University of Nevada

Reno

Geology of the Wild Horse canyon area,

Fox Range, Washoe County, Nevada

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science

in Geology

oy 1 y v

The thesis or Barnes 3. Dixon is approved:

University of Nevada

Reno 1.1

acknowledgements

I wish to express thanks to Dr. E.R. Larson who suggest ed the problem, and whose assistance and contributions prior to and during this project have been enormous. Special app­ reciation is also extended to my family for their patience and understanding. Thanks also goes to Rod Campbell who ably typed the manuscript.

James B. Dixon iii

GEOLOGY OF THE WILD HORSE CANYON AREA,

FOX RANGE, WASHOE COUNTY, NEVADA

By

James B. Dixon

ABSTRACT

The Wild Horse canyon area includes about 41 square kilometers on the western side of the Fox Range of southern Washoe County, Nevada. The area lies about 100 airkilomet- ers north of Reno, Nevada. Wild Horse canyon provides the major drainage channel of that protion of the Fox Range and empties into the eastern side of the Smoke Creek desert playa. Rocks in the canyon area range in age from Early .Meso­ zoic to Recent. The oldest rocks exposed in the area are the Triassic-Jurassic Nightingale sequence composed of met­ amorphosed, quartz-rich, argillaceous, sandy, fine-grained elastics and intercalated limestones. The Nightingale rocks were regionally metamorphosed, folded, and faulted by late- stage Sierran batholithic intrusions in the late-Middle Cretaceous period, and locally further dynamothermally met­ amorphosed by post-batholithic granodiorite intrusions in the Late Cretaceous. The Nightingale rocks were also intrud­ ed by several possible differentiates of the granodiorite magmas during Late Cretaceous time. The Mesozoic rocks of the canyon area have undergone two episodes of deformation, with each episode resulting in the development of two distinct structural grains. The earliest episode originated and ended in the Late Mesozoic period and developed an east-west and a north-south struc­ tural grain. Late Tertiary Basin and Range faulting initia­ ted the latest deformation episode developing a northwest and a northeast structural grain and forming the Fox Range horst. Continued horsting of the Fox Range fault block along Basin and Range structures has resulted in the folding of local Late Tertiary volcanics into a broad anticline. The axis of the anticline generally parallels the north-north­ east trend of the Fox Range fault block. The Late Cretaceous to Middle Tertiary period of erosion and peneplanation is represented locally by a few outcrops of stream channel deposits developed in the peneplaned Nightin­ gale sequence. Tertiary volcanism in the area began in the lower Oligo- cene with the eruption and flows of the South Willow formation. IV

The area was volcanically quiescent from the middle of the Oligocene until the lower to middle Miocene when dacite "in­ truded Basin and Range fault planes and related structures. The volcanic flows and sediments of the Pyramid seauence were deposited from the middle Miocene to Mio-Piiocene time. Erosion has oeen continuous in the elevated canyon areas since the beginning of the Pliocene epoch. In the ad­ jacent Smoke Creek desert, pluvial lakes formed lacustrine and subaarial deposits in Quaternary time. Mineralization in the Wild Horse canyon area is limited to gold-quartz veins developed in the post-batholithic Wild Horse mine granodiorite stock. V

CONTENTS

Page

Acknowledgements ...... ii

A b s t r a c t ...... iii

List of Illustrations ...... vii

Introduction ...... 1 Location and Accessibility ...... 1 Physiography ...... 4 Climate and Vegetation ...... 5 Wildlife ...... 6 Land U s e ...... 6 Scope of Investigation ...... 6 Method of Investigation ...... 7 Previous Investigations ...... 8

Mining History ...... 9

Lithologic Sequence ...... 10 G e n e r a l ...... 10

Rock Descriptions...... 14 Nightingale Sequence ...... 14 G a b b r o ...... 17 Granodiorite ...... 18 M o n z o n i t e ...... 19 North-South Mafic Dikes ...... 21 East-West Mafic D i k e ...... 23 Stream Gravels ...... 24 South Willow Formation ...... 25 Dacite Dikes ...... 26 Pyramid Sequence ...... 28 Lake Sediments ...... 32 Quaternary Alluvium ...... 33

Structural Geology ...... 34 Regional Framework ...... 34 Fox R a n g e ...... 35 Wild Horse Canyon A r e a ...... 38 F a u l t i n g ...... 38 F o l d i n g ...... 46

Economic Geology ...... 47 Wild Horse Mine Geology ...... 47 A l t e r a t i o n ...... 50 Ground Preparation ...... 51 vi

CONTENTS

Page Geologic History ...... 52

Correlations ...... 55 Nightingale Sequence ...... 55 Granodiorite ...... 57 South Willow Formation ...... 57 Pyramid Sequence ...... 57 Lake Sediments ...... 58

Summary ...... 59

Appendix ...... 61

References Cited ...... 62 V1X

LIST OF ILLUSTRATIONS

Page

Plate 1. Geologic map of the Wild Horse Canyon area ...... In Pocket

2. Cross-sections ...... in Pocket

3. View of the front of the Fox Range at the mouth of Wild Horse c a n y o n ...... 13

4. Blue-gray mica schists along fault plane at the mouth of Wii d Horse canyon . . . 16

5 „ Bed of recrystallized limestone inter­ calated in slates and phyilites of the Nightingale sequence ...... 17

6. Sills of monzonite containing xenoliths of Nightingale rocks in lower Wild Horse

7. North-south mafic dike along old fault in lower Wild Horse canyon ...... 22

8. East-west mafic dike along older fault in lower Wild. Horse canyon ...... 2 3

9. Folded dacite dike in Basin and Range Fault plane at the front of the Fox R a n g e ...... 27

10. View of the Wild Horse granodiorite stock and the workings of the Wild Horse mine as they appear today ...... 4 8

Figure 1A. Location of the Fox Range quadrangle in Washoe County ...... 2

IB. Route map to study area ...... 2

2. Map of the Fox Range showing location of the Fox Range anticline ...... 37

3. Block diagram, showing scissor effect of ro­ tation of faults during anticlinal folding. . 41

4. Diagramatic sequential formation of Fox Range anticline ...... 43 1

INTRODUCTION

Location and Accessibility

Wild Horse canyon, in the Cottonwood mining district

is located on the western slope of the Fox Range of southern

Washoe' County, Nevada (Figure 1A). The canyon empties into

the eastern side of the playa lake of the Smoke Creek desert.

The canyon is labeled and appears near the north-central portion of the Fox Range Quadrangle of the U.S. Geological

Survey's 15 minute topographic series.

The mouth of the canyon lies approximately 100 airkilo- meters north of the city of Reno, Nevada along a bearing of

N. 6°E. By road, the mouth of the canyon is some 130 kilo­ meters from Reno.

Access to Wild Horse canyon from Reno is gained by

traveling north on State Route 33 to Pyramid Lake and turn­ ing northwest along the western shore of the lake and con­ tinuing through Sutcliffe, Nevada (Figure IB). The paved

Route 33 changes to improved dirt 13 kilometers northwest of

Sutcliffe. Travel is continued along the improved dirt route for 44 kilometers to Sand Pass at the southern-most end of the Smoke Creek desert. Useful land marks at this location are the Western Pacific Railroad's gravel pit and construc­ tion crew house to the right or east of-the road. Also a rather large gravel pile is visible almost straight ahead of the driver when positioned in the road at the crest of

Sand Pass. Travel is continued on the main improved dirt 2

IC O ' 118* MS* (14*

Figure IA. Location of Fox Range Quadrangle in Washoe County.

c JO 20 30 40 30 60 70 80 b m m m a m i m m — ■ — ■> ■ ftiii'iii« — ..n lV

KILO M E S S P S

Figure IB. Route mop to study area.

® State Route improved dirt route

Jeep frail

Wesren Pacific Railroad

Reno 1

120- road around the western side of the gravel pile for about

1.2 kilometers to the junction of an unimproved jeep trail branching eastward off the main dirt road. This unimproved jeep trail crosses the southeastern tip of the Smoke Creek desert and continues northward paralleling the Smoke Creek desert and the railroad tracks until it reaches Gerlach,

Nevada approximately 88 kilometers from it's origin at Sand

Pass. The jeep trail into Wild Horse canyon turns off the

Sand Pass-Gerlach jeep trail approximately 29 kilometers from Sand Pass, or 51 kilometers from Gerlach. The trip to

Wild Horse canyon from Reno is thus noticeably shorter by way of Sand Pass.

Wild Horse canyon itself is about 6.4 kilometers in length, and is accessible by unimproved jeep trail continuing i throughout it's length. The mouth of the canyon lies about

1.6 kilometers east of the Sand Pass-Gerlach jeep trail or about 2.7 kilometers east of the Western Pacific's Smoke

Creek siding.

Access to Wild Horse canyon is most favorably attempted during dry periods. Travel along the Sand Pass-Gerlach por­ tion of the route becomes very difficult after even moderate precipitation due to the nature of the lake sediments through which rhe jeep trail passes. Within the canyon itself travel is not so seriously impaired by precipitation as the road passes mostly through hard-surface terrains. Snow in excess of 15 centimeters could however present problems on several 4 of the canyon bottora slopes. A good rule to follow when planning trips to the canyon area is to postpone for a few days after periods of precipitation accumulating more than about 1 centimeter.

