GEOLOGIC COLUMN

WE8TEHM EASTERN ,_^~^ -•" Formation contact Alluvium Qoii y Qal | Alluvium _. Fault -< Strike and dip of beds Sr j Sevier River formation Axis of flow folds Mt. Belknap Series Mt. Belknap Series Mar | Groy Hills Topogaphy from Groy Hills gloss Quadrangle, U.S. Geological Survey Gold Mt. Mgt Gray Hills arkosic ss. MtBelknop rhyolite [«br MM | Beover Hill tuff Indian Hollow tuff McTI Crescent Hill rhyolite Staley Pasture tuff [MSI Mit | Indian Hollow tuff Kimberly rhyolite JMkr Glassy dike Mkr| Klmberly rhyolite Mir j Indian Hollow rhyolite Delano Peak Series Delano Peak Series Delano Peok latite [opl Dpi 1 Delano Peak latite Bullion Canyon Series Bullion Canyon Series Tbc Bullion Canyon vofcanics (undivided)

TUSHAR FAULITIE ^FLA TSOUTH

GEOLOGIC MAP OF THE BEAVER CREEK AREA WEST OF MARYSVALE, MOLLOY AND KERR, PLATE 1 Geological Society of America Bulletin, volume 73

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 MARTIN W. MOLLOY Texaco, Inc., P. O. Box 3247, Ventura, Calif. PAUL F. KERR Dept. Geology, Columbia University, New Yor^, N. Y.

Tushar Uranium Area, Marysvale, Utah

Abstract: A thick sequence of late Tertiary volcanic Thin section and clay-mineral investigations yield tuff covers portions of the Tushar Plateau in south- information about the primary structure of the central Utah. Rhyolite of similar age fills parts of volcanic units and the chemical and mineralogical the valley to the east. Mines along reaction to alteration. Zeolites, quartz, and feldspar the valley produce uranium ore from deposits represent primary alteration features. Variations in genetically related to the tuff and rhyolite. The kaolinite, montmorillonite, and mixed-layer clay threefold relation of mountain tuff, valley rhyolite, suites reflect differences in the intensity of second- and uranium mineralization has been investigated ary alteration along fault zones. by field mapping and laboratory study of the Quartz monzonite intrusives and associated ore Mount Belknap volcanic group. bodies lie along a major east-west zone of weakness Field mapping defines changes in the volcanic which passes through Marysvale, Utah. Younger stratigraphy and shows the interfingering of the fault systems cut across this zone and define the regional tuff with the local valley . Primary north-south axis of the Tushar Range. Uranium features of compaction, intergradation, and flow mineralization and related alteration phenomena folding suggest unusual modes of air-fall aggregation have formed fissure veins along these younger fault and subsequent gravity flow for the emplacement trends. of some of the volcanic units.

CONTENTS Introduction 212 Figure Acknowledgments 212 1. Index map of the Marysvale area, Utah . . . 213 Rock units 214 2. Diagrammatic section of the Tushar Range . 215 Basement 214 3. Thin sections of volcanic and sedimentary rock Sedimentary sequence 214 units 216 Bullion Canyon volcanic group 214 4. Thin sections of Mount Belknap rock units. . 218 Delano Peak latite 214 5. A Gray Hills rhyolite flow 220 Dry Hollow latite 215 6. Volcanic rocks from the Tushar Range . . .221 Monzonitic intrusives 215 7. Air view of the crests of the Tushar Range from Delano Peak arkose 215 the south 222 Mount Belknap volcanic group 215 8. The Tushar Range from the west, above Blue Introduction 215 Lake 223 Indian Hollow rhyolite 216 9. Unconformity and Delano Peak latite at the Indian Hollow tuff 217 base of the Mount Belknap volcanic group 225 Crescent Hill rhyolite 217 10. Air view of Bullion Canyon volcanic rocks . . 226 Gray Hills arkose 219 11. Range in units of the Mount Belknap volcanic Gray Hills rhyolite 219 group 227 Beaver Hill tuff 219 12. Fault pattern and monzonite intrusives in the Staley Pasture tuff 220 Marysvale area 229 Gold Mountain tuff 222 Plate Facing Mount Belknap rhyolite 222 1. Geologic map of the Beaver Creek area west of Glass in the Mount Belknap volcanic group . 222 Marysvale, Utah 211 Clear Creek tuffs 224 Following Structure 224 2. Compaction features of the Marysvale Valley Tushar Range 224 volcanic rocks Emplacement of the volcanic units 226 3, Flowage features in the Mount Belknap rhyo- Basement latites 226 lites 220 Mount Belknap volcanic group 226 4. Flow glass, dike glass, and alteration phenomena Fault patterns 228 5. Gold Mountain and Joe Lott tuff units .... Mineralization and alteration 230 Facing Alteration features 230 6. Geologic map of the Clear Creek area north of Clay mineralogy 230 Marysvale, Utah 226 Uranium mineralization 232 7. General volcanic and sedimentary features. . . 230 Summary and conclusions 233 8. Thrust fault along Clear Creek, and silica concre- References cited 234 tions from dikes 231 Geological Society of America Bulletin, v. 73, p. 211-236, 12 figs., 8 pis., February 1962 211

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 212 MOLLOY AND KERR—TUSHAR URANIUM AREA, MARYSVALE, UTAH

Creek (Fig. 1) and 25 miles to the southeast INTRODUCTION near Antimony, Utah, tuff similar to the Mount The Tushar Range of southern Utah, a mod- Belknap is found. A strip along Beaver Creek erately tilted, sedimentary block submerged by (PI. 1), from the Tushar crests to the Sevier Tertiary volcanic rocks, forms a transition be- River valley and joining the Marysvale map tween the and the Basin and published by Kerr et al. (1951) provides an op- Range provinces. portunity for continuous observation of the Since the 1860's the Tushar Range has pro- Mount Belknap units. This area includes sev- duced diverse mineral groups ranging from eral recently discovered deposits of uranium potassium to gold. In several areas a dacite host near the summit of Mount Belknap. Mine rock contains veins of gold, silver, and copper roads, bulldozed across the crests of the Tushar localized by minor tensional fractures parallel Range, afford an unusual opportunity to study to the axis of the range. Nearby, similar fissures a particularly mountainous area hitherto ac- contain large deposits of vein alunite. Where cessible only on foot or horseback. erosion has exposed the underlying Plateau In this paper the color nomenclature of the sediments, related faults cut replacement de- Rock-color Chart (Goddard, 1948) has been posits of lead, zinc, silver, mercury, and selen- followed. The designation of volcanic units as ium. tuff and rhyolite rather than ignimbrite has Late Tertiary intrusions of quartz monzonite been adopted in order to avoid a decision con- are scattered throughout the Tushar Range. cerning origin, since the method of formation Fracture zones in these intrusives have furn- and emplacement of the Mount Belknap units ished significant production of uranium in re- is controversial. cent years. X-ray spectrochemical (fluorescence) analy- This long history of mining has led to a series ses of volcanic material from the Tushar Range of geologic reports on the area, beginning with have been published (Molloy and Kerr, 1960). C. E. Dutton in 1880. More recent work by Some of the conclusions from that paper con- Eardley and Beutner (1934) has reviewed the cerning stratigraphic correlation and alteration geomorphology, and Callaghan (1939) has de- phenomena are incorporated in this report. scribed the volcanic sequence in general terms. In 1951 the Division of Raw Materials of the ACKNOWLEDGMENTS U. S. Atomic Energy Commission sponsored a This study has been supported by the U. S. joint study with Columbia University on the Atomic Energy Commission, Divisions of Re- uranium mineralization near Marysvale, Utah. search and Raw Materials; a graduate fellow- The field work and laboratory study of this ship of the Union Carbide Ore Company, project (Kerr, Brophy, Dahl, Green, and division of Union Carbide Corporation; and re- Woolard, 1957) and a subsidiary report (Bethke search grants from the Department of Geology, and Kerr, 1954) investigated the sedimentary Columbia University. The co-operation of basement, volcanic units, and alteration phe- Messrs. Ben Bowyer of the Atomic Energy nomena accompanying mineralization in the Commission and Bertram L. Myerson is grate- Sevier River Valley. The relation between fully acknowledged. Mr. Allen Taylor of Phil- glassy dikes and uranium ore in the Marysvale lips 66 Petroleum Company has aided in dis- mines led to the present investigation of the cussing the field relations of the Mount Belknap Mount Belknap volcanic rocks. volcanic rocks. Mr. Frank Nallick has been The Mount Belknap volcanic group consists particularly helpful with his knowledge of of extensive deposits of tuff with local rhyolite uranium prospects in the Tushar Range and covering the crests of the Tushar Range and Mr. LeRoy Kmetzsch has kindly provided sequences of rhyolite with local tuff filling por- similar information on the Clear Creek area. tions of the Sevier River valley on the eastern The authors are indebted to Mr. Charles Steen side of the range. Glass flows are found at sev- of Utex Mining Company and Mr. Loren eral levels in the valley and glassy dikes, similar Atherlee for providing aircraft for aerial re- to those in the uranium mines, transect tuff and connaissance of the Tushar Range. rhyolite in the mountain and the valley vol- The many courtesies of the people in Marys- canic units. Uranium deposits are also found vale during the 2 field seasons are deeply ap- among the highest peaks of the range in tuff of preciated. Special thanks are extended to Mrs. Mount Belknap age. Bernice Palmer and Messrs. Harvey Gibbs, Fifteen miles to the northwest along Clear Max Parker, and Richard Ward. During both

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 ACKNOWLEDGMENTS 213

>-' = ar reacreev' ail yV Xl5 f l; I / ( bJf,MJM/JMtM/S///MS/A',;?,js/jsjs/j///ss///s////j/js,:'•m 1 -,.- ^ V^ ^-^^ / L;—^.-!-;:1^ 1 \^ SEVIER CO. ^

