Mineralium Deposita (199K) 33 : 4774'14 ( . srl'lllt!~r-V~rlag 199X
ARTICLE
C. G. Cunningham' J. D. Rasmussen' T. A. Stcycn R. O. Rye' P. D. Rowley S. B. Romberger' J. Selverstone Hydrothermal uranium deposits containing molybdenum and fluorite in the Marysvale volcanic field, west-central Utah
Received: 23 June 1997 I Accepted: 15 October 1997
Abstract Uranium deposits containing molybdenum \9-1 ~ Ma in a I km2 area. above a cupola of a com and fluorite occur in the Central Mining Area. near posite, recurrent. magma chamber at least 24 x 5 km Marysvale, Utah. and formed in an epithermal vein across that fed a sequence of 21- to 14-Ma hypabyssal system that is part of a volcanic/hypabyssal complex. granitic stocks. rhyolite lava flows. ash-flow tuffs. and They represent a known. but uncommon. type of de volcanic domes. Formation of the Central Mining Area posit; relative to other commonly described volcanic began when the intrusion of a rhyolite stock. and re related uranium deposits. they are young. well-exposed lated molybdenite-bearing, uranium-rich. glassy rhyolite and well-documented. Hydrothermal uranium-bearing dikes, lifted the fractured roof above the stock. A quartz and fluorite veins are exposed over a 300 m breccia pipe formed and relieved magmatic pressures. vertical range in the mines. Molybdenum. as jordisite and as blocks of the fractured roof began to settle back (amorphous MoS2), together with fluorite and pyrite, in place, flat-lying, concave-downward. "pull-apart" increase with depth. and uranium decreases with depth. fractures were formed. Uranium-bearing, quartz and The veins cut 23-Ma quartz monzonite, 20-Ma granite. fluorite veins were deposited by a shallow hydrothermal and 19-Ma rhyolite ash-flow tuff. The veins formed at system in the disarticulated carapace. The veins, which filled open spaces along the high-angle fault zones and flat-lying fractures. were deposited within 115 m of the ground surface above the concealed rhyolite stock. Editorial handling: R. Goldfarb Hydrothermal fluids with temperatures near 200°C, e.G. Cunningham (CSJ) IX 8 O H ,O '" -1 .5. 8DH ,o r-v -130. log /02 about -47 to US Geological Survey. 954 National Center. -50, and pH about (, to 7, permeated the fractured Reston. Virginia 20192 e-mail: cunningham(lL'usgs.gOY Fax: (703) 860-6383 rocks: these fluids were rich in fluorine. molybdenum, potassium, and hydrogen sulfide, and contained urani J.D. Rasmussen North American Exploration, 497 N. Main Street, um as fluoride complexes. The hydrothermal fluids re Kaysville. Utah 84037 acted with the wallrock resulting in precipitation of T.A. Steven uranium minerals. At the deepest exposed levels. wall US Geological Survey. MS 913, Box 25046. rocks were altered to sericite: and uraninite. coffinite, Denver Federal Center. Denver, Colorado 80225 jordisite, fluorite, molybdenite. quartz, and pyrite were R.O. Rye deposited in the veins. The fluids were progressively US Geological Survey. MS 963. Box 25046. oxidized and cooled at higher levels in the system by Denver Federal Center. Denver. Colorado 80225 boiling and degassing; iron-bearing minerals in wall P.O. Rowlev rocks were oxidized to hematite. and quartz, fluorite, US Geologi~al Survey, 6770 South Paradice Road. minor siderite, and uraninite were deposited in the Las Vegas, NY 89119 veins. Near the ground surface, the fluids were acidified S.B. Romberger by condensation of volatiles and oxidation of hydrogen Department of Geology and Geological Engineering. sulfide in near-surface, steam-heated. ground waters; Colorado School of Mines. 1516 lIIinois Street. wall rocks were altered to kaolinite, and quartz. fluo Golden. Colorado 80401-1887 rite, and uraninite were deposited in veins. Secondary J . Selverstone uranium minerals. hematite. and gypsum formed during Department of Earth and Planetary Sciences. Northrop Hall Room 14\. University of New Mexico. supergene alteration later in the Cenozoic when the 200 Yale Boulevard. NE. Albuquerque. upper part of the mineralized system was exposed by New Mexico 87131-1116 erosion. 478
(Bromfield et al. 19~Q). Since then. only sporadic ex Introduction ploration and development work has been undertaken and the mines are presently closed. The grade of ore at The most productive uranium deposits in the Marysvale Marysvale was typically a few tenths of a percent ura volcanic field of west-central Utah are in the Central nium (Walker and Osterwald 1963). The known vertical Mining Area (Fig. I). These uranium deposits contain occurrence of uranium-bearing minerals, according to significant quantities of fluorite and molybdenum. The mine workings and drill hole data. is at least 600 m deposits consist of hydrothermal quartz and fluorite (Tavlor ct al. 1951). veins that cut granitic and volcanic rocks and are in The Central Mining Area is located about 260 km terpreted to have formed in an epithermal vein system south of Salt Lake City and 5.6 km north-northeast of that is part of a volcanic/hypabyssal complex. They the town of Marysvale in the central part of the represent a known, but uncommon, type of deposit. Marysvale volcanic field. Callaghan (1939) published Other deposits having some similar features include the fIrst comprehensive study of the igneous rocks in the Jamestown. Colorado (Goddard 1935, 1946; Phair and area. Paul Kerr and his students P.M. Bethke. G.P. Shimamoto 1952: Kelly and Goddard 1969: Nash and Brophy, H.M. Dahl. J. Green. L.E. Woodward, and Cunningham 1973), Rexpar. Southern British Columbia N.W. Molloy, of Columbia University, under the aus (Preto 1978: Morton et a1. 1978: Curtis 1981), Maureen. pices of the US Atomic Energy Commission. prepared Northeast Queensland, Australia (O'Rourke 1975), Spor many reports on the area, which are summarized in Kerr Mountain. Thomas Range. Utah (Staatz and Osterwald et al. (1957) and Kerr (1968). Other pertinent publica 1956. 1959; Lindsey 1981); and Sierra de Pena Blanca. tions include Gruner et al. (1951), Walker and Osterwald Chihuahua. Mexico (Goodell 1985: George-Aniel et al. (1956), Gilbert (1957), Callaghan and Parker (1961), 1985). Relative to many of the other volcanic-related Willard and Callaghan (1962), Callaghan (1973), and uranium deposits described here. the Central Mining Shea and Foland (1986). The US Geological Survey Area deposits are young, well-exposed. and well-docu (USGS) began a study of the volcanic geology and ore mented. Uranium was discovered there in 1949 and at 5 deposits of the area in the mid-1970s that has continued least 6.4 x 10 kg of U 30X were produced by 1962 to the present. The volcanic setting and geochronology, and descriptions of other deposits in the volcanic field, are in Cunningham and Steven (1979a); Steven et al. (1979. 1984); Steven and Morris (1987) and Rowley et al.