Physiography

Wild Horse canyon provides the major drainage channel off the western slope of the Fox Range and is the only canyon accessible by vehicle along this portion of the range. Wild

Horse canyon as well as smaller canyons locally contain in excess of 10 springs, many of which ultimately drain into

Wild Horse canyon which itself drains into the Smoke Creek playa. Local ranchers indicate that most of these are in­ termittent during dry periods but water has been known to flow through Wild Horse canyon continuously for a number of years. Several of the springs such as Roseberry Spring near the head of the canyon have been catalogued and marked with plaques by the U.S. Geological Survey Water Resources Divi­ sion.

The topographic expression of the canyon area should be generally considered rugged. The maximum local relief is

948 meters— the lowest elevation being 1,084 meters above sea level a.t the northwest corner of the map area, and the highest at 1,952 meters above sea level in the southeast corner of the area. The majority of the hills and mountains of the area reach 60-70% slope. Climate and Vegetation

Rainfall in this area of the Fox Range is estimated to be from 20 to 25 centimeters annually, or equivalent to that of other ranges of northern Nevada, with similar elevations and latitude. Snowfall may account for half of this estima­ ted value. Apparently no records are available on snowfall in the area but the author witnessed 12 to 15 centimeters in the canyon bottom in the Spring of 1976. Snow remains sev­ eral days or weeks on the northeast facing slopes of the area with frozen ground persisting for some time after the melting of the snow during winter months.

■The mean annual temperature for the Fox Range is approx­ imately 7.4°C. The mean average for the month of January is

-0.8°C. The mean average for July is 20.1°C.

Vegetation of the area consists as follows; several varieties of grasses thrive in the wider canyon bottoms and along the lower slopes. Basin sagebrush (Artemesia trident- ata), rabbit brush (Chrysothamnus nauseosas), plateau goose­ berry (Ribes velutinum), and bitterbrush (Pershia tridentata) cover the intermediate slopes and some higher areas depend­ ing on soil character. High slopes and mountain crests are usually dominated by Juniper (Juniperus utatiensis) . A few large cottonwoods (Pcpulus fremontii) as well as dogwood

(Cornus californica) and several species of willow (Salix spp) occur along streams in the canyon bottoms. A few’ stands of aspen (Populus tremuloldes) survive along the 6 northeast facing slope of Wild Horse canyon where water

supplied by springs is apparently continuous.

Wildlife

Wildlife in the area is dominated by wild horses (Equus

caballus) after which the canyon takes it's name. Also pop­ ulating the area are chuckar (Aiectoris graeca), mule deer

(Odocoileus hemionus), and the standard desert suite of

coyote (Canis latrans) , jackrabbits (Lepus spp) , covtontails

(Sylvilagus spp), etc. The author had the pleasure of sight­

ing three antelope (Antelocapra americana) near the mouth of

Wild Horse canyon in early 1976.

Land Use

Although there has been at least one period of mining activity in the canyon area the. land is used exclusively for

cattle grazing, at the present time. A.portion of the land

in and around Wild Horse canyon is privately owned by the

W.B. Ceresola family of Wadsworth, Nevada. The remainder of

the land in the canyon area is controlled by the Western

Pacific Railroad.

Scope of Investigation

The purpose of this s tudy is to map at a larger scale

than in the previous investigation those Mesozoic rocks ex­ posed through the Cenozoic cover in the canyon area, and to describe and correlate when possible the local stratigraphic,

structural, and economic relationships. This has been done 7 with the hope that in the future other investigators will contribute to compile a detailed description and maps of the remaining portions of the Fox Range.

Method of Investigation

The ground geology of the area was mapped at a scale of

1:12,000 on an enlargement of part of the Fox Range Quadrang­ le, U.S. Geological Survey 15 minute topographic series base.

The majority of the ground within the map area was traversed on foot to locate geologic contacts and structural features.

Aerial photographs (scale approximately 1:24,000) were used to assist in locating the continuation of structural linea­ ments and lithologic contacts. The collected information was then transferred to the topographic base by graphic meth­ ods at a final scale of 1:12,000.

Attitudes of some bedded rocks as well as several fault planes were obtained by the three-point method or the use of structure contouring. Thin sections of all Mesozoic and Ter­ tiary rocks were prepared by Western Petrographic, Inc., of

Tucson, Arizona, and studied, to more accurately describe the local lithologies.

In keeping with the efforts of the scientists of the

United Stares to convert from the English tc- the metric sys­ tem of measurement, this study has been conducted with the use of metric values. The unfamiliar reader is referred to the appendix for conversion factors pertinent to this study. 8

Approximately 41 square kilometers were mapped and de­ scribed according to these methods.

Previous Investigations

The earliest mention of the Cottonwood district was that, of Hill (1915, p. 181-184) giving a general description of the geology of the district as well as a brief summary of the Wild Horse mine geology. Hill did not include a geologic map of any of the district in his report. Overton (1947) also gave a brief description of the district and Wild Horse mine geology apparently taken after Hill, and similarly did not include maps in his report.

The first investigation of the Cottonwood district pro­ viding a geologic map was a reconnaissance study by Bonham

(1969) which included all of Washoe and Storey Counties as well. q

MINING HISTORY

The Cottonwood mining district includes all of the Fox

Range beginning just north of Pyramid Lake and between the

Smoke Creek and San Emidio deserts. The southern portion of the district lies within the Pyramid Lake Indian Reservation and consequently has not been open to location. Gerlach and

Empire are the nearest towns and railheads.

Kill (1915) states that the oldest mines of the district were located in Cottonwood Canyon, 11 kilometers north of

Pah-rum Peak, on the east side of the mountains. These mines were worked as early as the 1870's and were then 200 kilo­ meters from the nearest mills and 160 kilometers from the nearest, railroads. By 1882 as many as 100 claims were lo­ cated in the district. The Cottonwood Canyon mines have been closed, since about 1900 .

According to Hill (1915), the vein at the Wild Horse mine was located in 1902. After the discovery the mine was worked by the Washoe-Lassen Mining Company of Susanville,

California. At the time of Hill's visit to the mine in 1912, a. 5-stamp mill had been erected on the property to treat quartz ores for gold and silver. Local sources say that the

Wild Horse mine closed in the early 1920's. According to

Overton (1947) the only active mine in the district in the early 1920's was the Sano mine near the southern end of the district. 10

Records of production of the district were apparently not kept during the working years. Bonham (1969) states that the only recorded production figures are for 0.68 metric tons of lead shipped in 1929, and that the district probably yielded little over $100,000 in total values.

The district has been largely a lead-silver producer with the majority of the gold production coming from the

Wild Horse mine.

LITHOLOGIC SEQUENCE

General

Rocks exposed in the Wild Horse canyon area range in age from early Mesozoic to Recent, representing a variety of lithologies and environments of deposition.

The oldest rock unit of the area is the marine Triassic-

Jurassic Nightingale sequence composed predominantly of fine­ grained elastics with lesser amounts of carbonate sediments.

The Nightingale sequence locally has been invaded and metamorphosed by at least two granitic intrusive waves re­ lated to late Sierran batholithic emplacement. The Late

Cretaceous intrusions range in composition from gabbroic to granodioritic. The intrusions furthered metamorphism, frac­ turing, and faulting of the local Nightingale sequence rocks.

Gabbroic stocks and isolated plugh are the oldest of the two granitic intrusions. Granodiorite stocks intrude the larger gabbro bodies leaving small, isolated roof pen­ dents of gabbro within the granodiorite masses. 11

Several other intrusive rock types are present in the

canyon area. The earliest of these are well exposed in Wild

Horse canyon as irregular masses, medium to very thick sills,

and generally east-west trending dikes of monzonite. The

monzonite intrusives account for a good deal of expansion

locally of the lower portions of the Nightingale stratigraphic

column. The monzonite also carries a fair amount of iron sul- >

fide, the first notable introduction of sulfide minerals into >

the local area.

Mafic dides of basic composition crop out in and along

the planes of old north-south trending faults in the lower

portions of Wild Horse canyon.

The last of the granodioritic intrusions is a mafic

dike cropping out along an older east-west trend-fault in

lower Wild Horse canyon. The outcrop displays breccia-pipe

characteristics of cobble-sized clasts of brecciated mafic

rock. Individual hornblende crystals up to 4 centimeters

long are scattered throughout the rock as well as clots of

brecciated hornblende up to 6 centimeters long.

These three rock types may have differentiated from

the granodiorite magmas by crystal settling, becoming more mafic as the lower portions of the magma chamber were emptied.

The rocks are Late Cretaceous, or Maestrechtian in age. 12

Late Eocene aged stream channel deposits developed in the Nightingale sequence which was being peneplaned during

Late Cretaceous to early Olxgocene time.

Earliest volcanist! in the area consists of pyroxene andesite, hornblende andestite, and hornblende dacite flows cut by hornblende-biotite dacite dikes. The volcanics rest unconformably on top of pre-Tertiary rocks and are cut by later intrusions along Basin and Range structures. The later intrusives and these earliest volcanic rocks are in turn unconformably overlain by Miocene and younger volcanics.

The unit appears very similar in composition and character to the South Willow formation described by Bonham (1969) and will be sc referred to hereafter. The rocks are early

Oligocene, or Chadronian in age (Bonham, 1969).