Figure 1. Index map of the Marysvale area, Utah. The areas covered by geologic maps of Beaver Creek (PI. 1) and Clear Creek (PI. 6) are indicated. Mount Belknap tuff extends south from Clear Creek through the Tushar Range toward Antimony, Utah.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 214 MOLLOY AND KERR—TUSHAR URANIUM AREA, MARYSVALE, UTAH

field seasons, many discussions with Mr. Richard direction successively younger units are en- Kennedy of the Missouri School of Mines, who countered. Approximately 5000 feet of alkalic is mapping the adjoining Bullion Canyon- latites and andesites including flows, agglom- Marysvale Valley area, have provided valuable erates, and tuffs of the Bullion Canyon volcanic insight into the regional distribution of Mount group (Callaghan, 1939) lies upon the erosional Belknap and Bullion Canyon units. Drs. Harry unconformity above the sedimentary sequence. M. Dahl, Jack Green, and Phillip L. Merritt The volcanic rocks have been intruded by have critically reviewed the manuscript and quartz monzonite and granite stocks, and latite have made many helpful suggestions for which porphyry sills. An erosional surface with con- the authors are grateful. siderable relief has cut into these volcanic rocks and intrusives to form the base for late Tertiary ROCK UNITS rhyolitic tuffs and flows. A particularly widespread member of the Basement Bullion Canyon sequence is the massive green- Sedimentary sequence. The Tushar fault ish-gray latite seen at Butler Beck workings forms the western side of the Sevier Valley along Deer Creek, at Indian Creek on the west- graben southwest of Marysvale, Utah, where ern side of the Tushar Mountains, and in the more than 2000 feet of Colorado Plateau sedi- large fault block on the north side of Beaver ments is exposed. Although associated mineral- Valley. Indistinct layering (8-10° SE dip) may ization has changed sandstones to quartzite and be distinguished from the air and in oblique air silicified the limestones, the strata are believed photographs. Flowage alignment of pheno- to represent units ranging from Mississippian crysts is absent in the dense latite. Although the to Jurassic. Included are familiar names such as rock appears unaltered on a fresh surface, thin Supai, Coconino, Kaibab, Moenkopi, Chinle, section examination reveals sericite, chlorite, and Carmel formations (Bethke and Kerr, clay minerals, magnetite, and calcite. The sodic 1954). orthoclase crystals are usually zoned; ortho- An erosional unconformity transects the clase is abundant; and augite is present. The Carmel remnants and the Navajo formation. groundmass is crystalline, either as fine feld- The overlying volcanic rock is separated into spar-quartz or as sericite-chlorite-clay altera- two mineralogically distinct groups by a pro- tion. In general, the Bullion Canyon units are found erosional surface. In the Pavant Range considerably more altered than the younger to the north these volcanic rocks overlie ex- volcanic rocks. tensive exposures of the Wasatch formation Delano Pea\ latite. In the Marysvale area which is believed to be Eocene and perhaps the Delano Peak latite overlies the erosional Oligocene. The oldest volcanic unit is therefore unconformity at the top of the Bullion Canyon younger than this portion of the Tertiary. The sequence. At the divide between Bullion and diagrammatic relations of units within the Beaver canyons the unit is a purple to red Tushar Range are shown in Figure 2. latite porphyry with numerous white feldspar Geochemical dating in progress at Brook- phenocrysts. The 2-3 mm phenocrysts are pre- haven National Laboratory (Bassett, Kerr, dominately oligoclase, partly altered, twinned, Schaeffer, and Stoenner, I960) has provided zoned, and accompanied by orthoclase, magne- absolute age values for the Marysvale volcanic tite, biotite, and hornblende which is rimmed rocks. Potassium-argon measurements indicate with iron oxide. The phenocrysts lie in a that glass, biotite, and feldspar samples range groundmass of brownish glass with minute from middle Oligocene to late Miocene. The crystals which mark a distinct flow structure. Mount Belknap units give dates clustered Many of the phenocrysts are sericitized and about 15-19 million years, suggesting late Mio- contain patches of calcite, and there are several, cene emplacement. Determination of the small, zeolite-filled cavities. Pb206-U238 ratio and radon leakage for sooty To the east, near the mouth of , pitchblende from Marysvale (Prof. J. L. Kulp, the Delano Peak latite merges with flows personal communication, 1961) dates the equivalent to the Dry Hollow latite. These are mineralization as Pliocene (9 million years, medium-gray to grayish-purple latites with ±2 million years). abundant, coarse (3-5 mm), glassy sanidine Bullion Canyon volcanic group. The sedi- phenocrysts. At the mouth of the valley there mentary basement exposed near Marysvale is are approximately equal proportions of sanidine moderately tilted to the northwest, and in that and albite-orthoclase. To the east, thin sections

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 ROCK UNITS 215

show a preponderance of sanidine (Dry Hollow berly on the east and north. Contact meta- latite), whereas to the west, albite and ortho- morphic effects are limited to narrow chill clase are most abundant, followed by oligoclase zones where the intrusive has cut Bullion Can- and orthoclase (Delano Peak latite). yon latite. Where the more porous tuffs of this Dry Hollow latite. In the Sevier River val- sequence are encountered, the alteration halo ley, east of the Tushar Range, widespread latite is extensive and colorful, consisting of hematite- flows (200-600 feet thick) lie at the base of the and limonite-stained clay and alunite. Mount Belknap and Joe Lott units. These Delano Pea\ ariose. At several points in flows are commonly purplish-gray but include Marysvale Valley a dusky red-purple arkose medium-gray, dusky-purple, reddish-brown, overlies the erosional unconformity at the top and dusky-brown members. Large, clear, frac- of the Bullion Canyon sequence. At Bear Hol-

Mt Belknop rhyohle Groy Hills fhyolite

-t- +• + +Bullion Canyon volconics +- +• + +• +

Figure 2. Diagrammatic section of the Tushar Range. The original structure of the range, uncomplicated by faulting, is represented from the Tushar peaks on the west to Marysvale Valley on the east, a distance of approximately 14 miles. The members of the Mount Belknap volcanic group are identified. Tuff units predominate in the mountainous west, whereas rhvolite is widespread in the eastern valley.

tured sanidine phenocrysts predominate in the low in Deer Creek, a mine road has cut through glassy groundmass and are accompanied by a 30-foot exposure of this dark, laminated sedi- orthoclase and oligoclase. Dark-brown biotite ment (Fig. 3B). In Beaver Valley this unit ap- is abundant and shows reaction nms of magne- pears as a thin lens on the north side, northeast tite. Quartz, hornblende, apatite, and augite of Copper Belt Peak. The erratic distribution, are also noted. Some larger sanidmes (up to 6 variable thickness, and coarse laminae suggest mm long) enclose small plagioclase crystals. accumulation of the arkose in stream channels Monzonitic intrusives. Marysvale lies near or ponds. the eastern end of an intrusive belt which ex- tends from Monroe, Utah, toward Pioche, Mount Eettyiap Volcanic Group Nevada. In the Marysvale area the monzonite, Introduction. The older, red phase of the quartz monzonite, and granite stocks or plugs Mount Belknap represents outpourings of intrude the Bullion Canyon volcanic group rhyolite and tuff from local vents in Marysvale (Fig. 3A) and are cut in turn by the pre-Mount Valley. Several may have been in eruption at Belknap erosional surface. There are several the same time, and the physical nature of the large intrusive masses in Indian Creek on the eruptives from a single vent probably varied western side of the Tushar Range and several from time to time. The red phase is a mosaic of small bodies along Deer Creek and near Kim- rhyolitic and tuffaceous units which are rela-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 216 MOLLOY AND KERR—TUSHAR URANIUM AREA, MARYSVALE, UTAH

lively uniform in composition. They represent foot thick Indian Hollow rhyolite, a dense, a series of local eruptions from a single magma moderate pink, porphyritic flow characterized chamber, breached by the graben faulting by 1-2 mm phenocrysts of golden biotite. which formed the Sevier River valley. Numerous phenocrysts of potassium feldspar, Indian Hollow rhyolite. The basal rhyolite with minor twinned plagioclase and quartz, of the Mount Belknap sequence is the 1000- give the rock a mottled appearance. Flowage