0 Monroe Peak (1988a, b, 1994). Brief overviews of the tectonic setting 112 15' Caldera are in Cunningham et al. (1994, 1997). Cunningham and Steven (I 979b ) suggested that the uranium vein system Central -~ of the Central Mining Area may be underlain by a cli mining area ./ max-type porphyry molybdenum deposit; that sugges ~;;J tion is still valid but is as yet untested. Brief, descriptive .,/ 30' / overviews. and abstracts that include the uranium de GH posits, are Steven et al. ( 1981 ); Cunningham et al. (1980) / Red Hills and Rasmussen et al. (1985). This study presents a ge / @ /' Caldera netic model of the deposits. Relevant major publications / BC // e on uranium veins in volcanic rocks include US Geo logical Survey (1963), Rich et al. (1977), Goodell and I ~ / Marysvale Waters (1981), and the International Atomic Energy I I Agency (1985). I I \ ~AR Geologic setting / ' ...... o 5km The Marvsvale volcanic field is one of the largest volcanic fields in the west~rn United States and is located in the High Plateaus I subprovince between the Colorado Plateau and the Basin and Range Provinces. The field consists of middle to upper Tertiary volc~nic rocks which unconformably overlie Mesozoic and lower Cenozoic sedimentarv rocks. Most of the volcanic rocks in the field Fig. I Location map of igneous rocks. volcanic features. and the are part of an inte;mediate-composition. calc-alkaline sequence Central Minim!: Area in the eastern source area of the Mount Belknap that was erupted from about 35 to 22 Ma. The Central Mining Volcanics. This shows 23 Ma Monroe Peak caldera: PS. 21 Ma Area lies at the western margin of the 23-Ma Monroe Peak caldera, porphyritic stocks and volcanic domes: FCC. 20 Ma fine-grained ~he largest caldera in the Marysvale volcanic field (Steven et al. granite: Red Hills Caldera. source of 19 Ma Red Hills Tuff: C H. 1984: Rowley et al. 1988a.b). A principle host for the uranium ore I g Ma Gray Hills Rhyolite Member: Be. 16 Ma Beaver Creek stock: deposits described here is a quartz monzonite porphyry called the A R. 14 !'vIa Alunite Ridge includes Alunite Ridge center and Deer C~ntral Intrusion (formerlv called the "Central intrusive" by Kerr Trail Mountain center et al. 1957) which is a late-stock associated with the Monroe Peak caldera. The Central Intrusion is surrounded by 2.1 !'vb n:rl
Geology SW--__~~~ Five significant rock types are shown on the geologic map and cross sections of the Central Mining Area (Fig. 3). The oldest is the 23-Ma Central Intrusion monzonite (Tci) that underlies much of the area. Dunk hase ( 1980) reported that the intrusion contains anoma lously high contents of uramum. thorium, and zirconium. The Central Intrusion was intruded by the 20-Ma fine-grained granite (Tmf) which crops out just Alunite Ridge Stock (AR) north of the mapped area and widens downward in the (14Ma) deeper levels of the mines of the Central Mining Area. The next unit the 19-Ma alkali rhyolite Red Hills Tuff, Central mining area caps a prominent north-trending ridge in the northeast o 5km above hidden stock ern part on the mapped area (Fig. 3), where it uncon (18 Ma) I I formably overlies the Central rntrusion . The source of the Red Hills Tuff is the Red Hills caldera just west of the Fig. 2 Diagrammatic sketch showing postulated relations in the roots Central Mining Area. All orthe aforementioned units are of source areas for the Mount Belknap Volcanics. Same units as Fig. I cut by brown. flow-banded. rhyolite dikes that are well 480
Fig . .'~ -+xl
Freedom 2 B mine Prospector Sunnyside Cloys mine shaft mines c Potts A c .~ ~ shaft A' ~ c C\J ~ ~ ~ Meters c ~ a ~o _f) Feet rn 7100 2150 0 (3 0 ____-_ ...... ti 2 Q) Qj 7000 a.
C "'S ell u- S S' Meters Feet Cloys :; 7400 2250 100 m :; mines ,If u..ell 7300 2000 7200 7100 2150 7000 2100 6900 6800 2050 6700
Prospector 0 mine c "'S ~ ell C\J u.. C' C 0 U Feet Meters Q) a. 2100 (/) 6900 IOU m e a.. 6800 2050 6700 II! 6600 2000 I Te' I 6500 1950 6400 6300 1900 6200 exposed just east of the mapped area (sample M689) and Fig. 3 A Geologic map and B-D cross sections of the Central Mining underground in mine workings (see "Kerr's dike", Area. Marysvale. Utah. Tci. 23-Ma porphyritic quartz monzonite Fig. 4; sample M408), on the Freedom 700 level cross-cut (Central Intrusion); Tmt: 20-Ma fine-grained granite: Tmr. 19-Ma Red Hills Tuff Member of the Mount Belknap Volcanics: glassy dikes. close to the VeA shaft (Fig. 3A)). Mineral separations of 18-Ma glassy rhvolite dike. Faults dashed where uncertain. dotted the glassy dikes contain minor biotite, hornblende. where c~nce~led:' har and half on down thrown side. Mine locations sphene, and molybdenite. The glassy dikes and uranium projected into the plane of the cross section deposits have similar spatial distributions·. The last sig nificant rock type is a breccia that is composed of cline and the Prospector mine, but the area where it rounded to sub angular fragments of the Central Intru would project to the surface is covered by colluvium. sion in a matrix of comminuted rock and glassy rhyolite The uranium deposits have been dated at 19-18 Ma and. based on drilling, forms a vertical pipe. The breccia (Cunningham et al. 1982). A 207Pb/204Pb_235 U ,204Pb is exposed underground between the Sugar. Daddy de- isochron age of 19.0 ± 3.7 Ma was obtained for 482
Tahle I Descriptions and locations of analyzed samples from the Central Mining Area. Marysvale. Utah
M29B Colorless Auonte vein sampks VCA-Freedom mine dump M49A Porphyritic. alkali rh yolite Jome M50 Fresh fine-grained granite. near the northern end of the Marysvale uranium district M93 Red Hills Tuff. US highway 89 at Deer Creek M270 Purple and clear fluorite veins cutting: silicilied quartz monzonite. Prospector mine dump M403 Vein of euhedral quartz crystals with younger tluorite Deepest exposure of the 2 vein. Prospector ll1ine. 300 level M407 Fluorite-quartz vein. end of drift under Sunnyside shaft M40XA Glassy rhyolite dike. ("Kerr's dike"). It is vertlcal and cuts quartz monzonite on the crosscut between the Prospector and Freedom mines. Zircon fission-track age or 1X . I ± O.X Ma and apatite age of 23 .3 ± 4.7 Ma (Cunningham et al. Il)X2) M444 Veins of euhedral quartz crystals cut by veins of purple Auorite Freedom 2 mine dump M547B Fluorite-quartz vein that cuts quartz monzonite. Same location as M701, M702. and M729 . Vertical 2A vein on the Freedom 800 level M600 Pyrite-bearing. sericite-bearin; . line-grained granite with miarolitic cavities. some lined hy purple Auorite Plumbic mine dump. Sericite age of 21 .1 ± 0.6 Ma (Cunningham et al. 1982) M602 Pyrite-bearing uranium vein. Rex vein. prospector 4 mine. 12 m from 2 zone M621 Fresh quartz monzonite. just west of Flattop hill M663 Purple fluorite vein. sample from portal of the fluorite vein on west side of district M667 Pyrite-fluorite-uranium veins in sample from dump of the Freedom mine M674 Altered. punky, quartz monzonite with pervasive hematite. cut by sooty uranium veins. M674A is the red wall rock. Prospector 4 mine M688 Relatively fresh. hard, quartz monzonite of the transition zone. cut by I-em-wide uranium vein that is bordered by I-cm-wide band of hematite. Sample M68XA is within the hematite