Dike intrusions of dacitic composition outcrop conspic­ uously in and along Basin and Range structures at the front of the Fox Range in the Wild Horse canyon area. One fairly continuous dacite dike can be seen along major frontal structures of the range as far south as Pole Canyon, and past Reynard Siding to the north of Wild Horse canyon

(Plate 3) . Within, the interior of the range in the Wild

Horse canyon area the. dikes follow smaller structures which often deviate from Basin and Range trends, and also occur as isolated plugs or small sills. Since no radiometric dates are available on this rock the age is estimated based on the stratigraphic evidence locally present. The unit 13

Plate 3. View of the front of the Fox Range at the mouth of Wild Horse canyon. The mouth of Wild Horse canyon is on the right. Light-colored outcrops on the face of the hills are the dacite dike in Basin and Range faults, cropping out al­ most continuously for 16 kilometers along the front of the range. Darker rocks hosting dikes are the Nightingale sequence.

occurs within non-active or simi-active Basin and Range

structures, and is overlain by volcanic rooks younger than

the South Willow formation. Since the unit intrudes along

Basin and Range structures, that were then already present,

the maximum age should not exceed lower Miocene or Arikare-

ean. The dikes are unconformably overlain by rocks of the

Pyramid sequence which are late-middle Miocene to Mio-

Pliooene, or Barstovian to early Clarandonian, so not young­

er than Barstovian. Consequently the age of the dikes is

then lower Miocene to middle Miocene, or Arikareean to

Hsmmingfordian. 14

line youngest volcanic rocks of the Wild. House canyon area are the unnamed portion of the Pyramid sequence mention­ ed by Bonham (1969, p. 28). Locally the unit consists of approximately 600 meters of basalt, basaltic andesite, and andesite flows, and volcanic mudflow breccias with inter- bedded lenses of sandstone and volcanic breccia conclomer- ates. As indicated above, the age of the Pyramid sequence is middle Miocene to Mio-Pliocene.

Sediments deposited by Lake Lahonton are present within the portion of the map area that extends into the Smoke

Creek desert. The deposits consist primarily of silt to boulder sized clastic sediments of all local rock types, and a few eolian sand dunes. The Lake Lahonton deposits are

Wisconsin in age.

Recent alluvium is not widespread in the area and is best developed in Wild Horse canyon itself'. Alluvium is also present in a few hanging valleys and at the mouths of other smaller canyons.

Rock Descriptions

Nightingale Sequence (TrJn)

The thick sequence of Triassic-Jurassic marine meta­ sediments of southern Washoe County were informally named the Nightingale sequence by Bonham (1969) after the exposures at the southern end of the Nightingale Range. The sequence is said to contain about 1,000 meters of metamorphosed, quartz-rich, argillaceous, and sandy clastic sediments with 15

inte^_ cala i.ec limestone ana doaomite in the nearby ranges o 1

the County. In the Wild Horse canyon area only about 300 to

350 meters of Nightingale rocks can be ascertained due to

the nature of the local exposures. Neither the upper or

lower depositional contacts are exposed in the canyon area.

The original composition of the Nightingale rocks local

ly is believed to have been sandy, quartz-rich, argillaceous

fine-grained clastic marine sediments with intercalated beds

of limestone; or quartz-rich mudstones and shale with inter-

becded limestone. These rocks have been dynamically and

thermally metamorphosed to slates, phyllites, mica schists,

and recrystallized limestones and marble. The slates and

phyllites contain about 50% quartz with 25% biotite and

muscovite. Tourmaline, actinolite, andalusite, feldspar,

and opaque minerals comprise the remaining 25% of the rock.

These elastics grade into blue-gray, spotted mica schists

along major structures (Plate 4). The schists consist of

40-60% quartz, 10-30% muscovite and biotite, and 10-15%

andalusite with minor amounts of tourmaline, feldspar, ac-

tinolite, and opaque minerals.

The dark gray to black slates and phyllites of the

Nightingale sequence locally are thin to medium bedded and

highly fractured and weather to grayish-black. Fracture

surfaces are. commonly coated with iimonitic material giving many outcrops a brownish-black appearance. The high degree

of fracturing promotes mechanical weathering resulting in 16 slope debris so extensive that outcrops are locally limited.

Plate 4. Slates and phyllites of the Nightingale sequence have been dynamothermally metamorphosed to blue-gray mica schist st left of photo at the mouth of Wild Horse canyon, Dacite dike in the plane of Basin and Range fault on right of adit. Adit was driven to explore along fault/intrusive contact.

Recrystallized limestone is composed of interlocking calcite grains averaging 3 millimeters in length, and small amounts of graphite and other opaque minerals. Marbleized limestones have essentially the same mineralogy but are gen­ erally of a finer-grained texture. Calc-silicate or skarn developments do not crop out in the canyon area.

Beds of recrystallized..limestone and marble account for not more than 5% of the total Nightingale thickness and ex­ 17

posures are therefore not abundant. Marble makes up only

about 20% of the Nightingale carbonates locally. Recrvstal-

lized limestone and marble beds range from 0.3 to 1.0 meter

in thickness (Plate 5). The limestone and marble beds are

highly fractured with calcite filling the fractures and both

from gray to dark-gray, resistant ledges.

Plate 5. Bed of recrystallized limestone inter­ calated in slates and phyllites of the Nightin­ gale sequence. Limestone bed is about 0.5 meters thick. Note fracturing of limestone and enclos­ ing slates and phyllites.

Gabbro (Kgs)

Gabbroic stocks and isolated, irregular plugs were the first intrusive to invade Nightingale rocks locally. The gabbros are composed primarily of plagioclase of composition 18 greater than Anso. Well twinned, euhedral to subhedral plagxoclase crystals commonly make no 40—60% of the rock.

Ophitic clinopvroxenes compose 35-45% of the rock with oc­ casional subhedral crystals present. Brown hornblende makes up o-10'o of the rock with minor amounts of apatite, sphene, and opaque minerals.

The brownish-black gabbro weathers reddish-brown and. forms boulder-strewn slopes around the outcrops of isolated plugs. Gabbro at the Wild Horse mine exists in the form of roof pendents in the younger granodiorite stock.

Granodiorite (Kgp)

Granodiorite is the most abundant intrusive rock in the

Wild Horse canyon area with a. combined area of outcrop total­ ling nearly 3 square kilometers. Granodiorite has intruded the older gabforcic Wild Horse mine stock leaving only isolat­ ed roof pendents of the older rock. Gabbro pendents are not noted in other granodiorite outcrops.

Bonham (1969, o. 8) states that biotite-hornblende granodiorite is the only type of granodiorite found in Washoe and Storey Counties. However, it appears that locally the ratio of biotite to hornblende often decreased in favor of hornblende as the dominant mafic mineral. Euhedral to sub- edral green hornblende and greenish-brown biotite together make up 20-25% of the rock. Euhedral to subhedral plagio- clase constitutes 35-50% of the granodiorite. The plagio- cla.se is commonly zoned and twinned and shows slight seri- 19 citization in many outcrops.

The groundmass of the granodiorite is made up of fine­ grained quartz and perthitic potassium feldspar in nearly equal amounts of 10-20%. Sphene, zircon, and opaque minerals are common accessories in the rock.

The granodiorite of the canyon area is fairly uniform in texture. It is phaneritic in fine-grained groundmass throughout the area except for one small outcrop at the mar­ gin of the Wild Horse mine stock. Apparently a border phase at this locality, the rock is more of an equigranular, fine­ grained, quartz diorite.

Monzonite (Kgm)

The granodiorite of the Wild Horse canyon area grades into a monzonitic phase. The monzonite occurs as thick to very thick sills, dikes, and irregular massive bodies in the canyon itself (Plate 6). The rock has been intensely altered and unaltered outcrops are essentially nonexistent. Thin y sections of the freshest available samples show that the rock has been pervasively chloritized during an early alter­ ation phase giving a greenish coloration to unbleached out­ crops. The chloritized rock is composed of 25-35% euhedral to subhedral, zoned, partially or completely sericitized plagioclase crystals, and 10-15% subhedral to anhedral, brownish-green, chlorite-rimmed hornblende. Interstitial pyroxenes make up 5-15% of the rock with pyrite ranging from

5-7% of the composition. The groundmass is composed of very 20 fine-grained, kaolinized potash feldspar, sericitized pla- gioclase, chlorite, and a little quartz. If primary biotite was originally present in the rock it has since been chlori- tized beyond recognition.

Plate 6. Sill of monzonite containing xenoliths of Nightingale rocks in lower Wild Horse canyon.

The plagioclase crystals as well as the plagioclase in the groundmass alter directly to sericite. Sericitization begins at the core of zoned crystals and progresses outward until the entire crystal is replaced forming psuedomorphs of 21

sericite after plagioclase. Hornblende alters directly to

chlorite.

The extensive chloritization and the absence of biotite,

primary or secondary, suggest a mineralizing system of rather'/'

high sulfide content.

The majority of the monzonite outcrops are extensively

weathered and bleached. This bleaching accompanied by kao-

linization has removed the greenish coloration of the rocks

producing buff-tan outcrops. As long as the monzonites have

not undergone bleaching and kaolinization pyrite remains un­

altered. After this advanced weathering pyrite alters to

goethite or limonite.