A

5 mm 5 mm Figure 3. Thin sections of volcanic and sedimentary rock units A. Monzonitic intrusive. A specimen from the west side of the Tushar Range shows coarse andesine (An), anhedral orthoclase (O), augite (Au), and magnetite (M) with secondary growths of biotite (B). B. Delano Peak arkosic sandstone. Poorly sorted, coarsely laminated, subangular stream debris includes decayed orthoclase (O) and oxidized magnetite (M) in a matrix containing limonite blebs (L) and recrystal- lized calcite (C). Crossed nicols C. Indian Hollow rhyolite. A highly porphyritic unit with coarse, clear, zoned sanidine (S), corroded golden biotite (B), and minor magnetite (M) in a glassy, slightly spherulitic matrix showing vague flowage lineation D. Indian Hollow tuff. Large, lenticular glass masses (G) with occasional cores of altered rhyolite (R) lie aligned in a matrix of glass shards and ash which is transected by minute veinlets of silica (S).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 ROCK UNITS 217

alignment of the crystals and rock fragments tions dip away in a radial arrangement from is more apparent in portions where the frag- vents mapped here by Kerr et al. (1957, p. 31), ments predominate over the phenocrysts. it is evident that the rhyolite issued from these On the east toward Marysvale Valley the valley vents. porphyritic texture predominates and the unit Callaghan (1939, p. 447) infers that, although is rarely tuffaceous. In the mountains toward some rock features are suggestive of welded the west the tuff interbeds increase in thick- tuff, the bulk of the material is flow breccia. ness between heavy ledges of porphyritic rhyo- Mackin (1960, p. 91, 95) has described similar lite. At Indian Hollow the rhyolite (Fig. 3C) features in ignimbrite (tuff) and concludes that is a moderately red porphyry. Toward the west the absence of lineation in the foliation plane it becomes progressively more gray and less red. indicates that the foliation is a compaction Indian Hollow tuff. Immediately above the structure. porphyritic portion of the Indian Hollow rhyo- The most distinctive feature of the Crescent lite is a fragmental tuff with a characteristic Hill rhyolite is a coarse foliation (an alignment platy fracture parallel to the bedding plane. A of elongate, angular autoliths in planes parallel degree of welding is indicated by the fracture to the base of the unit) with no evidence of surface breaking through the fragments of lineation (alignment of the fragments within rhyolite and tuff. A fine network of silica vein- these planes). Many fragments show a charac- lets (Fig. 3D) reinforces the matrix, but there teristic bleaching of their borders. Only in the is no indication of collapsed bubble structure immediate vicinity of the vents is there evi- to support an origin of welded tuff. The frag- dence of flowage in addition to compaction. ments are elongated and consist primarily of Features of flowage brecciation (the chaotic glass, commonly with a lenticular core of tumbling about of large blocks in a manner altered rhyolite or microcrystalline feldspar. similar to aa flows) have not been noted. The The matrix is devitrified glass, partly chlori- terminology for ash flows proposed by Smith tized and altered to clay minerals. (1960) does not seem applicable to these ma- The tuff thickens rapidly toward the west terials with predominantly air-fall character- and interfingers with the Gold Mountain tuff. istics. The unit becomes less fragmental, and a thin, The Indian Hollow tuff grades continuously mottled lens (20-50 feet) may be traced to upward into the Crescent Hill rhyolite at In- Mount Belknap (Fig. 2). The upper part of the dian Hollow. The tuff is foliated without tuff becomes rhyolitic and intergrades into the lineation or flowage and contains the same Crescent Hill rhyolite. aligned, elongate fragments as the rhyolite. Ex- At Indian Hollow a 2-foot gravel bed and planation of foliation in the tuff as a compaction several 6-8 inch ash layers occur within the phenomenon resulting from continuous air-fall upper portion of the tuff. The gravel contains of particles and fragments is customary and several coarse (1 cm), subangular fragments of reasonable. The inference is unmistakable that rhyolite in a rust-colored, arkosic matrix of the "rhyolite" had similar origin. orthoclase and oligoclase. Much of the feldspar Lithologic distribution suggests that three is extensively chloritized. These gravel and ash zones of differing rock type are arranged in a beds have not been distinguished beyond concentric pattern about the valley vents. Indian Hollow. Most remote from the vent is the area of com- Crescent Hill rhyolite. Dominating the red paction tuff (PI. 2, fig. 4) with fragment size sequence of the Mount Belknap volcanic group increasing inward. The middle zone is that of a in Marysvale Valley is a moderate red rhyolite compaction rhyolite (PL 2, fig. 1) without flow which is thick and widespread in the Central structure. The inner zone contains rhyolite Mining District east of the Sevier River. It is (PL 2, fig. 2; PL 3, fig. 1) with definite flowage characterized by aligned, elongate, dark gray- of contained fragments and phenocrysts. As the ish-red autoliths (PI. 2, figs. 1, 2), the coarsest intensity of the eruption increases, the zones of which (exceeding 5 inches in length) are increase in size and move outward. The start of found east and west of the Sevier River, op- the eruption may be marked by a compaction posite the mouth of Deer Creek. The base of glass (PI. 2, figs. 3, 5). Glass may be found in the Crescent Hill sequence may be marked by the lower portions of the rhyolitic members as a glass with similarly aligned autoliths (PI. 2, flattened droplets, and in the tuff as broken fig. 3). Between Crescent and Dome hills the fragments. rhyolite is thickest, and, as the coarse stratifica- The threefold division of the red eruptive

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 218 MOLLOY AND KERR-TUSHAR URANIUM AREA, MARYSVALE, UTAH r .0

5 mm 5 mm

5 mm Figure 4. Thin sections of Mount Belknap rock units A. Crescent Hill rhyolite. A specimen exhibiting the angular autoliths (A) of the compaction rhyolite, and the stretched fragments (F) of the flowage rhyolite. The latter are lined with growths of spherulitic sanidine (S) which form the characteristic bleached borders seen in the hand specimen. B. Gray Hills arkosic sandstone. An erosional unit at the base of the Gray Hills rhyolite, lying immedi- ately west of the Sevier River, contains subrounded glass (G), orthoclase (O), albite (A), and rhyolite fragments (R). C. Gray Hills rhyolite. Spherulites (S), some with cores of shattered, euhedral quartz (Q), form the dark, vitric laminae in this flow-folded gray rhyolite. Crossed nicols D. Staley Pasture tuff. This thick, massive gray tuff, which forms the base of the Mount Belknap sequence in the Tushar Range, is characterized by a mottled appearance caused by large, lenticular frag- ments of altered tuff (T) containing areas of calcite recrystallization (C), and numerous, angular fragments of iron-stained rhyolite (R). Sanidine phenocrysts are also noted.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 ROCK UNITS 219

sequence in Marysvale Valley is supported by placement of the red Mount Belknap valley thin section examination. The tuff (Fig. 3D) is rhyolites and the outpourings of glass marking distinguished by a porous groundmass of angu- the beginning of the younger gray phase lar glass shards and glassy envelopes surround- rhyolites. ing rock fragments. In the rhyolite (Fig. 4A) Gray Hills rhyolite. Another unusual mem- the matrix is massive, devitrified glass and ber of the Mount Belknap volcanic group is the cryptocrystalline feldspar. Incipient develop- Gray Hills rhyolite (Fig. 4C). Contorted flow ment of sanidine spherulites is noted, and the folding is characteristic of the thin laminae in bleached borders are apparently similar re- this medium gray unit. Isoclinal flow folds, growths of spherulitic sanidine around the overturned toward the valley, rim the large edges of rock fragments. (1500 feet thick) rhyolite mass which forms the The network of silicic veinlets noted in the Gray Hills. The undeformed laminae (PI. 3, tuff are coarser in the rhyolite and consist of fig. 2), composed of alternating grayish-black microcrystalline oligoclase and quartz. The glass and minute, light-gray fragments of quartz thin, glassy envelope surrounding some rock and potassium feldspar, parallel the base of the fragments in the rhyolite merges imperceptibly unit with minor crenulations. Flowage is at with the devitrified glass of the matrix, in dis- first symmetrical (PL 3, fig. 4); the contortions tinction to the separation of matrix and enve- then increase in complexity until the laminae lope in the tuff. This indicates partial fusion of become unrecognizable (PL 3, fig. 3). the matrix glass with the glassy envelope of the The extensive flowage pattern indicates that fragments, resulting from the greater heat con- the entire unit was plastic when it flowed from tent of the rhyolite during emplacement. a somewhat higher elevation and became em- Crystalline rock fragments are more abundant placed in the lowland which originally marked in the rhyolite. Fibrous feldspar lines minute the Gray Hills. The radial overturning of flow cavities, and there is evidence of local flowage folds from the center of the Gray Hills mass in the more plastic units of the Central Mining shows that the material descended in a tongue- District. The autoliths of the flowage rhyolite like mass into an earlier depression (Fig. 5). are less noticeable than are those in the com- Interlamellar quartz crystals are abundant, and paction rhyolite and are drawn out into tenu- cavities encrusted with clear, milky, and ous, undulating streaks up to 30 mm long, and amethystine quartz crystals are common in the 1-2 mm in diameter. Gray Hills. In Marysvale Valley the base of Gray Hills ariose. Stream channels have cut this unit is marked by a black obsidian flow into the eastern side of the Gray Hills and ex- (PI. 4, fig. 1). posed glass and yellowish-white arkosic sand- At the mouth of Beaver Creek, portions of stone at the base of the Gray Hills rhyolite. Gray Hills rhyolite which show only minor ef- Approximately 20 feet of sandstone (Fig. 4B) fects of flowage exhibit another form of primary is exposed, but the basal contact with the older structure. The laminae are first partly as- Mount Belknap red rhyolites is not seen be- similated, then become progressively discon- cause of faulting near the mouth of the stream tinuous (PL 4, fig. 3). Feldspar patches form bed. The greatest thickness of this unit is ex- around a core of quartz, indicating syngenetic posed above the Carson Prospect in Marysvale concentration of silica from the laminae, with Canyon. A small fault block has preserved feldspar remaining in the glassy areas from about 50 feet of well-bedded, moderately which silica was drawn. Secondary alteration sorted, light-gray arkosic sandstone with along fault planes has resulted in local, but in- laminae of varicolored, nugget-like feldspar tense alteration and silification of the rhyolite grains. The contact with the overlying Gray (PL 4, fig. 4). Hills rhyolite is well exposed, showing 18 inches Beaver Hill tuff. More than 50 feet of mas- of black glass at the base of the rhyolite. sive, light-gray tuff is exposed near the base of The continuity of this thin sedimentary unit the Gray Hills rhyolite in the vicinity of in an area immediately west of the Sevier River, Beaver Hill. Similar tuff beds have been recog- the excellent parallel laminae, and the rapid nized by Woolard (Kerr et al., 1957) north of thickening toward the present river basin indi- the mouth of Beaver Creek. A small number of cate that it was deposited in a shallow lake 1-2 mm fragments, mostly of lighter-colored which followed the present north-south course tuff, show no preferred alignment to disturb of the Sevier River. The arkosic sandstone the massive character of the host rock. Because marks a brief period of erosion between the em- of limited exposure and north-south faults at

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 220 MOLLOY AND KERR—TUSHAR URANIUM AREA, MARYSVALE, UTAH

Beaver Creek Gold Mt. Signal Peak

Figure 5. A Gray Hills rhyolite flow. A tongue of flow folded rhyolite descended from a higher elevation into a depression between Beaver and Deer creeks. One of the valley vents of the Mount Belknap red rhyolite is in the foreground.

the mouth of Beaver Creek, it has not been the base of the Joe Lott tuff and is at least 200 possible to determine the relationship of this feet thick. This massive, moderately fragmental tuff to the glass at the base of the Gray Hills unit is distinguished by numerous dark blotches rhyolite. which lie parallel to the bedding plane. These Staley Pasture tuff. At the base of the Mount are flattened discoids, probably of tuff, which Belknap volcanic sequence on the eastern are altered to a mixture of calcite and clay slopes of the Tushar Range is a thick, light- minerals stained black by manganese oxides. gray, mottled tuff (Fig. 4D). In the high moun- The tuff has a distinct platy fracture parallel to tain area this regional unit is more than 700 the bedding plane, and there is no lineation of feet thick. In Clear Creek the same unit forms the fragments in the foliation plane.

