UNIVERSITY OF NEVADA, RENO
Geology and Uranium Mineralization of the Hallelujah Junction, Red Rock Canyon Area, Lassen County, California, Washoe County, Nevada
A thesis submitted in partial fulfillment of the requirements for the degree of
Master of Science in Geology
by
Mark J. Cinque ill
November, 1979 1 kind
The thesis of Mark J. Cinque is approved: l h e . s u I3U 1
The-sis Advis'or ~ F
/O
Department Chairnfan
/
i University of Nevada, Reno * i } i1 \
November, 1979 11
ACKNOWLEDGEMENTS
The writer wishes to express his gratitude to Pathfinder
Mines Corporation for providing financial support for his thesis. Special thanks are due to Mr. Don Anderson, Manager, Exploration and Development Department, and Mr. Charles D.
Snow, District Geologist, Reno office, for approving the request for thesis support. In addition, Mr. Snow provided invaluable assistance and inspiration throughout the proj ec t .
Drill hole information was cordially supplied by Pathfinder Mines and Energy Fuels Corporation. Sincere thanks are due to the geologists of these offices for their cooperation.
Dr. L.T. Larson provided guidance and encouragement during the field work and text preparation, and his tireless efforts are greatly appreciated. Many thanks are due to the geologists of the Mackay School of Mines, Nevada Bureau of Mines and Geology, and the Reno exploration office of
Pathfinder Mines, for their helpful comments and discussions.
A very personal note of thanks is due Miss Kathleen Butler for typing this manuscript. Ill
ABSTRACT
The oldest rocks exposed within the mapped area are the Mesozoic granodiorite and associated plutonic rocks of Petersen Mountain. Middle Tertiary ash-flow tuffs unconformably overlie the plutonic rocks along the western flank of Petersen Mountain. Unconformably overlying the tuffs is a sequence of Pliocene fluviatile and lacustrine rocks exposed in Long Valley.
Uranium deposits occur within the ash-flow tuffs and the Pliocene sediments. The principal source of uranium for both types of deposits is believed to be the granodioritic rocks of Petersen Mountain, with subordinate amounts of uranium coming from the tuffs. The deposits within the tuffs are thought to be supergene veins formed by mobilization of uranium in a weathering environment and deposition in receptive porous zones. The deposits within the sedimentary rocks are believed to have.been formed by a geochemical cell (roll-front system) that moved westward through the sediments. IV
TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS...... ii
ABSTRACT...... iii ILLUSTRATIONS...... vi
INTRODUCTION...... 1
GENERAL STATEMENT ...... 1 PHYSIOGRAPHIC SETTING AND LOCATION ...... 1 METHODS AND PROCEDURES ...... 4 PREVIOUS WORK ...... 5 DESCRIPTIVE GEOLOGY...... 7
GENERAL STATEMENT ...... 7 MESOZOIC PLUTONIC ROCKS ...... 9 HARTFORD HILL RHYOLITE ...... 12 The Cooling Unit Concept ...... 12 Problems of Nomenclature ...... 16 Mapped Units ...... 18 Unit 1A lower (ThlAl) ...... 23 Unit 1A upper (ThlAuf ...... 25 Unit IB lower (ThlBiy ...... 25 Unit IB upper (ThlBu) ...... 26 Unit 2 (Tn2) ~ ^ 7 ~ ...... 28 Unit 5 (Th3) ...... 28 Upper Units Member (Thu) ...... 29 Age, Regional Correlation and Origin ...... 29
PLIOCENE FLUVIATILE AND LACUSTRINE DEPOSITS . . . 34
Problems of Nomenclature . . . . 34 Mapped Units ...... 37 Arkose (Tar) ...... 37 Interbedded Siltstone (Tst) 40 White, Water-Laid Tuff (Twt) . 43 Paleontology ...... 45 Environments of Deposition . . . 48
TERTIARY BASALT 51 V
Page QUATERNARY DEPOSITS ...... 54 STRUCTURAL GEOLOGY ...... 56
REGIONAL STRUCTURE ...... 56 STRUCTURE OF THE HALLELUJAH JUNCTION- RED ROCK CANYON A R E A ...... 59
URANIUM D E P O S I T S ...... 65
CHARACTER OF DEPOSITS ...... 65
Buckhorn M i n e ...... 6 5 Bastain Claims ...... 68 Lucky Day/Valley View P r o s p e c t ...... 71 RME P i t s ...... 71 Yellow Jacket Cuts ...... 73 Jeanne K., Barbara L., Herbal Prospect and scattered outcrops ...... 76
GENESIS OF THE URANIUM DEPOSITS ...... 78
Source of Uranium ...... 78 Mobilization and Deposition of Uranium ...... 83
EXPLORATION POTENTIAL ...... 89 BIBLIOGRAPHY ...... 91
APPENDIX I ...... 95
APPENDIX II...... 114 VI
ILLUSTRATIONS
Figures Page 1 Index Map of Thesis Area ...... 2
2 Physiographic Map of Thesis Area ...... 3
3 Classification System for Ash-flow Tuffs ...... 15
4 Photograph of Mesozoic Granodiorite and Mapped Units ThlAl and ThlAu ...... 19 5 Photograph of Mapped Units ThlAu and ThlBl .... 19
6 Photograph of Mapped Units ThlBl, ThlBu and Th2 . 20 7 Photograph of Mapped Unit T h 2 ...... 20
8 Photograph of Mapped Unit T h 3 ...... 21 9 Photograph of Mapped Unit T h u ...... 21 10 Stratigraphic Section of the Study Ar e a ...... 22 11 Correlation Chart for the Ash-Flow Tuffs of the Study Area ...... 24
12 Photograph of Arkosic Sediments at the Basal Contact of Mapped Unit T h l B l ...... 27
13 Photograph of Mapped Unit T a r ...... 38
14 Photograph of Leached Sandstone in Section 5, T. 23 N. , R. 18 E ...... 38
15 Photograph of Interbedded Siltstone Unit (Tst) . . 41
16 Photograph of White, Water-lain Tuff Unit (Twt) . 44 17 Photograph of Tertiary Basalt (Tb) ...... 52
18 Photograph of Tertiary Basalt Unconformably Overlying Unit T s t ...... 52
19 Photograph of Quaternary Boulder Train ...... 55 20 Photograph of Fractured, Altered and Limonite Stained Tuff of Unit ThlBl at the Buckhorn M i n e ...... 67 vi i
Figures Page 21 Photograph of Fractured and Brecciated Tuff of Unit ThlAl at the Bastain Prospect . . . 69
22 Photograph of Carbonaceous Material in Unit ThlAl at Bastain Prospect ...... 70
23 Photograph of Mapped Unit ThlBl at Lucky Day Prospect ...... 72
24 Photograph of Leached Sandstones and Interbedded Mudstones Exposed in RME P i t s ...... 72
25 Close-up of Leached Sandstones in RME Pits .... 74
26 Photograph of Leached Sandstones in Yellow Jacket C u t s ...... 74
27 Close-up of Leached Sandstones in Yellow Jacket C u t s ...... 75
28 Photograph of Leached Sandstones Exposed at Cornelia C. Prospect ...... 77
Plates (In Pocket)
1 Geologic Map of the Hallelujah Junction-Red Rock Canyon Area
2 Location Map - Uranium Occurrences
3 Cross Sections 1
INTRODUCTION
GENERAL STATEMENT
The principal objective of this study is to determine the genesis and geologic history of the uranium deposits within the area. This information will facilitate the uranium resource evaluation of this area and other areas with similar geology. The approach used was to 1) map and describe the general geology of the area; 2) describe and study the uranium deposits within it, and 3) develop a conceptual model for the genesis of these deposits.
PHYSIOGRAPHIC SETTING AND LOCATION
The area mapped includes the portions of Long Valley and the western flank of Petersen Mountain between
Hallelujah Junction and Red Rock Canyon (Fig. 1). The
California-Nevada border passes through the area, which includes portions of Lassen County, California, and Washoe
County, Nevada. It is accessible by U.S. 395, California
Highway 70, Red Rock Canyon Road, and numerous dirt roads.
Long Valley is bounded on the west by the Diamond Mountains, and on the east by Petersen Mountain (Fig. 2).
Long Valley Creek flows northward between these ranges, draining the entire area. It is fed by numerous creeks south of the area, and by Red Rock Canyon Creek and several intermittent streams within the area. 2
0 5 10 Figure 1- Miles Index map of thesis area. kj a ., . i 0 5 10 gi~E-j"ta... i Kilometers
4
The highest point in the area is Red Rock Peak, which
rises to an altitude of 7109 feet above sea level in the
center of Petersen Mountain. The lowest point is 4480 feet, on the bed of Long Valley Creek, at the northern border of
the area. The climate is semi-arid, with an average annual precipitation of 12 to 15 inches per year (Bonham, 1969) .
Summer temperatures may rise to over 100° F, and winter
lows may fall below 0° F. The vegetation consists primarily
of sagebrush and juniper trees.
METHODS AND PROCEDURES
Approximately 50 square miles were mapped between
September, 1978 and August, 1979. Sixty days were spent
in the field. The mapping was done on the Chilcoot and
Dogskin 15 minute quadrangles, which were enlarged to a
scale of 1 :20,000. Fifty-three thin sections were prepared and studied
from rock samples of typical and distinctive lithologies
collected in the area, and ten other rock samples were
analyzed for uranium content and daughter product equi
librium by the Ore Control Lab at Pathfinder's Lucky Me
Mine. Lithologic and radiometric logs from 13 drill holes within the area were incorporated into the structural and
stratigraphic interpretations. 5
PREVIOUS WORK
Numerous publications address some aspect of the geology of the study area, or discuss correlative rocks in other areas. There has been no definitive work done on the
Tertiary sedimentary rocks of Long Valley, or on the uranium deposits therein. It should be noted that these rocks have never been formally assigned to a recognized formation.
Several publications describe the Hartford Hill Rhyolite in Washoe and Storey Counties.
A description of the rocks of west-central Nevada is included in the Report of Investigations of the 40th
Parallel (King, 1878). The presence of sedimentary rocks in Long Valley was noted by Louderback (1907) , and Anderson
(1910) briefly describes the Tertiary sediments of this and other areas. Lindgren (1911) mapped the Tertiary gravels of the Sierra Nevada province.
McJannet (1957) was the first to describe the tuffs of the Red Rock Canyon-Pyramid Lake area. Van Couvering
(1962) mapped the Chilcoot quadrangle, and Burnett and
Jennings (1965) mapped the Chico AMS sheet. Bonham (1969) mapped the geology and described the mineral deposits of
Washoe County, Nevada, and Garside (1973) described the radioactive mineral occurrences of Nevada, including seven prospects within the present study area. Cupp, et al
(1977) evaluated the uranium favorability of the Hartford 6
Hill and Truckee Formations in this and other areas, and
Larson, et al (1978 p.214 ) discuss the uranium potential of the area in their report on the geologic framework and uranium favorability of the Great Basin. Hutton (1978) describes the middle Tertiary ash-flow tuffs of Dogskin
Mountain, 10 miles east of the study area. Drumheller (1979) studied the igneous rocks of Petersen Mountain. 7
DESCRIPTIVE GEOLOGY
GENERAL STATEMENT
There are three principal rock types within the mapped
area: Mesozoic plutonic rocks, middle Tertiary ash-flow
tuffs, and Pliocene fluvial and lacustrine deposits.
Quaternary alluvial deposits occur throughout the area.
The predominant lithology of the Mesozoic plutonic
rocks is a biotite-hornblende granodiorite, which comprises
the bulk of Petersen Mountain, and those parts of the
Diamond Mountains that lie within the study area. Grano
diorite also makes up the bulk of most of the adjacent ranges,
including Freds Mountain, Dogskin Mountain, the Bald
Mountains, Granite Peak and the Fort Sage Mountains
(Bonham, 1969).
Middle Tertiary ash-flow tuffs of the Hartford Hill
Rhyolite uncomformably overlie the plutonic rocks. They are exposed within the study area on the west flank of
Petersen Mountain and within Red Rock Canyon. They form
the mass of Seven Lakes Mountain, immediately to the north
of the mapped area, and outcrop on the northern end of
Dogskin Mountain. A thickness of more than IB 00 feet is
represented in the study area, although the complete section
of tuffs attains a thickness of over 4000 feet in Mullen
Pass and Dogskin Mountain (Bonham, 1969) . They are pre
dominantly vitric and vitric-crystal tuffs, which vary from 8
rhyolitic to andesitic. The tuffs have been subdivided
and mapped as individual cooling units, which were
emplaced over a 6 million year period, from 28 to 33 million years ago (Wallace, 1975, Hutton, 1978, Bingler,
1978). A sequence of fluviatile and lacustrine sandstones,
siltstones and water-lain tuffs unconformably overlie the
Tertiary volcanics and Mesozoic plutonic rocks. They
include a basal arkosic sandstone, an interbedded siltstone
and sandstone unit, and an uppermost white, extremely
fine grained, water-lain tuff. This water-lain tuff
contains 85% amorphous silica and is mined as a cement
additive. The sedimentary rocks occur in Long Valley and along the base of the west side of Petersen Mountain.
They range in thickness from 0 to more than 3000 feet, and are middle Pliocene in age (Gimlett, 1967, p. 12).
Lithologically similar rocks of the same age crop out in
Warm Springs Valley and Hungry Valley.
A basalt flow unconformably overlies the ash-flow tuffs and Pliocene sedimentary rocks at three separate
locations on the west flank of Petersen Mountain. The
precise age of this basalt was not determined, but its unconformable stratigraphic relationships in one locality
indicate a late or post-Pliocene age.
Four types of Quaternary deposits occur in the area: flood plain deposits, colluvium, pediment gravels, and boulder beds. 9
MESOZOIC PLUTONIC ROCKS
Plutonic rocks are exposed throughout most of Petersen and the Diamond Mountains (Plate 1). Compositional variations range from gabbro to granite, with granodiorite the predominant lithology. A typical sample of granodiorite is light to medium gray, massive, coarse grained, and hypi- diomorphic-granular. Outcrops generally weather to smooth rounded surfaces and abundant sandy debris, although exposures of fresh rocks are abundant in areas of high relief.