zone. Sample M688B is outside the hematite zone. 5-9 cm from the vein M689 Glassy rhyolite dike. Cuts quartz monzonite I km northeast of the Freedom mine. Zircon fission-track age is 19.0 ± 1.0 Ma (Cunningham et al. 191<2) M699 Pyrite-jordisite veins cutting fine-grained granite. Prospector mine. 300 level M701 Uranium-purple/clear fluorite-quartz veins cutting tine-grained granite. same location as samples M702 and M729A. 2A vein. Freedom 800 level M702 Fine-grained granite containing pyrite and cut by uranium-molybdenum veins, same location as samples M701 and M729A. 2A vein. Freedom 800 level M704 Fine-grained granite adjacent to the 2a vein on the Freedom 900 level. Sample M704A is bleached and altered, within 5 cm of the vein. and gives a sericite age of 20.5 ± 0.7 Ma (Cunningham el al. 1982). Sample M704B is fresher than M704A. 5- 10 em from the vein M706A Altered quartz monzonite containing sooty uranium veins and some hematite M711A Altered, limonite-stained. quartz monzonite adjacent to Seegmiller vein. contains pyrite. Freedom 700 (Prospector 300) level M714 Gray breccia containing angular fragments of quartz monzonite as much as several cm diameter in a gray matrix of rock flour and rhyolite glass. Sugar Daddy incline. 2 sublevel M715A Altered quartz monzonite. center of Seegmiller vein. by VCA headframe M715B Altered quartz monzonite. 10 em from center of Seegmiller vein M715C Altered quartz monzonite. 25 em from center of Seegmiller vein M715D Altered quartz monzonite. 36 cm from center of Seegmiller vein M715E Altered quartz monzonite. 51 cm from center of Seegmiller vein M715F Altered quartz monzonite. 183 cm from center of Seegmiller vein M729 Purple ftuorite-quartz-uranium veins containing jordisite and ilsemannite. Sample M729A is altered fine-grained granite adjacent to veins. Same location as M547, M701, and M702. 2A vein, Freedom SOO level M766A Altered. limonite-stained. quartz monzonite adjacent to high-grade uranium vein. Freedom 2 mine. depth unknown. Collected by Gerry Brophy \1767 Fluorite veins. Sample M767A is green tluorite and M767B is purple fluorite. Freedom mine. I vein. I upper level. about 30 m below present ground surface M768 Green. purple. and clear fluorite veins. Sample M76SA is green fluorite that appears younger than M768B (purple). 1A vein on the I upper level of the Freedom. about 30 m below present ground surface M770 Purple fluorite-quartz-uranium veins containing jordisite and ilsemannite. Sample M770A is purple. M770C is clear. and M770D is purple fluorite. Same location as M729A . 2A vein. Freedom 800 level M776 Pyrite-bearing quartz monzonite cut by fluorite veinlets. Drill site near south end of uranium mining district. sample is of drill core from the 267 111-284 m interval \1777 Pyrite-bearing. slightly-altered. tine-grained granite from dump of the Poison shaft. Mine workings do not extend rar from the shaft M793 Pyrite-bearing. tine-grained granite. Drill cllttings from hole LS 14-\. 116 m- 128 m interval M794 Pyrite-bearing fine-grained granite cut by uranium-fluorite bearing- Prospector I vein. Prospector mine. 500 level. west end of drift Pyrite-bearing fine-grained granite. Drill hole P4--1-C. depth 268 m Table 2 Analyses of Igneous rocks in the Marysvale Uranium District. Utah. Major oxides, and recalculated analyses on water-free basis. ;:rc: in weight percent. U and Th in ppm
Field No. M715A M715B M715C M715D M715E M715F M688A M688B M49A M704A M704B M50 M502 M408A Ident. Altered Altered Altered Altered Altered Altered Hematite Fresher Porph All Alt Fresh Red Hills Kerr's Tci Tci Tci Tci Tci Tci Tci Tci dome Tmf Tmf Tlllf Tlllr dike
Sine 57 .30 57.50 56.20 55.60 56.60 58.20 59.50 55 .80 71. 70 53.70 6740 6X.40 72.60 69.60 AI~O, IX.OO 17.00 16 .80 16.50 16.80 17.30 16.40 16.10 13.2() 19 .30 IJ.IO 13 .l)O 1240 11 .9() Fe~(), 3.71 5.D8 6.78 6.30 4.!W 4.96 2.54 2.37 1.10 4.23 334 1.15 (l,72 (J.55 FeO 0.38 0.32 0.45 0.88 1.89 1.35 2.64 3.21 0.44 0.59 0.62 1.07 O.2..t O.2..t MgO 1.10 1.10 1.30 I. 70 1.90 1.40 2.20 1.80 OA5 1.20 0.97 (H;tl 0.23 0.27 CaO 2.09 1.83 2.19 2.05 3.42 3.05 2.10 5.36 1.10 1.26 O.5..t 1.5 X 0.5:" 2.00 N Fig. 4 Underground photograph of verticaL now-handed. hrown. glassy dike ("Kerr's dike") cutting line-grained granite on the Freedom 700 crosscut near the VCA shaft. Dike is about 0.5 m wide and hends to the right above the hammer. The dike is locally Jevitritied and contains uranium and molybdenite. Host rock is the Central Intrusion quartz monzonite. The dikes are co-extensive with the uranium deposits and are interpreted to he apophyses on an underlying stock that is genetically related to the deposits. This is the location of sample M408 whole-rock pitchblende + fluorite vein samples. Another age of 16.5 ± 4.3 Ma was obtained using hydrothermal quartz in the veins as a natural external detector for Fig. 5 Vertical exposure of the Freedom 2A vein on the Freedom 800 fission tracks from adjacent pitchblende. Sericite from level showing the anastomosing network of dark yeinlets containing fluorite, uraninite. jordisite, coffinite, quartz, and pyrite cutting the the wallrock adjacent to the Freedom 2A vein on the lighter colored. sericitized, and pyritized fine-grained granite Freedom 900 level (the deepest available level of the mine) gives a K-Ar age of 20.5 ± 0.7 Ma, which is slightly older. considering that the uranium mineraliza tion postdates the 18.9 ± 0.7 Ma Red Hills Tuff in the Upper and Lower Cloys mines. Here. pitchblende and fluorite ore bodies occur within the tuff and argillically altered rocks extend along the contact with the under lying Central Intrusion. Structural geology The main uranium veins in the Central Mining Area occupy near-vertical faults that trend N. 55° to 65°E. (Fig. 3: Kerr et al. 1957: Callaghan 1973). The veins tend to be open space fillings that fill anastomosing structures as much as n.s m wide: an example is the Freedom 2A \cin on the Freedom 800 level (Fig. 5). Some of the Fig. 6 Flat-lying "pull apart" vein on the side of a drift on the richest ore occurs in steeply plunging shoots at fault Prospector 200 level. Vein material is mostly dark fluorite with uraninite intersections. Structures continue at depth but mineral and some pyrite. Light-colored host rock is the Central Intrusion quartz monzonite \vith local hematite. The "pull apart" motion was vertical ization \\'as non-economic. An important type of vein and the wall rock fragments can be fit back together if the vein material recognized during underground mapping is flat-lying, was removed. H([IIIII/(;'1' is at the hose of the vein. abundant in the upper levels of the mines. These veins selvages an(~ above that to more rcrvasive hematite are somewhat similar in appearance. but not in size. to (Fig. 38). Molyhdenum is widely present in the deposits the near-horizontal veins of the Panasqueira. Portugal. and in places IS as abundant as uranium. although it was tin-tungsten deposit structures that formed by erosional not recovered in the treatment process (Kerr 196X: Cal unloading and hydraulic pressures (Kelly and Rye 1(79). laghan 1973). It is present as molybdenite in the glassy but we ascribe a different origin to the Marysvale veins. dikes. Jordisite (amorphous MoS 2) is the common mo Most of the major mines are located in and adjoining lybdenum mineral in the veins at depth (Walker and Os blocks that have been uplifted relative to the surrounding tcrwald 1956). where it can be detected by what the miners area (Fig. 3). The Prospector mine block is an upthrown called the p-test (jordisite commonly alters to blue ilse block bounded by the Prospector I. 2. and 706 faults mannite (Mo,Ox . nH 20) when exposcd to urine). (Fig. 3 B). Southwest of the Prospector mine. across the Broken and mineralized ground within 30-40 m of Prospector 2 fault. the block is down-dropped and con the surface in the Central Mining Area contains a vari tains the breccia pipe. The wedge-shaped block between ~ty of oxidized uranium minerals. These secondary the Seegmiller vein and the Bertelsen vein. that includes minerals include uranopilite. uranophane. ~-urano the Sunnyside shaft and is adjacent to the yeA shaft. is an tile. schroeckingerite, autunite. meta-autunite. johan uplifted block relative to the adjacent blocks. The Free nite, tyuyamunite. rauvite. torbernite. meta-torbernite. dom I vein is down to the north and the Freedom 2A vein phosphuranylite. and zippeite (Walker and Osterwald is down to the south. indicating that the block between 1956: Kerr 1968). A new mineral umohoite ((U02) them is uplifted. The three blocks to the north of this. Mo04 . 4H20) was discovered at Marysvale (Brophy bounded successively on their south sides by the Freedom and Kerr 1951) where it has been found chiefly on the 1 vein. Freedom I A vein. and Cloys fault. step down to 3rd level of the Freedom Number 2 mine. It occurs just the north, and the Cloys fault is a reverse fault. The below the oxidized zone and may be supergene in origin mineralized block containing the Potts shaft and most of (Walker and Osterwald 1956). Carbonate. gypsum. and the underground workings of the Cloys mine (Fig. 3C) is iron and manganese oxides also appear to be mostly an anomalous block because it is down-dropped. like a secondary minerals. This relationship of supergene keystone block. The entire highly fractured area of the uranium minerals to the present surface indicates rela Central Mining Area thus seems clearly to have been tively late oxidation after late Tertiary erosion had ex uplifted relative to adjacent less fractured ground. and the posed the upper parts of the mineralized system. There is areas that were mineralized are generally uplifted the no reported enrichment of uranium in the assemblage of most. Post-mineralization northeast- and northwest secondary minerals overlying the deposit (Walker and striking basin-range faults cut the uranium veins. Osterwald 1956). Mineralogical data has been supplemented by chem ical analyses to assist characterizing the ore assemblage. Ore and gangue mineralogy Semiquantitative spectrographic analyses of vein sam ples show that the are contains as much as 3000 ppm As. A description of the mineralogy of the uranium deposits is 50 ppm Co. 500 ppm Sb, 1500 ppm Tl. trace W, and given in Walker and Osterwald (1956), however. para highly variable Th. Walker and Adams (1963) cite ana genetic information is sparse. Walker and Adams (1963) lyses of the are that indicate the presence of Au. Ag. Cu. report early quartz and chalcedony, pyrite. fluorite. and Pb. and Y. but they could not be confirmed in analyses adularia (which could not be confirmed in the present as part of this study. X-ray diffraction patterns of min studies) were followed by a period of brecciation. then eral separates have shown that minor scheelite is present deposition of these minerals continued along with in the 2A vein at the Freedom 800 level. marcasite. pitchblende, magnetite. hematite. and jordi site; carbonate formed mostly in the late stages along with supergene gypsum and iron and manganese oxides. The Alteration principal ore mineral mined at the Central Mining Area was pitchblende that was finely crystalline. granular to The uranium veins are surrounded by envelopes of al massive at depth, and sooty near the surface. Coffinite has tered rock that flare in width from several cm in the been identified from lower levels of the veins. Most deepest parts of the mines to as much as 2 m at the pitchblende is intimately associated with jordisite and surface (EI Mahady 1966: Kerr \968). In the deepest dark fluorite in anastomosing veinlets: in the deep levels accessible leveL the Freedom 900 level of the 2A vein. of the mines, banded purple fluorite is in places inter fine-grained granite wallrock is altered to sericite and grown with quartz. pitchblende. and coffinite. The fluo pyrite in a layer about 5 cm wide on either side of the rite content in most veins increases with depth (Walker vein; no hematite is evident. Sample M704A (Table 2) is and Osterwald 1956), and some fluorite veins are barren the sericitized equivalent of sample M704B. the freshest of metals. Pyrite content of the ore increases downward. equivalent of which is M50. Sericite replaces feldspar and makes up an estimated 15% of primary vein material and the mafic minerals are pyritized. X-ray diffraction in the deepest mine levels (EI-Mahady 1966). Pyrite gives studies show no kaolinite is present in the wallrock way upward, first in a transition zone with h~I?atite vein adjacent to the deep veins. The mineralogy changes systematically upwards. At 40 higher levels, pyrite content in the vein walls decreases, sericite is absent. and the veins are surrounded by a few D Green fluorite em-wide envelope of hematitized, but much less altered, wallrock. In this hematite transition zone, sample 30 o Clear fluorite M688A (Table 2), collected adjacent to the vein in the Central Intrusion, is the hematitized equivalent of • Purple fluorite M6888 (collected 5- 9 em from the vein). Sample 20 M688A is characterized by a strong hematite color. Near t23 Quartz the present ground surface, above the pyrite-hematite transition zone (Fig. 38), pyrite is virtually absent. the wallrocks near the veins are altered to kaolinite, sooty 10 pitchblende veins are associated with much more per vasively distributed hematite in the wall rocks, and the widths of vein alteration-mineral envelopes are on the order of a couple of meters. Sample M715 A-F was collected at the ground surface from the Seegmiller vein 100 150 200 250 300 Homogenization temperature (DC) outwards to 183 cm. Data in Table 2 show that as the vein is approached, hydration, SiO:!, and Al:!O:l gener Fig. 7 Histogram of fluid inclusion homogenization temperatures ally increase and FeO, MgO, and CaO decrease. from quartz and fluorite samples from the Central Mining Area, Marysvale. Utah Fluid inclusions and stable isotopes tends to occur in the deeper levels of the mines, and green fluorite tends to occur at higher levels. Homogenization temperatures and salinities of 132 fluid The histogram (Fig. 7) of fluid inclusion homogeni inclusions were measured in U~ quartz and fluorite sam zation temperatures for all samples shows a peak at ples. The samples and their locations are described in about 200 °C.Quartz fluid inclusion temperatures are Table I and the fluid inclusion data given in Table 3 are generally hotter (190-260 °C) than most of the fluorite plotted on Figs. 7 and 8. Fine-grained quartz is locally fluid inclusion temperatures, but there is overlap in the intergrown with fluorite. Clear and purple fluorite are 200-250 °C range. The median homogenization tem commonly part of the same crystaL with clear fluorite peratures of fluid inclusions in green and purple fluorite having purple growth bands: locally, however. massive is 190-200 °C, clear fluorite is slightly less at 170- purple fluorite occurs without clear fluorite. Green fluo ISO °e, and quartz is hotter at 210-220 °C. Coexisting rite is generally paragenetically younger than purple vapor and liquid-rich inclusions in fluorite were found in fluorite, but in places the sequence is reversed. Vein quartz the near-surface samples and indicate boiling occurred Table 3 Fluid inclusion data from quartz and Huorite veins in the Central Mining Area. Marysvale. Utah Sample Host ~umber of Th Salinity Comments number mineral inclusions "C wt. 0;;) NaCI equivalent M29B clear fluorite 12 135-190 0.18-0.53 Clear fluorite with purple rims M270 purple fluorite -') ISO-lSI 0 Primary inclusions along purple growth band M403 cuhedral quartz 13 201 - 271 0.0- 0.88 Euhedral quartz crystals that are pre-purple fluorite M407 purple fluorite .f 189-192 0.18- 0.35 Purp\e fluorite that is younger than green fluorite in same sample \1407 green fluorite 13 196-239 0. 18-0.53 Part of sample above M444 euhedral quartz 13 194-203 0. 18-2.24 Euhedral 'quartz crystals cut by purple fluorite M547B quartz I 230 Quartz intergrown with purple fluorite M663 purple Hum'ite II 125- 186 O.O- O.S3 Purple fluorite vein. Includes homogenization to vapor at 143 °C and 186 °C M701 purple Huorite 6 121 - 177 0.88·-2.57 Purple fluorite is interbanded with clear fluorite "1701 dear Huorite 5 162--203 0.35 -0 .88 Part of sample above \1729 quartz .3 230-258 2.07 "1767:\ l!reen fluorite 12 130·-203 0- 0.71 \V67B purple Huorite 11 192-197 0.71 --0.88 Part of sample above \176:-, clear fluorite 6 113-272 Data highly variable (boiling'?) M76XB purple fluorite 5 188-194 1.4 Part of sample above :v1770A purple tluorite 5 157--262 0. 18- 1.23 Data highly variable \1770C clear tluorile 5 120-167 Part of sample above \177()[) purple fluorite 6 207-215 1.05 - 1.23 Part of sample above 4X7 50 pcraturcs of inclusions in Ljuartz from the Freedom XOO level range from 230 -260 0(' and most or the inclusions in l1uorite (rom the same level range from IlJO-- 220 °C: 40 most homogenization temperatures in hoth minerals at this level are in the 2()O 24() 0(' range. Homogenization (/) Green fluorite c temperatures from inclusions in tluorite I'rom the Free o '(j) Clear fluorite dom I upper level. high in the mine. arc generally in the -5 30 range of 170- 200 °C . .S o Purple fluorite Measured fluid inclusion freezing point depression Q; temperatures were modeled on the system NaCI-H~O .0 E 20 Quartz and converted to NaCI cLjuivalents (Potter ct al. IlJ7X). :::::I Z The inclusion nuids are generally dilute. The median salinity is 0.0--0.5 wt.( ~~ ) NaCI cLjuivalent for quartz and for green and clear fluorite. and slightly higher. 0.5-- 1.0 10 wt. (Yc , NaCI cquivalent. fo r purple Iluoritc. A small, generally rhombic daughter mineral was noted in fluo rite of different colors: it did not dissolve upon heating o 2 3 to homogenization temperatures. Carbon dioxide as Weight % NaCI equivalent liquid or clathrates was not found in any samples. In Salinity generaL the hotter. more saline fluid inclusions are III Ljuartz and the cooier, least saline inclusio ns are in Fig. 8 HI stogram of fluid inclusion salinities from quartz and t1uor i t~ tluorite. samples from the Central Mining Area, Marysvale, Utah. Measured 1 A series of 8 Xo analyses were made on whole rock freezing point depression temperatures have heen modeled on the system NaCI-H ~ O and converted to NaCI equivalents (Potter et al. samples, feldspar separates, and vein quartz to examine (978) the water-rock history of fluids in the veins and wall rock (Table 4). 8 1X O values range from 0.2 to 4.0 for whole rock samples and from -3.7 to 6.0 for feldspar at these levels. Fluid inclusion temperatures in minerals phenocryst separates; wallrock samples collected adja deeper in the mine are hotter than those near the present cent to the veins vary systematically with depth be ground surface (Tables 1 and 3). Homogenization tem- coming heavier toward the ground surface (Fig. 9). Two Table 4 Stable isotope analyses 1X 61!10 (")I XO of rock, vein, and mineral Sample 8 O 8D 8~S samples from the Central number Who le rock Feldspa r Vein qua rtz Fluid in fluorite Pyrite Mining Area. Marysvale. Utah M50 1.3 - 2.0 M407 1.5 -1 33 M408 -1 .5 M547B 11.4 -137 -4.3 M600 4.8 M602 -5.0 M621 2.3 4.0 M667 -3.8 M674A 3.3 6.0 M688A 1.2 1.2 M688B 3. 1 4.0 M699 6.2 M702 -0.9 M704A - 3.7 0.0 M704B -2.7 -0.4 M706A 4.0 5.2 M711A - 2.4 -2.9 M715A 4.7 7.4 M715E 4. 1 3.9 M715F 3.9 4.5 M729A 0.2 M766A 1.3 3.7 M768A - 140 M770 -130 M776 0.9 M777 2.7 M793 -0.6 M794 -1.8 M846 6.6 composition volcanic centers (Cunningham et al. 1994; 1997). Fluorine is 0.07-0. 14 wt. % and uranium is 10.1- PRESENT DAY GROUND SURFACE \8 .2 ppm in these Mount Belknap volcanic rocks. The 7.4 3.9 4.5 Sr content of the glassy dikes associated with the ura 4.7 4.1 3.9 Shallow, 18 Ma ground waler nium deposits (675 ppm; Table 2) is the highest of any of the alkali rhyolites that were emplaced from 23 to 5.2 18 Ma and the silica content (76.76 wt. 0/0. recalculated 4.0 ~ volatile free) is also the highest in the same rocks. The 23 Ma quartz mODzonite southwestward progression of igneous activity within 6.0 the eastern source area probably reflects successive em .5 3.3 placement of shallow stocks above a composite high ;;;.~ o level magma chamber (Fig. 2). Peiffert et al. (1996), has ::i! ~ 1.2 4.0 shown fractional crystallization in peralkaline suites "C 1.2 3.1 leads to uranium mineralization. In the Marysvale sit ~ uation. the changes that took place with time, an in ~ -2.4 f 18 crease in silica and volatile content and decrease in .s~ Exchange zone from 20 Ma 0 0 -! deplered fluids phenocryst content, may reflect tapping successively ::i!--;U shallower levels of a zoned magma chamber or pro \ l-ioMa fme:;;med grani-; gressive differentiation of a high-level magma chamber -3.7 -2.7 with episodic input of thermal energy .. Structural constraints Interpretation of the structural evolution of the Central Mining Area is heavily dependent on underground mapping and rock-ore textural observations. Incipient Fig. 9 Diagrammatic cross section through the Central Mining Area basin-range faulting may have begun with the start of showing the relationships between igneous rocks, mineralized rocks, and stable isotope analyses. The OIRO data from feldspars (subscript rhyolite volcanism, about 3 Ma before the uranium de j.