PLATE 2. COMPACTION FEATURES OF THE MARYSVALE VALLEY VOLCANIC ROCKS Figure 1. Aligned, elongate autoliths in the Crescent Hill rhyolite; from Indian Hollow Figure 2. Polished surface showing bleached borders and local flowage around xenoliths in the Crescent Hill rhyolite; from the Central Mining District Figure 3. Glass at the base of the Crescent Hill rhyolite; from the Blackie Prospect, Central Mining District. The glass autoliths show slight flowage and are transected by silica and uranium-bearing veinlets. Figure 4. Foliation plane from a 2-3 cm thick plate of Indian Hollow tuff; from Indian Hollow Figure 5. Weathered surface of glass; from Antimony, Utah

PLATE 3. FLOWAGE FEATURES IN THE MOUNT BELKNAP RHYOLITES Figure 1. Flowage of autoliths showing lineation and foliation in the immediate vicinity of vent; from Dome Hill in Marysvale Valley Figure 2. Undeformed laminae in the Gray Hills rhyolite; from the Gray Hills Figure 3. Highly contorted and stretched laminae in the Gray Hills rhyolite; from the Gray Hills Figure 4. Symmetrical flow-folding, with "saddle reef" deposits of quartz crystals formed at the apex of the folds. The quartz crystals are surrounded by bleached areas of fine-grained feldspar; from the Gray Hills

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 ------COMPACTION FEATURES OF THE: MARYSVALE VALLEY VOLCANIC ROCKS

MOLLOY AND KERR, PLATE 2 Geological Society of America Bulletin, volume 73

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 ------

------

------

FLOWAGE FEATURES IX THE MOUNT BELKNAP RHYOI.ITES

MOLLOY AND KERR, PLATE 5! Geological Society of America Bulletin, volume 73

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 ------

FLOW GLASS, DIKI;. GLASS. \\I> ALTFKATION PHF.NOMKNA

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 ------

------

------GOLD MOUNTAIN AND JOE I.OTT TUFF UNITS

MOLLOY AMD KERR, PLATE 5 (Icological Society of America Bulletin, volume 73

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 ROCK UNITS 221

A F

5 mm 5 mm Figure 6. Volcanic rocks from the Tushar Range A. Kimberly rhyolite. Flowage-aligned, lenticular, microcrystalline feldspar (F), and phenocrysts of oligoclase (O) and quartz (Q), distinguish light gray lenses of rhyolite in the basal regional tuff of the Mount Belknap volcanic group. B. Gold Mountain tuff. Compaction effects are observed beneath some of the larger rhyolite fragments (R). Limonite masses (L), orthoclase fragments (O), and areas of iron staining (I) show vague alignment parallel to the bedding plane.

In many areas the tuff has been regionally color of nontronite or the rusty stains of iron altered, perhaps by self-contained steam, and oxide. contains a large proportion of clay. The rock The Staley pasture tuff contains lenses of is soft and friable with many light-colored tuff dense white rhyolite (Fig. 6A) up to 50 feet fragments which show the faint apple-green thick. The extensive talus slopes formed by the

PLATE 4. FLOW GLASS, DIKE GLASS, AND ALTERATION PHENOMENA Figure 1. Black obsidian flow at the base of the Gray Hills rhyolite; from Black Knob, east of the Gray Hills in Marysvale Valley Figure 2. Green glass and silica masses from Beaver dike, Beaver Valley. The dike contains glass, perlite, pitchstone, silica concretions, and clay alteration products. Figure 3. Partial destruction of laminae in the Gray Hills rhyolite by the growth of feldspar masses with cores of crystalline quartz; from the mouth of Beaver Creek Figure 4. Relict laminae after silification of the Gray Hills rhyolite; from a fault zone at the head of Indian Hollow Figure 5. Cooling fractures formed perpendicular to the walls of a glassy dike in the Freedom mine in the Central Mining District. Faint laminae parallel the walls of the dike.

PLATE 5. GOLD MOUNTAIN AND JOE LOTT TUFF UNITS Figure 1. Glass fragments in the Joe Lott tuff, from Long Valley. This grayish-white tuff is character- istically stained dark brown on the weathered surface. Figure 2. Iron stains formed parallel to the weathered surface of the Gold Mountain tuff, from Gold Mountain. The tuff is essentially non-fragmental, and the dark spots result from the oxidation of iron minerals. Figure 3. Columnar joints of the upper Joe Lott tuff, resting on 50 feet of pink, cavernous tuff which contains laminated ash and gravel strata, from the west side of Clear Creek. Telephone pole at the lower left indicates the scale. Figure 4. Cliff face in Clear Creek formed by 80-100-foot-high columnar joints in the Joe Lott tuff

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 222 MOLLOY AND KERR—TUSHAR URANIUM AREA, MARYSVALE, UTAH

Staley Pasture and Gold Mountain tuffs pre- X-ray spectrochemical analyses of several vent tracing the rhyolite in the field. Toward Gold Mountain tuff specimens (Molloy and Kimberly the rhyolite thickens and forms a Kerr, 1960, p. 931) show almost identical continuous unit. amounts of aluminum, silicon, potassium, cal- Gold Mountain tuff. The distinctive, cream- cium, manganese, iron, copper, and a trace of colored crests of Gold Mountain and Signal rubidium. The chemical data re-emphasize the Peak (Figs. 5, 7) are covered by a massive, homogeneity of this extensive tuff unit. porous tuff (Fig. 6B) which exceeds 2000 feet Mount Belfyiap rhyolite. The topmost 300 in thickness and dominates the Mount Belknap feet of Mount Belknap (Fig. 7) is capped by a

Mt. Belknap Mt. Barrette

: ^%%;.v 'S^^ 1»v ;''>; x^ ., i^Kfev.:?./i>xJ?i.:,' .'> !,.»••>'•,, i u « i ,'" > I

Figure 7. Air view of the crests of the Tushar Range from the south. Mount Belknap is capped by sub-horizontal rhyolite flows. The slopes and the crests of the other peak are covered by Gold Mountain tuff. Uranium prospects are found on Mount Barrette and Phillips Mountain.

sequence among the highest peaks of the distinctive, pinkish-gray rhyolite which shows Tushar Range. The tuff intergrades with the very coarse layering, particularly on the west- older Staley Pasture tuff and is more massive, ern side (Fig. 8). This unit does not appear without mottling, and considerably less affected elsewhere in the district and is believed to rep- by clay mineral and calcite alteration. Both resent the youngest member of the Mount units are part of the regional tuff which Belknap volcanic group. This massive, siliceous blankets the crests of the Tushar Range. The rhyolite resembles quartzite and breaks with origin of these regional mountain tuffs is dis- subconchoidal fracture. Small areas show tinct from the flowage and compaction rhyolite columnar jointing. and tuff which are distributed about local vents X-ray spectrochemical analyses (Molloy and in Marysvale Valley. Kerr, 1960, p. 933) show this unit to have the In the high mountain area the massive Gold highest silica content of the Mount Belknap Mountain tuff is fractured into thin plates (av. rhyolite, tuff, and glass specimens examined. 6 by 4 by J4 inch) which cover the ridges and Aluminum, potassium, calcium, and titanium form long talus chains. Columnar jointing is content are similar to gray rhyolites of the rare, and iron stain penetrates the rock in Mount Belknap volcanic group, and traces of parallel or concentric bands (PI. 5, fig. 2). phosphorus and rubidium occur.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 ROCK UNITS 223

Glass in the Mount Belknap volcanic group. glassy groundmass. The glass particles may be There are three types of glass in the Mount sharp, angular fragments or rounded, flattened Belknap volcanic sequence. Obsidian flows droplets. These glass types represent air-fall and mark the base of several rhyolite members; compaction of glass fragments with varying de- glassy dikes are found in monzonite of the grees of plasticity at the time of eruption. The uranium mine workings and within younger glass autoliths of some samples indicate flowage volcanic units; and some tuffs and compaction subsequent to compaction. Some of the ob- rhyolites contain a basal layer of aligned sidian breccias are sufficiently massive to ob- obsidian fragments and flattened droplets. scure alignment of glass autoliths and may rep-

Figure 8. The Tushar Range from the west, above Blue Lake. The unconformable contact of the Mount Belknap volcanic group with the older Bullion Canyon flows may be seen at two points on the extreme right. Talus chains from the gently dipping Mount Belknap tuff obscure Bullion Canyon outcrops which might otherwise occur in Blue Lake Valley.