Two widely separated samples of granodiorite from the
Diamond Mountains and one from Petersen Mountain were very similar in thin section, with the following mineral assem blages: plagioclase (A^^ 35) 40%, quartz 30%, potassium feldspars 15%, biotite 10%, hornblende 3%, magnetite 1%, sphene, zircon and apatite, less than 1%. Plagioclase feldspars are euhedral to subhedral, and range in composition from oligoclase to andesine. Normal and oscillatory zoning are well developed. Quartz is anhedral, commonly coarse grained, and generally strained. Potassium feldspars include orthoclase and microcline, which are subhedral to anhedral. Graphic intergrowths of quartz in oligoclase were observed in all three slides. A perthitic intergrowth of microcline and plagioclase was noted in a thin section of one rock specimen from the Diamond Mountains. Biotite is typically anhedral, and is occasionally altered to chlorite. Hornblende was a minor constituent in thin section, but it is more apparent 10
in hand sample, where it appears in euhedral form. Magnetite, sphene, zircon and apatite are commonly euhedral, and are present in trace amounts.
Aplite dikes and pods of granite occur throughout the areas of granodioritic rocks. One granite from the Diamond
Mountains contains 20% microcline, 20% orthoclase, 40% quartz,
15% plagioclase ( A ^ ^ j q ), 5% biotite and chlorite, and traces of magnetite. Granophyric textures are well developed
in the potash feldsaprs. An aplite dike of Petersen Mountain
is composed of quartz (50%) , orthoclase (30%) , plagioclase
(An^p) (15%), biotite, chlorite and magnetite (5%).
A quartz monzogabbro is in contact with the granodiorite of Petersen Mountain in section 8, T. 22 N . , R. 18 E. The rock is massive, melanocratic, coarse grained and hypidio- morphic granular. The amounts of minerals in one sample are as follows: andesine (An^^^) 40%, quartz 20%, potassium feldspar 10%, hornblende 15%, biotite and chlorite 15%, magnetite 3%, traces of epidote and zircon. The plagioclase has well developed oscillatory zoning. Hornblende crystals
are subhedral and commonly surrounded by reaction rims of biotite and chlorite.
The intrusive rocks of Petersen Mountain have been
studied by R.E. Drumheller (1979) who concludes that the
quartz monzogabbro was the earliest intrusive rock, followed
closely by the granodiorite and a third phase of intrusion
consisting of aplite and pegmatite dikes. 11
Plutonic rocks of Petersen Mountain intrude Triassic and early Jurassic metamorphic rocks of the Peavine Sequence
(Bonham, 1969) north and south of the mapped area. They are unconformably overlain by middle Tertiary volcanics. This evidence brackets the age of the intrusive from Jurassic to early Tertiary. All previous authors have considered the granitic rocks of western Nevada to be part of the northern extension of the Sierra Nevada batholith. No age determin ations have been made on intrusive rocks within the area of study, but ages of 88.8 +_ 2.6 million years and 91.2 +_ million years have been obtained for the pluton of the Granite Range, 75 miles northeast of Petersen Mountain (Bonham, 1969, and Smith, et al, 1971, respectively). These dates correspond with ages of Sierran batholithic rocks to the south (Curtis, et al, 1958 , Larsen, et al, 1958, Evernden and Kistler, 1970).
A Jurassic to Cretaceous age is generally assigned to the plutonic rocks within the study area. 12
HARTFORD HILL RHYOLITE
The Cooling Unit Concept
Ash-flow tuffs are the products of violent eruptions of siliceous magmas. Upon extrusion the mixtures of pyroclastic material and gas are semi-fluid, and flow down hill, initially filling gullies and channels in the pre eruption surface. Subsequent eruptions and flows continue to smooth out existing topography, and will generally reheat underlying flows and retard their cooling. When emplacement of flows has ceased for a substantial period of time, the interior ash-flows are able to cool together as a single unit. This unit is defined as an "ash-flow tuff cooling unit" (Smith, 1960, Ross and Smith, 1961). Units that consist of multiple flows, with or without visible partings, are called compound cooling units, whereas single ash-flows are termed simple cooling units.
Throughout the cooling history of a cooling unit two processes, welding and devitrification, affect the character of the resulting rock. Welding is the process whereby the pyroclastic groundmass, glass shards, and pumice lapilli are compacted and deformed by the lithostatic load and high temperatures of the flows. Devitrification occurs as glass shards, pumice fragments and volatiles crystalize. Both these processes result in the development of separate and often overlapping zones within the cooling unit. 13
The basal zone of an ash-flow cooling unit is cooled rapidly from contact with the underlying rocks. This chilled zone is non-welded, lacking collapsed and deformed particles.
Above this is a zone of partial welding which grades upward into a zone of dense welding. These zones result from in creasing lithostatic loads and temperatures (Smith, 1960).
The degree of welding decreases with decrease in the litho static load above the densely welded zone, which results in a second zone of partially and non-welded material.
Crystallization of vitric material begins immediately upon emplacement of the tuffs. The basal and uppermost chilled units form glassy zones because rapid cooling does not allow sufficient time for devitrification. Glassy zones may also result from intense welding, which prohibits crystal growth. Within the central portion of the cooling unit, crystallization is facilitated by slow cooling. The meta stable glass gradually forms axiolitic and spherulitic intergrowths of cristobalite and alkali feldspars (Ross and Smith, 1961).
Devitrification of the glass shards and pumice fragments releases volatiles which migrate along pressure and temperature gradients. They may form platy aggregates of tridymite, cristobalite, and alkalic feldspars in zones of low litho static pressure and high pore space. This vapor phase zone usually develops in the upper portion of the cooling unit.
Another type of devitrification occurs when ground waters migrate through the porous zones of the cooling unit. 14
These low temperature solutions hydrate the rock and promote the crystalization of zeolites and clays. This process, which occurs subsequent to cooling, is termed secondary devitrification.
The magmas from which ash-flow tuffs are formed contain phenocrysts of various minerals. A typical rhyolitic magma may contain quartz, alkali feldspar, plagioclase, biotite
and traces of hornblende and augite. The amount of each mineral present in a cooling unit is a reflection of the
original magma composition. Furthermore, the crystallization
history of the magma may be represented in the tuffs: an
increase in mafics and heavy minerals towards the top of the
cooling unit (normal crystal zoning) would result from gravi
tational settling of these crystals in the magma chamber prior to eruption. An increase of mafic and heavy minerals towards
the bottom of a cooling unit (reverse crystal zoning) could be
the result of a magma chamber that was tapped at it's bottom, and erupted these crystals first (Lipman, 1966). Thus, the phenocryst volume and ratio of minerals are important para
meters for characterizing a cooling unit. These factors,
combined with degree of welding, devitrification and the
presence of unconformities, indicate the contact between cool ing units.
This report will follow the petrologic classification
scheme of Cook (1965), in which the rock name of an ash-flow
tuff is assigned on the basis of the percentages of lithic,
crystal and vitric components of the rock (Fig. 3). It 15
LITHIC
(including pumice)
Figure 3- Classification system for ash-flow tuffs based on percentages of lithic, crystal and vitric components, (taken from Cook, 1965) 16 must be noted that this system uses the word "vitric" without regard to whether the rock is devitrified or not.
Problems of Nomenclature
The lowermost sequence of Tertiary pyroclastic rocks in southern Storey County, Nevada was named the Hartford
Hill Rhyolite by V.P. Gianella (1936) from exposures at
Hartford Hill. Gianella believed the formation consisted largely of flows, but Thompson (1956) recognized that rhyolitic pyroclastic material comprises most of the unit.
McJannet (1957) proposed the name Tule Peak Formation for the tuffs of the Pyramid Lake Red Rock Canyon area, but this name was not generally accepted. Bonham (1969) extended the name Hartford Hill Rhyolite to include all the Tertiary rhyolitic ash-flow tuffs of west-central Nevada that overlie Mesozoic basement and under lie the andesitic flows of the Alta and Kate Peak Formations.
Most of the rhyolitic tuffs, which cover 1/3 of southern
Washoe and Storey Counties, are lithologically dissimilar from the type section at Hartford Hill.
Wallace (1975) subdivided the Hartford Hill Rhyolite in the Virginia Mountains and Pah Rah Range (Fig. 2) into six individual cooling units. Cupp, et al (1977) recognized
Wallace's three lowermost units in Red Rock Canyon, and further subdivided the basal tuff (unit 1 of Wallace) into two units: unit 1A and IB. 17
The trend in recent publications has been to discontinue the use of the term Hartford Hill Rhyolite, and assign formational names to individual cooling units. Profett and
Profett (1976) assigned new formational names to the ash-flow tuffs of central Lyon County, Nevada. Bingler (1978) proposes seven new formational names for the cooling units in the
Carson City - Silver City district, which includes the type section of the Hartford Hill Rhyolite. Hutton (1978) uses the new formational names proposed by Bingler for similar tuffs on Dogskin Mountain. He also uses informal names for tuffs of the Dogskin Mountain area that have no litho logic counterpart in the Silver City area, and in this way completely avoids the use of the term Hartford Hill Rhyolite. In this report, the tuffs of Red Rock Canyon, Petersen
Mountain and Seven Lakes Mountain are referred to as the
Hartford Hill Rhyolite, and are subdivided into units 1A,
IB, 2, 3, and Upper Units member, as per Cupp, et al
Cl977), and Wallace (1975). This procedure was followed because correlation of these tuffs with cooling-units described by those authors proposing new names could not be done on the basis of lithologic similarity, thickness, or stratigraphic succession. In as much as it is not the purpose of this report to propose any new formal or informal names, it was deemed appropriate to abide by the subdivisions made by previous authors. 18
Mapped Units
In this study units 1A and IB are further subdivided into upper and lower zones. The mapped units of the Hartford Hill Rhyolite therefore include 1A lower (ThlAl), 1A upper
(ThlAu), IB lower (ThlBl), IB upper (ThlBu), 2 (Th2), 3 (Th3) and upper units member (Thu).
Units 1A, IB and 2 outcrop in Red Rock Canyon and along the west flank of Petersen Mountain (Figs. 4-7 ). Unit 3 crops out between Seven Lakes Mountain and Red Rock Canyon
(Fig. 8 ), and the upper units member (Thu) forms the south ern flank of Seven Lakes Mountain (Fig. 9 ).
The division of cooling units 1A and IB into upper and lower zones is made on the basis of changes in composition, mineralogy, degree of welding and crystallization. The purpose of the divisions was to facilitate mapping and to enhance the understanding of the stratigraphic position of the uranium deposits contained within these units (Fig. 10).
The upper units member (Thu) includes the upper portion of cooling unit 3, cooling unit 4, and parts of cooling unit
5, as defined by Wallace (1975). These are grouped together for several reasons: 1) they do not occur on Petersen
Mountain in association with uranium deposits, 2) the depositional contacts between these cooling units are not readily discernable within the mapped area, and 3) this member is separated from the lower section of tuffs by a major fault, and the stratigraphic relationship between the 19
Figure 4 - Mesozoic granodiorite and mapped unit ThlAl and ThlAu, exposed in Red Rock Canyon.
Figure 5 - ‘ m u m * Mapped units ThlAu and ThlBl exposed in Red Rock Canyon Figure 6 - Mapped units ThlBl, ThlBu and Th2 exposed in Red Rock Canyon. ThlBl and ThlBu are excavated for road-fill material in this area.
Figure 7 - Unit Th2 exposed in Red Rock Canyon. fl TTP T iT h U
Figure 9 - Upper units member (Thu) exposed in the Red Rock Canyon area Stratigraphic 5000' Tb 25' basalt position of erosional unconformity uranium occurrences
Twt 60Q' white, water-laid tuff 4500'
4000' Cornelia C, Jeannie K and Barbara L Tst 900' interbedded siltstones and sandstones 3500'
3000'
Tar 1500' arkosic sandstones and conglomerates 2500'
RME pits, Yellow Jacket cuts 2000' erosional unconformity a ? a 74 c A < A Thu thickness not determinable ' S ' w & v Q. -o =r A. numerous vitric to vitric crystal tuffs ^ A -= <7«=J A V ^ V £i V C=Z3 A c — A V7 iA A
Figure 10 - Stratigraphic section of the Hallelujah Junction - Red Rock Canyon area. 1" = 500' 23 two groups is not determinable within the mapped area.
Correlation of the mapped units with previous authors is shown in Figure 10, and the thicknesses and stratigraphic relationships of the tuffs are shown in Figure 11.
Unit 1A Lower (ThlAl)
Map unit 1A lower is a white to light gray, non-welded vitric to vitric-crystal tuff. It is the lower chilled zone of cooling unit 1A, and crops out very poorly forming gentle slopes. Fine grained volcanic ash comprises 60-90% of the rock; crystals constitute up to 20%; lithic fragments 10% and pumice lapilli 10%. A typical hand sample contains 10% crystals, several percent lithic fragments and pumice.
Carbonaceous material is usually present in trace amounts throughout the unit, and is abundant in some places (Fig. 22).
Dark gray cohesive clays are present in this unit at the
Bastain prospect, where it has been extensively fractured and brecciated. Clays constitute up to 50% of the cuttings from drill hole #1 (Appendix 1).
In thin section, the groundmass contains minor amounts of glass shards, and evidence of welding is lacking. Pheno- crysts (in order of abundance) are potassium feldspar
(sanidine), plagioclase, biotite and quartz. Lithic fragments of volcanic and plutonic rocks are present.
Unit 1A lower is up to 100 feet thick and unconformably overlies plutonic rocks in all outcrop areas. A soil horizon Hartford Hill Rhyolite Hartford Hill Rhyolite (Cupp et al, 1977) (Wallace, 1975)
Middle Tertiary tuffs Cooling Hartford Hill Rhyolite Cascade Middle Tertiary tuffs (Bineler. 1978) (present study) Member Unit 6 (Hutton, 1978) Maue -McCray Cooling Santiago Member Unit 5 Canyon Upper Rainbow Cooling Tuff Units Canyon Unit 4 tuff of Member Member Chimney Springs not named Cooling Cooling Cooling Unit 3 Unit 3 Nine Hill Eureka Unit 3 Tuff Canyon Tuff
Cooling Cooling Cooling tuff of Nine Hill Unit 2 Unit 2 Unit 2 Jackass Sp Tuff tuff of Dogskin Unit lBu Cooling not named Unit IB Mountain Cooling Cooling (T Leniham Unit 1B1 Unit 1 ______— Unit D H* Canyon l/> Tuff Unit lAu H-3 Cooling Cooling JO n 3 Unit 1A1 Unit 1A Unit C Mickey o? Pass Cooling H*-t Tuff Unit B 3 Mesozoic Mesozoic OQV) Basement Basement Unit A Figure 11 ■ Correlation chart for the stratigraphic sections of Cupp et. al. (1977), . Wallace (1975), Hutton (1978), Bingler (1978), and the present study, as modified from Hutton., (iy/8). 25 and up to 20 feet of arkosic sediments are present at the basal contact.
