within 3,000 feet of either the. quartz monzonite or quartz per- pnyry intiuoive. This spacial proximity would probably be further sub- stantiated if more of the dolomite were exposed, that is if the alluvium surrounding the mountain could be stripped away. Veins do not cut either intrusive at Chalk Mountain but the fact that both veins and intrusives are found on the mountain may imply a genetic connection. The reason for there being no veins in the intrusives may be simply because the. dolomite is more permeable and reactive to hydrothermal solutions.

Hydrothermal ore-forming solutions may be considered as being the very end stages of differentiation from a cooling, solidifying host magma. The series of events leading to ore emplacement at Chalk Moun- tain could be rather straightforward when viewed in this perspective.

The episode began with the rising and solidifying of a granitic ma gma during the Cretaceous at depth beneath what is now Chalk Mountain. As emplacement continued, difrerentiation took place and the quartz mon— zonite and the later stage quartz porphyry intruded the Mesozoic 70

dolomite. Concurrent and subsequent rormation of contact metamorunic

halos in the adjacent dolomite took place including s k a m zones immedi-

ately adjacent to the intrusives and complete recrystallization of the

dolomite up to 2,000 feet away from the intrusives.

The s k a m alteration and the recrystallization producing bleaching

preceded ore deposition in time and probably represented a separate

phase of hydrothermal activity, or a facies of a differentiating fluid

that would eventually develop into an ore—former. The ore deposition

crosscuts the s k a m zones, the recrystallized dolomite, and the un-

altered dolomite which proves that ore deposition was latest. These

ore-forming solutions may therefore have been the very end stage of

fluids emanating from the solidifying intrusive and probably came from

some unknown depth where the greatest mass of the cooling intrusive is

present. -^ne c>:posute of the quartz monzonite and quartz porphyry in-

trusive s at Chalk Mountain probably represents only the top of a

larger intrusive mass at depth.

As previously described, the most prominent zone of mineralization

is found along the eastern flank of the mountain and is outside the

bleached zone where the most intensive recrystallization of the dolo-

mite took place. Local recrystallized and bleached dolomite and minor

s k a m alteration is often associated specially to these veins, though,

and implies a direct genetic connection between the. three. It is here hypothesized than all these events are genetically related and ore dep-

osition was the end stage-of magmatic differentiation with the ore-

carrying solutions migrating the furthest from the intrusive mass and depositing the.;„r ore minerals when, equilibrium with the host environ-

ment was attained.

The mineralisation in the Westgate mining district is minor but

appears to be similar to that at Chalk Mountain and probably represents

a part of the same epoch of hydrothermal activity. There is no Meso-

zoic intrusive outcropping at Westgate that could genetically be relat-

ed to the sparse mineralization but the presence of skara alteration

implies there may be such an intrusive at shallow depth. Here again

the mineralization, in some cases, has a spacial proximity to the skara alteration. The hypothesized intrusive may or may not be related to that at Cnaik Mountain two miles to the west. The mineralization at

Westgate is confined to the Triassic Gabbs Formation, as at Chalk Moun- tain, and the lower part of the Sunrise Formation of Jurassic age. If more of this lower sequence of sediments were exposed or if erosion had progressed further, rhere would probably be more evidence of mineralization in the district. Also, much of the Westgate district is cover- ed oy the unreactive upper plate of the previously described thrust fault which would mask mineralization in the Mesozoic sediments beneath.

The presence of the ore deposits cn the east flank of Chalk Moun- tain and the minor mineralization on the west flank of the range at

Westgate leads one to speculate as to the possibility of additional economic deposits beneath the alluvium of the valley separating the districts. It seems highly probable that additional mineralization, though of unknown intensity, would be encountered if the Triassic section were reached in this valley, but this would probably be at depths 72

As previously described there is no apparent 2onation of elements or ore minerals in the districts except for the fact that mineralization is weaker at Westgate than at Chalk Mountain. The total distribution and extent of mineralization cannot be discerned now because of the Basin and Range faulting and subsequent filling of the valleys. It may be inferred, from known facts in the districts however, that the center cf mineralization appears to be around the intrusives of Chalk

Mountain. If this indeed is the case, there is the possibility that zinc or copper may be found at depth if we assume that this lead mineralization may be intermediate in the paragenesis of a more extensive, but unseen, zoned ore deposit.

The veins and ore deposits of both districts are oxidized to depths of hundreds of feet and the veins now are composed of limonites and other secondary minerals. It is not believed that leaching and migration of the ore: material has been extensive as the carbonate host rock acts as an effective neutralizer. ' For this reason it is not believed that an extensive zone of secondary enrichment is present at the water table, though minor local enrichment of silver may be a possibility.

