• University of Nevada

Reno

Geology and Uranium Content of kiddle Tertiary Ash-Flow Tuffs in the Southern Nightingale Mountains and Northern Truckee Range, Washoe County, Nevada

A thesis submitted in partial fulfillment of the

requirements for the degree of

Master of Science in Geology

by

Julia Anne Benham ih

July 1982 1

■•its LIMRAnr

The thesis of Julia Anne Benham is approved: Tkesi ' to

/68 >%

University of Nevada

Reno

July 1982 ii

ACKNOWLEDGMENTS

Mr. Harold F. Bonham, Jr. (Nevada Bureau of Mines and

Geology) suggested this area and topic. He and Dr. Donald

C. Noble (University of Nevada-Reno) provided guidance and erudite discussions throughout the investigation, and, most importantly, taught me how to work with, map, and interpret ash-flow tuffs. Thank you.

I acknowledge Dr. L. T. Larson, Dr. J. Lintz, Dr. J.

Nelson, and Dr. E. Kersten for their help and guidance.

My graduate education was supported by an H.E.W.

Fellowship, and partial thesis funding was provided by the

Mackay Research Fund. Dr. J. Firby was responsible in both areas of financial assistance and I sincerely thank him.

Many thanks to M. Todd, who served as my Field Assis­ tant during the summer of 1981.

Conoco, Inc. granted me the use of various equipment.

Many fellow graduate students and friends provided interest and encouragement throughout the project. Thank you: Delores Cates, Nancy Wolverson, Judith Wright-Clark,

Jaye Up De Graff, Peggy Hester McGrew, Fred and Kathy

Strawson, Larry and Sue Amateis, Dick and Sue Nosker, John

Brown, Glenn Gierzycki, Steve Park, Walter Crone, Doug Bruha,

Don Hudson, and Becky Weimer-McMillion.

Above all, my deepest and sincerest thanks to Michael

R. Smith for moral and emotional support, stimulating discussions, and constructive critical review throughout the investigation. iii

DEDICATION

To: Elzena Posey Benhara, my Grandmother,

(Winter, 1892 to Summer, 1981)

tor whom this thesis is dedicated in most

respectful, thoughtful and loving memory. IV

ABSTRACT

A composite thickness of 940 meters of nine, latitic to rhyolitic, Middle Tertiary ash-flow tuff cooling units, and

200 meters of the younger, overlying Chloropagus Formation are exposed in the southern Nightingale Mountains and northern

Truckee Range, Washoe County, Nevada. The basal six Middle Tertiary ash-flow tuffs were emplaced between 29.6 and 27 million years ago, and the upper three units were emplaced between 23.0 and 20.5 million years ago. All nine ash-flows are hydrothermally altered to a minor degree. Uranium is associated with fault zones or the Middle Tertiary ash-flows, where it occurs in or near the basal porous glassy zone of the Tuff of Rattlesnake Canyon. The Chloropagus Formation consists of andesitic to basaltic lava flows with minor dacite, ash-flows, and thin sedimentary horizons. Structure throughout this area is due to the interaction between Basin and Range extension and Walker Lane strike slip movement. V

TABLE OF CONTENTS

Page ACKNOWLEDGM E N T S ...... ii

DEDICATION ...... iii

ABSTRACT...... iv

ILLUSTRATIONS ...... vii

INTRODUCTION...... 1

GENERAL STATEMENT...... 1 LOCATION AND PHYSIOGRAPHIC SETTING ...... 3 METHODS AND PROCEDURES ...... 5 PREVIOUS WORKERS ...... 6 THE ASH-FLOW TUFF COOLING UNIT CONCEPT ...... 7

REGIONAL GEOLOGY ...... 11

NATURE AND DISTRIBUTION OF ROCK TYPES...... 11 TECTONIC SETTING ...... 12

DESCRIPTIVE GEOLOGY ...... 15 GENERAL STATEMENT...... 15 MESOZOIC R O C K S ...... 16

Nightingale Sequence ...... 16 Granitic Intrusive Rocks ...... 19

CENOZOIC ROCKS ...... 22

Middle Tertiary Ash-Flow Tuffs ...... 22 General Statement...... 22 Ash-Flow Tuff Cooling Unit A ...... 28 Ash-Flow Tuff Cooling Unit B ...... 33 Age and Correlation of Ash-Flow Tuff Cooling Units A and B ...... 35 Tuff of Rattlesnake C a n y o n ...... 37 Coyote Spring T u f f ...... 44 Tuff of Jackass S p r i n g ...... 49 Ash-Flow Tuff Cooling Unit E ...... 52 Age and Correlation of Ash-Flow Tuff Cooling Unit E ...... 55 Ash-Flow Tuff Cooling Unit F ...... 55 Age and Correlation of Ash-Flow Tuff Cooling Unit F ...... 57 Chimney Springs T u f f ...... 58 VI

Page Tuff of Gary's R i d g e ...... 60

Andesite Dikes ...... 63

Chloropagus Formation...... 66 Andesite, Basalt, and Basaltic andesite. . . 69 Welded Dacite...... 70 Ash-Flow Tuffs ...... 71 Miscellaneous Sedimentary Rocks...... 77 Emplacement of the Chloropagus Formation . . 80 Basalt Flows ...... 81

Surficial Deposits ...... 82

STRUCTURAL GEOLOGY ...... 84 GENERAL...... 84 LOCAL STRUCTURE...... 84 Regional Tilting ...... 85 Unconformities ...... 86 F a u l t s ...... 87

ECONOMIC GEOLOGY ...... 92

GENERAL...... 92 URANIUM MINERALIZATION ...... 92 OTHER PROSPECTS AND MINERALIZATION ...... 96 Crosby Mine...... 97 Other Prospects...... 98

GEOLOGIC HISTORY ...... 100

RECOMMENDATIONS AND CONCLUSIONS...... 104

REFERENCES CITED ...... 107

% vii

ILLUSTRATIONS

Figures Page 1 Index M a p ...... 2

2 Generalized distribution of 34 to 17 million year old silicic ash-flow tuffs in western Nevada...... 24

3 Photograph of the southwest facing escarpment of the southern Nightingale Mountains . 25

4 Stratigraphic column of Middle Tertiary ash- flow tuff cooling units in the southern Nightingale Mountains and northern Truckee Range, Nevada ...... 26

5 Histograms of phenocryst mineralogy of Middle Tertiary ash-flow tuffs in the southern Nightingale Mountains and northern Truckee Range, Nevada ...... 27

6 Correlation chart for Middle Tertiary ash-flow T u f f s ...... 29

7 Photograph of an andesite dike...... 65

8 Schematic, generalized stratigraphic section of the Chloropagus Formation in the northern Truckee Range and southern Nightingale Mountains, Nevada ...... 68

9 Histograms showing phenocryst mineralogy of ash-flow tuffs within the Chloropagus Formation in the northern Truckee Range, Nevada...... 73

10 Photographs of the basal vitrophyre and densely welded zone of the upper ash-flow tuff cooling unit (Tcafb) within the Chloropagus Formation in the northern Truckee Range 76

Plates

1 Geology of the southern Nightingale Mountains and northern Truckee Range, Washoe County, Nevada...... in pocket

2 Interpretive Cross Sections for the southern Nightingale Mountains and northern Truckee Range, Washoe County, Nevada...... in pocket

i 1

INTRODUCTION

GENERAL STATEMENT

The ultimate purpose of this study is to determine and interpret the stratigraphy and map the extent of uranium bearing ash-flow tuff cooling units in the southern Nightin­ gale Mountains and northern Truckee Range, Nevada (Figure 1).

Additional objectives of this study include: determination not only of the Middle Tertiary ash-flow tuff stratigraphy, but that of younger volcanic rocks as well; definition of the uranium—rich ash—flow tuff cooling units and the nature of any uranium mineralization; discussion of possible models relating welding, crystallization, weathering and erosion of these units to the formation of uranium deposits; and, finally, to hypothesize a possible tectonic regime responsi­ ble for this geologic setting.

This thesis consists of a geologic map (1:12,000)

(Plate 1), interpretive cross sections (Plate 2), and descriptions, based on hand and petrographic analysis, of the various ash-flow tuff cooling units and all other rock units encountered in the thesis area.

This paper utilizes and follows the terminology of Smith

(1960) and Ross and Smith (1960); those papers which best define, describe and explain ash-flow tuffs and the cooling unit concept. CALIFORNIA 3

LOCATION AND PHYSIOGRAPHIC SETTING

Situated along the southeastern margin of Winnemucca Dry Lake, near the junction of Washoe, Pershing, and

Churchill Counties, this study area is in the southern

Nightingale Mountains and northern Truckee Range, Washoe

County, Nevada (Figure 1). The area mapped, approximately

15 square miles (24 sq. km.), lies north of the town of

Nixon, within the northeast quarter of the U.S.G.S. fifteen

minute quadrangle map of Nixon, Nevada; in parts or all of:

sections 33, 34 and 35, T.25N., R.24E. and sections 2, 3, 4,

9, 10, 11, 14, 15, 16, 17, 19, 20, 21, 22, 23, 26, 27, 28,

29, 30, 31, 32, 33, 34 and 35, T.24N., R.24E., Mount Diablo Base and Meridian (Plate 1).

The Nightingale Mountains and northern Truckee Range

are an essentially continuous north-south trending mountain

block which rises abruptly on its western side from the val­

ley occupied by Winnemucca Dry Lake (Figure 1). Elevations

range from 5,840 ft (1,780 m) in the southern Nightingale

Mountains, to slightly less than 4,000 ft (1,220 m), on the alluvial plain proximal to Winnemucca Dry Lake. The topo­ graphy in the southern Nightingale Mountains is abrupt, with steep cliffs and deeply incised stream canyons. In contrast, south of Coyote Canyon, topographic relief in the northern

Truckee Range is relatively gentle with a tendency toward more subtle, rolling hills. Topographic relief is a direct reflection of lithology. The southern Nightingale Mountains 4

are dominantly composed of ash—flow tuffs which are highly

resistant to erosion and form very steep slopes and prominent cliffs; whereas, the rocks south of Coyote Canyon, in the

northern Truckee Range are dominantly andesite, basalt, la-

tite, limestone, schist, and granite, and tend to form less

resistant hills and a much more moderate relief.

Drainage from all points of this study area is into

Winnemucca Dry Lake. All streams are intermittent, and only

one perennial spring (Creel Spring, Efc, Sec. 28, T.24N., R.24E.) exists in the entire area.

Vegetation consists of low brush such as: sagebrush,

Indian rice grass, rabbit brush, and other plants typical of

the Great Basin. A few cottonwood trees exist near the edge

of Winnemucca Dry Lake and one exists at Creel Spring. This

area is shown in Houghton and others (1975) report as part of

the steppe and grassland vegetation zone.

A variety of fauna exists in this area, including;

insects, arachnids, lizards (collard, horned, leopard, whiptail, et cetera), birds (golden eagles, western owls,

red tail hawks, crows, sparrows, seagulls, and red breasted

nut hatches), rabbits (cottontail and jackrabbits), snakes

(rattlesnakes and racers), coyote, and mustangs.

Climate in this region can be characterized as arid with high seasonal and daily temperature contrasts, with periodic high velocity winds. Typical temperatures in summer are frequently over 100°F (38°C), and winter temperature highs 5

average about 40°F (4°C). Precipitation is generally in the

form of rain, rarely as snow, and occurs as winter storms or summer showers.

This area is accessible via a county maintained unpaved

road, which travels north from Nixon, Nevada, along the east

side of Winnemucca Dry Lake (Figure 1). A few unmaintained

roads occur throughout the area, on which travel by conven­

tional passenger vehicle is not recommended (Plate 1).

METHODS AND PROCEDURES

Field work was initiated in the fall of 1980, continued

intermittently throughout the spring of 1981, and completed

during the summer months of 1981. Mapping was done on a

topographic base map at a scale of 1:12,000; made from an

enlarged portion of the U.S.G.S. 15 minute Nixon, Nevada,

quadrangle. Aerial photographs (approximately 1:40,000)

were further utilized during mapping.

Over 180 thin sections were made from all ash-flow tuff

cooling units, younger volcanic rock units, and other dis­

tinctive lithologies. Point counts were done on each ash-

flow tuff cooling unit as well as other rock units in the

area, and counts of 350 to 700 are the basis for mineralogi- cal percentages.

Radioactivity was measured via scintillometer (Mount

Sopris model Scl32) and identification of uranium minerals was aided by x-ray diffraction. Uranium analyses from 6

Bendix Field Engineering Corporation Open File Report

PGJ 037 are further utilized in this report (Hurley, et al, 1980) .

PREVIOUS WORKERS

This area was first mapped and described in minor, yet

surprisingly quite accurate, detail in 1877 as part of a

reconnaissance geologic study along the 40th parallel, from

Wyoming to the California/Nevada border (Hague and Emmons,

1877; King, 1878). I.C. Russell (1885) mentions Winnemucca

Lake (which then had a water depth of 85 ft) as part of his

monograph on the history of Lake Lahontan.

Smith and Guild (1942) described the tungsten deposits

in the Nightingale District, just east of the map area, and

recognized that Tertiary volcanics rested unconformably upon

pre-Tertiary metasedimentary rocks.

Bonham (1960) mapped the area (1:24,000) via reconnais­

sance and aerial photographs, but did not publish his data

until 1969, where it is included in his report on Washoe

County at a scale of 1:250,000. Rai (1968) mapped a larger

area, also at 1:24,000 (within which this present study is

included), with the intent to document the extent of the pre-

Tertiary metasedimentary rocks. Both Rai (1968) and Bonham

(1960; 1969) mapped the Tertiary pyroclastic volcanic rocks as the Hartford Hill Rhyolite.

Garside (1973) noted the radioactive occurrences and 7

prospects in this study area.. Most recently, the uranium

mineralization was briefly reported on, and tuffs mapped in

reconnaissance fashion, by Bendix Field Engineering Corpora­

tion (Hurley, et al, 1980). In the southern Nightingale

Mountains, several uranium claims, initially claimed in the

early 1950's., are currently held by the following: A. and G. Wilson; J. and H. Craig; J., A., and L. Moser; and, F. Wilson.

THE ASH-FLOW TUFF COOLING UNIT CONCEPT

The terminology and descriptions of ash-flow tuff cooling units used in this report are after those defined by Ross and and Smith (1960) and Smith (1960). These are classic studies of pyroclastic rocks and their terminology is commonly used by geologists working in volcanic terrains in Nevada and elsewhere.

Briefly, ash-flow tuffs are emplaced as turbulent, gaseous flows as a result of violent, explosive eruptions of siliceous magma from calderas, volcano-tectonic troughs, or fissures. Ash-flows travel, or flow, as dense, turbulent, semi-fluid mixtures of gas, very fine grained ash-particles, phenocrysts, and fragments of wall rock, torn away from the sides of the vent during eruption. They initially flow down any previously developed drainage pattern, and subsequent flows tend to smooth out and cover existing topography, effectively forming a flat plains-like geomorphology. 8

Often, ash-flow tuff units are very thick, and can be

on the order of hundreds of meters thick. Thick ash-flow

sheets, or units, are usually the result of emplacement of

several ash-flows in relatively rapid succession. Generally,

after the emplacement of ash—flows has ceased, the interior

portion of the ash—flow pile will cool together as a single

cooling unit. If volcanic activity were to resume before the

ash-flow pile had cooled completely and new ash-flows were

deposited, a compound cooling unit would result. Both simple

and compound cooling units are recognized by Ross and Smith (1960) and Smith (1960).

After emplacement, during cooling, two separate but

intimately overlapping and corresponding zones develop: welding and primary devitrification.

Welding occurs as a result of compaction due to the

lithostatic load on still hot material. Therefore, welding

is dependent on the heat retention properties of the ash-

flow and the thickness of the ash-flow. Welding reduces the

permeability and porosity of an ash-flow in zones of in­ creased compaction. Typically, welding increases downward

in the ash-flow as lithostatic load increases. In this thesis, the densely welded zone is defined as that zone where pumice and glass shards have been completely compacted, are aligned and deformed. This alignment, due to welding, is perpendicular to the direction of downward directed compaction force and is referred to as eutaxitic structure 9

(Ross and Smith, 1960; Smith, 1960).

Devitrification, or primary crystallization of glass,

occurs immediately after deposition and results in the fine

grained intergrowth of alkali feldspar + cristobalite and/or

tridymite. However, a basal vitrophyre which usually occurs

at the base of the ash—flow pile is part of the densely

welded zone, and occurs as the result of a quenching effect

of hot ash-flow tuff against an underlying cold surface.

Devitrification occurs throughout the ash-flow unit as

cooling progresses and gaseous volatiles are given off. In

the lower, densely welded zones, where porosity and perme­

ability are nearly eliminated, devitrification is a solid

state process (zone of devitrification). Upward in the ash-

flow unit, where there is a high degree of porosity and permeability, vapors are present, and the ash-flow is partial­ ly welded to nonwelded, devitrification occurs in the pres­ ence of vapors and crystallizing products fill pore spaces (vapor phase zone).

Secondary devitrification usually occurs after the ash- flow sheet is completely cool and is the result of supergene effects, and, in particular, of migration of ground water through the tuff pile.

The phenocryst content can vary vertically in ash-flow tuffs with varying mafic and felsic components. Phenocrysts represent the composition and crystallization history of the magma, and variations in phenocryst content are probably a 10

reflection of a zoned magma chamber and/or a tapping of the magma chamber at different levels (Hildreth, 1979).

Cooling units may be of large areal extent. Previously developed topography and drainages will affect not only the areal extent of an ash—flow sheet, but the welding and devitrification zonations as well. 11

REGIONAL GEOLOGY

NATURE AND DISTRIBUTION OF ROCK TYPES

Rocks in southern Washoe County, northwestern Churchill

County and southwestern Pershing County (these later two

counties are proximal to the study area) consist of Triassic/

Jurassic (?) metamorphic rocks, Cretaceous granitic rocks,

and Tertiary rhyolitic ash-flow tuffs, andesite and basalt flows.

In southern Washoe County, Miocene to Pliocene andesites, basalts, and minor intercalated sedimentary rocks are by far

the dominant rock types (Bonham, 1969). Silicic, rhyolitic ash-flow tuffs, stratigraphically the base of the Tertiary in western Nevada, are minor in exposure relative to the widespread younger Tertiary volcanics (Bonham, 1969). In southern Washoe County, Mesozoic metasedimentary rocks occur as isolated outcrops or as roof pendants.

Northwestern Churchill County, located just east of the study area, contains widespread Pliocene basalt flows and very minor exposures of Mesozoic metasedimentary and granitic rocks (Willden and Speed, 1974).

