Tertiary Volcanic Rocks and 

Uranium in the Thomas Range 

Northern Drum Mountains,

GEOLOGICAL SURVEY PROFESSIONAL PAFSH 1221
Tertiary Volcanic Rocks and 

Uranium in the Thomas Range and 

Northern Drum Mountains, 

Juab County, Utah

By DAVID A. LINDSEY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1221

The geologic setting and controls of uranium 

mineralization in a volcanic environment

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1982
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Catalog No. 82-600501

For Sale by the Branch of Distribution, U.S. Geological Survey 

1200 South Eads Street, Arlington, VA 22202
CONTENTS

Introduction 

Stratigraphic section 

Age and correlation 

Description of rock units 

Diorite 

Mt. Laird Tuff 

Joy Tuff

Crystal tuff member 

Landslide breccias 

Breccia at Spor Mountain and breccia at Wildhorse 

Spring . Megabreccia of the northern Drum Mountains 

Dell Tuff

Beryllium tuff member 

'P/wnfivrntif* rfivnlif'A iri^inl^^i* - . . -

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Description of rock units Continued 

Topaz Mountain Rhyolite Continued 

Stratified tuff 

Structural geology 

Thomas caldera 

Chemical composition of volcanic rocks 

Relationship of volcanism to mineralization 

Uranium in the beryllium tuff member of the Spor

Uranium in stratified tuff of the Topaz Mountain

Rhyolite 

A model for uranium deposits and some suggestions for

exploration 

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ILLUSTRATIONS

FIGURE 1. Map showing location of the Thomas Range and Drum Mountains, other geographic features, and mineral occurrences 

2. Geologic map of Tertiary rocks in the Thomas Range and northern Drum Mountains 

3-13. Photographs and photomicrographs showing:

3. Drum Mountains Rhyodacite 

4. Mt. Laird Tuff 

5.

6.

7.

8.

9.

10.

11.

12.

13.

Crystal tuff member of Joy Tuff 

Black glass tuff member of Joy Tuff 

Landslide breccia at Spor Mountain and Wildhorse Spring 

The Dell Tuff 

Needles Range(?) Formation 

Beryllium tuff member of the Spor Mountain Formation 

Porphyritic rhyolite member of the Spor Mountain Formation 

Topaz Mountain Rhyolite 

Stratified tuff in the Topaz Mountain Rhyolite

14. Aerial photographs of the ring fracture zone of the Thomas caldera, showing the Joy graben and its northward extension in The Dell 

15. Geologic map of The Dell 

16. Maps showing structural evolution of the Thomas Range and northern Drum Mountains 

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10

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ill
IV CONTENTS

Page

FIGURE 17. Graphs showing chemical composition of igneous rocks compared to age 39

18. Graph showing total alkalis (Naj,O plus K2O) versus SiO2 content of igneous rocks - 42

19. AFM diagram of igneous rocks _____ 43

20. Histograms showing the abundance of uranium and thorium in the beryllium tuff member of the Spor Mountain

Formation . .... 48

21. Diagrams showing distribution of altered zones, beryllium, thorium, uranium, and thorium:uranium in the beryllium

tuff member of the Spor Mountain Formation at the Roadside beryllium mine 49

22. Histograms showing the abundance of BeO, uranium, and thorium, and a scattergram showing segregation of BeO and

uranium in mineralized zones of the beryllium tuff member of the Spor Mountain Formation 50

23. Diagram showing location of geochemical samples in two ore lenses in the Yellow Chief mine 52

TABLES

Page

TABLE 1. Revised stratigraphy of the Tertiary rocks of the Thomas Range and northern Drum Mountains 6

2. Summary of radiometric ages for volcanic formations of the Thomas Range and northern Drum Mountains 8

3. Analytical data for new fission track ages of igneous rocks in Juab and Millard Counties 9

4. Comparison of mineral composition of volcanic rocks of the Thomas Range and northern Drum Mountains 11

5. Section of Mt. Laird Tuff in drill hole east of Topaz Mountain 12

6. Analytical data for uranium content of zircon in volcanic rocks of the Thomas Range 44

7. Chemical analyses of samples from two ore lenses in the Yellow Chief mine 51

8. Chemical analyses of igneous rocks of the Thomas Range and Drum Mountains 60
TERTIARY VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE 

AND NORTHERN DRUM MOUNTAINS, JUAB COUNTY, UTAH

By DAVID A. LlNDSEY

ABSTRACT

The Thomas Range and northern Drum Mountains have a history 

of volcanism, faulting, and mineralization that began about 42 m.y. 

(million years) ago. Volcanic activity and mineralization in the area 

can be divided into three stages according to the time-related occurrence of rock types, trace-element associations, and chemical composition of mineral deposits. Compositions of volcanic rocks 

changed abruptly from rhyodacite-quartz latite (42-39 m.y. ago) to 

rhyolite (38-32 m.y. ago) to alkali rhyolite (21 and 6-7 m.y. ago); 

these stages correspond to periods of chalcophile and siderophile 

metal mineralization, no mineralization!?), and lithophile metal 

mineralization, respectively. Angular unconformities record 

episodes of cauldron collapse and block faulting between the stages 

of volcanic activity and mineralization. The youngest angular unconformity formed between 21 and 7 m.y. ago during basin-andrange faulting.

Early rhyodacite-quartz latite volcanism from composite 

volcanoes and fissures produced flows, breccias, and ash-flow tuff of 

the Drum Mountains Rhyodacite and Mt. Laird Tuff. Eruption of 

the Mt. Laird Tuff about 39 m.y. ago from an area north of Joy 

townsite was accompanied by collapse of the Thomas caldera. Part 

of the roof of the magma chamber did not collapse, or the magma 

was resurgent, as is indicated by porphyry dikes and plugs in the 

Drum Mountains. Chalcophile and siderophile metal mineralization, 

resulting in deposits of copper, gold, and manganese, accompanied 

early volcanism.

The middle stage of volcanic activity was characterized by explosive eruption of rhyolitic ash-flow tuffs and collapse of the 

Dugway Valley cauldron. Eruption of the Joy Tuff 38 m.y. ago was 

accompanied by subsidence of this cauldron and was followed by 

collapse and sliding of Paleozoic rocks from the west wall of the 

cauldron. Landslides in The Dell were covered by the Dell Tuff, 

erupted 32 m.y. ago from an unknown source to the east. An ash 

flow of the Needles Range(?) Formation was erupted 30-31 m.y. ago 

from an unknown source. Mineralization probably did not occur during the rhyolitic stage of volcanism.

The last stage of volcanism was contemporaneous with basin-andrange faulting and was characterized by explosive eruption of ash 

and pumice, forming stratified tuff, and by quiet eruption of alkali 

rhyolite as viscous flows and domes. The first episode of alkali 

rhyolite volcanism deposited the beryllium tuff and porphyritic 

rhyolite members of the Spor Mountain Formation 21 m.y. ago. 

After a period of block faulting, the stratified tuff and alkali 

rhyolite of the Topaz Mountain Rhyolite were erupted 6-7 m.y. ago 

along faults and fault intersections. Erosion of Spor Mountain, as 

well as explosive eruptions through dolomite, provided abundant 

dolomite detritus to the beryllium tuff member. The alkali rhyolite 

of both formations is fluorine rich, as is evident from abundant 

topaz, and contains anomalous amounts of lithophile metals. Alkali 

rhyolite volcanism was accompanied by lithophile metal mineralization which deposited fluorite, beryllium, and uranium.

The structure of the area is dominated by the Thomas caldera and 

the younger Dugway Valley cauldron, which is nested within the 

Thomas caldera; the Thomas caldera is surrounded by a rim of 

Paleozoic rocks at Spor Mountain and Paleozoic to Precambrian 

rocks in the Drum Mountains. The Joy fault and Dell fault system 

mark the ring-fracture zone of the Thomas caldera. These structural 

features began to form about 39 m.y. ago during eruption of the Mt. 

Laird Tuff and caldera subsidence. The Dugway Valley cauldron 

sank along a series of steplike normal faults southeast of Topaz 

Mountain in response to collapse of the magma chamber of the Joy 

Tuff. Caldera structure was modified by block faulting between 21 

and 7 m.y. ago, the time of widespread extensional faulting in the 

Basin and Range Province. Vents erupted alkali rhyolite 6-7 m.y. 

ago along basin-and-range faults.

Uranium mineralization was associated with the stage of alkali 

rhyolite volcanism, extensional basin-and-range faulting, and 

lithophile metal mineralization; it occurred at least 11 m.y. after the 

end of the caldera cycle. Uranium, derived from alkali rhyolite 

magma, was concentrated in trace amounts by magmatic fluids and 

in potentially economic amounts by hydrothennal fluids and 

ground water. Hydrothennal fluids deposited uraniferous fluorite 

as pipes in carbonate rocks of Paleozoic age on Spor Mountain and 

uranium-bearing disseminated deposits of fluorite and beryllium in 

the beryllium tuff member of the Spor Mountain Formation. 

Uranium of hydrothennal origin is dispersed in fluorite and opal. 

Uranium in fluorite may be tetravalent(?) but that in opal is probably hexavalent; no primary minerals of tetravalent uranium are 

known to occur. Ground water has concentrated significant ores of 

hexavalent uranium minerals in the beryllium tuff member of the 

Spor Mountain Formation at the Yellow Chief mine and is probably 

also responsible for widespread low concentrations (O.OX percent) of 

uranium that occur separately from beryllium ore in the beryllium 

tuff member. More deposits of the Yellow Chief type may occur in 

down-faulted sections of beryllium tuff beneath the Thomas Range. 

The ground-water ores show no evidence of a reducing environment; 

instead, precipitation of hexavalent uranium minerals occurred by 

evaporation, by decline in concentration of complexing ions such as 

carbonate, or by some other mechanism. Reducing environments 

for hydrothennal deposits must be sought around rhyolite vents 

and in a hypothesized pluton of alkali rhyolite composition beneath 

Spor Mountain; for ground-water deposits, reducing environments 

may occur in basin fill such as that of the Dugway Valley cauldron.

INTRODUCTION

The Thomas Range (fig. I) contains important 

deposits of fluorspar, beryllium, and uranium. These 

mineral deposits are associated with a sequence of 

volcanic rocks that extends into the northern Drum

1
VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

EXPLANATION 

Rodenhouse Wash Mineral occurrence

GREAT SALT LAKE 

DESERT Dugway Range 

uadrangle

SIMPSON 

MOUNTAINS/ V

TOOELE COUNTY

JUAB COUNTY VALLEY 

DUGWAY

oneycombs ^ I SPRINGS 

cc.

Q_ 

CO

rf>\ \-s, I FLATS

SEVIER DESERT

JUAB COUNTY

Topaz Mountain 

quadrangle <f\ MILLARD COUNTY DRUM 

MOUNTAINS

SVLITTLE DRUM 

MOUNTAINS

LONGfA 

RIDGE V \

FIGURE 1. Map showing location of the Thomas Range and Drum Mountains, other geographic features, and mineral occurrences shown by a pick and hammer and the following symbols: U, uranium; Be, beryllium; F, fluorite; Mn, manganese; Cu, 

copper; Pb, lead; Zn, zinc; Au, gold; and Ag, silver.

Mountains and contains intermediate-composition 

flow rocks and tuffs, rhyolitic ash-flow tuffs, and large 

volumes of alkali rhyolite. The area is part of an eastwest belt of mineral deposits, volcanic and intrusive 

rocks, and aeromagnetic high anomalies called the 

Deep Creek-Tintic mineral belt (Shawe and Stewart, 

1976; Stewart and others, 1977), which also contains 

the beryllium belt of western Utah (Cohenour, 1963). A 

similar sequence of volcanic rocks, but with no known 

mineral deposits, crops out in the Keg Mountains, east 

of the Thomas Range.

The volcanic rocks of the Thomas Range were first 

mapped and divided into two groups by Staatz and 

Carr (1964). Shawe (1972) reclassified the volcanic 

rocks of the Thomas Range into three assemblages, (1) 

flows and agglomerates, (2) ash-flow tuffs, and (3) 

rhyolite flows and tuff, all of which he was able to map 

throughout the area of the Drum Mountains, Keg 

Mountain, and Desert Mountain. Geochronologic

studies (Lindsey and others, 1975) confirmed much of 

Shawe's (1972) three-fold classification of the volcanic 

rocks of the region. Shawe also concluded that eruption of voluminous ash-flow tuffs of his middle assemblage was followed by caldera collapse in the Thomas 

Range and at Keg Mountain and Desert Mountain. 

The Joy fault was interpreted as part of the ring fracture of the Thomas caldera, and such ring fractures 

were believed to have provided conduits that localized 

the deposits of ore-forming fluids. The northern Drum 

Mountains also were mapped by Newell (1971), who 

confirmed the general outline of the caldera model 

there. Recent mapping in southwestern Keg Mountain 

by Staub (1975) did not confirm the Thomas caldera 

ring fracture that was projected there by Shawe (1972). 

The mineral deposits of the Thomas Range were 

studied by Staatz and Osterwald (1959) and Staatz and 

Carr (1964), who described the fluorspar pipes and 

uranium occurrences there. Beryllium deposits in tuff
STRATIGRAPHY

were discovered at Spor Mountain in 1959, and studies 

of these deposits related them to fluorspar mineralization and rhyolite volcanism in the Thomas Range 

(Staatz and Griffitts, 1961; Shawe, 1968; Park, 1968; 

Lindsey, 1977). The manganese deposits of the Detroit 

district in the Drum Mountains have been studied by 

Crittenden and others (1961), and the area's potential 

for gold, copper, and other mineral deposits has been 

examined by mapping and geochemical surveys 

(Newell, 1971) and geochemical studies of jasperoid 

found there (McCarthy and others, 1969).

This report describes the geology of Tertiary rocks 

and uranium occurrences of the Thomas Range and 

northern Drum Mountains in detail, and proposes a 

model relating volcanism, tectonism, and mineralization for the area. The present study was conducted in 

response to the current (1978) high interest in uranium 

exploration in the Thomas Range and vicinity. Reconnaissance studies of the uranium potential of the area 

have been made (Leedom and Mitchell, 1978; Texas Instruments Incorporated, 1977), and the uranium 

potential of the area has been evaluated by drilling 

(Morrison, 1980), but recent geologic investigations 

(Lindsey, 1978a) indicated that the history of volcanism and tectonism in the Thomas Range and Drum 

Mountains was still not sufficiently understood to 

relate it to uranium mineralization and to provide reliable guides for uranium exploration. A new stratigraphic framework, resembling that of Shawe (1972) in 

general outline but differing in many details, was 

developed from field mapping, geochronologic, petrographic, and trace-element studies, and a new geologic 

map of the area was prepared (Lindsey, 1979a). The 

results of the new mapping are summarized in figure 2 

for reference here, but the reader should consult Lindsey (1979a) for details. The present report supersedes 

an earlier open-file report (Lindsey, 1979b).

ACKNOWLEDGMENTS

Many employees of the U.S. Geological Survey provided chemical analyses and assisted with other 

aspects of this study; they are acknowledged in the 

text and tables. D. R. Shawe, by many discussions of 

the area studied, J. C. Ratte' , by discussion of 

cauldron-subsidence concepts, and R. K. Glanzman, by 

obtaining samples for uranium and thorium analyses, 

have contributed importantly to this report. L. M. 

Osmonson and Ezekiel Rivera made mineral separations, and Louise Hedricks prepared photomicrographs. I thank many industry exploration geologists, 

including M. J. Bright, W. A. Buckovic, R. H. Dorman, 

D. E. Gorski, S. M. Hansen, S. H. Leedom, P. L.

Nieson, J. M. Pratt, and W. A. Spears, for information 

about the geology of the area; and I also thank W. L. 

Chenoweth and D. A. Sterling of the U.S. Department 

of Energy and Charles Beverly, M. C. Callihan, C. M. 

Freeman, and R. D. Cole of Bendix Field Engineering 

Corp. for geologic information and coordination with 

U.S. Department of Energy plans and programs in the 

area. I thank the employees of Brush Wellman for their 

cooperation and hospitality during fieldwork.

STRATIGRAPHY 

STRATIGRAPHIC SECTION

The Tertiary rocks of the Thomas Range and northern Drum Mountains are divided into nine formations 

(table 1). As revised here, the stratigraphy corresponds 

generally to the former subdivision of stratigraphic 

units into oldest (flows and agglomerates), middle (ashflow tuffs), and youngest (rhyolite flows and tuffs) 

assemblages or groups (Shawe, 1972; Lindsey and 

others, 1975). Each of these groups is characterized by 

a particular style of volcanism and is separated by an 

angular unconformity. Minor unconformities occur 

between some formations, also.

All of the volcanic rocks with the possible exception 

of the Needles Range(?) Formation have local sources. 

Flows of the Drum Mountains Rhyodacite were probably extruded from local fissures and from central 

volcanoes in the Black Rock Hills and Little Drum 

Mountains (Leedom, 1974) about 42 m.y. ago. Quiet 

eruption of rhyodacite gave way to explosive eruption 

of ash-flow tuff from vents north of Joy townsite in the 

Drum Mountains and east of Topaz Mountain in the 

Thomas Range. The first explosive eruptions, from the 

Drum Mountains vent, deposited tuff of intermediate 

composition (Mt. Laird Tuff), whereas all later eruptions, from 38 m.y. to 32 m.y. ago, deposited rhyolitic 

tuff. One or more of these eruptions deposited the 

crystal tuff member of the Joy Tuff over an area that 

extends from Fish Springs Flat to Desert Mountain, a 

distance of 60 km (kilometers). Collapse of cauldron 

walls 39-32 m.y. ago left landslide deposits of megabreccia and breccia interbedded with lava flows and 

ash-flow tuffs. Near the end of explosive rhyolite volcanism, about 30-31 m.y. ago, an ash-flow tuff of the 

Needles Range(?) Formation was deposited in part of 

the area. All later volcanism consisted of explosive 

eruption of ash and quiet extrusion of alkali rhyolite 

lava as flows and domes 21 m.y. ago (Spor Mountain 

Formation) and 6-7 m.y. ago (Topaz Mountain 

Rhyolite).
4 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

113°15' 10' 05' 113*00'
STRATIGRAPHY

Alluvium and colluvium (Quaternary)

Topaz Mountain Rhyolite (Upper

Miocene)

Flows and domes of alkali rhyolite

Stratified tuff

Drum Mountains Rhyodacite (upper 

Eocene) Flows and breccias

Angular unconformity A

Undifferentiated rocks (Devonian to

Angular unconformity D

Angular

Spor Mountain Formation (Lower 

Miocene)

Porphyritic rhyolite member 

Beryllium tuff member 

unconformity C

Needles Range (?) Formation 

(Oligocene)

Dell Tuff (Oligocene)

Landslide breccia (Oligocene?) 

Megabreccia in northern Drum 

Mountains, Age relations with 

other landslide breccias unknown

Landslide breccia (Oligocene?) 

Breccia at Spor Mountain

Landslide breccia (Oligocene?) 

Breccia at Wildhorse Spring 

Joy Tuff 

Black glass tuff member (lower

Oligocene)

Crystal tuff member (lower Oligocene and upper Eocene)

unconformity B

Mt. Laird Tuff (upper Eocene) 

Ash-flow tuff and brecda

Mt. Laird Tuff equivalent (upper 

Eocene)--Intrusive dike and 

plugs

Diorite (upper Eocene?) Intrusive 

plugs

Precambrian) Limestone, dolomite, quartzite, and shale

Contact Dashed where approximately located

Fault, dotted where concealed, ball 

and bar on downthrown side; 

concealed faults located from 

aerial photographs (Lindsey, 

1979a)

Probable vent area

Structural margin of Thomas caldera 

Queried where location uncertain

Structural margin of Dugway Valley 

cauldron Queriedwhere location 

uncertain

Mines and prospects in Tertiary rocks

1 North End (Be)

2 Taurus (Be)

3 Monitor (Be)

4 Roadside mine (Be)

5 Fluro mine (Be)

6 Rainbow (Be)

7 Blue Chalk mine (Be)

8 Oversight (Be, U)

9 Buena no. 1 (U)

10 Claybank (Be, U)

11 Hogsback(Be)

12 Yellow Chief mine (U)

13 Autunite no. 8 (U)

14 Sigma Emma (Be)

FIGURE 2. Geologic map of Tertiary rocks in the Thomas Range and northern Drum Mountains (modified from Lindsey,

1979a).

Four angular unconformities record periods of 

cauldron collapse, faulting, and erosion in the Tertiary 

section; these unconformities have regional extent 

throughout the volcanic field of the Thomas Range, 

Drum Mountains, and Keg Mountain. Unconformity 

A, at the base of the section, records some prevolcanic 

period or periods of uplift and erosion of uncertain age. 

Unconformity B lies beneath the 38 m.y.-old crystal 

tuff member of the Joy Tuff, and records subsidence of 

the Thomas caldera that accompanied eruption of the 

Mt. Laird Tuff about 39 m.y. ago. Erosional detritus is 

lacking above the unconformity, indicating that it is

mainly constructional. Cauldron collapse resulting 

from eruption of the Joy Tuff 38 m.y. ago was followed 

by deposition of the DeU Tuff 32 m.y. ago and the 

Needles Range(?) Formation about 30-31 m.y. ago. 

Unconformity C represents a 9-m.y. period of 

quiescence after ash-flow eruption; it is partly constructional and partly erosional, as indicated by erosional detritus in the Spor Mountain Formation of 21 

m.y. ago. Unconformity D, between the Spor Mountain Formation and the Topaz Mountain Rhyolite, 

formed during basin-and-range faulting between 21 

and 6-7 m.y. ago.
6 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH 

TABLE I. Revised stratigraphy of Tertiary rocks in the Thomas Range and northern Drum Mountains

[Leaders ( ), information not available or not relevant; <, less than; ~, about]

i ? Description of rocks Age (m.y.) Map symbol

___________________________________________ _______ ______________________________________(fig- 2)

Alluvium and colluvium (Quaternary): Alluvial pediments and stream deposits of poorly sorted gravel, sand, and <1 Qal 

clay; colluvium covering slopes; playa sediments; beach sand and gravel deposits and lake-bottom clays 

deposited by Lake Bonneville at elevations below about 1,580 m elevation.

Unconformity

Topaz Mountain Rhyolite (Upper Miocene): Flows and extrusive domes of gray to red, topaz-bearing alkali rhyolite, 6.3-6.6 Ttm^ ^ 3 

black vitrophyre, and interbedded units of tan stratified tuff. Tuff units (Ttmt) seldom exceed 30 m in 

thickness, are local in extent, and have unconformable bases. Rhyolite contains sparse cryatals of quartz, 

aanidine, biotite, and plagioclase except locally at Antelope Ridge and lower part of Pismire Wash, where 

phenocrysts are abundant. Maximum thickness of rhyolite about 700 m.

Angular unconformity D

Spor Mountain Formation, porphyritic rhyolite member (Lower Miocene): Flows, domes, and plugs of gray to red 21.3 Tsp 

porphyritic alkali rhyolite; rhyolite contains abundant phenocrysts of dark quartz, sanidine, plagioclase, and 

biotite and abundant microscopic topaz in groundmasa. Maximum thickness about 500 m.

Spor Mountain Formation, beryllium tuff member (Lower Miocene): Stratified tan vitric tuff and tuffaceous breccia

with abundant clasts of carbonate rocks. Tuff includes thin beds of ash-flow tuff, bentonite, and (21-3) Tsb 

epiclastic tuffaceous sandatone and conglomerate in The Pell. Hydrothermal alteration of tuff to clay, 

fluorspar, and potassium feldspar widespread. Maximum thickness about 60 m.

Angular unconformity C

Needles Range(?) Formation (Oligocene): Simple cooling unit of pink to gray to red-brown ash-flow tuff with 31.4 Tnr

abundant small crystals of plagioclase, hornblende, and biotite. Partially welded. Tuff fills paleovalleys

on northwest side of Drum Mountains. Maximum thickness about 30 m.

Dell Tuff (Oligocene): Gray to pink rhyolitic ash-flow tuff that contains abundant crystals of euhedral quartz, 32.0 Td 

sanidine, biotite, and plagioclase in poorly welded to unwelded matrix of devitrified shards and pumice. 

Tuff resembles older Joy Tuff but may be distinguished from Joy by presence of abundant large quartz 

bipyramids and loose, ashlike weathering aspect. Maximum thickness about 180 m at north end of The Dell.

Landslide breccia, megabreccia of the northern Drum Mountains (Oligocene?): Megabreccia of Cambrian limestone (<37) Tld 

and dolomite overlying Joy Tuff in northern Drum Mountains. Megabreccia retains original stratigraphy of 

Cambrian strata but contains intensely brecciated and rotated clasts of Cambrian rocks locally. Maximum 

thickness about 60 m.

Landslide breccia, breccia at Spor Mountain (Oligocene?): Breccia of Ordovician and Silurian dolomite, limestone, (32-42) Tls 

and quartzite. Breccia retains original stratigraphy of Paleozoic rocks near breakaway zone at crest of Spor 

Mountain but passes east into breccia with clasts of various strata mixed together and faintly stratified. 

Maximum thickness estimated very approximately at 80 m.

Landslide breccia, breccia at Wildhorse Spring (Oligocene?): Breccia of mixed angular to subround clasts of (32-42) Tlw 

Paleozoic rocks and Drum Mountains Ihyodacite in a matrix of rhyodacite fragments. Breccia underlies and 

passes laterally into breccia at Spor Mountain and is stratified at top. Maximum thickness estimated very 

approximately at about 20 m.

Unconformity

Joy Tuff, blark glass tuff member (Lower Oligoeeno): Simple rooting unit nf rhyolitic ash-flow tuff with sparse -37 Tjb 

crystals of sanldlne, quartz, plaglnr- lase, and blntlte, and llthlc fragments nf limestone and volcanic rock. 

Most of tuff Intensely welded, with abundant comparted black pumlre In lower part. Upper unwelded part is 

tan and contains abundanr light-enlured pumice. Maximum thickness about 3H m.

Joy Tuff, crystal tuff member (Lower Oligocene and Upper Eocene): Gray-pink to red-brown rhyolitlc ash-flow tuff 38-0 Tjc 

with abundant euhedral and broken crystals of quartz, sanidine, plagioclase, and biotite in moderately welded 

matrix of devltrtfled shards. Lower 10 m of tuff contains abundant compacted black pumice; light-colored pumice 

present higher in section. Tuff contains abundant accessory sphene and rare cognate inclusions of lathlike 

plagioclase, biotite, and sphene that aid in distinguishing It from Dell Tuff. Welding strong near probable vents 

east of Topaz Mountain, where foliation near vertical and breccia occurs. Maximum thickness about 180 m at the 

type locality.

Angular unconformity B

Mt. Laird Tuff (Upper Eocene): Pink quartz latitic ash-flow tuff with abundant euhedral crystals of white ~39 Tml 

plagioclase (10 mm long), bronze biotite (5 mm across), and hornblende. Quartz phenocrysts with resorbed outlines 

occur in tuff northeast of Thomas Range. Pumlceous breccia and hydrothermally altered tuff occur near probable 

vent north of Joy townsite. Dikes and plugs of porphyry (Tmli) very similar to Mt. Laird Tuff crop out 3 km south 

of Joy townsite are included in unit. Maximum exposed thickness about 80 m, but 500 m of tuff interbedded with 

tuffaceous lacustrine sediments in subsurface of Dugway Valley.

Diorite (Upper Eocene): Plugs of dark-gray, massive, fine-grained dlorite intrude Paleozoic strata 3 km southeast (39-42) Tdi 

of Joy townsite. Diorite contains abundant calcic plagioclase and hornblende.

Drum Mountains Rhyodacite (Upper Eocene): Rusty weathering black rhyodacite flows and breccias with phenocrysts of ~42 Tdr 

intermediate composition to calcic plagioclase and pyroxene in an aphanitic to glassy matrix. Modally, rock 

is hypersthene andesite, but chemical analyses show rock to be rhyodacite in classification of Rittmann (1952). 

Unit includes some interbedded tuffaceous sandstone and laharic debris flows in Black Rock Hills and some 

aphanitic flow or dike rocks near Joy townsite. Maximum thickness about 240 m in Black Rock Hills.
STRATIGRAPHY

TABLE 1. Continued

Description of rocks' Age (m.y.)" Map symbol 

(fig. 2)

Angular unconformity A

Undifferentiated rocks (Devonian to Precambrian): Limestone, dolomite, quartzlte, and shale. Formations are Dp 

differentiated on maps of Staatz and Carr (1964) for Thomas Range and Newell (1971) and Crittenden and others 

(1961) for Drum Mountains. Maximum thickness exceeds 1,200 m.

Many units have unconformable tops, so that the original thickness has been reduced by erosion.

Ages are averages of all valid potassium-argon and fission-track dates (table 3); ages in parentheses are inferred from stratigraphic 

relationships.

^Average of two ages of Needles Range(?) Formation of Little Drum Mountains is 31.4 m.y.; average age of the Needles Range Formation is 

30.4 m.y. (Armstrong, 1970).