If one must ‘'pigeonhole" the ore deposits of Chalk Mountain and

Westgate into a distinct classification it may be best, from this writer's point of view and from a knowledge of this report, to say that they partially fit both the category cf igneous metamorphic deposits and mesothermal deposits as described by Park and MacDiarmid (1964). APPENDIX

Geochemical. Analyses, values reported in ounces per ton for Au and Ag

and in parts per million for Cu, Pb, and Zn unless otherwise noted,

Selected Samples from Surface mineralisation on Chalk Mountain

Sample No. Description Au Ag Cu Pb Zn

1 6" ox. vein in skarn Cu oxides 0.0 1980 125 310

2 1' Fe ox, vein in skarn and dolomite 0.0 220 250 210

3 l f Fe ox. vein 0.0 640 250 720

4 1 1/2’ Fe ox. vein dark red to black 0.04 0.20 90 300 2220

5 6" Fe ox, vein with skarn 0.30 2350 750 5750

6 i' Fe ox. vein 0.30 325 7.1% 9400

7 6M dark Fe ox. vein 0,50 700 11.5% 6.6%

8 small Fe ox. vein with skarn 0.04 71.4 300 8,5% 8100

9 minor Fe ox. mixed with magnetite and skarn 0.0 50 200 740

10 3' Fe ox. cone with skarn and magnetite 0.0 270 125 640

11 narrow Fe ox. and skarn a one 0.0 1750 150 6650

12 l f Fe ox. zone with galena minor skarn 0.02 3.6 875 33.2% 1.2%

13 1 1/2' Fe ox. vein along thrust minor magnetite and skarn 0.7 800 1700 150

14 Massive magnetite with minor Fe ox. 0.0 0.0 100 200 1040 74

Sample No. Description Au Ag Cu Pb Zn

15 Dump sample containing skarn with Fe ox. 0.6 7600 1.5% 5500

16 1* Fe ox. vein galena boxworks 0.02 3.6 360 1850 3800

17 4* Fe ox. vein minor Cu ox. staining 0.10 4.1 250 6000 1.3%

18 1 ! Fe ox. vein 0.02 6.9 370 28,9% 4.6%

19 2* Fe ox. vein with minor skarn 0.0 2.4 3080 2.7% 7350

20 Black 1* Fe ox. vein 2.0 270 5900 2.2%

21 narrow Fe ox. vein 0.08 1.9 1200 9.9% 1.5%

22 Dump sample from shaft 1.0 405 6.5% 2.3%

23 1? black Fe ox. vein 0.5 885 200 1020

24 6' Fe ox. vein 0.3 270 1200 205

25 3 ’ weak Fe ox. zone silieified 0.08 0.04 180 2.5% 360

26 6” Fe ox, vein 0.06 0.14 140 2100 2440

27 2' Fe ox. vein 0.02 3.0 270 9.3% 8750

Samples from within the Chalk Mountain Mine

28 1’ Fe ox. vein 0.3 175 5.8% 9200

29 s tope sample."random 1.3 115 1.9% 4850

30 stope sample-random 0.0 0.14 325 1.2% 5.0%

31 3“ galena and Fe ox, vein 0.02 68,5 325 74,3% 4110

32 3" Fe ox. vein-same vein as sample #31 (no galena) 0.14 0.7 675 26.1% 7.9%

33 stope sample-vein at 3* wide 0.6 75 1.2% 1.0% 75

triple No. Description Au Ag Cu Pb Zn

34 randon scope sample 0.0 75 5600 3665

35 weak 2' Fe ox. zone 0.0 0.0 81 4350 3.1%

36 scope sample from 1T wide galena vein 0.0 64.9 100 73,0% 3830

37 Fe ox. sample off wall 0.6 155 8250 2.9%

Selected samp lea from kestgate

38 small pit in yellow alteration 1.0 50 120 85

39 6" weak Fe ox. veins parallel to bedding minor s k a m 0.0 270 200 600

40 2" Fe ox. vein 0.0 0.0 1180 1.6% 5750

41 6,! Fe ox. vein 3.0 4170 3.6% 6.2%

42 wall rock in adit showing primary galena and sphalerite 0.02 0.28 120 3650 5750

43 Fe ox. 1.0 85 2.2% 2.6%

44 Fe ox. 2.0 1400 8.5% 2.2%

45 3' Fe ox. vein with galena 0.02 36.4 6840 43.0% 7200

46 1 ’ Fe ox. vein 0.0 3.9 1230 7.7% 1900

47 small pit along thrust dull yellow 0.2 50 200 250

48 6" Fe ox. vein 0.0 0.0 120 150 325 76

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