Unit 1A Upper (ThlAu)
Unit 1A upper is a light gray to buff moderately welded vitric tuff, that outcrops as a light to medium reddish brown cliff former. Fine grained tuffaceous ground- mass constitutes 90-95% of the rock, phenocrysts of sanidine plagioclase and quartz compose 5-10%, and trace amounts of volcanic lithic fragments are present. Limonite staining is abundant on fracture surfaces, and in hand sample the rock has a distinctive vitric-granular texture. Unit 1A upper is 120 feet thick and it's lower contact with ThlAl is sharp and conformable.
Unit IB Lower (ThlBl)
Unit IB lower is a light to medium gray slightly welded vitric to vitric-crystal tuff that outcrops as a poorly indurated slope former. It is the lowermost chilled zone of cooling unit IB, a compound cooling unit. A typical hand sample is composed of 80% fine grained gray volcanic ash and glass shards, 10% white to buff pumice lapilli, 7% phenocrysts and 3% lithic fragments of plutonic and volcanic rocks.
Relative proportions of these constituents vary, and there are pumice, lithic and crystal-rich zones within the unit. 26
The phenocrysts are potassium feldspar and plagioclase in nearly equal amounts, quartz and biotite. Pumice lapilli are commonly up to 1 inch in longest dimension and distinctly
flattened. Unit IB lower is 125 feet thick, and it unconform- ably overlies unit ThlAu and granodiorite, with up to ten
feet of arkosic sediments present at the contact in some
locations (Fig. 12). Unit IB Upper (ThlBu)
Unit IB upper consists of multiple flows of light gray, yellow, tan and light brown vitric to vitric-crystal tuffs.
The tuffs are moderately to densely welded and the unit crops out as a moderately indurated cliff former. While individual
flows vary in the relative amounts of fine grained volcanic ash, crystals, pumice, lithic fragments and glass shards, most hand samples contain around 90% fine grained ash and pumice. Phenocrysts include potassium feldspars, plagio clase, biotite and quartz. Devitrification of the fine grained material is generally minor, but extensive development of clays is notable in some areas, especially near the Buck- horn Mine. The origin of these clays is discussed in the uranium deposits section.
Unit IB upper is 400 feet thick in Red Rock Canyon, and
substantially thicker in the vicinity of the Buckhorn Mine, where a thickness of 700 feet is exposed before the unit is
covered by Pliocene sediments. The lower contact with
ThlBl is sharp and unconformable. Units IB lower and upper
28 comprise a compound cooling unit, consisting of numerous flows of limited extent that shared a common cooling history.
Unit 2 (Th2)
Unit 2 is a simple cooling unit consisting of a single moderately to densely welded, light to dark gray vitric tuff.
It crops out as a well indurated reddish to dark brown cliff former, and is composed chiefly of fine grained ash, abundant, black, flattened pumice fragments and glass shards.
Phenocrysts of feldspar, quartz and biotite are a very minor constituent. The abundant flattened pumice fragments are as much as several inches in longest dimension, and comprise up to 50% of the rock, making this unit very distinctive.
Unit 2 is approximately 500 feet thick in Red Rock Canyon, and it's lower contact with ThlBu is unconformable.
Unit 5 (Th3)
Unit 3 is a light gray to tan vitric to crystal-vitric tuff. It is slightlyto moderately welded and crops out as a mildly indurated slope former, tan to light brown in color.
Phenocrysts comprise up to 30% of the rock, and include quartz, sanidine, plagioclase and biotite. The groundmass contains minor pumice and lithic fragments. Primary and secondary devitrification are evident throughout much of the unit.
This unit is 150 feet thick in Red Rock Canyon, where it is truncated by faults and buried by alluvium. It's contact with Th2 is poorly exposed but seems conformable, and it appears to be a simple cooling unit. 29
Upper Units Member (Thu)
The upper units member contains numerous lithologies.
Within the mapped area, the bulk of the unit consists of a brick red vitric-crystal to crystal tuff that crops out as a moderately resistant cliff former. Phenocrysts of sanidine, quartz, plagioclase and occasionally biotite, comprise up to 50% of the rock, which is moderately to densely welded. The second most common lithology is a white to light gray moderately welded crystal-vitric tuff with a similar suite of phenocrysts. These two rocks make up the strikingly scenic north end of Red Rock Canyon.
Other less common lithologies included in this unit are a densely welded light gray to buff vitric tuff, with up to 10% phenocrysts of sanidine and quartz, and a pinkish-purple, moderately welded vitric-crystal tuff. The stratigraphic relationships of this unit are not determin able within the mapped area, but these rocks, which crop out on Seven Lakes Mountain, are stratigraphically higher than the section exposed in Red Rock Canyon, which rests on
Mesozoic basement rocks.
Age, Regional Correlation and Origin
Radiometric age dates ranging from 22 to 28 million years before present are reported for the Hartford Hill
Rhyolite. Ages of 27.9 and 22.1 million years were 30 obtained by Wallace (1975) for the lowermost and uppermost cooling units in the Virginia Mountains. Evernden and
James (1964) report a radiometric age of 22.7 for one tuff within the formation, and Bingler (1978) determined ages ranging from 28.0 to 21.8 for the ash-flow tuffs in the Virginia Range. The Hartford Hill Rhyolite overlies the Pah Rah Formation, a middle Tertiary andesitic mudflow breccia, in the Pah Rah Range. Elsewhere it overlies
Mesozoic intrusive rocks. It is overlain by the Alta
Formation and the Pyramid Sequence, which are middle Miocene and Mio-Pliocene in age, respectively. This stratigraphic and radiometric evidence indicates that the Hartford Hill
Rhyolite is early to middle Miocene in age.
Bonham (1969) correlates the Hartford Hill Rhyolite with the Delleker Formation (a rhyolitic tuff) of north eastern California, and the Valley Springs Tuff of the central Sierra Nevada. The Hartford Hill Rhyolite is temporally correlative with an extensive group of silicic ash-flow tuffs which were erupted in the western United
States from 28 to 21 million years ago (Silberman and McKee, 1976). The volcanic centers appear to have formed an arcuate belt, concave to the northeast, that extended from central Oregon to southeastern Utah (Noble, 1972).
Sufficient stratigraphic work has not yet been done to determine the lithology, extent and correlation of all the ash-flow tuff units in western Nevada. 31
The radiometric ages reported by Wallace (1975) and
Bingler (1978) indicate a six million year period of emplacement for the Hartford Hill Rhyolite. It is generally agreed that time breaks of up to 2 million years occurred between deposition of successive cooling units (Wallace,
1975, Cupp, et al , 1978, Bingler, 1978, Hutton, 1978).
The source area of the tuffs lies outside the mapped area, although the precise location is unknown. Wallace (1975) favors a cauldron source near the northern tip of the Pah Rah range. Evidence for this hypothesis includes the greater thickness of the tuffs in this area, the greater number of cooling units, and the presence of a quartz monzonite intrusive body of the same age in the area. Hutton (1978) believes the two lower cooling units on
Dogskin Mountain (the McKisnick Springs and tuff of Dogskin
Mountain, see Fig. 4) were emplaced in quick succession from
28 to 27 million years ago from a source near Dogskin
Mountain. He postulates a.source further away and probably to the south for the upper series of tuffs. Bingler (1978) describes the field evidence for sources of the tuffs as rare, and suggests that they were erupted from widely separated calderas in west-central Nevada. There is substantial evidence that the two lowest cool ing units (ThlA and ThlB) were initially deposited within channels or gullies. The distribution of these units is known to be quite restricted; the distribution of unit ThlA 32
in the vicinity of the Bastain pits (Plate 2) is defined by drill holes 1, 2 and 3 (Plates 1 and 2), and is depicted on cross section B-B' (Plate 3). The outcrops and drill hole information suggest that this unit was deposited in a channel or depression approximately 1000 feet wide with a generally north-south trend.
Channel deposition for units ThlA and ThlB is also suggested by their well developed chilled, non-welded zones, which are approximately 100 feet thick. Development of chilled zones would be enhanced if the initial ash-flows were restricted to wet, sandy channels. The abundant surface and ground waters would tend to quench the first flows before welding could occur.
The overlapping character of the upper portions of cooling units ThlA and ThlB is further evidence of channel deposition. Throughout the study area, map unit ThlBu tends to cover up ThlBl, and ThlAu tends to extend over ThlAl. This would be expected if deposition occurred in channels; the latter flows would be deposited in the higher and wider portions of the channels.
It seems likely that cooling unit 2 (Th2) was not deposit ed within channels, but was deposited as a flat, sheet-like mass. This conclusion is supported by the fact that unit 2 has no chilled zone, is not overlapped by cooling unit 3, and correlates with the Nine Hill Tuff of Dogskin Mountain and the Carson City-Silver City area. It seems likely that any unit with such a wide distribution could not have been restricted to channels. Cooling unit 3 (Th3) and the tuffs of the upper units member also appear to be sheet-like deposits, based on the distribution of these units on Seven Lakes Mountain.
* 34
PLIOCENE FLUVIATILE AND LACUSTRINE DEPOSITS
Problems of Nomenclature
The sedimentary rocks of Long Valley have never been formally assigned to any previously defined formation, nor has a formational name and status been applied to them.
Two main formational names, Truckee and Coal Valley, have been defined for Pliocene sedimentary rocks and associated volcanics which occur in basins south of Peavine Peak
(8 miles northwest of Reno). Similar Pliocene rocks that cover extensive areas north of Peavine Peak have not been formally named. These rocks were deposited in dis connected basins throughout northwestern Nevada, and never formed a continuous sequence over a large area (Thompson and White, 1964).
The name Truckee Formation was originally applied by
King (1878) to a sequence of sedimentary and associated volcanic rocks in the Hot Springs Mountains of Churchill
County, Nevada, and in the Trinity Range of Pershing County.
King included within the Truckee Formation sediments of the Reno and Verdi areas, although he noted that they were somewhat different in lithologic character than the type section in the Hot Springs Mountains. At the type section the Truckee Formation consists of a basal basalt tuff, tuffaceous sandstone, diatomite and thin flows of olivine b a s a l t . 35
Axelrod (1956) applied the name Coal Valley Formation to a sequence of sedimentary and associated pyroclastic rocks occurring in Coal Valley, southern Lyon County, Nevada.
The formation consists of fluviatile and lacustrine shales, diatomites, sandstones, conglomerates, andesite and rhyolite tuffs and thin coaly beds, with interbedded andesite flows and breccias. Axelrod considered derivation of much of the clastic material from the Kate Peak Andesites (Mio-Pliocene) to be an important distinguishing feature of the Coal Valley Formation (1956, p. 31-32).
The age of the Truckee and Coal Valley Formations has been determined on the basis of mammalian faunas, fossil floras and radiometric dates of interbedded volcanics.
Bonham (1969) cites the age of the Truckee to be late
Clarendonian to Hemphillian (early to middle Pliocene) and Axelrod (1956) and Evernden and James (1964) determine the age of the Coal Valley to be early Clarendonian to
Hemphillian (Mio-Pliocene to middle Pliocene). Axelrod
(1956) points out that the Truckee and Coal Valley Formations interfinger in northern Lyon County.
Application of these two formational names to the
Pliocene fluviolacustrine sequences of northwestern Nevada has been a point of contention for many years, and remains so today. Axelrod (1958, 1962) extended the name Coal Valley
Formation to Pliocene rocks in the Verdi and Chalk Hills area, which previous authors (including King, 1878) had assigned to the Truckee Formations. Axelrod reassigned these rocks on the basis that clastic sediments in these areas are characterized by andesitic debris and interbedded volcanics, whereas the Truckee Formation typically contains basaltic tuffs and flows and elastics derived from basaltic rocks. Thompson and White (1964) point out that these lithologic differences are not regular or consistent, and use the name Truckee Formation for the Pliocene sediments of the Verdi and Chalk Hills area, citing the general application of the name of Miocene and Pliocene fluviatile and lacustrine deposits in northwestern Nevada.
Bonham considers the lithologic differences and strati graphic associations noted by Axelrod to be sufficiently distinctive to assign the bulk of the Pliocene sedimentary rocks in southern Washoe and Storey Counties to the Coal
Valley Formation (1969, p. 37-38). He confidently applies the name Truckee Formation only to those Pliocene sediments in the northern end of the Truckee Range (east of Pyramid
Lake). Cupp, et al (1977) apply the name Truckee Formation to the sediments of Warm Spring, Hungry and Long Valleys, noting that this is the common practice among workers in the Reno area.
In as much as it is not the purpose of this report to determine stratigraphic nomenclature, the sediments of Long Valley will be referred to as Pliocene fluviatile and lacustrine deposits. 37
Mapped Units
The sedimentary rocks of Long Valley include sandstones, conglomerates, siltstones and shales, which constitute a stratigraphic thickness of up to 3000 feet. These rocks were subdivided into three mapped units; a lowermost arkose, an interbedded siltstone and sandstone unit, and an upper, white water-lain tuff. Anomalous concentrations of uranium occur in the lower two units (Fig. 10).
Arkose (Tar)
The arkose member consists of fine to coarse grained sandstones and conglomerates, with minor amounts of siltstones and shales. It outcrops as resistant knobs and ridges in sections 25, 35 and 36, T. 24 N., R. 17 E., and sections 30 and 31, T. 24 N., R. 18 E.. The unit is thickly bedded and is approximately 1500 feet thick.