4A single age of 36.4±l-7 m.y. on the Mt. Laird Tuff was determined (table 3), but the true age of the Mt. Laird Tuff Is estimated at about 

39 m.y., because it underlies the 38.0-m.y.-old crystal tuff member of the Joy Tuff.

AGE AND CORRELATION

An attempt was made to date each rock unit in the 

Thomas Range and Drum Mountains, using both old 

and new data (table 2). The ages have been revised to 

reflect additional dating and recently adopted decay 

constants used in calculating ages by the potassiumargon and fission-track methods. Methods used for 

fission-track dates reported here are described by 

Naeser (1976) and Naeser and others (1978).

The new fission-track ages extend the history of 

volcanism in western Utah to almost 42 m.y. ago 

(tables 2 and 3). A single age of 41.8±2.3 m.y. on zircon 

from the Drum Mountains Rhyodacite is somewhat 

older than two whole-rock potassium-argon ages of 

38.2±0.4 m.y. (revised from 37.3±0.4 m.y., reported 

by Leedom (1974), to account for change in constants) 

for flow rocks that overlie the rhyodacite in the Little 

Drum Mountains. The zircon age is considered reliable 

because it is not subject to the effects of weak alteration that pervades much of the rhyodacite; also, uranium and the resultant fission tracks are distributed 

uniformly in the zircon dated so that counting errors 

that may attend dating of zoned grains are not a 

problem.

The age of the crystal tuff member of the Joy Tuff is 

estimated at 38.0±0.7 m.y. (table 2) by eight fissiontrack dates on sphene, zircon, and apatite. The date of 

the Joy Tuff marks the onset of extensive eruptions of 

rhyolitic ash-flow tuff. A single zircon age of 36.4 ±1.6 

m.y. on the Mt. Laird Tuff, which unconformably 

underlies the Joy Tuff, is probably not significantly 

different from the age of the Joy Tuff. Accordingly, the 

true age of the Mt. Laird Tuff is believed to be about 

39 m.y. The age of the black glass tuff member of the 

Joy Tuff, which overlies the crystal tuff member, was 

checked by a single determination of 37.0±4.1 m.y. on 

sphene. The age is supported by the close chemical and 

spatial association of the two members of the Joy Tuff

and sets them apart from the younger Dell Tuff, which 

has an average age of 32.0± 0.6 m.y. as determined by 

10 fission-track dates on zircon, sphene, and apatite.

The age of the ash-flow tuff correlated with the 

Needles Range(?) Formation (Pierce, 1974) was determined to test that correlation. Tuff of the Needles 

Range(?) Formation was described from outcrops 

south of the Little Drum Mountains (Leedom, 1974; 

Pierce, 1974), and mapped by me (Lindsey, 1979a) 

along the northwest side of the Drum Mountains. 

Fission-track ages of 30.6±1.2 m.y. on zircon and 

32.2±3.6 m.y. on apatite are in accord with assignment of the tuff to the Needles Range Formation, 

which was estimated by Armstrong (1970) to be 30.4 

m.y. old (age adjusted to account for different decay 

constants).

The 21-m.y. age of the Spor Mountain Formation is 

confirmed by a new zircon date of 21.5 ±1.1 m.y. on the 

dome of porphyritic rhyolite near Wildhorse Spring. H. 

H. Mehnert dated the porphyritic rhyolite flow at the 

Roadside mine at 21.2±0.9 m.y. by the potassiumargon method on sanidine (Lindsey, 1977). An age of 

18.1 ±4.6 m.y. was obtained for porphyritic rhyolite at 

the east side of The Dell but was not used in estimating the average age (table 2) because only a single grain 

of zircon could be dated.

An attempt to date zircon from bentonite in the 

Yellow Chief mine, a facies of the beryllium tuff 

member of the Spor Mountain Formation, was not successful. Detrital zircon from the bentonite makes up 

about 10 percent of the total zircon and is dated as 

28.3±1.8 and 40.0±7.0 m.y. old; it reflects the ages of 

the older volcanic rocks. The remaining 90 percent of 

the zircon has high track densities and could not be 

dated; it contains 3,700-7,500 ppm (parts per million) 

uranium, which is well above that of zircon from the 

older volcanic rocks and within the range of high 

uranium content typical of zircon in the overlying porphyritic rhyolite member. Field relationships pre-
VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

TABLE 2. Summary of radiometric ages for volcanic formations of the Thomas Range and northern Drum Mountains

[Ages determined from some formations in nearby areas are included. All fission-track ages on zircon, sphene, and apatite have been recalculated using Xj-j=1.551X10~'yr~' and 

Xp=7.03X10"17yr~'- All K-Ar ages on sanidine have been recalculated using decay constants for 40K of X£ =0.581X10'loyr~ 1 and XB =4.962X10''°yr"' 

and «K/K=1.167X10-4]

Sample No. Mineral dated Age ±2o Average age 

(millions of years) (± standard error of the mean)

,1

Topaz Mountain Rhyolite:

Younger flow, Topaz Mountain 177

Do T40-TRA2

no T40-TRA3

Older flow, Topaz Mountain T52-TR-A

Do T5 2-TR-B

Older flow, Pismire Wash T50-TR-A

Spor Mountain Formation:

Porphyritic rhyolite member T53-TR-B 

Do TT26

Needles Range(?) Formation, U229 

Little Drum Mountains.

Ho U229

Dell Tuff T43-A3

n 0 T4 3-A 3

Do T43-A 3

Do T42-A 3

n 0 T42-A3

Do T4 2-A 3

Do T54-A

nell Tuff,

Keg Mountain Pass K20-A 3

Do K48-A 3

Do K48-A 3

Joy Tuff:

Black glass tuff member U141

Crystal tuff member T51-A 3 

Do T51-A 3 

Do U18B 

no U32 

Do U34 

Do U56

Crystal tuff member,

Desert Mountain TJ238

Crystal tuff member,

Picture Rock Hills U240

Mt. Laird Tuff U57 

Drum Mountains Rhyodacite D10A

Armstrong (1970, table 3). 

^E. H. McKee, oral commun. , 1975. 

Lindsey and others (1975). 

4H. H. Mehnert, oral commun., 1976, 1978.

Sanidine

 do 

Zircon 

__do 

 do 

Sanidine 

Zircon

Apatite

Zircon

Sphene

Apatite

Zircon

Sphene

Apatite

Zircon

Rphene 

 do 

Zircon

Sphene

Apatite

Sphene

 do 

 do 

 do 

 do 

Zircon

 do 

 do 

 do 

6.1±0.4 

6.3±0.4 

6.3±0.3 

6.2±0.3 

6.4±0.3 

6.8±0.3

21.2±n 

21.5±1 -.1}

30.6±1.2 

32.2±3.6

30.7±6.3

2fi.5±1.2

32.8

33.0±1.3

32.4±1.4

33.3

29.4±1.3

33.6±1.8 

32.5±1.6 

33.R±1.3

37.0±4.1

40.0

38.5±2.0

39.7±3.4

39.4±2.8

38.4±4.0

36.4±2.8

34.5±1.3 

36.9±1.7 

36.4±1.6 

41.8±2.3

6.3±0.1 n.y. 

6.3±0.1 m.y.

21.3±0.2 m.y. 

31.4±O.P m.y.

32.0±0.fi m.y.

38.0±0.7 m.y.

sented in following sections of this report indicate that 

the age of the bentonite and the rest of the beryllium 

tuff member is close to that of the overlying porphyritic rhyolite.

The history of igneous activity is approximately the 

same in the Thomas Range, Drum Mountains, Keg 

Mountain, and Desert Mountain (Shawe, 1972; Lindsey and others, 1975). Each range has local intrusive 

and volcanic rocks, however, and only the ash-flow 

tuffs provide stratigraphic markers that extend into 

all of the ranges. The "Keg Spring andesite and latite" 

of Erickson (1963) is 39.4±0.7 m.y. old and is confined 

to the northwest part of Keg Mountain. The "Keg 

Spring" is overlain by the Mt. Laird Tuff north of Keg 

Pass; thus, it occupies the same stratigraphic position 

at Keg Mountain as the Drum Mountains Rhyodacite 

does in the Thomas Range. A newly recognized stock

of granodiorite (Staub, 1975; H. T. Morris, oral commun., 1976), dated here at 36.6 ±1.6 m.y. by the 

fission-track method on zircon, intrudes the "Keg 

Spring andesite and latite" west of Keg Pass. Both of 

these rocks are unconformably overlain by the crystal 

tuff member of the Joy Tuff in the Picture Rock Hills, 

which suggests that the zircon age of the granodiorite 

stock may be about 1-2 m.y. too young. The Dell Tuff 

unconformably overlies the "Keg Spring andesite and 

latite" and the Mt. Laird Tuff north of Keg Pass, 

where three fission-track ages yield an average of 

33.1±0.4 m.y. (Lindsey and others, 1975). The crystal 

tuff member of the Joy Tuff is well exposed on the east 

side of Desert Mountain, where it has been dated at 

34.5 ±1.3 m.y. by the fission-track method. This age 

may be too young because the tuff has been intruded 

by the stock of Desert Mountain (Shawe, 1972); the
DESCRIPTION OF ROCK UNITS 

TABLE 3. Analytical data for new fission-track ages of igneous rocks in western Juab and Millard Counties

(All ages determined by author by external detector method. Neutron dose determined by C. W. Naeser]

Sample No.

U26 

U12B-1 

U229 

U141 

U18B -

U57 

U29B 

U10A 

' Compi 

2 Zlrct

Rock unit

Spor Mountain Formation, 

porphyritic rhyolite 

member.

Spor Mountain Formation, 

beryllium tuff member 

(bentonlte at Yellow 

Chief mine).

Needles Range(?) Formation

Joy Tuff, black glass 

tuff member.

Joy Tuff, crystal tuff 

member.

Mt. Laird Tuff 

Granodiorite of Keg - 

Mountain (Staub, 1975).

Drum Mountains Rhyodacite

ited using x D-1.551xlO~ 10 /yr 

>n believed to be detrital: i

Geographic

North of Spor 

Mountain.

 do 

South of 

Little Drum 

Mountains.

Northeastern 

Drum 

Mountains.

East of Topaz 

Mountain.

Topaz 

Mountain.

Northwest ofJoy townsite.

Mountain.

Picture RockHills.

Northwest ofJoy townsite.

Western Keg 

Mountain.

The Dell 

Sample Mineral Number of Induced tracks 

location grains Fossil tracks in detector Neutron dose

NE1/4 

T. 12

T. 12

NW1/4 

T. 12

NE1/4 

T. 16

SE1/4 

T. 12

NE1/4 

T. 13

SW 1/4 

T. 13

T. 13

NE1/4 

T. 13

NE1/4 

T. 14

NW1/4 

T. 12

NW1/4 

T. 13

NE1/4 

T. 14

SE1/4 

T. 12

SW1/4 

T. 12

and X F="7.03xlO~ 17 /yr. 

ibundant (90 percent) zi

Number Density Number 2X density 

counted (tracks/cm2 ) counted (tracks/cm^) (Neutrons/en/)

sec. 9, Zircon 5 730 1.13xl07 463 1.4U107 4.56xl0 14 

S., R. 12 W.

S., R. 12 W.

sec. 36, do2 4 321 4.57xl06 338 9.63xl06 9.97xlO U 

S., R. 12 W.

 do2 2 346 9.62xl06 257 1.43xl07 9.97xl0 14

sec. 23, do 10 919 5.87xl06 861 l.lOxlO7 9.61xl014 

S., R. 11 W.

S., R. 12 W.

sec. 36, Sphene 12 262 l.OlxlO6 1,017 7.85xl06 4.81xl0 15 

S., R. 11 W.

sec. 10, do 12 488 l.BBxlO6 350 2.70xl06 9.55xl0 14 

S., R. 11 W.

S., R. 11 W.

S., R. 11 W.

S., R. 11 W.

S., R. 6 W.

sec. 23, do 8 1,005 8.80xl06 782 1.37xl07 9.61xl014 

S., R. 10 W.

sec. 21, do 8 966 6.63xl06 886 6.08xl06 1.12xl0 15 

S., R. 11 W.

sec. 24, do 6 727 6.48xl06 598 1.07xl0 7 l.OlxlO 15 

S., R. 10 W.

sec. 35, do 8 518 3.00xl06 345 4.00xl06 9.33xl0 14 

S., R. 12 W.

X D , total decay constant for 238U; X p , decay constant for spontaneous fission of 238U. 

rcon with high U content could not be dated.

Age ±2<r(m.y.) 1

21.5±1.1 

18.1±4.6

28.3±1.8

40.0±7.0

30.6±1.2

32.2±3.6 

29.4±1.3

37.0±4.1

39.7±3.4

39.4±2.8

38.4±4.0

36.4±2.8 

34.5±1.3

36.9±1.7

36.4±1.6

36.6±1.6

41.8±2.3

granitic fades of the stock was emplaced about 28-31 

m.y. ago (Lindsey and others, 1975; Armstrong, 1970), 

and that event may have reset slightly the zircon age 

of the tuff. Stratified tuff and rhyolite considered to be 

equivalent in part to the Topaz Mountain Rhyolite 

were erupted from Keg Mountain about 8-10 m.y. ago. 

Alkali rhyolite and basalt north of Fumarole Butte 

were erupted about 6-7 m.y. ago (Mehnert and others, 

1978; Peterson and others, 1978). The last volcanic activity in the region was the eruption of basaltic lava at 

Fumarole Butte 0.88 m.y. ago (Peterson and others, 

1978).

DESCRIPTION OF ROCK UNITS 

DRUM MOUNTAINS RHYODACITE

The oldest formation of volcanic rocks in the mapped 

area consists of dark, rusty-brown-weathering flows

and flow breccias having the chemical composition of 

rhyodacite and, to a lesser extent, quartz latite1 . Named the Drum Mountains Rhyodacite for exposures in 

the Drum Mountains (Lindsey, 1979a), the rhyodacite 

crops out discontinuously around Spor Mountain and 

is well exposed in the Black Rock Hills, where it is 

believed to have been erupted from a small central 

volcano. It is about 240 m (meters) thick in the Black 

Rock Hills and about 150 m thick in the southern part 

of The Dell. Volcaniclastic sandstone and laharic breccias are interbedded with the lower part of the rhyodacite in the Black Rock Hills. Some dark, aphanitic 

flow rocks were mapped with the more porphyritic 

rhyodacite near Joy townsite; these are seen under the 

microscope to have similar petrographic features. 

Much rhyodacite in the northern Drum Mountains and

Nomenclature of Rittmann (1952) is used throughout this report, unless otherwise 

stated.
10 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

around Spor Mountain is broken into blocks about 

100-300 m across of diverse orientation; these blocks 

may reflect shattering of the formation as it subsided 

into the Thomas caldera. The rhyodacite everywhere 

unconformably overlies rocks of Paleozoic age and is 

overlain by the Mt. Laird Tuff locally, by the Joy Tuff 

at many localities, and by landslide breccia, Dell Tuff, 

and the Spor Mountain Formation around Spor 

Mountain.

The rhyodacite contains an average of 35 percent 

euhedral phenocrysts of plagioclase and pyroxene as 

large as 3-4 mm (millimeters) in a groundmass of 

plagioclase microlites and glass (table 4, fig. 3). 

Plagioclase is mainly andesine and labradorite, but 

sodic rims are common; pyroxene is mainly hypersthene, but lesser amounts of augite are present, and 

pigeonite has been reported (Staatz and Carr, 1964). 

Euhedral magnetite is abundant, traces of quartz occur at a few localities, and very small amounts of accessory apatite and zircon also occur locally. Along the 

east side of Spor Mountain, the mafic minerals in the 

rhyodacite are commonly altered to chlorite and brown 

hydromica(?).

The texture of the rhyodacite indicates that the 

phenocrysts were not in equilibrium with the magma 

at the time of eruption and cooling of the groundmass. 

Intratelluric crystallization of the phenocrysts 

evidently proceeded in an andesitic magma; somehow, 

either by addition of another magma, contamination 

with wall rocks, or differentiation, the melt became 

more silicic and alkalic than the original magma, so 

that plagioclase phenocrysts were partially resorbed 

and overgrown with sodic rims. The interiors of some 

plagioclase have a sieve texture of holes filled with 

glass; this texture may indicate change in composition 

of the melt or, alternatively, of temperature and 

pressure. When erupted, the remaining lava cooled 

quickly to form microlites and glass. Chemical 

analyses confirm that the groundmass of the rhyodacite is much more silicic (60-67 percent SiO2) than 

the phenocrysts would indicate. The modal phenocryst 

content suggests the rock is a hypersthene andesite, 

but most chemical analyses show it to be rhyodacite 

and quartz latite in the classification of Rittmann 

(1952).

DIORITE

Plugs of diorite crop out in the Drum Mountains 3 

km southeast of Joy townsite; they were first mapped 

by Crittenden and others (1961) as quartz diorite and 

were examined only briefly by me. Fresh specimens of 

the rock are dark brown in color, like the Drum Mountains Rhyodacite, but the rock is phaneritic, unlike the

aphanitic-porphyritic texture of the rhyodacite. In thin 

section, the rock is seen to contain about 56 percent 

elongate, subparallel grains of calcic plagioclase and 

about 22 percent hornblende that is highly altered. 

Other minerals include about 7 percent anhedral brown 

biotite, about 9 percent anhedral potassium feldspar, 5 

percent magnetite, and accessory apatite. Although 

Crittenden and others (1961) called the rock quartz 

diorite, no quartz could be identified with certainty. 

The diorite is assigned a probable Eocene age on the 

basis of overall compositional similarity to other rocks 

of that age and on the basis of crosscutting by dikes 

considered equivalent to the Mt. Laird Tuff. Crittenden and othrs (1961, pi. 20) mapped dikes of quartz 

monzonite porphyry cutting the diorite. Reexamination of their quartz monzonite dikes 2-3 km south and 

southwest of Joy townsite showed them to be nearly 

identical to the Mt. Laird Tuff. Similar but hydrothermally altered dikes cut one diorite plug, indicating that 

the diorite is older than the Mt. Laird Tuff.

FIGURE 3. Photomicrograph of Drum Mountains Rhyodacite 

showing partially resorbed plagioclase (P) and clusters of 

hypersthene (H) and plagioclase in a groundmass of microlites. 

Crossed polars. Sample from sec. 20, T. 13 S., R. 11 W.
DESCRIPTION OF ROCK UNITS 11

TABLE 4. Comparison of the mean and range of mineral composition and the occurrence of accessory minerals in volcanic rocks in the

Thomas Range and northern Drum Mountains

[Mineral composition estimated from point counts on three to six thin sections and cobaltinitrite-stained slabs of each rock type by C. A. Brannon. Accessory minerals identified by 

binocular microscope and X-ray methods by the writer using mineral concentrates prepared by heavy-liquid and electromagnetic separation. Range of values are in parentheses. Tr, 

trace; x, present; leaders ( ), not present; <, less than]

Potassium-feldspar

Hypersthene and 10(1.5-13) 

augtte.

Rock fragments ----- 

Opaque minerals 2(1-6)

Other accessory Tr. 

minerals .

Matrix Glass and 

composition. crystals.

2

0.3(0-1)

17(9-29) 

3(1-5) 

6(0-11) 

4(1-5)

Tr. 

1.5(0-3) 

0.8(0-2.4)

68(61-72)

Devitrified 

shards and 

pumice.

3

24(15-35)

3(0-8) 

Tr.

1(0-13) 

Tr. 

1.0(0-2)

4

Mineral composition

5(3-7) 

3(2-6) 

5(1-7) 

<1(0-1)

8(6-11) 

<1(0-1) 

<1(0-1)

79(77-89)

Devitrified Glassy to 

shards and devitrified 

pumice. shards and 

pumice.

5

, in percent

20(14-25) 

21(16-26) 

8(5-11) 

3(1-5)

<1(0-1) 

Tr. 

Tr.

68(40-54)

Devitrified 

shards and 

pumice.

6

3(0-5)

31(26-37) 

7(2-11) 

9(6-13) 

0.7(0-1)

<1(0-1) 

2.6(1-4) 

Tr.

46(47-52)

Glassy to 

devitrified 

shards and 

pumice.

789

18(13-27) 6(4-10) 16(12-10) 

19(12-29) 7(3-10) 13(11-14)

1(0-3) Tr. Tr.

Tr. <1(0-1) <1(0-1) 

Tr. Tr. Tr.

60(46-72) 87(74-96) 68(63-73)

Ttevitrified Devitrified Flevitrified 

glass. glass. glass.

Occurrence of accessory minerals

X

X

X

X 

X 

X 

X

X

X 

X

X

X 

X 

X 

X

X

X

X

Tr. Tr. Tr.

XXX

Rock units

1. Drum Mountains Rhyodacite

2. Mt. Laird Tuff.

3. Crystal tuff member of Joy Tuff.

4. Black glass tuff member of Joy Tuff.

5. Dell Tuff.

6. Needles Range(?) Formation.

7. porphyritlc rhyolite member of Spor Mountain Formation.

8. Flows and domes of alkali rhyolite, Topaz Mountain Rhyolite.

9. Local flows and domes of porphyritic alkali rhyolite, Topaz 

Mountain Rhyolite.

MT. LAIRD TUFF

The Mt. Laird Tuff, called plagioclase crystal tuff by 

Staatz and Carr (1964, p. 78-79), is about 80 m thick in 

the type locality near Mt. Laird in the north-central 

Drum Mountains. It overlies the Drum Mountains 

Rhyodacite and is unconformably overlain by the Joy 

Tuff. The Mt. Laird Tuff has a wide but scattered 

distribution: it crops out northeast of the Thomas 

Range, where it overlies rocks of Paleozoic age (Staatz 

and Carr, 1964), and in the northern part of Keg Mountain, where it overlies the "Keg Spring andesite and 

latite" of Erickson (1963). The formation is more than 

500 m thick in the subsurface 2-3 km east and northeast of Topaz Mountain, where drilling has revealed a 

section consisting of (1) 95 m of ash-flow tuff, (2) 62 m 

of tuffaceous sediments, (3) 75 m of ash-flow tuff, (4) 85 

m of tuffaceous sediments, and (5) 199 m of ash-flow 

tuff to the bottom of the hole (table 5). The drilled section contains three major intervals of ash-flow tuff, 

and variation in size and type of lithic inclusions 

within these suggests that each may consist of more 

than one ash flow. Much of the tuff is partly welded, 

devitrified and further altered to clay and calcite.

Outcrops of the Mt. Laird Tuff vary from gray to 

pink to lavender, and the tuff can be recognized easily 

in the field by conspicuous white plagioclase phenocrysts as much as 10 mm long and bronze-colored bio-
12 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS. UTAH

TABLE 5. Section ofMt. Laird Tuff in drill hole east of Topaz Mountain

[Revised, after study of core and thin sections, from log by Charles Beverly of hole 7 drilled for Bendix Field Engineering Corp. Drill hole is in SW1/4NW1/4 sec. 3, T. 13 S., R. 11 W. Ground elevation, 1,669 m; total depth, 608 mj

Top of unit 

(m below 

surface)

Formation Description of core and thin sections

Topaz Mountain Rhyolite Crystal-poor alkali rhyollte flow.

70- Joy Tuff; crystal tuff member Crystal-rich (quartz, sanidine, 

biotite, and plagioclase) 

rhyolitic welded ash-flow tuff.

90- Mt. Laird Tuff (top Crystal-rich (plagioclase, biotite, 

approximate). hornblende) quartz latitic welded

ash-flow tuff. Thin sections at 

98 m show glassy, perlitic matrix; 

at 152 m, matrix is devitrified and 

crystals are euhedral and mostly 

unbroken. At 158 m, matrix is 

devitrified but contains relict 

pumice, and broken crystals are 

common.

185 Mt. Laird Tuff 

187 Mt. Laird Tuff

249- Mt. Laird Tuff324- Mt. Laird Tuff409- Mt. Laird TuffFault(?) breccia.

Tuffaceous sediments of probable 

lacustrine origin include 

carbonaceous laminated mudstone, 

siltstone, sandstone, and minor 

conglomerate. Ash-flow tuff 

interbedded with sediments below 239 

m. Thin sections at 202 m are of 

tuffaceous sandstone containing 

altered pumice, plagioclase, 

biotite, quartz, and lithic 

fragments of rhyodacite, tuffaceous 

siltstone, quartzite, and carbonate 

rock; all fragments are angular. 

At 210 m, same as at 202 m but has 

birefringent clay or sericite. At 

240 m, unwelded ash-flow tuff 

containing broken plagioclase, 

biotite, quartz, hornblende 

altered to calcite, altered pumice, 

and lithic fragments of flow rock.

Crystal-rich (plagioclase, biotite, 

quartz, and hornblende), latitic 

ash-flow tuff containing abundant 

lithic fragments of carbonate rock 

and volcanic rocks to 292 m and 

below 305 m. Minor to no welding. 

Thin sections at 277 m and 306 m 

contain many broken crystals, relict 

pumice, and an altered matrix.

Tuffaceous sediments of probable 

lacustrine origin include black 

laminated carbonaceous mudstone 

containing pyrite, siltstone, 

sandstone, minor conglomerate and 

some interbedded ash-flow tuff. 

Sediments contain abundant graded 

bedding, flame structure, mud chips, 

and cross-lamination. Thin sections 

at 335 and 392 m contain abundant 

angular quartz, plagioclase, 

biotite, carbonate rock, and 

altered volcanic rock fragments.

Crystal-rich (plagioclase, biotite, 

altered hornblende) quartz latitic 

welded ash-flow tuff. Large lithic 

clasts to 433 m. Thin sections at 

514 m show euhedral and unbroken 

crystals accompanied by relict 

pumice in a devitrified matrix. At 

575 m, many crystals are broken and 

large resorbed quartz crystals are 

common.

608- Mt. Laird Tuff Bottom of hole.
DESCRIPTION OF ROCK UNITS

13

tite phenocrysts as much as 5 mm across. As seen 

under the microscope, the tuff contains 9-29 percent 

plagioclase, 1-5 percent red-brown biotite, and as 

much as 11 percent hornblende and 5 percent pyroxene 

(table 4, fig. 44). Crystals in the upper part of the Mt. 

Laird, at both the type locality and the drilled section, 

are remarkably euhedral and unbroken for an ash-flow 

tuff, and some of this rock, with no preserved vitroclastic texture, may possibly be flow rock instead of 

tuff. Broken crystals and relict pumice and shards are 

common in the rest of the tuff (fig. 4B). Northeast of 

the Thomas Range, the tuff differs from the type locality in that it contains conspicuous quartz. Plagioclase 

(labradorite) in the Mt. Laird Tuff is typically zoned; 

some is euhedral but other crystals have sieve texture 

and rounded, resorbed boundaries and zones and overgrowths that follow these boundaries. Hornblende is 

partly to completely altered to iron oxide minerals, and 

locally, plagioclase is altered to chlorite and sericite. 

Accessory magnetite and traces of apatite and colorless zircon are present. The groundmass of the tuff 

near Joy townsite contains abundant plagioclase 

microlites and small hornblende crystals that are 

altered to iron oxide minerals; all are set in a matrix 

that was probably glassy, but evidence of vitroclastic 

texture has been destroyed by alteration.

Study of core and thin sections from a drill hole 

(table 5) that penetrated the Mt. Laird Tuff east of 

Topaz Mountain confirms the abundance of ash-flow 

tuff in the formation. Only the upper 70 m contains 

large, euhedral crystals with little or no vitroclastic 

texture preserved. The matrix is perlitic glass at 8 m 

but is completely devitrified at 62 m below the top of 

the Mt. Laird Tuff. The devitrified matrix resembles 

the upper part of the Mt. Laird 3-4 km northwest of 

the type locality. Perhaps the uppermost part of the 

Mt. Laird is a flow, but more likely it formed by eruption of a very hot, viscous ash flow. Below the top 70 m 

of Mt. Laird Tuff the drilled section contains much 

ash-flow tuff distinguished by broken crystals and 

relict pumice and shards (fig. 4JB). At 485 m below the 

top of the tuff, near the bottom of the drill hole, large 

crystals of resorbed quartz are common, as also in tuff 

northeast of the Thomas Range.

Two intervals of tuffaceous sediments are interbedded with the Mt. Laird Tuff in the subsurface east 

of Topaz Mountain (table 5). The sediments contain 

laminated carbonaceous mudstone, siltstone, sandstone, and conglomerate (figs. 4C, D) derived mainly 

from the Mt. Laird ash flows and Drum Mountains 

Rhyodacite. The sediments contain abundant graded 

bedding, flame structures, mud chips, and cross 

lamination; conglomerates are matrix supported, indicating a probable mud-flow origin. Such features, 

taken together, indicate rapid sedimentation. 

Mudstones contain small amounts of organic carbon

and pyrite, indicating that they may have been 

deposited in stagnant water. Samples of mudstone 

were searched for pollen and microfossils that might 

reveal evidence about the depositional environment, 

but only abundant pollen of conifers was found; such 

pollen is not diagnostic of age or depositional environment (D. J. Nichols, written commun., 1980). The Mt. 