North-South Mafic Dikes (Kgb)

Mafic dikes of basic composition crop out within the

planes of several old north-south faults in and along the walls of lower Wild Horse canyon (Plate 7). The basic com­ position of the dike differentiates of the granodioritic magmas suggests that their source was relatively low in the magma chamber where crystal settling had been accomplished to a moderately advanced stage.

The grayish-black, brownish-black weathering rock forms dikes along the fault planes from 1 to 3 meters wide. The rock is composed of 20-35% euhedral to subhedral crystals of plagioclase, 20-25% of subhedral to anhedral pyroxenes, and

20-25% subhedral to anhedral hornblende in a very fine-grain­ ed groundmass of plagioclase, quartz, and mafic minerals. Plate. 7. North-south mafic dike along an old fault of the same trend in the lower portion of Wild Eorse canyon.

The dikes are closely related in space and time to the more acid monzonitic differentiate of the granodiorite mag­ mas. The dikes crop out in the vicinity of one of the larg­ er monzonite bodies of the area. The mafic dikes in their north-south fault planes cut several monzonite sills as well as a relatively large monzonite body in lower Wild Horse canyon. The north-south faults developed soon after the in­ trusion of the monzonite and the intrusion of the mafic dikes in the fault planes must have followed soon after the north- south faulting, if not contemporaneous with faulting. 23

East-West Mafic Dike (Kgd)

The last granodioritic intrusion is a single mafic dike

exposed along the plane of an older east-west fault in the

northern one-half of section 33, T. 30 N., R. 21 E. (Plate 8).

The dark grayish—black dike ranges from 4 to 18 meters wide

and weathers dark brownish-black.

Plate 8. Dark outcrop paralleling skyline is the east-west mafic dike in lower Wild Horse canyon. North-south mafic dike cropping out in right fore­ ground.

The breccia-pipe characteristics of the rock have al­ ready been outlined above. Internally the rock consists of as much as 65% mafic minerals. Euhedral to subhedral crys­ tals of orthopyroxene compose 30-35% of the rock with euhed­ ral to subhedral hornblende crystals constituting 25-30% of 24 nhe lock. Euhedral <_o subhedral plagioclase accounts for

J —10'° t]ie composition. Quartz and opaque minerals make up about 5% each of the total composition. The groundmass oi the rock is very fine-grained and composed of quartz and mafic minerals.

A well developed flow foliation of crystals and mineral grains is recognizable in the section of the rock, the folia­ tion paralleling the sides of the dike. Some slickened sides are found along the contacts of the dike with the country rock. It is impossible to tell at the outcrop whether these reatures are a result of post-intrusive age movement along the fault or if they are striations due to flowage at the time of intrusion.

Stream Gravels (Ts)

Some stream channel deposits crop out in the Wild Horse canyon area. The deposits are developed on the Nightingale erosional surface which was being peneplaned during Late

Cretaceous to Lower Tertiary time. The deposits consist of poorly cemented, poorly sorted, silt to boulder sized clasts of all older local rocks, and. cobbles of staurolite schist and serpentinite^introduced from, outside the. area. The de­ posits are relatively small and do not exceed 5 meters in thickness and not over 800 square meters in area. Assays of the stream gravels show no gold or platinum metal values. 25

South Willow Formation (Tsw)

The South Willow formation locally consists oredominant-

ly of pyroxene andesite and hornblende andesite flows, with

a few thin flows of hornblende dacite composition. Horn- blende-biotite dacite dikes cut the flows locally.

Pyroxene andesite flows typically contain phenocrvsts of augite, hypersthene, and zoned plagioclase in approximate­

ly equal amounts. Accessory minerals are opaque, and the groundmass is composed of fine-grained pyroxenes and plagio­ clase .

Hornblende andesite consists of brown hornblende, pyro­ xene, and zoned plagioclase phenocrysts in nearly equal amounts in a fine-grained groundmass of potash feldspar, plagioclase, hornblende and quartz. Hornblende dacite flows share essentially the same mineralogy but with an increase of quartz and potash feldspar in the groundmass. Hornblende- biotite dacite dikes are also composed of the same overall mineralogy but with a notable increase in biotite and have a generally larger crystal size.

The flow rocks of the South Willow formation are non- resistant and readily form slopes in most places. The flow rocks are generally light to tan-brown at the outcrop. Dike rocks are more resistant and stand several meters above the enclosing rocks. These are slightly darker in color ranging from brown to light grayish-black. 26

T!ig oOuth Willow rormatxcn locally does not seem to con­ tain olivine basalt flows, flow breccias, or mudflow breccias as it does at it's type locality in South Willow Creek in northern Washoe County. Occasional gravel-strewn slopes littered with well rounded, pebble to cobble sized clasts suggest that the formation's stream gravel beds may be pre­ sent but no consolidated outcrops of this material were lo­ cated .

Dacite Dikes (Td)

Dikes, small sills and isolated plugs of dacite crop out extensively in the Wild Horse canyon area. Most con­ spicuous of these intrusions are large dikes generally par­ alleling the front of the Fox Range, emplaced in and along the planes of Basin and Range faults. One large dike crops out almost continuously along the western side of the range between Pole Canyon and Reynard Siding, a distance of almost

16 kilometers (Plates 3 and 9). Dacite dikes such as this one reach 'widths up to 60 meters. Apophyses of the larger dikes decrease in width to a few centimeters. Sills are generally a few centimeters to a few meters thick. Dacite plugs or irregular massive bodies commonly occupy areas of

2 to 3 hundredths of a square kilometer.

Composition of the dacite varies little over the canyon area. Overall the rock averages 5-15% euhedrai to subhedral pla.giocla.se laths up to 4 millimeters in length, and 1-4% euhedrai to subhedral hornblende and biotite set in a very 27

fine-grained matrix of quartz, and plagioclase with small amounts of potassium feldspar. At a few outcrops a slight

increase in biotite is noted with hornbiend remaining about

the same.

Plate 9. Dacite dike in Basin and Range fault plane at the front of the Fox Range in an un­ named canyon north of Wild Horse canyon. A small north-south mafic dike marked by arrows is cut by the fault and the dacite dike. Re­ current movement of fault has folded the da­ cite dike.

Dike intrusions commonly form the largest outcrops of dacite along Basin and Range related faults in the canyon 28 area, with sills and irregular masses somewhat smaller in overall dimension. However, one large sill is exposed in i_he S \< ^ of section 27, T . j 0 N . , R. 21 E . reaching thick" nesses as great as 25 meters (Plate 1 and Plate 2, section

B-B'). The dacite intrusive in section 27 discordantly followed the Basin ana Range fault in the lower portions of the Nightingale rocks, and changed to a concordant attitude m part, locally following she northwest-dipping bedding of the Nightingale rocks to form the largest concordant body of dacite in the Wild Horse canyon area. Also found in the

SWhjj of this section are massive xenoliths of Nightingale rocks enclosed in dacite.

The dacite forms resistant features at most outcrops.

Larger dikes often stand 15 to 20 meters above the surround­ ing host rocks. The rock weathers whittish-tan to yellowish- brown from cream-buff to light tan fresh rock.

Pyramid Sequence (Tps/Tpsb)

The Pyramid sequence is the youngest volcanic unit with­ in the Wild Horse canyon area and accounts for 50% of the total outcrop of the area. The name Pyramid sequence was applied by Bonham (1969) to include the Chloropagus forma­ tion, the Pyramid formation of MacJannet, and the old Gregory formation of Rose. Also included in the sequence by Bonham was an unnamed suite of volcanic and sedimentary rocks ex­ tensively cropping out in the Fox Range and other nearby ranges in southern Washoe County. 29

Ti/e unnamed portion of the Pyramid sequence exposed in

the Wild horse canyon area consists largely of basalt, ba­

saltic andesite, andesite flows, and volcanic mudflow brec­

cias with interbedded lenses of sandstone and volcanic brec-

cia conglomerates. These units reach some 600 meters in

uotal thickness near the southeast corner of the map area.

The rhyolitic and dacitic flow tuffs characteristic of the

Chloropagus, Pyramid, and Gregory formations apparently did

not extend to the Wild Horse canyon area.

Basalt flows typically weather brownish-red at the out­

crop and are grayish-black to black on freshly broken sur­

faces. Textures of the basalts range from very fine aphanitic

to phaneritic, vesicular and amygdaloidal.

Fine-grained, aphanitic basalt flows contain a small percentage (<5%) of olivine as a characteristic mineral with eubedral to subhedral plagioclase and clinopyroxene in about equal amounts in a fine-grained matrix of plagioclase, pyro­ xene, and a little pyrite. The more phaneritic basalt flows contain no visible olivine but are characterized by abundant, euhedral to subhedral plagioclase laths to 2.5 millimeters

long with euhedral to subhedral clinopyroxene in a matrix of plagioclase, pyroxene, and lesser amounts of pyrite. Both flow types are vesicular or sccriaceous at their tops with some units containing amygdules of calcite. or opaline mater­ ial. The basalt flows range from 3 to 10 meters thick. 30

Basaltic andesite and andesite flows commonly weather

to a brownish color and are grayish-brown on fresh surfaces.