In southwesternmost Pershing County, bordering the north edge of the study area, rocks consist of Cretaceous grano- diorite with small, limited exposures of Triassic/Jurassic (?) metasedimentary rocks (Johnson, 1977). 12

TECTONIC SETTING

At the end of the Cretaceous and early part of the

Tertiary, subduction of the Pacific and Farallon Plates was

underway on the western margin of the North American Plate.

This subduction, which began in the Triassic and had probably

triggered many major orogenic events throughout the Mesozoic,

continued into the Cenozoic. This was an Andean-type margin

that had produced a relatively continuous mountain range, or highland area, (known as the Ancestral Cordillera) that

extended from Canada to Mexico (Bateman and Wahrhaftrig,

1966; Nilsen and McKee, 1979). Therefore, the widespread and quite voluminous middle to late Cenozoic volcanic rocks were deposited subaerially and were subject to topographic barriers, previously developed drainage patterns, and ero- sional processes very soon after deposition.

Axelrod (1956; 1966) documented paleoclimate and ele­ vations during the middle Tertiary, based on fossil plants found in fresh water sediments intercalated between volcanic flows. The evidence suggests that elevations in west- central Nevada did not exceed 2,000 ft and that the Sierra

Nevada to the west did not exceed 3,000 ft during this time.

Furthermore, the climate was subhumid and supported conifer, deciduous and hardwood forests (Axelrod, 1956). He also noted that a gradual change to a cooler and drier climate occurred in the later part of the Tertiary; a fact that is recorded on all continents (Axelrod, 1956). 13

Numerous workers have documented the various volcanic

episodes which occurred in the Great Basin throughout the

Cenozoic. These various volcanic episodes have been divided

into generalized groupings, based on their ages, lithology

and spatial location (Stewart and Carlson, 1976; Silberman,

et al, 1976; Stewart, 1980; et cetera). The significance

of these divisions is in the fundamental differences in

tectonic setting. The divisions are: a) 43 - 34 million

year old (MYO) rocks, dominantly andesite; b) 34 - 17 MYO

rocks, dominantly rhyolitic to quartz latitic ash-flow tuffs;

c) 17-6 MYO rocks consisting of basalts and bimodal

assemblages of basalt and rhyolite, with voluminous andesitic

volcanics in westernmost Nevada; and, d) 6 - 0 MYO basalt

flows and even lesser extrusions of rhyolite flows and domes,

and andesite flows (Stewart and Carlson, 1976; Silberman,

et al, 1976; Stewart, 1980). These divisions illustrate

that throughout the Cenozoic, the pattern of volcanism across

the Great Basin changed significantly with time, and the loci

of activity migrated west-southwest. They also confirm that

tectonic events were episodic rather than continuous (McKee, 1971; Stewart and Carlson, 1976; Stewart, 1980).

Most authors attribute the early and middle Cenozoic

volcanic activity to the convergent plate boundary, along

the continental margin to the west of the Great Basin. At

17 million years, the onset of bimodal volcanism occurred coevally with Basin and Range faulting (Cole and Armentrout, 14

1979). This latter volcanisra is believed to reflect a

"cracking" of crustal rock, allowing for the upwelling of

primary mafic magmas (Noble, 1972). The cause of this

extension is interpreted as: a) oblique tension across a

broad belt of extension resulting from rearrangement of

plate boundaries (Atwater, 1970); b) as the result of

subduction of a spreading center (McKee, 1971); c) mantle

upwelling or 'back-arc' spreading behind an active subduction

zone (although active volcanism continued in the Great Basin

after subduction ceased) (Christiansen and Lipman, 1972; Scholz, et al, 1971).

Tertiary volcanic rocks in this thesis are included within the 34 - 17 and 17-6 MYO divisions. 15

DESCRIPTIVE GEOLOGY

GENERAL STATEMENT

Rocks of Mesozoic, Middle to Late Tertiary, and

Quaternary ages are exposed in the study area. Mesozoic

rocks are metamorphic and granitic. Tertiary rocks are

dominantly volcanic, consisting of Middle Tertiary silicic

ash-flow tuffs and Late Tertiary andesite and basalt flows.

Quaternary rocks are Pleistocene lakebed sediments and

Holocene alluvial deposits (Plate 1).

Mesozoic metasedimentary and metamorphic rocks occur in

southern Washoe County as isolated outcrops or as roof

pendants (Bonham, 1969). In this study area, these rocks are

exposed mostly throughout the northern Truckee Range, some­ times in contact with later Mesozoic granitic rocks. Meso­ zoic granitic rocks are mostly exposed in the southern por­ tion of the study area and are believed to be an extension of the Sierra Nevada Batholith (Bonham, 1969; Smith, McKee, Tatlock, et al, 1971).

Tertiary volcanic rocks unconformably overlie the older,

Mesozoic rocks. Middle Tertiary (Late Oligocene to Early

Miocene) ash-flow tuffs occur dominantly in the northern portion of the map area and to a lesser extent in the southwestern portion. Nine ash-flow tuff cooling units are exposed in this study area and these tuffs attain a maximum composite thickness of 940 m.

t: 16

Late Miocene volcanic rocks are dominantly andesite and basalt flows with minor dacite lavas and pyroclastic flows, and intercalated sedimentary rocks; gravels, siltstones, and sandstones. These rocks crop out dominantly in the northern

Truckee Range, where they attain a composite thickness of

200 m and are considered to be part of the Chloropagus Formation.

Minor exposures of Pliocene basalt flows crop out in the very southern portion of the study area.

Quaternary deposits occur throughout the area.

MESOZOIC ROCKS

Nightingale Sequence (Jns)

The oldest exposed rocks in this thesis area are meta- sedimentary and metamorphic rocks (designated Jns, Plate 1).

These rocks are correlated with the rocks of the Nightingale

Sequence of Bonham (1969). The Nightingale Sequence was named for metasedimentary rocks exposed in the southern

Nightingale Mountains and northern Truckee Range (Bonham,

1969). Bonham (1969) described this sequence as dominantly metasedimentary with minor intercalated metavolcanic rocks.

Carbonate rocks of this sequence generally crop out along the western side of the study area. Sedimentary rocks and volcanic (?) rocks, which have been metamorphosed to hornfels, chlorite schist, and amphibole mica schist, occur further east along the central and eastern portions of the

t * 17

study area. These rocks occur within the northern Truckee

Range and in isolated exposures in the southern portion of the southern Nightingale Mountains. The thickness of this unit is estimated to be about 305 m, as the basal contact was not observed. The upper contact is unconformable with younger rock units. This upper contact can either be a fault contact or, more typically, a nonconformable, depositional contact on which the Tertiary volcanic section was deposited.

Rocks of the Nightingale Sequence tend to form massive outcrops.

Limestone and dolomite are very thinly 1 mm) to thickly 1 m) bedded, locally cross bedded, complexly folded, recrystallized, and are usually dark gray to blue- gray (with occasional tan). Thin calcite veinlets often crosscut the unit. When these rocks are in close proximity to intrusive contacts, they are altered to skarn (of variable thickness) consisting of garnet, wollastonite, quartz, actin- olite, epidote as late fracture coatings, and sulphides.

Locally present are sericite plus quartz veins. In the northern portion of the study area, in the Nightingale

Mountains, these carbonates become increasingly carbonaceous, weather platey and fissile, and are black to dark gray.

Quartzites are most common in the metasedimentary sequence in the northern Truckee Range. These rocks are white to tan and are intercalated with argillaceous and carbonate rocks. Hematite often coats fracture surfaces.

t i 18

The metamorphic rocks include chloritic talc, chlorite

mica schist, hornfels, and amphibole mica schist. These

rocks are typically resistant to erosion and form massive,

resistant, steep hillsides and mountains. They are light to

dark green, tan and black. Chloritic talc occurs intimately

adjacent to amphibole mica schist and weathers easily. Dark

green chlorite mica schist exhibits well developed foliation

cleavage. Hornfels that occur throughout the southern por­

tion of the field area are typically very dark green to dark

gray or even black, and are very fine grained to aphanitic

in hand sample. As seen in thin section, plagioclase laths

(z 4 mm) are intergrown with amphibole crystals 6 mm)

(now altered to sericite) and subrounded opaque crystals.

Orange to tannish-white very fine grained cryptocrystalline

lenticular bodies of quartz are intercalated within the

hornfels, and are possibly former quartz sandstone/siltstone

bodies (although all evidence of a sedimentary nature has

been obliterated). Amphibole mica schist has large horn­

blende (^ 8 mm) porphyroblasts, and in thin section, there

are two generations of crystal growth, as evidenced by two

distinct crystal sizes within one mineral phase. The

larger crystals account for 35% of the rock and include;

hornblende (40-45%), plagioclase (45-50%)(^ 2 mm), and opaque crystals (5-10%). The second generation, smaller crystals (65% of the rock) are hornblende (60-65%), opaques

(10%), and plagioclase microlites (30-35%) which are crudely 19

aligned.

Rocks of the Nightingale Sequence were deposited in a

marine environment possibly adjacent to a magmatic arc.

These rocks were later deformed and metamorphosed during late Mesozoic orogenic activity.

According to Bonham (1969), these rocks are lithologi­

cally similar (and probably correlative with) rocks wide­

spread in northern and western Pershing County, where a

fossil, dated at lowermost Jurassic, was found. Therefore,

an age of lower Jurassic is tentatively assigned to the pre-

Tertiary metasedimentary and metamorphic rocks of the

Nightingale Sequence in this study area.

Granitic Intrusive Rocks (Kg)

Granitic intrusive rocks occur in the study area and

are most abundant along the western margins (designated Kg, Plate 1).

This rock type is moderately resistant to erosion and

forms rounded hillsides which range from pinkish-tan to medium gray.

Intrusive rocks of this study area are biotite granite

and biotite-hornblende granodiorite which, in this study area,

have been grouped together, based on similar age and rock

type. These rocks are in intrusive contact with the Jurassic

Nightingale Sequence and in unconformable depositional con­ tact with overlying Tertiary volcanic rocks. A thin paleo- soil horizon occurs along the contact between these granitic 20

rocks and younger volcanics.

Average specimens of the plutonic rocks are medium to

coarse grained hypidiomorphic granular. In thin section,

granite (by far the dominant rock type of the two intrusive

types) contains large crystals of; microcline (30-40%),

anhedral quartz (30%), biotite (10%), euhedral plagioclase

(20-25%), and small, anhedral opaque crystals (5-10%).

Many plagioclase crystals have centers which have altered

to limonitic material, and a few crystals are sieved with

small biotite and opaque crystas. Even some opaque crystals are sieved with plagioclase crystals. Biotite crystals are euhedral and small (^ 1 mm). Anhedral quartz appears to fill in space between other previously developed crystals,

Quartz veining, from 2.5 cm to 60 cm wide, is common near intrusive contacts with older, Jurassic metasedimentary rocks. A knob of coarse grained white quartz, related to this intrusive, occurs uphill from a garnet skarn

(in the NEJ, Sec. 21, T.24N., R.24E.) developed in the older metasediments. In some areas, quartz veins are pod or lenticularly shaped and have associate^ hematite and limonite staining (probably the alteration product of sul­ phides) .

Equigranular granodiorite is dark gray, medium grained, and contains; hornblende (20%), biotite (15%), plagioclase

(40-45%), quartz (15%), orthoclase (15%), and opaque crystals

(less than 5%). There is a crude lineation developed in 21

this rock type as the crystals are roughly aligned in one direction. Both quartz and orthoclase crystals are anhedral, average 2 mm, and exhibit ophitic texture; that is these minerals have grown around all the other minerals present and are in optical continuity with them. Quartz crystals are often strained. Plagioclase crystals average 3 mm, and are euhedral to subhedral. Hornblende and biotite crystals tend to occur in clusters, are subhedral, and exhibit ophitic to subophitic textures, with plagioclase crystals having grown into these crystals. Hornblende crystals aver­ age 3 mm and biotite crystals 2.5 mm. Opaque crystals are small, less than 0.2 mm, are usually cubic and sieved into biotite and hornblende crystals. Occurring in minor to trace amounts are zircon and apatite as inclusions in plagio­ clase crystals.

The intrusive rocks in this study are presumed to be late Mesozoic in age, as these rocks intrude the Jurassic

Nightingale Sequence. Several authors, including Bonham

(1969), have suggested that the Mesozoic plutonic rocks of western Nevada are extensions of the Sierra Nevada Batholith.

Plutons in the Granite Range, about 45 mi (72 km) north of this thesis area, have been dated at between 91.02 and 93.48 million years old, corrected dates (Bonham, 1969; Smith, et al, 1971; Dalrymple, 1979). This indicates an age of middle

Late Cretaceous and corresponds to dates obtained from the

Sierra Nevada Batholith, to the southwest (Evernden and 22

Kistler, 1970). Therefore, granitic rocks in this study area

are assigned an age of Late Cretaceous.

CEN0Z0IC ROCKS

Middle Tertiary Ash-Flow Tuffs

General Statement

Middle Tertiary ash-flow tuffs which occur in this study area have formerly been known as the Hartford Hill

Rhyolite. These ash-flow tuffs are exposed in the south­ western Nightingale Mountains and in portions of the northern

Truckee Range, particularly near the southwestern boundary of this study area (Plate 1).

Gianella (1936) first used the name "Hartford Hill

Rhyolite" for exposures of rhyolite and quartz latite vol­ canic rocks exposed near Silver City, Nevada. This name was used in reference to the widespread, rhyolitic volcanic flows which form the base of the Tertiary section in western

Nevada. Thompson (1956) recognized the pyroclastic nature of these rocks and formally named the formation, "Hartford Hill Rhyolite Tuff."

More recently, Bingler (1973; 1977; 1978a; 1978b), working in the northern Wassuk Range and particularly at the type locality of the "Hartford Hill Rhyolite Tuff," docu­ mented a diverse sequence of quartz latitic to rhyolitic welded ash-flow tuff cooling units. Using ash-flow cooling 23

unit concepts, Bingler (1978a) found that each of these tuff units are individually mappable and came from widely separate sources. He concluded that these ash-flows are genetically unrelated and that it served no useful purpose to continue using the name, "Hartford Hill Rhyolite Tuff."

Since then, several authors have studied these volumin­ ous ash-flow tuffs in western Nevada and have added tremen­ dously to the understanding of the ash-flow tuff stratigraphy (Figure 2).

In this study area, these Middle Tertiary ash-flow tuffs were mapped and subdivided into nine individual cooling units: six ash-flows occur in the southern Nightingale Mountains and three ash-flow tuffs in the northern Truckee Range.

Five of the six ash-flow tuff cooling units are exposed along the steep, southwest facing escarpment of the southern

Nightingale Mountains, and are shown in Figure 3.

A stratigraphic column, showing the relative position of the ash-flow tuffs is illustrated in Figure 4. Although compositionally quite similar, those tuffs in the southern

Nightingale Mountains either lack or contain very small amounts of quartz as a phenocryst constituent, whereas, those tuffs in the southwestern portion of the study area contain relatively abundant, often vermiculated, quartz (Figure 5).

The ash-flow tuffs in the southern Nightingale Mountains compositionally/mineralogically correlate with ash-flow tuffs

fc 24

Figure 2. Generalized distribution of 34 to 17 million year old silicic ash-flow tuffs in western Nevada (modified after Stewart and Carlson, 1976; and Bingler, 1978a). Most of these exposures were formerly known as the Hartford Hill Rhyolite Tuff. The numbers refer to the location of recent studies which have reported on these Middle Tertiary ash-flow tuffs, and include the following: 1) This study; 2) Bon­ ham (in press); 3) Wallace (1975); 4) Hutton (1978); 5) Deino (in press); 6) Geason (1980); -7) Bingler (1977; 1978a); 8) Proffett and Proffett (1976); 9) Hudson and Oriel (1979); 10) Bingler (1973; 1978b); 11) Hardyman (1978); and, 12) Ekren, et al (1980).

100m

50 m

0 m

Figure 4. Stratigraphic column of Middle Tertiary ash-fl tuff cooling units present in the southern Nightingale Mountains and northern Truckee Range, Nevada. 27

. ^Th-u. ] -IL_ :■~n-r-T-^ Ash-Flow Tuff Cooling Unit F Chimney Springs Tuff Tuff of Gary's Ridge

LOWER UPPER

1 Tuff of Jackass Springs Ash Flow Tuff Cooling Unit E

LOWER MIDDLE UPPER

Coyote Spring Tuff

SASE LOWER MIDDLE TOP JZtu Tuff of Rattlesnake Canyon

LOWER MIDDLE UPPER

H b m n A s h - F l o w Tuff Cooling Unit A Unit B

Figure 5. Histograms of phenocryst mineralogy of Middle Tertiary ash- flow tuffs in the southern Nightin­ gale Mountains and northern Truckee Range, Nevada. 28

from 28.6 to 27.0 million years old which occur near the

base of the Middle Tertiary ash-flow tuff assemblage in and

north of the Pah Rah Range, in western Nevada. Those ash-

flow tuffs which occur in the northern Truckee Range within

the southwestern portion of the study area correlate to

younger tuffs that occur toward the top of the same ash-flow

tuff section. A correlation chart of this area to six other

ash-flow tuff sections is illustrated in Figure 6.

The compositional classification of Streckheisen (1979)

and the textural classification for ash-flow tuffs of Cook

(1965) are utilized throughout this report.

Ash-Flow Tuff Cooling Unit A (Toa)

The lowest cooling unit exposed in the southern Nightin­

gale Mountains is informally designated Ash-Flow Tuff Cooling

Unit A (Toa; Figure 4; Plates 1 and 2). This unit crops

out in nearly continuous exposure along the steep cliffs of

the southwestern Nightingale Mountains, and is a crystal-

lithic, latitic ash-flow tuff (Figures 3 and 5; Plate 1).