Laird sediments were probably deposited in a lake that 

formed during eruption of the Mt. Laird Tuff and attendant cauldron subsidence.

Three features of the Mt. Laird Tuff indicate that it 

was erupted from the vicinity of Mt. Laird in the Drum 

Mountains. (1) At the type locality and also 3-4 km 

northwest of Mt. Laird, the lower part of the tuff contains a distinctive breccia of abundant large (30 cm or 

centimeters) pumiceous tuff clasts in a similar matrix 

(fig. 4B). Such monolithologic breccias may represent 

fallback into the vent during eruption. (2) Massive to 

layered porphyritic rock of Mt. Laird mineralogy but 

resembling flow rock overlies the breccia at Mt. Laird 

and nearby; layers in the porphyritic rock are inclined 

gently and consist of dark-gray lenses 1-5 cm thick 

(fig. 4F) that extend many meters. In thin section, the 

porphyritic rock is seen to contain many euhedral 

crystals but also layers of small broken crystals and 

lenses of devitrified matrix with sparse phenocrysts; 

these features and the layering seen in outcrops are undoubtedly the product of flowage, but it is unclear 

whether such flowage features formed by eruption as a 

flow or as a hot, viscous ash flow that became densely 

welded and flowed after eruption. In either case, proximity to the source of the Mt. Laird magma is indicated. (3) An area of kaolinitic and pyritic alteration, 

now oxidized, occurs around a small breccia pipe on the 

northwest side of Mt. Laird; alteration affected the Mt. 

Laird Tuff but did not affect the Joy Tuff, which overlies the Mt. Laird Tuff nearby. Intrusion of breccia and 

alteration evidently represent late venting of fluids 

from the source magma of the Mt. Laird Tuff.

An area of porphyry intrusion and mineralization in 

the Detroit mining district of the Drum Mountains 

may be part of the Mt. Laird magma chamber that did 

not collapse or may represent resurgence and shallow 

intrusion of the Mt. Laird magma after eruption of the 

tuff. Small dikes and plugs of porphyritic rock, 

mapped as quartz monzonite porphyry by Crittenden 

and others (1961), resemble the Mt. Laird Tuff. Most 

intrude rocks of Cambrian and Precambrian age, but 

one dike intrudes a diorite plug of probable Eocene 

age. The westernmost group of dikes retains most of 

their original plagioclase phenocrysts, but mafic 

minerals and matrix have been hydrothermally 

altered. The eastern group contains only relict textures 

of feldspar and biotite phenocrysts. Study of X-ray diffraction patterns and thin sections show that the 

eastern intrusions are altered to quartz, kaolinite, well-
14 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

)Qk- .^ VJ*,' ~jfc"''fey*

P ^:-:^-*>r^ <

. ,iv:/ ^<'.*T<i£&i W^ft .-*** ^^1^-^::^^

^-" . >.v.lif." ' R. > ,

^.*; -. v -> -'-^ y^ 4M^tr Jw^ep-, P;

crystallized sericite and minor pyrophyllite; such 

alteration is typical of the phyllic zone of the Lowell 

and Guilbert porphyry copper model (Hollister, 1978). 

The easternmost plug contains a stockwork filled with 

jarosite and is partly surrounded by quartzite pebble

breccias derived from the country rock of the Precambrian and Cambrian Prospect Mountain Quartzite. The 

intrusions of porphyritic rock are believed to be 

apophyses from a buried pluton (Newell, 1971) that is 

the uncollapsed or resurgent part of the magma 

chamber of the Mt. Laird Tuff. The area of porphyry 

intrusion and mineralization is only 4-6 km south of 

the probable vent breccias of the Mt. Laird Tuff, and
DESCRIPTION OF ROCK UNITS 15

1 cm , P

FIGURE 4. Mt. Laird Tuff. A, Photomicrograph of ash-flow tuff 

showing crystals of plagioclase (P), hornblende (H), and biotite (B) 

in a devitrified matrix; vitroclastic texture is poorly preserved. 

Sample is from drill core from 158 m below surface at SWJ- NWJsec. 3, T. 13 S., R. 11 W. B, Photomicrograph of ash-flow tuff 

showing broken crystals and vitroclastic texture, from drill core 

from 216 m below surface at same locality as A. C, Core of tuffaceous sandstone and siltstone from 391 m below surface at 

same locality as A; note flame structure, graded bedding (top), 

and carbonaceous laminae. D, Core showing carbonaceous 

siltstone and mudstone with mud chips and matrix-supported 

conglomerate of probable mudflow origin (right core). Left core is 

from 401 m below surface; right core is from 334 m below surface; 

both from same locality as A. E, Probable vent breccia in the Mt. 

Laird Tuff northwest of Joy townsite, NE^ sec. 21, T. 14 S., R. 11 

W. F, Layered porphyry that overlies vent breccia northwest of 

Joy townsite, same locality as E; porphyry contains large 

euhedral crystals of white plagioclase and dark biotite in a 

layered, foliated matrix.

the original mineralogy of the porphyry so closely 

matches that of the tuff that the two are considered tc 

have been derived from the same magma.

JOY TUFF

The Joy Tuff was named for excellent exposures in 

hills 3-5 km northwest of Joy townsite in the Drum

Mountains (Lindsey, 1979a). The tuff consists of two 

members: a lower crystal tuff member about 180 m 

thick and an upper black glass tuff member about 30 m 

thick. In the type locality, the crystal tuff member 

overlies the Mt. Laird Tuff and the Drum Mountains 

Rhyodacite along angular unconformity B. The black 

glass tuff member is present only in the northeastern 

part of the Drum Mountains and southeast of Topaz 

Mountain; it overlies the crystal tuff member.

CRYSTAL TUFF MEMBER

The crystal tuff member of the Joy Tuff extends 

from Fish Springs Flat to the east side of Desert 

Mountain, a distance of 60 km (fig. 1). It is well exposed in the Joy graben (fig. 5A), where it is 180 m 

thick in hills northwest of Joy townsite and 70 m thick 

on the south side of Topaz Mountain, in the Picture 

Rock Hills of southwestern Keg Mountain, where it is 

about 150-200 m thick, and on the east side of Desert 

Mountain (fig. 1). Locally, the lowermost 10 m contains abundant compacted black pumice fragments 

(fig. 5B); the degree of welding and amount of black 

pumice decrease upward. Sections of the tuff in cliffs 

northwest of Joy townsite have a distinct parting that 

may be a partial cooling break within a compound cooling unit (fig. 5A). No compositional zoning of the 

crystal tuff is evident.

The source of the Joy Tuff is believed to be the 

Dugway Valley cauldron, which is generally east of 

Topaz Mountain and perhaps in the subsurface northeast of Topaz Mountain. The tuff east of Topaz Mountain is strongly welded and contains steeply dipping 

(60°-90°) foliation of somewhat variable northeasterly 

strike, in contrast to the more friable and less welded 

tuff with gentle dips in the Joy graben. Monolithologic 

tuff breccia having unrotated and slightly rotated 

clasts in a welded matrix occurs east of Topaz Mountain (fig. 5C), and similar breccia was penetrated by 

drill holes 3 and 5 km north of Topaz Mountain. One 

small outcrop of crystal tuff in the northeast Drum 

Mountains has near-vertical foliation. Such welded tuff 

breccia and tuff with steep foliation may indicate 

vents; possibly, the north-south distribution of such 

occurrences along faults may indicate a system of 

eruption fissures. Alternatively, it is possible that the 

tuff breccias formed by collapse of the ash flow into the 

cauldron during eruption. Such might be the explanation for large areas of steeply dipping foliation in the 

tuff; these could be large blocks that fell into the 

cauldron.

The crystal tuff member ranges from gray to reddish 

brown in outcrop; it contains abundant broken crystals 

of sanidine and quartz, and lesser quantities of
16 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

%.J^'J; 3AV?-^~,3s>,^- -. ^ : - ^

^Jfcrr^;^^fa ,-jj 54 --^S^w^^.^r- 1 ^ ->**

"** ' <'* '" >-»*'' ' ' ;*£ ^ ^ - 'JE; :s*t« * ' .-.* ^^k'****-*/ $T-'/ " ' '" ->r-J

FIGURE 5. Crystal tuff member of Joy Tuff. ^4, Section of crystal 

tuff in the type locality (sec. 22, T. 14 S., R. 11 W.). B, Compacted 

black pumice in lower 10 m of tuff (sec. 29, T. 13 S., R. 11 W.). C, 

Probable vent breccia of crystal tuff fragments in welded matrix 

of the same. D, Photomicrograph showing broken crystals of 

quartz and sanidine with lesser plagioclase (P) and biotite (B). E, 

Photomicrograph of cognate inclusion (lower right half of 

photograph) containing abundant sphene (S). Photomicrographs 

(crossed polars) of sample from sec. 5,T. 13 S., R. 11 W.

plagioclase (oligoclase-andesine), brown biotite, and 

euhedral magnetite (table 4, fig. 5D). The crystals are 

as much as 4 mm across and average about 65 percent 

of the tuff. Sphene, apatite, allanite, and colorless zircon occur as accessory minerals, but large euhedral 

crystals of sphene are so abundant and conspicuous 

that they can be seen in some hand specimens. Abundant sphene and the darker, more compact appearance 

of outcrops are the most reliable characteristics that 

can be used to distinguish the Joy Tuff from the Dell 

Tuff, which it resembles closely. The matrix of the Joy 

Tuff consists of welded shards that have been devitrified and altered to potassium feldspar and cristobalite. 

Lithic inclusions of the Drum Mountains Rhyodacite 

occur in the crystal tuff member east of Topaz Mountain, and distinctive cognate inclusions consisting of 

normally zoned plagioclase, brown biotite, sphene, and 

interstitial glass occur in the tuff in the canyon north 

of Topaz Mountain (fig. 5E). The cognate inclusions 

contain smaller, unbroken phenocrysts, in contrast to 

the tuff, and evidently represent an early stage of 

crystallization of the Joy Tuff within the magma 

chamber. Chemical analyses show that the crystal tuff 

member is a rhyolite.
DESCRIPTION OF ROCK UNITS 17

BLACK GLASS TUFF MEMBER

The upper member of the Joy Tuff is a simple cooling 

unit of ash-flow tuff. It consists of three zones that 

show differing degrees of welding (fig. 6A): a densely 

welded lower zone consisting almost entirely of black, 

glassy pumice, a partly welded middle zone of gray, 

glassy tuff containing locally abundant compacted 

black pumice (fig. 6J3), and an upper zone of unwelded 

tan, glassy tuff. At some localities the welded zones 

are more readily divisible into a lower black, glassy 

zone and an upper gray-brown, devitrified zone; both 

are overlain by unwelded tan tuff. The tuff crops out 

east of Topaz Mountain (Staatz and Carr, 1964, p. 84) 

and in the northeastern Drum Mountains. A section 

about 30 m thick is well exposed in sec. 36, T. 13 S., R. 

11 W., where all three of the welding zones occur in approximately equal thickness, and where the tuff is 

overlain by a landslide of Tertiary age. The limited 

local extent of the black glass tuff, together with its 

small thickness and densely welded lower zone, indicate that it was a local hot ash flow erupted from the 

Dugway Valley cauldron.

The black glass tuff contains an average of only 13 

percent small inconspicuous crystals of quartz,

sanidine, zoned plagioclase, brown biotite, and 

magnetite (table 4, fig. 6Q. Sphene is a conspicuous accessory mineral, as it is in the underlying crystal tuff 

member. Inclusions of the crystal tuff, Mt. Laird Tuff, 

Drum Mountains Rhyodacite, and Paleozoic rocks are 

common in all zones. Glass shards and pumice compose the rest of the tuff; these are highly compacted 

and deformed in the welded zones (figs. 6J3, 6Q but are 

uncompacted, porous, and partly altered to clinoptilolite in the upper, unwelded zone.

The correct stratigraphic assignment of the upper 

unwelded zone of the black glass tuff is critical to the 

age of the overlying landslide megabreccias and their 

role in the volcanic and tectonic history of the area. 

The unwelded zone resembles tuff that is interbedded 

with younger rhyolite flows and was mapped as waterlaid tuff of the youngest volcanic group by Shawe 

(1964). The unwelded zone is not clearly stratified, 

however, and its contact with the underlying welded 

zone is conformable and sharply gradational. Chemical 

analyses show the composition of the unwelded zone to 

be consistent with leached tuff that had the same composition as the underlying welded zones. Stratified 

tuffs interbedded with young alkali rhyolite have a 

distinct trace-element signature comprised of anomalous beryllium, lithium, niobium, uranium, thorium, 

and yttrium that is not present in any of the black 

glass tuff, including the upper, unwelded tuff. The 

unwelded zone has been partly altered to clinoptilolite 

and leached of alkali and alkaline-earth elements, and 

perhaps some trace elements, but such leaching does 

not remove the distinctive trace-element signature of 

the younger tuffs (Lindsey, 1975).

LANDSLIDE BRECCIAS

Large masses of breccia interpreted by me as landslide deposits and related debris flows occur at Spor 

Mountain and in the northeastern part of the Drum 

Mountains; they are associated with the time of ashflow eruption and probably record collapse of caldera 

walls. The breccias were mapped as three lithologically 

and geographically distinct informal units: breccia at 

Spor Mountain, breccia at Wildhorse Spring, and 

megabreccia of the northern Drum Mountains.

BRECCIA AT SPOR MOUNTAIN AND BRECCIA 

AT WILDHORSE SPRING

These breccias are spatially and genetically 

associated. The breccia at Spor Mountain consists of 

angular clasts of Paleozoic rocks as large as 1 m across 

in a matrix of the same material (fig. 1A). Breccia at 

Wildhorse Spring contains subrounded clasts (0.5 m or 

less) of Paleozoic rocks and rhyodacite in a matrix of
18 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

Megabreccia of Drum Mountains

Upper zone unwelded

Middle zone welded

Lower zone welded

FIGURE 6. Black glass tuff member of Joy Tuff. A, Zones of 

welding in the type locality of the black glass tuff, tanging from the 

densely welded, black lower zone, the partly welded middle zone, to 

the unwelded upper zone. Also shown is the landslide megabreccia 

of the northern Drum Mountains, which overlies the unwelded zone 

of the black glass tuff. B, Specimen of tuff from the partly welded 

middle zone, showing compacted black pumice. C, Photomicrograph 

showing glassy shards and pumice (P) and lithic inclusions (I) of 

rhyodacite in middle zone; crossed polars. Samples B and C from 

outcrop in sees. 25 and 26, T. 13 S., R. 11 W.
DESCRIPTION OF ROCK UNITS 19

*-t^ i^JL p-t^^p, ^P^- * '«;

SJ^S'*^

^*e

^-^La* *>»., « ^'^noB^ " - §

^rapp^^'^'

^?^y!l^'te£ :,«-,

FIGURE 7. Landslide breccia. y4, Breccia at Spor Mountain, showing clasts of mixed lithologies derived from rocks of early 

Paleozoic age on Spor Mountain (sec. 27, T. 12 S., R. 12 W.). B, 

Breccia at Wildhorse Spring, showing large clasts of rhyodacite 

and Paleozoic rocks in a matrix of pulverized rhyodacite (sec. 15, 

T. 12 S., R. 12 W.). C, Contact between breccia at Wildhorse

Spring and overlying breccia at Spor Mountain, which contains 

clasts 1 m in diameter. Note layering defined by concentration of 

clasts along the contact between breccias, above map case and 

pick (sec. 16, T. 12 S., R. 12 W.). D, Breccia at Spor Mountain, 

showing monolithologic crackle breccia of dark dolomite in the 

breakaway zone (sec. 27, T. 12 S., R. 12 W.).

pulverized rhyodacite (fig. IB). The two units are 

associated locally but can be distinguished readily in 

the field. The breccia at Spor Mountain overlies the 

Drum Mountains Rhyodacite along the east side of 

Spor Mountain and overlies the breccia at Wildhorse 

Spring 1 km southwest of that locality (fig. 7Q. The 

breccia at Spor Mountain is overlain by the Dell Tuff 

in the northern part of The Dell and probably west of 

the Yellow Chief mine, although relations near the 

mine may be complicated by faulting. Thus, stratigraphic relationships indicate that the age of the breccias is between 42 and 32 m.y., the ages of the 

rhyodacite and Dell Tuff, respectively (table 2).

The breccias have long been considered to be of intrusive origin (Staatz and Carr, 1964, p. 85), but new

evidence presented here indicates that they formed 

from landslides and related debris flows along a steep 

escarpment that followed the present north and east 

sides of Spor Mountain. Three types of evidence support my view that the breccias are landslide deposits: 

(1) The breccia at Spor Mountain contains rotated and 

mixed clasts of Paleozoic dolomite and limestone and 

Ordovician Swan Peak Quartzite in a nonigneous 

matrix (fig. 7A); it can be traced continuously 

westward from the east side of Spor Mountain to 

monolithologic dolomite breccia having healed fractures between unrotated and slightly rotated fragments (fig. ID). Both of these facies were mapped as 

breccia at Spor Mountain (Lindsey, 1979a). The monolithologic dolomite breccia retains the original stratig-
20 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

raphy of the Paleozoic rocks and resembles the 

"crackle breccia" of Tertiary landslides in Arizona described by Krieger (1977). All stages of brecciation 

from bedded but fractured dolomite to breccia of 

mixed Paleozoic lithologies are present in a small area 

about 2 km north-south and less than 0.3 km east-west 

that extends along the east side of the crest of Spor 

Mountain; I interpret this area as the breakaway zone 

for a landslide. (2) Both the breccias at Spor Mountain 

and at Wildhorse Spring are for the most part massive 

and unsorted, but they are faintly bedded locally, and 

the contact between the breccias southwest of Wildhorse Spring is distinctly bedded (fig. 7C). Massive 

structure and clasts of mixed lithology, set in abundant fine material, indicate turbulent mass transport, 

but bedding in the breccias indicates a transport 

mechanism capable of partial sorting of fragments. 

Massive breccias may represent landslides that moved 

when on steep slopes or when saturated by moisture, 

and faintly bedded breccias may represent watersaturated debris flows deposited during heavy rainfall. 

(3) Only Paleozoic rocks are present as clasts and 

matrix of the breccia at Spor Mountain; Paleozoic 

rocks and rhyodacite are both present as clasts and 

matrix of the breccia at Wildhorse Spring; nowhere is 

an ingeneous matrix other than fragments of rhyodacite present. A few areas of breccia occur in small 

plugs of intrusive alkali rhyolite on Spor Mountain, 

such as at the Blowout mine, but these are related to 

later faulting and to intrusion of the rhyolite.

The breccias at Spor Mountain were formed by 

sliding and flowage off a northerly trending breakaway 

zone on Spor Mountain, west of Eagle Rock Ridge; the 

slides traveled east and northeast over a valley floor 

covered by rhyodacite. Somewhat similar breccias 

form by collapse of caldera walls (Lipman, 1976), by 

landsliding along upthrown fault blocks (Krieger, 

1977), and by sliding along scarps of erosional origin 

(Shreve, 1968); the geologic environment of ash-flow 

eruption and cauldron collapse in the Thomas Range 

favors an origin by collapse of caldera walls. The breccia at Spor Mountain probably began moving as coherent masses (fractured and brecciated dolomite) that 

disintegrated into landslides (breccia with rotated 

clasts of mixed lower Paleozoic lithologies) and debris 

flows during heavy rainfall. Where slides and debris 

flows traveled over rhyodacite, they incorporated 

rhyodacite detritus into them, forming the breccia at 

Wildhorse Spring. Evidently, the two most important 

agents responsible for formation of the breccias were 

steep scarps and abundant moisture. There is no 

reason to believe that the slides traveled on a cushion 

of air, as was concluded for the Blackhawk slide 

described by Shreve (1968), in part because the total

distance traveled was probably not more than 5 km. 

Localization of sliding and flowage along north- and 

northwest-trending scarps that follow the Dell fault 

system suggests that these faults originated during 

times of ash-flow eruption and cauldron collapse 42 to 

32 m.y. ago, even though the faults now displace rocks 

as young as 21 m.y. old.

MEGABRECCIA OF THE NORTHERN DRUM MOUNTAINS

This megabreccia overlies the upper, unwelded zone 

of the black glass tuff member of the Joy Tuff in an 

area of about 5 km2 (square kilometers) (fig. 6A). It occurs as more or less coherently bedded slabs, some 

hundreds of meters across in size, of Cambrian limestone and dolomite; locally, the slabs show large-scale 

contortion (fig. 6A) and intense brecciation. Breccia 

zones have been thoroughly recemented with fine particles of Cambrian rocks. The underlying tuff has not 

been appreciably disturbed. A breakaway zone may be 

preserved along the east side of the north-central 

Drum Mountains, where small amounts of breccia crop 

out in an upthrown fault block of Cambrian rocks that 

contains the same formations as the breccia.

The megabreccia of the Drum Mountains was first 

identified as a late Tertiary gravity slide (Shawe, 1964) 

and later as a Quaternary landslide (Newell, 1971). According to both interpretations, the megabreccia overlies water-laid tuff of the youngest volcanic group. The 

tuff is considered by me to be the upper unwelded zone 

of the black glass tuff member of the Joy Tuff; this interpretation permits the megabreccia to be as old as 37 

m.y. and within the age range of the breccia at Spor 

Mountain and Wildhorse Spring. The top of the tuff is 

nearly flat and has not been eroded; thus, sliding must 

have taken place soon after deposition of the tuff so 

that it was protected from erosion.

DELL TUFF

The name "Dell Tuff" was given (Lindsey, 1979a) to 

slightly welded, gray to pink, rhyolitic ash-flow tuff 

that crops out extensively in The Dell; the tuff appears 

to be a simple cooling unit. A section about 180 m 

thick is exposed at the north end of The Dell (fig. SA). 

The Dell Tuff unconformably overlies the Drum Mountains Rhyodacite and landslide breccia at Spor Mountain and is unconformably overlain by stratified tuff 

and tuffaceous sandstone of the beryllium tuff member 

of the Spor Mountain Formation at the Yellow Chief 

mine, by the beryllium tuff and porphyritic rhyolite 

members of the Spor Mountain Formation elsewhere in 

The Dell, and by the Topaz Mountain Rhyolite at the
DESCRIPTION OF ROCK UNITS 21

V '*,, 1 mm ,' -

FIGURE 8. The Dell Tuff. A, Type locality of the Dell Tuff, north 

end of The Dell (sees. 23 and 24, T. 12 S., R. 12 W.), showing Topaz 

Mountain Rhyolite unconformably overlying faulted Dell Tuff. 

Ttm, Topaz Mountain Rhyolite; Ttmt, stratified tuff in Topaz 

Mountain Rhyolite; Td, Dell Tuff; Pz, Paleozoic rocks; exposed 

faults shown by heavy solid lines; concealed faults shown by 

heavy dotted lines. B, Photomicrograph of the Dell Tuff, showing 

vitroclastic texture of devitrified shards and abundant crystals of 

quartz (Q), sanidine (S), plagioclase (P), and biotite (B). Plain light. 

Sample from SE}- sec. 2, T. 13 S., R. 12 W.

north end of The Dell. As much as 200 m of Dell Tuff 

crops out northeast of Keg Pass at Keg Mountain. 

There, it unconformably overlies the "Keg Spring 

andesite and latite" of Erickson (1963) and the Mt. 

Laird Tuff, and is unconformably overlain by stratified 

tuff and alkali rhyolite of the Topaz Mountain 

Rhyolite.

The source of the Dell Tuff has not been located. The 

tuff is limited to the Thomas Range and Keg Mountain, indicating that the source should be within or 

near these ranges. Intrusive rocks of about the same 

age and composition as the Dell Tuff occur as dikes 

northwest of Keg Mountain and in the granite facies of 

the stock of Desert Mountain.

The Dell Tuff was called quartz-sanidine crystal tuff 

by Staatz and Carr (1964, p. 80-81) because it contains 

large (5 mm or larger) euhedral crystals of quartz and
22 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

sanidine and lesser amounts of plagioclase and brown 

biotite (fig. 8B). The tuff contains an average of 52 percent crystals (table 4). Minor euhedral magnetite and 

accessory allanite, sphene, apatite, and colorless zircon 

are present, also. The matrix consists of white pumice 

and shards that are only slightly welded. Tuff along 

the east side of The Dell and at Keg Pass is so poorly 

indurated that exposures weather to loose crystals in 

soft, ashy soil. Pumice and shards show both vaporphase crystallization and devitrification textures, but 

hydrothermal fluids have partially altered some of the 

tuff to smectite and sericite and introduced minor 

amounts of fluorite. Chemical analyses show that the 

tuff has a rhyolitic composition.

NEEDLES RANGE(?) FORMATION

A simple cooling unit of ash-flow tuff that closely 

resembles the Wah Wah Springs Tuff Member of the 

Needles Range Formation crops out discontinuously 

along the west side of the Drum Mountains from the 

Sand Pass road southward to the Little Drum Mountains. It is also well exposed for 25 km south from the 

Little Drum Mountains through the Red Knolls and 

Long Ridge, where it forms long escarpments about 30 

m high. The tuff was first identified in outcrops on the 

south flank of the Little Drum Mountains by Leedom 

(1974) and Pierce (1974), whose petrographic studies 

demonstrated its similarity to the Needles Range Formation. Ages reported here (table 3) are in accord with 

this correlation. Its thickness is difficult to judge but 

is probably near that of the 30 m estimated south of 

the Little Drum Mountains. The Needles Range(?) Formation is about 70 m thick in the subsurface south of 

The Dell, at the center of sec. 23, T. 13 S., R. 12 W., 

where it overlies about 25 m of Dell(?) Tuff and Drum 

Mountains Rhyodacite. The Needles Range(?) unconformably overlies lower Paleozoic rocks, Drum Mountains Rhyodacite, and Mt. Laird Tuff along the west 

side of the Drum Mountains, but it has not been 

observed in contact with the Joy and Dell Tuffs at the 

surface. The tuff fills valleys in Paleozoic dolomite and 

Drum Mountains Rhyodacite in sees. 23 and 26, T. 14 

S., R. 12 W.

Tuff mapped as Needles Range(?) Formation is typically red brown and crystal rich, the crystals being 

small (1-2 mm) plagioclase (26-37 percent) of intermediate composition, conspicuous biotite (2-11 percent), small green-brown hornblende (6-13 percent), 

and minor resorbed quartz, magnetite, and hypersthene (fig. 9). Accessory minerals include apatite and 

colorless zircon; no sphene was observed. The matrix is 

glassy to devitrified pumice and shards that have been

moderately welded. Collapsed black pumice and lithic 

inclusions of Mt. Laird Tuff occur in the lower part of 

the tuff as far south as the Little Drum Mountains and 

Long Ridge; the lithic inclusions are abundant in the 

drilled section of tuff south of The Dell. A few meters 

of tan, crudely stratified, loosely indurated tuff containing stones of Mt. Laird Tuff and Drum Mountains 

Rhyodacite crop out in the basal part of the Needles 

Range(?) Formation about 0.5 km south of the Sand 

Pass road.

The correct stratigraphic assignment of. the tuff is 

still uncertain, although I assigned it to the Needles 

Range Formation (Lindsey, 1979a) because of the 

tuff's close resemblance, in petrography and age, to 

the Wah Wah Springs Tuff Member. The type locality 

of the Needles Range Formation is in southwestern 

Utah, about 150 km south of the Drum Mountains 

(Best and others, 1973; Cook, 1965). The Wah Wah 

Springs Tuff Member has been traced from its type 

locality as far north as the southern House Range (fig.

FIGURE 9. Photomicrograph showing crystal-rich tuff of the 

Needles Range(?) Formation. Quartz (Q), plagioclase (P), biotite 

(B), hornblende (Hb), pyroxene (Py), and magnetite (M), in a 

matrix of devitrified shards. Crossed polars.
DESCRIPTION OF ROCK UNITS 23

1), where I examined it in an effort to resolve the problem of correlation with tuff mapped as Needles 

Range(?) in the Drum Mountains. The two tuffs compare closely in general aspect and in petrography and 

mineralogy; only minor differences in lithic inclusions 

and preservation were noted. Lithic inclusions of Mt. 

Laird Tuff are, of course, not present in the tuff in the 

House Range, but they are noticeable at Long Ridge 

and in the Drum Mountains. The thickest section 

found to date by drilling just inside the Thomas 

caldera contains abundant inclusions of Mt. Laird 

Tuff, indicating a possible source within that structure. The possibility that the tuff is not Wah Wah 

Springs is also suggested by differences in paleomagnetic direction between the two tuffs (Shuey and 

others, 1976). Further study is needed to resolve the 

correlation of tuff mapped as Needles Range(?) in the 

Drum Mountains.

SPOR MOUNTAIN FORMATION

The Spor Mountain Formation consists of two informal members, the beryllium tuff and an overlying porphyritic rhyolite, both of which in most places occur 

together and are restricted to the vicinity of Spor 

Mountain. The beryllium tuff member contains all of 

the important deposits of beryllium and uranium discovered in the Spor Mountain area as of 1980, and the 

21-m.y.-old porphyritic rhyolite is an important 

reference point for establishing the onset of alkali 

rhyolite volcanism, the maximum age of beryllium and 

uranium mineralization, and the maximum age of 

block faulting in this part of the Basin and Range Province. The close spatial association of the two members 

of the Spor Mountain Formation indicates that eruption of the porphyritic rhyolite followed soon after 

deposition of the beryllium tuff.