Andesites are generally of the pyroxene group in most of the

flow units although some flows contain as much as 15% of

green to brown hornblende crystals averaging 4 millimeters

long.

Interbedded with, these basalt, basaltic andesite, and

andesite flows are thick sequences of volcanic mudflow brec­

cias and thinner t.uffaceous sandstones interbedded with vol­

canic breccia conglomerates. Mudflow breccias reach as much

as 50 meters of thickness in a single unit. Composition of

these are of sand to boulder sized clasts of scoriaceous ba­

salt, basaltic andesite, and andesite cemented in a mixture of mud and silt. The cementing matrix is usually soft and

friable and slopes form readily where the mudflow breccias

lie between the more resistant volcanic flow rocks. These volcanic mudflow breccias account for as much as 30% of the total Pyramid sequence thickness in the Wild Horse canyon area.

Within 60 meters of the bottom of the Pyramid sequence

in the canyon area is a unit composed of grayish-white vol­

canic breccia conglomerate with interbedded lenses of gray­

ish-white tuffaceous sandstone (Tpsb). The unit is approx­ imately 30 meters thick and serves as a convenient marker bed wThen approaching the bottom of the Pyramid sequence

locally. The unit is composed of 0.5 meter thick lenses of 31

fine-grained, well sorted, subangular, friable, sandstone of

volcanic detrital material in a tuffaceous, clayey cement

interbedded in volcanic breccia conglomerates composed of pebble to boulder sized clasts of basaltic to andesitic mat­

erial, vitrophyre and obsidian. Both members are well stra-

tified, poorly to moderatly well cemented and form slopes at most localities. Sorting is especially poor in the volcanic breccia conglomerate and the texture becomes very coarse to­ ward the top of the unit where boulders of vitrophyre reach

2 meters in diameter locally. The upper 10 meters of the unit become highly welded at several localities by the over­ lying volcanic flows.

Scattered pieces of silicified wood occur throughout the unit. One silicified log, 2 meters long and 0.5 meters in diameter was located. Regretably, no genetic data is presently available on this fossil plant.

Two volcanic vents within the Pyramid sequence are found in the Wild Horse canyon area. One forms the promin-

T~! ent hill in the NE% of section 35, T. 30 N. , R. 21 r,. , and the other in the NE% of section 4, T. 29 N. , R. 21 E. Both of these are andesitic in composition. The rocks of the vent in section 35 weather reddish-brown and are grayish-black in fresh samples. These rocks contain up to 15% of euhedral to subhedral, brownish-black hornblende crystals averaging 5 millimeters in length. Crudely developed columnar jointing is present on the southwest and northeast sides of the hill 32 containing the vent. Flow material from this vent covers approximately 12 square kilometers.

The vent in section 4 is of similar composition to that of the northern vent. However, the andesite at this loca­ tion has a more vitric groundmass and hornblende makes up only about 8-10% of the rock. Columnar jointing is not devexoped around this vent. A strong rhombehedral fracture pattern is developed in some of the rocks of this vent giv­ ing an appearance of columnar jointing when viewed from a distance. Close examination of this fracture system shows rhombehedral shaped blocks to 1 meter in length and width and 0.5 meters in thickness. This fracture pattern is re­ sulting in a weathering process by exfoliation simi- parallel to the rhomb faces producing ellipsoidal talus boulders downslope.

Lake Sediments (Ql)

Morrison and Frye (3965) have studied and fully describ­ ed the lacustrine and subaerial deposits of the Lahonton basin. The interested reader is referred to their paper for a complete account of the Pleistocene deposits.

Locally, the reworked lake sediments grade from boulder sized clasts around the margins of the lake, to silt sized material toward the center of the lake, all derived from the older adjacent source lands. The interior portions of the lake are composed mostly of clays and silts and minor amounts of sand. Near the foot of the mountains sediments consist 33

of boulder to sand sized clasts of all older local rocks

with size decreasing basinward. Terraces developed by re­

cession of the lake are still preserved along this portion

of rhe Fox Range and elsewhere throughout northwestern Nevada.

A few sand dunes are preserved in the Smoke Creek playa

near the canyon area. The dunes were probably longitudinal

in character since they are oriented normal to the prevailing

southwesterly winds. The dunes are no longer advancing and

have, been stabilized by vegetation for some time.

Tufa deposits are quite common in much of the Lahonton

basin. In the Wild Horse canyon area, tufa is developed on

the outliers of Pyramid sequence rocks down-faulted into the

lake at the southwestern corner of the map area.

Quaternary Alluvium (Qal)

As previously mentioned, alluvium is not widely devel­

oped in the Wild Horse canyon area. The lack of development

of alluvium in the area is due to the "V" shape of most of

the valleys containing narrow channels commonly eroding to

bedrock, and the high angle of the adjacent slopes. Colluvial

deposits in hanging valleys and lower-angle slopes have also been mapped as alluvium in this study.

The most extensive development of alluvium in the area is in Wild Horse canyon itself. At it's widest area, allu­ vium probably exceeds several tens of meters in thickness where the canyon intersects with the Scout Canyon drainage.

At this location a crude regolith is developed. A local 34 ranther -^-hdicated thac the slluvidl gravels in the upper por — tion of Wild Horse canyon, below the Wild Horse mine yielded small values of placer gold in the early 20th century.

Composition of the alluvium is of silt to boulder sized clasts of all older upslope rock types. Locally it is un­ consolidated and composed of angular to subangular smaller

fragments and poorly rounded cobbles and boulders.

STRUCTURAL GEOLOGY

Regional Framework

The literature on the rocks exposed in Washoe County

suggests that there have been two main periods of deformation in the area. The earliest of these was Late Mesozoic and the most recent was late Tertiary to Quaternary. During the Paleozoic, the evidence suggests that the area was the site of deposition of eugeosvnclinal facies volcanic and sedimentary rocks.

The Late Mesozoic episode of deformation is represented by complex structures developed in the pre-Tertiary rocks resulting in folding, faulting, and regional metamorphism prior to the intrusion of granitic plutons in Cretaceous time. Pluton invasion resulted in further dynamothermal metamorphism of the pre-Tertiary rocks.

The Late Tertiary deformation period resulted in the formation of the Basin and Range horst and graben system.

Two strongly defended theories on the mechanisms of Basin 35 and Range development, prevail in the literature; that pre­ dicted on regional tension (Thompson and White, 1964), and that predicted on regional compression (Moody and Hill, 1956.

Sales, 1966) .

The tensional theory suggests that the Basin and Range province was elevated during the Late Tertiary and Quaternary time. The resulting extension of the crust permits high- angle faulting to develop to release the tensional stress.

The regional compression theory relies on wrench fault tectonics relating the origin of Basin and Range structures to major strike-slip or wrench faults such as the Walker

Lane lineament. These type major strike-slip faults are for­ med by horizontal compressive forces acting within the earth's crust, with near-surface rocks responding by faulting into horsts and grabens, en-echelon to underlying or nearby shears.

It is beyond the scope of this investigation to defend or attack either of these theories. The important point is that two periods of deformation have occurred in Washoe

County, the latter most pronounced and best preserved.

Evidence of the older deformation period is often destroyed at least in part by the younger, or is often masked by Tert­ iary cover.

Fox Range

The Fox Range is a north-trending fault block, tilted to the east and bounded on the west by northwest to northeast­ trending, high-angle, Basin and Range type faults with several 36 hundred meters of dip—slip displacement. Tertiary rocks on the eastern side of the range dip as much as 30 degrees east­ ward. The eastward tilt of the range is largely responsible

j-or the exposures of pre—Tertiary rocks along the range's western, side. Several northwest to northeast-trending, high- angle faults in the interior of the range also affect Meso­

zoic exposures on the western flank.

Some folding is developed in the Fox Range. The earli­ est examples occur along major faults and adjacent to grano- diorite intrusive bodies. Bonham (1969, p. 49) states that the syntectonic granodiorite pluton at the north end of the range is the earliest local intrusion of this type and that the Mesozoic rocks that it intrudes were metamorphosed, fold­ ed, and foliated penecontemporaneously with intrusion. The granodiorite bodies in the central and southern portion of the range are post-tectonic. Bonham also concludes that the metamorphic rocks of the northern part of the range are dis­ tinctly more foliated and of a higher metamorphic grade than those of the central and southern portions of the range.

Later folding has formed from the continued horsting of the Fox Range block, resulting in the folding of Tertiary volcanics as well as underlying rocks into a broad anticline

(Figure 2 and 4) . The trend, of the axis of the fold varies from N. 10°E. to N. 40°E., with the crest of the fold gener­ ally lying near the highest portions of the Fox. Range (Fig­ ure 2) . Figure 2. Map of the Fox Range showing the approx­ imate loca.tion of, the. Fox Range anticline. Wild Horse canyon area is outlined along'bounding faults in heavy, dashed black lines. Scale 1:29,400.. 38

^'-ia.rj/ volcanic _ocks cropping out on cither side of the Fox Range at elevations as low as those of the Smoke

Creek and San Emrdio deserts , have been brought to these levels as the limbs of the anticline.

Wild Horse Canyon Area

Faulting

The Wild Horse canyon area is in the central part of the Fox Range, in the area of post tectonic granodiorite plutons and generally lower grade metamorphic rocks.