The flow glass at the base of the valley rhyo- resent flow breccias. However, the alignment lites (PL 4, fig. 1) is massive and dark gray to of fragments in the rhyolitic and tuffaceous black. In places the glass is perlite or pitchstone, units is regionally controlled, showing neither but the usual form is obsidian. the local reversals of dip nor the chaotic The red, brown, and green dike glass (PI. 4, tumbling of blocks associated with the advance fig. 2) may be massive, brecciated, or show cool- of flow breccia. The glass fragments are accom- ing fractures perpendicular to the dike walls panied by angular rhyolite and tuff xenoliths (PL 4, fig. 5). Alteration may form perlite, which support the air-fall hypothesis. Litho- silica concretions, and clay masses. Glassy dikes logically similar units have been found in the in quartz monzonite of the Central Mining basal Crescent Hill rhyolite at Indian Hollow, District are altered to clay and then partly re- and near Antimony, Utah, 25 miles south. placed by uranium minerals as the intensity of X-ray spectrochemical analyses of intrusive alteration increased. and surficial glass (Molloy and Kerr, 1960, p. A third type, compaction glass (PI. 2, figs. 932) show relatively constant content of alu- 3, 5), includes members ranging from obsidian minum, silica, manganese, and copper. There to glassy tuff in density. Their unifying feature are frequent traces of strontium, phosphorus, is the presence of aligned glass particles in a rubidium, and zirconium. The dike glass is dis-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 224 MOLLOY AND KERR—TUSHAR URANIUM AREA, MARYSVALE, UTAH

tinguished from the flow glass by lower potas- the Long Valley area east of the Sevier River sium and higher calcium content. The dike and north of Marysvale. glass shows a closer chemical affinity to the tuff sequences of the mountains than to the rhyo- STRUCTURE lites of the valley vents. Tushar Range Clear Cree/^ Tuffs The Hurricane fault forms the western A structural depression in the Clear Creek boundary of the Colorado Plateau, and its area has permitted the accumulation of consid- northern extension marks the western side of erable thickness of several regional lithologic the Tushar Range. A series of faults including units. Dry Hollow latite, Joe Lott tuff, and the Tushar bound the eastern side of the range. Sevier River gravels have been deposited upon The mountains are a north-south trending a basement of Bullion Canyon latite and tuff. horst block, approximately 50 miles long and The Sevier River formation is well exposed 15 miles wide. On the north, the Clear Creek at several points in the area. At Clear Creek, divide marks the merging of the Tushar and stream erosion has exposed a long, tilted section Pavant Ranges. To the south, the range is more of laminated gravels which form the alluvial properly called the Tushar Plateau and contains fan at the mouth of the canyon. The ash-white a series of 11,000-foot peaks rising above the color of the underlying Joe Lott tuff is unmis- main plateau which gradually descends toward takable in many of the gravel layers. On the the Markagunt Plateau. south side of the canyon laminated lake gravels The Tushar Range is transitional between are exposed over a large area with erosion pin- the Basin and Range Province and the Colo- nacles and buttes suggestive of Bryce Canyon rado Plateau. The extensive volcanic rocks that and Cedar Breaks. Indian dwellings are found cover the fault block duplicate similar features in several of the narrow, vertical gullies which in the Basin and Range Province to the west. cut into the gravel mass. The Coconino, Chinle, and Moenkopi forma- The unconformable contact between the tions of the basement complex belong to the Sevier River formation and the older Joe Lott stratigraphy of the Colorado Plateau. tuff is seen along the north side of the canyon. These main features were recognized by Angular conglomerate lies above the uncon- Dutton in 1880 (p. 169-187). He also noted the formity near Shady Dell in Marysvale Canyon. alluvium-buried faults which mark the sides of The Joe Lott consists of four phases aggre- the mountain block, the numerous step faults gating about 1500 feet thick. At the base is a which repeat the volcanic stratigraphy into the massive gray tuff equivalent to the Staley mountain mass, the sequence of volcanic erup- Pasture tuff. Above lies almost 1000 feet of tions which submerge the Plateau sediments, light-gray tuff equivalent to the Gold Moun- and the erosional unconformities. Butler (But- tain tuff, separated by the fourth member, a ler, Loughlin, Heikes, et al., 1920, p. 536-568) 50-foot thick bed of pink tuff (PI. 5, fig. 3), observed that some of the east-west canyons which is stratified and contains laminae of owe their position to fault zones, that glacial arkosic gravel. Both the upper and lower mas- activity took place in the higher parts of the sive tuff are conspicuously welded and form range, and that intrusive rocks form part of the columnar joints up to 100 feet high (PI. 5, fig. mass which blocks the Sevier River Valley 4). The upper tuff contains fragments of Gray north of Marysvale. He also recorded several Hills rhyolite, and about 15 feet of laminated large masses of intrusive quartz monsonite on white ash beds appears in the upper portion of the western side of the range. An alignment of the lower tuff. A geologic map of the eastern similar intrusive rocks reaches west from Mon- portion of Clear Creek is presented in Plate 6. roe, Utah, through Marysvale, toward Pioche The tuff, rhyolite, latite, and glass fragments in eastern Nevada. These intrusives apparently in the Joe Lott (PI. 5, fig. 1) are roughly equi- mark a major east-west zone of crustal weakness dimensional. The groundmass is fresh and and are closely related to the considerable highly pumiceous, ranging from a very light mineralization in their vicinity. gray to medium dark gray, and then to dusky Uranium deposits are found near or within brown. The pale grays predominate in the monzonite stocks in Marvsvale Valley on the Clear Creek area, except for brown staining on eastern side of the Tushar Range. Prominent the columnar joints. The brown shades are monzonitic and dioritic dikes are present near found in the thinner portions which blanket the gold and silver mines of the Kimberly area

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 STRUCTURE 225

at the north end of the Tushar Range (Butler, Horn Silver deposits to those of the base-metal Loughlin, Heikes, et al, 1920, p. 539). Similar mines in the Marysvale district. The ores are dikes are present in the Deer Trail mine which similar metallogenically, and both the replace- produces lead, zinc, silver, and some gold from ment and fissure bodies show common relation- replacement deposits in Permian limestones ship to post-monzonitic mineralization. above and below the Coconino quartzite. Sev- The larger intrusive masses have also in-

Anaular Unconfor uLty. Delano Peak Latite

Figure 9. Unconformity and Delano Peak latite at the base of the Mount Belknap volcanic group. Mount Belknap tuff of the Gold Mountain sequence (left) rests unconformably on the Delano Peak latite along Bullion-Beaver Divide. The latite has flowed over the edge of a plateau formed by the Bullion Canyon volcanic sequence (upper right) into an ancient valley of possible glacial origin. In this view, looking east at the head of Beaver Meadow, the ancient valley profile and subsequent fill are both visible. There is extensive iron staining along the angular unconformity between the Bullion Canyon and Delano Peak latites.

eral small monzonitic bodies in Deer Creek are fluenced local structure by opening fissures for accompanied by uranium and gold mineraliza- later mineralization, by producing halos of tion. alunite-clay alteration, by doming local areas In the San Francisco Mountains, 60 miles of volcanic strata, and by producing a barrier west, the Horn Silver mine (Butler, 1913, p. which temporarily blocked the Sevier River 164) has produced lead-silver ores with associ- valley north of Marysvale. The several levels of ated zinc and copper from Cambro-Ordovician terrace gravels on the upstream side of the limestones intruded by quartz monzonite. Here barrier are ascribed by Butler and others to the the deposition of ore closely followed the in- accumulation and later removal of alluvium trusion, and differences in the deposits are at- thus trapped in the valley floor. Indeed, the in- tributed to variations in the host rock and dis- trenched meanders in Marysvale Canyon along tance from the intrusive body. Butler (1913, U. S. Highway 89 form one of the deepest, p. 133) remarks upon the similarity of the most accessible exposures of volcanic strata and

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 226 MOLLOY AND KERR—TUSHAR URANIUM AREA, MARYSVALE, UTAH

associated intrusive bodies in the southwestern purple-grays predominating toward Marysvale United States. Valley. Mount Belknap volcanic group. Figures 2 Emplacement of the Volcanic Units and 11 show that tuff units predominate in the Basement latites. Two of the three latite se- mountainous west, whereas rhyolites are abun- quences in the Tushar Range (Bullion Canyon dant in the eastern valley. With the exception and Delano Peak) underlie the Mount Belknap of the Kimberly rhyolite, the flows of the flows and tuffs (Fig. 9). In the areas studied Mount Belknap sequence are local units de- (north and east of the Tushar Range), the Dry rived from nearby vents. Some of the valley Hollow latite underlies the Mount Belknap, rhyolites are excellently layered and show a

Figure 10. Air view of Bullion Canyon volcanic rocks. In this sketch, looking north over the Bullion-Beaver Divide toward Mount Belknap, a coarse stratification is apparent in slightly inclined Bullion Canyon latites. Bulldozer tracks crisscross Phillips Mountain.