Angular unconformities separate the Spor Mountain 

Formation from older rocks (unconformity C) and from 

the younger Topaz Mountain Rhyolite (unconformity 

D) (table 1). The two members of the Spor Mountain 

Formation are conformable to one another. The 

beryllium tuff member unconformably overlies 

Paleozoic rocks, Drum Mountains Rhyodacite, and Joy 

Tuff (locally, in the subsurface only) west of Spor 

Mountain, and Drum Mountains Rhyodacite and Dell 

Tuff in The Dell. Unconformity C is partly of erosional 

origin as is evident from abundant detritus of older 

volcanic rocks in the beryllium tuff member in The 

Dell. Unconformity D is erosional as indicated by 

angular relationships in The Dell. There, the Spor 

Mountain Formation dips as much as 20° west along

the east side of The Dell, where it is unconformably 

overlain by flat-lying Topaz Mountain Rhyolite.

BERYLLIUM TUFF MEMBER

About 60 m of the beryllium tuff member occurs 

beneath the porphyritic rhyolite member in drilled sections and beryllium mines south and west of Spor 

Mountain. In The Dell, about 60 m of the tuff, including bentonite and tuffaceous sandstone, is exposed 

beneath porphyritic rhyolite in the Yellow Chief mine, 

and a partial section of 22 m occurs at the Claybank 

prospect. Drilling in the southern part of The Dell 

showed that the beryllium tuff thins from about 60 m 

near Spor Mountain to thin, discontinuous lenses on 

the east side of The Dell.

The beryllium tuff member contains five fades: (1) 

tuffaceous breccia containing abundant clasts of 

altered and unaltered dolomite (fig. 10A B), (2) thinly 

stratified tuff containing a few small lithic fragments 

(fig. 10C), (3) bedded, massive ash-flow tuff containing 

sparse to moderate lithic fragments, (4) bentonite composed mostly of altered pumice (fig. IOD), and (5) tuffaceous sandstone and conglomerate (fig. 10.D). Detailed study of textures in the beryllium tuff member 

shows that much of fades 1 and 2 are block-flow, ashflow, and ground-surge deposits (Bikun, 1980). The 

dominance of a particular facies varies from one section to another. Tuffaceous breccia containing distinctive dolomite clasts that have been hydrothermally 

altered to fluorite, clay, and silica minerals (Lindsey 

and others, 1973) grades laterally and vertically into 

ash-flow tuff and thinly stratified tuff containing 

sparse dolomite clasts in the vicinity of the beryllium 

mines. These facies persist eastward into The Dell, 

where tuffaceous breccia containing carbonate clasts 

comprises only a minor part of a section dominated by 

ash-flow tuff, bentonite, and tuffaceous sandstone and 

conglomerate. In The Dell, tuffaceous breccia containing altered dolomite clasts occurs in thin zones at the 

top of the tuff, as at the Hogsback prospect and the 

west side of the Yellow Chief pit; thin ash-flow tuffs 

are interbedded with bentonite at the Claybank prospect. The Yellow Chief mine section (modified from 

Bowyer, 1963) illustrates the composition of the 

beryllium tuff member in The Dell:

Porphyritic rhyolite (southwest wall of pit only); top of

section.

Tuffaceous breccia with altered dolomite clasts 

Bentonite - - 

Limestone pebble and cobble conglomerate with

tuffaceous sandstone matrix 

Thickness in 

meters

3

15
24 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

Tuffaceous sandstone with abundant quartz and sanidine 

derived from Dell Tuff 

Tuffaceous conglomerate and sandstone; cobbles and 

boulders mostly rhyodaciteThickness in 

meters

18

18

Stratified tuff containing volcanic-rock fragments (only 

exposed south of pit in 1977, 8 m reported by Bowyer, 

1963, when pit was freshly dug) 8 

Dell Tuff; bottom of section.

A partial section at the Claybank prospect illustrates 

the marked local variation in thickness of tuffaceous 

sandstone and conglomerate in the lower part of the 

beryllium tuff member:

Fault and remnants of mineralized tuffaceous breccia

containing altered dolomite clasts; top of section. 

Bentonite 

Massive ash-flow tuff containing volcanic-rock

fragments 

Bentonite 

Massive ash-flow tuff containing volcanic-rock

fragments and compacted pumice 

Bentonite 

Massive ash-flow tuff containing volcanic-rock

fragments, minor quartzite, and compacted pumice 

Tuffaceous sandstone containing abundant quartz and

sanidine derived from Dell Tuff 

Thickness in 

meters

0.5 

3

2

7

1.5

Drum Mountains Rhyodacite; bottom of section.

Studies of the beryllium tuff member by thin section 

and X-ray diffraction methods (Lindsey and others, 

1973) show that most of the clasts in the tuff are 

dolomite from formations exposed on Spor Mountain, 

nodules derived from the alteration of dolomite to 

calcite, silica minerals, clay and fluorite, and 

fragments of volcanic rocks and of Ordovician Swan 

Peak Quartzite. Dolomite fragments dominate in the 

breccia facies, and volcanic rock fragments dominate 

in thinly stratified tuff and ash-flow tuff. The matrix of 

all three facies contains abundant pumice and shards 

that range from glassy to completely altered; diagenetic alteration formed clinoptilolite and possibly 

potassium feldspar, and hydrothermal alteration produced smectite, sericite, potassium feldspar, and cristobalite. About 10-20 percent broken crystals of 

quartz, sanidine, plagioclase, and biotite are scattered 

through the matrix.

PORPHYRITIC RHYOLITE MEMBER

The porphyritic rhyolite member crops out as flows 

in The Dell and south and west of Spor Mountain and 

as domes and small plugs that may have been the 

source of some of the flows and of the pyroclastic 

debris in the beryllium tuff member. The largest dome 

(6 km2) is both intrusive and extrusive; it intruded the 

Drum Mountains Rhyodacite west of Wildhorse

''

Spring and reached the surface, as judged from local 

lenses of beryllium tuff penetrated by drilling beneath 

the rhyolite nearby. The intrusive contact is well exposed in NEJ sec. 17, T. 12 S., R. 12 W. A small dome 

in sec. 10, T. 13 S., R. 12 W., is only 0.25 km2 in area; it 

overrode an apron of stratified tuff. Only small remnants of the tuff apron have survived erosion; they are 

exposed in prospects at the northwest and southwest
DESCRIPTION OF ROCK UNITS 25

( Limestone conglomerate 

containing weeksite ore

 r*- -t *-J* ^ .A**t " - V ^i_-- ** ^ ;s5 '-: *T te^^t^;,

D

FIGURE 10. Beryllium tuff member of the Spor Mountain Formation. A, Tuffaceous breccia containing abundant clasts of carbonate rock 

in the Monitor pit (sec. 7, T. 13 S., R. 12 W.). Pick shows scale. B, Closeup view of tuffaceous breccia containing clasts of carbonate 

rock. C, Thinly stratified tuff containing sparse clasts of volcanic and sedimentary rock, south of Blue Chalk pit (sec. 9, T. 13 S., R. 12 

W.). D, Section showing bentonite and epiclastic tuffaceous sandstone and conglomerate at the Yellow Chief mine (sees. 25 and 36, T. 

12 S., R. 12 W.).

sides of the dome. Both the tuff and the margin of the 

rhyolite dome have been incompletely altered to clay 

and uraniferous fluorite. Several small domes of porphyritic rhyolite occur along faults in The Dell; they 

are north of the Claybank prospect, south of the 

Yellow Chief mine, southeast of the Hogsback prospect, and perhaps east of the Oversight prospect (see 

fig. 15 for prospect locations). A large vent may 

underlie the Thomas Range east of The Dell, where 

more than 80 m of porphyritic rhyolite extends east 

beneath stratified tuff of the Topaz Mountain 

Rhyolite.

The porphyritic rhyolite contains abundant (average 

of 40 percent) phenocrysts of euhedral sanidine, 

quartz, plagioclase (andesine) and biotite; the 

phenocrysts are conspicuous (3 mm across) and distinguish the rhyolite from most of the Topaz Mountain 

Rhyolite (table 4). The dome west of Wildhorse Spring 

contains as much as 60 percent phenocrysts locally

and quartz and sanidine phenocrysts as much as 5 mm 

across (fig. 11A). Sanidine phenocrysts are commonly 

chatoyant. Biotite occurs in the rhyolite as euhedral 

black phenocrysts containing abundant magnetite inclusions, in contrast to the ragged, red appearance of 

biotite in the Topaz Mountain Rhyolite. Small pink zircon occurs in trace amounts; the zircon is distinctive 

from that of older volcanic rocks in that it contains an 

average of about 10 times as much uranium. Southwest of Spor Mountain, the rhyolite contains numerous dark inclusions; the inclusions contain conspicuous 

(10-20 mm) large, zoned plagioclase crystals with vermicular intergrowths of plagioclase, potassium 

feldspar, and glass.

The groundmass of the porphyritic rhyolite, as seen 

under the microscope, is commonly a coarse-grained 

(0.1-0.2 mm) mosaic of crystals, although fine-grained 

and spherulitic textures are common, also. The groundmass consists mostly of potassium feldspar and cristo-
26 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

*" *"*<

'', ><**>''

#*'% * A* ft'

w>*&"

: '--asr^-i, v-r^y^«r»

FIGURE 11. Photomicrographs of porphyritic rhyolite member of the Spor Mountain Formation. A, Porphyritic rhyolite at Wildhorse 

Spring, showing abundant phenocrysts of quartz (Q) and perthitic sanidine (S). Crossed polars. Sample from sec. 9, T. 12 S., R. 12 W. B, 

Groundmass of porphyritic rhyolite, containing abundant acicular and prismatic topaz (high relief, labeled T). Plain light. Sample from 

Hogsback prospect, sec. 35, T. 12 S., R. 12 W.

halite and lesser disseminated magnetite, fresh graybrown biotite, and cavity fillings of quartz and topaz 

(fig. IIB). The groundmass biotite is unique because it 

is completely fresh and free of magnetite; it occurs also 

as overgrowths on the earlier formed biotite phenocrysts, which are brown and filled with magnetite inclusions. Chemical analyses show that the porphyritic 

rhyolite is borderline between rhyolite and alkali 

rhyolite in the classification of Rittmann (1952). The 

rhyolite is clearly allied to the younger Topaz Mountain Rhyolite; both have high alkali and fluorine contents and anomalous traces of beryllium, lithium, 

niobium, tin, thorium, uranium, and yttrium.

TOPAZ MOUNTAIN RHYOLITE

The Topaz Mountain Rhyolite is a large complex of

alkali rhyolite flows, domes, and interbedded local 

units of stratified tuff. The entire complex unconformably overlies older volcanic rocks that have been 

faulted and tilted. Unconformities are also present 

beneath stratified tuff that separates older from 

younger flows of the Topaz Mountain Rhyolite. These 

unconformities do not show visible angular discordance and may be constructional in part, but paleotopography is clearly visible beneath stratified tuff at 

Topaz Mountain and in the upper reaches of Pismire 

Wash. Radiometric dates indicate quiescent intervals, 

of perhaps 100,000 years on Topaz Mountain to perhaps 500,000 years in Pismire Wash, between eruption 

of major flows and domes.

The Topaz Mountain Rhyolite contains distinctive 

sequences of lithologies (fig. 12A) that are repeated 

several times in vertical sections; the sequence is 

stratified tuff, breccia, vitrophyre, and flow-layered
DESCRIPTION OF ROCK UNITS 27

rhyolite (Staatz and Carr, 1964, p. 86-90). Flows that 

extend several kilometers or more are commonly 

underlain by stratified tuff, whereas some domes and 

short flows have only breccia or vitrophyre at their 

bases. Staatz and Carr (1964) divided the Topaz Mountain Rhyolite into five subgroups having implied 

lateral continuity, but my mapping shows that each 

subgroup is actually a complex of flows and domes 

derived from numerous vents. Flows and domes have 

been mapped separately by me (Lindsey, 1979a) as far 

as was practical, but the location of the contacts between adjacent flows remains obscure in many places.

FLOWS AND DOMES OF ALKALI RHYOLITE

Flows and extrusive domes of alkali rhyolite have 

been mapped and their vent areas identified on the 

basis of lines of viscous flowage and shearing (fig. 

12B). These flowage lines are visible on aerial 

photographs and were identified on the ground as 

lenses of steeply dipping vitrophyre and layered 

rhyolite. Large flows were erupted from the vent on 

Topaz Mountain and another vent in the northeastern 

Drum Mountains; each flow extends west-southwest 

about 4 km from the vent. A complex of at least four 

domes and short flows was erupted from separate 

vents along Antelope Ridge. Other vents are north and 

south of Colored Pass and north of Pismire Wash. 

Much of the rhyolite between Topaz Mountain and 

Pismire Wash issued from a fissure vent that is clearly 

discernible on aerial photographs as a lineament 

bounded by subparallel flow lines. Some of the vents 

were first located by Staatz and Carr (1964), who 

measured the attitude of flow layering throughout the 

rhyolite. These attitudes and additional observations 

by me were used to confirm the orientation of flow 

lines seen on aerial photographs.

Vitrophyre and rhyolite evidently accumulated to 

considerable heights around vents and then spread 

laterally under their own weight and by force from injection of new lava within vents, producing concentric 

flowage lines of sheared vitrophyre and flow-layered 

rhyolite. Williams (1932, p. 142-145) described a 

similar origin for the dacite domes of Lassen Peak, 

Calif. Vitrophyre zones in the Topaz Mountain RhyoKeg Mountain

FIGURE 12. Topaz Mountain Rhyolite. A, Aerial view looking north 

at Topaz Mountain, showing section of stratified tuff (Ttmt), vitrophyre (V), and alkali rhyolite (Ttmr) that issued from vent on 

top of Topaz Mountain. Crystal tuff member of Joy Tuff (Tjc) and 

Drum Mountains Rhyodacite (Tdr) downdropped to west along 

the east side of Joy graben by the Topaz Mountain fault (bars and 

balls indicate downthrown sides of subsidiary faults), which is 

covered by Topaz Mountain Rhyolite (Ttmt and Ttmr) on Topaz 

Mountain, and lineament of buried Topaz Valley fault (arrows); 

Pz, Paleozoic rocks. B, Aerial veiw looking northeast at isolated 

flow of alkali rhyolite in northeastern Drum Mountains (centered 

at sec. 7, T.14 S., R. 10 W.), showing alinement of dark lenses of 

vitrophyre (arrows) that are concentric around the vent area.
28 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

lite were formed at the outer chilled margin of each succeeding injection of lava. Lenses of vitrophyre and 

parallel flow layering in rhyolite dip steeply; in short 

flows and in domes, the dip is generally toward the 

vent, but in larger flows and in flows between vents the 

dip of layering may be either toward or away from the 

vent. Horizontal lineations, perpendicular to the overall direction of flowage and in the plane of layering, are 

common in some flows; lineations are defined by rodshaped vesicles and sets of parallel striations in both 

vitrophyre and rhyolite. Rhyolite on Antelope Ridge 

evidently cooled prior to flowage because it contains 

vesicles with geopetal structures of layered silica 

which were originally deposited in horizontal position 

but which now dip steeply toward the vent. The rhyolite also contains many small-scale flowage folds that 

vary considerably in attitude; these folds probably 

formed near the vent and became reoriented during 

outward spreading of the rhyolite.

The Topaz Mountain Rhyolite typically contains 

about 13 percent phenocrysts that consist mainly of 

euhedral sanidine, partially resorbed quartz, and lesser 

amounts of red oxidized biotite and plagioclase (table 

4). Small flows and domes in Dugway Valley and at 

Antelope Ridge contain as much as 37 percent phenocrysts. The groundmass is composed entirely of finegrained quartz, cristobalite, potassium feldspar, and 

scattered tiny grains of calcite and specular hematite. 

Traces of zircon of high uranium content (as much as 

11,000 ppm) occur as tiny (0.05-0.10 mm), subround, 

gray to pink grains. Cavities filled with vapor-phase 

quartz, opal, and topaz are common, and locally these 

cavities contain fluorite, pink beryl, garnet, bixbyite, 

and pseudobrookite (Staatz and Carr, 1964, p. 

102-108). Both mosaic and spherulitic groundmass 

textures are common. Chemical analyses show that 

most of the flows and domes are alkali rhyolite. The 

distinctive composition of the rhyolite anomalous 

fluorine, beryllium, lithium, niobium, uranium, 

thorium, and yttrium has been noted often (Staatz 

and Griffitts, 1961; Shawe, 1966; Lindsey, 1977).

FIGURE 13. Stratified tuff in the Topaz Mountain Rhyolite. A, 

Stratified tuff filling paleovalley in Joy Tuff east of Topaz Valley 

(sec. 9, T. 13 S., R. 11 W.). Inclination of strata is believed to 

result from initial dip and compaction. B, Pumice and scattered 

dark obsidian fragments in air-fall tuff on Antelope Ridge (sec. 13, 

T. 13 S., R. 11 W.). C, Angular fragments of volcanic rock and 

limestone (at pencil point) in tuff west of Topaz Valley (sec. 16, T. 

13 S., R. 11 W.).

'if":v*S|

'- f^^wSL \*j ^^-^^^^ i--'~".^^ ^-^ftP*^'' '**' - *'*.-'r*~~'

^^^^tf^-f 

 #* 'T/y - -^ ^^^^ '» - * 

T'M/ -" ^^^M

 ^«^*F5&. - A;-"-"'»-*>'S-*- : 5f»t .' * ^«^ka&*f^

^m'28SgiSg!i
STRUCTURAL GEOLOGY 29

STRATIFIED TUFF

Stratified tuff occurs at many places beneath flows 

of alkali rhyolite in the Thomas Range. At any one section, the tuff is overlain by breccia, vitrophyre, and 

alkali rhyolite flows. The tuff units are rarely more 

than 30 m thick and vary greatly in thickness over 

short distances; the thickest sections occur in 

paleovalleys over older formations and flows of Topaz 

Mountain Rhyolite. A paleosurface having more than 

30 m of relief (fig. 13-A) was developed on older alkali 

rhyolite, the Joy Tuff, and Paleozoic rocks beneath 

Topaz Mountain.

Detailed study of textures and structures in the tuff 

indicates deposition by air fall, ash flow, and ground 

surge (Bikun, 1980). Massive to stratified tuff consisting mostly of pumice is probably of air-fall origin 

(fig. 13fi); such air-fall tuffs are abundant in the 

southern part of The Dell. Massive unsorted beds of 

pumice and lithic fragments (top of fig. 13-A, fig. 13C) 

in tuff around Topaz Mountain resemble thin ash-flow 

tuffs described by Sparks and others (1973). The tuff 

has been interpreted previously as a water-laid deposit 

(Shawe, 1972; Lindsey, 1977) because of extensive stratification (fig. 13A), abundant clasts derived from 

underlying formations (fig. 13C), and unconformities 

at the base of the tuff. Epiclastic tuffaceous sandstone 

containing abundant quartz and sanidine derived from 

the Dell Tuff comprises most of the tuff 1| km north of 

Wildhorse Spring; such sandstone is probably water 

laid. Angular fragments of older volcanic rocks as 

large as 1 m in diameter are locally abundant in the 

tuff (Lindsey, 1975, fig. 3fi); these may have been 

blasted from nearby vents or washed into the tuff from 

nearby paleohills.

The uppermost few meters of tuff are reddened and 

fused to welded tuff by overlying flows of alkali 

rhyolite. This fusing contrasts with the welding of ashflow tuffs, which generally are most densely welded 

toward the middle and lower parts of cooling units. 

Similar fused tuffs beneath rhyolite flows have been 

described from southern Nevada (Christiansen and 

Lipman, 1966).

Pumice and shards are the dominant constituents of 

the tuff (fig. 135); these accompany variable amounts 

of mostly volcanic rock fragments (fig. 13C) and less 

than 20 percent broken crystals consisting of quartz, 

sanidine, plagioclase, biotite, and traces of heavy 

minerals. Some of the tuff is glassy, but large areas 

have been altered by diagenesis to clinoptilolite, 

potassium feldspar, and cristobalite. Chemical analyses indicated that the tuff has a rhyolitic composition

that, before diagenesis, was probably very close to the 

composition of the alkali rhyolite. Additional details of 

the mineral and chemical composition of the tuff have 

been reported elsewhere (Lindsey, 1975).

STRUCTURAL GEOLOGY

The Thomas Range and northern Drum Mountains 

are part of a caldera complex that has been extensively 

-modified by basin-and-range block faulting. The 

Thomas caldera (Shawe, 1972) is the major structural 

feature of the area (fig. 2); its western outline is 

delineated by the ring fracture zone of the Joy fault 

and the Dell fault system, and by faults now buried 

beneath Topaz Mountain Rhyolite in the Thomas 

Range. The remainder of the caldera margin has been 

projected in the western part of Keg Mountain (Shawe, 

1972), outside the area of this study. The Thomas 

caldera contains, nested within it, the younger 

Dugway Valley cauldron. The Dugway Valley cauldron, whose topographic expression is now obscure, 

underlies much of Dugway Valley; its western margin 

is marked by faults and landslide deposits in the 

northeastern Drum Mountains, but its east side has 

not been located. The margin of a third cauldron may 

account for landslide deposits north of Spor Mountain, 

but other evidence for its existence is lacking. Basinand-range structures cut across the caldera complex; 

two of the most prominent are the Joy graben and the 

horst of the north-central Drum Mountains. Highlands 

at Spor Mountain and the northwestern and central 

Drum Mountains compose the rim of the caldera complex; these also have been modified by basin-and-range 

faulting.

Outlines of the Thomas caldera and the Dugway 

Valley cauldron are based partly on the distribution of 

exposed and buried faults and partly on the thickness 

and distribution of ash-flow tuffs and landslide 

deposits. In other well-studied caldera complexes, such 

as those of the San Juan Mountains (Steven and Lipman, 1976) and southern Nevada (Byers and others, 

1976), ash-flow tuffs associated with caldera collapse 

ponded to great thicknesses within the source caldera 

but are thin or absent outside the margin of the 

caldera; landslides along the caldera margins left 

lenses of breccia and megabreccia interbedded with the 

intracauldron ash flows (Lipman, 1976). All of these 

features have been identified within the Thomas 

caldera and Dugway Valley cauldron, and are used 

here to establish the extent and history of cauldron 

subsidence.
30 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

FIGURE 14. Aerial views of the ring-fracture zone of the Thomas caldera, showing the Joy fault and its northward extension in The Dell. A, 

View looking northwest from above Joy townsite, showing the arcuate trace of the Joy fault (arrows) and the caldera rim of the northwestern Drum Mountains. , Cambrian sedimentary rocks; Tdr, Drum Mountains Rhyodacite; Tml, Mt. Laird Tuff; Tjc, crystal tuff 

member of Joy Tuff. B, View looking north at The Dell, showing The Dell fault system, remnant of the caldera rim of tilted Paleozoic 

rocks on Spor Mountain, and the location of mines and prospects, which are the Yellow Chief mine (YC), Bell Hill Mine (BH), Claybank 

(C), Hogsback (H), Oversight (O), and Buena No. 1 (B). Location of superposed stream (SS) may mark the position of The Dell drainage 

system during time of unconformity C and deposition of the beryllium tuff member of the Spor Mountain Formation. Pz, undifferentiated Paleozoic rocks; Ttmr, Topaz Mountain Rhyolite.

The faulted structure of the caldera complex, also including postcaldera basin-and-range faults, has been 

revealed by mapping of numerous lineaments in the 

alluvium and the Topaz Mountain Rhyolite (fig. 2). 

These lineaments are interpreted as concealed faults; 

little or no offset is visible in alluvium and Topaz 

Mountain Rhyolite along most of the lineaments, but 

the lineaments are visible on aerial photographs as a 

result of slight fault movement, compaction, or settling of the covering unit. Many of the lineaments pass 

into well-exposed faults in rocks older than the Topaz 

Mountain Rhyolite, thus confirming their identification as concealed faults. Fracturing and small offsets 

are visible along lineaments in Topaz Mountain Rhyolite at Colored Pass and east of Topaz Mountain, as 

noted also by Staatz and Carr (1964).

THOMAS CALDERA

The Thomas caldera began to subside about 39 m.y. 

ago in response to eruption of the Mt. Laird Tuff from 

vents in the northern Drum Mountains. More than 500 

m of Mt. Laird Tuff and interbedded lacustrine sediment accumulated in a depression as much as 15-25 

km across. Later eruptions of the Joy Tuff (38 m.y. 

ago) and the Dell Tuff (32 m.y. ago) largely filled the 

caldera, and local cauldron collapse in Dugway Valley 

and perhaps north of Spor Mountain modified the 

Thomas caldera. In the Thomas Range and Drum 

Mountains, the western half of the Thomas caldera is 

well-defined by (1) the ring-fracture zone of the Joy 

fault and the Dell fault systems, (2) the caldera wallcollapse breccia at Spor Mountain, and (3) the confine-
STRUCTURAL GEOLOGY 31

ment of most of the post-subsidence Joy and Dell Tuffs 

within the caldera.

Initial subsidence of the Thomas caldera is believed 

to have accompanied eruption of the Mt. Laird Tuff 

about 39 m.y. ago. The source of the Mt. Laird Tuff 

was near Joy townsite, as indicated by tuff breccias 

associated with probable vents there and by intrusive 

plugs and dikes resembling Mt. Laird Tuff in the Drum 

Mountains south of Joy townsite. The intrusive equivalents may represent resurgence of Mt. Laird magma 

outside the caldera, but none occurs within the caldera 

or in the ring fractures. Subsidence of the caldera was 

most pronounced in Dugway Valley, where numerous 

ash flows of the Mt. Laird Tuff flowed into a lake, leaving more than 500 m of tuff and lacustrine sediments.

The south and west walls of the Thomas caldera are 

defined by an extensive system of ring fractures, 

faults, and lineaments that extend nearly the entire 

length of the mapped area (figs. 14A, B). The ring fracture of the caldera is exposed as the Joy fault in the 

Drum Mountains, where it has both a structural and 

topographic expression. The Joy fault is exposed only 

in a few prospects in jasperoid and iron-stained Cambrian rocks near Joy townsite. The Joy fault connects 

with the Dell fault system along the east side of Spor 

Mountain (figs. 14B, 15) and extends northward

beneath the Topaz Mountain Rhyolite as a system of 

lineaments. Lineaments in alluvium near the junction 

of the beryllium road and the Sand Pass Road and in 

Topaz Mountain Rhyolite, 2 km northwest of the road 

junction, may be the surface expression of buried 

faults in the ring-fracture zone where it extends 

northward into The Dell. All northerly trending faults 

mapped in The Dell are downthrown on the east side, 

indicating that they form part of a complex, steplike 

boundary on the west side of the Thomas caldera at 

this locality (fig. 15). The buried northwest corner of 

the caldera may be quite angular, as indicated by 

prominent intersecting north- and east-trending buried 

faults in the northern part of the Thomas Range.

There is good evidence that the ring fracture and 

topographic wall of the Thomas caldera existed as 

early as 38 m.y. ago. The 38-m.y. crystal tuff member 

of the Joy Tuff is completely confined to the east side 

of the Joy fault and does not occur on the west side of 

the Drum Mountains, where the Needles Range(?) 

Formation lies directly on the Mt. Laird Tuff. The 

distribution of the Joy Tuff indicates ponding against 

the scarp of the Joy fault 38 m.y. ago. A small occurrence of Joy Tuff has been found by drilling in the subsurface of Fish Springs Flat west of Spor Mountain, 

however, indicating that the caldera wall did not block
32 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

EXPLANATION 

Quaternary rocks

Alluvium and colluvium

Tertiary rocks 

Topaz Mountains Rhyolite (6-7 m.y.)

Alkali rhyolite flows and domes of 

topaz-bearing alkali rhyolite

Stratified tuff-vitric and zeolitic tuff underneath and interbedded with alkali 

rhyolite

Angular unconformity 

___ Spor Mountain Formation (21 m.y.)

Porphyritic rhyolite member flows, 

domes, and plugs of topaz-bearing 

alkali rhyolite

FIGURE 15.

Beryllium tuff member tuffaceous 

breccia containing clasts of carbonate rocks; stratified tuff, bentonite, 

and tuffaceous sandstone 

Angular unconformity

Dell Tuff (32 m.y.) rhyolitic ash-flow tuff 

containing abundant crystals of 

quartz and sanidine 

Unconformity

y\///Z\ Landslide breccia, breccia at Spor

Mountain breccia with clasts derived from rocks of early Paleozoic 

age

Angular unconformity

r\Hv;.y Drum Mountains Rhyodacite (42 m.y.) 

flows and breccias of porphyritic 

rhyodacite

Angular unconformity 

Paleozoic rocks

|MRi Undifferentiated dolomite, lime 

stone, quartzite, and shale

 Contact

 I Fault Dashed where inferred, dotted

where concealed by younger strata. 