The color of the sandstones is determined by the degree to which they are oxidized. Fresh sands observed in drill cut tings are light to dark gray. Fresh sandstone exposures are buff to light brown with weathered surfaces darker brown (Fig. 13).
Some sandstones are leached; they have a bleached appearance, carbonaceous material has been removed, iron oxides have been redistributed into orange-brown limonitic bands, and feldspar grains are altered to white clays (Fig. 14).
Individual sandstone beds vary from massive (greater than
50 feet thick) to several inches thick, with a typical bed 38
Figure 13- Typical outcrop of sandstone unit (Tar) in Red Rock Canyon
sandstones exposed in section 11, T. 23 N . , R. 39
between one and two feet thick. Grains vary in size from silt to boulders, but the majority of the sandstones consist of moderately well sorted medium to coarse sand. Grains are angular to sub-angular, and include quartz (40%), plagio- clase (35%) , potassium feldspar (10%) , hornblende and biotite
(7%), lithic fragments (5%), iron oxides and carbonaceous material (3%). Lithification is typically poor, with iron oxides the only cement.
Sedimentary structures found within the sandstones include cross-bedding, graded bedding, cut and fill structures and ripple marks. Cross-bedding is more common in the lower part of the unit, and cross-bed troughs are up to four feet wide. Graded bedding is occasionally well developed, with graded beds ranging from six inches to two feet thick. Some beds display a consistent reduction in grain size from pebbles to fine sand. More, commonly, however, a pebble bed occurs at the base of a medium to coarse grained sandstone.
Cut and fill structures and ripple marks are usually found
in the finer sands towards the top of the unit.
Individual beds are discontinuous along strike, and
generally thin out within 20 to 30 feet. Planar beds become predominant higher in the section, and some of these beds have been traced for % mile.
Conglomerate beds are typically one to several feet
thick, and consist of pebbles, cobbles and boulders of
granitic rocks. A few clasts of volcanics were observed, 40
but these are very minor. Sorting is poor in the conglom erates, with boulders up to 12 feet in diameter contained within beds of pebbles and sand. Most of the conglomerates consist of cobbles less than six inches in diameter in a coarse sandy matrix. Weathering of the granitic fragments is usually extreme, and they can often be broken apart by handling.
The lower contact of the arkose member is poorly exposed, but is known to be depositional upon granitic rocks from drill hole logs. The tuffs of the Hartford Hill
Rhyolite are not found in most of the drill hole logs, and
it is presumed that this unit was never deposited very far west of it's present outcrops on Petersen Mountain (see p.
62). The initial sandstones were deposited on an undulating surface, and with an irregular distribution and thickness.
Interbedded Siltstone (Tst)
The middle unit of the sedimentary section consists of
interbedded siltstones and sandstones, with lesser amounts of mudstones and lignitic shales (Fig. 15)• The unit is exposed in sections 12, 13 and 24, T. 23 N., R. 17 E., and
sections 7, 18 and 19, T. 23 N., R. 18 E., where it forms
gentle slopes. Thickness of beds varies from less than one
inch to tens of feet, with typical sand and silt beds of
one foot. A thickness of 900 feet is represented in drill
hole logs. 11 41
Figure 15 - Interbedded siltstone unit (Tst) exposed in section 7, T. 23 N . , R. 18 E. The siltstones and sandstones are yellow, buff, tan and gray, and generally weather to a light brown. The siltstones are poorly sorted, containing abundant sand grains and occasional pebbles. Induration is poor, and cement is generally lacking.
Many of the fine sands and silts exhibit straight and cross laminations, and soft sediment deformation features such as pinched and plucked beds, slumping and inclusions. Medium and coarse grained sandstones display cut and fill structures and cross-bedding. Beds lense out along strike due to changes in lithology and texture.
Some of the sandstones of this unit are leached: their appearance is like that of the leached sandstones of unit Tar
(p. 38). General areas of leaching within both units are dis tinctive due to a rusty-brown discoloration of leached outcrops and soils derived from leached outcrops. Areas of apparent leaching are shown on the geologic map (Plate 1).
Mudstones and lignitic shales comprise about 51 of the unit. They are typically darker than the enclosing sands and silts, and are well sorted. These finer grained beds terminate rapidly along strike, and are generally less than five feet thick.
The base of the interbedded siltstone unit is arbitrary, and has been defined as the point where the lithology changes from greater than 501 silts to greater than 50% sand. This contact is gradational and poorly exposed. 43
White, Water-Lain Tuff (Twt)
The uppermost unit consists of a very fine grained water-lain tuff. It is poorly exposed in sections 12, 13,
14, 23, 24, 25, 26, 35 and 36, T. 23 N., R. 17 E.. The tuff is massive, and attains a thickness of 600 feet. This rock is mined for "pozzolanite"; a sulfate-retarding cement a d d i t i v e .
The tuff is creamy white to light gray (Fig. 16), and becomes darker when wet. Bedding is indistinct, being occasionally demonstrated by a thin silty, organic lamination.
The bulk of the rock consists of particles that are silt sized or finer. Amorphous rhyolitic ash comprises approxi mately 751 of the material, with intermixed pumiceous and diatomaceous material representing 20% of volume. Glass shards, quartz and feldspar grains and clays make up the remaining 5%. Moderate lithification is provided by compaction of the ash, without any cements. Laminations are common in the white tuff, and always present in the minor amount of interbedded siltstones near the base of the unit. Wavy laminations and ripple marks are also present.
The lower contact with the underlying siltstone member is exposed in section 12, T. 23 N., R. 17 E., where it is seen to be gradational, with interbedded siltstones and tuffs giving way to massive white tuff within 200 feet of stratigraphic section. Figure 16 - White water-lain tuff unit (Twt) exposed in section 11, T. 23 N . , R. 17 E. 4 5
An economically important property of this rock is it's unusually high proportion of free silica. This
characteristic, combined with it's small particle size, provides a high SiC^ activity, which retards the corrosive
effects of sulfates on cement prepared with up to 251 of
such a pozzolan. Since natural waters contain significant
amounts of sulfates, this material is an important con
stituent of cements used in construction of dams, bridges,
and other structures that are in contact with water bodies.
The mining and processing operation is performed by
Lassenite Industries, Inc. The rock is mined in section 11,
T. 23 N., R. 17 E., by elevating scrapers. Grinding,
calcining and packaging are carried out in the mill, located
adjacent to the railhead in section 2, T. 23 N., R. 17 E..
Paleontology
A preliminary report entitled "Paleontology, Paleo-
ecology and Correlation of Pozzolan Bearing Lake Beds in
Long Valley, California" was prepared by Dr. James R. Firby,
Associate Professor of Geology, University of Nevada,
Reno. His report was part of a private study conducted
for Lassenite Industries, Inc., titled "Geological Report
on the Pozzolanic Deposits near Hallelujah Junction, Lassen
County, California". The findings of Dr. Firby are
summarized below. 46
Twenty four taxa of diatoms were distinguished from
samples of the white tuff unit. Individual species were not identified, but three forms were recognized to be
common to the Coal Valley Formation, exposed at Verdi,
Nevada. The presence of diatoms indicates an abundance of silica in solution in the lacustrine waters.
Plant fossils were recovered from two localities;
southwest % of section 7, T. 23 N., R. 17 E., low in the section, and southwest % of southwest % of section 1,
T. 23 N., R. 17 E., approximately 1400 feet stratigraphically higher. Six taxa were identified within the sediments; the willows Salix truckeana Chaney (Black Willow), Salix boisiensis (Knowlton), an unidentified species of pine-
Pinus sp., Nymphaeites nevadensia Axelrod (a water lily), the pond weed Potamogeton verdiana Axelrod, and a rush (?)
Typha s p ..
All the specifically identifiable plants found within the Long Valley sediments also occur within the Coal Valley
Formation, at Verdi, ten miles west of Reno. These two floras are considered equivalent by Dr. Firby. As previously noted, Axelrod (1958) cites an early middle Pliocene
(Hemphillian) age for the Verdi Flora, and Evernden and
James (1964) report a radiometric age of 5.7 x 10^ years for an andesitic tuff interbedded with the leaf bearing horizon at Verdi. 47
Three different habitats are represented by the Long
Valley Flora. Pinus sp. is a forest element that was probably restricted to the upland portions of the region.
Salix truckeana and Salix boisiensis are reparian woodland elements, which occupied stream, lake and swamp margins in
lowland regions. A shallow swamp environment is indicated by the presence of the pondweed Potamogeton and the water
lily Nymphaeites. Climatic conditions inferred by the Long Valley and Verdi faunas are an average rainfall of 20-25
inches per year, precipitated mainly in winter, warm to hot summers and mild winters.
Ostracods and gastropods were collected from the white tuff member, in the southwest % of section 1, T. 23 N.,
R. 17 E.. Ostracods are abundant at this locality, but are non-diagnostic. Three species of gastropods were
identified; Parapholys gester: Hanna, Planorbis sp., and the rissoid snail Amnicola sp..
The specimens of Parapholys gesteri from Long Valley were determined to be an earlier stage of development of the same species which occurs in late Pliocene lake beds,
in Mono Basin, California. Geologic ranges of the other
gastropods are non-diagnostic. A shallow, relatively quiet environment of deposition
in a fresh water lake is indicated by Parapholyx and
Planorbis. The latter is an air breathing gastropod which
rises to the surface periodically. The depth at the site 48
of deposition would be no deeper than 30 feet, and probably
as shallow as a few inches. Amnicola is a gill breather,
and does not tolerate ephemeral habitats or a pH greater than 8.5 or lower than 6.5.
Mammalian bone and tooth fragments were collected in
sections 14 and 23, T. 23 N., R. 17 E.. The fauna includes
the horse Pliohippus proversus Merriam, an elephant (?)
Mastodon sp., an antelope (antilocaprid) and a camel
(camelid). The geological span of the horse species is non-diagnostic. The association of antelope, horse, camel
and elephant is common in Pliocene environments similar to
that inferred for the Long Valley area.
Environments of Deposition
The three mapped units of the Pliocene sediments
of Long Valley were deposited in different environments.
The lower arkose was laid down by a fluvial system. The : . # y i # £ f 1 ’5 3 interbedded siltstone member accumulated in a nearshore
lacustrine setting, and the white tuff was deposited in a
tranquil offshore lacustrine environment. The presence of
interbedded siltstones, mudstones and lignitic shales indicates the existence of various marginal and subsidiary
environments within the larger depositional regime of each
unit. A high energy environment is indicated for the lower
arkose by the large size of boulders in the interbedded 49
conglomerates, the coarse grain size of the sandstones, the
lack of silt sized material and the magnitude of current
features. A short transport distance is indicated by the
presence of unstable minerals (feldspars, biotite and
hornblende) and the lack of sorting and rounding of sand sized grains.
V h * Current direction indicators noted in the field are So T? . ?7 non-diagnostic. The axes of many cross-bedding troughs and C ...... ——...... scour features are approximately east-west, although a
diversity of orientations are typically present within a
single roadcut or outcrop. It seems probable that streams
flowed eastward and westward from adjacent highlands, and:ift
coalesced to form a principal drainage channel in the
basin. The predominance of sandstones in the northern end
of Long Valley suggests that this stream's drainage was towards the north.
The interbedded siltstones and sandstones of the second
unit indicate a fluctuating energy of the depositional environment. The siltstones were deposited in a nearshore
lacustrine environment, but sufficiently offshore to be
largely unaffected by wave action and coarse sediment
load of inflowing streams. The sandstones of this unit
were deposited in the shoreline area of the lake, where
inflowing streams deposited their bed loads. Changes in the
level of the lake, stream flow, and sediment load of the
streams altered the position of these two adjacent environ
ments, and produced the interbedding of sands and silts. 50
The deposition of the white tuff occurred in an off shore environment, where contribution ofdetrital material was minimal. Lack of sedimentary structures indicates a quiet environment, and the paleontological evidence previously discussed indicates a shallow depth, approximately neutral pH and sufficient circulation to provide oxygenation of the waters. The source of the volcanic material was ash falls associated with regional rhyolitic volcanism.
The presence of lignitic shales and brown organic layers in the arkose and interbedded siltstone members indicates the existence of marginal swamp environments.
Swamps probably developed near the lake shore and adjacent to fluvial channels. The minor mudstones and claystones interbedded with the sandstones also represent deltaic and swamp environments. 51
TERTIARY BASALT
Basalt crops out at three separate locations within the mapped area, along the western flank of Petersen
Mountain. Each of these areas of outcrop is quite small; the largest encompassing approximately 4 acres. In section 19, T. 23 N., R. 19 E., the basalt unconformably overlies units
Thlbu and Thlbl of the Hartford Hill Rhyolite. In section
30 of the same township a basalt unconformably overlies unit
Tst of the Pliocene sedimentary rocks (Figs. 17, 18). The basalt at this locality is approximately 25 feet thick, and the siltstones immediately adjacent to it's contact are baked; they are indurated and altered to a darker brown color.
Twelve hundred feet east of this outcrop another small, discontinuous outcrop of basalt overlies unit ThlAu. The basalt in these three outcrops is very similar petro- graphically, and is composed of plagioclase, A n ^ ^ (30%), augite (10%) , olivine (5%) and accessory minerals apatite and iron oxide, in a dark brown vitric matrix. The texture
is intersertal and pilotaxitic, with sub-parallel laths
and microlites of plagioclase enveloped by the glassy
groundmass and iron oxides. At each outcrop area the basalt
is closely associated with one or several faults. This
fact, along with the outcrop pattern of small, separated
outcrops, tends to indicate that the basalt was extruded
along the faults. The contact relationships of the outcrop 52
Figure 17 - Tertiary basalt outcrop in the vicinity of the Bastain prospect.
Figure 18 - Tertiary basalt unconformably overlying unit Tst of the Pliocene sedimentary rocks in the vicinity of the Bastain p r o s p e c t . 53 in section 30 indicate that the basalt is younger than the
Pliocene sedimentary rocks, and is therefore late Pliocene to Quaternary. This interpretation differs from that of
Cupp, et al (1977) , who consider the basalt to be a basal flow of the Pliocene sedimentary section. The present, late Pliocene-Quaternary interpretation is supported by the
fact that none of the numerous drill holes in Long Valley encounter any basalt, and several pass through the Pliocene sedimentary rocks directly into granodiorite (drill holes
3, 5, 7, and 8, Appendix I). 54
QUATERNARY DEPOSITS
The Quaternary deposits of Long Valley were divided
into four mapped units: Quaternary alluvium (Qal), colluvium
(Qcl) , pediment gravels (Qpg) and boulder trains (Qbt).