This unit disconformably overlies metasedimentary and

metamorphic rocks of the Nightingale Sequence, and this

contact is marked by an unconsolidated, porous ash and weathered paleosoil horizon. South of Coyote Canyon, this

unit rests nonconformably on granitic rocks, and petrified wood occurs along the contact at this locality. This unit

is overlain, disconformably, by either Ash-Flow Tuff Cooling Ill VI COOLING UNIT 6 ASH FLOW COOLING / UNIT \ UNIT 5 VII / \ V 6 II COOLING IV / y SANTIAGO UNIT 4 / TUFF OF CHIMNEY CANYON CHIMNEY TUFF OF / \ COOLING GARY'S RIDGE GARYS RIDGE y SPRINGS. \. TUFF CHIMNEY SPRINGS / UNIT 3 CHIMNEY CHIMNEY TUFF SPRINGS TUFF SPRINGS TUFF SPRINGS TUFF UNNAMED TUFF / i EUREKA ASH FLOW NINE HILL NINE COOLING _ NINE HILL / CANYON NINE UNIT A TUFF UNIT F_ TUFF \ HILL TUFF / TUFF \ V HILL TUFF / COOLING COYOTE COYOTE TUFF SPRING -'Tsir’Fnjw’l SPRING \ NINE HILL TUFF OF / UNIT TUFF UNIT E / / \ JACKASS SPG TUFF TUFF DRY TUFF OF / 1 TUFF OF ^ VALLEY TUFF OF L \ JACKASS . LENIHAN DOGSKIN PIUTE CREEK \ \ SPRING / TUFF MOUNTAIN ! CANYON A X COOLING UNNAMED COYOTE TUFF PORCUPINE o / /; UNIT D TUFF OF SPRING MOUNTAIN COOLING RATTLESNAKE ASH FLOW // TUFF TUFF ' 2 UNIT C CANYON / / f UNIT W ERN r* Torranrar \ / i u COOLING TUFF OF MICKEY HANDLE 2 2 UNIT B CANYON IIARRYS / / l/> RATTLESNAKE PASS * SPRING / / , o COOLING W CANYON 2 ■UNIT A----- TUFF i / \ \ LONG ASH FLOW TUFF UNIT B a s h f l o w VALLEY / /.' \ // ASH FLOW \ UNIT TUFF UNIT CONSTANTS // \ 1 / A 2 AMBON!

Figure 6. Correlation chart for Middle Tertiary ash-flow tuffs of (from left to right): I) Seven Lakes Mountain area (Deino, in press); II) Dogskin Mountain (Hutton, 1978); III) Virginia Mountains (Wallace, 1975); IV) Bacon Rind Flat, Pah Rah Range (Bonham, in press); V) This study); VI) Olinghouse District, Pah Rah Range (Geason, 1980); and, VII) Virginia Range (Bingler, 1978a). 30

Unit B or the Tuff of Rattlesnake Canyon. The contact with

the overlying Ash-Flow Tuff Cooling Unit B is marked by a

porous glassy zone and a change in welding, and the contact

with the Tuff of Rattlesnake Canyon is marked by deep channel­

ing and angular discordance (Figure 4; Plates 1 and 2).

Ash-Flow Tuff Cooling Unit A is a simple cooling unit,

although it consists of multiple flows. This unit has a

lower, densely welded zone which grades upward to partially

welded. In the lower, densely welded zone, the unit is

usually red both on fresh and weathered surfaces and crops

out as a steep slope or cliff. This grades upward through a

purple colored (fresh and weathered surfaces), very steep

slope former into the partially welded zone. The upper, partially to nonwelded zone is greenish yellow-orange, on both

fresh and weathered surfaces. Ash-Flow Tuff Cooling Unit A can attain a maximum thickness of approximately 260 m.

This unit is composed of: 15% pumice fragments; 20% lithic fragments; and, 50% phenocrysts, set in a devitrified matrix of fine grained ash particles and glass shards.

Pumice fragments are white to tan, average 4 cm (al­ though they can occur up to 23 cm) in length, and are com­ pacted with up to 10:1 elongation, thus imparting a well- developed eutaxitic structure to this unit. In thin section, pumice fragments are devitrified, with fine grained textures, and contain tridymite, indicative of vapor phase devitrifi— cation. In most pumice fragments that occur in the partially 31

welded zone, the vesicular nature is not recognizable due to

the high clay content (probably celadonite). Within the

densely welded zones, spherulitic structures developed in

pumice fragments have hematite within and surrounding them.

Many pumice fragments have been completely replaced by cal-

cite. In the very densely welded portion, lensoid gas

cavities are common and can be up to 30 cm in length. Very

coarse grained quartz and alkali feldspar have replaced

pumice fragments in the very densely welded portion of this

unit as well. Despite the high percentage of phenocrysts

in this ash-flow tuff, only up to 10-25% phenocrysts occur within pumice fragments.

Lithic inclusions are subrounded, average 3 cm or less

in size, and consist of Mesozoic metasedimentary and meta-

morphic fragments.

Phenocrysts constitute 22% at the base of the unit,

32% at the top of the unit, and reach a maximum of 50-54%

in the central, moderately densely welded portion (Figure 5).

Phenocrysts in this unit include: sanidine (17-35%);

plagioclase (42-52%); biotite (7-23%); hornblende (3-10%);

opaque minerals (^ 13%); and, in trace amounts, augite,

quartz, apatite, and zircon. All phenocrysts are subhedral, with some well formed faces and some broken faces. Pheno­

crysts were probably euhedral but were broken on eruption.

Plagioclase phenocrysts average 2.5 mm (or less), and they

have been at least incipiently altered to calcite and/or

4 32

montmorillonite (?). Plagioclase phenocrysts have a ten­

dency to cluster and a few are sieved with opaques. Hematite

occurs along many twin planes in plagioclase crystals. Tiny

crystals of apatite and zircon occur as inclusions in plagio­

clase and sanidine phenocrysts. Sanidine crystal content

increases from 17 to 35% (of the total phenocryst content)

from the base to the top of this unit. These crystals aver­

age 2.5 mm but can occur, rarely, as large as 4 mm. Like

plagioclase crystals, sanidine phenocrysts have altered to

calcite and clays, although not as pervasively. Many sani­

dine crystals exhibit perthitic texture and many are embayed,

while still others cluster. Biotite crystals, which occur

in relative high abundance in this unit (23%), show a de­

crease in percentage from the base upward (23% to 7%)(Figure

5). These crystals average 1 mm but can be as large as 3 mm.

Most biotite phenocrysts exhibit very dark pleochroism or are

nearly opaque along their cleavages, an indication of oxida­ tion. A few biotite crystals are pseudomorphs after horn­ blende. Minor pleochroic halos occur in biotite crystals from the central, moderately welded portion of this unit. Many biotite crystals have been altered to chlorite. Hornblende phenocrysts average 0.5 mm but can occur up to 1.5 mm. These phenocrysts are pervasively altered most commonly to chlorite, oxychlorite, and some biotite crystals. A few crystals were altered/oxidized and are opaque. In some cases, all that re­ mains of the former hornblende crystal is an opaque skeletal 33

form. Hornblende phenocryst content remains relatively

constant throughout the unit, although it is most abundant

in the central part of the unit (Figure 5). Opaque crystals

have orange halos and most are small (0.5 mm). They are

disseminated throughout the matrix, and most are anhedral.

Both quartz and augite crystals are very small (^ 0.3 mm).

The groundmass exhibits welding textures typical of

ash-flow tuffs. In partially welded zones, glass shards

retain their tricuspate shapes and are commonly only incipi-

ently devitrified. In moderately densely welded to very

densely welded zones, glass shards exhibit axiolitic

devitrification and are compacted and aligned parallel,

bending around lithic fragments and phenocrysts. Typically,

glass shards account for over 85% of the total groundmass

and the remaining amount consists of very fine grained ash,

sometimes altered to clays or even hematite or limonite.

Minor calcite veinlets crosscut this unit.

Evidence of propylitic hydrothermal alteration is seen

nearly pervasively throughout the section, although it is

more obvious toward the upper portions of the unit. The

very to moderately densely welded zones of this ash-flow are

devitrified with standard devitrification textures, but the

upper, partially welded zone has undergone vapor phase

alteration and secondary effects.

Ash-Flow Tuff Cooling Unit B (Tob)

Disconformably overlying Ash-Flow Tuff Cooling Unit A, 34

in two locations, is a maximum of approximately 26 m of

crystal-vitric latitic ash-flow tuff, designated Ash-Flow

Tuff Cooling Unit B (Tob; Figure 4; Plates 1 and 2). This unit is exposed in two isolated localities in the southern

Nightingale Mountains (Figure 3; Plate 1).

Upper and lower contacts are unconformable, and both are marked by porous glassy zones as well as significant changes in composition and welding characteristics. However, the upper contact with the Tuff of Rattlesnake Canyon exhibits pronounced angular discordance. The lower contact with Ash-Flow Tuff Cooling Unit A is marked by the weathered horizon of the upper partially welded zone of unit A and by a basal porous glassy zone. This unit forms blocky, steep slopes, is yellow-orange on fresh and weathered surfaces, and is partially welded throughout its vertical extent.

This ash-flow contains: 32% phenocrysts; 2% lithic fragments; 6% pumice fragments; and, 60% groundmass.

Phenocryst constituents include; plagioclase (44%), sanidine (35%), opaque minerals (10%), biotite (5%), horn­ blende (4%), and quartz (2%)(Figure 5). Trace amounts of apatite and zircon occur as inclusions in plagioclase. Both sanidine and plagioclase phenocrysts are 2.5-3 mm in size.

Plagioclase phenocrysts are usually unaltered, sieved with small opaque crystals, and tend to occur in clusters. Sani­ dine crystals are blocky and perthitic. Biotite crystals are z 1 mm and are ophitic with feldspar crystals. Opaque 35

and hornblende crystals are small (0.3-0.5 mm). Opaque minerals are granular and anhedral. Most hornblende pheno- crysts are altered to chlorite and clays (sericite?).

Lithic fragments are small 1 mm), rounded to angular, and consist mostly of siltstone and andesite particles.

Pumice fragments are rare, typically small 1.5 mm) and light brown, due to limonite staining. Pumice fragments are devitrified with very fine grained spherulites and are slightly compacted, up to 2:1 elongation.

The groundmass is composed of 85-90% glass shards.

Shards are incipiently devitrified and many are compacted and bent around other constituents. Between shards, the matrix is composed of very fine grained ash and dust which has been altered to clay (celadonite?).

Age and Correlation of Ash-Flow Tuff Cooling Units A and B

Ash-Flow Tuff Cooling Unit A's emplacement was re­ stricted by older, previously developed drainage and paleo- topography on Mesozoic rocks. The outcrop pattern of the contact between Ash—Flow Tuff Cooling Unit A and Mesozoic rocks indicates that there was a moderate amount of paleo- relief. However, this unit eventually buried and smoothed the older topography. Therefore, as the outcrop pattern for the contact between Ash-Flow Tuff Cooling Units A- and B indicates, unit B was probably emplaced on a relatively flat topography.

Ash-Flow Tuff Cooling Unit A is inversely zoned, with 36

the mafic components decreasing while felsic components in­

crease stratigraphically upward in the section. This suggests

a tapping of the magma chamber at deeper levels (Hildreth,

1979; Noble, 1981, personal communication).

Differences in mineralogy and welding, as well as evi­ dence of a complete cooling break between Ash-Flow Tuff

Cooling Units A and B, distinguishes these two units. How­ ever, as no channel features or paleosoil horizon were seen along the disconformity between units, this indicates that a relatively short span of time elapsed between their emplace­ ment .

Ash-Flow Tuff Cooling Unit A correlates to an unnamed unit in the Pah Rah Range, which is not exposed at the Bacon

Rind Flat section, but underlies the Tuff of Rattlesnake

Canyon (exposed at Bacon Rind Flat) elsewhere in the Pah Rah

Range (Figures 2 and 6)(Bonham, 1982, personal communication).

In this study area, both Ash-Flow Tuff Cooling Units A and B underlie the Tuff of Rattlesnake Canyon. A K/Ar age date of

28.6 +0.9 million years (sanidine) is reported for the Tuff of Rattlesnake Canyon (Bonham, in press).

Major channel features and significant erosion were observed along the disconformity between the overlying Tuff of Rattlesnake Canyon and Ash-Flow Tuff Cooling Units A and

B; where channels had eroded down through unit B into unit

A and, because Ash—Flow Tuff Cooling Unit B crops out only in a few places, possibly that unit was nearly completely 37

eroded off prior to the emplacement of the Tuff of Rattle­

snake Canyon. This suggests that a significant amount of

time elapsed between the emplacement of Ash-Flow Tuff

Cooling Units A and B and the overlying Tuff of Rattlesnake

Canyon. An estimated hiatus of perhaps one million years

occurred, thus assigning the underlying units, A and B, a

possible age of at least 29.6 to 29.0 million years respectively.

Tuff of Rattlesnake Canyon (Tore)

Disconformably overlying Ash-Flow Tuff Cooling Units A

and B, and in places disconformably overlying Mesozoic meta-

sedimentary rocks, is as much as 275 m of pumice—bearing,

quartz latitic, crystal-vitric ash-flow tuff. Bonham (in

press) has informally assigned the name, Tuff of Rattlesnake

Canyon to this tuff at Bacon Rind Flat in the Pah Rah Range

(Figures 2 and 6). In this report, the name Tuff of Rattle­

snake Canyon is used for rocks of this lithology which over­

lie Ash-Flow Tuff Cooling Units A and B, based on similar

lithology and mineralogy (Tore; Figure 4; Plates 1 and 2).

In this study area, this ash-flow tuff is, for the most part, a simple cooling unit, although a multiple flow ash- flow sheet. This unit correlates to two ash—flow cooling units at Bacon Rind Flat, the Tuff of Rattlesnake Canyon and an overlying unnamed ash-flow tuff unit (Figure 6). The lower half of this ash-flow tuff (in this study area) con­ 38

tains few biotite crystals and correlates well with the Tuff

of Rattlesnake Canyon. The upper half of this ash-flow tuff

( m this study area) contains fairly abundant biotite and

correlates well with the overlying unnamed ash-flow tuff unit

seen at Bacon Rind Flat (Figure 6). Because this ash-flow

tuff, in this area, is a simple cooling unit and correlates

well with the two units described at Bacon Rind Flat, this

indicates that the two units seen at that locality are very

close in age. Therefore, the name Tuff of Rattlesnake

Canyon is used in this area, rather than assign a new, in­ formal name to this unit.

This unit is extensively exposed along the southwestern

side of the southern Nightingale Mountains and just south of

Coyote Canyon (Figure 3; Plate 1). A zone of porous glassy

tuff, the basal most part of this unit, overlies the non to

partially welded, upper portions of Ash-Flow Tuff Cooling

Units A and/or B and defines the contact between the units.

In a few places, where Ash-Flow Tuff Cooling Units A and B

do not exist, the Tuff of Rattlesnake Canyon rests dis- conformably on older limestone and marble of the Nightingale

Sequence. A red soil horizon, approximately 1.5-3 m, and the basal porous glassy zone of this ash-flow tuff marks the con­ tact between the Tuff of Rattlesnake Canyon and the older, pre-Tertiary rocks.

The Tuff of Rattlesnake Canyon is disconformably over- lain, in most locations, by the porous glassy zone of the 39

overlying Coyote Spring Tuff. However, in one locality, a

thin paleosoil horizcm with large 1 m) stumps of petrified wood was observed.

The exposed thickness of this ash-flow tuff varies

dramatically, reflecting underlying paleotopographic troughs

and ridges, but the lithology remains constant in lateral

extent. This tuff is usually pink to brownish red (on both

fresh and weathered surfaces) although minor bleaching, to

orange, tan, or green was observed along joint surfaces.

An orange/red hydrothermal breccia was observed

crosscutting this ash—flow tuff in a few places. This breccia

zone is usually less than 1.5 m thick and is composed of

angular to rounded fragments of this ash-flow tuff in a matrix of hematitic jasperoid.

The densely welded zone of the Tuff of Rattlesnake

Canyon comprises the lower 75% of the thickness of the en­

tire section. This zone weathers to form extremely steep

cliffs with moderately to well developed columnar jointing.

The upper, partially welded zone of the Tuff of Rattlesnake

Canyon weathers into rounded slopes, typically pink, and

appears to be exfoliating. The basal portion of this unit

is composed of three very thin 1 m), discontinuous,

partially welded, but discolored and silicified, cooling units overlain by an approximately 1 m thick porous glassy zone

and a brown, black, or olive green crystal-rich 4.5-6 m

thick vitrophyre. This vitrophyre is thickest in those areas 40

where the ash-flow tuff filled paleotroughs. In such troughs

there can occur, as noted in one area, multiple vitrophyric

zones, generally a basal vitrophyre overlain by a thinner and lenticularly shaped second.

The Tuff of Rattlesnake Canyon is composed of approxi­ mately: 15-40% phenocrysts; 5-10% pumice fragments; 5% lithic fragments; and, 45-60% groundmass.

This unit is compositionally zoned felsic to mafic

(base to top). Biotite content increases from ^ 10% to approximately 20% about one-half to one-third upward in the unit, while still in the densely welded zone of this ash- flow (Figure 5). Phenocryst content varies from about 15%, at the base, to 40% near the top (Figure 5). Phenocrysts occurring in this unit include; sanidine (15-52%), plagio- clase (31-50%), biotite (^ 10-20%), opaque minerals (5-17%), and minor amounts of quartz (^ 5%) and hornblende (trace to

7%)(Figure 5). Augite, apatite, and zircon occur in trace amounts. All phenocrysts are, or probably were, well formed with many faces (euhedral) but most have been broken, prob­ ably during eruption. There is also a tendency for the phenocrysts to cluster. Both sanidine and plagioclase phenocrysts average 2 to 3 mm in size. Sanidine crystals exhibit a decrease in percent upward in the unit (Figure 5).

Many of these phenocrysts are partially argillized, others have hematite on fracture surfaces, and many are perthitic.

A few of these crystals are carlsbad and baveno twinned, and 41

are sieved with very fine grained opaques. Many feldspar

crystals are embayed. Plagioclase phenocrysts increase in

content from the base to the top of this ash-flow (Figure 5).

Most plagioclase crystals exhibit some dendritic and skeletal forms and albite and carlsbad twinning. Many plagioclase crystals are altered either incipiently or nearly completely to calcite or clay (montmorillonite?). Many of these crystals have also undergone at least partial albitization, where albite twinning has been obliterated. Minor mantling by matrix, in optical continuity with the phenocrysts, were noted around a few plagioclase crystals. A few plagioclase phenocrysts are sieved with small biotite crystals, particu­ larly near the base of this unit, and minor inclusions of zircon and apatite crystals. The most striking feature of this unit is the dramatic increase in biotite in the upper half of this ash-flow (Figure 5). Most biotite crystals average up to 2 mm, and many have been at least partially oxidized and are now opaque along fractures and their outer rim. Many exhibit a green pleochroic scheme, suggesting that those biotite phenocrysts have been chloritized.

Opaque minerals are probably magnetite and ilmenite, and are disseminated throughout the matrix. These crystals are generally small, ranging from less than 0.01 mm to less than

0.5 mm. Many opaque minerals have a near cubic shape. Quartz phenocrysts are less than 0.3 mm. Hornblende phenocrysts are less than 1 mm and are altered to calcite and sericite(?),

« 42

leaving an opaque rind. Hornblende crystal content increases

slightly from the base to the top of the ash-flow. Augite

crystals are rare and occur most commonly in the basal vitro- phyre.