~) are shown in normal print whereas data from whole rocks posits were formed. The 1:24 000 scale mapping (Cun (subscript H'r) are in italics. The samples are shown in their relative ningham and Steven 1979b; Rowley et al. 1988a) has vertical positions. The values are shown in their relative horizontal shown that the area is dominated by largely post-min positions: the data shown adjacent to the mineralized vein was collected immediately adjacent to the vein and values shown farther to eralization northeast- and northwest-striking basin the right were collected successively further away. Detailed sample range faults; Osterwald (1965) pointed out that the locations are in Table 1. Relatively fresh 23 Ma quartz monzonite has Central Mining Area is located where these regional 1X 6 O feld spar of 4.0 and whole rock of 2.3. Relatively fresh fine faults change direction. Detailed surface and under grained granite (M50) has OIHO feldspar of -2.0 and whole rock of 1.3 ground mapping has shown that the area corresponding to the Central Mining Area was uplifted relative to the samples of vein quartz gave 8 1R O values of 1.5 and 11.4. surro unding area (Cunningham and Steven 1979b) prior In addition. 80 was examined in inclusion fluids in to this late faulting. The present study demonstrates that fluorite and values for four samples range from -140 to the distribution of uranium deposits corresponds to the -130. These data place constraints on the relative distribution of individual uplifted blocks. abundance of magmatic and meteoric water components A structural model is proposed (Fig. lOA-C) to ex in the hydrothermal fluid. The 834S in pyrite have a large plain the correspondence of the uranium- and molyb range of-5.0 to 6.6 05 samples. Table 4) and constrain denum-bearing rhyolite dikes with the mineralized area the source of sulfur in the hydrothermal system. and the significance of the breccia pipe and "pull apart" textures in the flat-lying veins. In the model, a rhyolite stock was emplaced into a fractured area, and as mag Discussion matic pressure increased, differentially uplifted roof blocks bounded by high-angle faults. Glassy dikes, that Detailed surface and underground geologic mapping, are apophyses of the stock, were intruded along the combined with regional relations and geochemical/iso growing faults and these dikes filled faults that offset the topic fluid inclusion data. provides insight into the contact between the Central Intrusion and fine-grained genesis of the hydrothermal uranium deposits in the granite (Fig. lOA). A breccia pipe formed and relieved Central Mining Area. Lead and strontium isotopic evi the upward magmatic pressure (Fig. 108). As this dence indicates that the Mount Belknap Volcanics. pressure was relieved, the uplifted blocks began to settle \v'hich are part of the alkali rhyolite end member of the back, but tended to "hang up" so that flat-lying, "pull himodal suite. were derived by partial melting of the apart" structures formed by differential downward hatholith that had previously fed the intermediate movement. particularly near high angle faults (fig. IOC). Hydrothermal fluids activated hy the rhyo lite stock permeated the hroken carapace and mineral ized the high-angle and nat-lying faults. Fluid inclusion constraints An estimate of the depth of mineralization comes from fluid inclusion data. The highest temperature at which most fluid inclusions in fluorite from the Freedom I upper level homogenize to liquid is about 195°C; these inclusions contain about O.g-l.4 wt. (X) NaCI equivalent. and arc associated with sporadic vapor-rich inclusions. These are some of the higher salinities in the deposit and the presence of vapor-rich inclusions suggests sporadic boiling of the fluid as does this slight increase in salinity A of the fluids near the surface. The depth beneath the water table at hydrostatic pressures and boiling condi tions was about 145 m with a pressure of about 14 bar (Haas 1971) at the Freedom 1 upper level which equates to about 115 m of cover having been eroded from above the present ground surface. Fluid pressures. deduced from fluid inclusion homogenization temperatures in 1~ some shallow deposits such as the McLaughlin. Cali fornia, hot-spring gold deposit. and lulcani. Peru. vol canic dome precious-metal deposit, however. indicate pressures in the upper 200 m of the deposit generally exceeded hydrostatic and episodically exceeded litho static (Deen et al. 1994; Sherlock et al. 1995). Modeling the Marysvale data on a lithostatic gradient at 2.7 g/cm 3 gives a depth of about 53 m to the Freedom I upper level or about 23 m having been eroded. At these shal low depths, fluid pressures would be expected to be B generally hydrostatic and only increase toward litho static during episodes of deposition of quartz and fluo rite in the vein structures. This depth of 115m, is reasonable on geologic grounds because the deposit is adjacent to the Red Hills caldera that formed slightly before the deposit and the Red Hills Tuff on the caldera rim would have been piled higher than it is at present. Mineralogical and thermochemical constraints The mineralogical associations in the ore, together with 1 ~ thermochemical data, place geochemical constraints on the environment of ore deposition. The vertical zonation of wallrock alteration adjacent to the veins documents the upward changes in the hydrothermal fluid and its interaction with the wallrocks. In the deepest levels of the .-nine. the sericite-pyrite-fluorite-uraninite-q uartz as c semblage and lack of hematite suggests that H~S was the dominant sulfur species, the oxygen fugacity was low, Fig. lOA-C Model for the fonnation of the uranium deposits of the and that the fluids were near neutral. The presence of Central Mining Area. Marysvale. Utah. A Intrusion of a rhyolite stock quartz and coffinite in these deep ores indicates that the and associated dikes and lifting up of blocks above the stock. B F om1ation hydrothermal system was commonly saturated with sil ofa breccia pipe thus relieving pressure. C Blocks return to fonner position ica. Thermochemical constraints suggest the deep fluids but fonn flat-lying fractures containing "pull apart" textures that are then mineralized bv U-Mo-F fluids associated with the stock. A stockwork had log ./02 of about -47 to -50 and pH about 6 to 7 molybdenum deposit is postulated to be near the top oftl'le rhyolite stock (Fig. II A-C). The general correspondence of fluorite 490 A B -20 -20 HSO~ -30 -30 Alunite 0 0, .Q -40 -40 ...... --.. Kaolinite ~ ...... ~ .. -50 -50 2 3 4 5 6 7 2 3 4 5 6 7 pH pH Sulfur species I ron species Alteration minerals C -20 Uranium in solution -30 -40 -50 2 3 4 5 6 7 pH Uranium species ---- Uranium solubility _. -_. -_. and uraninite in the ore suggests that a uranium-fluorine Fig. llA-C Oxygen fugacity-pH diagram showing the distribution complex was probably the transporting agent for ura of alteration minerals. sulfur species, uranium speCies, and uranium solubility in the Central Mining Area uranium deposit. Boundaries nium. Romberger (1984) has shown that fluoride com are calculated activities. Conditions based on fluid inclusion and plexes. in particular uranyl trifluoride, are important in mineralogical data are 200 0c, 0.5 m NaCL 100 ppm F, 100 ppm S, the neutral region. Uranous fluoride complexes are im 10 ppm Fe. 100 ppm K. p CO~ = 1 atm. L U = 0.1 ppm. quartz portant in reducing environments but especially so at present. Coffinite has a stability that is close to that of urani nile + quartz, therefore the