Quaternary alluvium consists of water transported
sediments derived from surrounding areas. This unit was mapped along Long Valley Creek, and in the central part of the
area. Two distinct types of sediments are included within
this unit: flat lying, stratified, flood plain deposits exposed within cuts of Long Valley Creek, and unconsolidated, non-stratified material in transport by intermittent streams
and Long Valley Creek.
Within the central portion of Long Valley, in the
southern quarter of the mapped area, Quaternary colluvium covers the Pliocene sedimentary rocks. The colluvium consists of detritus and soils derived from adjacent and underlying rocks, and slope wash from enclosing highlands.
Pediment gravels are well developed adjacent to the
fronts of the ranges on both sides of Long Valley. The pediment gravels consist primarily of boulders of granite
in a sandy, granitic matrix. They were formed by the action
of landslides, soil creep and alluvial fan deposition.
The oldest Quaternary unit in the area is a series of
disconnected boulder trains. They are comprised of boulders
of granitic rocks up to 20 feet in diameter (Fig. 19). 55
Individual boulder trains are up to 1.5 miles in length,
and h mile wide. They are cut by the Quaternary alluvium and buried by pediment gravels.
Figure 19 - Quaternary boulder train exposed in Highway 395 roadcut. 56
STRUCTURAL GEOLOGY
REGIONAL STRUCTURE
There are two principal components of regional structure in western Nevada; a right-lateral strike-slip fault system, and Basin and Range normal faulting.
A belt of subparallel, en echelon faults extends from
Beatty, Nevada to Honey Lake, California, and is known as the Walker Lane. This zone separates two groups of mountain ranges: to the north and east of Walker Lane the ranges have a north-south to N. 15 E. trend, whereas ranges to the south and west of it generally trend northwest. Estimates of the amount of right-lateral offset that has occurred along this shear zone area on the order of 80 to 120 miles (Bonham, 1969).
Movement is generally believed to have begun in Mesozoic time and continued to present.
The Walker Lane is considered by Moody and Hill (1956) to be a wrench-faulting tectonic system in which a conjugate set of shear directions develops in response to regional compression. One shear direction resulting from a north- south compression would be that of the Walker Lane (N. 40-45
W.), which is also the orientation of the right-lateral San
Andreas fault. Furthermore, a right-lateral shear zone with such an orientation would produce two second order fault sets; a right-lateral system with a trend of approximately north-south, and a left-lateral system with a trend of 57
approximately N. 60-80 E. Faults with these trends and
senses of offset are well developed in the Walker Lane and adjacent areas.
Western Nevada has been affected by extensive normal
faulting which has produced most of the present ranges, and
is therefore termed Basin and Range faulting. This episode of faulting began approximately 17 million years ago, and continues through present (McKee, 1971). The widely accepted tensional theory for the origin of the Basin and
Range province proposes that the area was elevated during the Late Tertiary, which resulted in expansion of the crust.
The expansion was accomplished by development of normal
faults of tensional origin and the resultant graben and horst
structures. These structures are primarily responsible
for the elevation of the ranges and the downdropping of the basins in this physiographic province.
It is as yet undetermined which of the two structural regimes, wrench faulting of tensional normal faulting, is predominant in Western Nevada. E.R. Larson (in Larson, L.T., et al, 1978, p. 88) suggests that normal faults may have developed along zones of weakness formed by strike-slip
faulting. Bonham (1969) considers Washoe and Storey Counties
to occupy a transitional zone between the Basin and Range,
Sierra Nevada and Cascade provinces. The area may therefore be affected by structural elements of all of these provinces. 58
The main branch of the Walker Lane shear zone passes
through Winnemucca Valley, and continues north of Fort Sage
Mountains until it is buried by Quaternary lake deposits of Honey Lake (Fig. 2). A group of faults which parallel
the main zone cut the Hartford Hill Rhyolite and higher volcanic assemblages of Seven Lake Mountain. Normal faults with trends of approximately north-south are abundant in
the vicinity of the mapped area; such faults are important
structures of Petersen, Fort Sage and Seven Lakes Mountains. 59
STRUCTURE OF THE HALLELUJAH JUNCTION-RED ROCK CANYON AREA
The predominant structural feature of the Hallelujah
Junction-Red Rock Canyon area is Basin and Range faulting, which has resulted in the formation of a graben structure.
The portion of Long Valley included in the study area is a downdropped block, separated from the relatively uplifted blocks of Petersen and the Diamond Mountains by high angle normal faults. The faults along which this displacement has occurred on the eastern side of Long Valley are moderate ly well exposed on the west side of Petersen Mountain. The western boundary faults of the Long Valley graben are not
exposed, but their existence is substantiated by the
attitude of the Pliocene sedimentary rocks on the western
side of Long Valley.
There are two major unconformities within the study
area: the erosional unconformity between the Mesozoic plutonic rocks and the Miocene tuffs, and the angular unconformity between the Miocene tuffs and the Pliocene
sedimentary rocks. In addition, minor unconformities
separate individual ash-flow tuff cooling units. The un
conformities have a profound effect on the distribution of
the tuffs, and are therefore important structural features
of the area.
The principal faults responsible for the uplift of
Petersen Mountain have trends between N. 25 E. and N. 30 W.
(Plate 1). The lack of variations in the trend of this north- 60 south group of faults over topography of strong relief indicates that they are steeply dipping, mostly to the west
(Plate 3). The faults at the northern end of Petersen
Mountain offset the arkose member of the Pliocene sedimentary rocks into juxtaposition with granodiorite. In the central portion of the range the generally north trending faults cut the Hartford Hill Rhyolite. Further to the south the major faults lie outside the mapped area to the east, and offset only granodiorite.
The principal displacement along the north trending faults on Petersen Mountain is vertical, with the western side usually downthrown. Two exceptions to this are the northeasternmost fault shown on Petersen Mountain and the fault that forms the western boundary of the sliver of
Pliocene arkose that projects southward into the northern tip of Petersen Mountain. These two faults have downdropped their eastern blocks. The amount of lateral displacement across the north-south faults is difficult to estimate, but would seem to be minor in comparison to vertical offset, which is estimated to range from several hundred to more
than 1000 feet. This estimate is based on the limited
stratagraphic evidence and topographic relief.
The north trending faults offset the middle Miocene
tuffs of the Hartford Hill Rhyolite, and are therefore
late Miocene or younger in age. These faults also cut
Pliocene rocks, indicating that movement occurred in post-
Pliocene time, and perhaps as late as Recent time. 61
The numerous east-west trending faults exposed on
Petersen Mountain and the individual east-west fault on
Seven Lakes Mountain (in section 32, T. 24 N., R. 18 E.) cut the Hartford Hill Rhyolite and are steeply dipping structures with largely vertical displacement. The Seven Lakes Mountain fault (the longest east-west fault mapped in the study area) dips to the south and has downdropped its northern block.
In addition to a substantial vertical displacement (perhaps as much as 1000 feet) a significant amount of rotation has occurred, as can be seen by the disparity of strike and dip of the tuffs across the fault. The east-west faults of
Petersen Mountain are generally limited to the Miocene tuffs and Mesozoic granodiorite, which suggests that this series of faults is older than the north-south set.
The northeast trending faults of Petersen Mountain cut the Hartford Hill Rhyolite and the Pliocene sedimentary rocks, although they do not offset the north-south faults.
The northeast faults are steeply dipping and have up to 1500 feet of lateral displacement at the Bastain claims.
The consistent westward dip of the Pliocene sedimentary rocks in Long Valley must be explained by the structural
interpretation. Perhaps the simplest hypothesis is the assumption that high angle faults have downdropped the western side of Long Valley to such an extent that the entire
graben is tilted towards the west. The difficulty with this interpretation is that it requires the western graben 62
faults to have displacements on the order of 6,700 feet
(the vertical distance between either end of a bed dipping
25° over a horizontal distance of three miles), while surface expression of these faults is entirely lacking. A second structural hypothesis that explains the westward dip of the sediments is that the eastern side of Long Valley and
Petersen Mountain have been uplifted relative to the western side of Long Valley and the Diamond Mountains. Although the relative sense of displacement on the north trending faults of Petersen Mountain is downdropped on the west, actual move ment of both sides of the fault could be up, with the eastern side moving further than the western side. This would accomplish the tilting of the sediments without requiring faults of such large magnitude on the west side of Long
Valley.
The unconformities that separate the tuffs of the
Hartford Hill Rhyolite from the underlying granodiorite and overlying Pliocene sedimentary rocks limit the distribution of the tuffs south of Red Rock Canyon to a narrow band on the west side of Petersen Mountain. The absence of the
Hartford Hill Rhyolite under the Pliocene sedimentary rocks of Long Valley is supported by drill holes 3, 5, 7, and 8.
These holes pass through the Pliocene sediments directly into granodiorite, without encountering any ash-flow tuffs
(Appendix 1). The limited distribution of the Hartford Hill 63
Rhyolite is, therefore, the result of restricted areas of deposition and removal by erosion.
The areas of deposition of cooling units 1A and IB were restricted to a Miocene channel or depression that is presently located along the western flank of Petersen Moun tain. The evidence of channel deposition of these units is discussed on page 32. Although these units cooled separately, the proximity of outcrops and limited thickness of unit 1A (225 feet) suggests that they were contained by the same general channel. Basin and Range faulting sub sequent to deposition has tilted the rocks and the channel, so that the depositional surface now dips towards the west
(Plate 3).
The irregularity of this surface may be the cause of the increased thickness of unit ThlBu, south of the Buckhorn
Mine area. The unit is at least 700 feet thick in this area, and the additional thickness could have resulted from a deeper channel or basin in the Miocene surface, which was filled by the ash-flows of unit ThlBu.
Cooling unit 2 is present on the western flank of
Petersen Mountain in sections 18 and 19, T. 23 N., R. 18 E., where it overlies unit ThlBu. Unit 2 is not present south of section 19, and i t ’s absence is probably due to erosion that occurred primarily in late Miocene. This age for the removal of cooling unit 2 is suggested by the general lack of tuff fragments in the Pliocene sediments. South of section 64
7, T. 22 N., R. 18 E., all the tuffs of the Hartford Hill Rhyolite have been removed by erosion.
It seems unlikely that unit 3 and the individual cooling units of the upper units member were ever deposited in the area that is presently the west side of Petersen
Mountain. If these units were deposited and later removed by erosion, that erosion would had to have occurred before the deposition of the Pliocene sedimentary rocks, because as previously noted, the component of tuff fragments in the sandstones and conglomerates is minor. An alternative explanation for the absence of the upper 1400 stratigraphic feet of the Hartford Hill Rhyolite from Petersen Mountain is non-deposition: subsequent to the deposition of cooling unit 2, the topography of the Miocene surface was such that ash-flows were mainly restricted to the area that is now
Seven Lakes Mountain. In such a case cooling unit 3 and higher units would not have been deposited in the Petersen
Mountain area, or could have been deposited as much smaller, thinner units in this area (Plate 3). 65
URANIUM DEPOSITS
CHARACTER OF DEPOSITS
The uranium deposits and occurrences exposed within the study area are of two distinct types: those contained in the tuffs of the Hartford Hill Rhyolite, and those within the
Pliocene sedimentary rocks. The first group includes the
Buckhorn Mine, the Bastain claims and the Lucky Day/Valley
View prospect, and the latter group includes the RME pits, the Yellow Jacket claims, and the Jeanne K., Barbara L. and
Cornelia C. prospects (Plates 1 and 2). The deposits are described below, and the genesis of the mineralization is discussed in the following section.
Buckhorn Mine
The Buckhorn Mine is in the unsurveyed portion of section 31, T. 23 N., R. 18 E.. Workings consist of two adits, one on either side of a northwest trending ridge that contains the mine, and one inclined shaft on the west side of this ridge. The underground workings are all caved, but were never very extensive. Garside (1973) reports total production as over 400 tons of ore, with a grade in excess of .2% U^Og.
The mineralization is contained within the lower 100 feet of unit ThlBu of the Hartford Hill Rhyolite. This unit is offset by numerous faults in the general vicinity of the 66
Buckhorn Mine (Plate 1). These faults cause a repetition of the ThlBl-ThlBu contact, and have caused extensive fractur ing of these units. The area of fracturing includes the entire northwest trending ridge that contains the mine (Plate
2). The fractures are generally one to five inches wide and less than one foot in linear extent, and are closed; the presence of a fracture is indicated by breccia and/or gouge
(Fig. 20)• The tuffs of the fractured zone (which consists mostly of unit ThlBu) are extensively altered to clay, and are permeated by iron oxides, which impart a yellow color to them (Fig. 20). The clays are the products of secondary devitrification, which has obscured or obliterated the original ash-flow textures and crystals. In areas of intense fractur ing, alteration to clays is complete, and the rock consists of white clays and iron oxides. In addition to the iron oxides that permeate the altered tuffs, limonite coatings are present on many fracture surfaces.
The area of mineralization is approximately 50 feet wide and extends 200 feet along the ridge that contains the mine. The mineralized zone is contained within the much larger area of fracturing and alteration. Garside (1973) reports grades as high as .51% U^Og, but two grab samples
(samples 3 and 4, Appendix 2) from the mine area yielded grades of .19 and .13% U^Og. Gummite is the chief uranium mineral, although uranophane and autunite are also present
(Garside, 1973, Hetland, 1955). Scaler assays of samples 67
Figure 20 - Fractured, altered, and limonite stained tuffs of unit ThlBu near the Buckhorn Mine. 68
3 and 4 indicate that enrichment of uranium over daughter products has occurred (Appendix 2).
Bastain Claims
The Bastain claims are in unsurveyed section 29,
T. 23 N., R. 18 E., .7 miles north of the Buckhorn Mine.
Development work consists of two bulldozer trenches and numerous drill holes.
Uranium mineralization is contained within unit ThlAl of the Hartford Hill Rhyolite. This unit is extensively fractured and brecciated in the area of the Bastain bulldozer pits (Fig. 21), and is overlain by unit ThlAu. The outcrop area of the tuffs is terminated to the south by a northeast trending fault immediately south of the pits.