Pumice fragments range in size from 4 mm to 18 cm but

average less than 5 cm. In the densely welded portions of this unit, lensoid gas cavities are common, as well as highly compacted pumice fragments. Both pumice fragments and lens­ oid gas cavities exhibit an elongation ratio of up to 20:1; imparting a well developed eutaxitic structure to the rock.

Pumice fragment color varies from white to tan at the base to lavender, white, orange, and yellow in the upper, partially welded zone of this unit. In this upper, partially welded zone, pumice fragments average 5 cm, but can occur up to

10 cm, and are usually very fluffy in appearance. In thin section, in the basal, most densely welded zone of this cooling unit, pumice fragments are devitrified completely with very coarse grained intergrowths of alkali feldspar and quartz. However, in less densely welded zones, most pumice fragments have spherulites developed within them, with a 'rind' along the inside edge of near axiolitic tex­ tural type growth. Many of these have an opaque rim (around the outside edge of the pumice fragment) consisting of hema­ tite. In the upper, partially welded zone, pumice fragments have been subjected to vapor phase devitrification, as in­ dicated by the presence of tridymite.

i 43

The content of lithic fragments increases slightly, up­ ward in the unit, from 4% to 7%. Fragments are typically

angular and less than 10 cm in length. These lithic inclu­

sions consist of (from most to least abundant); basalt,

andesite, shaley limestone, and granitic fragments.

Groundmass textures are 'typical' of thick ash-flow

tuffs (Smith, 1960; et cetera). Glass shards are smashed,

aligned, and bent around phenocrysts and lithic fragments in

densely welded zones. In the very densely welded zone,

however, glass shards are unrecognizable and the groundmass

is devitrified with very coarse grained textures (considered

almost granophyric). In the upper, partially welded zone,

glass shards are orange (plane light), tricuspate, and de­

vitrified with axiolitic textures, and are usually set in a dusty matrix high in hematite content. Minor calcite

veining occurs toward the basal portion of this unit.

Like Ash-Flow Tuff Cooling Unit A, the Tuff of Rattle­

snake Canyon’s emplacement was controlled by the pre-existing

paleotopography or erosional surface (paleotroughs and paleo-

ridges) developed on Ash-Flow Tuff Cooling Units A and B.

This unit shows normal compositional zoning, from

felsic to mafic, indicating a zoned magma chamber, with high

silica content at the top of that chamber (Hildreth, 1979).

An age of 28.6 +0.9 million years (K/Ar, sanidine) is reported for this unit at Bacon Rind Flat, in the Pah Rah

Range (Bonham, in press). This date is stratigraphically consistent with an average age of 28.1 +0.85 million years 44

obtained on the overlying Coyote Spring Tuff, also at Bacon

Rind Flat (Garside, et al, 1981).

Coyote Spring Tuff (Toes)

The Tuff of Rattlesnake Canyon is disconformably over-

lain by up to 122 m of biotite-rich latitic crystal-vitric

tuff. This ash-flow tuff has been assigned the informal name of Coyote Spring Tuff by Bonham (in press) for exposures in

the Pah Rah Range. In this report, the name Coyote Spring

Tuff is adopted for rocks of similar lithology in the southern Nightingale Mountains, and designated Toes (Figure 4; Plates 1 and 2).

This ash-flow tuff is extensively exposed throughout the southern Nightingale Mountains (Figure 3; Plate 1). The basal contact is disconformable and marked by a porous glassy zone of basal Coyote Spring Tuff resting on the partially welded zone of the Tuff of Rattlesnake Canyon. In one local­ ity (NWi, SEJ, Sec. 34, T.25N., R.24E.), a thin paleosoil horizon was observed which contained large (^ 1 m) stumps of petrified wood. The upper contact is marked by a paleosoil horizon with numerous petrified wood fragments up to 0.5 m long. This upper, disconformable contact is a significant, recognizable marker horizon.

The Coyote Spring Tuff is a multiple flow ash-flow cooling unit which can be traced laterally from a simple cooling unit to a compound cooling unit to three simple

i 45

cooling units. Where this unit is a simple cooling unit,

it is composed of thick lower and upper zones and a central

partially to densely welded zone which weathers with blocky outcrops. These three zones can each be separate cooling

units, each separated by thin porous glassy zones and paleo- soil horizons containing petrified wood fragments, as well as three zones within a simple cooling unit. The thick upper and lower zones account for over 85% of the entire unit.

The central zone accounts for about 15% of the thickness of this unit and exhibits the greatest variability in thickness (accentuating paleotroughs and ridges). This central, partially to densely welded zone is a blocky cliff former, weathers a dark reddish brown, and on fresh surface is tan to pink. The densely to partially welded upper and lower zones weather greenish-lavender, greenish-pink, and grayish green on both fresh and weathered surfaces. These zones form very steep slopes and exhibit platey weathering.

This ash-flow tuff is composed of approximately; 27-44% phenocrysts, 15-20% pumice fragments, 3-9% lithic fragments, and 27-55% groundmass.

Phenocrysts consist of: plagioclase (51-61%); sani- dine (5-20%); biotite (12-29%); opaque minerals (5-10%); and minor to trace amounts of hornblende (2-7%), quartz (^ 3%), and zircon (^ 2%)(Figure 5). Phenocrysts are mostly sub- hedral, but were probably euhedral and were broken during eruption of this ash-flow tuff. Plagioclase phenocrysts 46

range from 1 to 4 mm in size and retain a relatively con­

stant percentage throughout the ash-flow (Figure 5). Many plagioclase phenocrysts are incipiently to completely altered

to calcite. Phenocrysts exhibit dendritic, skeletal, and embayed forms, as well as minor albitization. Plagioclase crystals are sometimes sieved with tiny biotite, opaque, and zircon crystals. Sanidine phenocrysts have also been at least incipiently replaced by calcite. These phenocrysts average less than 2 mm and a few are perthitic. Sanidine content decreases slightly upward in this unit (Figure 5).

Biotite crystals occur in grains up to 2 mm and many are bent hnd kinked. Many biotite grains are oxidized and have nearly opaque centers or portions within. Opaque minerals are typically anhedral, small (^ 5 mm), and occur disseminated throughout the matrix. Many, if not most, of the opaque minerals are the oxidized products after biotite alteration.

A few have cubic outlines, and most have orange/red rims of hematite and/or limonite. Hornblende occurs in very minor to trace amounts and in very small crystals (^ 2 mm). Only in the upper portions of this ash—flow do hornblende crystals occur well formed and unaltered. Elsewhere in this unit, hornblende phenocrysts have been altered to calcite with opaque rims. Quartz phenocrysts occur in trace amounts and are very small in size (^ 0.2 mm).

Pumice fragments throughout this unit are somewhat com­ pacted, but are still fluffy, and green in color, as they

4 47

have been altered to celadonite. In the central zone, pum­ ice fragments are usually purple, lavender, and white.

Throughout the unit they are compacted with from 4:1 to 10:1 elongation ratios. Fragments range in size from 1 to 3 cm

and increase, slightly, m abundance, from 13 to 19% upward

unit. In thin section, pumice fragments are devitri- fied with standard devitrification textures; spherulites are

well developed. No clay occurs within spherulites, although

pumice fragments can have calcite and limonite within. Pum­ ice fragments contain less than 15% phenocrysts.

Lithic fragments are usually angular and range in size

from ^ 1 mm to 1 cm. The constituent lithic fragments are

carbonaceous limestone, andesite, and other ash-flow tuff fragments.

Glass shards are usually orange (p!ane light), axioliti-

Caliy devitrlfled. and are bent, smashed, and aligned, de­ pending on the relative degree, or intensity, to which

welding has occurred. In the basal portions of this ash-flow

tuff, calcite occurs replacing glass shards. Throughout the Coyote Spring Tuff, the matrix is composed of fine ash and

dust particles, which appear as brown, cloudy material in

thin section. Celadonite is a common matrix constituent, and

probably is the weathering/alteration product of glassy ash

and dust. Celadonite is uncommon in the central zone of this

unit. In this same zone, limonite pseudomorphs after pyrite occur near joint surfaces.

The Coyote Spring Tuff was emplaced on top of the Tuff 48

Qf Rattlesnake Canyon, which was, although slightly undulating,

a relatively flat topographic surface. Variability in

thickness of this ash—flow tuff, reflecting flow down troughs

and ridges is best observed in the central, blocky weathering

zone (which can be from 0 to 20 m thick). This ash-flow tuff

overrode a stand of trees in at least one area, and later on, itself, supported a forest.

Thickness of this ash—flow reaches a maximum in the

southern part of the southern Nightingale Mountains and

a minimum in the very northern portion of the area (Plate 1).

Zoning is absent and phenocryst percentage remains

fairly constant throughout the ash—flow. Welding and devitri­

fication are typical in this unit, hydrothermal alteration

is prevalent, and later low temperature, possibly groundwater alteration is common.

A composite K/Ar age date of 28.1 +0.85 million years

(biotite and plagioclase) is reported for this unit, at

Bacbn Rind Flat in the Pah Rah Range (Garside, et al, 1981).

The Coyote Spring Tuff occurs extensively throughout

the Pah Rah Range and at least as far north as the Seven Lakes

Mountain area, where this unit is informally known as the

Porcupine Mountain Tuff (Deino, in press)(Figure 6). The

Coyote Spring Tuff correlates to Cooling Unit 1 of Wallace

(1975) in the Virginia Mountains area, the tuff of Dogskin

Mountain (Hutton, 1978), and a tuff underlying the Nine Hill and overlying the Lenihan Canyon Tuff in the Carson City area

(Bingler, 1978a; Deino, in press)(Figure 6).

i 49

Tuff of Jackass Spring (Tojs)

Approximately 140 m of pumice-bearing vitric-crystal,

quartz latitic ash-flow tuff disconformably overlies the

Coyote Spring Tuff. This ash-flow tuff unit is tentatively

correlated with the tuff of Jackass Spring, an informal name

assigned by Hutton (1978) for exposures on northern Dogskin

Mountain (Figures 2 and 6), on the basis of similar mineralogy

and mineral percentages. This unit is designated Tojs (Figure 4; Plates 1 and 2).

This ash-flow tuff crops out throughout the eastern side of the southern Nightingale Mountains (Plate 1). In most

exposures, this unit is a multiple flow simple cooling unit.

It is generally tabular to planar in shape but was deposited on an irregular surface making thickness variable.

Contacts with under and overlying units are disconform- able. Contacts are marked by very thin (^ 1 m thick) porous glassy zones and, in the case of the underlying Coyote Spring

Tuff, a thin paleosoil horizon with abundant petrified wood.

This ash-flow exhibits normal welding and devitrification features, and it is devitrified throughout. Throughout the exposed extent of the tuff of Jackass Spring on both fresh and weathered surfaces, the unit is usually brownish-red to pink, with minor color variations, to tan, orange, yellow, and white, due to bleaching along fractures and joint surfaces. Lise- gang banding is common in the upper portions.

The tuff of Jackass Spring is well indurated throughout 50

its thickness and crops out as steep cliffs and slopes. In

lower densely welded portions, this unit weathers via blocky to polygonal crumbly/hackly. In upper, partially welded

zones, this unit forms resistant, rounded, step-like out­ crops .

A basal vitrophyre is from 1.5 to 4.6 m thick and is

overlain by a zone of densely welded ash-flow tuff up to

125 m thick. This basal vitrophyre is hydrated, green—gray

to dark brownish-gray and has small fractures throughout.

This ash-flow tuff contains: lithic fragments (*1-8%');

phenocrysts (10-20%); pumice fragments (5-10%); and, groundmass (62-84%).

Lithic fragments are subrounded to angular and average

^ 4 mm, with a maximum size of 2.5 cm. Rock types represent­

ed include: andesite, andesite dike material, granite,

quartzite, carbonaceous limestone and siltstone, and ash- flow tuff fragments.

This unit exhibits slight zoning, felsic to mafic, up­ ward in the section (Figure 5). The tuff of Jackass Spring contains an average from 10-20% phenocrysts (which are well formed but broken) including; plagioclase (27-35%), sani- dine (40-45%), quartz (5%), biotite (trace to 10%), and small

(*■ 0.3 mm) opaque minerals (3-5%) (Figure 5). Hornblende

(2-5%) and pyroxene (trace) crystals occur only in the lower portions of this ash-flow, particularly in the basal vitrophyre and both average *■ 0.5 mm. Plagioclase and 51

sanidine phenocrysts average 1.5 to 2 mm. Many plagioclase

phenocrysts are at least incipiently altered to calcite and

clays (montmorillonite?) throughout the unit, but, in particu­ lar, are altered in greatest abundance in the moderately to

very densely welded zones. Sanidine crystals are usually

unaltered and are not perthitic, but exhibit a hackly

fracture, characteristic to this unit. Sanidine is the

dominant phenocryst in the lower portion of this unit and

is subordinate in the upper portions (Figure 5). Biotite

crystal content is variable but increases upward in the unit

from nil to trace amounts at the base, to about 10% at the

top. A few biotite crystals have been altered to opaque

material, however, most are well formed and unaltered and usually ^ 2 mm in size.

Pumice fragments are white-tan, pink, and lavender in

hand sample and average 2.5 cm in length, but commonly occur up to 7.6-10.2 cm in length. Pumice fragments in this unit contain very few phenocrysts 10%). From 4:1 to 10:1

elongation of pumice is common in densely welded zones. In thin section, pumice fragments from densely welded zones can be rimmed by red-orange hematite and/or limonite and usually are coarsely devitrified with large spherulites, visible in hand specimen. Calcite replaces a few pumice fragments. In some cases, the inside rim of pumice fragments have axiolitic type texture and the centers are spherulitic.

The groundmass contains a high content of glass shards and is crosscut by numerous very thin veinlets of chalcedony.

i 52

The groundmass exhibits standard primary devitrification

minerals/textures, but minor secondary alteration, addition

of minor amounts of celadonite and calcite is pervasive

throughout the unit. Glass shards are devitrified with

axiolitic textures. In the basal vitrophyre, the glassy matrix has altered to clays along numerous fractures; possibly due to hydration.

The tuff of Jackass Spring was deposited on an undula­

ting surface. In general, the underlying topography was

nearly flat and this ash—flow tuff was emplaced over the

forest covered slopes of the Coyote Spring Tuff.

The tuff of Jackass Spring correlates to the informally named Dry Valley Tuff in the Seven Lakes Mountain area, where a K/Ar age date of 27.0 +0.2 million years (sanidine) is reported (Deino, in press)(Figure 6). According to

Deino (in press), this ash-flow tuff occurs "near Mills

Peak and at Doyle Crossing, Lassen County, California,

30 km southwest and 47 km northwest of Seven Lakes Mountain."

Therefore, the tuff of Jackass Spring was, originally, quite an areally extensive ash-flow tuff.

Ash-Flow Tuff Cooling Unit E (Toe)

Overlying the tuff of Jackass Spring is as much as 60 m of green, brown, and purple crystal-vitric, latitic ash-flow tuff, here informally designated Ash-Flow Tuff Cooling Unit E (Toe; Figure 4; Plates 1 and 2).

This ash-flow tuff is exposed along a north-south trend­ 53

ing ridge along the eastern side of the study area in the

southern Nightingale Mountains (Plate 1). The basal contact

is marked by a thin (1 m) porous glassy zone which is often

covered by debris from upslope. The basal glassy zone is

poorly indurated, easily eroded, and pale green to orange/

rust. The upper contact is not seen, as the upper portion of this unit has been eroded.

This ash-flow tuff typically forms very steep, debris covered, rounded slopes, with the densely welded portions

forming cliffs best exposed in drainages. This unit also weathers platey or friable in partially to densely welded zones. Variations in color are common in the densely welded portion.

This unit is composed of approximately; 25-45% pheno- crysts, 15-25% pumice fragments, 5-10% lithic fragments, and 15-40% groundmass.

Well formed, but broken, phenocrysts in this ash-flow tuff include: plagioclase (35-45%); sanidine (25-45%); biotite (10-25%); small opaque crystals (5-10%); and, minor amounts of quartz (5%)(Figure 5). Zircon and apatite crystals were seen in trace amounts as inclusions in feldspars.

Hornblende phenocryst outlines were seen in few thin sections, and those crystals have been altered to calcite and were sieved with biotite and feldspars. Plagioclase and sanidine crystals average 2 mm in size. Sanidine crystals are some­ times perthitic and plagioclase is sometimes albitized. A 54

few of the centers of plagioclase phenocrysts in the lower, partially to densely welded zone have been at least incipi- ently altered to chlorite and clays (montmorillonite?) with some calcite. Biotite phenocrysts are moderately abundant throughout this ash-flow tuff and average 1-1.5 mm. In the very densely welded zone, biotite are nearly opaque. In the partially welded to densely welded zone, some biotite phenocrysts have an orange rim or halo and are granular in appearance. All opaque crystals tend to be ^ 0.5 mm and are usually anhedral. Many have celadonite surrounding them.

Quartz crystals are small, some have thin zones of devitri- fied matrix overgrowth, and they tend to occur in clusters.

Pumice fragments average 2 cm and exhibit such color variations as green, lavender, tan and white. They are devitrified with spherulitic intergrowths of alkali feldspar and quartz in the densely welded zone, and they tend to have opaque 'rinds' and ragged edges. Minor tridymite was observed in pumice fragments from the lower partially welded zone as well as minor secondary alteration with the addition of cal­ cite. Throughout this cooling unit, pumice fragments are aligned and compacted in varying degrees from 3:1 to 5:1, thus imparting eutaxitic structure to the ash-flow.

Lithic fragments are usually small 1 cm), angular to subrounded, and composed of several different lithologies.

Such heterolithologies observed in this unit include; ande­ site, basalt, siltstone, quartzite, and ash-flow tuff frag­ 55

ments.

The groundmass is usually brown and cloudy and composed of fine ash and dust. A few glass shards are discernable

which commonly exhibit axiolitic devitrification textures.

When glass shards are not discernable, the matrix is typi­

cally devitrified with coarse grained devitrification tex­ tures. Celadonite and calcite are present in less densely

welded zones, but are virtually nonexistent in the very

densely welded, devitrified portion.

Age and Correlation of Ash-Flow Tuff Cooling Unit E

Ash-Flow Tuff Cooling Unit E was emplaced on a slightly

undulating paleotopography developed on the tuff of Jackass Spring.