but is reported (Callaghan, 1939, p. 449) to be steep, concentric dip inward toward the six ex- interbedded with the Joe Lott in the southern trusive centers mapped in Marysvale Valley part of the range. (Kerr et al, 1957, p. 26, 49-52; PI. 4, fig. 1). The Bullion Canyon latites are the oldest Subangular autoliths, oriented parallel to the flows of the basement complex. Beyond the base of the unit, characterize the Indian Hollow Marysvale source area mapped by Kerr et al. tuff, Crescent Hill rhyolite, and the basal (1957), which contains many local and highly Crescent Hill glass. These three units represent variable units, the Bullion Canyon is com- a single volcanic sequence with three varying monly represented by a massive, coarsely strati- components—glass, rhyolitic autoliths, and fied, greenish-gray latite (Fig. 10). tuffaceous matrix. The Crescent Hill glass South of Mount Belknap and at the head of marks the start of a volcanic episode and con- Bullion Canyon, an 800-foot thick reddish- sists of air-fall and compaction aggregation of purple latite porphyry overlies the Bullion glassy droplets in the Crescent Hill vicinity. On Canyon latites and dips gently west. At the the fringes of the glass depositional area, at head of Beaver Valley this Delano Peak latite Indian Hollow, this basal unit is 5-10 feet has spilled over the edge of the high plateau thick, with a few, small, glassy droplets in a into an ancient valley, forming a sheet about tuffaceous or devitrified matrix. Near the 50-100 feet thick (Fig. 9). The more rapid center of the eruptive area, the glass droplets cooling of this thin unit has caused variation in have accumulated to 200 feet and have partly the coloring of the groundmass, with grays and flowed together into a massive glass layer. This

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 STRUCTURE 227

// 11 1 _ •f = a + "a ? ^S-H 5 s *• ° i = Sg. S 8. u " »• *- a •o o ; ; * ~ e .as u 1 L— _ t. ^ a, o3

-t*-l J J 01 -c o o 5 "" 2 '£ = |C« i wj in ^ b i- c 3 o c >, o « J= •** M «s --5 •fi ^ 0^ c

£? E

V si i o C at •a" 1 « a I §! o | "fl - ^ S

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 228 MOLLOY AND KERR—TUSHAR URANIUM AREA, MARYSVALE, UTAH

flowage reflects greater mass, higher tempera- regional, mottled gray unit approximately 750 ture, and proximity of the source. feet thick. Above, lies the regional Gold After the initial glassy phase, the Crescent Mountain tuff (PI. 7, fig. 1), a slightly frag- Hill eruptives become rhyolitic but persist in mental, yellowish-gray tuff exceeding 2000 feet the unusual air-fall aggregation mode of ac- in thickness. The youngest volcanic unit pre- cumulation. Close to the source, at Dome Hill, served is the 300 feet thick rhyolite cap on Crescent Hill, and the mouth of Deer Creek, Mount Belknap. Laminated lake gravels of the the unit appears to be a massive rhyolite flow. Sevier River formation lie upon the Joe Lott Closer examination reveals aligned, discoidal equivalent of the Gold Mountain tuff in the autoliths with bleached borders lying within a Clear Creek area (PL 7, fig. 2). More than 1000 rhyolite matrix formed by compaction of feet of Joe Lott tuff has accumulated in this smaller, similar particles (PI. 2, fig. 2; PL 3, basinal area, with about 200 feet of gray tuff fig. 1). equivalent to the Staley Pasture at the base. As Autoliths with bleached borders are most cooling was more rapid on the mountain crests, abundant in a semicircular zone between the the prominent columnar feature of the Joe flowage rhyolite at the center of the eruptive Lott tuff did not develop in the Gold Moun- area and Indian Hollow on the west. The bleach- tain tuff. In the valley basins the tuff was ing appears to result from a sufficiently high "ponded" and the heat escaped more slowly, particle temperature after air-fall to permit permitting the layers to weld together and marginal reaction (growth of spherulitic sani- form gigantic columns (PL 7, fig. 3). The total dine), but insufficient to fuse the particles into area covered by the Gold Mountain tuff and a homogeneous mass. Reaction borders around equivalents is about 40 miles long and 20 miles fragments are rare west of Indian Hollow; the wide, or 800 square miles. autoliths, which exceed 4-5 inches near Cres- cent Hill, are an inch or less in length. Fault Patterns The upper unit of the eastern, rhyolitic se- The structural pattern of the Tushar Range quence is a laminated, flow-folded gray rhyo- is that of a mountain horst with a valley graben lite. The presence of both cristobalite and on the east. In the Marysvale area the graben quartz indicates high temperature and rapid is bounded by the Sevier and Tushar fault sys- cooling. The minute thickness and horizontal tems. The Tushar fault and the northern ex- extent of individual laminae in the Gray Hills tension of the Hurricane fault bound the rhyolite suggest that flowage was not the mountain block. original method of deposition. Alternating In the Beaver Creek area west of Marysvale ashy beds of similar composition and thickness there are four main fault systems (Fig. 12). The indicate that the vitreous layers are similar in predominant trend is that of the Sevier fault, origin, but more viscous at the time of deposi- which runs north-south for 200 miles through tion and changed by compaction into glass. central and southern Utah. The next distinct The overturning of the flow folds outward from trend begins just east of the Tushar fault and the center of the Gray Hills suggest that the continues to the west. This system includes the entire unit moved as a plastic mass into a Tushar fault and forms a series of northwest- former depression. trending step faults with the eastern side down- The tuff units of the Mount Belknap first dropped. The dip of the Tushar fault plane appear as interbeds within the rhyolites. West may be seen in three places—in the workings of the Tushar fault, the rhyolites of the valley of the Deer Trail mine, on the west side of sequence disappear and tuff constitutes the en- Copper Belt Peak, and along the 80-foot high, tire Mount Belknap stratigraphy, except for slickensided escarpment between the Bullion the Indian Hollow rhyolite. Lenses of Kimberly Canyon latite and the Staley Pasture tuff north rhyolite appear within the Indian Hollow tuff of Copper Belt Peak. At all three locations the and then within the Staley Pasture tuff, as the fault plane dips 75° E. or more. The movement former intergrades into the gray, western along the Tushar fault system may be ac- counterpart. The Kimberly rhyolite thickens counted for by a series of normal, gravity faults from 10-foot lenses in Beaver Creek to exten- downdropped on the valley side (PL 1, Sec- sive exposures in the Kimberly mining area tions A-A', B-B'). Toward the crests of the north of Mount Belknap. Tushar Range the movement is progressively The Staley Pasture tuff is the basal member reversed, with the western side downdropped. of the Mount Belknap sequence in the moun- This reversal probably anticipates a similar sys- tains west of Marysvale and forms a massive, tem of downdropped steps on the western

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 STRUCTURE 229

slopes of the mountains. The faults of the Sevier River Valley. The Indian Hollow trend Tushar system continue across Beaver Creek, is developed in Clear Creek as a series of major through Bullion Canyon, Cottonwood Creek, faults which terminate the Monroe and Pavant and south toward Tenmile Creek. To the north ranges on the east and west sides, respectively. the Tushar system cuts through the Kimberly The valley graben holds the towns of Monroe, mining district and extends toward Clear Creek. Joseph, Elsinore, and Annabella. The north- A third fault system is seen in Indian Hollow west trend of the Tushar and the northeast as a northeast trend which predominates in the trend of the Indian Hollow fault systems form

NORMAL FAULT J--t-*- THRUST FAULT INTRUSIVE BODIES EXTRUSIVE CENTERS

Figure 12. Fault pattern and monzonite intrusives in the Marysvale area. The intrusives trend east-west across Monroe, Marysvale, Delano Peak, and a portion of Beaver quadrangles. The largest bodies and the uranium mineralization occur at the intersection of the main northeast and northwest valley fault trends. The Mount Belknap extrusive centers are found in this immediate area. Compiled in part from Callaghan (1939) and Kerr et al. (1957) with supple- mentary field reconnaissance

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 230 MOLLOY AND KERR—TUSHAR URANIUM AREA, MARYSVALE, UTAH

the eastern boundary of the Tushar Range. MINERALIZATION AND ALTERATION The zone of intrusive rocks and uranium mineralization studied by Kerr et al. (1957) is Alteration Features in Marysvale Canyon at the intersection of the Transformations and modifications of rock two trends. This area is also the source of the material resulting in ore formation is here re- Crescent Hill rhyolite and the Indian Hollow ferred to as mineralization; those that do not tuff. The two fault trends seem similar in age, result in the formation of ore are called altera- although the evidence is not conclusive. Both tion. offset and are younger than the fourth trend. The numerous mineral deposits in the Marys- As noted by Butler et al. (1920, p. 539), vale area are accompanied by large alteration many of the east-west canyons in the Tushar zones. Seven alteration stages have been dis- Range owe their position to fault zones. The tinguished in the area studied by Kerr et al. phrase "fault zones" is particularly expressive (1957) and include slight, moderate, and in- of the series of offset faults which mark Beaver tense argillic alteration, silicification, mixed Creek (Fig. 12). This east-west system probably alunitic-argillic aureole, alunite, and zeolitiza- marks tensional fractures across the axis of the tion. Permeable volcanic units such as tuff show range. The similarity in trend between the belt widespread alteration zones along fractures, of intrusives and the fault zones suggests that while similar fractures in dense flows show the two are related. Regional features of in- relatively restricted alteration effects. How- trusion and mineralization indicate that they ever, confining and concentrating hydro- may be contemporaneous. thermal solutions in the denser rocks may re- The Tushar fault system in Beaver Valley is sult in more intense alteration. not so prominently expressed in the topography The Bullion Canyon volcanic rocks are con- as it is in Bullion Canyon. The Mount Belknap siderably more altered than are the younger tuff does not offset sharply to form escarpments Mount Belknap units. Intense alteration in the as do the older latite flows. The Tushar move- former results in the production of a mont- ment, which exceeds 3000 feet at Deer Trail morillonite-illite-quartz suite along prominent Mountain, dies out toward the north. These fractures within or close to the intrusives, and factors result in less vertical offset and in a the formation of diffuse zones of kaolinite- multiplicity of small faults which distribute the alunite-quartz in the more porous volcanic main fault movement. rocks. The Clear Creek area (PI. 6) provides an un- The contrast in alteration between the Bul- usually well exposed example of thrust faulting lion Canyon and the Mount Belknap units, and (PI. 8, figs. 1, 4). The escarpment formed by the presence in the Bullion Canyon volcanic Joe Lott tuff has overridden younger Sevier group of intense alteration halos about mon- River gravels. The Sevier River formation is zonite intrusives indicate at least two periods contorted and faulted in front of the thrust. of alteration. The older period is characterized Several minor faults contain siliceous veins up by alunite, the younger by uranium mineraliza- to a foot thick, formed of silica, opal, and tion. brecciated gravels of the Sevier River. Autunite and radioactive opal were deposited along sev- Clay Mineralogy eral of these veins, apparently as mineralization X-ray diffraction analyses by Kerr et al. remobilized from earlier deposits. Immediately (1957, p. 109-137) demonstrate that hydro- in front of the thrust, a 12-foot wide clay seam thermal alteration of the Bullion Canyon vol- has formed parallel to one of the siliceous veins canic group results in progressive changes in and has been excavated to more than 50 feet. clay mineralogy.