Ball and bar on downthrown side. Concealed faults located by aerial photograph interpretation

* Vents for Spor Mountain Formation 

K Mines and prospects

1 Oversight prospect (uraniferous fluorsparin fault)

2 Claybank prospect (Be and U in beryl1 ' ' ' lium tuff along fault)

3 Hogsback prospect (Be and U in beryllium tuff)

4 Buena No. 1 (uraniferous opal in Topaz 

Mountain Rhyolite)

5 Yellow Chief mine

 Geologic map of The Dell, showing the setting of uranium mines and prospects (modified from Lindsey, 1978b). Base from U.S. Geological Survey orthophotograph taken in 1976 and enlarged to 1:40,000.

2 KILOMETERS

all westward flow of ash north of the Drum Mountains. 

Some movement of the Joy fault occurred after 38 m.y. 

ago, as indicated by faulted contacts of the Joy Tuff.

The ring fracture of the Thomas caldera has a long 

history of movement in The Dell; here, movement can 

be inferred or documented between 42 and 32 m.y. ago,

32 and 21 m.y. ago, and 21 and 7 m.y. ago (fig. 15). 

Reinterpretation of the breccia at Spor Mountain as a 

landslide deposit that may have formed along a fault 

scarp between 42 and 32 m.y. ago indicates that this 

part of The Dell fault system was active at that time. 

Restriction of the Dell Tuff to the east side of Spor
STRUCTURAL GEOLOGY 33

Mountain indicates ponding against the scarps of the 

Dell fault system 32 m.y. ago. Faults of the Dell 

system cut the Dell Tuff and older rocks throughout 

The Dell; they cut the Spor Mountain Formation in the 

central part of The Dell but are seen as lineaments 

covered by the Spor Mountain Formation in the southern part of The Dell (fig. 15). These relationships are interpreted to mean that the faults moved prior to 21 

m.y. ago, probably during collapse of the Thomas 

caldera, and were rejuvenated in some sections during 

basin-and-range faulting after 21 m.y. ago.

DUGWAY VALLEY CAULDRON

The Dugway Valley cauldron is defined by (1) an area 

of vent breccias in the crystal tuff member of the Joy 

Tuff north and east of Topaz Mountain, (2) restriction 

of the black glass tuff member of the Joy to an area 

south of the vent breccias, and (3) the occurrence of 

faults and landslide breccias along the cauldron 

margin in the northeastern Drum Mountains (fig. 2). 

The Dugway Valley cauldron contains much greater 

thicknesses of volcanic rocks than the western segment of the Thomas caldera. The center of the Dugway 

Valley cauldron lies northeast of Antelope Ridge; the 

west side of the cauldron consists of three fault blocks 

that are downthrown stepwise into Dugway Valley. 

The Topaz Valley fault marks the west side of the 

faulted blocks and extends north beneath Topaz Mountain. The Antelope Ridge west fault is exposed in the 

northeastern Drum Mountains, where it dropped the 

black glass tuff member of the Joy Tuff and the overlying landslide megabreccia of the northern Drum Mountains down on the east side. The fault extends north in 

the subsurface west of Antelope Ridge. The Antelope 

Ridge east fault extends south from Antelope Ridge 

into the subsurface. The northwestern extent of the 

margin of the Dugway Valley cauldron is problematical, but it may extend northward from Topaz Mountain and then eastward, encircling Dugway Valley. 

Faults in the western part of the Keg Mountain area, 

mapped by Shawe (1972) as the east side of the 

Thomas caldera, may also mark the east side of the 

Dugway Valley cauldron.

The Dugway Valley cauldron collapsed 38 m.y. ago 

during eruption of the Joy Tuff from vents along the 

western side of Dugway Valley. Large areas of tuff 

breccia in Joy Tuff along the west margin of the 

Dugway Valley cauldron probably formed during 

simultaneous eruption and collapse. After eruption of 

the black glass tuff member of the Joy Tuff, which was 

restricted to the Dugway Valley cauldron, the west 

wall of the cauldron collapsed and slid over the

cauldron fill of Joy Tuff, forming the megabreccia of 

the northeastern Drum Mountains.

BASIN-AND-RANGE STRUCTURE

Faults that formed during early development of 

basin-and-range structure in Miocene time are widespread in the Thomas Range and northern Drum 

Mountains and have modified the caldera structure extensively. Modification was accomplished by rejuvenation of earlier faults that were formed during cauldron 

subsidence and by initiation of new, north-trending 

faults. As a result of early basin-and-range faulting, 

the Joy graben and an adjacent horst in the northcentral Drum Mountains probably were formed. Numerous faults of early basin-and-range age cut the rim 

outside the Thomas caldera.

The Joy graben was formed partly by subsidence of 

the Thomas caldera and partly by basin-and-range 

faulting. All of the structures associated with the ring 

fracture of the Thomas caldera, such as the Joy fault 

and the Dell fault system, were formed during caldera 

subsidence. The Dell fault system was rejuvenated 

after 21 m.y. ago, as shown by large offsets of the Spor 

Mountain Formation. The east side of the Joy graben 

and the adjacent horst of the north-central Drum 

Mountains probably were formed during early basinand-range faulting. The age of the horst is a problem 

for the caldera model; the horst had to be absent for 

caldera-filling ash flows of the Joy Tuff to travel west 

across the present location of the horst to the caldera 

wall, but a highland had to exist soon after eruption of 

the Joy Tuff so that the landslide megabreccia of the 

northeastern Drum Mountains could slide over the 

Dugway Valley cauldron. The eastern side of the Joy 

graben is defined by the Schoenburger Spring fault, 

which strikes northwest and intersects the northtrending Topaz Mountain fault in the north-central 

Drum Mountains. The Topaz Mountain fault passes 

beneath the Topaz Mountain Rhyolite (fig. 12A), where 

it is alined with a system of lineaments and vents in 

rhyolite that extends north into the Dugway Range, a 

distance of about 20 km. These structures cut across 

the caldera and therefore are considered to be of basinand-range origin.

The rim of the Thomas caldera contains numerous 

faults of early basin-and-range age. Almost all major 

faults on Spor Mountain, whatever their trend, extend 

beyond the Paleozoic rocks and cut the Spor Mountain 

Formation and volcanic rocks of Oligocene and Eocene 

ages (figs. 2 and 15). Previous mapping did not show 

offset of volcanic rocks by many faults, so only a few 

faults were assigned a Tertiary age (Staatz and Carr,
34 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

1964, p. 128-129). Faulting of early basin-and-range 

age is most evident around the beryllium mines southwest of Spor Mountain, where northeasterly and easterly trending high-angle normal faults displace the 

Spor Mountain Formation by as much as 200 m or 

more, throwing the beryllium tuff and porphyritic 

rhyolite members into large blocks that have been 

tilted 15°-30° northwest. East of Spor Mountain, 

easterly and northeasterly trending faults extend into 

The Dell, where they cut volcanic rocks as young as the 

Spor Mountain Formation, also. The Drum Mountains 

are traversed by many westerly, northwesterly, and 

northerly trending faults, some of which offset rocks of 

Tertiary age. Faults cut the Drum Mountains Rhyodacite at many places southwest of Joy townsite. 

South of the Sand Pass Road, faults cut rocks as 

young as the Needles Range(?) Formation.

Basin-and-range faulting was accompanied by 

voluminous eruptions of alkali rhyolite. The first eruption, of the Spor Mountain Formation 21 m.y. ago, predated most basin-and-range faulting, although the two 

may have been closely related. After most early basinand-range faulting had ceased, the Topaz Mountain 

Rhyolite was erupted 6-7 m.y. ago. Basin-and-range 

faults, and earlier faults rejuvenated by basin-andrange faulting, were a major influence on the locus of 

eruption of the Topaz Mountain Rhyolite. Numerous 

vents in the Topaz Mountain Rhyolite are at or near 

fault intersections beneath the rhyolite; these include 

vents in the north-central Thomas Range, south of Colored Pass, on Topaz Mountain (fig. 12A), on Antelope 

Ridge, and in the northeastern Drum Mountains (fig. 

12B). Vents north of Colored Pass and in the northeastern part of the Thomas Range occur near faults or 

along their projections beneath rhyolite. The vent 

north of the head of Topaz Mountain was a fissure; the 

extent of the fault is still visible as an east-west lineament between subparallel flow lines in rhyolite. Only 

minor block-faulting continued after eruption of the 

Topaz Mountain Rhyolite.

STRUCTURAL EVOLUTION

The structural evolution of the Thomas Range and 

Drum Mountains during part of Tertiary time can be 

inferred from the distribution of volcanic rocks and the 

age of fault movements, as already discussed. The evidence has been integrated in a series of maps depicting 

the most probable structural evolution of the region 

(fig. 16), although evidence for the existence and age of 

some events is lacking or equivocal. It must be emphasized that the degree of confidence in the maps 

decreases for those showing earlier events; the maps of 

Eocene and Oligocene time are partly conjectural, and 

those for Miocene time are mainly factual.

The map of the time of unconformity B (fig. 16A), 

about 38-39 m.y. ago, reflects the eruption of the Mt. 

Laird Tuff and accompanying subsidence of the 

Thomas caldera. Both the Mt. Laird Tuff and accompanying lacustrine sediments accumulated in Dugway 

Valley, whereas the tuff was erupted over a wide area 

beyond the caldera margin. Eruption of the Mt. Laird 

Tuff followed eruption of flows, breccias, and tuffs 

from small central volcanoes in the Black Rock Hills 

and the Little Drum Mountains, and possibly from 

fissures in the Thomas Range and Drum Mountains. 

The Joy Tuff had not been erupted yet.

The Thomas caldera was filled with the mostly intracauldron Joy and Dell Tuffs 38-32 m.y. ago (figs. 

16B and 16Q. Eruption of the Joy Tuff 38-37 m.y. ago 

was accompanied by subsidence of the Dugway Valley 

cauldron, a subsidiary depression nested within the 

larger Thomas caldera. This subsidence left a segment 

of the Thomas caldera as a high rim relative to the 

Dugway Valley cauldron; the rim collapsed and slid 

over the west side of the cauldron after 37 m.y. ago 

(fig. 16B). Also, between 42 and 32 m.y. ago the wall of 

the Thomas caldera at Spor Mountain collapsed; this 

event may have accompanied initial subsidence of the 

Thomas caldera or may reflect additional subsidence of 

the caldera during eruption of the Joy Tuff. Filling of 

the Thomas caldera was completed 32 m.y. ago by 

eruption of the Dell Tuff from an unknown source 

within the Thomas caldera (fig. 16Q. The Dell Tuff 

covered some of the landslide breccia at Spor Mountain and ponded against scarps of the Dell fault 

system.

The interval between deposition of the intracauldron 

ash-flow tuffs (about 32 m.y. ago) and the Spor Mountain Formation (21 m.y. ago) must have been a period 

of erosion of the caldera rim, including Spor Mountain, 

and perhaps of volcanic highlands in the Thomas 

Range (fig. 16Q, but the only depositional record of 

such erosion is in The Dell, where sandstone and conglomerate occur in the beryllium tuff member of the 

Spor Mountain Formation. There is no evidence for 

cauldron resurgence (Smith and Bailey, 1968) during 

this interval; such resurgence should be evident from a 

moat filling of thick volcaniclastic sediments, intrusive 

rocks, and lavas along the margin of the caldera. These 

rocks would have formed soon after ash-flow eruption 

and cauldron subsidence. None of these features has 

been observed. The beryllium tuff is a local member 

that rarely exceeds 60 m in thickness; its conformable 

relationship with the overlying 21-m.y.-old porphyritic rhyolite member suggests an age close to the latter 

rather than close to that of the ash-flow tuffs. The 

oldest postcauldron intrusive rocks and flows belong 

to the 21-m.y.-old porphyritic rhyolite, indicating a 

hiatus in local volcanism of nearly 11 m.y.
STRUCTURAL GEOLOGY 35

The tuff, tuffaceous sediments, and rhyolite of the 

Spor Mountain Formation were deposited on the erosional surface that became unconformity C 21 m.y. ago 

(fig. 16D). Spor Mountain began to rise above a body of 

alkali rhyolite magma and to shed detritus into the 

surrounding area; plugs and vents of the porphyritic 

rhyolite member formed highlands to the north and 

east of Spor Mountain and erupted volcanic detritus 

that became incorporated in the beryllium tuff. A 

small south- and southwest-flowing stream system 

developed in The Dell and received detritus from Spor 

Mountain on the west and volcanic highlands on the 

east. This detritus collected in local tilted fault blocks, 

such as the one at Yellow Chief mine. The stream in 

The Dell flowed across the southern end of present-day 

Spor Mountain, where its probable location is marked 

by a modern entrenched drainage system that has been 

superposed across the structural grain of the Spor 

Mountain block (marked by SS on fig. 14B). Fish 

Springs Flat lay directly downstream from The Dell 

and received detritus from it and from smaller streams 

that probably drained southwest from Spor Mountain. 

At the same time, small air falls and flows of ash and 

pumice from nearby vents covered the drainage basin 

and may have been partly reworked by small streams. 

The tuffs and sediments deposited in this volcanicsedimentary environment a mixture of tuffaceous 

breccia, stratified tuff, and bentonite of ash-flow, 

ground-surge, and air-fall origin (Bikun, 1980) and 

epiclastic tuffaceous sandstone and conglomerate compose the beryllium tuff member of the Spor 

Mountain Formation.

Unconformity D represents a long period of block 

faulting and erosion, from 21 m.y. to 7 m.y. ago, that 

followed the eruption of the porphyritic rhyolite 

member of the Spor Mountain Formation (fig. 16E). 

The period coincides with the onset of widespread extensional tectonism in the Basin and Range Province 

(McKee and others, 1970; Noble, 1972). During this 

time, some faults that formed by cauldron subsidence 

in the Thomas Range and Drum Mountains were rejuvenated, blocks of Paleozoic rocks were uplifted at 

Spor Mountain and in the Drum Mountains, and the 

Spor Mountain Formation was broken into many fault 

blocks and eroded, so that only part of it remains.

Voluminous flows and domes of the Topaz Mountain 

Rhyolite began to erupt from vents along old faults 

about 7 m.y. ago; they covered much of the erosional 

surface of unconformity D in the Thomas Range (fig. 

16F). Quiescent periods of as much as 0.5-m.y. duration interrupted intense rhyolitic volcanism. By the 

end of volcanism at 6 m.y. ago, rhyolite flows and 

domes had coalesced to form a large plateau; upland 

areas of the Thomas Range are still flat, attesting to 

the once great extent of the plateau surface. During

the early stages of eruption, the rhyolite vents erupted 

abundant pumice and rock fragments which, mixed 

with detritus from nearby hills, filled valleys and lowlying areas with stratified tuff. The depression left by 

the Dugway Valley cauldron was nearly filled with tuff 

and rhyolite at this time.

SOME UNSOLVED PROBLEMS

I have sought to integrate the structural development and volcanism of the Thomas Range and northern Drum Mountains into one model which relies on a 

framework of caldera subsidence and early basin-range 

faulting. The model may require modification if new information can be obtained concerning some problems. 

These include (1) the location of the east side of the 

Thomas caldera, (2) the occurrence of large areas of 

basement within the Thomas caldera that might date 

from the time of subsidence, (3) the apparent small 

volume of ash-flow tuff erupted from the caldera at the 

time of initial subsidence, and (4) the peculiar absence 

of individual tuffs from parts of the caldera.

Much work remains to be done to locate and verify 

the east side of the Thomas caldera and the adjacent 

Keg caldera proposed by Shawe (1972). The east side of 

the Thomas caldera was projected through the western 

part of Keg Mountain by Shawe (1972), but detailed 

mapping by Staub (1975) in the Picture Rock Hills did 

not confirm evidence for the ring fracture there. The 

occurrence of thick sections of Joy Tuff in the Picture 

Rock Hills and of the Dell Tuff east of Keg Pass suggests that the caldera margin may be farther east than 

was projected by Shawe. Dikes and plugs of rhyolite to 

quartz latite in northwestern Keg Mountain have been 

dated at 32 m.y. (Lindsey and others, 1975); they may 

be intrusions along a ring-fracture zone, which was interpreted to be that of the Keg caldera by Shawe 

(1972). The distribution of intracauldron tuffs and of 

possible ring-fracture-related intrusive rocks and 

faults should aid in locating the Thomas caldera and 

smaller calderas nested within it. Additionally, breccias that might indicate tuff vents and collapse of 

caldera walls should be sought; wall-collapse breccias 

might be found in the northern part of Keg Mountain, 

where volcanic and sedimentary rocks predating ashflow eruption abound.

The presence of basement areas dating from the time 

of cauldron subsidence may provide insight into the 

process of subsidence. Such an area of basement is the 

horst of the north-central Drum Mountains, a large 

block (about 14 by 4 km) of Paleozoic rocks that probably has existed since about 37 m.y. ago. The horst 

provided the source from which landslide megabreccias slid over the 37-m.y.-old black glass tuff member 

of the Joy Tuff after subsidence of the Dugway Valley
36 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

DUGWAY

VALLEY

CAULDRON

113°00'

113°00'
STRUCTURAL GEOLOGY

113°00'

37

TOPAZ

?\ MOUNTAIN 

FAULT

113°00'

8 KILOMETERS

J

EXPLANATION

Topaz Mountain Rhyolite Contact lines 

show individual flows and domes

Spor Mountain Formation Arrows show 

direction of stream and ash flow inferred for beryllium tuff member

Landslide deposits Arrows show direction of sliding

Ash-flow tuffs erupted during or after the 

Dugway Valley cauldron Includes Joy 

Tuff and Dell Tuff

Flows, breccias, and ash-flow tuff erupted 

priortoor during the Thomas caldera 

Includes Drum Mountains Rhyodacite 

and Mt. Laird Tuff

Sedimentary rocks of Devonian to Precambrian age 

Undifferentiated rocks (Fonly)

Contact

Fault dotted where concealed, ball and 

bar on downthrown side. Concealed 

faults located by aerial photograph interpretation

Probable vent

 Structural margin of caldera or cauldron 

Queried where location uncertain

FIGURE 16. Maps showing structural evolution of the Thomas Range and northern Drum Mountains. A, Surface of 

unconformity B, about 39-38 m.y. ago, after eruption of the Mt. Laird Tuff and subsidence of the Thomas caldera. B, 

Results of eruption of the Joy Tuff 38-37 m.y. ago, accompanying subsidence of the Dugway Valley cauldron, and collapse and sliding of cauldron walls. C, Surface of unconformity C, about 30-21 m.y. ago. The Thomas caldera has been 

filled with intracauldron Joy and Dell Tuffs. The Needles Range(?) Formation has been omitted. D, Volcanism and 

sedimentation at the time of eruption of the Spor Mountain Formation 21 m.y. ago. E, Surface of angular unconformity 

D, resulting from block-faulting and erosion 21-7 m.y. ago. F, Results of volcanism 7-6 m.y. ago after eruption of the 

Topaz Mountain Rhyolite.
38 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

cauldron. The horst did not exist 38 m.y. ago, when the 

crystal tuff member of the Joy Tuff flowed west to the 

wall of the Thomas caldera. Although the horst probably owes much of its present relief to early basinrange faulting, its early existence seems likely. Two explanations for its origin are (1) uplift over resurgent 

magma, and (2) differential subsidence within the 

caldera complex. Uplift by resurgence is considered 

unlikely because no intrusive rocks or lava flows are 

associated with the horst. Consideration of differential 

subsidence leads to the interesting possibility that the 

entire caldera complex may reflect incomplete or variable degrees of subsidence. Drilling has revealed a high 

area of Paleozoic rocks beneath Topaz Mountain Rhyolite near Colored Pass, at the head of Pismire Wash in 

the Thomas Range, that may indicate another horst. If 

other positive areas are found within the caldera complex, and if these can be shown to be related to fault 

blocks that predate basin-range structure, then incomplete, differential subsidence will be verified.

The problem of insufficient ash-flow volume for subsidence may reflect inadequate information about the 

true extent and thickness of ash-flow tuffs. The volume 

of the Mt. Laird Tuff, believed to be associated with 

subsidence of the Thomas caldera, is somewhat 

roughly estimated at 50-100 km3 (cubic kilometers). 

This volume seems small for the size of the Thomas 

caldera, which is at least 15-25 km across. The accuracy of the volume estimate is severely affected by 

lack of information about the extent of the 

500-m-thick section east of Topaz Mountain. Additional drilling or geophysical study is needed to determine the volume of tuff beneath Dugway Valley. The 

combined volume of the Joy and Dell Tuffs, which 

filled the Thomas caldera and overflowed it to the east, 

is estimated at approximately 150-200 km3 if one 

assumes a once-continuous distribution of 180 m of 

tuff from the Joy fault to Desert Mountain. Such a 

volume of tuff is compatible with the collapse of the 

Dugway Valley cauldron and perhaps additional undiscovered cauldrons.

Individual formations of ash-flow tuff are absent 

from parts of the caldera; such distribution would not 

be expected if a single large area subsided at once. 

Although the distribution of individual tuffs is incompletely known because of cover and gaps in subsurface information, enough is known to suspect that additional structures related to cauldron subsidence will 

be found. Such an example is the absence of the Mt. 

Laird and Joy Tuffs in the caldera throughout the west 

half of the Thomas Range, and of the Dell Tuff from 

the complementary part of the caldera. Such a distribution may be caused by a separate time of subsidence 

for the northwest part of the caldera, perhaps later

than the 39-38 m.y. eruptions of the Mt. Laird and Joy 

Tuffs. Nevertheless, no structure delineating a separate subsidence feature is evident. A second explanation is that of differential subsidence; the northwest 

part of the caldera may not have subsided as much as 

the south and east parts; only after these were filled 

with tuff could the Dell Tuff flow into the northwest 

part of the caldera. A third explanation is that long 

periods of erosion removed so much ash-flow tuff that 

the present distribution beneath rhyolite flows and 

alluvium is spotty. Long gaps in the recprd of volcanism, as shown by the ages of volcanic formations, provided time for extensive erosion. If so, the detritus 

from such erosional epochs must have been carried beyond the volcanic field, because no volcanic sediments 

in the area are of sufficient volume to account for the 

expected amount of detritus. Available evidence does 

not permit a definitive explanation for the absence of 

individual tuffs from parts of the Thomas caldera.

CHEMICAL COMPOSITION OF 

VOLCANIC ROCKS

ROCK TYPES

Most of the volcanic rocks of the Thomas Range and 

northern Drum Mountains are believed to have been 

derived from local vents. Only the ash-flow tuff of the 

Needles Range(?) Formation may be from a distant 

source. Thus, from geographic considerations, magmas beneath the Thomas Range and Drum Mountains 

could have produced the entire volcanic sequence and 

the small plutons associated with it, with the possible 

exception of the Needles Range(?) Formation. Accordingly, the composition of all volcanic units except the 

Needles Range(?) is compared (fig. 17).

There are three time-dependent rock types among 

the indigenous rocks of the Thomas Range and northern Drum Mountains. In the classification of Rittmann 

(1952), the three types are (1) rhyodacite and quartz 

latite of 42 and 39 m.y. ago (Drum Mountains Rhyodacite and Mt. Laird Tuff), (2) rhyolite of 38 to 32 m.y. 

ago (Joy and Dell Tuffs), and (3) alkali rhyolite of 21 

and 7-6 m.y. ago (Spor Mountain Formation and 

Topaz Mountain Rhyolite) (fig. 17). Rhyodacite and 

quartz latite, erupted as both flows and tuffs, are 

characterized by about 60-67 percent SiO2, 14-19 percent A12O3,4-8 percent total iron as Fe2O3, 2-4 percent 

MgO, 4-6 percent CaO, 2-4 percent each of Na2O and 

K2O, and 0.6-1.1 percent TiO2 (major oxides have been 

recalculated to total 100 percent and to exclude volatile components). The rhyodacite and quartz latite are 

about the same age and close in overall composition to 

the intermediate-composition volcanic rocks in the Lit-
CHEMICAL COMPOSITION OF VOLCANIC ROCKS 39

r- Porphyntic rhyolite member, -Porphyritic rhyolite member,

1 Spor Mountain Formation Spor Mountain Formation

Crystal tuff member, Joy Tuff--]

80

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j-lntrusive diorite (age estimate 

r- Or urn1 Mountains Rhyodacite 

Rhyodacte^qUrtz '-Dell Tuff 

Latit? j Rhyolite

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Total new analyses

17

40 30 20

AGE IN MILLIONS OF YEARS

_LL I I I __l_________I______ 

90221 1 1

Total published analyses

_L_LU-_L_-_J_________I 

10

FIGURE 17. Chemical compositions, including trace elements, compared to the ages of igneous rocks in the Thomas Range and 

northern Drum Mountains. Major oxides have been recalculated 

to total 100 percent and to exclude volatile components. Line pattern along the base of some diagrams shows limit of detection 

when some samples were below the limit. Analyses of major ox5 1 366 4 11 

Total new analyses

17

ides from the literature (shown by triangles) are from Staatz and 

Carr (1964, p. 110), Newell (1971, p. 38), and Hogg (1972, p. 180). 

New chemical analyses (shown by x's) are in table 8.
40 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

Crystal tuff

2000 

S

Q_ 

Q_

Z 1000

«3 

CO

0 

40

S

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of 

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40

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200

S

Q. 

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nf 

O

0 

5

rPorphyritic rhyolite member, 

Spor Mountain Formation 

member, Joy Tuff-i 

Mt. Laird Tuff-j rBlack glass tuff member Joy Tuff

rlhtrusive diorite (age estimated) T .. 

J I >L DU j » i Topaz Mountain -i r-C rum Mountains Rhyodacite i Rhyolite I 

Rhyodacite-quaVtz r-Oell Tuff j 

Latite i Rhyolite! Alkali rhyolite

1 L: ' 1 * I I i 1

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1 I 1

XXXX X 

X XXXXXXXX X -

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XX X 

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 50 

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Spor Mountain Formation 

member, Joy Tuff-! 

Mt. Laird Tuff-j rBleck glass tuff member, Joy Tuff

(-Intrusive diorite (age estimated) T ., i l I i _. , . Topaz Mountain i pDrum Mountains Rhyodacite Rhyolite

Rhyodacitejcuajrtz rDell Tuff 

Latjte Rhyolitej Alkali rhyolite

 L^ii i i f 

XX

xxxxx 

x -

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AGE IN MILLIONS OF YEARS

___ Li U^_ l i i .....--..-.

5 1 366 4 11 17

AGE IN MILLIONS OF YEARS

5 1 366 4 11 

Total new analyses

17

FIGURE 17. Continued

tie Drum Mountains described as shoshonitic by Leedom (1974) and Pierce (1974). Rhyolite, erupted as ashflow tuffs, and alkali rhyolite, erupted as flows and 

tuffs, both contain 74-79 percent SiO2, 11-14 percent 

A1 2O3, 1-2 percent total iron as Fe2O3, and less than 1 

percent MgO; both are of calc-alkalic affinity, as is

Total new analyses

FIGURE 17. Continued

shown below. The two rock types differ from each 

other in that the alkali rhyolite contains slightly less 

CaO than the rhyolite (commonly less than 1.0 percent 

but as much as 2 percent, versus 1-2 percent for the 

rhyolite), more NazO (generally 3-4 percent versus 1-3 

percent for the rhyolite), slightly more K2O (generally 

4-5.5 percent versus 3-5 percent for the rhyolite), and 

less TiO2 (generally 0.05-0.15 percent versus 0.2-0.3
CHEMICAL COMPOSITION OF VOLCANIC ROCKS 41

pPorphyntic rhyolite member, 

I Spor Mountain Formation

-Porphyritic rhyolite member, 

Spor Mountain Formation

Crystal tuff

Rn

40

I 30

Z 

c- 20 

to

10

0

2000

ct 

2 1000

c/f

0

200

a.

Q.

z 100

0

100

a. 

a.

Z

0 

5

member, Joy Tuff-j 

Mt. Laird Tuff-j j pBlack glass tuff member. Joy Tuff 

-Iptrusive diorite (age estimated) T .. 

-Drum Mountains Rhyodacite j Rhyolite '" 1 

Rhyodaci'te-cjuEirtz -Dell Tuff 

L'atite' Fhyolite Alkali rhyolite

: 1 i j '

XXXX -

XXXXXX XX

i I ' I 

X

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XX X

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1 i j

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' 1 1 w; 1 1

XXXXXXX XXX

XXXX XXX XXXX*

0 40 30 20 10 C 

AGE IN MILLIONS OF YEARS

51366 4 11 17 

Total new analyses

FIGURE 17. Continued

Crystal tuff

20

a. 

a.

Z 10

0

200

CL

a. 

z 100

j-T

0

100

a. 

a.

Z 

D

20

a. 

a.

Z 10

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0

^ 100

Q_ 

Q.