This ash-flow tuff is not compositionally zoned, but is

quite lithologically similar to the upper portions of the

underlying tuff of Jackass Spring. No evidence of a major

erosional period between this ash-flow tuff and the under­

lying tuff of Jackass Spring was observed, and these two units

are separated by a thin porous glassy zone. Therefore, it

can be inferred that Ash-Flow Tuff Cooling Unit E and the

tufft of Jackass Spring are very close in age. No direct

correlation could be made for this ash-flow tuff with ash- flow tuffs outside of this study area (Figure 6).

Ash-Flow Tuff Cooling Unit F (Tof)

The basal unit of three ash-flows which occur in the 56

southwestern portion of the study area is a crystal-vitric rhyolitic ash-flow tuff cooling unit. This unit is limited in exposure, and is here informally designated Ash-Flow Tuff

Cooling Unit F (Tof; Figure 4; Plates 1 and 2).

The lower contact of this unit is not exposed, however, the upper contact is disconformable and this unit is some­ times in fault contact with younger tuffs or basaltic andesite flows of the Chloropagas Formation. The upper disconform- able contact with the Chimney Springs Tuff is marked by a thin porous glassy zone. This ash-flow tuff is a simple cooling unit and exhibits typical welding and devitrification gradational zonations.

This unit has an at least 4.6m thick, black vitrophyre exposed near the base. This ash-flow tuff is typically orange-pink in the lower densely welded zone and light green to lavender in the upper zone, on both fresh and weathered surfaces. It forms steep, rounded slopes, often covered with debris from overlying units, and can be up to 14 m thick.

Ash-Flow Tuff Cooling Unit F contains abundant pumice fragments (35%) and phenocrysts (20%), few lithic fragments

(2-5%), and a devitrified groundmass (43-45%).

Pumice fragments are usually white, range in size from

6.4 mm to 2.5 cm, and exhibit vapor phase devitrification in the upper, partially welded portion of this ash-flow.

Phenocrysts present in this ash-flow tuff include; plagioclase (30%), quartz (15-20%), sanidine (35%), biotite

# 57

(trace to 10%), and opaque minerals (2-8%). Augite crystals were observed in trace amounts. Plagioclase, quartz, and sanidine crystals all average 1.5 mm or less in size. Some plagioclase phenocrysts are sieved with opaques and very fine grained ash. Quartz phenocrysts are slightly embayed and have oscillatory extinction (strained?). Biotite phenocrysts increase in abundance upward in the ash-flow tuff, average

0.5 mm, and generally have red oxidation halos in the upper part of the unit. A few opaque crystals have orange halos and many are nearly cubic in shape.

Lithic fragments are angular particles consisting dominantly of older, metasedimentary rocks which average z 6.4 mm in size with minor basaltic fragments.

Glass shards comprise about 60% of the groundmass and can be light gold (in plane light). They exhibit axiolitic devitrification textures. Minor amounts of celadonite occur within the groundmass.

Age and Correlation of Ash-Flow Tuff Cooling Unit F

This ash-flow tuff underlies the Chimney Springs Tuff, which has a reported K/Ar age date of 23.0 +0.2 million years

(sanidine) from the Seven Lakes Mountain area (Deino, in press). Since no major erosional features were seen along the contact between this ash-flow tuff and the overlying

Chimney Springs Tuff, and a thin porous glassy zone separates the two units, it is inferred that Ash-Flow Tuff Cooling Unit 58

F is just slightly older than the Chimney Springs Tuff.

Because the lower, basal contact of Ash-Flow Tuff

Cooling Unit F is not seen, it is difficult to interpret

this unit's emplacement history. However, by comparison with the other Middle Tertiary ash-flow tuffs, it is reason­

able to conclude that Ash-Flow Tuff Cooling Unit F was

emplaced over at least a slightly undulating topography.

Chimney Springs Tuff (Tochs)

Overlying Ash-Flow Tuff Cooling Unit F is an approxi­ mately 11 m thick section of crystal-vitric rhyolitic ash-

flow tuff cooling unit tentatively correlated with the in­ formally named Chimney Springs Tuff of Bonham (in press)

(Figure 6). This correlation is based on lithologic similar­ ity and this unit is designated Tochs (Figure 4; Plates 1 and 2).

This ash-flow tuff is a simple cooling unit which crops out to a very limited extent in the southwestern portion of the study area (Plate 1). Both upper and lower contacts are marked by porous glassy zones. However, this unit can be in fault contact against other rock units.

The majority of the exposed thickness of the Chimney

Springs Tuff is densely welded. The upper, partially to nonwelded portion is thin, not well indurated, weathers easily, and is easily covered by debris from upslope.

In this area, the Chimney Springs Tuff is usually pur­ plish orange, red and sometimes white-tan. The porous glassy zone at its base is white. This unit forms a steep, blocky cliff which forms an overhang above the easily erodable, 1.2 m thick basal porous glassy zone.

This ash-flow tuff contains abundant phenocrysts

(35-40%), with sparse lithic fragments 5%) and pumice fragments (5-10%).

Phenocrysts are usually well formed, but partially

broken, and include: large sanidine (50%) and quartz (30%)

crystals; plagioclase (10%); biotite (5%); and opaque

minerals (5%)(Figure 5). Hornblende and zircon phenocrysts

can occur in trace amounts, with zircon crystals usually

occurring as inclusions in sanidine phenocrysts. Blocky

sanidine crystals average 1.5-2 mm and are sometimes perthitic.

Quartz phenocrysts are vermicular and up to 2 mm. Plagio­

clase phenocrysts are usually 1 mm and a few have hematite on fracture surfaces. Biotite crystals are sparse and average

1 mm. Opaque crystals are typically near cubic in shape and are probably magnetite and/or ilmenite.

Lithic fragments are very sparse, average L 4 mm, and include andesite and older(?) ash-flow tuff material.

Pumice fragments are usually nondiscernable in the densely welded zone, but can be light orange in color. They are usually devitrified.

The groundmass of this ash-flow tuff is composed of glass shards (approximately 75%) and very fine grained ash and dust particles. The latter material imparts a slightly cloudy appearance to the matrix. Glass shards are small and 60

usually exhibit axiolitic devitrification textures. Emplacement of the Chimney Springs Tuff was probably controlled by paleodrainage developed on the underlying Ash- Flow Tuff Cooling Unit F.

A K/Ar age date of 23.0 +0.2 million years (sanidine) is reported for the Chimney Springs Tuff in the Seven Lakes

Mountain area (Deino, in press)(Figures 2 and 6).

The Chimney Springs Tuff is areally extensive. It has been recognized as far south as southern Washoe Valley and possibly correlates with an unnamed unit which underlies the Santiago Canyon Tuff in the Carson City/Silver City area

(Trexler, 1978; Bingler, 1978a)(Figure 6). The Chimney

Springs Tuff occurs throughout the Pah Rah Range and is recognized in the northern Dogskin Mountain and Seven Lakes Mountain areas (Hutton, 1978; Geason, 1980; Bonham, in press;

Deino, in press)(Figures 2 and 6). This study area is probab­ ly near the distal end of the original ash-flow sheet, as the thickness of this unit at this locality is much less than at other localities, particularly those exposures in the

Pah Rah Range (Figure 6)(Wallace, 1975; Geason, 1980; Bon­ ham, in press).

Tuff of Gary's Ridge (Togr)

Disconformably overlying the Chimney Springs Tuff is a purple to lavender, crystal to crystal-vitric rhyolitic ash- flow tuff cooling unit. This ash-flow tuff is tentatively 61

correlated to the informally named tuff of Gary's Ridge of

Bonham (in press) and designated Togr (Figure 4; Plates 1 and 2)

This correlation is based on stratigraphic position and similar lithologic description of the unit described at

Bacon Rind Flat, in the Pah Rah Range (Bonham, in press)

(Figure 6).

This ash-flow tuff crops out throughout the southwestern portion of the study area, and it is the most extensive of the three ash-flows described in this portion of the study area (Plate 1).

Upper and lower contacts are unconformable. The lower contact with the Chimney Springs is a disconformable zone marked by the porous glassy zone at the base of the tuff of

Gary's Ridge. This porous glassy zone is white-gray and very thin, less than 1 m thick. The upper contact is an angular unconformity and the tuff of Gary's Ridge is overlain by basalts and andesites of the younger Chloropagus Formation.

Some contacts of this ash-flow tuff with other units are faults.

The tuff of Gary's Ridge is usually purple to lavender, but can be tan and pale green, and is a simple cooling unit which shows typical zonation in welding characteristics. The lower very densely welded zone is well indurated and resistant to erosion, forming steep hillsides. The upper less densely welded to moderately welded portions are far less indurated and form rounded hills and a fairly low relief topography. 62

This ash-flow tuff can attain at least 31 m in thickness.

The unit contains abundant pumice fragments (20-25%), phenocrysts (35-40%), and few lithic fragments (0-5%).

Pumice fragments are usually lavender to tan and can exhibit an elongation ratio of from 6 to 20:1 in the densely welded zone. Pumice fragments average 2 cm and typically weather out easily leaving remnant pumice fragments, or fiamme. Many are sperulitically devitrified and have a very thin dark rind of opaque material around them.

Phenocrysts include: sanidine (30%); plagioclase

(30-35%); quartz (10-15%); biotite (15%); and, opaque crystals (5%)(Figure 5). Hornblende, apatite, and sphene crystals were seen in trace amounts. Phenocrysts are gener­ ally aligned, length parallel to the plane of compaction developing a eutaxitic structure, especially in the densely welded zone. Sanidine crystals are slightly perthitic.

Plagioclase, sanidine, biotite, and quartz phenocrysts all tend to be large, averaging 1.5 to 2 mm. Most plagioclase crystals are blocky and unaltered, but a few are at least incipiently altered to clays (montmorillonite?). Quartz crystals also tend to be blocky and are typically at least somewhat vermiculated. Many biotite phenocrysts are bent and are opaque along cleavage planes. Opaque crystals have irregular to near cubic shapes and are probably magnetite and/or ilmenite or even pyrite.

Lithic particles average 3.5 mm or less in size and 63

consist mainly of basalt and ash-flow tuff fragments.

Glass shards, which comprise about 60 to 70% of the

groundmass, are usually devitrified with axiolitic textures. Shards are also usually clear, but can be gold colored in

plane light. The remainder of the matrix is devitrified with

a coarse grained texture, and is composed of ash and tiny granular opaque crystals.

This ash-flow tuff possibly correlates with Cooling Unit

4 of Wallace (1975), in the Rainbow and Mine Canyon areas

of the Virginia Mountains, with the lower half of Ash Flow

Unit 6 of Geason (1980), in the Olinghouse District of the

Pah Rah Range, and with the Santiago Canyon Tuff of Bingler (1978a), south of the Truckee River in the Carson City/Silver

City area (Figures 2 and 6). According to Bingler (1978a),

the Santiago Canyon Tuff crops out as far south as the Terrill

Mountains (north of Schurz, Nevada). A composite K/Ar age

date of 20.5 and 21.8 million years (sanidine and biotite)

is reported for the Santiago Canyon Tuff (Bingler, 1978a).

These dates are consistent with stratigraphic relations seen in this study area.

Andesite Dikes (Tad)

Andesite dikes have intruded and crosscut the Middle

Tertiary ash-flow tuffs throughout the study area (designated Tad; Plates 1 and 2).

Dikes weather green to orange, and brown, and are green 64

to tannish orange on the fresh surface. These rocks

weather friable to platey and are easily eroded. Along

their contacts with ash-flow tuffs, 'baked' border zones

were produced within the host rock, up to 1.2 m thick. These

'baked' zones along the contacts with ash-flow tuffs render

the host rock a purplish gray color and are resistant to

weathering thus forming small ridges on either side of the dikes (Figure 7).

Andesite dikes range in thickness from 9.1 m to less

than 0.3 m. They have undulating shapes, often pinching and

swelling along their lateral extent. They dip near vertical and strike approximately east-west.

Dikes are aphanitic to porphyritic with large pheno-

crysts of plagioclase and sometimes hornblende visible in

hand specimen. In thin section, phenocrysts, averaging

3-4 mm, comprise about 40% of the rock and are set in a very

fine grained matrix of a smaller version of the same crystals.

These include; plagioclase laths (45-50%), hornblende

(30-35%), and small opaques (15-20%). Plagioclase and hornblende crystals have largely been altered to clays and calcite. These alteration products can also occur through­ out the matrix.

Based on crosscutting relations, the andesite dikes are younger than the ash-flow tuffs, which they cross cut, and older than the overlying Chloropagus Formation, as they do not transect that formation. Therefore, the age of the 65

Figure 7. Photograph of an andesite dike, illustrating the resistant, rib-like outcrop pattern to the 'baked' zones on either side of the dike (in Color). 66

andesite dikes is probably Middle Miocene.

Numerous other workers have noted the occurrence of

dikes, ranging in composition from basalt to andesite,

as crosscutting Middle Tertiary ash-flow tuffs, in western

Nevada, and have postulated that these dikes could possibly

have been feeders for the overlying Chloropagus Formation.

Chloropagus Formation

In this study area, sedimentary rocks and ash-flow tuffs

are intercalated between andesite, basaltic-andesite, basalt, and dacitic volcanic rocks. These rocks crop out most

extensively south of Coyote Canyon, in the northern Truckee

Range, as well as in the northeast corner of the study area, and are correlated to the Chloropagus Formation (Plate 1).

The Chloropagus Formation, originally named and described by Axelrod (1956), has since been studied further by Bonham

(1969) and Rose (1969). In this study area, these rocks have also been mapped and identified as the Chloropagus

Formation by both Bonham (1960; 1969) and Rai (1968).

In this study area, the Chloropagus Formation attains a composite thickness of 200 m. It unconformably overlies the Middle Tertiary ash-flow tuff cooling units, with angu­ lar discordance, but can rest nonconformably on Jurassic metasedimentary rocks of the Nightingale Sequence depending on the degree of erosion, non-deposition, or faulting. In a few places, contacts are faults. The Chloropagus Forma­ 67

tion is unconformably overlain by lense-shape bodies of white

tan tuffaceous siltstone and vesicular basalt. However, over

most of the study area, an upper contact was not seen. The

Chloropagus Formation in the northern Truckee Range and

southern Nightingale Mountains was deposited on an irregular

paleotopographic surface.

The lower, depositional contact can be seen in places to

be a lenticular shaped, 4.6 m thick section of tan-gray/

yellow/brown gravelly sandstone, sandy siltstone, and minor

gravel conglomerate. Where these sediments are not present,

the lower contact may be seen as a very thin 20 cm) red

paleosoil horizon which had probably developed on the older, pre-Chloropagus rocks.

Surficially, the formation forms steep but rounded

hillsides. It is generally massive but can have individual

flows which form thin, rib-like outcrops on hillsides. The volcanic rocks of this formation are typically brown to reddish brown to dark grayish purple on the weathered surface and usually dark gray to black on the fresh surface.

A schematic stratigraphic column showing the relative position of flows with intercalated sediments and ash-flows is illustrated in Figure 8 (see also Plates 1 and 2). The irregular, unconformable nature of contacts between flows with­ in the formation are probably due to an irregular topography caused by periods of erosion between deposition of subsequent lava flows (Figure 8). 68

Figure 8. Schematic, generalized stratigraphic section of the Chloropagus Formation in the northern Truckee Range and southern Nightingale Mountains, Nevada. Tcs is the symbol used to designate miscellaneous sedimentary horizons which form thin, isolated exposures. 69

For purposes of this study, rocks of the Chloropagus

Formation have been subdivided into four volcanic units and

a miscellaneous sedimentary rocks unit. These units are

designated as follows: Tcab, andesite, basalt, and basaltic

andesite; Ted, welded dacite; Tcafa and Tcafb, two ash-flow tuff units; and, Tcs, sedimentary rocks (Plates 1 and 2).

Andesite, Basalt, and Basaltic andesite (Tcab)

The dominant volcanic rock types are andesite, basalt,

basaltic andesite, and all are included together as the unit

Tcab. Basalt is more abundant toward the top of this unit.

This andesite to basalt section can be up to 110 m thick, and

can have lenses of hematitic-rich volcanic sedimentary rock intercalated within.

Basalt is usually aphanitic. In thin section, the

basalts consist of approximately 10—12% larger phenocrysts

(0.5-0.2 mm) which include olivine (20%), augite (15%),

opaque minerals (^20%), spinel (^10%), and plagioclase laths

(40%), set in a very fine grained matrix of the same minerals.

Plagioclase microlites in the matrix exhibit pilotaxitic tex­

tural alignment. The centers of olivine crystals are commonly

altered to orange-red iddingsite. There is a tendency, in some flows, for the larger phenocrysts to cluster.

Andesites contain approximately 25—30% phenocrysts of: plagioclase (50%), red oxyhornblende (30%), and sometimes biotite (20%). Oxychlorite and clays can occur as an altera­ 70

tion product of both biotite and hornblende. These larger

phenocrysts are set in a groundmass of a smaller version of the same crystals; plagioclase microlites, and oxyhornblende, with green opaque clay.

Basaltic andesites are nearly equigranular and consist of plagioclase laths (60-65%), hornblende (25%), opaque

minerals (10%), and clinopyroxene (probably augite)(5%).

Chlorite and oxychlorite occur in minor amounts as an altera­

tion product, probably after the mafic minerals. The opaque

minerals are small (^ 0.1 mm), are sieved in hornblende and chlorite, and occur as irregular bodies between crystals.

Plagioclase crystals average 2 mm in length, and hornblende

crystals are altering to chlorite and average z 1 mm. Clino­

pyroxene crystals are anhedral and appear to fill space be­

tween plagioclase laths, as well as other crystals. They

are surrounded by hornblende and chlorite crystals.

These flows are aphanitic to weakly porphyritic in

hand sample. The flow tops are non- to strongly vesiculated.

The vesicles are often filled with chalcedony, calcite,

or even caliche. In some places, flows have an associated

air fall ash beneath them or a red paleosoil horizon. In one locale spatter-type features were seen (Center, Sec. 14, T.24N., R.24E.).

Welded Dacite (Ted)

Welded dacite crops out in the northern Truckee Range in various locations, and as mentioned by Bonham (1969) and 71

Wallace (1975) it also occurs in the Mullen’s Pass area

near Pyramid Lake. It exhibits a pyroclastic nature, with

smashed lavender colored pumice fragments imparting a eutaxi-

tic structure. This unit is designated, Ted (Plate 1).

Within the study area, this unit can form very steep

slopes, even cliffs, and exhibit all features of a simple ash-flow tuff cooling unit; a basal porous glassy zone and

vitrophyre overlain by a densely welded zone which can be at

least 30 m thick. Laharic breccias are common within this

unit and can be about 8 m thick. Laharic breccias are com­

posed of large blocks (^ 30.5 cm) of welded dacite randomly

oriented in a lavender colored dacite volcanic ash matrix.