PLATE GENERAL VOLCANIC AND SEDIMENTARY FEATURES Figure 1. Mount Belknap, capped with rhyolite, slope of Gold Mountain tuff, from the southwest Figure 2. "Bryce Canyon" erosional effect produced in laminated gravels of the Sevier River formation, from the south side of Clear Creek basin Figure 3. Joe Lott tuff on the west side of Sam Stowe Canyon along Clear Creek. The pink, cavernous tuff (left foreground) separates upper and lower (lower right) columnar tuff sequences.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 ------

------

------GENERAL VOLCANIC: AND SEDIMENTARY FEATURES

MOLLOY AND KERR, PLAT!''. 7 (•'• iloiju \! Socict\ o( America Bulletin, vnlun e 7 •>

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 MINERALIZATION AND ALTERATION 231

Initial alteration stages are marked by the X-ray diffraction studies indicate that the change of feldspar to kaolinite and montmoril- intensity of alteration in the Mount Belknap lonite, pyroxene to carbonate and montmoril- specimens is much less than in the Bullion Can- lonite, olivine to serpentine, and by the oxida- yon volcanic group. Alunite has not been recog- tion of magnetite. Kaolinite and montmoril- nized among the alteration minerals. The lonite then increase m nearly equal proportions. Mount Belknap alteration suite includes kao- Illite first appears in minor amounts and later linite, montmorillonite, illite, mixed-layer increases to characterize the most intense montmorillonite-illite, silica, calcite, and zeo- alteration. lites. Some minerals reflect primary, syngenetic The Mount Belknap volcanic rocks in the alteration, while others are secondary. Beaver Valley area show the development of Alteration of the Mount Belknap units may clay minerals, especially along fault zones. Iron be divided into three primary features pro- staining, silification of rock units, and the duced during consolidation, and two secondary growth of silica lithophysae are other forms of features, usually developed along fault zones. alteration. The primary features are: (1) contact-meta- To provide a basis for the analysis of altera- morphic bleaching of autolith borders in Cres- tion materials from the Tushar Range, a series cent Hill tuff near the Marysvale Valley ex- of reference clay minerals was examined by X- trusive center; (2) growth of quartz crystals ray diffraction. A Philips X-ray diffractometer, within the Gray Hills rhyolite and simultane- equipped for pulse height analysis, recorded ous bleaching and destruction of surrounding standard patterns for kaolinite, dickite, halloy- laminae; (3) deposition of zeolitic masses in the site, nontronite, montmorillonite, illite, atta- Joe Lott tuff and as vug fillings in the Dry pulgite, and pyrophyllite from the set of ref- Hollow-Delano Peak latites. erence clay minerals studied as Research The major secondary changes are silification Project 49 of the American Petroleum Insti- and the formation of clay minerals. Hematite tute (Kerr et al., 1951). Oriented samples from and limonite staining is characteristic of the the original chemical analyses were subjected bands of argillic alteration that follow the faults to glycolation and heat treatment. The result- of the Tushar trend. Iron staining is also ex- ing diffractometer patterns and interpretation tensive at the erosional contact between the have been published (Molloy and Kerr, 1961). Bullion Canyon and Delano Peak latites along Approximately 50 alteration samples from the Bullion-Beaver Divide west of Copper Belt the Marysvale area have been examined by X- Peak. X-ray spectrochemical analysis of clay ray diffractometer techniques of orientation, alteration developed along faults in the Marys- glycolation and heat treatment. Orientation vale area (Molloy and Kerr, 1960, p. 930) shows enhances the intensity of basal (001) reflections enrichment of iron, titanium, aluminum, and of sheet silicate lattices, improving the sensi- calcium, and depletion of phosphorus. tivity with respect to clay minerals. Heating Silicification in the form of lithophysae or destroys certain clay lattices at specific tem- concretions is common within the rhyolites peratures, differentiating kaolinite and chloritic parallel to the bedding plane. As the silica con- minerals, and aiding in the identification of cretions grow, the relict bedding is destroyed other minerals in the residue. The combination and crusts or coatings of chalcedony develop. of glycolation and heat treatment is useful in These features are extensively developed in distinguishing mixed-layer clay. the Mount Belknap rhyolites along fault planes

PLATE 8. THRUST FAULT ALONG CLEAR CREEK, AND SILICA CONCRETIONS FROM DIKES Figure 1. Escarpment formed along the front of the Joe Lott tuff (right) overriding younger, contorted Sevier River gravels (left). The thrust curves from the upper left down to the lower right corner. Figure 2. Radial silica concretions in black chalcedony; from a dike along a fault zone between Phillips Mountain and Mount Belknap Figure 3. Shattered, ameboid concretions of chalcedony in green perlite; from Beaver dike in Beaver Valley Figure 4. Close-up view of the sole of the Clear Creek thrust, showing shattered Sevier River gravel beneath the overthrust Joe Lott tuff

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 232 MOLLOY AND KERR—TUSHAR URANIUM AREA, MARYSVALE, UTAH

near the mouth of Beaver Creek. The fault polymorphs (illite), indicates primary modifica- zones have been saturated with intrusions of tion before consolidation, rather than secon- reddish-brown and pale-green glass, and dary alteration. "pseudo-bauxite", consisting of siliceous litho- Mixed-layer montmorillonite-illite is the physae in a glassy groundmass (PI. 8, fig. 2), has dominant alteration mineral along faults in the developed. Bands of lithophysae and clay have Mount Belknap sequence. Illite, montmoril- formed in the Indian Hollow rhyolite, parallel lonite, and widespread limonite-hematite stain- to the bedding and up to 200 feet away from ing are also present. Where rhyolitic units have the glassy dike in Beaver Creek. Siliceous con- been bleached, significant amounts of kaolinite cretions, many with cavernous centers, lie are present as alteration of feldspar pheno- aligned in bands within the dike parallel to the crysts. Development of kaolinite is character- sides of the 15-foot-wide zone of brown, red, istic of bleached zones, not of faulted areas. and green glass and pitchstone. These features Fault zones are more restricted in the rhyo- represent late magmatic effects, particularly lite than in the tuff. Hematite staining is more those of a hydrothermal or hot-spring nature. intense in the former, and montmorillonite is X-ray diffractometer techniques reveal that common. A trace of illite is found in the post-emplacement modifications of the Mount alteration of both rock types. Illite and mont- Belknap volcanic rocks consist of development morillonite increase with alteration. Mixed- of cryptocrystalline quartz and feldspar in the layer clays are universal in the clay alteration, groundmass, deposition of zeolites in the but illite dominates the montmorillonite-illite bleached tuffs, formation of calcite, sericite, mixed-layer material in the more highly altered and chlorite, and production of a clay mineral specimens. Along some fault planes, bright- suite which includes kaolinite, montmorillonite, green gouge and breccia has developed. X-ray illite, and mixed-layer montmorillonite-illite. diffraction identifies this clay material as a Mixed-layer members predominate in the member of the montmorillonite group, prob- clay suite. Illite is minor, and montmorillonite ably nontronite. is well developed in fault gouge. Calcite result- Two trends of alteration are developed along ing from ground water deposition and chlorite fault zones in the Mount Belknap volcanic form an identifiable trace in the X-ray patterns. rocks: (1) the clay content greatly increases, The more altered samples show distinct calcite principally by the production of mixed-layer veinlets, but alunite, a prominent alteration montmorillonite-illite with illite dominating mineral in Bullion Canyon volcanic rocks, has the more intense alteration; (2) illite and mont- not been noted in any of the 150 patterns ex- morillonite develop as separate minerals. In the amined. center of extensive alteration zones, 60 feet or The Joe Lott tuff in the Clear Creek area more in width, illite is dominant at the expense contains considerable zeolite (principally stil- of the mixed-layer clays. At the fault plane it- bite) and minor feldspar. Clay minerals and self, however, montmorillonite is well devel- quartz are absent, indicating that glass shards oped, also at the expense of the mixed-layer in the matrix are only slightly decomposed. clays. At the mouth of Beaver Valley the Gray Hills rhyolite has been modified to form three Uranium Mineralization alteration types—blotchy or variegated rhyo- The Mount Belknap volcanic rocks are lite, clay seams, and hematite-stained zones. younger than most of the mineralization in the The first consists of blotchy areas in the cores Tushar Range. They are not the host rock for of rhyolite masses in which the laminae are pro- the gold, silver, lead, zinc, copper, or significant gressively obliterated, and of white, sugary alunite deposits found in the district. These veins which transect these areas. This primary ores are restricted to the Bullion Canyon vol- feature suggests progressive plastic deformation canic group and the quartz monzonite in- of the rhyolite and injection of the more mobile trusives. The main period of mineralization ap- portion into fractures. This is supported by pears to have preceeded the formation of the field relations and by the absence of clay in the erosional unconformity at the base of the diffraction patterns. The blotchy areas and Mount Belknap units. sugary veins consist almost entirely of feldspar Uranium mineralization in the district, how- with minor quartz. There is fine-grained, well- ever, postdates the Mount Belknap. At the crystallized muscovite in the sugary veins. uranium mines in Marysvale Valley, the basal Muscovite, rather than poorly crystalline mica Mount Belknap rhyolite overlies the erosional