5

member. Joy Tuff-j 

Mt. Laird Tuff-j r Black glass tuff member, Joy Tuff 

Hntru'sive diorite (age estimated) , ,. 

rOruro1 Mountains Rhyodacite ^0° ~ 

Rhyodacit^-q'uartz rDell Tuff 

tatjte | Rhyolite Alkali rhyolite

' i I f '

I I * I I

XXX

xxxxx

XX XXXXXXXX 

XXXXXXJto

J

~" \ I I I

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XX XXXX XXXXXX XXXXXX 

X

I I I I -,::.-;"; ; n.'::U

i ! i

I I I I

X

_____ y^^"" XX

X XXX 

X

XXX X

xx»r

XXX X# X x 

X X

I I I I

1 4yX

xl« 1 1 1

0 40 30 20 10 0 

AGE IN MILLIONS OF YEARS

i i I I i - - .... I

percent for the rhyolite). Alkali rhyolite also differs 

from rhyolite in that it contains as much as 0.77 percent fluorine (Staatz and Carr, 1964, p. 110; Shawe, 

1966). The high fluorine content of alkali rhyolite is reflected by the widespread occurrence of topaz in that 

rock.

Rocks intermediate in composition between rhyodacite-quartz latite, rhyolite, and alkali rhyolite are not

5 1 366 4 11

Total new analyses

FIGURE 17. Continued

17

present (fig. 17). Shifts in composition from one rock 

type to the next are abrupt, not gradual. For example, 

the age of the Mt. Laird Tuff is intermediate between 

the Drum Mountains Rhyodacite and the Joy Tuff, but 

the composition of the Mt. Laird Tuff closely resem-
42 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

bles that of the Drum Mountains Rhyodacite and is 

distinctly different from the slightly younger rhyolite 

of the Joy Tuff.

Comparison of total alkali versus silica contents of 

all three types indicates that they are within the field 

of subalkalic suites (fig. 18) (Irvine and Baragar, 1971). 

The rocks also fall within the calc-alkalic field of Irvine 

and Baragar on an AFM diagram (fig. 19). The three 

rock types tend to define separate fields in the alkalisilica and AFM diagrams, however, thus supporting 

the previous observation that volcanic products having compositions intermediate between these rock 

types are not present. The fields of rhyolite and alkali 

rhyolite occur close together and might be considered 

as one, but the rocks of each type plot as a coherent 

group having only partial overlap. Soda equals or exceeds K2O in most of the rhyodacites and quartz latites, but K2O dominates NaaO in the rhyolites and 

alkali rhyolites.

TRACE-ELEMENT ASSOCIATIONS

The three rock types are sharply defined by their 

trace-element association; rhyodacite-quartz latite contains chalcophile and siderophile metals, rhyolite has a 

scarcity of most trace elements, and alkali rhyolite contains lithophile metals (fig. 17). Rhyodacite and quartz 

latite contain trace amounts of copper (5-50 ppm),

nickel (7-100 ppm), and vanadium (100-200 ppm) that 

far exceed the amounts of these elements in rhyolite 

and alkali rhyolite. The most obvious characteristic of 

the rhyolitic tuffs is their relative scarcity of most 

trace elements. An exception is barium, which is more 

abundant (500-1,000 ppm) in rhyolite than in the alkali 

rhyolite (less than 200 ppm). Rhyodacite and quartz latite contain slightly more barium (1,000-1,500 ppm) 

than the rhyolite. In contrast to the other rocks, the 

alkali rhyolites contain traces of lithophile elements 

such as beryllium (generally 3-15 ppm),. lithium (as 

much as 700 ppm), molybdenum (as much as 7 ppm), 

niobium (30-200 ppm), tin (as much as 50 ppm), yttrium (20-100 ppm), ytterbium (3-10 ppm), uranium 

(generally 10-20 ppm), and thorium (50-80 ppm). The 

abundance of lithophile elements is slightly different in 

alkali rhyolites of two ages: the older alkali rhyolite of 

the Spor Mountain Formation contains more gallium, 

lithium, niobium, tin, yttrium, and ytterbium, and less 

molybdenum and uranium than the Topaz Mountain 

Rhyolite.

The consistent associations of trace elements confirm the classification of volcanic rocks in the area into 

three distinct types. In particular, trace-element abundances reveal significant differences between the character of rhyolite and alkali rhyolite. Whether all three 

are cogenetic members of a series is not known. Sharp 

distinctions between the three rock types and a time10

Rhyolite

Rhyodacite-quartz latite

EXPLANATION

Published New 

analyses analyses

50 60 70 

SiO2, IN PERCENT

80

Topaz Mountain Rhyolite 

Spor Mountain Formation, porphyritic rhyolite member 

Dell Tuff

Joy Tuff, black glass tuff member 

Joy Tuff, crystal tuff member 

Mt. Laird Tuff 

Intrusive porphyry, equivalent to Mt.

Laird Tuff (somewhat altered) 

Intrusive diorite 

Drum Mountains Rhyodacite

FIGURE 18. Total alkalis (Na^O plus K2O) versus SiO2 content of igneous rocks of the Thomas Range and northern Drum Mountains. 

Analyses have been recalculated to total 100 percent and to exclude volatile components. Previously published analyses are from 

Staatz and Carr (1964, p. 110), Newell (1971, p. 43), and Hogg (1972, p. 180). Line separating alkalic from subalkalic rocks from Irvine 

and Baragar (1971).
CHEMICAL COMPOSITION OF VOLCANIC ROCKS 43

dependent distribution of each are facts which require 

a unique evolution, if not a unique parentage, for each 

type.

The trace-element associations in the three rock 

types are believed to be of magmatic origin. Only fresh 

rocks were selected for analysis from compact flow 

rocks and tuffs. Except for the unwelded upper part of 

the black glass tuff member of the Joy Tuff, which was 

sampled to aid in stratigraphic assignment of that 

rock, very porous tuffs were not utilized. Rhyolitic ashflow tuffs do not show effects of hydrothermal alteration and do not contain anomalous concentrations of 

trace elements except locally along faults and in The 

Dell. Geochemical anomalies are widespread in the beryllium tuff member of the Spor Mountain Formation

and in stratified tuff of the Topaz Mountain Rhyolite 

(Lindsey and others, 1973; Lindsey, 1975), but associated flows of alkali rhyolite appear fresh and compact, 

with only local areas affected by mineralization.

The magmatic origin of uranium in alkali rhyolite 

was checked by comparing the uranium content of zircon from the three rock types (table 6). Uranium is a 

highly mobile element, and the uranium measured in 

whole rocks could possibly have been introduced by 

hydrothermal fluids or ground water. Zircon, a highly 

resistant mineral to chemical alteration, contains 

magmatic concentrations of uranium that would not 

be expected to change after crystallization. Microphenocrysts of zircon commonly contain zones of high uranium content and do not lose this characteristic in

EXPLANATION

Published New 

Analyses analyses

0.9 Fe2Os (new analyses, where total

Fe expressed as Fe2Os) 

FeO or FeO+0.9 Fe2O3 (Published analyses)

Na2O+K2O

Topaz Mountain Rhyolite 

Porphyritic rhyolite member, 

Spor Mountain Formation

Dell Tuff

Black glass tuff member. 

Joy Tuff

Crystal tuff member. Joy 

Tuff

Mt. Laird Tuff

Intrusive porphyry, equivalent to Mt. Laird Tuff 

(somewhat altered)

Intrusive diorite

Drum Mountains 

Rhyodacite

Rhyodacite-quartz 

°latite DD

MgO

FIGURE 19. AFM diagram of igneous rocks of the Thomas Range and northern Drum Mountains. Previously published analyses are 

from Staatz and Carr (1964, p. 110), Newell (1971, p. 43), and Hogg (1972, p. 180). Line separating tholeitic from calc-alkalic rocks 

from Irvine and Baragar (1971).
44 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

TABLE 6. Analytical data for uranium content of zircon in volcanic rocks of the Thomas Range 

[Uranium determined by author; neutron dose determined by C. W. Naeser. Estimated ages of volcanic rocks are listed for comparison]

Sample 

No.

U15 

U26 

U12B-1 

Rock unit

Topaz Mountain Rhyollte 

Porphyrltlc rhyollte member of Spor 

Mountain Formation.

Bentonlte at Yellow Chief mine 

(beryllium tuff member of Spor 

Mountain Formation).

Sample location, 

T. 12 S., R. 12 W.

SW1/4 sec. 14 

NE1/4 sec. 9

SW1/4 sec. 36 

NW1/4 sec. 36

Number 

grain

Alk;

20 

20

20 

A3 , 10 

B4 ,20

of Induced track

s Number 

counted

ill rhyollte

597 

794

703 

250 

494

s In detector

Density 

(tracks /cm2 )

1.24xl07 

1.65xl07

1.47xl07 

9.49xl06 

1.03xl07

Neutron dose 

(neutrons/cm )

9.06xl0 13 

9.23xl0 13

9.14xl0 13 

9.97xl014 

1.05xlO U

Uranium

Average

8,000 

10,000

9,500 

570 

5,800

(ppm) 1 Es

Range of

5,300-18,000 

2,300-16,000

7,000-13,000 

200-1,300 

3,700-7,500

tlmated age

(m.y.)

26.3-6.8 

21.3

21.3 

5 21.3 

5 21.3

Rhyollte

T54-A SE1/4 sec. 26 

NW1/4 sec. 36

20 

10

717 

493

1.50xl07 

2.05xl07

l.OOxlO 15 

1.24xl0 15

880 

970

360-2,000 

630-1,400

32.0 

32.0

Rhyodacite-quartz latite

U10A Drum Mountains Rhyodaclte SW1/4 sec. 35 8 345 2.00xl06 9.33xl014 130 50-130 42

'u (ppm) - Induced track density (S.SSxlO 1 )/neutron dose. Concentrations of U are rounded to two significant digits. 

Age estimated by association with dated flows.

Population A Is about 10 percent of total zircon and contains low-U zircon dated at 28.3±1.8 m.y. and 40.0±7.0 m.y. 

Population B Is about 90 percent of total zircon and contains hlgh-U zircon that could not be dated. 

Based on geologic relations and correlation of hlgh-U zircon (population B) with volcanism related to porphyritic rhyollte of Spor Mountain Formation.

altered or weathered rocks. Zircon from the volcanic 

rocks of the Thomas Range shows a high concentration 

of uranium in alkali rhyolite, with individual crystals 

containing as much as 1 percent uranium. Uranium in 

zircon from rhyodacite-quartz latite and rhyolite does 

not exceed 2,000 ppm. The high concentration of uranium in zircon of alkali rhyolite is strong evidence that 

this element was concentrated in the alkali rhyolite 

magma

The uranium content of all volcanic rocks in the 

Thomas Range and Drum Mountains is high compared 

to that of volcanic rocks from erogenic belts. Only two 

rock samples having less than 4 ppm uranium were analyzed from the Thomas Range-Drum Mountains area, 

whereas uranium from volcanic rocks in orogenic belts 

is commonly as little as 1-4 ppm. Volcanic rocks collected from two transects across the central Andes 

generally did not contain more than about 4 ppm uranium (Zentelli and Dostal, 1977). Volcanic rocks of 

basaltic to rhyolitic composition in northern New Zealand contain less than 3.4 ppm uranium (Ewart and 

Stipp, 1968), and rocks of basaltic to intermediate composition in island arcs north of New Zealand generally 

contain less than 1 ppm uranium (Ewart and others, 

1977). The uranium content of the Utah rocks is not 

unique in the western United States, however, where 

rhyolitic rocks containing 5-20 ppm uranium and more 

are common (Zielinski, 1978).

RELATIONSHIP OF VOLCANISM TO 

MINERALIZATION

The time-dependent rock types are associated with 

distinctive types of mineralization. Rhyodacite-quartz

latite volcanism was associated with chalcophile and 

siderophile metal mineralization, rhyolite volcanism 

was probably barren, and alkali rhyolite volcanism was 

associated with lithophile metal mineralization. The 

types of mineralization are characterized broadly by 

the same trace elements that mark the types of igneous rocks.

Early rhyodacite-quartz latite volcanism was associated with sulfur-rich chalcophile mineralization, as illustrated by the occurrence of copper minerals and 

jarosite in small plutons that may overlie an unexposed mineralized pluton in the Joy area (Newell, 

1971). Siderophile mineralization, as illustrated by the 

manganese, iron, gold-bearing jasperoid, and kaolin 

deposits of the Drum Mountains (Crittenden and 

others, 1961; McCarthy and others, 1969; Newell, 

1971), probably belongs to the period of chalcophile 

mineralization when acid, sulfur-rich solutions leached 

these metals from country rock, concentrated them in 

fissures and fault zones, and left leached areas of pure 

clay. Most chalcophile and siderophile mineralization 

in the Drum Mountains appears to have been confined 

to rocks older than the Joy Tuff.

Most of the occurrences of the lithophile metals, including beryllium and uranium, are in the beryllium 

tuff member, which is the oldest representative of 

alkali rhyolite volcanism. Deposition of fluorite pipes 

in Paleozoic rocks on Spor Mountain also probably accompanied alkali rhyolite volcanism. Sulfur is believed 

to have been sparse or absent during lithophile metal 

mineralization. Some beryllium, fluorite, and uranium 

were deposited after eruption of the Topaz Mountain 

Rhyolite 6-7 m.y. ago (Lindsey and others, 1975; Zielinski and others, 1977), but I now believe that late
CHEMICAL COMPOSITION OF VOLCANIC ROCKS 45

mineralization was relatively minor because of the lack 

of mineral deposits in the Topaz Mountain Rhyolite 

and their relative abundance in the older Spor Mountain Formation. The major period of lithophile metal 

mineralization probably occurred between 7 and 21 

m.y. ago, when deposits of low-grade beryllium, uranium, and fluorite were formed in the beryllium tuff 

member of the Spor Mountain Formation. There is no 

evidence to associate uranium or other lithophile metal 

mineralization with the caldera cycle.

ORIGIN OF VOLCANIC ROCKS

The volcanic rocks of the Thomas Range and the 

northern Drum Mountains were formed in three distinct stages, much as originally proposed by Shawe 

(1972). (1) Older flow rocks and tuffs of rhyodacitequartz latite composition were erupted from small central volcanoes and fissures in late Eocene time. Initial 

subsidence of the Thomas caldera accompanied eruption of the Mt. Laird Tuff. Volcanism was accompanied by emplacement of small plutons and by chalcophile and siderophile metal mineralization. (2) The composition of volcanic products switched abruptly to 

rhyolitic ash-flow tuff in latest Eocene and Oligocene 

time. Eruption of tuff was accompanied by collapse of 

the Dugway Valley cauldron. Rhyolitic ash-flow volcanism in latest Eocene and Oligocene time probably 

was not followed by cauldron resurgence and probably 

was not accompanied by mineralization. (3) Alkali 

rhyolite volcanism accompanied early basin-and-range 

faulting in Miocene time, producing tuffs, flows, and 

domes. Block faulting of basin-and-range type rejuvenated earlier faults and was accompanied by local erosion and sedimentation that mixed detritus with the 

tuffs. Fluorine and lithophile metal mineralization accompanied alkali rhyolite volcanism. Reconnaissance 

studies by Shawe (1972) and me indicate that, in general, the Tertiary history of Keg Mountain resembles 

that of the Thomas Range and Drum Mountains.

All of the volcanic rocks could have originated by 

crustal fusion; alternatively, they may have originated 

by partial melting of the mantle and contamination 

with considerable crustal material. These ideas are 

compatible with strontium isotope evidence for continental felsic rocks in general (Hedge, 1966) and with 

strontium isotopic compositions of plutonic rocks 

nearby (Moore and others, 1979). The high content of 

SiO2, alkalis, thorium, and uranium in the entire 

volcanic pile supports a crustal contribution and is in 

contrast to the low content of these elements in volcanic rocks that are clearly derived from the mantle or 

from oceanic crust (e.g., Ewart and others, 1977).

Abrupt switches from rhyodacite-quartz latite to 

rhyolite to alkali rhyolite volcanism through time, accompanied by contemporary changes in magmatic 

trace-element associations and types of mineralization, 

suggest that each magma was produced by fusion of a 

different region of the mantle or crust, or that differentiation proceeded in three steps. If the magmas 

originated by fusion, it may have been partial or complete; if complete, differentiation may have been 

necessary to concentrate lithophile elements. Petrographic evidence for an ancestral mafic magma for the 

Drum Mountains Rhyodacite and Mt. Laird Tuff permits a magmatic history beginning with fusion in the 

upper mantle followed by upward migration and major 

contamination with crustal material. The magmas 

were deep enough, or lacked the necessary vapor pressures, to prevent collapse of the roofs of magma 

chambers during most rhyodacite-quartz latite volcanism, except during eruption of the Mt. Laird Tuff. The 

Mt. Laird magma may have migrated to a relatively 

high level in the crust before explosive eruption and 

crustal subsidence, as indicated also by plugs and 

dikes of Mt. Laird composition in the Drum Mountains. The magma chambers of the rhyolitic tuffs were 

shallow enough and the vapor pressure great enough 

for subsidence to occur during eruption. Such rhyolitic 

tuffs have been regarded as differentiates of calcalkalic magma like the Drum Mountains Rhyodacite 

(Lipman and others, 1972).

The extreme enrichment of lithophile metals in the 

alkali rhyolite magmas may have resulted from differentiation or from fusion of Precambrian rocks already 

enriched in these elements. Origin by differentiation 

would require a large volume of mafic magma; there is 

no evidence for mafic magmas nearby except the basalt at Fumarole Butte. Origin by crustal fusion would 

require lithophile-metal-rich rocks nearby. Granitic 

and clastic metasedimentary terranes of Precambrian 

age are exposed beneath Paleozoic rocks nearby, as in 

the Deep Creek Range (Bick, 1966), at Granite Peak 

(Fowkes, 1964), and in the Simpson and Sheeprock 

Mountains (Cohenour, 1959). Partial fusion or remobilization of Precambrian terrane having a high content 

of lithophile metals has been proposed as a source for 

alkali rhyolite magma and mineralizing solutions that 

formed beryllium deposits at Spor Mountain (Moore 

and Sorensen, 1978). Metamorphism of Precambrian 

terrane during Tertiary time has been documented for 

the Grouse Creek and Raft River Mountains, about 

200 km north of the Thomas Range (Compton and 

others, 1977). Reheating of Precambrian granitic terrane may have occurred also at Granite Peak, where 

Precambrian biotite-gneiss and granite are cut by 

beryl-bearing pegmatite veins of Tertiary age only 20 

km north of the Thomas Range (Park, 1968; Moore and 

Sorensen, 1978).

The volcanic history inferred for the Thomas Range
46 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

and Drum Mountains supports the broad outlines of 

volcanism and basin-and-range development as proposed by Lipman and others (1972) and Christiansen 

and Lipman (1972) and elaborated more recently by 

Snyder and others (1976). They proposed that calcalkalic volcanism of early to middle Tertiary age 

developed in response to subduction of the Farallon 

plate beneath the American plate, that calc-alkalic 

volcanism and subduction ended in Miocene time when 

the Farallon plate was consumed west of the basinand-range province, and that the basin-and-range 

structure and fundamentally basaltic volcanism, including bimodal basalt-rhyolite, developed in response 

to crustal attenuation caused by strike-slip movement 

between the Pacific and American plates. Two imbricate, easterly dipping subduction zones, one under the 

present basin and range, and the other under the 

Rocky Mountains, were proposed to account for calcalkalic volcanism of the type represented by rhyodacite-quartz latite and rhyolite in the Thomas Range 

and Drum Mountains.

In their model, Lipman and others (1972, p. 237) 

noted the Thomas Range as one of two areas of volcanic rocks having high K2O contents indicative of an 

anomalously shallow depth to the western Benioff 

zone, thus possibly reflecting a zone where the descending slab was decoupled from the crust. Correct 

estimation of the depth to the Benioff zone in western 

Utah is complicated, however, by the very slight systematic variation of K2O with SiO2 in rhyodacite and 

quartz latite (Hogg, 1972; Leedom, 1974). In any case, 

the switch from calc-alkalic to fundamentally basaltic 

volcanism, represented in western Utah by alkali rhyolite, occurred at 21 m.y., somewhat early for the platetectonic model as refined by Snyder and others (1976).

Alkali rhyolite volcanism was probably contemporaneous with extensional basin-and-range faulting in 

the Thomas Range and Drum Mountains, as indicated 

by widespread faults in all rocks 21 m.y. and older and 

by only a few faults in rocks 6-7 m.y. old. Extensional 

tectonism in the Basin and Range Province is generally regarded as having begun about 16-17 m.y. ago 

(McKee and others, 1970; Noble, 1972). In the Thomas 

Range, basin-and-range faulting included rejuvenation 

of caldera ring fractures, as indicated by evidence for 

recurring movement along the faults of The Dell.

URANIUM OCCURRENCES

Uranium occurs in four diverse settings in the 

Thomas Range: (1) in fluorspar pipes in Paleozoic rocks 

of Spor Mountain, (2) associated with beryllium

deposits in stratified tuff and tuffaceous breccia of the 

beryllium tuff member of the Spor Mountain Formation, (3) in tuffaceous sandstone and conglomerate of 

the beryllium tuff member at the Yellow Chief mine, 

and (4) in veinlets of opaline SiO2 in volcanic rocks of 

all ages.

Production and reserves are limited, but resources 

may be large. Fluorspar has been produced from the 

pipes, but no production of uranium has been reported. 

The uranium deposits in the beryllium tuff member are 

large but of low grade (O.OX percent); no uranium has 

been recovered as a byproduct of beryllium mining. 

The only production of uranium from the area has been 

from the Yellow Chief mine, which produced more than 

90,000 metric tons of ore having a grade of 0.20 percent U3O8 (Bowyer, 1963). Veinlets of uraniferous opal, 

such as those at the Buena No. 1 and Autunite No. 8, 

have been prospected but none has produced ore. Analyses of samples from several prospects indicate a maximum grade of about 0.2 percent uranium (Staatz and 

Carr, 1964, p. 152-154).

All of the uranium occurrences were formed as a 

result of alkali rhyolite volcanism that accompanied 

early basin-and-range faulting; some were formed by 

hydrothermal activity and others by ground-water 

leaching of the products of volcanism. The fluorspar 

pipes, beryllium deposits, and probably the uraniferous opal veinlets were formed by hydrothermal activity that accompanied volcanism. No primary uranium minerals have been found; uranium in fluorspar 

pipes and in nodules and veinlets in the beryllium 

deposits is dispersed in fluorite and opal. Yellow secondary uranium minerals are common. Deposits at the 

Yellow Chief mine consist entirely of secondary 

uranium minerals that were precipitated from ground 

water; the uranium there probably was derived by 

ground-water leaching of nearby hydrothermal 

deposits or uranium-rich volcanic rocks.

URANIUM IN FLUORSPAR PIPES

Uraniferous fluorspar pipes are numerous 

throughout Spor Mountain along faults and fractures 

in Paleozoic rocks; no new study of the pipes was done, 

and the information presented in this report is summarized from Staatz and Carr (1964). Analyses of 

fluorspar show a range of 0.003-0.33 percent uranium 

(Staatz and Carr, 1964, p. 135). Some of the most uraniferous fluorspar (0.10-0.20 percent range of U3O8) occurs near the Bell Hill Mine, at the southeastern end of 

Spor Mountain. Most of the pipes are small, the largest 

reported is 47 by 32 m, and they commonly diminish in 

size at depth (Staatz and Carr, 1964, p. 130).
URANIUM OCCURRENCES 47

URANIUM IN THE BERYLLIUM TUFF MEMBER 

OF THE SPOR MOUNTAIN FORMATION

The beryllium tuff member contains low-grade concentrations of uranium as well as economic deposits of 

beryllium. Epiclastic tuffaceous sandstone, a facies of 

the beryllium tuff member at the Yellow Chief mine, 

contained minable quantities of beta-uranophane; this 

deposit is discussed separately. Deposits of beryllium 

in tuff include the North End, Taurus, Sigma Emma, 

Monitor, Roadside, Fluro, Rainbow, and Blue Chalk 

claims southwest of Spor Mountain, and the Oversight, Hogsback, and Claybank claims in The Dell (fig. 

2). Most of these beryllium deposits contain uranium, 

also. Previous work has indicated that uranium in the 

beryllium tuff member is most abundant in and near 

beryllium ore (Park, 1968; Lindsey and others, 1973), 

but new data presented here indicate that abundant 

uranium commonly occurs separate from beryllium 

ore.

Uranium of hydrothermal origin (as much as 2,000 

ppm) occurs with beryllium in the structure of fluorite 

and opal in zoned nodules in the beryllium tuff member 

(Lindsey and others, 1973; Lindsey, 1978a). Fissiontrack maps of fluorite and opal nodules show uniform 

and zonal distributions of uranium, but no point 

sources that might indicate the presence of pitchblende or other uranium minerals. Such uranium must 

have been deposited with fluorite as a result of the 

breakdown of stable fluoride complex ions of beryllium 

and uranium (Lindsey and others, 1973), and thus is 

regarded to have been introduced by hydrothermal 

fluids. The absence of pyrite, other sulfide minerals, 

and tetravalent uranium minerals, even in unweathered fluorite-silica nodules, indicates that these 

minerals were not stable during hydrothermal 

mineralization.

The conduits for hydrothermal fluids that mineralized the beryllium tuff member are not exposed at the 

beryllium mines southwest of Spor Mountain but may 

be revealed by occurrences of fluorite, beryllium, and 

uranium in prospects on Spor Mountain and in The 

Dell. Uranium occurs with beryllium and fluorite in 

plugs and domes of the Spor Mountain Formation and 

along fault zones that cut the Spor Mountain Formation. Small plugs of porphyritic rhyolite, along faults 

in Paleozoic rocks near the crest of Spor Mountain (fig. 

15), were affected by uraniferous fluorspar mineralization. The fringes of at least two domes of porphyritic 

rhyolite that overrode aprons of tuff were mineralized. 

One such dome, in NW4L sec. 10, T. 13 S., R. 12 W. on 

the southern part of Spor Mountain (fig. 2), intruded 

Paleozoic rocks at a prominent fault intersection and

overrode an apron of tuff previously erupted from the 

vent; mineralizing fluids rose from the vent and 

deposited uraniferous fluorite in brecciated wall rocks 

of Paleozoic age, in the tuff apron, and in the outer part 

of the rhyolite dome. A grab sample of fluorite-rich 

rhyolite from the margin of the dome contained 1,000 

ppm uranium and 50 ppm beryllium. Similarly, mineralized tuff underlies the small dome of porphyritic 

rhyolite south of the Yellow Chief mine in The Dell (fig. 

15). Mineralized gouge of tuff and rhyolite occurs in 

the Dell fault system at the Oversight and Claybank 

prospects in The Dell (figs. 14B, 15). A grab sample of 

fluorite-rich gouge from the fault at the Oversight 

prospect contained about 250 ppm uranium and 700 

ppm beryllium. Nearby, drillholes have penetrated 

mineralized beryllium tuff beneath porphyritic 

rhyolite.

Yellow secondary uranium minerals occur dispersed 

in the beryllium tuff member, and these form ore 

deposits at the Yellow Chief mine. Such accumulations 

are evidently of ground-water origin and may reflect 

leaching and remobilization of magmatic or hydrothermal uranium in the beryllium tuff member. Little is 

known about the possible residence of uranium in other 

minerals, such as smectite, which makes up most of 

the tuff in altered areas.

The overall abundance of uranium and thorium in 

part of the beryllium tuff member of the Spor Mountain Formation is indicated by analyses of drill-hole 

cuttings from southwest of Spor Mountain (sees. 8 and 

9, T. 13 S., R. 12 W.; fig. 20). These cuttings, supplied 

to the U.S. Geological Survey by the Vitro Minerals 

Corp., are from drill holes that penetrated most of the 

tuff section and thus should adequately reflect the 

overall character of the tuff in the area of the drill 

holes. Histograms (fig. 20) are skewed toward high 

values of uranium and thorium, indicating that the 

underlying frequency distributions may have been 

modified by processes that concentrated uranium and 

thorium in the tuff. The original (premineralization) 

content of uranium and thorium in the tuff is probably 

approximated by the modes of about 20 ppm uranium 

and about 80 ppm thorium. The resulting thorium: 

uranium ratio of about 4:1 is a reasonable value for 

these elements in igneous rocks, and it is close to that 

of about 5:1 for the overlying porphyritic rhyolite 

member of the Spor Mountain Formation. The porphyritic rhyolite contains a range of 50-79 ppm 

thorium (average of 62 ppm) and 8-17 ppm uranium 

(average of 11 ppm).