Autunite is the principal uranium mineral in the pits, where it forms fracture coatings on the pumaceous tuff and is associated with carbonized wood (Fig. 22). Mineralization in holes drilled by Pathfinder Mines Corp. is generally associated with medium to dark gray cohesive clays and carbonaceous material of unit ThlAl. Two samples collected from the Bastain pits yielded chemical assays of .15% and
.04% U^Og. Beta-gamma scaler assays of the same samples determined grades of .24 and .13%, indicating that uranium has been selectively enriched. ------*...... • .....
Figure 21 - Fractured and brecciated tuff of unit ThlAl in one of the Bastain bulldozer trenches. 70
Figure 22 - Carbonaceous material in unit ThlAl exposed in the Bastain trenches. 71
Lucky Day/Valley View Prospect
The Lucky Day/Valley View prospect is in section
18, T. 23 N., R. 19 E., .9 miles north of the Bastain claims.
Development work consists of several bulldozer cuts (Fig. 23).
Weak uranium mineralization is contained within unit
ThlBl of the Hartford Hill Rhyolite. Grades as high as .2%
U^Og are reported by Garside (1973) , but chemical analysis of one grab sample made for this study yielded a grade of
.004% U^Og. Mildly anomalous radioactivity is encountered
throughout much of the bulldozed area of the prospect. The
slightly welded pumaceous tuffs are not fractured or brec- ciated, although several faults are mapped in the vicinity.
RME Pits
The RME pits are two bulldozer cuts dug by Rocky Mountain Energy Corp., in unsurveyed section 30, T. 24 N., R. 18 E., north of Red Rock Canyon Road (Plate 2). These two pits
are approximately 3 feet deep and 6 feet wide, and expose
leached sandstone beds up to several feet thick, with inter- bedded siltstones and lignitic shales (Fig. 24).
The mineralized horizon is a dark brown, lignitic mudstone.
The rock is soft, friable, lightweight and contains abundant
carbonaceous material. Several six inch to two foot beds
of this lithology are present in the general area of the RME
pits, and are mineralized to varying extent. This lithology 72
A £
$ J5
Figure 23 - Bulldozer cuts expose Unit ThlBl at the Lucky Day prospect. I s 73
is typical of ore horizons in Rocky Mountain Energy Corpor
ation's drill holes (Joe Johnson, RME Geologist, personal
communication). The enclosing sandstone beds are leached;
they have a bleached appearance, abundant limonite staining,
alteration of feldspars to clay and removal of carbonaceous material (Fig. 25) .
Two samples were analyzed chemically and by X-ray
fluorescence. The results of both procedures were in close
agreement: grades of .11% U^Og and .15% U^Og were obtained
for the two samples which were selected in the field for
their high scintillometer readings. Beta-gamma analysis
of the same two samples provided scaler assays of .15%
U_0o and .17% U_0o. These results are indicative of a e 3 8 e 3 8 disequilibrium in favor of uranium.
Yellow Jacket Cuts
The Yellow Jacket cuts consist of four bulldozer
trenches in unsurveyed section 5, T. 23 N., R. 18 E.. The
prospect pits expose sandstone beds up to six feet thick,
with interbedded siltstones. Many of the sandstones in
these cuts are leached, and the sandstone shown in Figure 26
is one of the best examples of leaching found in the area.
This outcrop is devoid of carbonaceous material, the re
distribution of iron oxides is apparent, and alteration of
feldspars to clay is moderately well developed; most
feldspar grains have a dull, earthy lustre, and a few are
totally altered to white clays (Figs. 26 and 27). Figure 25 - Close-up of leached sandstone in RME pits.
Figure 26 - Leached sandstone exposed in Yellow Jacket cuts. Note limonite staining. Figure 27 - Close-up of leached sandstone exposed in Yellow Jacket cut. Alteration of feldspars to clays is well advanced in this o u t c r o p . 76
Uranium mineralization is restricted to the interbedded siltstone beds. These siltstones are less than one foot thick, white to tan, with limonite staining commonly present at their contacts with the sandstone. They are generally well sorted and some contain up to 25% tuffaceous material.
Two grab samples yielded chemical assays of .055 and .041%
U^Og (samples 7 and 8, Appendix 2). Scaler assays for the same two samples indicate enrichment of uranium over daughter products has occurred (Appendix II).
Cornelia C., Jeanne K. and Barbara L. Prospects
The Cornelia C. and Jeanne K. prospects are in section 7, T. 23 N., R. 18 E., and the Barbara L. pros pect is in section 18 of the same township (Plate 2).
These prospects are grouped together because of their similar geological features. Development work at each prospect consists of one to.three bulldozer trenches.
The rocks exposed by the cuts of these three locations are interbedded sandstones, siltstones, mudstones and clay- stones. The sandstones and siltstones are generally oxidized, and are buff to light brown in color. Some sandstones in the cuts at the Cornelia C. prospect appear to be leached
(Fig. 28), but this is exceptional.
The mudstones and claystones exposed in these cuts are generally less than one foot thick, and vary in color from 77
light brown to dark gray. Many of these fine grained beds
contain abundant carbonaceous material, and it is the carbon- rich beds that are mineralized. The mineralization is
generally weak, although no chemical analyses were performed,
scintillometer readings of 1.5 to 3.0 times background are typical.
Figure 28 - Leached sandstone exposed in Cornelia C. trench. 78
GENESIS OF THE URANIUM DEPOSITS
The two types of uranium deposits within the study area, those contained within tuffs of the Hartford Hill Rhyolite and those contained within the Pliocene sedimentary rocks, were formed by different mechanisms operative in different environments. The deposits within the tuffs appear to be fracture related supergene veins formed by downward percola tion of uranium-bearing solutions into porous, fractured zones. The deposits within the sediments appear to be epigenetic concentrations formed by movement of ground waters with deposition of uranium at chemical and/or physical interfaces. In both cases, the principal source of uranium is thought to be the granodioritic rocks of Petersen Mountain, although some contribution of uranium from the tuffs seems p o s s i b l e .
Source of the Uranium
There are three possible sources of uranium ions within the geologic setting of the study area: hydrothermal solutions, the Hartford Hill Rhyolite tuffs and the granitic rocks of
Petersen Mountain. Hydrothermal solutions are considered unlikely by the complete lack of evidence of hydrothermal activity. Nowhere within the study area were hydrothermal argillization, propylitization or other forms of hydrothermal alteration, minerals or textures observed. The alteration 79
associated with the uranium mineralization (leaching, limonite
staining, secondary devitrification of tuffs) is due to
supergene processes described in the following pages.
It is possible that .the tuffs of the Hartford Hill
Rhyolite were the principal source of uranium for the deposits contained within them, but several lines of evidence argue against this premise. The areas of mineralization within
the tuffs are not related to depositional features such as individual cooling units or non-welded zones, but rather are associated with fracture zones related to post-depositional
faulting. Furthermore, within the study area the tuffs are mineralized only where they are in close proximity to the
granodioritic rocks of Petersen Mountain, a very fortuitous
spatial arrangement if the mineralization were derived from
the tuffs. However, it is conceivable that the fracture
systems provided permeable zones within the tuffs, and that migrating ground waters flowed through these zones redistribut
ing and concentrating the uranium from one unit into another.
The Hartford Hill Rhyolite tuffs may also have contri buted uranium to the deposits within the Pliocene sedimentary
rocks. An important factor in the analysis of this hypothesis
is the behavior of uranium upon deposition and devitrification
of ash-flow tuffs. A widely held concept regarding the re mobilization of uranium originally deposited in tuffaceous
rocks is that the majority of the initial uranium content
of tuffs is released into permeating solutions upon devitri 80 fication (Rosholt, et al, 1971). In such a case, the uranium so released from the Miocene tuffs would be local- lized or transported out of the area long before the sediments were deposited in Pliocene time. However, the fate of uranium following it's release from glass shards during diagenesis is by no means known with any degree of certainty, and there is evidence that little or no migration of uranium occurred during diagenesis of the Tascostal Formation, an Oligocene glass-rich volcanic sediment (Walton, 1978) .
The behavior of uranium during diagenesis of the Hartford
Hill Rhyolite has not been determined, so it is possible that the majority of the initial uranium content remained in the tuffs until erosion or processes other than primary devitrification removed it.
There are two mechanisms by which uranium in the tuffs could have entered the sediments; erosion of the tuffs could have contributed clastic material to the Pliocene sediments, or migrating ground waters could have leached the tuffs and then entered the sediments after their deposition. The first mechanism could not have contributed a very large part of the uranium presently in the sediments, because less than 101 of the clastic material in the Pliocene sediments is derived from tuffs. The second mechanism may have contributed a significant amount of uranium to the sediments. However, if leaching by ground waters obtained uranium from the tuffs, it seems likely that ground waters would also transport uranium 81
from the granodiorite mass of Petersen Mountain to the sediments. The post-Pliocene outcrop area of the grano diorite was most likely much greater than the outcrop area of the tuffs (as it is today), so it seems reasonable to assume that leaching ground waters would obtain more uranium from the granodiorite than from the tuffs. However, since area of outcrop is not the only factor affecting contributions of uranium to ground waters, the tuffs could have contributed significant amounts of uranium to the Pliocene sediments in this way.
Several lines of indirect evidence suggest that the source of the uranium ions is the granodioritic rocks of
Petersen Mountain. The evidence includes the amount of uranium released by weathering and erosion of the granitic rocks of Petersen Mountain, the spatial relationships of the uranium deposits to Petersen Mountain, and the apparent supergene origin of the deposits within the Hartford Hill
R h y o l i t e .
Uplift and erosion of the Petersen Mountain block began in Miocene time, and the steep slopes and rugged topography of its' western flank suggest that uplift has continued to present. The presence of large boulders of granodiorite and the predominance of granitic detritus in the Pliocene sedi mentary rocks indicates that a high rate of erosion of grano dioritic rocks occurred during Pliocene time. The average concentration of uranium in plutonic rocks of the Sierra 82
Nevada Batholith is reported by C.W. Dodge (1972) to be
4.3 ppm. Although this concentration is in the range of trace elements, it is sufficient to assure that substantial quantities of uranium would be released into the surface environment as weathering and erosion of large amounts of granodiorite occurred.
The spatial relationships of the uranium deposits is also suggestive of a granodiorite source. All of the uranium deposits of the area are located on or near the western flank of Petersen Mountain. While not conclusive of a causal relationship, this fact suggests some correlation between the ore deposits and the mountain. This spatial relationship is readily explained by release of uranium upon weathering and erosion, with deposition of uranium in chemical and physical traps that existed in the tuffs and sediments along the western front of the range.
The evidence of a supergene origin for the uranium deposits within the tuffs of the Hartford Hill Rhyolite
(described below) is further evidence of a granodioritic source of the uranium. A supergene origin suggests that the uranium was derived from somewhere up the hydrologic gradient. In as much as the only rocks that presently lie up-gradient from the tuffs are granodiorite, these are indicated as having been the source. 83
Mobilization and Deposition of the Uranium
As the tuffs and plutonic rocks of Petersen Mountain were uplifted, weathered and eroded, some of their uranium content was dissolved by the oxygenated ground and surface waters that accomplished the weathering and erosion. The uranium-bearing solutions that flowed from the granitic
rocks of Petersen Mountain westward over the Hartford Hill
tuffs exposed on the west flank of the range are believed
to be responsible for the uranium deposits within these tuffs.
Where the solutions encountered faults that had brecciated
and fractured the rocks they percolated into the tuffs and deposited all or part of their dissolved uranium. Deposition
of uranium was brought about by adsorbtion onto clays that
are abundant in the fractured areas, and by local reductive
chemical environments within the tuffs, which resulted from
carbonaceous material locally present within some of the flo w s .
In addition to depositing uranium, the solutions en hanced the secondary devitrification of the tuffs, which
resulted in the formation of more clays, which could then
adsorb more uranium. This process could continue until the
fractured area was mineralized down to a depth at which
lithostatic pressure reduced permeability to such a degree
that solution flow was halted. Another possibility for the
control of downward mineralization is the water table - when
the present or paleo water table was reached, any remaining 84
uranium could be deposited due to the change to reducing conditions. However, this downward limit of mineralization would be subject to changes in the water table.
The evidence that the deposits in the Hartford Hill tuffs of the study are supergene veins formed in the above manner include the following:
1) All deposits are fracture related. While there
is an abundance of outcrop area of the Hartford
Hill Rhyolite on the west side of Petersen Mountain,
the tuffs are mineralized only where faulted and
fractured. Faults that offset the tuffs without
brecciating them are rarely very weakly mineralized,
and generally not mineralized at all.
2) The tuffs of the uranium deposits are extensively
devitrified. Devitrification and alteration to
clays is greatly enhanced by the presence of
abundant solutions.
3) The largest such deposit (Buckhorn Mine) is located
in the largest drainage channel of Petersen
Mountain. Although the age of this drainage
channel is not known for certain, the assumption
that it did exist during the period of mineralization
allows for the suggestion that because this area
had the greatest amount of uranium-bearing solutions
available to it, the largest deposit was formed. 85
4) The limonitic alteration within the general area
of the Buckhorn Mine apparently resulted from the
addition of iron to the fractured tuffs. The
mobilization of iron could well be accomplished
by the same solutions that transported uranium,
and it's deposition also controlled by receptive
zones within the tuffs.
While some of the uranium carried by ground waters was removed from further transportation in the above manner, most of the solutions flowed past the narrow band of outcrop of
Hartford Hill Rhyolite and continued basinward with little or no depletion of uranium. Solutions that did so during the erosional period between the deposition of the Miocene tuffs and the deposition of the Pliocene sedimentary rocks, flowed down the paleo-drainage and were removed from the area.
During the period of deposition of the fluviolacustrine sediments, the ground waters eventually entered the Pliocene rivers, streams and lake. Syngenetic deposition of uranium may have occurred in the marshes and swamps that bordered the lake and the fluvial-lacustrine interfaces. The presence of highly carbonaceous mudstones interbedded with sandstones and siltstones indicates that these environments did exist.