The dacite is typically lavender to purple and porphyri-

tic. In thin section, this unit contains about 25% pheno-

crysts which consist of; plagioclase (70-75%), hornblende

(15%), biotite (5-19%), and opaque minerals (5%). Most

phenocrysts are small, less than 1 mm, except plagioclase

laths which average 5 mm and are easily visible in hand

specimen. The groundmass consists of small plagioclase microlites and opaque dust. Pumice fragments account for about 30% of this unit and are devitrified and compacted, with up to 20-25:1 elongation, and contain less than 5% phenocrysts within. No lithic inclusions were observed.

Ash-Flow Tuffs (Tcafa and Tcafb)

Two thin ash-flow tuff cooling units, separated in 72

part by a thin tuffaceous sandstone, unconformably overlie

the welded dacite and basalt/andesite (Figure 8; Plate 1).

In turn, these ash-flow tuffs are unconformably overlain by

basalt (and some andesite) of the upper Chloropagus Formation.

These two ash-flow tuffs weather to form a cuesta, with the

densely welded portion of the upper cooling unit forming the

resistant dip slope. These two units crop out only in the

very southern and southeastern portions of the study area,

and are very different mineralogically, texturally, and in overall lithology from the older, Middle Tertiary Ash-Flow Tuffs.

The lower unit, designated Tcafa, is orange on both fresh and weathered surfaces, at least 12 m thick, and weathers very crumbly and friable. This unit, where exposed, is partially welded, and contains fluffy pumice fragments

(50-65%), about 5-10% very small phenocrysts, and 35% lithic fragments. This unit is considered a lithic-vitric andesitic ash-flow tuff.

Phenocrysts consist of plagioclase (71%), pyroxene (7%), and opaques (21%)(Figure 9). Lithic fragments are angular,

^ -*-n diameter, and consist of andesite and metasedimen— tary rock fragments.

This lower ash—flow tuff is only incipiently devitrified and the matrix, between few recognizable shards, is filled with celadonite and other clays.

An irregular, lenticular shaped tuffaceous sandstone 73

UPPER

a Ash-Flow Tuff B (Tcafb)

(Tcafa)

100- Figure 9. Histograms showing o/ KEY phenocryst mineralogy of ash- /o flow tuffs within the Chloro- pagus Formation in the northern Truckee Range, Nevada. fcha □

°.‘t- o O o ^ ^ ^<7 °- o ^ T £ ^ O 4) «* ^ 74

occurs between the two ash-flow tuffs and is described in the

sedimentary rocks section of this chapter (Tcs; Figure 8; Plate 1).

The upper ash-flow tuff, designated Tcafb, attains an

approximate maximum thickness of 23 m (Figure 8; Plate 1).

Color of this unit varies from orange to green in the lower

densely welded portions to white-tan in the upper partially

to nonwelded zones of the unit. This upper portion is less

resistant to weathering than the lower, densely welded por­

tion, which is well indurated. This lower densely welded

portion forms a resistant dip slope on a cuesta.

This upper unit consists of 15-17% lithic fragments,

9-20% phenocrysts at the base increasing to 39-56% at the top, and 15-26% pumice fragments. This ash-flow tuff also exhibits a felsic to mafic zonation, from base to top, and is considered a crystal-lithic andesitic ash-flow tuff.

Lithic fragments include small angular pieces of basaltic- andesite, andesite, sandstone, and even smaller clasts of ash-flow tuff.

Phenocryst content increases upward in the unit, and includes: plagioclase (48-57%); sanidine (7-18%); horn­ blende (3-5% at the base, to 22-24% near the top); biotite

(3%); and opaque minerals (9-21%)(Figure 9). Minor to trace amounts of small quartz, pyroxene (hypersthene + augite), apatite, and zircon occur in this unit. Most pheno­ crysts were probably well formed but were broken on eruption. 75

Plagioclase crystals are less than 2.5 mm in size. Sanidine

crystal content decreases from 18% at the base to 7% at the

top (Figure 9). Sanidine crystals are sometimes poikilitic

and sieved with matrix or can exhibit perthitic textures.

Hornblende phenocrysts are usually small (<* 0.1 mm), can be

sieved with opaque crystals, often occur as skeletal forms,

are often altered to hematite(?) and chlorite(?), and increase

in abundance toward the top of the ash-flow (Figure 9).

Pyroxene phenocrysts (5-8%) occur in the lower portion of

this unit, and biotite phenocrysts (3%) occur in the upper

portion (Figure 9). Many pyroxene and biotite phenocrysts

are altered, partially to completely, to opaque minerals.

Accessory minerals, apatite and zircon, occur in trace amounts

as inclusions in other phenocrysts and scattered throughout the matrix.

The rock is composed of approximately 50% pumice fragments

in the lower, densely welded portion of this ash-flow tuff, and

this is probably due to the higher degree of welding, in this portion, which has increased the pumice fragment density. Pum­

ice fragments are glassy at the base of the unit, particularly where they grade upward from a basal black vitrophyre (Figure

10). These glassy pumice fragments are black, perlitic, and compacted with up to 10:1 elongation. Many fragments are

30 cm in length but most average 5 cm. The basal zone of black, glassy pumice, set in an orange colored densely welded devitrified zone is about 2.5 m thick. Above this ash-flow°tnffh^ o ? raPhS °5 ?£e ^a^al vitrophyre and densely welded zone of the upper Truckee Ranee Nevadf ^ T h (Tca^ ) Wlthln the Chloropagus Formation in the northern from ’ Nevada* .These illustrate the gradational zonation (over less than 15 cm) J V1p 3 ? yr? Upwar? lnto the densely welded zone, as well as the degree of compac- are still g l a s ^ a n d ^ r ^ COmpacted approximately 10:1, and black pumice fragments and a h * e r for f °neS are devitrified. The vitrophyre occurs at the bottom of r L o , f scale, occur m each photograph. Photo B shows the lower one-third of the same area shown in Photo A. (in Color) “ 77

zone, lensoid gas cavities occur localized where former pumice

fragments were. Occurring in minor amounts is a second

type of pumice fragment. These are smaller 5 mm) and are

incipiently devitrified with small spherulites. In the

densely welded zone pumice (as fiamme) have small vaguely

discernable spherulites with a high clay content due to weathering and secondary devitrification effects. Toward

the top of the unit, pumice are fluffy, incipiently vapor phase devitrified (with wispy texture), and still retain their vesicular structure. Throughout this unit, pumice fragments contain very few phenocrysts.

Groundmass textures are typical throughout the vertical extent of the ash—flow in the various zones of welding and devitrification. Celadonite occurs in the groundmass and imparts a cloudy nature to the matrix. In the upper, non- to partially welded zone, vapor phase type devitrification is pervasive. The majority of the groundmass consists of glass shards.

This upper ash-flow tuff was deposited on an irregular surface, as its upper zones sometimes rest unconformably on underlying basaltic-andesite and dacite without the pres­ ence of the densely welded zone.

Miscellaneous Sedimentary Rocks (Tcs)

Thin lenticular shaped bodies of sedimentary rock occur intercalated within the volcanic rocks of the Chloropagus 78

Formation, designated Tcs (Figure 8; Plate 1). The sedi­

mentary rock grouping includes various lithologies; paleo-

spils, sandstone, tuffaceous sandstone, gravels, and silt- stone .

In numerous localities between lava flows, red paleo—

soil horizons occur and reach a maximum of 2.5 m thick. These

soil horizons developed subaerially and are evidence showing that this was a highland area exposed to erosion.

Along the basal portion of the Chloropagus Formation

is a gravelly sandstone and sandy siltstone, with minor gravel

conglomerate. This unit crops out in isolated places along the basal contact of the formation. It is a lenticular

shaped unit, at most 9 m thick, whose shape and composition

are indicative of a channel fill deposit. These sandstones

are medium to coarse grained and lithic-rich, being composed

almost entirely of basalt and andesite clasts with a minor

fraction of metasedimentary clasts. All constituents are

subrounded to rounded, and there are three size fractions

crudely discernable: 1 mm (most dominant); 0.5 mm; and

1.5-2 mm. These sandstones are grain supported but contain a clay cement. Calcite fills in parts of the matrix.

Where this basal sandstone is absent, usually a red paleo- soil horizon developed.

Between the two ash-flow tuffs (Tcafa and Tcafb), a medium to coarse grained tuffaceous sandstone occurs, as previously alluded to, in the upper part of the Chloropagus 79

section (Figure 8; Plate 1). This arkosic sandstone

is tan to bluish gray on both fresh and weathered surfaces,

and can be up to 4.5 m thick. Beds of coarse and fine

material are from 2 to 4 ram thick, contain 25% matrix, and

are mud and clay supported. The constituent clasts (75%

of the rock) include: basaltic andesite lithic fragments

(40%); and broken phenocrysts of plagioclase (40%), opaque minerals (15%), quartz (5%), and pyroxene (trace). All

constituents range in size but average less than 1 mm and are mostly subrounded to subangular.

Minor, thin sandstone and siltstone units occur inter-

bedded with basaltic andesite in the lower portion of the

Chloropagus Formation (Figure 8). These are extremely friable

and often covered, but they are quite similar to the basal sandstone unit previously described.

Outcrops of gravel occur as several, small, lense-shaped,

isolated exposures in the southeastern part of the study area.

These rock units attain an approximate maximum thickness of

4.5 m. In one outcrop, this gravel unit merges with the green

densely welded portion of the upper Chloropagus ash-flow

(Tcafb). In another area, this same gravel is overlain by

well bedded sandstone and siltstone which are similar in

lithology to the arkosic sandstone that occurs between the ash-flows (Tcafa and Tcafb). These gravels weather easily and crop out either in stream canyons or in the low places along ridges. Gravels are yellow orange to gray on fresh 80

and weathered surfaces, are very porous, and contain approxi­ mately 80-85% constituent clasts. These clasts, all of which

occur in subequal amounts, include: basalt, vesicular

basalt, and andesite fragments, all of which average 1 cm

or less and are subangular. These are set in a mud/clay

matrix containing numerous small broken fragments of the above material.

Fine grained sediments are white to tan and also occur in limited exposures throughout the southern portion of the

study area, intercalated between andesite and basalt flows.

These sediments also form lense-shaped bodies which are more than 4.5 m thick and are of limited lateral extent. One

silicified ash horizon, in the southern part of the area

(NWi, Sec. 27, T.24N., R.24E.), can be traced laterally for

about 760 m and is interbedded between basalt flows. One

small pod of claystone, composed of white, poorly to moderately indurated clay particles exhibits ripple marks and cross

bedding with all constituent clay particles aligned.

Emplacement of the Chloropagus Formation

The volcanic rocks of the Chloropagus Formation were deposited subaerially, under terrestrial conditions. Lava flows were deposited in previously developed drainages, and could have blocked drainages, thereby damming streams and forming small lakes.

The age of the Chloropagus Formation is from 14.3 to 81

14.9 +1.5 million years old, based on corrected dates, from

Evernden and James (1964) and Bonham (1969)(Dalrymple, 1979).

These dates indicate an age of Late Miocene.

Basalt Flows (Tb)

Vesicular basalt flows crop out in the southeastern portion of the study area in a very few isolated exposures,

and is designated Tb (Plate 1). These rocks unconformably overlie rocks of the Chloropagus Formation with angular discordance, which can be marked by a very thin, lenticular white tan tuffaceous siltstone. The upper contact has been eroded and is not observable.

These basalt flows form thin veneers which are resis­ tant to weathering and form the tops of hills and peaks.

Flows reach a maximum thickness of 13.5-15 m. They exhibit well developed columnar jointing and flow tops are commonly vesiculated. Most of this basalt is dark brown gray/black on the weathered surface and black on the fresh surface.

Basalt flows are aphanitic, but in thin section, contain large, average 1 mm, euhedral phenocrysts of olivine and plagioclase set in a very fine grained matrix of olivine, clinopyroxene, opaques, and, dominantly, plagioclase micro— lites. Olivine crystals commonly alter to iddingsite along fractures and in some centers. Opaques are usually anhedral and tend to fill space between plagioclase microlites.

Just to the south of this study area, Bonham (1969) found that these basalt flows interfinger with the lower- 82

Middle Pliocene Truckee Formation, and that olivine basalt flows could be the upper member of that formation. There­

fore, the basalt flows, Tb in this area, are tentatively

correlated to those described by Bonham (1969) on the basis

of similar lithology and stratigraphic position. To date,

the author is unaware of any age dates from this unit, and

because, as Bonham infers, these basalt flows, at least in

part, interfinger with the upper Truckee Formation, they are

of probable Middle to Upper Pliocene in age.

These basalt flows exhibit features typical of basalt

lava flows deposited subaerially; columnar jointing and

vesiculated tops. These flows were deposited subaerially.

Surficial Deposits

Much of this study area is covered by unconsolidated,

surficial material of Quaternary age; particularly along

the western margin, bordering Winnemucca Dry Lake, and

in the canyons and arroyos throughout the area (Plate 1).

All Quaternary deposits unconformably overlie older, con­

solidated rocks, on which they form a thin veneer. Two types

of Quaternary, alluvial deposits were mapped: landslide

deposits (Qls) and undifferentiated alluvium (Qal)(Plate 1).

Landslide deposits are composed of large (several meters

long) blocks of ash-flow tuffs, nearly completely cemented with tufa, and randomly oriented.

The undifferentiated alluvial deposits include (from oldest to youngest): Lake Lahontan sediments; aeolian

sand; stream channel sediments; and, alluvial fan detri­ tus .

Lake Lahontan sediments include beach terrace deposits of sand, silt and gravels, and calcareous tufa. Where

beach terrace deposits crop out in Coyote Canyon, there are

numerous small gastropod shells weathering out of the exposures. Deposits of calcareous tufa resemble the three varieties

described and illustrated by Russell (1885). These deposits

can be found up to elevations just exceeding 4200 ft (1280 m),

in greater abundance along the northwest side of the

northern side of the northern Truckee Range, covering as a

thin veneer the much older consolidated rocks of the range.

Aeolian sands are medium to fine grained, moderately sorted, and tend to occur on the eastern, or lee, sides of

ridges, most often in the southern portion of the area.

Dunes occur on the alluvial plain west of the southern

Nightingale Mountains and east of the Winnemucca Dry Lake Basin.

Alluvial fan and stream channel sediments are composed of angular, poorly sorted debris derived from local sources.

Alluvial fan deposits are just slightly older than stream channel sediments.

Lake Lahontan deposits and alluvial fan deposits are currently undergoing dissection. 84

STRUCTURAL GEOLOGY

GENERAL

Two dominant regional structural zones, the Basin and

Range and the Walker Lane, transect this thesis area. The

Basin and Range is a series of near north-south trending

horsts and grabens which extend across the entire width of

the Great Basin. Basin and Range structure, an extensional feature, began approximately 17 million years ago. The

Walker Lane strike slip system parallels the southwestern

Nevada border and is composed of a series of northwest trending right-lateral strike slip faults which have an average strike of N.40°W. This zone extends from southern Nevada to northern California, and has been studied in some detail, particularly in the central portions. The Walker

Lane trends essentially parallel to, and exhibits the same sense of movement as the San Andreas fault zone to the west.

The timing of movement along the Walker Lane and its rela­ tion to Basin and Range structure is unclear, however, move­ ment probably began in the Mesozoic and has continued throughout the Cenozoic (Silberman, et al, 1976).

LOCAL STRUCTURE

In general, the southern Nightingale Mountains and northern Truckee Range are a nearly continuous east-dipping horst block (as noted by Bonham, 1969). Rock units dip to the east (from 4° to 33°) and are well exposed along a steep, 85

west facing escarpment along the southwestern side of the

southern Nightingale Mountains (Figure 3; Plate 1).

Mesozoic rocks exhibit the oldest observed deformational features. Rocks of the Nightingale Sequence have been

folded, crenulated, faulted and metamorphosed. This deforma­

tion is probably the result of Late Mesozoic orogenic events,

including the emplacement of granitic intrusive rocks during

the Cretaceous. The contact between metasedimentary and

granitic rocks trends generally northeast-southwest in the

northern Truckee Range and nearly north-south in the southern

Nightingale Mountains (Plate 1). These granitic rocks, which

crop out in isolated exposures, are believed to be part of a large stock.

Structures in the Cenozoic rocks, which are dominantly

volcanic, consist of: regional tilting, numerous unconform­

ities (either between Cenozoic rock units or Mesozoic and

Cenozoic rock units), and faults (Plates 1 and 2).

Regional Tilting

Tertiary rocks throughout the study area dip to the east. Middle Tertiary ash—flow tuff cooling units strike, in the southern Nightingale Mountains, from N.56°E. to

N .65°W. and dip an average of 24° to the east. Strike in the northern Truckee Range ranges from N.30°W. to N.6°W. with an average dip of 31° to the northeast. Attitudes for these rocks were taken on well developed eutaxitic structure. 86

In the Chloropagus Formation, rocks usually strike

northwest, varying from N.30°W. to'N.6°W. and average dip

is 31° to the northeast. In the Chloropagus Formation,

individual flows form rib-like outcrops on hillsides which, m a few places, form very large open 'apparent' folds.

These features are depositional and reflect underlying

topography as well as the nature of the lava flow. However,

attitudes were taken on many of the vesiculated, but planar, rib-like outcrops of this formation.

Pliocene basalt flows strike about N.22°W. and dip to the east at about 41°.

All Tertiary rocks dip to the east with similar atti-

tudes. Therefore, by inference, regional tilting occurred

after deposition of these rock units, and these rocks are all part of a larger east-dipping horst block.

Unconformities

All Tertiary rock units, particularly volcanic pyro­ clastic and lava flows, are separated by unconformities.

Unconformities represent a time hiatus in volcanic activity, however short or long their duration, during which erosional processes were prevalent. Therefore, unconformity surfaces parallel the attitudes of overlying volcanic flows and strike nearly north-south with variable dips to the east. Most unconformities are considered disconformities as only ero­ sion occurred before the deposition of an overlying unit 87

without tilting of previously deposited strata. A few

unconformities are considered nonconformities and occur

where the Tertiary section rests on Cretaceous granitic rocks.

Faults

The majority of faults in the southern Nightingale

Mountains trend north-northeast, although a few trend north­

east. All andesite dikes within the entire study area (of

which the vast majority occur in the northern part of the

study area) trend approximately east-west and dip vertically

and were probably emplaced along previously developed fault

zones. In the northern Truckee Range, faults dominantly

trend northeast, with very few, minor, faults which trend

either north-south, northwest, or east-west (see Sec. 27, 28, 29, T .24N., R.24E.; Plate 1).