The occurrence of uranium and thorium relative to 

beryllium in tuff is illustrated by analyses of cuttings 

from drill holes at the Roadside beryllium mine (Grif-
48 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

70

60

50

Q- 40

30

20

10

Geometric mean = 29.8 ppm 

Number of samples=237

100 200 

URANIUM IN PPM

300 400

30

20

10

Geometric mean=80.3 ppm 

Number of samples=230

n

o 

B

100 200 

THORIUM IN PPM

300 400

FIGURE 20. Histograms showing the abundance of A, uranium, 

and B, thorium in the beryllium tuff member of the Spor Mountain Formation, as shown by analysis of 237 samples from drill 

holes southwest of Spor Mountain. Analyses by delayed-neutron 

method by H. T. Millard, Jr., A. J. Bartel, R. J. Knight, J. P. Hemming, J. T. O'Kelley, and R. J. White.

fitts and Rader, 1963; and Lindsey and others, 1973) 

(fig. 21). At the Roadside mine, beryllium-fluorite 

mineralization and attendant hydrothermal alteration 

were generally concordant to bedding and were most 

intense in the upper part of the tuff. Feldspathic tuff

makes up the uppermost 18 m and lowermost 9 m of a 

drilled section; incompletely altered argillic tuff lies 

between the intervals of feldspathic tuff (fig. 2L4). The 

upper 6 m of tuff contains beryllium ore (fig. 2 IB) and 

conspicuous fluorite, the underlying 18 m of tuff contains abundant calcite and lithium (in trioctahedral 

smectite), and the lower half of the tuff contains abundant dolomite. This alteration sequence reflects increasing intensity of hydrothermal alteration from bottom to top. Thorium (fig. 21C) is most abundant in and 

near the beryllium ore, but uranium (fig. 2LD) is most 

abundant below the beryllium ore. Thus, at the Roadside mine, uranium tends to occur separately from 

beryllium ore.

The relationships among altered zones, beryllium 

ore, and uranium at the Roadside mine are believed to 

be typical at Spor Mountain, but they do not hold at all 

of the mines. For example, beryllium ore occurs at 

more than one horizon below the top of the beryllium 

tuff member at the Taurus mine, and the maximum 

concentration of uranium occurs with that of beryllium 

in holes drilled through the beryllium tuff by the 

Anaconda Co. (Park, 1968) and by Bendix Field 

Engineering Corp. (Morrison, 1980).

The abundance of both uranium and thorium in drillhole cuttings of tuff supports a magmatic-hydrothermal origin for these elements, but wide variation in 

the thoriumruranium ratio indicates leaching and 

reconcentration of uranium with respect to thorium in 

some of the tuff. If an overall thoriumiuranium ratio of 

about 4:1 is used as a guide, it is evident that tuff having a higher thorium:uranium ratio may have been 

leached of uranium, whereas tuff having a lower 

thoriumruranium ratio may have been enriched. A 

thoriumiuranium ratio of about 1 is associated with 

the highest concentrations of uranium in the drill-hole 

cuttings (fig. 211?), indicating enrichment by either 

hydrothermal fluids, or more likely, by ground water.

Additional evidence that uranium concentrations occur in the beryllium tuff member separately from those 

of beryllium is provided by analyses of drill-hole cuttings of selected mineralized zones (Glanzman and 

Meier, 1979) (fig. 22). These data are from drill cuttings 

provided to the U.S. Geological Survey by the Anaconda Co. from the east side of Fish Springs Flat (sees. 

6, 7, 8, 18,19, T. 13 S., R. 12 W.; sec. 1, T. 13 S., R. 13 

W., and sec. 31, T. 12 S., R. 12 W.). Cuttings of tuff 

were selected from zones showing visible evidence of 

alteration and mineralization, such as clay and fluorite. 

The abundance of BeO, uranium, and thorium in the 

selected cuttings show the expected effect of mineralization. Histograms of BeO and uranium (figs. 22A and 

22B) are skewed positively toward high concentrations, as would be expected if some beryllium and
URANIUM OCCURRENCES 49

2 30 m 30 m

LZZI

Dioctahedral Trioctahedral Potassium 

smectite smectite feldspar

cn

25-50 50-100 100-150 

THORIUM, IN PARTS PER MILLION

30 m -

Hole 

1

Hole 

2

Hole 

3

30 m

B <50 50-100 100-1000 >1000 

BERYLLIUM, IN PARTS PER MILLION

Hole 

1

Hole 

2

n 10-25 25-50 50-100100-150150-200 

URANIUM, IN PARTS PER MILLION

Hole 

3

30 m -

_ <2 2-3 3-4 >4 

E THORIUM: URANIUM

FIGURE 21. Distribution of (A) altered zones (Lindsey and others, 1973), (B) beryllium (Griffitts and Rader, 

1963), (Q thorium, (D) uranium, and (E) thorium:uranium in the beryllium tuff member of the Spor Mountain 

Formation at the Roadside mine. Uranium and thorium analyzed by delayed-neutron method by H. T. 

Millard, A. J. Bartel, R. J. Knight, J. P. Hemming, J. T. O'Kelly, and R. J. White.

uranium had been concentrated by mineralizing fluids. 

Many samples selected as having been mineralized did 

not prove to be of beryllium ore grade, however, so that 

the samples cannot be regarded as representative of 

beryllium ore. Thorium is generally more abundant 

(about 100 ppm) in mineralized tuff than in the tuff 

overall (about 80 ppm), but the frequency distribution 

of thorium is not skewed positively (fig. 22Q. The most

significant aspect of the data is the separate occurrence of uranium from beryllium, as shown by a scattergram (fig. 22D) of 48 samples having 0.1 percent or 

more BeO or 100 ppm or more uranium. The scattergram defines two distinct groups of samples, one 

having high beryllium (0.1-1.42 percent BeO) and one 

having high uranium (100-556 ppm). There is no correlation between beryllium and uranium. This relation
50 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

30

20

10

20

£ 10

m

20

10

109 samples 

(13 blanks)

ra !" I In n n I _Lo_

0.20 0.40 0.60 0.80 1.00 

BeO, IN PERCENT

1.20 1.40 1.60

122 samples

100 200 300 400 500 

URANIUM, IN PARTS PER* MILLION

600

118 samples 

(4 blanks)

0 100 200 300 400 500 

THORIUM, IN PARTS PER MILLION

uuu

0500

^

cc400

UJ 

Q.

cc 300

^

2

-200

ID

2 

cc 100

n

I I

x \

\

\

x\

- Uranium i

mineralized i

tuff ,'

- x /

r,' x/

? /

* /

*^ /Not 

x_Z /plotted ^,-

X """" X X

x* x v x

I x I

I I I I I

EXPLANATION

48 samples having &100 ppm U

or s=o.1 percent BeO

x 1 sample _

* 2 samples

-

-

^ ,. Beryllium mineralized

-" * ^^^ tuff

x A y v x ^^^- ., " __ .

X v A X "~^

v A y v ¥">>

X X l I I * I I \

0.2 0.4 0.6 0.8 1.0 

BeO, IN PERCENT

1.2 1.4 1.6

FIGURE 22. Histograms showing the abundance of BeO, uranium, 

and thorium, and a scattergram showing segregation of BeO and 

uranium in drill cuttings of mineralized zones in the beryllium tuff 

member of the Spor Mountain Formation. Data from Glanzman 

and Meier (1979).

corroborates that seen in the Roadside drill-hole cuttings (fig. 21), where uranium tends to occur below 

beryllium ore.

YELLOW CHIEF MINE

The Yellow Chief mine is in a tilted fault block of 

volcanic rocks in The Dell (fig. 15). Tuffaceous sandstone and conglomerate in the lower part of the 

beryllium tuff member of the Spor Mountain Formation is the host for uranium ore at the Yellow Chief 

mine (fig. IOD). Westward tilting of the fault block has 

brought the Spor Mountain Formation down against 

the Dell Tuff along the footwall of the fault that marks 

the west side of the block. The fault at the west side of 

the Yellow Chief has been active for a long time, both 

before and after 21 m.y. ago; at the pit it downdropped 

a small erosional remnant of the 21-m.y.-old porphyritic rhyolite and beryllium tuff members. Farther 

south in The Dell, the fault is exposed in the Dell Tuff 

but is covered by the Spor Mountain Formation, with 

no displacement of that unit. Small faults having less 

than 1-m displacement cut the tuffaceous sandstone 

and conglomerate in the pit. The stratigraphic section 

at the Yellow Chief has been described in detail under 

the discussion of the beryllium tuff member of the 

Spor Mountain Formation.

The ore at the Yellow Chief is beta-uranophane 

(Ca(UO2 )2 (SiO3 )2 (OH)2 -5H2O), a pale-orange-yellow 

mineral (Bowyer, 1963). It occurs in lenses as much as 

6 m thick and 90 m long that are approximately concordant to the bedding in tuffaceous conglomerate and 

sandstone (fig. IOD). The yellow mineral weeksite 

(K(U02)2(Si2O5)3-4H2O) occurs in lenticular zones less 

than 1 m thick and 10m long in the limestone conglomerate that overlies the tuffaceous sandstone (Staatz 

and Carr, 1964, p. 156). Both of these minerals occupy 

interstices and fractures and coat sand grains and 

clasts in the conglomerate. Schroekingerite 

(NaCa3(UO2)(CO3)3(SO4)F-10H2O) has been reported in 

veinlets in the tuffaceous sandstone (Staatz and Carr, 

1964, p. 157) but has not been found in most of the ore. 

The host rock is partially altered to smectite, and some 

limestone clasts in the conglomerates have altered 

shells, but evidence for intense hydrothermal alteration is lacking. Small bits of earthy yellow jarosite are 

scattered in the tuffaceous sandstone and conglomerate, and coarsely crystalline calcite cement and veinlets are widespread.

Two lenses of ore exposed on the face of the Yellow 

Chief pit (fig. IOD) were sampled to check for 

geochemical haloes and possible associations of trace 

elements that might suggest clues to the origin of the 

Yellow Chief ores. One ore lens sampled contains weeksite; the other, believed to be more typical of the 

Yellow Chief ore, contains beta-uranophane. The ore 

lenses were mapped in the field using a hand-held 

scintillator, and a series of samples was taken across 

the middle and ends of each ore lens (fig. 23, table 7). 

Equivalent uranium (eU), analyzed by a counting technique, and uranium determined by delayed-neutron 

analysis, are about the same in the ore lenses, indicating that uranium there is approximately in 

equilibrium. Equivalent uranium exceeds the concen-
URANIUM OCCURRENCES 51

TABLE 7. Chemical analyses of samples from two ore lenses in the Yellow Chief mine

[Samples are located in figure 23. Values are in parts per million, ell by beta-gamma sealer by H. G. Neiman; U and Th by delayed-neutron method by H. 

T. Millard, Jr., A. J. Bartel, R. J. Knight, C. L. Shields, C. M. Ellis, R. L. Nelms, and C. A. Ramsey; P by specific-ion-electrode method by H. G. 

Neiman and Patricia Guest; Be, Li, Cu, V, Cr, and Pb by six-step semiquantitative spectrographic method by J. C. Hamilton. Ag and Mo not found 

at detection limits of 0.5 ppm and 3 ppm, respectively, by the spectrographic method. Leaders ( ), no data; < , less than]

Sample

No.

i

2 

3 _____

4 _____

5 _____

6 

7 

Sample source

Samples

Ore in conglomerate. 

Barren conglomerate.

 do 

Ore in conglomerate. 

Barren conglomerate.

eU

from

30 

90 

910 

50 

50 

30 

290 

70

Samples of sandstone

1 _____

2 _____

3 _____

4 _____

5 _____

6 

7 _____

g

Outside ore lens.

__do 

Outside ore lens.

__do 

_ doOutside ore lens.

40 

120 

80 

40 

3,130 

610 

2,200 

40

U

vicinity

15 

100 

1,002 

34 

21 

10 

286 

49

Th

of

57 

68

14 

14 

14

21

F Be

weeksite

3,600 

6,100 

800 

1,400 

1,200 

600 

800 

1,300

from vicinity of

21 

76 

39 

16 

3,343 

602 

2,316 

31

28 

35 

26 

22

33

1,200 

1,400 

1,500 

1,000 

1,700 

1,300 

1,400 

1,500

15 

30 

20 

15 

30 

10 

15 

15

Li

lens

200 

1,000 

150 

100 

<50 

<50 

<50 

<50

uranophane

50 

20 

20 

30 

30 

30 

30 

5

<50 

100 

<50 

<50 

100 

100 

<50 

<50

Cu

5 

7 

10 

7 

7 

3 

7 

7

lens

10 

10 

10 

7 

7 

10 

15 

15

V

10 

30 

30 

50 

70 

30 

30 

30

100 

70 

70 

70 

70 

70 

150 

100

Cr

15 

30 

70 

100 

100 

10 

70 

100

70 

50 

100 

50 

30 

30 

70 

70

Pb

70 

70 

300 

300 

100 

50 

70 

30

30 

30 

30 

70 

15 

15 

70 

150

tration of uranium outside the ore lenses because at 

low concentrations (100 ppm and less) the content of 

potassium and thorium accounts for a significant part 

of the eU. Fluorine, beryllium, and lithium, which 

would be expected to indicate intensity of hydrothermal mineralization associated with beryllium deposits, 

show no systematic change in abundance across the 

middle or ends of the ore lenses. Fluorine and lithium 

are most abundant in the bentonite at the top of the pit 

wall, indicating that beryllium-related mineralization 

affected the clay-rich bentonite most. Uranium content 

of the bentonite is 100 ppm immediately above the 

weeksite ore, but only 15 ppm 1 m above the ore. Copper, vanadium, chromium, lead, silver, and molybdenum were analyzed by spectrographic methods to 

determine whether the ores might chemically resemble 

those of the Colorado Plateau; no enrichment of these 

elements, except perhaps lead, and no systematic 

changes were noted for either of the ore lenses.

The uranium deposits at the Yellow Chief mine are of 

uncertain origin. The paucity of fluorite and beryllium 

in the uranium ore suggests that the Yellow Chief 

deposits were not formed by the hydrothermal mineralization that produced the fluorspar and beryllium 

deposits. Overall, the ores do not resemble those of the 

Colorado Plateau, inasmuch as they do not contain

uranous minerals, organic matter, or pyrite, although 

jarosite, noted in the host sandstone, might represent 

oxidized pyrite. Metals that are locally abundant in the 

beryllium ores (such as manganese, lithium, and zinc) 

and in the Colorado Plateau ores (such as vanadium, 

copper, and molybdenum) are not concentrated in the 

Yellow Chief ore. Uranium at the Yellow Chief may 

have been introduced by ground waters bearing silica 

and uranyl carbonate complex ions that dissociated 

when the calcite cement precipitated.

URANIFEROUS OPAL

Uranium occurs in the structure of opaline silica in 

fracture fillings in tuff at many places in the Thomas 

Range. The best known and most uraniferous opal occurs in the crystal tuff member of the Joy Tuff at the 

Autunite No. 8 prospect on the east side of Topaz 

Mountain. Fracture fillings of uraniferous opal occur 

also in the Topaz Mountain Rhyolite above the Autunite No. 8 prospect, at the Buena No. 1 prospect 

(Staatz and Carr, 1964, p. 152-154), and west of Topaz 

Valley; in the Dell Tuff in The Dell; with massive uraniferous opal in the beryllium tuff member of the Spor 

Mountain Formation at many locations; and in the
52 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

Lens of weeksite ore

3r ,n^e

Lens of beta-uranophane ore

FIGURE 23. Locations of geochemical samples (table 7) in two ore 

lenses in the Yellow Chief mine. Location of lenses is shown in 

figure 10D. A, Lens of weeksite ore at the top of limestone pebble 

and cobble conglomerate. B, Lens of beta-uranophane ore in tuffaceous sandstone and conglomerate.

dome of the porphyritic rhyolite member of the Spor 

Mountain Formation near Wildhorse Spring. Reconnaissance suggests that at least minute quantities of 

uraniferous opal can be found in any tuff in the 

Thomas Range. The fracture fillings are generally less 

than 1-2 cm wide and occur both singly and in zones 

less than 10 m wide and 30 m long. None has produced 

ore, and their small size and extent indicate that they 

are not of economic interest themselves.

The opal in fracture fillings is generally zoned, the 

zoning being defined by varying sizes of fibrous 

crystallites oriented perpendicular to the walls of the 

vein. The opal fluoresces bright yellow green under 

ultraviolet light, so that it can be readily distinguished 

from ordinary opal in the Thomas Range, which does 

not fluoresce. Calcite, quartz, fluorite, weeksite, and 

perhaps other secondary uranium minerals are commonly associated with the opal. Fission-track maps 

show that uranium is concentrated in the opal parallel

to the zoning, so that there are large variations in 

uranium content between zones only a fraction of a 

millimeter thick (Zielinski and others, 1977).

The uraniferous opal is probably of hydrothermal 

origin; more specifically, such opal may have 

precipitated in hot springs. Both massive replacement 

opal and opal in fracture fillings occur in hydrothermal 

beryllium deposits in the beryllium tuff member of the 

Spor Mountain Formation. Uranium-lead apparent 

ages indicate that deposition of opal began 21 m.y. 

ago, during or soon after eruption of the host Spor 

Mountain Formation and beryllium-fluorite mineralization, and that deposition of opal continued episodically until 3 m.y. ago (Ludwig and others, 1980). The 

close temporal relation between the onset of opal formation, beryllium-fluorite mineralization, and igneous 

activity indicates a genetic relation, also. Many of the 

fracture fillings show strong zoning of uranium concentration, which suggests wide fluctuation in the supply, rate, or conditions of precipitation of uranium. If 

silica, calcite, and fluorite were the major phases in 

equilibrium with the fluids, a likely mechanism for controlling the rate of precipitation was change in temperature or pressure. Fluctuating temperature or pressure 

and the presence of fluorite support a hydrothermal 

source. For opal at the Autunite No. 8 locality, 

temperatures of deposition in the range of 36 °C or less 

were estimated from oxygen-isotope composition 

(Henry, 1979), but this opal may have been deposited 

far from its presumed hydrothermal source.

URANIUM IN STRATIFIED TUFF OF THE 

TOPAZ MOUNTAIN RHYOLITE

A survey of stratified tuff in the Topaz Mountain 

Rhyolite showed that its original chemical composition 

was nearly identical to that of the associated alkali 

rhyolite and that large areas of the once-vitric tuff had 

been altered to zeolite (Lindsey, 1975). Alteration was 

accomplished by open-system ground-water leaching 

of the major alkalis, Na2O and K2O, and of minor 

elements including fluorine, rubidium, manganese, and 

lead. No analyses of uranium were available for the 

study reported in 1975, but the eU content, which is 

dependent on ^K, thorium, and uranium, was found to 

decline during zeolitization. The tuff was weakly affected by beryllium-fluorite mineralization, also.

A random selection of 20 samples used in the 1975 

survey was analyzed for uranium and thorium by the 

delayed-neutron method by H. T. Millard, Jr., Cynthia 

McFee, and C. A. Bliss of the U.S. Geological Survey. 

These analyses show that the stratified tuff contains 

7-23 ppm uranium (average of 16 ppm) and 44-71 ppm 

thorium (average of 55 ppm). Zeolitic tuff does not con-
URANIUM OCCURRENCES 53

tain appreciably less uranium and thorium than vitric 

tuff, but the small number of samples may be insufficient to detect differences. The analyses do show that 

the overall uranium and thorium content of the tuff is 

very close to that of alkali rhyolite. Seventeen samples 

of alkali rhyolite showed 6-20 ppm (average of 16 ppm) 

and 36-76 ppm thorium (average of 62 ppm). Although 

the wide range of uranium content in some of the tuff 

and rhyolite suggests that uranium has been mobile, 

study of similar tuffs at Keg Mountain (Zielinski and 

others, 1980) indicates that most of the uranium has 

not been flushed from the tuff to be concentrated in ore 

deposits. No concentrations in excess of trace amounts 

of uranium, except in uraniferous opal, are known in 

the Topaz Mountain Rhyolite.

A MODEL FOR URANIUM DEPOSITS AND SOME 

SUGGESTIONS FOR EXPLORATION

The uranium (and other lithophile metal) mineralization of the Thomas Range was associated with extensional block faulting (early basin-and-range faulting) 

and fluorine-rich alkali rhyolite volcanism beginning 

21 m.y. ago. Uranium and other lithophile metals did 

not accompany the caldera cycle, which was complete 

by 32 m.y. ago. A hiatus of 11 m.y. separates uranium 

mineralization at Spor Mountain from the caldera 

cycle; thus uranium clearly is not associated with the 

caldera cycle or with earlier magmas that contained 

abundant sulfur and chalcophile metals.

Uranium in the Thomas Range has been concentrated by (1) magmatic fluids, (2) hydrothermal fluids, 

and (3) ground water. These methods of concentration 

correspond respectively to the (1) initial magmatic, (2) 

pneumatogenic, and (3) hydroallogenic classes of volcanogenic uranium deposits proposed by Pilcher 

(1978). All of these fluids circulated in an environment 

of extensional faulting and alkali rhyolite volcanism, 

however, and their only relation to the caldera cycle 

was their introduction through fractures that were 

formed during cauldron subsidence and later reactivated during basin-and-range faulting. Uranium was 

concentrated by magmatic fluids in the beryllium tuff 

member (about 20 ppm) and in alkali rhyolite (10-20 

ppm) of both the Spor Mountain Formation and the 

Topaz Mountain Rhyolite. Particularly large amounts 

of uranium, beryllium, and fluorine were present in the 

magma of the Spor Mountain Formation, which underlay the vicinity of Spor Mountain and which was 

erupted as the beryllium tuff and porphyritic rhyolite 

members. Uranium was further concentrated in hydrothermal fluids rising through conduits that were 

opened by early basin-and-range faulting. Such 

faulting tapped fluorine and lithophile-metal-rich

fluids in the top of alkali rhyolite magma that underlay 

the vicinity of Spor Mountain. The fluids deposited 

uraniferous fluorite in pipes along faults and fault intersections on Spor Mountain (Staatz and Carr, 1964, 

p. 130-148) and spread laterally into the beryllium tuff 

member, where they deposited disseminated fluorite, 

beryllium, lithium, and uranium (Lindsey, 1977). 

Uranium of hydrothermal origin was deposited in the 

structure of fluorite and opal; no tetravalent uranium 

minerals have been found. Uranium in the structure of 

fluorite at Spor Mountain may be tetravalent, but this 

possibility remains unproven.

The oxidizing chemical environment of hydrothermal uranium mineralization at Spor Mountain contrasts with the reducing chemical environments at 

McDermitt, Nevada-Oregon (Rytuba and Glanzman, 

1978), and Marysvale, Utah (Cunningham and Steven, 

1978). Pitchblende, pyrite, and fluorite are common in 

uranium ores at both McDermitt and Marysvale, but 

pitchblende and pyrite have not been found at Spor 

Mountain. At McDermitt, hydrothermal fluids containing sulfur, fluorine, mercury and uranium mineralized volcanic domes and moat sediments of the McDermitt caldera complex. At Marysvale, uranium-bearing 

hydrothermal fluids contained abundant fluorine, 

which complexed with uranium, and minor concentrations of sulfur, which was in the reduced state; 

uranium probably traveled as a tetravalent fluoride 

complex and was precipitated as the fluids cooled and 

the pH rose by reaction with wall rocks (Cunningham 

and Steven, 1978). At Spor Mountain, uranium probably traveled in hydrothermal fluids as hexavalent 

fluoride and SiO2 complexes; precipitation of uranium 

occurred in response to precipitation of fluorite and 

SiO2 and accompanying breakdown of complex ions. 

Fluorite and SiO2 were probably precipitated by cooling of fluids; reaction of the fluids with carbonate rock 

and porous tuff caused the pH to rise and resulted in 

widespread smectite and potassium-feldspar alteration 

(Lindsey and others, 1973). Ground water has 

redistributed hydrothermal uranium at Spor Mountain 

and concentrated it locally in deposits of secondary 

minerals.

The potential for finding magmatic or hydrothermal 

deposits of pitchblende in the Thomas Range is uncertain. No pitchblende or other tetravalent uranium 

mineral has been found in the near-surface environment exposed at Spor Mountain, indicating that no 

reducing agent or environment was present to precipitate large amounts of uranium. No reducing agent, 

such as pyrite or carbonaceous matter, has been 

observed in the fluorspar pipes or in the beryllium tuff 

member by me, but such reductants might occur in 

deep environments. Some Paleozoic carbonate rocks on
54 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

Spor Mountain are fetid and might serve as a local 

reducing environment along the walls of fluorspar 

pipes and rhyolite vents. The discovery of carbonaceous and pyritic lakebeds in the Mt. Laird Tuff of the 

subsurface northeast of Topaz Mountain indicates yet 

another reducing environment. The most favorable 

areas are near vents for alkali rhyolite lava, where the 

lava has passed through carbonate rocks, or Tertiary 

lakebeds that contain carbonaceous matter. A mineralized plug of porphyritic rhyolite, in the southern part 

of Spor Mountain, and vents of alkali rhyolite at Topaz 

Mountain, Antelope Ridge, and the northeastern 

Drum Mountains fit this criterion.

A deep hydrothermal environment at Spor Mountain 

may be present near a hypothetical pluton of alkali 

rhyolite that was the source of the beryllium tuff and 

porphyritic rhyolite of the Spor Mountain Formation, 

and of the beryllium-fluorite-uranium mineralization 

that followed. The vicinity of intrusion of the 

hypothesized pluton can be predicted from the distribution of the Spor Mountain Formation around Spor 

Mountain and the intensity of mineralization 

associated with that area. Also, Spor Mountain may 

have been lifted trap-door fashion by the pluton after 

eruption of the Spor Mountain Formation 21 m.y. ago. 

The hinge of the trap door would have been west of the 

mountain, and the region of greatest uplift would be 

along the Dell fault system where it displaces Spor 

Mountain Formation against Paleozoic rocks. Maximum upward projection of the pluton might be expected about 1 km west of Eagle Rock Ridge. The 

depth to the pluton is probably unpredictable without 

geophysical data, and the nature of the ores, if any, 

that might be associated with it seems equally difficult 

to predict. If sufficiently reducing environments are 

present at depth, there is a possibility of finding large 

concentrations of uranium and perhaps other metals. 

Anomalous traces of molybdenum, tin, and tungsten 

near the surface, concentrated in manganese oxide 

minerals near beryllium ore, may be generally indicative of other metals that would be expected below 

(Lindsey, 1977).

The Spor Mountain Formation and the Topaz Mountain Rhyolite are the most favorable source rocks for 

uranium deposited by ground waters. In general, the 

alkali rhyolites contain 10-20 ppm uranium, which is 

approximately two to four times as much as other volcanic rocks in the area. The beryllium tuff member 

probably contained 20 ppm uranium at the time of 

deposition and local concentrations of 2,000 ppm or 

more were added in hydrothermal fluorite and silica. 

The stratified tuffs of the Topaz Mountain Rhyolite 

also contained 10-20 ppm uranium. The tuffs of both

formations are porous and were initially glassy, and 

they are between relatively impermeable (though not 

totally impermeable) rhyolite and other rocks, so that 

they provided a good conduit for fluids to leach and 

transport uranium. Thus, it is not surprising that 

many occurrences of secondary uranium minerals, including the Yellow Chief deposits, are in the beryllium 

tuff member of the Spor Mountain Formation. Much 

secondary uranium, probably deposited by ground 

water, occurs in the beryllium tuff member, as shown 

by the separate occurrence of uranium and beryllium in 

the tuff. The Yellow Chief deposit, which consists entirely of secondary uranium minerals, probably was 

formed by ground water also.

The potential for finding oxidized uranium deposits 

formed by ground water in the beryllium tuff member 

of the Spor Mountain Formation is moderate. The scarcity of reductants to precipitate uranium in tuff indicates that deposits of reduced uranium minerals such 

as uraninite and coffinite, which usually occur in 

deposits of the Colorado Plateau and Wyoming types, 

will not be found unless a hydrologic basin favorable to 

entrapment and precipitation of uranium can be 

located; none is exposed. The beryllium tuff member 

southwest of Spor Mountain has been drilled extensively in the search for both beryllium and uranium, so 

it is not likely that large high-grade deposits could 

have been overlooked. There may be much potential, 

however, for uranium-bearing tuff of low grade (O.OX 

percent). The best possibility for discovery of additional uranium deposits formed by ground water may 

be concentrations of secondary uranium minerals like 

those at the Yellow Chief. Favorable host rocks in The 

Dell may extend beneath the Thomas Range, where 

they may be expected in down-faulted blocks covered 

by Topaz Mountain Rhyolite.

Finally, another area of possible but very uncertain 

uranium potential, as yet mostly unexplored, is in the 

subsurface of Dugway Valley north and east of Antelope Ridge. The potential hosts there are stratified tuff 

of the Topaz Mountain Rhyolite and lakebeds of the 

Mt. Laird Tuff. Drilling and testing of Dugway Valley 

to 500-1,000 m will be necessary to evaluate its 

uranium potential. The cauldron environment there is 

a passive factor favoring occurrence of uranium; subsidence of both the Thomas caldera and the Dugway 

Valley cauldron created a depression filled with lakebeds, tuffaceous sediments, and ash-flow tuffs that 

could act as a reducing trap for uranium leached by 

ground water from adjacent highlands of alkali 

rhyolite and tuff in the Thomas Range and Keg 

Mountain.
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Shuey, R. T., Caskey, C. F., and Best, M. G., 1976, Distribution and 

paleomagnetism of the Needles Range Formation, Utah and 

Nevada: American Journal of Science, v. 276, p. 954-968.

Smith, R. L., and Bailey, R. A., 1968, Resurgent cauldrons, in Coats, 

R. R., Hay, R. L., and Anderson, C. A., eds., Studies in 

volcanology (Howell Williams volume): Geological Society of 

America Memoir 116, p. 613-662.