uranium solubility for coffinite is closely lower pHs; hydroxides are stable at neutral pHs (Green approximated by the calculations for uraninite. Pyrite is stable in the and Kerr 1953: Langmuir 1978; Romberger 1984: Parks H 2S field. A Sulfur species and alteration minerals; B iron species and Pohl 1988). Sulfidation of the mafics. as observed at tOQ:ether with arrOlI' showing chanQ:e of conditions as fluids moved to the deepest mine levels. can precipitate uranium and free sh~llower parts of the syste~: C ~ranium species and solubility with the uraninite field shaded tluorine by the reaction U02 F; + FeO (in silicates) ~2H::S = U02 + FeS2 + 3F- + 2H-t- + H 20. Fluorite \vould be deposited as calcium was released from altered thermal fluid can also act as a reductant for U02F 3, plagioclase. as well as by cooling (Holland and Malinin releasing F- to solution. Higher in the system, as sericite 1979) of the fluid. The presence of H2S in the hydro- plus pyrite in the wall rock gave way to hematite selv- 491 ages, the formation of hematite and precIpItation of after the tine-grained granite was emplaced. The is'XO in uraninite resulted from the coupled redox reaction both feldspar and whole rocks adjacent to the veins U02 FJ + 2FeO (in silicates) + H20 = U02 + Fe20:< becomes systematically heavier upward: whole rock + 3F- + 2HT. At the top of the system, pervasive 6 1:-10 changes from 0.2 per mil at depth to 4.7 per mil at hematite plus kaolinite and a lack of pyrite are consis the surface and feldspar 8 1X O changes from -3.7 per mil tent with condensation of volatiles and mixing \vith at derth to 7.4 per mil at the surface. This, combined shallow, steam-heated waters, resulting in oxidation of with the constraints on depth of penetration of the fluids H 2S, decrease in pH, and reaction of the fluids with the from the chemical changes, argues for the pattern of wall rock (Fig. II A-C; Barton et al. 1977: Henley I YX4: isotope signatures in the wall rocks adjacent to the veins Reed and Spycher 1985). being due to illteraction with the isotopically heavy water in an 1X Ma hydrothermal fluid. This isotope enrichment in the wallrocks occurs where the hydro Isotopic constraints on fluids and minerals thermal alteration typically extends further from the veins. This increased extent of alteration probably co The chemistry of the wallrocks documents the changes incides with the degassing of acid volatiles from hydro in the chemistry of the hydrothermal fluids and the ex thermal fluids, as suggested by evidence of boiling in the tent that they penetrated into the wallrock adjoining the fluid inclusions of shallow samples. This boiling may veins over the vertical range of the mines. The fluid was have resulted in further 6 1xO enrichment in the hydro potassium-rich as shown by the relative increases in K 20 thermal fluids. Also, as shown later from isotopic evi in samples adjacent to the veins (Table 2). At the deepest dence. this area in the veins was also a mixing zone levels of the mine, the distribution of sericite (sample between the hydrothermal fluids and overlying ground M704A, Table 2) shows the significantly reactive fluids water. Thus, alteration in the wall rocks surrounding the penetrated about 5 em from the vein. The elevated K 20 upper part of the veins at Marysvale probably formed in and uranium contents in sample M704B (collected about a setting analogous to that in the Creede, CO mining 10 cm from the vein) shows hydrothermal fluids pene district where the formation of a clay cap resulted from trated at least that far. In the transition zone above, the degassing of hydrothermal fluids and condensation hematitically altered rock adjacent to the vein (sample of acid volatiles in the overlying steam-heated ground M688A) contained 194 ppm uranium (Table 2), slightly water (Barton et al. 1977; Bethke 1984). altered rock about 10 cm from the vein (sample M688B) The 80 of the inclusion fluids in the fluorite range contained 25 ppm uranium, and unaltered Central In from -130 to -140 per mil. The average 80 value of trusion quartz monzonite (sample M621) contains only magmatic fluids in the Marysvale area was probably 13 ppm uranium. At the ground surface (samples M715 close to -70 per mil (e.g., Beaty et al. 1986; and Rye A-F) uranium values significantly above background 1993). It is clear that the hydrothermal fluids were values have been detected as much as 50 cm into the wall dominantly meteoric although they probably contained rock (Tables I and 2). The elements K, Mo, and W magmatic components including a small percentage of follow a pattern similar to uranium. In contrast, thorium magmatic water. The 81g0H~O of the hydrothermal fluids was enriched in vein samples as much as twice, but from the 800 foot level of the Freedom 2A vein in the generally showed little, if any, enrichment in the wall Freedom mine is about 1.5 per mil as calculated from the rock. A fission track study of U 235 distribution adjacent 8 1H O of quartz and a depositional temperature of to veins in the Central Intrusion showed nuclide move 230°C. This calculated value indicates that the meteoric ment by diffusion as much as 5 cm (Shea 1982). water hydrothennal fluids were highly exchanged. Such Oxygen isotope studies of the rocks in the Central exchanged fluids are consistent with the 180 enriched Mining Area by Shea and Foland (1986) indicate that feldspars and whole rocks in the wall rock alteration in 18 the rocks underwent massive subsolidus hydrothermal the upper part of the vein systems. The 8 0 H,o of fluids exchange with circulating meteoric water; and our data in equilibrium with vein quartz about 33 m higher in the confirm this and show that much of the exchange was Sunnyside mine was between -7.9 and -10.9 per mil as prior to mineralization. The sample of the fine-grained calculated from 8 18 0 values from quartz and from a granite (M50) that could be collected furthest away from depositional temperature range of 190 to 240°C. Vein the mining area has a 8 180 whole rock of 1.3 and a 8 180 quartz was obviously deposited in a fluctuating mixing of feldspar of -2.0 and the corresponding sample of the zone between highly exchanged meteoric water hydro quartz monzonite (M621) has a 8 18 0 whole rock of 2.3 thermal fluids and probably steam-heated largely unex and a 8 180 of feldspar of 4.0 (Table 4). The spatial re changed ground water much as in the Creede, CO lation of 8 18 0 values of wallrock feldspars and whole hydrothermal system (Foley et al. 1989: Rye et al. 1988). rocks in a vertical section through the mines is presented The 834S values of sulfides range from -5.0 to 6.6 per schematically in Fig. 9. Samples collected away from mil. There is no consistent variation of the 834S values in the veins and near the fine-grained granite-quartz mon time and space in the veins, and the variations can not zonite contact show low 8 180 values for feldspar that are easily be assigned to changes in the physical chemical similar to regional values and probably resulted from environment of are deposition. The 834S values of drill isotopic exchange with meteoric waters about 20 Ma, core in the volcanic rocks underlying the district range 492 from -15.3 to 