In the southern Nightingale Mountains, a major north- south range bounding normal fault occurs along the eastern edge of Winnemucca Dry Lake, which bends to a northwest- southeast strike as it trends into the northwestern part of the study area. This fault has an approximate vertical displacement of at least 480 m; based on elevation differ­ ences from the alluvial plain to the top of the ridge in the southern Nightingale Mountains. Two normal faults occur to the east of this range bounding fault and parallel its trend

(see Sec. 34, T.25N., R.24E.; Plate 1). Within the fault zones, the host rock has often been brecciated and silicified and can contain large sparry calcite crystals and some clay. 88

The eastern-most of these three major, normal, northwest

trending faults exhibits the opposite displacement from the

two faults it parallels to the west. That is, along this

eastern-most fault, rocks have been displaced downward on the

east side. Andesite dikes striking generally east-west

were displaced in right lateral fashion (about 5 m horizontally)

by this fault. Minor northeast trending faults occur in

the northwestern part of the area (Ei, Sec. 33, T.25N., R.24E.;

Plate 1), which are truncated by and therefore are older

than the north-northwest striking faults. Many of these

fractures display displacement of less than 1 m. However, the largest of these northeast striking faults in the

southern Nightingale Mountains (N.35°E.)(E|, Sec. 33, and

W2, Sec. 34, T.25N., R.24E.; Plate 1) is a normal fault which

dips vertically and displays approximately 100 m of vertical

displacement. Another northeast trending fault (N.20°E.;

Ei, m i , Sec. 3, T.24N., R.24E.; Plate 1), exhibits approxi­

mately 60 m of total displacement along a series of step-like

faults. One low angle fault (listric?) occurs in the Wi-> Sec. 34, T.25N., R.24E. (Plate 1). Along the slip plane,

breccia clasts (which consist of the Tuff of Rattlesnake

Canyon and contain uranium mineralization) are rounded and

spheroidal and occur within the now argillized basal porous

glassy zone of the Tuff of Rattlesnake Canyon. Rounding of

the clasts probably occurred during fault movement. The now argillized, basal porous glassy zone of the Tuff of Rattle- snake Canyon could have provided a relatively frictionless

plane along which movement could occur. Displacement was

probably a result of landslide movement due to oversteepened

slopes; which were formed by previously developed northwest

trending steeply dipping normal faults which crosscut the area.

In the northern Truckee Range, a major northeast

trending fault (N.40°E.) splays along strike into two major

faults (which trend N.25°E. and N.55°E.)(Sec. 11, 14, 15,

22, 23, 27, and 28, T.25N., R.24E.; Plate 1). These faults

dip from near vertical to a slight inclination to the west.

Movement, which is difficult to determine due to the nature and distribution of rock types, is estimated to be at least

50 m of dip slip displacement, based on the relative position of rock units and crosscutting relationships. However, there is a rotational component of movement, as this major north­ east fault abruptly terminates at Creel Spring (Ej, Sec. 28,

T.24N., R.24E.; Plate 1). Therefore, scissor-type rota­ tional movement is indicated. Two north—south trending normal faults (Sec. 28 and 33, T.24N., R .24E.; Plate 1) ex­ hibit approximately 15 m of displacement. A minor, north­ west-southeast trending fault, in the NE|, Sec. 28, T.24N.,

R.24E. (Plate 1), exhibits approximately 30 m of horizontal displacement. Very few, minor, east-west striking faults occur in the very southern and southwestern portions of the study area (SWi, Sec. 27, and NWJ, Sec. 29, T.24N., R.24E.). 90

These faults exhibit minor vertical displacement of at least a few meters.

The boundary of the two major regional structural zones, the Basin and Range and the Walker Lane, trends through this

thesis area. Fault patterns and their relative movement in

this area display the effects of both major zones. The

southern Nightingale Mountains are crosscut predominantly by approximately north to south trending normal faults, which

are a structural feature more typical of the Basin and Range

style (Wright, 1976; Stewart, 1980; et cetera). Whereas,

in the northern Truckee Range, the dominant structural feature

is a major set of northeast (N.25°E. to N.55°E.) striking

faults. Although the sense of movement is difficult to

determine, these faults exhibit the same strike as left-

lateral first order conjugate shears to the right-lateral

Walker Lane system (Bell, 1982, personal communication;

Bell and Slemmons, 1982; Bell and Slemmons, 1979; Sanders and Slemmons, 1979).

Regionally, a northeast trending (N.40°E.) left-lateral

strike slip zone has been documented which extends, in width,

from Coyote Canyon to the Carson sink (known as the Carson

Lineament in this latter area) and includes the Olinghouse

fault zone in the Pah Rah Range (which has had movement along its length in historic time)(Bell, 1982, personal communica­ tion; Bell and Slemmons, 1982; Geason, 1980; Sanders and Slemmons, 1979; Wright, 1976). 91

Principal stresses, as shown generalized by Wright (1976) xllustrate the possible orientations of greatest stress directions of both the Basin and Range and Walker Lane

Structural systems, but as Tchalenko (1970) illustrates, shear zones can develop complex fault patterns to each shear zone relative to principal stress directions. Therefore, fault patterns in this study area are a reflection of the complex interaction between these two major structural zones and the various orders of conjugate shearing. 92

ECONOMIC GEOLOGY

GENERAL

This study area is within the Nightingale District,

which incorporates the southern Nightingale Mountains,

northern Truckee Range, and the southern part of the

Sahwave Range (to the east of this study area)(Bonham, 1969).

Most of this district is in Pershing County, however, several

prospects and a mine are located in the Washoe County portion

of the district, and are also in this thesis area (Plate 1).

The Nightingale District is best known for tungsten ores which Bonham (1969) discusses in concise detail. However,

occurrences of uranium, gold, silver, molybdenum, copper, and other metals are present in the district" (Bonham, 1969, p.70).

URANIUM MINERALIZATION

In the southern Nightingale Mountains, uranium deposits were first explored in the mid-1950's. Most of the claims occur along the southwestern escarpment of those mountains and are known as: the Lucky Day, Pegmatite, Nemrex Group, and Lucky Strike Claims. All of these claims are current, active claims and consist of numerous bulldozer cuts and an adit, approximately 61 m long. Bonham (1969) briefly re­ ported on uranium prospects in this area, and Garside (1973) concisely described the prospects at this locality. Bendix

Field Engineering Corporation recently reviewed this area as part of the National Uranium Resource Evaluation (NURE) 93

program (Hurley, et al, 1980).

Bendix geologists sampled rocks from this area and had

numerous analyses performed for uranium and other elements.

The analysis of 14 elements, other than uranium, from miner­

alized samples taken from the southern Nightingale Mountains

revealed that these tuffs contain "anomalous amounts of boron,

copper, manganese, molybdenum, vanadium, and yttrium when

compared with mean element abundance for 343 volcanic rocks

samples for the Reno quadrangle" (Hurley, et al, 1980).

These anomalous amounts probably reflect a high volatile

content in the original magma from which these ash-flow tuffs

originated, and/or mobilization of these elements on devitri­ fication, not unlike the release of uranium.

In the southern Nightingale Mountains, uranium mineraliza­ tion in the form of weeksite, carnotite, uranophane, and

autunite(?) occurs in unconformities, near the basal porous

glassy zone of the Tuff of Rattlesnake Canyon, and in fault

zones (Hurley, et al, 1980). Background radioactivity was

determined at 200 cps, and in the areas of the unconformities and fault zones, counts range from 250 to 10,000 cps.

Where the basal, porous glassy zone of the Tuff of Rattle­ snake Canyon disconformably overlies Ash-Flow Tuff Cooling

Unit A (NWi, NEJ, Sec. 3, T.24N., R.24E.; Plate 1), uranium content varies from 625 ppm to 3640 ppm, and radioactivity was measured at 6,000 cps. Argillization and minor zeoliti- zation are present and uranophane and autunite(?) were identified. Uranium content in the still glassy, vitro- 94

phyric zone of the Tuff of Rattlesnake Canyon, at 30 ppm, is

less than that in the underlying porous glassy zone but more

than the uranium content in the overlying densely welded, devitrified zone (6, 8, and 11 ppm).

In fault zones, uranium content is quite variable from one place to another in the study area. A northeast trending high angle fault in the northwestern portion of the Nightingale

Mountains, near the 'Lucky Day Mine' (NEJ, SEi, Sec. 33, T.25N. R .24E.; Plate 1), yielded a uranium content of 1350 ppm to

1810 ppm, with uranium content in the surrounding wallrocks

considerably lower (85 ppm and 15 ppm) (Hurley, et al, 1980).

Associated alteration includes calcite veining, silicifica-

tion, limonitization, and argillization. The deposit at the 'Lucky Day Mine’ is associated with a low angle fault

(listric?). Uranium content is 1810 ppm with radioactivity

measured at 6,000 to 7,000 cps. Uranium, in the form of

uranophane, weeksite, and carnotite (Hurley, et al, 1980), occurs in rounded, spheroidal clasts of the Tuff of Rattle­ snake Canyon set in a sheared, argillized, porous glassy matrix. In contrast, toward the southern end of the Nightingale

Mountains, a northwest trending fault yielded low uranium values of 11, 18, and 48 ppm, with limonitization and argilli­ zation as common alteration products.

In the southwestern portion of the study area, no uranium mineralization was seen nor has there been any prospecting activity, and radioactivity ranges from 150 cps to 250 cps. 95

In a porous, nonwelded zone which exists between Ash-Flow Tuff

Cooling Unit F and the overlying Chimney Springs Tuff, 400 cps was noted.

Most known uranium occurrences in southern Washoe County

that are related to Middle Tertiary rhyolitic ash-flow tuffs

(formerly known as the Hartford Hill Rhyolite Tuff), are

either confined to unconformable, stratigraphic boundaries,

or are controlled by structural features (Cupp, et al, 1977).

Deposits associated with stratigraphic boundaries occur in

the basal porous glassy nonwelded zones of ash-flow tuffs and are known as 'infiltration deposits' (Holmes, 1972). This

type of deposit can occur either at the disconformable boun­ dary between tuffs, within the volcanic pile, and/or at the non- to disconformable contact with much older, pre-Tertiary rocks at the base of the ash-flow tuff assemblage (Hutton,

1978). Uranium deposits controlled by structural elements occur in faults and are associated with basalt and andesite dikes (although andesite dikes occur in this study area, no uranium mineralization occurs in association with them). The source of the uranium in structural traps, or faults, origina­ ted from either pre-existing infiltration deposits or was

liberated from the devitrified tuffs by erosion and decomposi­ tion of that tuff" (Hutton, 1978, p.85).

Crystalline rocks have shown a uranium content from 20 to 30% less than its related obsidian (Rosholt and Noble, 1972;

Zielinski, 1978). Release of uranium by primary crystalliza­ ‘iftteitLr.s/ejr.r.'y-^

96

tion (devitrification) occurs during the post-emplacement,

cooling and compaction of ash-flow tuffs and is largely a

solid state process (Henry and Tyner, 1978). During

primary devitrification (when uranium is released), axiolitic and spherulitic intergrowths of cristobalite and tridymite

with alkali feldspar, accompanied by the loss of halogens and

the oxidation of iron, are produced in what was formerly

glass shards or massive glass (vitrophyre) (Smith, 1960;

Lipman, et al, 1969). Once uranium is released, and is in

the oxidized state, it is readily susceptible to leaching

by groundwater. The nonwelded, porous glassy zones or paleo-

soil horizons, which often contain carbonaceous matter, occur at the base of individual ash-flows and provide permeable

avenues for uranium-bearing solutions to flow through. There­

fore the volume of uranium released, as a result of devitrifi­

cation, is conceivably sufficient enough to contribute to

the formation of infiltration deposits (Holmes, 1972). Faults and shear zones have variable permeability and usually high clay and gouge content. Fluids could easily flow through open, permeable fault zones and conceivably be trapped in areas of high clay content. If solutions are uraniferous, the uranium could be adsorbed by clays (Hutton, 1978).

OTHER PROSPECTS AND MINERALIZATION

Within the northern Truckee Range, numerous non-uranium prospects and the Crosby Mine occur within pre-Tertiary rocks 97

and have been reported on, in concise detail, by Bonham

(1969). All prospects in this area are inactive and appear to have been idle for some time.

Crosby Mine

The Crosby Mine occurs in Sec. 21, T.24N., R.24E. (Plate

1). From this mine small tonnages of tungsten ore were

produced between 1942 and 1956, and the mine has remained idle since (Bonham, 1969). This mine has several hundred feet

of workings including at least two adits and several shafts.

No attempt at examination of the underground workings of this mine nor any examination via ultraviolet lamp was made by

this author. However, Bonham (1969) provides a geologic map of the lower level mine workings.

The Crosby Mine is a tungsten-bearing tactite body in the skarn zone developed along the contact between meta- sedimentary, carbonate rocks, and biotite—bearing granite, the assumed source of mineralizing fluids in this area. The granite is altered to endoskarn within a few feet of the contact with altered carbonate rocks, now exoskarn. The calcsilicate assemblage developed in the exoskarn zone in­ cludes; dominantly grandite, wollastonite, actinolite, quartz, and epidote (as a fracture coating). Sulphides were seen, in minor amounts, in exoskarn. Actinolite occurs dominantly in endoskarn. Quartz veining, locally containing sericite, occurs in the granite, adjacent to the skarn contact. 98

A knob of very coarse grained white quartz, related to the

intrusive, occurs uphill and just east of the mine area and

contains large, but broken, quartz crystals. This knob of

quartz has been previously reported as a "large, plug-shaped,

quartz-rich pegmatite" which contains minor scheelite (Bonham,

1969). However, no feldspars or other typical pegmatite miner als were observed by this author.

The Crosby Mine skarn zone is 150 to 215 m long, along

the intrusive/metasedimentary contact. This contact contin­ ues to trend uphill, out of the skarn zone, where the meta-

sedimentary rocks grade from carbonates to schists and horn- f els.

Other Prospects

Numerous small prospects occur in Mesozoic metasedimen-

tary rocks throughout the northern Truckee Range, generally south of the Crosby Mine (Plate 1).

Many small, unnamed prospects occur to the south of the

Crosby Mine. Quartz-sulphide mineralization generally occurs along the contact between amphibole mica schist and fractured

quartzites. Sulphides with hematitic jasperoid occur on

fracture surfaces in the quartzite and as small veins in

schist. Chalcopyrite has oxidized to cuprite followed by

malachite in thin veinlets. These veinlets are surrounded by a thin selvage of tremolite. Derivatives of the fluids which were responsible for 'skarnizing' at the Crosby Mine are probably responsible for the quartz-sulphide veining 99

formed further away from the intrusive in the metasedimentary rocks of the Nightingale Sequence.

Mineralization, seen to the southwest of the Crosby Mine, in Sec. 20, T.24N., R.24E. (Plate 1) (currently known

as the Midnight Claims), is in the form of copper staining

(as Chrysocolla) on fracture surfaces, in otherwise fresh,

steeply dipping limestone. The copper is probably related

to the nearby granite body, which is in close proximity

and is the intrusive body present at the Crosby Mine. Mines/ prospects in this area are also inactive.

Bonham (1969) briefly reports on those called the Black Warrior Peak Claims (now known as the "N Claims") which occur in the very southeast corner of the map area in Sec. 26,

T .24N., R.24E. These prospects are similar to those formed

just south of the Crosby Mine, and have been explored via

shallow pits and short adits. In general, slates and shales are crosscut by thin veins of material which were once sul­ phide-bearing but now dominantly contain the oxidized products. According to Bonham (1969), prospects are on quartz veins, located in granodiorite near the intrusive contact with phyllite and hornfels and contain minor amounts of sulphides, including; pyrite, galena, sphalerite, tetrahedrite, and their oxidized products. Originally, these veins were prospected for gold and silver (Bonham,

1969). Prospects in the area can also occur in "areas of pyritized granodiorite and hornfels" (Bonham, 1969). 100

GEOLOGIC HISTORY

Metasedimentary and metamorphic rocks of the Jurassic

Nightingale Sequence are the oldest rocks exposed in the study

area. These rocks are thought to have been deposited in a

eugeosynclinal environment. These rocks were subsequently

regionally metamorphosed and orogenically intruded, mineral­

ized, and uplifted by granodiorite and granitic plutons, of

probable Sierran affinity, during the Late Cretaceous.

During the period of time between the Late Cretaceous

and Late Oligocene, rocks of the former orogenic belt were

subjected to erosion and elevations probably did not exceed

2,000 ft. The Walker Lane right-lateral strike slip fault zone probably became active during this time as well.

Volcanic pyroclastic activity began during the Late

Oligocene. The oldest two ash-flow tuffs (Ash-Flow Tuff

Cooling Units A and B) were deposited between 29.6 and 29.0

million years ago. Following this deposition, an approxi­

mately one million year hiatus in volcanic activity elapsed

prior to the emplacement of the overlying Tuff of Rattlesnake

Canyon. During this hiatus significant erosion of Ash-Flow

Tuff Cooling Units A and B occurred. Channels, as deep as

125 m, developed on top of these two ash-flows, often mimicking the paleotroughs and ridges of the underlying pre-Tertiary rocks paleotopography.

Following this erosional period, the Tuff of Rattlesnake Canyon, a thick, zoned ash-flow tuff, was emplaced about 28.6 101

million years ago. The Tuff of Rattlesnake Canyon's flow was

generally restricted to the previously developed channels, and in this area, this unit is closer to its source than at Bacon

Rind Flat in the Pah Rah Range (Bonham, in press)(Figure 6).

After emplacement of this unit, a short hiatus occurred, during

Which a thin paleosoil (with a few trees) and minor channeling- developed in the partially welded top.

Approximately 28.1 million years ago, the Coyote Spring Tuff was emplaced, and was followed, again, by a long period

of erosion and nondeposition. A paleosoil horizon, with

forest, developed on top of the Coyote Spring Tuff during this hiatus.

The tuff of Jackass Spring was deposited on top of the Coyote Spring Tuff, on an undulating, but relatively flat

surface. Ash-Flow Tuff Cooling Unit E was emplaced imme­

diately after the emplacement of the tuff of Jackass Spring.

Three ash-flow tuff cooling units, Ash-Flow Tuff Cooling

Unit F, the Chimney Springs Tuff, and the tuff of Gary’s Ridge

crop out in the southwestern portion of the study area, and

if they were deposited in the area of the southern Nightin­ gale Mountains, they have since been eroded off. Ash-Flow

Tuff Cooling Unit F and the Chimney Springs Tuff were emplaced approximately 23 million years ago, very shortly after one another, as no erosional features or paleosoil horizons were observed along the contacts between these two units. The overlying tuff of Gary's Ridge was emplaced between 21.8 and 20.5 million years ago on a relatively flat surface. 102

All nine of these ash-flow tuffs are hydrothermally

altered to a minor degree and exhibit some supergene effects.