Snyder, W. S., Dickinson, W. R., and Silberman, M. L., 1976, Tectonic implications of space-time patterns of Cenozoic 

magmatism in the western United States: Earth and Planetary 

Science Letters, v. 32, no. 1, p. 91-106.

Sparks, R. S. J., Self, S., and Walker, G. P. L., 1973, Products of ignimbriteeruptions: Geology, v. l,no.3,p. 115-118.

Staatz, M. H., and Carr, W. J., 1964, Geology and mineral deposits 

of the Thomas and Dugway Ranges, Juab and Tooele Counties, 

Utah: U.S. Geological Survey Professional Paper 415,188 p.

Staatz, M. H., and Griffitts, W. R., 1961, Beryllium-bearing tuff in 

the Thomas Range, Juab County, Utah: Economic Geology, v. 

56, no. 5, p. 946-950.

Staatz, M. H., and Osterwald, F. W., 1959, Geology of the Thomas 

Range fluorspar district, Juab County, Utah: U.S. Geological 

Survey Bulletin 1069,97 p.

Staub, A. M., 1975, Geology of the Picture Rock Hills Quadrangle, 

Juab County, Utah: Salt Lake City, University of Utah unpublished M.S. thesis, 87 p.

Steven, T. A., and Lipman, P. W., 1976, Calderas of the San Juan 

volcanic field, southwestern Colorado: U.S. Geological Survey 

Professional Paper 958,35 p.
REFERENCES CITED 57

Stewart, J. H., Moore, W. J., and Zietz, Isidore, 1977, East-west

patterns of Cenozoic igneous rocks, aeromagnetic anomalies,

and mineral deposits, Nevada and Utah: Geological Society of

America Bulletin, v. 88, no. 1, p. 67-77. 

Texas Instruments Incorporated, 1977, Aerial gamma-ray and

magnetic survey of the Delta area, Utah final report: U.S.

Energy Research and Development Administration Open-File

Report GJBX-18(77), [102 p.]. 

Williams, Howell, 1932, The history and character of volcanic

domes: University of California Department of Geological

Sciences Bulletin, v. 21, no. 5, p. 51-146. 

Zentelli, M., and Dostal, J., 1977, Uranium in volcanic rocks from

the central Andes: Journal of Volcanology and Geothermal

Research, v. 2, no. 3, p. 251-258.

Zielinski, R. A., 1978, Uranium abundances and distribution in 

associated glassy and crystalline rhyolites of the western 

United States: Geological Society of America Bulletin, v. 89, 

no. 3, p. 409-414.

Zielinski, R. A., Lindsey, D. A., and Rosholt, J. N., 1980, The 

distribution and mobility of uranium in glassy and zeolitized 

tuff, Keg Mountain area, Utah, U.S.A.: Chemical Geology, v. 

29, no. 1, p. 139-162. '

Zielinski, R. A., Ludwig, K. R., and Lindsey, D. A., 1977, Uraniumlead apparent ages of uraniferous secondary silica as a guide for 

describing uranium mobility, in Campbell, J. A., ed., Short 

papers of the U.S. Geological Survey Uranium-Thorium Symposium, 1977: U.S. Geological Survey Circular 753, p. 39-40.
TABLE 8

[Silica, A) A,, total iron as Fe2O,, CaO, K2O, TiO2, and some MnO by X-ray fluorescence by J. S. Wahlberg: MgO and Na,O by 

atomic absorption by C. A. Gent, V. M. Merritt, and H. G. Neiman. Equivalent uranium (eU) by beta-gamma sealer by H. G. 

Neiman, V. M. Merritt, and C. A. Gent. Manganese and boron through zirconium by six-step semiquantitative spectrographic 

method by R. G. Havens and F. E. Ldchte. For samples Sp-0, Sp-1, and Sp-2, lithium by atomic absorption by V. M. Merritt. 

Uranium and thorium by delayed neutron method by H. T. Millard, Jr., Cynthia McFee, C. A. Bliss, C. M. Ellis, and V. C. 

Smith. N, not detected; L, detected but below the limit of detection; <, less than. Leaders ( ), not determined. Analyses do 

not total 100 percent because HJO and volatile constituents were not determined; also, SiO2 by X-ray fluorescence is subject to 

considerable but unmeasured error]
60 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH 

TABLE 8. Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains

Lower limit

of detection U3 U10A

Composition in

Si02 

A1 203 

Fe 203 - 

MgO 

CaO 

Na20 

K20 

Ti02 

MnO l 0

Ba 

Be 

Co

Co 

Cr \jL

Cu 

Ga 

La 

Li 

Mo 

Nb 

Nd 

Ni 

Pb 

Sc 

Sn 

Sr 

V 

Y 

Yb 

Zr 

eU 

U 

rm_ in 

Footnotes at end of table.

.05;. 0001

20 

1.5 

1 

150 

3

1 

1 

5

30

50

3

10

70

5

10

5

10

5 

7 

10

1

10

10

59 

18 

3.7 

.51

5.7

3.6 

3.8 

.92 

<.05

Composition

N 

1,500 1, 

3 

200 

10

20 

10 

20

150

N

N

30

100

10

30

7

N

1,000 

100 

30

3

200

20 

6 

26

62 

14 

4.2 

1.65

4.5

3.11 

3.3 

.77 

<.05

in parts

N 

000 

2 

L 

10

150 

15 

20

70

N

N

20

70

30

20

15

N

500 

100 

30

3

150

30 

4 

19

U33

percent

63 

14 

3.8 

3.69

3.4

2.98 

2.2 

.60 

.090

per million

L 

1,500 

3 

N 

30

200 

15 

30

70

N

7

15

N

70

30

15

N

700 

150 

20

3

200

30 

4 

19

U37

62 

13 

4.6 

2.82

6.2

2.93 

2.9 

.81 

.077

N 

1,500 

2 

L 

15

150 

7 

20

70

N

N

15

70

30

30

15

N

700 

150 

30

3

150

29

4 

17

U65

61 

15 

4.2 

3.49

4.1

3.43 

2.8 

.90 

.0065

L 

1,500 

1.5 

300 

20

150 

30 

30

150

N

N

15

150

100

20

30

N

1,000 

200 

30

3

300

40 

6 

30
TABLE 8 61

TABLE 8. Chemical analyses of igneous rocks from the Thomas Range and northern Drum MountainsContinued

Si02 

A1 2°3 

Fe 2°3 

MgO 

CaO 

Na20 

K20 

Ti02 

MnO 

Ba 

Be 

Ce 

Co 

Cr 

Cu 

Ga 

La 

Li 

Mo 

Nb 

Nd 

Ni 

Pb 

Sc 

Sn 

Sr 

V 

Y . 

Yb 

Zr 

eU 

U 

TV. _

U233

53 

14 

6.8 

3.6

5.1

2.68 

1.9 

1.0 

.12

N

700

N 

N

15

30

5

15

L

N

L

N

N

7

10

30

N

700 

150

15

2

100

10

2 

i i

U57

Composition in

60 

15 

5.7 

3.24

5.3

2.81 

2.1 

.80 

.090

Composition in parts

L

1,000 

1.5

N

20

70

30

30

70

N

N

10

70

15

20

30

N

1,000 

200

30

3

150

20

4

1 9

U62

percent

59 

15 

5.5 

3.49

5.1

2.90 

2.3 

.80 

.090

per million

L

1,000 

1.5

N

20

70

50

30

70

N

N

10

70

15

70

30

N

1,500 

200

30

3

150

30

4

i *

2 U222

55 

13 

5.2 

2.20

3.9

2.85 

2.7 

.90 

.09

N

1,000

L

N

15

10

20

20

50

N

L

N

N

10

15

15

N

1,000 

150

15

1.5

100

20 

4

1 A

Footnotes at end of table.
62 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

TABLE 8. Chemical analyses of igneous rocks from the Thomas Range and northern Drum MountainsContinued

Si02 

A1 2°3"~

Fe 2°3 

MgO 

CaO 

Na20 

K2 0 

Ti02 

MnO 

U32

74 

11 

.70 

.33

.77

3.1 

4.5 

.067 

.11

U34

70 

12 

1.

1.

3. 

4.

U43

Composition in

76 

13 

4 1.3 

39 .46

6 1.4

18 3.30 

3 3.6 

29 .30 

057 .065

Composition in parts

TJ_ __ .

Be 

Ce 

Co 

Cr 

r>.. __ _

Ga 

Lid

Li 

Mo 

Nb 

Nd 

Ni 

Pb 

o 

Sn 

Sr 

V _ 

Y 

Yb 

Zr 

eU 

U 

Th 

L 

700 

1.5 

L

N

1 

1.5 

15 

70 

N

N

15

N 

N 

30

L 

N

200 

20 

20

2 

70

20 

7 

21

20 

1,000 

2 

N

L

1. 

1. 

30 

70 

N

N

15

N 

N 

30

L 

N

300 

30 

30

3 

70

20 

6 

19

20 

1,000 

3 

N

L

5 7 

5 3 

20 

L 

N

N

15

N 

N 

30

5

N

500 

30 

15

1.5 

100

40 

7 

25

U49

percent

73 

13 

1.6 

.96

1.7

2.90 

3.1 

.30 

.065

per million

L 

700 

3 

N

L

3 

3 

30 

50

N

N

15

N 

L 

30

7 

N

500 

30 

20

2 

100

20 

5 

22

U56

75 

14 

1.3 

.46

1.3

3.30 

3.6 

.20 

.065

30 

1,000 

3 

N

L

7 

1.5 

30 

L 

N

N

15

N 

L 

30

5

N

300 

30 

15

1.5 

100

30 

7 

25

T51-A

73 

12 

1.4 

.31

1.4

2.93 

3.9 

.21 

.022

20 

700 

1.5 

L

L

1 

1.5 

15 

L 

N

L

15

L 

L 

30

L 

N

300 

20 

15

1.5 

50

30 

7 

22
TABLE 8 63

TABLE 8. Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains 

Continued

Sp-0 U141B U141A

Composition in

Si02 

A1 2°3"

Fe 2°3" 

MgO 

CaO 

Na20 

K20 

Ti02 

MnO 

B_ _

Ba 

Be 

v^c

Co 

Cr 

Cu 

Ga 

La 

Li 

Mo 

Nb 

Nd 

Ni 

Pb 

31-

Sn 

Sr 

V 

Y 

Yb 

Zr 

eU 

U _

Th 

70 

12 

1.1 

.37

1.46

3.03 

4.80 

.20 

.039

20 

700 

1 

150

N

5

3

20

70

<10

L 

L

N 

5 

20

5

N

200 

20 

20

1

100

50 

8 

27

74 

13 

1.2 

.47

1.2

3.18 

4.0 

.20 

.039

Composition

30 

1,000 

2 

200

L

10

3

30

100

N

L 

15

70 

L 

50

5

N

300 

30 

20

2

100

50 

9 

23

71 

13 

1.3 

.51

1.7

2.73 

4.2 

.30 

.039

in parts

30 

1,000 

2

L 

L

15

5

30

70

N

N 

20

70 

L 

30

7 

N

300 

30 

15

2

100

40 

8 

23

U155

percent

76 

13 

1.2 

.49

1.1

3.06 

4.4 

.30 

.039

per million

30 

1,500 

2 

L 

L

15

3

20

70

N

N 

20

70 

L 

30

7 

N

300 

30 

20

2

100

30 

8 

24

3 Sp-l

76 

9.9 

.9 

.42

1.44

1.00 

4.82 

.20 

.026

L 

500 

1 

150

N

7

2

15

50

<10

N 

L

N 

L 

15

N 

N

150 

15 

10

1

100

40 

5 

21

3 Sp-2

66 

12 

1.0 

1.23

2.10

1.99 

4.13 

.20 

.013

L 

500 

1 

150 

5

10

2

20

70

20

N 

L

N 

5 

30

5

N

500 

20 

10

1

100

30 

4 

27

Footnotes at end of table.
64 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

TABLE 8. Chemical analyses of igneous rocks from the Thomas Range and northern Drum MountainsContinued

Si02 

A1 2 03

Fe 2°3 

MgO 

CaO 

Na 20 

K2 0 

Ti0 2 

MnO 

B _ 

Ba 

Be 

Ce 

Co 

Cr 

Cu 

Ga 

La 

Li 

Mo 

Nb 

Nd 

Ni 

Pb 

Sc 

Sn 

Sr 

Y 

Yb 

Zr 

eU 

U__ 

Th 

U78

77 

13 

1.3 

.61

.50

.80 

6.4 

.20 

.065

L 

1,000

3

N

L

10

1.5

30

50

N

N

15

N

N

30

5

N

200

30

15 

1.5

70

50

4 

23

U84

Composition in

76 

12 

1.3 

.73

1.1

2.10 

4.0 

.20 

.09

Composition in parts

L 

500

5

N

N

3

1.5

30

50

N

N

15

N

N

30

L

N

200

20

15

1.5

70

50

5 

21

T42-A

percent

70 

12 

1.3 

1.16

1.1

2.13 

3.6 

.25

per million

30

700

3

N

3

2

2

20

30

N

N

20

N

N

30

7

N

200

20

20 

2

70

30

2 

24

T54-A

71 

12 

1.3 

.8

3.5

.79 

4.9 

.23 

.096

L

700

3

L

L

2

1.5

15

50

L

N

15

L

L

30

L

N

150 

20

15 

1.5

70

30

6 

21
TABLE 8 65

TABLE 8. Chemical analyses of igneous rocks from the Thomas Range and northern Drum MountainsContinued

U2A U7A U11A

Composition in

Si02 

A1 2°3~~ 

Fe 203 ~

MgO 

CaO 

Na 20 

K2 0 

Ti02 

MnO 

Ba 

Be 

Co V/C

Co 

Cr 

Cu 

Ga 

La 

Li 

Mo 

Nb 

Nd 

Ni 

Pb 

Sc 

Sn 

Sr 

V 

Y 

Yb 

Zr 

eU 

U 

Th 

68 

11 

1.1 

.16

.98

3.8 

4.7 

.072 

<.05

50 

100

10 

L

N

N 

2 

30

70

150

N

100

70

N

100

N

15

30 

15 

100

15

70

50 

12 

79

64 

12 

.83 

.11

.52

4.2 

4.5 

<.05 

.053

Composition

30 

30

15 

L

N

N 

1 

50

70

300

N

70

70

N

70

N

30

15 

10 

70

10

70

40 

11 

54

72 

13 

.89 

.11

.64

3.9 

4.9 

.050 

.053

in parts

30 

30

10 

L

N

N 

N 

30

70

300

N

100

N

N

30

N

50

15

N 

70

10

70

60 

10 

55

U20A

percent

73 

12 

1.1 

.10

.65

3.8 

4.8 

.079 

.063

per million

30 

150

15 

L

N

N 

N 

50

70

300

N

150

100

N

50

N

15

20 

7 

200

20

150

50 

14 

75

U97

72 

15 

.8 

.20

.80

3.5 

4.5 

<.05 

.039

20 

70

7 

L 

N

3 

3 

100

50

150

N

150

N

N

70

N

20

15 

N 

50

7

100

70 

8 

56

U122

73 

15 

.9 

.14

.20

4.1 

4.3 

<.05 

.065

L 

30

10 

L

N

N 

1 

150

50

700

N

200

N

N

70

L

30

15 

30 

70

10

70

50 

13 

50
0c VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

DO

TABLE 8. Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains 

Continued

T53-TR-A T53-TR-B

Si02 

A1 2°3 

Fe 2°3 

MgO 

CaO 

Na20 

K20 

Ti02 

MnO 

B _ 

Ba 

Be 

Pf» Oc

Co 

Cr 

Cu 

Ga 

La 

Li 

Mo 

Nb 

Nd 

Ni 

Pb 

Sc 

Sn 

Sr 

V 

Y 

Yb 

Zr 

eU 

U 

Th 

76 

14 

1.

3. 

4.

70 

15

15 

L 

N

L

L

30

70

300

3

150

100

L

30

L

30

15 

L 

70

15

150

60 

17 

71

Composition in

74 

13 

5 1.5 

13 .08

54 .47

60 3.73 

9 4.7 

053 .055 

034 .036

Composition in parts

30 

20

7 

L 

N

L

L

30

70

300

3

150

150

L

30

L

30

15 

L 

100

15

150

50 

11 

72

U21C

percent

73 

12 

1.2 

.10

.72

3.8 

5.0 

.072 

.050

per million

30 

150

10 

L 

N

N

1

50

70

700

N

100

70

N

50

N

20

70 

10 

70

10

70

50 

12 

64

U26

72 

11 

1.4 

.08

1.7

4.1 

4.0 

.25 

<.050

20 

50

50 

N 

N

N

N

30

L

300

N

100

N

N

50

N

20

15 

N 

70

10

70

40 

13 

51

U100

74 

12 

.6 

.11

1.7

3.98 

3.5 

<.05 

.090

L 

50

30 

N 

N

3

N

70

L

200

N

70

N

N

70

L

15

70 

7 

70

7

100

40 

15 

53
TABLE 8 67

TABLE 8. Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains 

Continued

Si02 

A1 2°3 

Fe2°3 

MgO 

CaO 

Na20 

K2 0 

Ti0 2 

MnO 

Ba 

Be 

VjC

Co 

Cr 

Cu 

Ga 

Ijd

Li 

Mo 

Nb 

Nd 

Ni 

Pb 

DU

Sn 

Sr 

V 

Y 

Yb 

Zr 

eU 

Th 

U15

74 

12 

1.

.

3.

4.

20

30

15

N 

N

N

1. 

30

N

150

N 

100

N

N

50

N 

10

30 

10

70

7

100

60

15 

56

U16

Composition in

72 

12 

1 1.2 

23 .18

52 .67

5 3.5 

6 4.8 

071 .056 

053 .055

Composition in parts

20

30

15

N 

N

N 

5 1

30

L

150

N 

100

N

N

50

N

15

30 

10

70

7

100

50 

17

53

U72

percent

78 

11 

.80 

.09

.80

2.68 

4.8 

.090 

.065

per million

L

50 

10

L 

N

30

1 

50

70

L

3 

70

N

N

70 

L

15

70 

L

30

5

150

60

1 Q

68

T50-TR-A

75 

12 

.90 

.20

1.4

1.95 

6.6 

.069 

.100

L

15 

7

L 

N

L

L

15

50 

L

L 

50

L

L

30 

L

N

300 

L

20

5

70

60

19 

59

T50-TR-B

76 

12 

.84 

.07

.66

3.51 

4.6 

.073 

.036

L

7

7 

L

N

L 

L

20 

50

L

3

50

N

L

50 

L

N

20 

L

20

5

70

50

17 

61
68 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

TABLE 8. Chemical analyses of igneous rocks from the Thomas Range and northern Drum Mountains 

Continued

T52-TR-A T52-TR-B T13-TR-A T13-TR-B T21-TR-A T21-TR-B

Composition in percent

Si02 

A1 2°3 

Fe 20 3 -

MgO 

CaO 

Na20 

K2 0 

Ti02 

MnO 

B _

Ba 

Be 

VjC

Co 

Cv

Cu 

Ga 

La 

Li 

Mo 

Nb 

Nd 

Ni 

Pb 

DC

Sn 

Sr 

V 

Y 

Yb 

Zr 

eU 

U 

Th 

75 

12 

.97 

.16

1.4

3.25 

4.7 

.092 

.048

L 

20

7 

L 

N

L 

L 

15

70

100

3 

50 

L 

L 

50

L

N

50 

L 

20

5

100.

50 

18 

61

75 

12 

1.0 

.11

2.5

3.14 

4.7 

.110 

.047

Composition

L 

70 

7 

L 

N

L 

L 

15

70

100

3 

30 

70 

L 

50

L 

N

100 

L 

20

3

70

50 

17 

65

75 

12 

.98 

.06

1.0

3.71 

4.7 

.079 

.071

in parts

L 

10 

15 

L

N

L 

L 

20

50

100

5 

70 

L 

L 

50

L 

N

15 

N 

30

7

100

60 

20 

64

77 

12 

1.00 

.06

.86

3.58 

4.8 

.095 

.053

per million

L 

15 

10 

L 

N

L 

L 

20

70

100

5 

70 

L 

L 

50

L

N

15 

L 

30

7

70

60 

17 

74

73 

12 

1.0 

.15

1.3

3.51 

4.7 

.087 

.061

L 

10

7 

L 

N

L 

L 

20

70

100

7 

70 

L 

L 

50

L 

N

30 

L 

30

7

100

50 

19 

73

62 

10 

.93 

.19

1.6

3.49 

4.9 

.087 

.053

L 

20

7 

L 

N

L 

L 

20

70

100

3 

70 

L 

L 

50

L 

N

70 

7 

20

5

100

50 

18 

76
TABLE 8 69

TABLE 8. Chemical analyses of igneous rocks from the Thomas Range and northern Drum MountainsContinued

T40-TR-A T40-TR-B SP-13 U74 T03-TR-A T03-TR-B

Composition in percent

Si02 

A1 2°3~"

Fe 2°3" 

MgO 

CaO 

Na20 

K20 

Ti02 

MnO 

 

Ba 

Be 

Ce 

Co 

Cr 

Cu 

Ga 

La 

Li 

Mo 

Nb 

Nd 

Ni 

Pb 

Sc 

Sn 

Sr 

V 

Y 

Yb 

Zr 

eU 

U 

Th 

64 

10 

.92 

.12

.66

3.39 

4.6 

.090 

.043

L 

5

7

L

N

L 

L

20

70

L

5

50

L

L

50

L

N

50 

L 

30

7

100

50 

15 

67

62 

11 

1.0 

.18

.66

3.34 

4.9 

.100 

.050

Composition

L 

7

7

L

N

L 

L

15

70

L

3

50

L

L

50

L

N

50 

L 

30

5

100

50 

15 

68

74 

11 

.68 

.02

.55

3.45 

4.2 

.087 

.032

in parts per

L 

15

5

150

N

L 

1

20

70

50

5

15

N

N

50

L

N

10 

L 

50

5

100

60 

13 

63

75 

11 

1.0 

.19

1.3

2.76 

4.4 

.10 

.039

million

L 

200

5

L

N

L 

15

30

70

L

3

30

N

N

70

N

N

70 

20 

50

5

100

40 

6 

36

71 

12 

.91 

.04

.46

4.07 

4.5 

.050 

.12

20 

5

7

N

N

L 

L

20

L

100

3

50

L

L

70

N

N

7 

N 

70

7

70

50 

20 

51

74 

12 

.96 

.05

1.1

3.58 

4.7 

.070 

.050

L 

10

7

N

N

L 

L

20

L

L

3

50

L

L

70

L

N

15 

L 

50

7

70

40 

16 

59
70 VOLCANIC ROCKS AND URANIUM IN THE THOMAS RANGE AND DRUM MOUNTAINS, UTAH

TABLE 8. Chemical analyses of igneous rocks from the Thomas Range and northern Drum MountainsContinued

No. Rock type Location

U3 Drum Mountains Rhyodacite, flow SE1/4SW1/4, sec. 2, T. 13 S., R. 12 W.

U10A do SE1/4SW1/4, sec. 35, T. 12 S., R. 12 W.

U33 do NW1/4NW1/4, sec. 21, T. 13 S., R. 11 W.

U37 do SW1/4NW1/4, sec. 30, T. 13 S., R. 12 W.

U65 do NW1/4NW1/4, sec. 16, T. 14 S., R. 11 W.

U233 Intrusive diorite SW1/4SE1/4, sec. 36, T. 14 S. , R. 11 W.

U57 Mt. Laird Tuff SE1/4NE1/4, sec. 21, T. 14 S., R. 11 W. 

U62 do NE1/4SW1/4, sec. 16, T. 14 S., R. 11 W.

U222 Intrusive porphyry, slightly SW1/4SW1/4, sec. 35, T. 14 S., R. 11 W. 

altered, equivalent to 

Mt. Laird Tuff.

U32 Joy Tuff, crystal-tuff member NW1/4NW1/4, sec. 22, T. 13 S., R. 11 W.

U34 do SE1/4NE1/4, sec. 20, T. 13 S., R. 11 W.

U43 do SE1/4SW1/4, sec. 17, T. 13 S., R. 11 W.

U49 __do SW1/4SW1/4, sec. 5, T. 13 S., R. 11 W.

U56 do SE1/4NE1/4, sec. 21, T. 14 S. , R. 11 W.

T51-A do NE1/4NW1/4, sec. 10, T. 13 S., R. 11 W.

Sp-0 Joy Tuff, black glass tuff SW1/4SE1/4, sec. 25, T. 13 S., R. 11 W.

member, basal black welded

zone. 

U141B do NW1/4NE1/4, sec. 36, T. 13 S., R. 11 W.

U141A Joy Tuff, black glass tuff NW1/4NE1/4, sec. 36, T. 13 S., R. 11 W.

member, middle gray welded

zone. 

U155 do NW1/4SW1/4, sec. 6, T. 14 S., R. 11 W.

Sp-1 Joy Tuff, black glass- tuff SW1/4SE1/4, sec. 25, T. 13 S., R. 11 W.

member, upper unwelded zone. 

Sp-2 do SW1/4SE1/4, sec. 25, T. 13 S., R. 11 W.

U78 Dell Tuff NE1/4NW1/4, sec. 26, T. 11 S., R. 11 W. 

U84 do SE1/4NE1/4, sec. 2, T. 13 S., R. 12 W. 

T42-A do NW1/4NW1/4, sec. 36, T. 12 S., R. 12 W. 

T54-A do NE1/4SE1/4, sec. 26, T. 12 S. , R. 12 W. 

U2A Spor Mountain Formation, NW1/4NW1/4, sec. 11, T. 13 S., R. 12 W.

porphyritic rhyolite member,

flow.

U7A do SE1/4SW1/4, sec. 36, T. 12 S., R. 12 W. 

U11A do SE1/4SW1/4, sec. 35, T. 12 S., R. 12 W. 

U20A do NW1/4SE1/4, sec. 9, T. 13 S., R. 12 W.
TABLE 8

TABLE 8. Chemical analyses of igneous rocks from the Thomas Range and northern Drum MountainsContinued

71

No. Rock type Location

U97 __do NW1/4NE1/4, sec. 25, T. 12 S., R. 12 W.

U122 do SW1/4SE1/4, sec. 25, T. 12 S., R. 12 W.

T53-TR-A do SW1/4NE1/4, sec. 8, T. 13 S., R. 12 W.

T53-TR-B do SW1/4NE1/4, sec. 8, T. 13 S., R. 12 W.

U21C Spor Mountain Formation, NE1/4NW1/4, sec. 10, T. 13 S., R. 12 W. 

porphyritic rhyolite member, 

plug.

U26 Spor Mountain Formation, NW1/4SE1/4, sec. 9, T. 12 S., R. 12 W.

porphyritic rhyolite member,

extrusive dome. 

U100 do NE1/4SE1/4, sec. 8, T. 12 S., R. 12 W.

U15 Topaz Mountain Rhyolite, older SW1/4SW1/4, sec. 14, T. 12 S., R. 12 W.

flow.

U16 do NW1/4SE1/4, sec. 22, T. 12 S., R. 12 W. 

U72 do NE1/4NE1/4, sec. 19, T. 12 S., R. 12 W. 

T50-TR-A do NE1/4NE1/4, sec. 28, T. 12 S., R. 11 W. 

T50-TR-B do NE1/4NE1/4, sec. 28, T. 12 S., R. 11 W.

T52-TR-A Topaz Mountain Rhyolite, younger NW1/4SW1/4, sec. 15, T. 13 S., R. 11 W.

	flow.

T52-TR-B do NW1/4SW1/4, sec. 15, T. 13 S., R. 11 W.

T13-TR-A do SW1/4NE1/4, sec. 1, T. 13 S., R. 12 W.

T13-TR-B do SW1/4NE1/4, sec. 1, T. 13 S., R. 12 W.

T21-TR-A do NE1/4SE1/4, sec. 28, T. 13 S., R. 11 W.

T21-TR-B do NE1/4SE1/4, sec. 28, T. 13 S., R. 11 W.

T40-TR-A Topaz Mountain Rhyolite, younger NW1/4NE1/4, sec. 16, T. 13 S., R. 11 W.

flow.

T40-TR-B do NW1/4NE1/4, sec. 16, T. 13 S., R. 11 W. 

Sp-13 do NW1/4NW1/4, sec. 16, T. 13 S., R. 11 W.

U74 Topaz Mountain Rhyolite, dome. NE1/4NW1/4, sec. 26, T. 12 S., R. 11 W.

T03-TR-A do SW1/4SW1/4, sec. 14, T. 13 S., R. 11 W.

T03-TR-B do SW1/4SW1/4, sec. 14, T. 13 S., R. 11 W.

limits of detection for MnO are 0.05 percent by X-ray fluorescence; 0.0001 by 

six-step spectrographic method.

2Rock has been partly altered by hydrothermal fluids. 

3 Alkali metals may have been leached by ground water.

U.S. GOVERNMENT PRINTING OFFICE: 576-034 1982