These areas would have been extremely chemically reductive, and uranium in the lake and stream water could have been precipitated out.
Although syngenetic deposition may be responsible for the anomalous uranium content of some carbonaceous mudstones, 86
it is unlikely that the mechanism is responsible for the bulk of the uranium content of the Pliocene sedimentary rocks.
Concentrations of uranium within the sediments are not restricted to carbonaceous mudstones, but occur in carbon- poor siltstones as well. Furthermore, there is substantial evidence of that post-depositional leaching of the sedimentary rocks has occurred, (Figs. 24-28) and the mobilization and deposition of uranium would certainly be affected by this p r o c e s s .
After the deposition of the Pliocene sedimentary rocks, uranium-bearing ground waters flowing off Petersen Mountain would have entered the sedimentary rocks in Long Valley.
These oxygenated ground waters would be capable of oxidizing the host rock, leaching any uranium contained within them, and transporting this uranium and uranium derived from the plutonic and volcanic rocks of Petersen Mountain down the hydrologic gradient. As additional oxygenated ground and surface waters entered the Pliocene sedimentary rocks a geochemical cell (roll front system) could have formed. In this system a leached zone is developed in the host rock, and precipitation of uranium occurs at the edge of this zone in response to a change from oxygenating to reducing conditions.
Deposition of uranium may also have occurred within the leached zone where carbonaceous beds caused locally reducing conditions, or by adsorbtion of uranium onto clay minerals of interbedded siltstones and mudstones. 87
Immediately after deposition and diagenesis, reducing conditions most likely existed in the Pliocene sediments.
Reducing conditions would result from the presence of pyrite and carbonaceous material, and both are present in drill cuttings from unoxidized Pliocene sedimentary rocks of the study area (Drill holes 3-13, Appendix 2). Surface and ground waters flowing from Petersen Mountain into Long Valley were in all probability in chemical equilibrium with the atmosphere, and would therefore contain 7 or 8 ppm oxygen (Granger and
Warren, 1978) . This oxygen concentration is sufficient to dissolve and transport hexavalent uranyl ions, and to cause oxidation and leaching of formerly reduced sedimentary rocks
(Shawe and Granger, 1965).
Oxidation of the host rock by a particular volume of migrating ground water would continue until the oxygenating potential of the water was depleted. This water would then continue to pass through the rock without causing any further changes in it's reduced condition. Any uranium present in the oxygenated waters would be deposited when they pass into a reducing environment. As additional oxygenated waters entered the aquifer, additional host rock would be oxidized, and the position of the redox interface would migrate down the hydro- logic gradient. Uranium originally deposited at an initial redox interface would be continually redissolved and redeposit ed further down-dip. This process could continue until an impermeable barrier was reached, the end of the aquifer was approached or the ground water hydrologic regime was c h a n g e d .
Deposition of some uranium could have occurred within the leached zone of Pliocene sedimentary rocks. Highly carbonaceous interbedded mudstones and shales would have caused local chemically reductive environments, and uranium may have been precipitated as a result of the change in chemistry. Also, clay minerals of interbedded siltstones, mudstones and claystones in the leached zone may have adsorbed uranium from the oxygenated solutions. The former mechanism may be responsible for the mineralization of the carbonaceous mudstones of the RME pits, Cornelia C.,
Barbara L. and Jeanne K. prospects, and the latter mechanism could be the method by which the siltstones and claystones of the Yellow Jacket cuts were mineralized. Although no roll-front mineralization within permeable sandstones is exposed, mineralization of this type could exist below the present surface of the Pliocene sedimentary rocks. 89
EXPLORATION POTENTIAL
Exploration potential of the Hartford Hill Rhyolite within the study area is judged to be minimal: no previously undiscovered areas of mineralization within the study area are believed to exist, because the outcrop area has been thoroughly prospected, both by the writer and numerous other prospectors and geologists. A hidden deposit with no surface expression seems unlikely in light of the proposed supergene mechanism of mineralization. Potential for development of substantial new reserves of ore at the known prospects seems unlikely, since the vertical extent of mineralization is probably limited by the extent of fracturing, and it is very doubtful that the zones of high porosity extend to any great depth.
Exploration potential within the Pliocene sediments is judged to be moderate. Exploration activities should attempt to determine the extent of mineralization contained in the fine grained and carbonaceous beds of the leached section, and to determine the existence and location of a roll-front. To accomplish the first of these objectives, a series of holes could be drilled on the east side of Long
Valley. This would provide information on the extent and position of the leached zone, as well as indicate how much uranium this zone contained. The second objective could be realized by drilling as close to the estimated position of the roll-front as possible. The area near Highway 395 might be 90 a reasonable place to begin. If leached ground was found to exist, the next hole should be drilled to the west; if fresh ground was found, the next hole should be located to the east. Several east-west tiers of drill holes would be necessary to determine the changes in position of the roll- front in a north-south direction. A north-south distance of h mile between rows of drill holes might be most effective in determining the general position of the roll-front over an area of several square miles. Inasmuch as the central portion of the main area of apparent leaching lies immediately east of section 36, T. 24 N., R. 17 E., the first tier of holes might be located in this section. 91
BIBLIOGRAPHY
Anderson, Robert, 1910, Geology and oil prospects of the Reno region: U.S. Geol. Survey Bull. 381, p. 475-493.
Axelrod, D.I., 1956, Mio-Pliocene flora from west-central Nevada: California Univ. Pub. Geol. Sci., v. 33, p. 91-160.
...... , 1958, The Pliocene Verdi flora of western Nevada: California Univ. Pub. Geol. Sci., v. 34, no. 2, p. 91-160.
...... , 1962a, A Pliocene Sequoiadendron forest from western Nevada: California Univ. Pub. Geol. Sci., v. 39, no. 3, p. 195-267.
...... , 1962b, Post-Pliocene uplift of the Sierra Nevada, California: Geol. Soc. America Bull., v. 73, no. 2, p. 183-197.
...... , 1966, Potassium-argon ages of some western Tertiary floras: Am. Jour. Sci., v. 264, no. 7, p. 497- 506.
Bailey, R.V., and Childers, M.O., 1977, Applied Mineral Exploration with Special Reference to Uranium: Westview Press, 542 p.
Bingler, E.C., 1978, Abandonment of the name Hartford Hill Rhyolite Tuff and adoption of new formational names for Middle Tertiary ash-flow tuffs in the Carson City- Silver City area, Nevada: U.S. Geol. Survey Bull. 1457D.
Bonham, H.F., 1969, Geology and mineral deposits of Washoe and Storey Counties, Nevada: Nevada Bur. Mines Bull. 70, 140 p.
Burnett, J.L., and Jennings, C.W., 1965, Geologic map of California, Olaf P. Jenkins edition, Chico sheet: Calif ornia Div. Mines and Geology.
Cook, E.F., 1965, Stratigraphy of Tertiary volcanic rocks in eastern Nevada: Nevada Bureau of Mines Report 11.
Cupp, G.M., Leedom, S.H., Mitchell, T.P., and Allen, D.R., 1977, Geology, uranium deposits, and uranium favor- ability of the Hartford Hill Rhyolite and Truckee Formations, southwestern Washoe County, Nevada, and eastern Lassen County, California: B.endix field engi neering corporation, open file report GJBX-16(77). 92
Dodge, C.W., 1972, Trace element contents of some plutonic rocks of the Sierra Nevada batholith: U.S. Geol. Survey Bull. 1314-F.
Drumheller, R.E., 1978, The petrochemistry of the allanite- bearing granitoids, Red Rock Area, Washoe County, Nevada: unpublished M.S. thesis, Univ. of Nevada, Reno.
Evernden, J.F., and James, G.T., 1964, Potassium-argon dates and the Tertiary floras of North America: Am. Jour. S c i . , v. 262, p. 945-974.
Evernden, J.F., and Kistler, R.W., 1970, Chronology of emplacement of Mesozoic batholithic complexes in California and western Nevada: U.S. Geol. Survey Prof. Paper 623, 42 p.
Garside, L.J., 1973, Radioactive mineral occurrences in Nevada: Nevada Bur. Mines and Geology Bull. 81, 121 p.
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96
Drill Hole Lithologic Logs
DRILL HOLE 1 PATHFINDER MINES
Depth (feet) Description Total Depth - 250 feet
0-20 Light brown sandy soil.
20-50 White to tan tuff and 50% brown sticky clay. Tuff is fine grained and contains abundant feldspar crystals.
50-70 White to tan tuff, without clay. White and clear feldspar crystals up to % inch.
70-80 40% tan tuff as above, 60% dark gray cohesive clay.
80-90 Dark gray cohesive clay. Cuttings consist of clay balls without rock frag ments .
90-100 Dark gray clay and medium gray tuff. Tuff is fine grained and contains carbonaceous material.
100-110 Light brown tuff, medium gray tuff, and medium gray clays. Brown tuff derives color from abundant iron oxides.
110-200 White to light tan vitric crystal tuff: white and clear feldspar crystals, up to 20%, very fine grained biotite, a few percent. Some 5 foot cutting piles contain up to 50% gray cohesive clays, and limonite stains on some grains.
200-230 Light gray tuff, very fine grained. Feldspar crystals, 10%, biotite flakes a few percent. Gray cohesive clays comprise 25% of cuttings.
230-250 No cuttings last 20 feet. 97
DRILL HOLE 2 PATHFINDER MINES
Depth (feet) Description Total Depth - 400 feet
0-15 Light to medium brown soil: overburden.
15-80 Interbedded siltstones and sandstones: buff to light brown, abundant limonite staining - oxidized. Silty from 15-45', sandier below. Sub-rounded grains of quartz, feldspars (altered to clay), biotite and lithic fragments of granite and tuffs.
80-165 Buff to yellowish brown densely welded ash-flow tuff. Fine grained to glassy, with peculiar blocky texture. Limonite staining present throughout. Lower 15’ includes up to 10% lithic fragments of similar volcanics.
165-346 Slightly to non-welded ash-flow tuff, off-white, light gray and medium gray, fine grained and includes abundant lithic fragments of granite and volcanics throughout. Matrix is mostly fine grained ash but locally contains up to 50% light to dark gray clays. Carbonaceous material abundant from 165-215' and occasionally present below this. Feldspar and biotite crystals present below 215'.
346-380 Granite wash: sub-angular grains of plagioclase, quartz and biotite. Abundant limonite stained surfaces.
380-400 Fresh granite (granodiorite). Large multi mineralic grains containing plagioclase, K-feldspar, quartz and biotite. Very hard, slow drilling. 98
DRILL HOLE 3 PATHFINDER MINES Depth (feet) Description Total Depth - 1110 feet
0-17 Overburden: dark brown clay rich soil.
17-40 Very fine grained sandstone: buff to light brown - oxidized. Well sorted (very fine grained sand and silt), well rounded grains of quartz, feldspar, biotite (altered to gold color) and granitic rock fragments.
40-140 Sandstones and siltstones (151). Buff to light brown in color from 40 to 115' - oxidized. Unoxidized below 115', light to medium gray color. Sands are fine to coarse grained, moderately well sorted, rounded to sub-rounded, and include quartz, feldspars, granitic fragments, biotite and chlorite. Siltstones contain up to 25% fine sands. No visible carbonaceous material or pyrite.
140-245 Siltstones and interbedded sandstones (15%). Light to medium gray, well sorted - siltstones contain less than 5% sands. Thinly bedded with occasional shales. No visible pyrite, traces of carbonaceous m a t e r i a l .
245-285 Fine to medium grained sandstone. Medium gray color. Poorly sorted, sub-rounded to sub-angular grains of quartz, feldspars, biotite, granitic and volcanic fragments. Traces of carbonaceous material.
285-365 Fine to medium grained sandstones with interbedded siltstones (30%). Light to medium gray, poorly sorted sandstones as above (245-285'). Siltstones also poorly sorted. No carbonaceous material or pyrite observed.
365-430 Interbedded siltstones (60%), and sand stones (40%). Medium gray color, as above (285-365'). Fairly abundant carbonaceous material throughout, and fine grained pyrite noted below 395'. 99
DRILL HOLE 3 Continued
Depth (feet) Description
430-560 Fine to coarse grained sandstone with interbedded siltstones (20%) . Sandstones mostly medium grained, well sorted, sub rounded to sub-angular, with grains of feldspar, quartz, biotite, granitic and volcanic fragments. Sands are quite fresh - biotite is black although there is no visible pyrite. Siltstones as a b o v e .
560-620 Siltstones with minor sandstones (10%) . Siltstones light to medium gray, well sorted (5% san d ) , and abundant c a r b o n aceous material (carbonaceous laminations and small pieces of carbon trash - 1/8 inch long).
620-685 Interbedded sandstones and siltstones (50%). Light to medium gray, poorly sorted and poorly consolidated (no actual cuttings, just grains).
685-815 Thickly interbedded sandstones and silt stones: light to dark gray. Beds are up to 10' thick and consist of predominant ly silts or sands. Lithologies as above. One additional feature: some sandstones contain laminations of dark mafic minerals - magnetite, illmenite.
815-950 Siltstone: light to medium gray with abundant white tuffaceous siltstones. Well sorted and better lithified than overlying siltstones. Also more carbon aceous material (some beds contain 25% carbon trash). Pyrite present throughout, but locally very abundant, and forms shiny gold colored laminations occasionally Calcium carbonate cement fairly common - imparts a glassy lustre to some surfaces. White siltstones comprise 30% of overall cuttings, but up to 90% of some cutting piles (i.e. 815-820', 835-840'). This appears to be lassenite or pozzolanic material. No pyrite, calcite or carbon aceous material, cemented by high silica c o n t e n t . 100
DRILL HOLE 3 Continued
Depth (feet) Description
950-1000 Medium grained silty sandstone. Sub- angular to sub-rounded grains of quartz, feldspar, granite and volcanic fragments and biotite, with 40% silts and traces of carbonaceous material. Pyrite and calcium carbonate present in trace amounts.
1000-1100 Silty arkosic sandstone. Well rounded to sub-angular grains of quartz, feldspar, biotite and up to 15% granitic fragments. Abundant carbonaceous material and fine grained pyrite. Sands have a "granite wash" appearance. Some interbedded siltstones are present and contain abundant carbonaceous material that im parts a black color to these rocks.