The timing of this alteration, and the introduction of fluids,

was possibly subsequent to the emplacement of each pyroclastic flow. The hydrothermal alteration of these ash-flow tuffs

indicates either a proximal location of these tuffs to their

source area, and the hot fluids, or that the alteration is due to later Tertiary igneous activity.

In the southern portion of this study area, the paleo- topography was of higher elevation during the time of this

pyroclastic activity. In the southern Nightingale Mountains and northern Truckee Range, Middle Tertiary rhyolitic-type

pyroclastic volcanism ended approximately 20.5 to 20 million years ago. The source(s) for these ash-flow tuffs, based

mostly on increased thickness (being nearer source(s)), is

believed to be to the west, probably from the Pah Rah Range.

A hiatus in volcanic activity occurred at the end of this type of silicic volcanism from 20 million years ago until

17 million years ago; a date coincident with the onset of

Basin and Range faulting. Volcanism resumed and thick,

extensive flows of andesite and basalt were extruded. The

area of this study was probably a relative paleohigh region,

as andesite, basalt, dacite, and minor silicic ash-flow tuff rocks of the Chloropagus Formation were deposited, approxi­ mately 14.5 million years ago, three million years after inception of Basin and Range extensional tectonics. The 103

rocks of the Chloropagus Formation disconformably rest on top

of the undulating paleotopographic surface developed on the older, Late Oligocene to Early Miocene rhyolitic ash-flow

tuff cooling units and Mesozoic metasedimentary, metamorphic,

and granitic rocks. These flows could have blocked drainages

during their emplacement, thus damming streams and thereby

forming lakes. Thin sedimentary deposits of stream and lake

sediments and airfall ash are often intercalated between volcanic flows.

Volcanic activity, responsible for the Chloropagus

Formation, ceased approximately 14 million years ago. A last phase of volcanism resumed during the Middle to Upper

Pliocene, with the emplacement of basalt flows.

Uplift and eastward tilting of the Nightingale Mountains and Truckee Range occurred due to interaction between Basin and Range extension and Walker Lane strike slip movement.

During the Late Pleistocene, Lake Lahontan occupied

Wmnemucca Lake valley and adjoining valleys and canyons; attaining levels of at least 4,400 ft (by present day eleva­ tions ) .

Since Pleistocene time, sediments derived from local sources have been deposited on alluvial fans extending from the mountain ranges, and Basin and Range faulting as well as

Walker Lane movement probably continues until the present day.

Holocene erosional processes have formed many deeply incised stream canyons, and are currently dissecting this area today. 104

recommendations and conclusions

This study identifies nine ash-flow tuff cooling units

of Late Oligocene age, formerly included in the Hartford Hill

Rhyolite Tuff, and also recognizes four volcanic units with a miscellaneous sedimentary unit (composed of many thin

sedimentary horizons) as part of the Miocene Chloropagus

Formation in the southern Nightingale Mountains and northern Truckee Range.

Each of the nine ash-flow tuff cooling units is distinct and mappable on the basis of mineralogy, textural character­

istics, variations in welding, and the presence of bounding

disconformities. Of the nine ash-flow tuffs identified in

this study area, six are exposed in the southern Nightingale

Mountains and three in the northern Truckee Range. Five of

of the nine recognized in this study area are tentatively

correlated to units informally named in other areas by pre­ vious authors.

The six ash-flow tuffs which were identified i.n the

southern Nightingale Mountains, were deposited over an

approximately 2.5 million year time span (from 29.6 to 27.0 million years). Most of these ash-flow tuffs had significant

erosional periods between emplacement of subsequent units.

All of the six units in this portion of the study area contain a very small percentage of quartz as a phenocryst constituent.

Three of these ash-flow tuff cooling units tentatively cor­ relate to ash-flow tuff cooling units recognized elsewhere 105

m western Nevada by other workers. The three ash-flow tuffs

identified in the northern Truokee Range, were deposited be­ tween 23.0 and 20.5 million years ago. All three of these

ash-flow tuffs contain fairly abundant quartz phenocrysts

and two of the three tentatively correlate to ash-flow tuff units recognized in the nearby Pah Rah Range.

The present expression of the southern Nightingale Mountains and northern Truckee Range is that of an eastward tilted horst block, typical of Basin and Range extension.

However, faults exposed in the northern Truckee Range are

related to the Walker Lane right-lateral strike slip fault

zone. Where the Basin and Range and Walker Lane regional

Structural zones overlap, there is a high degree of structural complexity.

Known uranium occurrences in the southern Nightingale Mountains are generally associated either with the Tuff of

Rattlesnake Canyon or occur in fault zones. Where uranium

deposits are associated with ash-flow tuffs, they occur most often in or near the basal porous glassy zone.

The underlying pre-Tertiary metasedimentary rocks have received little attention. A study of the stratigraphy and internally complex structures within these rocks could offer insight into an understanding of their depositional and tectonic settings in western Nevada.

In summary, this study is one in a series of investiga­ tions which interpret the stratigraphy of Middle Tertiary silicic ash-flow tuffs, particularly those units which occur 106

north of the Truckee River. Further study of these rooks

' should be undertaken to not only determine the distribution of the individual cooling units, but to determine the loca­

tion and geometry of their source vent areas. This should

be important in light of the known uranium content associated with these Middle Tertiary ash-flow tuffs.

These tuffs form the base of the Tertiary section in western Nevada. A better understanding of their areal

distribution, structural history, source area, and chemistry would shed light on the tectonic regime responsible for this type of volcanism and the relationship to later, more mafic, volcanic activity. 107

references cited

Atwater, T. , 1970, Implications of plate tectonics for the enozoic tectonic evolution of western North America1 Geol. Soc. America Bull., V. 81, no. 12, pp. 3513-3535.

Axelrod, D.I., 1956, Mio-Pliocene floras from west-central Nevada: Calif. Univ. Pub. Geol. Sci., V. 33, 322 p.

-----j. 1966, Potassium-argon ages of some western Tertiary floras: Amer. Jour. Sci., V. 264, no. 7, pp. 497-506.

Bateman, P.C., and Wahrhaftrig, C., 1966, Geology of the ianrra N?vada; Geology of northern California, Bull. 190, Calif. Div. of Mines and Geol., pp. 107-169.

Bel1, E.J and Slemmons, D.B., 1979, Recent crustal movements in the central Sierra Nevada-Walker Lane region of California-Nevada: Part II, The Pyramid Lake right- S^-1P fault zone segment of the Walker Lane: Tectono- physics, V. 52, pp. 571-583.

-----, 1982, Neotectonic analysis of the northern Walker Lane western Nevada and northern California fabstr.] : Geol. Soc. America, Abstr. with Programs, V. 14, no. 4, pp. 148

Bonham, H.F. Jr., 1960, Reconnaissance geologic map of the Nightingale Mountains and Truckee Range, Nevada: Unpub­ lished Geologic Map, Southern Pacific Railroad Co.

-----, 1969, Geology and mineral deposits of Washoe and Storey Counties: Nev. Bur. of Mines and Geol. Bull r -7 A H A - 1- •

-----, in press, Geology of the Moses Rock Quadrangle, Nevada.

Bmgler, E.C., 1973, Oligocene welded tuff sequence in the northern Wassuk Range, central-western Nevada [abstr.] : Geol. Soc. America, Abstr. with Programs, V. 5 no 1 pp. 12. > • >

1977, Ash-Flow stratigraphy of the " Hartford Hill Rhyolite" in the type area [abstr] : Geol. Soc. America Abstr. with Programs, V. 9, no. 4, pp. 389-390.

, 1978a, Abandonment of the name Hartford Hill Rhyolit e Tuff and adoption of new formation names for middle Tertiary ash-flows in the Carson City-Silver City area Nevada: U.S. Geol. Survey Bull. 1457-D, 19 p. 108

-----, 1978b Geology of the Schurz 15 minute quadrangle andanSnr^i Geol., Map No.Counties- 60. Nevada: Nev. Bur. of Mines

Brooks, H 1956, Geology of a uranium deposit in the Vire:inia Thesislnnn-WaSh°+ Coifnty’ Nevada: unpublished [ M . S . f Thesis, University of Nevada, Reno, 50 p. J

Christiansen r l and Lipman, P.W., 1972, Cenozoic volcanism and plate tectonic evolution of the western United

ST? LTblT- m -’ edf '’ Paciflc Section Society of Econ. Paleon and Mineral., pp. 297-323. on’

Cook, E.F 1965, Stratigraphy of Tertiary volcanic rocks n eastern Nevada: Nev. Bur. of Mines and Geol., Kept. 11, 60 p. ’

Cupp G M. Eeedom, S.H., Mitchell, T.P., and Allen, D.R., ~ ’ uranium deposits and uranium favorabilitv of the Hartford Hill Rhyolite and Truckee Formation cTJ:eStr T / aSh°e County- Nevada, and eastern Lassen County, California: Bendix Field Eng. Corp., Grand Junction, Colorado, prepared for, U.S. Dept, of Energy contract no. E(05-1)-1664. GJBX-16(77). 61 p.

Dalrymple, G.B 1979, Critical tables for conversion of pp.r55l-560r°m °ld t0 neW constants: Geology, V. 7,

D e m o , A . , in press, Stratigraphy, petrology, and geochronology califorma-Nevada:CalTfori?. Mr h 1*7 a^ 33 ~ p. flow tuffs> Seven Lakes Mountain,

Ekren E .B., Byers, F.M., Hardyman, R.F., Marvin, R.F., and S i l k r m lit. , 1980, Stratigraphy, preliminary petrology indfh^mr H^rUSt^ al features of Tertiary volcanic rocks the Gabbs Valley and Gillis Ranges, Mineral County, Nevada. U.S. Geol. Survey Bull. 1464, 54 p. Evernden, J.F and tho t ' i d J-T-> 1964, Potassium-argon dates and the Tertiary floras of North America: Amer. Jour Sci . V. 262, no. 8, pp. 945-974.

Evernden J.F. and Kistler, R.W., 1970, Chronology of emplace- w“ ?da o T ° r C ?0nf lexes ln California and western iNevaaa. U.S. Geol. Survey Prof. Paper 623, 42 p. 109

Garside, L.J., 197o, Radioactive mineral occurrences in Nevada: Nev. Bur. of Mines and~Geol™Bull 81, 121 p

-----M}’fcKeehaScKee, ’HH• H., 'Fi«Sf 1981, hie^ Radiometric R 'P" Sllbe™ages of “ volcanic . M.L., and and Nevada"1- J°JkS and hydrothermal mineralization in Nevada determinations run under the USGS-NBMG cooperative program: Isocbron/West, no. 30, pp. 11-19. cooperative

Geason D.L. 1980, The geology of a part of the Olinghouse

LfM - • SingT.J Thesis, h M i ^ 1Cn University ’-WaSh°+ C°Unty'of Nevada, Nevada: Reno, unpublished 118 p.

Gianelia, V.P., 1936, Geology of the Silver City district and the southern portion of the Comstock Lode Nevada- Nevada Univ. Bull., V. 30, no. 9, 108 p. da •

Hague, A. and Emmons, S.F., 1877, Descriptive Geology: U.S. Geol. Exploration of the 40th Parallel Rept., V. II.

Hardyman R F 1978, Volcanic stratigraphy and structural g m f Gl^lls Canyon quadrangle, northern Gillis Range, Mineral County, Nevada: unpublished [Ph d ] Dissertation, University of Nevada, Reno, 248 p.

Henry C D and Tyner G.N., 1978, Alteration and uranium + 5r?r? rhyolltlc igneous rocks: Examples from the Mitchell Mesa Rhyolite, Santana Tuff, Chinati inUFin,1?SpGr°U? ’pand Allen ComPlex> Trans-Pecos Texas; in Final Report Formation of Uranium ores by Diagenesis of Volcanic Sediments; for Bendix Field Eng Corp subcontract no. 77-067-E. S

Hildreth, W., 1979, The Bishop Tuff: Evidence for the origin zonation ^ silicic magma chambers: 1^ Ash Flow Tuffs, C.E. Chapin and W.E. Elston eds., Geol 8oc. America opec. Paper 180, pp. 43-76.

Holmes P.J., 1972 Infiltration uranium deposits in ash-flow RenoS ’65UpPUbllShed M 'S‘ Thesis’ University of Nevada,

Houghton, J.G., Sakamoto, C.M., and Gifford, R.O. 1975 Nevada's weather and climate: Nev. Bur. of Mines and Geol., Sp. Pub. 2, 78 p.

Hudson, D.M., and Oriel, W., 1979, Geologic map of the Map^64ln RangS’ Nevada: Nev. Bur. of Mines and Geol.,

Huriey B .w., Johnson, C.L., Cupp, G.M., Mayerson, D.L., Dodd * 'A•, and Berg, J.C., 1980, Uranium resource evaluation,’ Reno quadrangle, Nevada and California: Bendix Field En°-. Corp., Grand Junction, Colorado, prepared for, U.S Dept^ oi Energy, contract no. DE-AC13-76GJ01664, PGJ 037, pp". 31-34 110

Hutton, R.A., 1978, Geology and uranium content of middle Tertiary ash-flow tuffs in the northern part of Dogskin Mountain, Nevada: unpublished [M.S.] Thesis, University of Nevada, Reno, 103 p.

Johnson, M.J., 1977, Geology and mineral deposits of Pershing County, Nevada: Nev. Bur. of Mines and Geol., Bull. 89 115 p.

King, C., 1878, Systematic Geology: U.S. Geol. Exploration of the 40th Parallel Rept., V. I.

Lipman, P.W., Christiansen, R.L., and Van Alstine, R.E., 1969, Retention of alkalis by calc-alkalic rhyolites during crystallization and hydration: Amer. Mineral., V. 54, pp. 286-291.

McKee, E.H., 1971, Tertiary igneous chronology of the Great Basin in the western United States — implications for tectonic models: Geol. Soc. America Bull., V. 82, pp. 3497-3501.

Nilsen, T.H., and McKee, E.H., 1979, Paleogene paleogeography of the western United States: iui Cenozoic Paleogeography of the western United States, Pacific Coast Paleogeography Symposium 3; Armentrout, J.M., Cole, M.R., and Terbest, H. Jr., eds., Pacific Section Soc. Econ. Paleon. and Mineral., pp. 257-276.

Noble, D.C., 1972, Some observations on the volcano-tectonic evolution of the Great Basin, western United States: Earth and Planetary Sci. Letters, V. 17, pp. 142-150.

Proffett, J.M., and Proffett, B.II., 1976, Stratigraphy of the Tertiary ash-flow tuffs in the Yerington district, Nevada: Nev. Bur. of Mines and Geol., Rept. 27, 28 p.

Rai, V.N., 1968, Geology of a portion of the Nightingale and Truckee Ranges, Washoe and Pershing Counties, Nevada: unpublished M.S. Thesis, University of Nevada, Reno, 45 p.

Rosholt, J.N. Jr., and Noble, D.C., 1969, Loss of uranium from crystallized silicic volcanic rocks: Earth and Planetary Sci. Letters, V. 6, pp. 268-270.

Ross, C.S., and Smith, R.L., 1960, Ash-Flow tuffs: their origin, geologic relations and identification: U.S. Geol. Survey Prof. Paper 366, 81 p.

Russell, I.C., 1885, Geological history of Lake Lahontan, a Quaternary lake of northwestern Nevada: U.S. Geol. Survey Mon. 11. Ill

Sanders, C.O., and Slemmons, D.B., 1979, Recent crustal movements in the central Sierra Nevada-Walker Lane region of California-Nevada: Part III, The Olinghouse fault zone: Tectonophysics, V. 52, pp. 585-597.

Scholz, C.H., Barazangi, M., and Sbar, M.L., 1971, Late Cenozoic evolution of the Great Basin, western United States, as an ensialic interarc basin: Geol. Soc. America Bull., V. 82, pp. 2979-2990.

Silberman, M.L., Stewart, J.H., and McKee, E.H., 1976, Igneous activity, tectonics, and hydrothernal precious- metal mineralization in the Great Basin during Cenozoic time: Amer. Inst, of Mining, Metal. and Petrol. Eng. Trans., V. 260, pp. 253-263.

Smith, J.G., McKee, E.H., Tatlock, D.B., et al, 1971, Age and chemistry of plutons in northwestern Nevada and their relationship to the Sierra Nevada and Idaho batholiths [abstr.] : Geol. Soc. America, Abstr. with Programs, V. 3, pp. 197.

Smith, R.L., 1960, Zones and zonal variations in welded ash flows: U.S. Geol. Survey Prof. Paper 354-F, pp. 149- 159.

Smith, W.C., and Guild, P.W., 1942, Tungsten deposits of the Nightingale district, Pershing County, Nevada: U.S. Geol. Survey Bull. 936-B, pp. 39-58.

Stewart, J.H., and Carlson, J.E., 1976, Cenozoic rocks of Nevada: Nev. Bur. of Mines and Geol., Map 52.

Stewart, J.H., 1980, Geology of Nevada: Nev. Bur. of Mines and Geol., Sp. Pub. 4, 136 p.

Streckheisen, A., 1979, Classification and nomenclature of volcanic rocks, lamprophyres, carbonatites, and metitic rocks: Recommendations and Suggestions of the IUGS Subcommission on the Systematics of Igneous Rocks: Geology, V. 7, pp. 331-335.

Tchalenko, J.S., 1970, Similarities between shear zones of different magnitudes: Geol. Soc. America Bull., V. 81, pp. 1625-1640.

Thompson, G.A., 1956, Geology of the Virginia City quadrangle, Nevada: U.S. Geol. Survey Bull. 1042-C, pp. 45-77.

Trexler, D.T., 1977, Geology of the Carson City 7-1/2' quad­ rangle: Nev. Bur. of Mines and Geol. Environmental Folio, Carson City area. 112

Wallace, A.B., 1975, Geology and mineral deposits of the Pyramid district, southern Washoe County, Nevada: unpublished [Ph-D.] Dissertation, University of'Nevada, Reno, 162 p.

Willden, R., and Speed, R.C., 1974, Geology and mineral deposits of Churchill County, Nevada: Nev. Bur. of Mines and Geol., Bull. 83, 95 p.

Wright, L., 1976, Late Cenozoic fault patterns and stress fields in the Great Basin and westward displacement of the Sierra Nevada block: Geology, V. 4, pp. 489-494.

Zielinski, R.A., 1978, Uranium abundances in associated glassy and crystalline rhyolites of the western U.S.: Geol. Soc. America Bull., V. 89, pp. 409-414.