The exposures of Mesozoic rocks in the map area are primarily structurally controlled, bounded on the northeast, east, and southwest sides by high-angle Basin and Range faults. The area enclosed by these boundry faults includes some 18 square kilometers of outcrop and lends some evidence as to the character of the Late Mesozoic deformation period mentioned earlier.

The earliest Mesozoic structural features are a few east-west trending, high-angle faults such as the one exposed in the northern portions of sections 33 and 34, T. 30 N.,

R. 21 E. The western portion of this fault contains the east west mafic dike described on page 23, that intrudes along the fault plane. A few other east-west structures are ex­ posed in the canyon area but these do not contain basic in­ trusions of the above type within their planes. 39

Cutting the east-west trending faults are several gen­ erally north-south trending, high-angle faults. One of these offsets the east-west fault that contains the east—west mafic dike in the northern one-half of section 33, T. 30 N., R. 21

E., righi. laterally to the south. Right lateral offset of older east-west faults by a north-south structure is also found in the SW?s of section 35, T. 30 N., R. 21 E. These

i-h — south faults and the others of similar attitude shown on Plate 1 represent a structural alignment indicating a torque or strike—slip shear component of stress underlying the Mesozoic rocks of the canyon area..

The north-south faults contain the north-south mafic dikes described on page 21, and are cut by Basin and Range faults and dacxte dikes contained by the Basin and Range structures (Plate 9). Several of the north-south faults have undergone one or more periods of recurrent movement such as the structures in the western portions of section

2, T. 29 N., R. 21 E., and section 35, T. 30 N ., R. 21 E., and in the eastern portion of section 34, T. 30 N., R. 21 E.

These faults cut rocks as young as the South Willow formation and dacite bodies of Tertiary age. Also the Scout Canyon north-south fault and the north-south faults in the western portion of section 34, T, 30 N., R. 21 E „, have undergone recurrent movement displacing rocks of the Late Tertiary

Pyramid sequence. 40

The formation of, and the original movement along the north-south faults within the Mesozoic rocks marked the end of the Late Mesozoic deformation episode. By the time of the original movement along these structures, the granodiorite plutons as well as their earliest two differentiates had already been emplaced.

The formation of Basin and Range structures initiated the Late Tertiary to Quanternary deformation: episode. Basin and Range faulting locally began about lowest Miocene and many of the structures were intruded by dacite soon after their formation. Basin and Range related structures are now characterized by dacite intrusions with their planes. These include the Lost Creek fault and the Wild Horse fault bound­ ing the Mesozoic exposures in the canyon area. Although both these faults are Basin and Range age, the Wild Horse fault is the oldest of the two as the Lost Creek fault ter­ minates at it is intersection with the Wild Horse structure.

The Wild Horse and Lost Creek faults are both rotation­ al faults with the hinge line lying near the front of the

Fox Range in the Wild Horse canyon area. Rotational move­ ment along these bounding faults results in normal movement southeast of the hinge line, and reverse movement northwest of the line. Dip-slip naturally increases farther from the hinge line with the scissor effect of rotational movement

(Figure 3). Figure 3. Block diagram showing scissor effect of rotational movement along the Wild Horse and part of the Lost Creek fault about the rotational hinge line as the Fox Range anticline was formed. Hachured areas represent the dip-slip of the bounding faults. The area within the hachured block is the Wild Horse canyon area Mesozoic block. Southwest-facing oiane of hachured block is the Wild Horse fault; the south­ east and northeast-facing planes of the hachured block are the Lost Creek fault. 42

Rotational movement along the Wild Horse and Lost Creek faults resulted from uplift in the central part of the range that formed the Fox Range anticline. The uplift activated these bounding faults as well as the reverse Scout Canyon fault (Figure 4). Rocks of the Pyramid sequence were not displaced by the Wild Horse, Lost Creek, and Scout Canyon faults until anticlinal folding had been essentially com­ pleted. After folding had reached a maximum! these faults fr­ actured and offset Pyramid rocks. Pyramid sequence volcanics were displaced as much as 600 meters along the Lost Creek fault, and probably several hundred, meters along the eastern one-half of the Wild Horse fault, east of it's intersection with the Scout Canyon structure. Along part of the- western one-half of the Wild Horse fault, west of the Scout Canyon structure, Pyramid rocks were folded into a monocline above the Wild Horse fault. Pyramid volcanics in the monocline dip a few? degrees to the northeast on the down-dropped

(northeast.) side of the Wild Horse fault. The only exposure of the western one-half of the Wild Horse fault is in the

SW% of section 33, T. 30 N., R. 21 E. The Wild Horse fault at. this location is reverse as it crops out northwest of the hinge line.

Faulting is common in the volcanic rocks of the Pyramid sequence. Although no preferred pattern of structures is developed in the volcanics, many faults have no doubt devel-. oped in response to stresses formed during anticlinal folding. 43 Stage A

Stage B

Stage C

Stage D

Figure 4. Explanation on following page. Explanation of Figure 4.

Tertiary Rocks Mesozoic Rocks

Diagramatic sequential formation of the Fox Range anticline in cross section envolving blocks along cross section D-D', Plate 1. Not to scale. SC— Scout Canyon fault. WH— Wild Horse fault. LC— Lost Creek fault. Blocks are numbered 1-4 from southwest to northeast. Stage A; Prefolding. Scout Canyon fault present as well as Basin and Range Wild Horse and Lost Creek faults. Stage B; Uplift and folding underway. Faults become active in Mesozoic rocks. Tertiary rocks not yet shear­ ed. Block 3 moves up relative to block 2 across Wild Horse fault. Stage C; Folding is completed. Uplift continues in blocks 2 and 4 and is maximum in block 4. Faults cut volcanic rocks. Movement across Wild Horse fault changes directions with continued uplift. Stage D? Movement along faults has apparently ceased. Erosional depth and general appearance of modern topography and present lith­ ologic age relationships are expressed. n m n 5 wmm

Basin and Range structures continue to be active in the NW% of section 32, T. 30 N., R. 21 E., where downfolded Pyramid

sequence rocks have been step-faulted into the Smoke Creek playa.

Small scale gravity sliding has occurred locally involv­

ing rocks of the Pyramid sequence, resulting in displacement or detachment of small blocks of volcanic breccia conglomer­

ate (Tpsb) .. Where as this member of the sequence normally crops out 60 meters above the lower contact of the sequence,

in the NW% of section 2, T. 29 N. , R. 21 E., a block of the conglomerate lies in part on top of Nightingale rocks and partially on the lowest Pyramid basalts. The bedding in

this slide block dips steeply to the southeast.

In the canyon northeast of the Wild Horse mine a rather

large detachment block of the volcanic breccia conglomerate overlies Nightingale rocks and granodiorite. The erosional top of the block now lies at an elevation nearly equal to

the lowest depositional contact of the Pyramid sequence rocks. The block is highly fractured internally and bedding displays various attitudes. ‘ Apparently the tuffaceous and clayey cementing material of the volcanic breccia conglomerate has provided sufficient lubrication to accomodate easy down-slope sliding of this unit at these locations.

Gravity sliding has also occurred in the SE% of section

33, T. 30. N., R. 21 E., envclving lowest Pyramid sequence 46 rocks as well as volcanic breccia conglomerate (Tpsb) and a

few meters of Pyramid rocks above the conglomerate. The block at this location bounded on each side by older north-

south planes of weakness, apparently slid off the monocline developed along the Wild Horse fault as the dip of the fold

steepened.

In summary, the east—west and north-south fault systems of the Wild Horse canyon area originared during the Late

Mesozoic deformation period after the emplacement of the gabbroic and granodioritic plutons. The end of the Mesozoic deformation episode was marked by the end of the original movements of the north-south faults. Basin and Range fault­ ing formed the Fox Range and controlled the development of the Mesozoic exposures in the Wild Horse canyon area, in conjunction with Late Tertiary and Quaternary folding. Re­ current movement has taken place along several of the old north-south faults and Basin and Range strucrures since the deposition of Late Tertiary volcanic rocks.

Folding

Except for the above mentioned anticlinal and monoclinal structures, folding is not developed on a large scale in the canyon area. Drag folds in the Mesozoic rocks are locally developed along major structures (Plate 2, section B-B').

Recurrent movement of one of the major frontal Basin and

Range faults is indicated by folding of the dacite dike in the fault plane. Small flowage folds are often seen near 47 the planes of major structures, the mechanism apparently being passive-flow

ECONOMIC GEOLOGY

Wild Horse Mine Geology

Workings at the Wild Horse Mine consist of 6 adits, several shallow shafts and pits, and one open stope 6 to 9 meters deep, 0.6 to 1.2 meters wide and 30 meters long ac­ cording to Bonham (1969, p. 58-59). All the adits are now caved and most of the shafts and pits as well as the open stope are filled or partially filled by slumping of the walls

(Plate 10). Therefore, the following descriptions of the underground workings of the Mine are based largely on

Bonham's observations at the time of his visit.