1100-1110 Granite wash: grains of quartz, feldspar, biotite and granite fragments cemented together with silica and pyrite. Very hard drilling. DRILL HOLE 4 ENERGY FUELS CORP Depth (feet) Description Total Depth - 2000 feet
0-75 Gray to brown, medium to fine grained siltstone. Moderate oxidation, includes some gravel and clay.
75-360 Gray, fine grained siltstone. Moderate oxida tion, sparse carbonaceous material.
360-400 Lost circulation, no cuttings.
400-1140 Gray, fine grained siltstone. Unoxidized, contains sparse carbonaceous material.
1140-1180 Gray siltstones and sandstone. Medium to fine grained, unoxidized, no carbonaceous material. Entire hole is unoxidized below.
1180-1260 Light gray, medium grained sandstone. Abundant carbon-lignitic. Also silty.
1260-1305 Light gray sandstone and conglomerate. Medium to coarse grained, sparse carbon, grains of biotite, feldspar, hematite?.
1305-1340 Gray, sandstone, siltstone and conglomerate: Fine to medium grained, sparse carbon, large shale fragments.
1340-1440 Lost circulation - no cuttings.
1440-1790 Light gray, coarse grained arkose: large grains of quartz, feldspars and biotite. Well sorted, lacks carbonaceous material.
1790-1830 Light gray coarse grained arkose: like above but contains silt.
1830-1880 Poor recovery- no cuttings.
1880-1950 Gray arkose and large gray mudstone fragments.
1950-2000 Gray to brown, fine grained tuffaceous siltstone. 102
DRILL HOLE 5 ENERGY FUELS CORP.
Depth (feet) Description Total Depth - 185 feet
0-40 Brown gravel and clay, moderate oxidation.
40-115 Gray siltstone, fine grained, some carbon, moderate oxidation, poorly consolidated.
115-130 Medium grained arkose, no carbon, unoxidized, silty, granite fragments.
130-185 Granite, light gray, medium to coarse grained 103
DRILL HOLE 6 ENERGY FUELS CORP.
Depth (feet) Description Total Depth - 1760 feet
0-155 Brown coarse grained arkose. Moderate oxidation, increasing silt with depth.
155-175 Gray, fine grained siltstone, moderately oxidized. Sparse carbonaceous material.
175-215 Brown conglomerate, medium to coarse grained, weak oxidation, no carbon. Some sands and silts included.
215-250 Gray conglomerate, medium to coarse grained, weak oxidation.
250-750 Gray, fine grained siltstone. Sparse carbonaceous material.
750-900 Gray, medium to coarse grained sandstone. Some silt.
900-980 Gray sandstone and conglomerate, unoxidized, no carbonaceous material.
980-1080 Gray, fine to medium grained sandstone. Unoxidized, silty. Unoxidized below this depth.
1080-1140 Gray sandstone and siltstone. Fine to medium grained, sparse carbonaceous material.
1140-1165 Gray, fine grained tuff. Some sand.
1165-1195 Gray, medium grained silty sandstone.
1195-1220 Gray tuff.
1220-1270 Gray sandy tuff.
1270-1310 Tuff.
1310-1510 Tuffaceous sandstone.
1510-1535 Tuff.
1535-1555 Tuffaceous sandstone. DRILL HOLE 6, Contin.
Depth (feet) Description
1555-1590 S a n d stone.
1590-1635 Tuffaceous sandstone
1635-1760 Arkosic sandstone. 105
DRILL HOLE 7 ENERGY FUELS CORP.
Depth (feet) Description Total Depth - 1900 feet
0-30 Light brown sandstone. Moderate oxidation, fragments of rhyolite and shale.
30-55 Light brown conglomerate, fine to coarse grained, moderate oxidation, shale fragments. 55-560 Light gray, fine grained mudstone. Weak oxidation, some carbonaceous material, well sorted.
560-620 Gray mudstone and sandstone. Abundant carbonaceous material with fragments of lignite.
620-725 Light gray, fine to medium grained sandstone. Unoxidized, some carbonaceous material, lignite fragments.
725-765 Gray, fine grained siltstone and sandstone, some carbonaceous material. Unoxidized below this depth.
765-885 Gray, fine grained sandstone. Sparse carbonaceous material, some silts.
885-945 Gray, medium grained sandstone and con glomerate, some rhyolite fragments.
945-1320 Gray, medium to fine grained sandstone.
1320-1450 Gray and brown, medium to very coarse grained conglomerate. Fragments of sandstone, siltstone and volcanics.
1450-1470 Lost Circulation.
1470-1575 Conglomerate as above.
1575-1660 Gray, medium to coarse grained conglomerate. Sparse carbonaceous material, rhyolite fragments.
1660-1730 Brown to gray, medium to coarse grained conglomerate. Sandstone, siltstone and volcanic fragments. 106
DRILL HOLE 7, Contin.
Depth (feet) Description
1730-1850 Gray, medium to fine grained sandstone. Sparse carbonaceous material, silty. 1850-1890 Gray to brown, very fine to medium grained tuffaceous sandstone. Contains fine ash and siliceous fragments.
1890-1900 Gray and brown, coarse grained granite. Abundant biotite. 107
DRILL HOLE 8 ENERGY FUELS CORP.
Depth (feet) Description Total Depth - 1500 feet
0-25 Light brown silty gravel.
25-160 Light brown to white tuff. Fine grains, some sand. Weak oxidation.
160-185 Light brown, fine to medium grained arkose.
185-235 Light brown, fine to medium grained tuffaceous siltstone. Weak oxidation.
235-280 Light brown to light gray silty tuff. Fine to medium grained, some sand. Unoxidized.
280-310 Light gray to gray, fine to medium grained silty tuff. Unoxidized below this depth.
310-435 Light gray, medium to coarse grained, sandy tuf f.
435-455 Light gray, medium to coarse grained arkose. Abundant granitic fragments.
455-500 Light gray to brown, fine to medium grained s a n d stone.
500-535 Light gray, medium to fine grained silty s a n d stone.
535-730 Gray, fine grained sandstone and siltstone. Some carbonaceous material, lignite fragments.
730-760 Gray, medium grained silty sandstone. Some carbonaceous material.
760-780 Light brown, fine grained silty tuff.
780-920 Gray to brown, fine to medium grained silty s a n d s t o n e .
920-940 Gray to brown, medium to coarse grained s a n d stone.
940-980 Gray, fine grained sandstone. Silty.
980-1020 Gray, very fine grained tuff. DRILL HOLE 8, Contin.
Depth (feet) Description
1020-1080 Tuff with fragments of rhyolite.
1080-1160 Gray, coarse grained conglomerate.
1160-1320 Arkosic sandstone.
1320-1400 Light gray, medium to coarse grained sandstone. Some granite fragments. 1400-1455 Gray, fine grained tuff. Some sand.
1455-1500 G r a n i t e . 109
DRILL HOLE 9 ENERGY FUELS CORP.
Depth (feet) Description Total Depth - 1300 feet
0-40 Brown, fine to coarse grained gravel.
40-95 Brown, fine grained siltstone. Moderately oxidized.
95-460 Gray, medium to fine grained siltstone. Weakly oxidized, sparse carbonaceous material. 460-480 Gray to brown, medium to very coarse conglomerate. Weakly oxidized, large granite fragments.
480-560 Gray to brown, fine to medium grained sandstone
560-590 Light gray, medium to fine grained sandstone. Unoxidized below this depth.
590-635 Gray to brown, fine to medium grained sand stone and siltstone.
635-820 Light gray, fine grained silty sandstone.
820-850 Fine to medium grained, gray sandstone and siltstone.
850-870 Fine to medium grained, gray sandstone.
870-1130 Gray, fine to medium grained sandstone and siltstone.
1130-1195 Gray tuffaceous conglomerate. Fine to medium grained, sparse carbonaceous material.
1195-1225 Gray to brown, very fine to medium grained tuffaceous sandstone. Sparse carbonaceous m a t e r i a l .
1225-1235 Brown, very fine grained tuff.
1235-1295 Gray to brown, very fine to medium grained tuffaceous arkose.
1295-1300 Gray, medium grained arkose. Granitic fragments. 110
DRILL HOLE 10 ENERGY FUELS CORP.
Depth (feet) Description Total Depth - 790 feet
0-25 Brown overburden. Gravel and clay.
25-420 Gray, fine grained siltstone. Some carbon, unoxidized.
420-440 Lost circulation.
440-460 Gray, medium to fine grained siltstone and sandstone. Unoxidized.
460-475 Light gray, medium to coarse grained arkose. Unoxidized below this depth.
475-550 Fine to medium grained, gray silty sandstone.
550-595 Gray, fine to medium grained siltstone and sandstone. Some carbonaceous material.
595-790 Dark gray, fine grained siltstone. Abundant carbonaceous material. Ill
DRILL HOLE 11 ENERGY FUELS CORP.
Depth (feet) Description Total Depth - 1400 feet
0-50 Brown gravel, fine to coarse grained.
50-60 Gray, fine grained siltstone. Moderate o x i d a t i o n .
60-80 Brown, fine to medium grained sandstone. Moderate oxidation.
80-215 Gray siltstone. Weak oxidation, sparse carbonaceous material.
215-275 Light gray, medium grained arkose.
275-295 Gray, fine to medium grained sandstone and siltstone.
295-350 Gray, fine grained siltstone.
350-450 Fine to medium grained, gray tuffaceous siltstone.
450-560 Gray, fine grained siltstone.
560-620 Tuffaceous siltstone.
620-910 Gray, fine grained siltstone, some sand.
910-960 Light gray, medium to coarse grained arkose.
960-1305 Light gray, fine to coarse grained conglomerate. Sparse carbonaceous material. Includes granitic and tuff fragments, some s i l t .
1305-1400 Conglomerate. Moderate oxidation - hematite staining. Abundant granitic fragments, b i o t i t e . 112
DRILL HOLE 12 ENERGY FUELS CORP.
Depth (feet) Description Total Depth - 1500 feet
0-60 Light brown sandstone, siltstone and con glomerate. Moderate oxidation.
60-755 Dark gray, fine grained mudstone. Some carbonaceous material, unoxidized, well sorted. Very easy drilling.
755-760 Dark gray, fine to medium grained sandstone. Unoxidized below this depth.
760-815 Dark gray siltstone and mudstone. Fine grained, some carbon.
815-835 Dark gray siltstone and sandstone.
835-875 Gray, fine grained siltstone. Some carbon.
875-1000 Gray, fine to medium grained siltstone and sandstone. Some carbon.
1000-1060 Gray, sandstone and siltstone. Fine to coarse grained. Some carbon.
1060-1150 Gray, fine to medium gray siltstone and sandstone.
1150-1190 Gray to green, fine grained siltstone. Some carbon, some sand.
1190-1390 Gray to brown, fine grained siltstone and mudstone. Sparse carbonaceous material.
1390-1500 Gray to green, fine grained siltstone. 113
DRILL HOLE 13 ENERGY FUELS CORP.
Depth (feet) Description Total Depth - 1440 feet
0-20 Brown sand, alluvium.
20-120 Brown, fine to medium grained silty sandstone. Weakly oxidized.
120-140 Gray to brown, fine to medium grained sand stone and siltstone. Sparse carbon.
140-190 Brown, fine to medium grained silty sandstone. Weakly oxidized.
190-280 Gray to brown, fine to medium grained siltstone and sandstone. Weakly oxidized, sparse carbonaceous material.
280-740 Fine to medium grained, gray siltstone. Unoxidized, some carbonaceous material.
740-785 Gray, medium to fine grained arkose. Unoxidized below this depth.
785-920 Gray, medium to coarse grained silty arkose. Sparse carbonaceous material.
920-930 Light gray, coarse grained arkose. Sparse carbonaceous material, very hard.
930-1130 Light gray,.fine to coarse grained silty arkose. Abundant granitic fragments, biotite, and some shale.
1130-1340 Light gray, medium to coarse grained tuffaceous arkose. Sparse carbonaceous mat e r i a l .
1340-1390 Light gray to white, fine to coarse grained tuffaceous arkose.
1390-1440 Gray, fine to coarse grained silty arkose. Sparse carbonaceous material.
115
Beta-Gamma Analytical Procedure
The beta-gamma analytical procedure is based on the 2 38 fact that uranium (U ) and it's immediate, short half-life 2 ^ ^ 2 ^ y| products (Th , Pa ) emit beta radiation, whereas gamma radiation is emitted most strongly by Pb21/* and Bi2"^, which are distant daughter products of uranium. Calculations of the concentration of uranium in a sample can be made by measuring either beta or gamma radiation, and if the instru ments used are properly calibrated, and if the sample is in isotopic equilibrium, the concentration values obtained by each method would be identical. In such a case it is clear that:
beta assay = gamma assay and therefore,
2x(beta assay) - gamma assay = gamma assay = beta assay.
The quantity 2 beta - gamma is known as the scaler assay, and is equal to the chemical assay for a sample in equi librium. Scaler assays are useful for samples that are out of equilibrium. There are two possible cases; disequilibrium in favor of uranium, and disequilibrium in favor of daughter products. In the first case, the addition of uranium causes an increase in the beta assay, and therefore, the scaler assay will be higher than a chemical assay of the same sample. In the latter case, the addition of daughter products or removal of uranium yields a higher gamma assay, 116 which results in a lower scaler assay. Thus, by comparing the scaler assay with the chemical assay of a particular sample, an estimate of the equilibrium condition can be m a d e . Sample # Location Description 2xBeta Gamma Scaler Chemical X-ray Assay Assay Assay
1 RME Pits mudstone .20 .05 .15 .109 .112 2 RME Pits mudstone .23 .06 .17 .150 .147 3 Buckhorn Mine tuff .43 .19 .24 .187 .187 4 Buckhorn Mine tuff .29 .14 .15 .134 .123 5 Bastain Cuts tuff .44 . 20 .24 .147 .163 6 Bastain Cuts tuff .21 .08 .13 .043 .043 7 Yellow Jacket siltstone .16 .05 .11 .055 .053 8 Yellow Jacket siltstone .16 .06 .10 .041 .040 9 Lucky Day tuff .01 .00 .01 .004 .004 10 Lucky Day tuff .02 .01 .01 .008 .007
1