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 MINERALIZATION AND ALTERATION 233

unconformity which cuts the monzonite in- chalcedony veins have developed in the Sevier trusive. At the erosional contact a uranium River gravels in front of the thrust. Coatings vein from the monzonite blossoms out to pro- of autunite and droplets of radioactive yellow duce a pocket of high-grade ore and then con- opal occur along fracture planes in the veins. tinues as a narrow, unproductive channel into An adit and trench have been opened on the the rhyolite. The porosity of the erosional property, but the occurrences are submarginal. debris at the unconformity has resulted in at There is no evidence of primary uranium least three such ore bodies—at Bullion Mon- mineralization, and the secondary minerals are arch, Potts Fraction, and Lucky Strike mines. probably remobilized from deposits at depth Scattered occurrences of uranium mineraliza- which have been cut by the faults. tion are found on the eastern side of the Tushar Range and to the north in Clear Creek; none SUMMARY AND CONCLUSIONS has yet proved to be ore grade. Mines are op- The volcanic area of the Tushar Range pro- erating in altered Bullion Canyon latites at vides an interesting assemblage of volcanic Indian Creek on the western slopes of the stratigraphy, fault patterns, alteration phe- Tushar Range, but the occurrences there have nomena, and mineralization. not been studied. Study of the Mount Belknap volcanic group Radioactivity is associated with small mon- indicates that more than 5000 feet of tuff and zonite bodies in Deer Creek, with fault zones rhyolite accumulated from local vents in the and erosional unconformities in Beaver Creek, valley floor and regional sources in the high and along silica veins associated with thrust mountain area. Tuff, rhyolite, and glass associ- faulting in Clear Creek. ated with the valley vents show common Three areas have been developed at the head features of compaction. The intergradation of of Beaver Creek (PL 1)—Mount Belknap valley units suggests that air-fall aggregation northeast ridge (Phillips Mountain), Mount may be the means of emplacement, not only Belknap southeast ridge (Mount Barrette), and of tuff but also of glass and rhyolite units. south of Mount Belknap overlooking Blue Glassy dikes associated with uranium minerali- Lake. Two adits were opened at the first loca- zation may have been the source of local tuff tion by Phillips 66 Petroleum Company in units, but the regional thickness of tuff which 1956. The radioactive minerals are autunite covers the Tushar crests was probably not and uranophane in a matrix of argillaceous tuff, erupted from vents in the immediate area. and occur along fracture planes, in vesicles, and Stratification in the tuff and rhyolite, which disseminated in the porous Gold Mountain tuff. form the 12,000-foot crests of Mount Belknap On the crest of Mount Barrette, southeast of and Mount Baldy, shows that these peaks are Mount Belknap, several veins of fine-grained remnants of a volcanic mass which filled an silica and purple fluorite in Gold Mountain tuff ancient valley. Correlation of the Mount have been prospected by Mr. Frank Nallick. Belknap tuff with similar sequences to the The radioactivity in this vein material is prob- north and east indicates that an area of at least ably caused by pitchblende, the X-ray diffrac- 800 square miles is covered by this eruptive tion pattern of which is obscured by similar sequence. patterns of fluorite and pyrite which are pres- The flow fold pattern in one of the rhyolites ent in much larger quantity. This material is indicates an unusual emplacement by air-fall the only primary mineralization found in the and subsequent gravity flow into a depression high mountain area and is closely related to in Marysvale Valley. Several types of primary similar vein material being mined in the in- alteration are developed in this rhyolite, sug- trusive area of Marysvale Valley. gesting a range in plasticity and contained heat Many radioactive "hot spots" occur along at the time of slumping. Differences in colum- opalitic rhyolite veins southeast of Clear Creek. nar jointing and degree of welding in tuff On the north side of the canyon an interesting deposited on mountain crests and in valley occurrence associated with thrust faulting has basins indicates varying heat content at the been developed by Mr. LeRoy Kmetzsch and time of consolidation. others. The Joe Lott tuff has overridden the The correlation of volcanic units is often younger Sevier River formation along a north- complicated by local variation in texture, east trending escarpment about 2 miles west of density, and color. Nevertheless, concentric the junction of U. S. Highway 89 and Utah 13. distribution of such variants about previously Several minor faults together with opal and mapped vents in Marysvale Valley show that

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 234 MOLLOY AND KERR-TUSHAR URANIUM AREA, MARYSVALE, UTAH

distance from the source may be a primary Mount Belknap specimens show the change of controlling factor in determining whether a feldspar phenocrysts to kaolinite in bleached volcanic unit will be glass, rhyolite, or tuff. areas, development of mixed-layer clays along The fault zones of the Marysvale area may fault zones, increase in illite with increased be resolved into four patterns. Two of these intensity of alteration, and the formation of are related and form an oblique shear set which montmorillonite in gouge and breccia zones. outlines the mountain horst and valley graben. Uranium mineralization is found in and near The third is older, and apparently is related the Marysvale Valley intrusives and on the to the main period of mineralization and the western side of the Tushar Range. Ore deposi- belt of monzonite intrusivcs that passes through tion along fissures forms productive uranium the Central Mining District north of Marys- vein deposits which have been continuously vale. The intrusive belt probably is the source mined since discovery in 1949. The epithermal of gold, silver, copper, lead, and zinc minerali- nature of the deposit is demonstrated by the zation in the Marysvale mining district and monzonitic host rock, associated volcanic se- deposits to the east and west. However, the quences, occurrence as veins and fissure fillings, intrusive rocks predate the uranium minerali- the clay mineral suite, and the accompanying zation. The final fault pattern is the major fluorite and jordisite (molybdenum sulphides). Sevier system which extends almost to Salt Similar pitchblende-quartz-fluorite-pyrite veins Lake City from the Arizona border, passing in Gold Mountain tuff, south of the crest of east of the Marysvale area. Mount Belknap, indicate that primary uranium Preliminary potassium-argon dating indicates mineralization penetrated into the main mass an age of late Miocene for the emplacement of of the Tushar Range. Most of the radioactive the Mount Belknap volcanic rocks. This agrees occurrences in the Mount Belknap volcanic with scant stratigraphic relationships which sequence are secondary, consisting primarily indicate Oligocene to Pleistocene, post-Wasatch of autunite and uranophane, together with age for the entire volcanic pile, including the uraniferous yellow opal. Siliceous veins con- older Bullion Canyon group. taining secondary uranium minerals are found Alteration features are less well developed in in gravels of probable late Pleistocene age. the Mount Belknap than in the Bullion Canyon Nearby hot springs indicate that this type of volcanic sequence. X-ray diffraction studies of remobilization is going on at present.

REFERENCES CITED Bassett, W. A., Kerr, P. F., Schaeffer, O. A., and Stoenner, R. W., 1960, K-Ar ages, Marysvale, Utah- Tertiary volcanic rock (Abstract): Geol. Soc. America Bull, v. 71, p. 1822 Bethke, P. M., and Kerr, P. F., 1954, Uranium occurrences in the older sedimentary rocks of the Marysvale district: U. S. Atomic Energy Comm., RME-3096 (Pt. 1), p. 60-74 Butler, B. S., 1913, Geology and ore deposits of the San Francisco and adjacent districts, Utah: U. S. Geol. Survey Prof. Paper 80, 212 p. Butler, B. S., Loughlin, G. F., Heikes, V. C., et al., 1920, The ore deposits of Utah: U. S. Geol. Survey Prof. Paper 111, 672 p. Callaghan, E., 1939, Volcanic sequence in the Marysvale region in southwest-central Utah: Am. Geophys. Union Trans., p. 3, p. 438-452 Dutton, E. C., 1880, Geology of the high plateaus of Utah: U. S. Geog. and Geol. Survey Rocky Mtn. Region, 307 p. Eardley, A. J., and Beutner, E. L., 1934, Geomorphology of Marysvale Canyon and vicinity, Utah: Utah Acad. Sci. Proc., v. 11, p. 149-159 Goddard, E. N., Chairman, 1948, Rock color chart: Natl. Research Council (Distributed by Geol. Soc. America), 6 p. Kerr, P. F., et al., 1951, Preliminary reports, reference clay minerals: Am. Petroleum Inst. Research Project 49, Repts. 1-8 Kerr, P. F., Brophy, G. P., Dahl, H. M., Green, J., and Woolard, L. E., 1957, Marysvale, Utah, uranium area: Geol. Soc. America Special Paper 64, 212 p. Mackin, J. H., 1960, Structural significance of Tertiary volcanic rocks in southwestern Utah: Am. Jour. Sci., v. 258, p. 81-131

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021 REFERENCES CITED 235

Molloy, M. W., and Kerr, P. F., 1960, X-ray spectrochemical analysis: an application to certain light elements in clay minerals and volcanic glass: Am. Mineralogist, v. 45, p. 911-936 1961, Diffractometer patterns of A.P.I, reference clay minerals: Am. Mineralogist, v. 46, p. 583-605 Smith, R. L., 1960, Ash flows: Geol. Soc. America Bull., v. 71, p. 795-842

MANUSCRIPT RECEIVED BY THE SECRETARY OF THE SOCIETY, AUGUST 22, 1960

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/211/3416898/i0016-7606-73-2-211.pdf by guest on 24 September 2021