The mine workings explore two mineralized fault zones which cut the granodiorite stock. The open stope explores the upper, oxidized portion of a N. 60°E. to N. 80°E. trend­ ing fault which dips steeply to the northeast. Several of the adits with their portals 120 meters or so above the can­ yon floor apparently intersect this fault plane a few tens of meters below the surface. The other mineralized fault zone has an attitude of N. 60°W., and dips 15 to 30 SW

(Bonham, 1969, p. 58) and is probably explored by the main adit near the small cabin about 6 meters above the canyon

floor judging front the geometry of the granodiorite stock and the attitude of the fatilt. Plate 10. View of the Wild Horse granodiorite stock and the workings of the Wild Horse mine as they appear today. The open stope is on the top- left of the stock. The main adit portal is just to the right of the cabin

The mineralized fault zones range in thickness from

0.6 to 6 meters wide and are fractured, sheared, and alter­ ed. Quartz stringers and veins up to 0.6 meters wide occur in the fault zones. The quartz veins and the altered gran­ odiorite contain pods and disseminations of pyrite and chalcopyrite according to Bonham. Oxidation is complete and extends to 15 meters below the present surface.

The Wild Horse mine stock is a post-batholithic, gran­ odiorite, gold-quartz vein bearing, intrusive type. It has already been established that the granodiorite plutonic rocks of the Wild Horse canyon area are post-tectonic aged while the granodiorite plutons at the north end of the Fox 4 9

Range are syntectonic m age. A samole collected by Bonham

(described on page 9 of his report) from the Granite Range batholith, a portion of which crops out at the northern tip of the Fox Range, was dated by K-Ar methods at 88.8 (±2.6)

years old, or late-Middle Cretaceous. Kistler and others (1971) have studied and divided Sierran batholithic emplacement into 5 Plutonic phases. They have determined by radiometric methods that the Cathedral Ridge portion of the batholith lying near the center of the the Sierra Nevada range is the youngest or last phase of plutonism ranging in age from 80-90 million years old, or Late Cretaceous. The

88.8 (±2.6) million year old date of the Granite Range bath­ olith of Bonham fits well, into the Cathedral Ridge age range of Kistler et al (1971) putting the age of the granodiorite pluton of the northern Fox Range at late-Middle Cretaceous to early-Late Cretaceous, of syntectonic. The post-tectonic, post-batholithic granodiorite plutons of the Wild Horse can­ yon area are required to be slightly younger than the late- phase intrusions. Probably the 88.8 million year date minus the factor of 2.. 6 million years, or 86.2 million years old would be the earliest possible estimate of the age of the post-batholithic plutons locally.

The Wild Horse mine gold-quarts veins are the only not­ able mineralization in the canyon area. Veining is poorly developed or absent in the remainder of the area, thus limit­ ing desirable sampling locations. Chip samples from selected 50 outcrops do however, show silver mineralization consistent throughout the granodioritic intrusions and their differen­ tiates, ranging from 3 to 6 grams per metric ton. These values of silver also apply to samples assayed from the late

Eocene stream channel deposits. Bonham sampled some of the mineralized zones of the Wild Horse mine. His assays re­ vealed no gold but showed up to 130 grams oer metric ton of

silver. Samples from the mine assayed during this investi­ gation similarly showed no gold values, and only 4 grams per metric ton silver. Copper in the form of fine-grained chalcopyrite assays from 3 to 9 grams oer metric ton in the area with lead at 6 grams and zinc at 4 grams per metric ton.

Assays show no tungsten or molybdenum in the canyon area.

Alteration

The most extensively altered rock in the Wild Horse canyon area is the monzonite. As mentioned earlier, the rock is pervasively chloritized and chloritization probably represents the earliest alteration phase of the rock. No biotite, secondary or primary is now present in the rock.

Such instances of absence of secondary biotite and intensive chloritization is said to indicate a high sulfide-content mineralizing system, where in the presence of abundant sul­

fide the excess iron produced during chloritization combines with sulfide to produce .pyrite as opposed to the iron form-

ing secondary biotite in the absence of sulfide (Moore, 1576, personal communications. Larson, 1976, personal communica- 51 tions) .

Most of the monzonite is thoroughly sericitized as well as chloritized. Plagioclase phenocrysts as well as plagio- clase in the groundmass in many cases is completely replaced by sericite. Potash feldspar in the groundmass has been partially kaolinized. Sericitization and kaolinization in the monzonite post-dates the early-phase chloritization.

hater bleaching and kaolinization of most of the mon­ zonite has removed the green coloration of the chloritized rock and also oxidizes pyrite to goethite and limonite.

Sericite is also found in most outcrops of granoaiorite.

Judging from Bonham's description of the veins in the Wild

Horse mine, sericitization is apparently well advanced in the granodiorite wall rock.

Ground Preparation

In the Wild Horse canyon area, the general condition of the Mesozoic ground and it's capacity to accept injections of ore-bearing fluids is excellent. The Nightingale rocks were made brittle during Mesozoic deformation and have sub­ sequently become highly fractured rendering them, very suscep- table to vein development.

The Nightingale sequence locally displays two interest­ ing stages of advancement in ground preparation involving two rock, types that intrude it. The earliest stage involved the intrusions of monzonite. The monzonite occurs as dis­ cordant bodies up to a certain level in the Nightingale 52 sequence, and then becomes concordant and remains at nearly the same level throughout it's extent, apparently restricted to that level by some resistant feature in the Nightingale rocks. Some later dacite intrusives assume a concordant nature always above the monzonite sills. These situations seem to indicate that Basin and Range or some earlier fault system was required to sufficiently fracture the resistant

Nightingale barrier to allow intrusives to exceed a certain height in the: Nightingale sequence.

Faulting has promoted ground preparation in the area by the formation of 4 prominent structural grains. In review the directions are from oldest to youngest; east-west, north- south, and the Basin and Range northwest and northeast. Re­ current movement along the latter 3 directions has continued to fracture younger flow rocks after they were deposited.

GEOLOGIC HISTORY

The Triassic-Jurassic Nightingale sequence and it's equivalents were the last eugeosynclinal facies (Osmond,

1960) deposition in Washoe County. Sierran batholithic in­ vasion began in California as early as the late-Middle Tri- assic and proceeded discontinuously until approximately the late-Middle Cretaceous. Emplacement of the granitic rocks occurred, in 5 intervals spaced at 30 million years, each epoch taking 10 to 20 million years to complete (Kistler and others, 1971). The eugeosynclinal rocks were folded, fault­ 53

ed, and regionally metamorphosed prior to the intrusion of

The granitic plutons during the Cretaceous. The Nightingale

sequence of the Wild Horse canyon area had undergone this

type of deformation prior to the emplacement of the Granite

Range granodiorite pluton at the north end of the Fox Range

in the late-Middle Cretaceous. The intrusion of late-stage

plutons such as the Granite Range batholith resulted in fur­

ther thermal and dynamic metamorphism of the Triassic-Juras­

sic aged volcanics and sediments (Bonham, 1969). Post-

batholithic age intrusions of granodioritic composition

closely followed the late-Middle Cretaceous plutons invasion,

forming post-batholithic type, gold-quartz veins such as the

Wild Horse mine deposit.

The intrusion of post-batholithic granodiorite was soon 4 followed by a period of east-west faulting in the canyon

area in Late Cretaceous time. This early faulting apparent­

ly sufficiently prepared the lower portions of the Nightin­

gale sequence for invasion of relatively large volumes of

monzonite possibly differentiated from the granodioritic

magmas. The Late Cretaceous monzonite did not exceed a

certain height in the Nightingale sequence probably because

of the state of the ground preparation existing at the time.

Immediately following the intrusion of the monzonite,

a system of north-south trending, high-angle faults develop­ ed in the canyon area in the late Cretaceous. The faults were soon after intruded by mafic dikes of basic composition 54

along their trend. The north-south faulting ended the Meso­

zoic period of structural deformation. Soon after the mafic

dikes intruded along the north-south faults, a breccia-pipe

intrusion of basic composition invaded one of the older lo­

cal east-west faults as the final intrusive of the grano-

diorite magmas.

Tertiary volcanism began in the Wild Horse canyon area

with the eruption and flows of the South Willow formation in

the lower Oligocene. The formation of the andesite flows

and dacite dikes of the South Willow formation marked the

close of the long period of erosion and peneplanation that

began locally in the Late Cretaceous and extended to the end

of the Eocene epoch.

Basin and Range faulting began in Washoe County about

earliest Miocene forming topographic highs such as the Fox

Range and lows such as the Smoke Creek desert. In the Wild

Horse canyon area, several parallel frontal Basin and Range

faults were developed as well as faults of the same age in

the interior of the range. Early Miocene dike intrusions of

dacitic composition intruded several of the Basin and Range

faults soon after the faulting began. Sills of dacite also

intruded the upper portions of the Nightingale sequence, al­ ways above the older monzonite sills. Another local period of erosion prevailed between the end of the deposition of the lower Oligocene South Willow formation and the early Miocene dacite intrusions. 55

eruptions of the Pyramid sequence volcanics began in the late-middle Miocene epoch and flows developed until the close of the Miocene. Pyramid sequence eruptions and flows marked the last period of deposition in the Wild Horse canyon area of the Fox Range. Continued horsting of the Fox Range fault block has folded the Pyramid rocks into a broad anti­ cline since their deposition. Erosion has been continuous in the elevated Wild Horse canyon area since the beginning of the Pliocene epoch.

During the Quaternary period, glacial and periglacial climatic changes resulted in lacustrine and subaerial deoosi- tion in the intermontane basins of Washoe County. The Wis­ consin aged Lake Lahonton was the last to occupy the Smoke

Creek desert playa having receded only in Recent time. The lake reworked alluvial debris forming gradational sediments from the paleo-shoreline to the center of the lake. The last recession of the lake developed terraces at various water-levels as the lake withdrew. Relatively dry periods prevailing after the lake's last recession resulted in eoli- an deposition of longitudinal sand dunes at several places around the lake.

In Recent time erosion has been continuous within the mountain range. Alluvium has been deposited in wider canyon bottoms such as in Wild Horse canyon and at the mouths of other canvons. 56

CORRELATIONS

Nightingale Sequence

Mesozoic metasediments and metavolcanics are widespread in the western Cordillera, in the Foothill region of Calif­ ornia, and in the Coast Ranges of California and Oregon.

Probably many of the Triassic-Jurassic rocks of these areas are correlative at least in part with the Washoe and Storey

County exposures of Nightingale sequence rocks. The Nightin­ gale Sequence has only recently been designated as a separ­ ate formation and only appears in the literature under the name of Nightingale sequence in the report of Bonham (1965).

Thus the sequence is not widely known at. present as a dis­ tinct lithology.

The Triassic-Jurassic metasediments of the Foothill re­ gion of California were described and divided relatively early by Lindgren (1894). Taliaferro (1942) correlated por­ tions of the Foothill rocks and Coast Range rocks of Calif­ ornia with Triassic-Jurassic rocks of southwestern Oregon.

The Nightingale rocks of western Nevada are probably correla­ tive in part with the Triassic-Jurassic rocks of these areas.

The thick sequence of Triassic-Jurassic metasediments and metavolcanics described by Noble (1962) in the Pine Nut

Range in Douglas County are believed by the author to be

Nightingale sequence rocks. Langenheim and Larson (1973) described a. similar sequence of rocks in the Virginia City area that are probably also Nightingale sequence deposits. 57

The author has examined outcrops of rocks on the western side of the Trinity Range in Pershing County that he believes are also Nightingale sequence rocks.

Granodiorite

The Granite Range granodiorite pluton at the northern end of the Fox Range is equivalent to the Cathedral Ridge aged Sierran batholith of Kistler et al (1971). The slight­

ly younger post-batholithic aged granodiorite plutons of the

Wild Horse canyon area are undoubtedly correlative with sev­ eral of the presently undated granodiorite plutons cropping out in Washoe and Storey Counties mentioned by Bonham (1969, p. 8-9).

South Willow Formation

In agreement with Bonham (1969, p. 12) parts of the

South Willow formation lithologies .locally aopear similar to the Ingalls formation of the Blairsden, California area des­

cribed by Durrell (1959). Bonham also correlated the South

Willow rocks to the Wheatland formation of the eastern Sac­

ramento Valley, and with the Cedarville series of the Warner

Range in northeastern California.

Pyramid Sequence

Bonham 61969) has widely correlated rocks of the Pyra­ mid sequence with similar aged and lithologic units of Calif­

ornia and western Nevada. According to Slemmons (1966) the

Pyramid sequence is in part correlative to the Relief Peak 58 formation of the central Sierra Nevada which includes rocks included in the Kate Peak formation of Washoe and Storey

Counties.

Lake Sediments

Morrison and Frye (1965) have widely correlated physical- stratigraphic units and geologic-climate units and soils with the rock-stratigraphic units and soils of the Lake Lahonton basin. From comparisons of the Eastern Midwest, Southern

Great Plains, Rocky Mountains, Lake Bonneville area, and the

Lake Lahonton area they have prepared two time-stratigraphic unit names for the Wisconsin aged geologic features of these areas. The suggested names are the Eetza-Churchill stage for the lower 50,000 years of the Wisconsin, and the Sehoo-

Toyeh stage for the latest 25,000 years of the Wisconsin stage. 59

SUMMARY

Several hundred meters of quartz-rich, argillaceous, clayey, clastic sediments and carbonates of the Nightingale sequence were the last marine deposition to occur in the

Wild Horse canyon area of the Cordilleran eugeosyncline.

These rocks were regionally metamorphosed to slates, phyl- li.tesf recrystallized limestones, and marble in the canyon area by Sierran batholithic emplacement- The last local in­ trusion of batholithic proportion and age was the Granite

Range bathoiith at the north end of the Fox Range intruded in the Late-Middle Cretaceous. In the Wild Horse canyon area, post-bathoiithic, gold-quartz vein bearing plutons in­ truded locally, followed by the intrusion of a series of three differentiates of the granodiorite magmas. A Late

Mesozoic deformation period occurred after the intrusion of the post-batholithic granodiorite plutons and ended before the close of the Cretaceous period.

Two periods of volcanism prevailed in the Wild Horse canyon area during the Tertiary period. The earliest orig­

inated at the beginning of the Oligocene and continued to

the middle of the Oligocene epoch, and the last began at the middle of the Miocene epoch and continued until Mio-Pliocene time.

Basin and Range faulting began locally about lower Mio­

cene and has been active, even until Recent time. Basin and

Range development initiated the Late Tertiary to Quaternary

GO deformation episode. Dacite dikes intruded many of the Basin and Range structures soon after their formation. Continued horsting along Basin and Range and older structures has fold­ ed Late Tertiary volcanics and older rocks into a broad anti­ cline in the Fox Range and a monocline in the Wild Horse canyon area.

The elevated areas of the Wild Horse canyon area as well as most of the Fox Range have been subjected to erosion- al processes since the beginning of the Pliocene. In the intermontane basins, lacustrine and subaerial deposition has occurred during the Quaternary period.

Mineralization in the canyon area is limited to the gold-quartz vein development in the granodiorite stock of the Wild Horse mine.

Several of the local rock types are correlative with lithologically similar units in western Nevada, northern

California, and southwestern Oregon. 61

APPENDIX

METRIC AND ENGLISH SYSTEM VALUES

AND CONVERSION FACTORS

linear Measurorient

Metric English Conversion 10 millimeters = 1 centimeter 2.54 centimeters = 1 inch 100 centimeters - 1 meter 1 meter - 39.4 inches 1,000 meters = 1 kilometer 1 meter = 3.28 feet 0.304 meter = 1 foot 1.6 kilometers = 1 mile

Area Measurenent

2.56 square kilometers = 1 square mile

Weight

Metric English Conversion 1.000 grams = 1 kilogram 28.4 grams = 1 ounce 1.000 kilograms - 1 metric ton 454 grams = 1 pound 908,000 grams - 1 short ton 0.908 metric ton = 1 short ton

Temperature

Fahrenheit to Celsius Celsius to Fahrenheit

°C = °F - 32 °F = 32 + 1.8 (°C) 1.8 6 2

REFERENCES CITED

Bonham, H.F., 1969, Geology and mineral deposits of Washoe and Storey Counties, Nevada: Nevada Bur. Mines Bull.70.

Durrell, C . , 1959, Tertiary stratigraphy of the Blairsden Quadrangle, Plumas County, California: California Univ. Pub. Geol. Sci., v. 34, no. 3, p. 161-192.

Hill, J.M., 1915, Some mining districts in northeastern California and northwestern Nevada: U.S. Geol. Survey Bull. 594.

Kistler. R.W., Evernden, J.F., and Shaw, H.R., 1971, Sierra Nevada plutonic cycle: Part I. Origin of composite granitic batholiths: Geol. Soc. America Bull. v. 82, no. 4, p. 853-868.

Langenheim, R.L., and Larson, E.R., 1973, Correlations of Great Basin stratigraphic units: Nevada Bur. Mines Bull. 72.

Larson, L.T., 1976, University of Nevada, Reno; personal communications.

Lindgren, W. , and Turner, H.W. , 1894 , U.S.. Geol. Survey Atlas, Placerville folio (no. 3).

Moody, J.D., and Hill, M.J., 1956, Wrench-fault tectonics: Geol. Soc. America Bull., v. 67, no. 9, p. 1207-1246.

Moore, D.G., 1976, Exxon Co., U.S.A., Tucson, Arizona; per­ sonal communications.

Noble, D.L., 1962, Geology of the Mesozoic rocks in the southern part of the Pine Nut Range, Douglas County, Nevada: Geol. Soc. America Special Paper no. 73, p. 54.

Osmond, J.C., 1960, Tectonic history of the Basin and Range Province in Utah and Nevada: Mining Eng., v. 12, no. 3, p. 251—265.

Overton, T.D., 1947, Mineral resources of Douglas, Ormsby, and Washoe Counties: Univ. Nev. Bull. no. 9, v. XLI: Geol. and Mining Series No. 46.

Sales, J.K., 1965, Structural analysis of the Basin and Range Province in terms of wrench-faulting: unpublished Ph.D. dissertauion, Univ. of Nev. 63

Slemmons, D.B., 1966, Cenozoic volcanism of the Central Sierra Nevada, California: Calif. Div. Mines Bull. 190, p. 199-214.

Taliaferro, N.L., 1942, Cenozoic history and correlation of the Jurassic of Southwestern Oregon and California: Geol. Soc. America Bull., v. 53, no. 1, p. 71-112.

Thompson, G.A., and White, D.E., 1964, Regional geology of the Steamboat Springs area, Washoe County, Nevada: U.S. Geol. Survey Prof. Paper 458-A.