5.1 per mil as determined from isotope probably gypsum and some of the shallow hematite al studies of the alunite deposits in the area (Cunningham teration. et al. 1984). The range of 6 14S values of pyrite in the veins is consistent with an isotopically variable source. Acknowledgements The authors thank Steve Ransom for his as The large range of 834S values in the sulfur source region sIstance with computer drafting. Energy Fuels Corporation for allowing access to thc mines and allowing and supporting JDR's may ultimately be due to the fact that prior to volcanism participation in the study, We appreciate thought-provoking and Jurassic evaporite beds originally underlay the district helpful reviews hy Paul Barton. Philip Gnodell. Richard Grauch. (Cunningham et al. 19X4), and repeated magma gener Jean Duhessy. B.A. Hofmann. and Bill Miller. ation and hydrothermal activity resulted in mixing of the evaporitic sulfate. the products of thermal reduction of the evaporitic sulfate, and magmatic sulfur in magmas References and fluids. Anderson JJ. Rowley PD (1975) Cenozoic stratigraphy of south western High Plateaus of Utah. In: Anderson JJ. Rowlev PD. Fleck RJ. Nairn AEM (cds) Cenozoic geology of southw~stern Conclusions High Plateaus of Utah. Geol Soc Am Spec Pap 160: pp I- 51 Barton PB J r. Bethke PB. Roedder E (1977) Environment of ore The hydrothermal uranium deposits of the Central deposi lion in the Creede mining district. San Juan mountains, Colorado: Part Ill. Progress toward interpretation of the Mining Area, Marysvale. Utah. were deposited in the chemistry of the ore-forming fluid for the OH vein. Econ Geol fractured roof above a stock from a recurrently active 72: 1-24 magma chamber during episodic magmatic evolution of Beaty DW. Cunningham CG. Rye RO. Steven TA. Gonzalez that chamber. The stock lifted overlying rocks as fault Urien E (1986) Geology and geochemistry of the Deer Trail Pb blocks and led to formation of a breccia pipe; escape of Zn-Ag-Au-Cu manto deposits. Marysvale district. west-central Utah. Econ Geol 81: 1932-1952 magma and volatiles from the breccia pipe relieved Best MG. McKee EH. Damon PE (1980) Space-time-composition magmatic pressure, and as the blocks settled back. flat patterns of late Cenozoic mafic volcanism, southwestern Utah lying fractures with '''pull apart" textures formed locally. and adjoining areas. Am J Sci 280: 1035-1050 The source stock was intruded into a cluster of older Bethke PM (1984) Controls on base- and precious-metal mineral ization in deeper epithermal environments. US Geol Surv stocks that shielded the fluids from interaction with the Open-File Rep 84-890: 40 sedimentary rocks that underlie the volcanic field and Brophy GP. Kerr PF (1951) Hydrous uranium molybdate in are exposed on Deer Trail Mountain. Marysvale ore. Ann Rep June 30. 1952 to April 1. 1953, US The wall rocks were altered by low I Xo meteoric water Atomic Energy Comm. RME-3046: 45-51 Bromfield CS. Grauch RI. Otton JK. Osmonson LM (1982) Na fluids prior to mineralization and probably right after tional Uranium Resource Evaluation of the Richfield quad the intrusion of the 20 Ma granite. Mineralization in the rangle. Utah. US Department of Energy Rep PGJ/F-044(82): Central Mining Area was related to intrusion of the 94 p. 13 pIts 18 Ma stock. Geologic reconstruction and fluid inclu Budding KE. Cunningham CG, Zielinski RA. Steven TA. Stern sion and stable isotope data support a near surface (23- CR (1987) Petrology and chemistry of the Joe Lott Tuff member of the Mount Belknap Volcanics. Marysvale volcanic 115 m) depth of formation of the deposit near the fluc field. west-central Utah. US Geol Surv Prof Pap 1354: 47 p tuating interface between an exchanged meteoric water Callaghan E (1939) Volcanic sequence in the Marysvale region in and overlying steam-heated ground water. Episodes of southwest-central Utah. Trans Am Geophys Union 20(3): ore deposition from the hydrothermal fluids were 43!:S-452 Callaghan E (1973) Mineral resource potential of Piute County, probably rapid, as suggested by the triggering action of a Utah and adjoining area. Utah Geol Min Surv Bull 102: 135 p breccia pipe. the presence of jordisite (amorphous Callaghan E. Parker RL (1961) Geology of the Monroe Quad MoS2), and the generally fine-grained nature of the ore. rangle, Utah. US Geol Surv Geol Quad Map GQ-155. Scale Isotopically exchanged, U-Mo-F-H S bearing, meteoric 1:24.000 2 Cunningham CG. Steven TA (1979a) Mount Belknap and Red water hydrothermal solutions with at least 3% salinity Hills calderas and associated rocks. Marysvale volcanic field. entered the fractures at depth at about 240 ± 20°C and west-central Utah. US Geol Surv Bull 1468: 34 deposited a uraninite-fl uorite-pyrite-coffinite-q uartz Cunningham CG. Steven TA (l979b) Uranium in the Central mineral assemblage in the veins and altered the wall Mining Area. Marvsvale District, west-central Utah. US Geol rocks to sericite. The temperature varied from about Surv Misc Invest S-eries Map 1-1177 Cunningham eG. Steven TA (1979c) Geologic map of the Deer 240°C to 190 °C over the 300 m vertical interval of the Trail Mountain-Alunite Ridge mining area. west-central Utah. exposed mine workings. These fluids boiled and the US Geol Surv Misc Invest Series Map 1-\230 degassed volatiles probably condensed in overlying Cunningham CG. Steven TA. Rasmussen JD (i 980) Volcanogenic steam-heated ground water. The resulting changes in ura;ium deposits associated with the Mount Belknap volc;nics. Marysvale \'olcanic field. west-central Utah (Abstr). Energy oxidation state and pH of these fluids led to the hematite exploration in the 80·s. Annual meeting of the southwest sec selvages and kaolinitized wall rocks in the upper part of tion. Am Assoc Petrol Geol EI Paso. Texas. Feb. 25-27. 1980. the ore bodies. Uranium is interpreted to have been P 22 transported mostly as uranyl trifluoride complexes and Cunningham CG. Ludwig KR. Naeser CWo Weiland EK. Mehnert H H. Steven T A. Rasmussen JD (1982) Geochronology of hy deposited as fluids reacted with the wallrocks and mixed drothermal uranium deposits and associated igneous rocks in with sl~:\m-heated ground water. Supergene alteration in the eastern source area of the Mount Belknap volcanics. th~ area led tn the formation of secondary minerals and :vtarysvak. Ctah. Econ Geol 77: 453-463 Cunningham CG. Rye RO. Steven TA. Mehnert H H (1 YX4) Ori hydrothermal ore deposits. Jllhn WIIe\ and SUl1s . New York. gins and exploration significance of replacement and velO-type PI"' 4615()~ alunite deposits in the Marysvale volcanic field. west cenlral I nternational Atomic Energy Agency ( [9S5) L ranium deposits in Utah. Econ Geol 79 : 50 -71 volcanic rocks. IAEA-TC-..J.90719. Proc Tech Comm Mtg. EI Cunningham CG. Steven TA. Rowlcy PD. Naeser CWo \1ehnert Paso. Texas. 2 ·5 April [9H4. International Atomic Energy HH. Hedge CEo Ludwig KR (1994') Evolution of volcanic rocks Agency. Vienna . ..J.6H p and associated ore deposits in the Marysvale volcanic field. Kelly We. Goddard EN ( 1%9) Telluride ores of Boulder County. Utah. Econ Geol ~9: 2003- 2005 Colorado. Geol Soc Am Mem 109: 2:'7 r Cunningham eG. Rye RO. Bethke PM. Logan MAV (llJ90) Kellv We. Rye RO (1979) Geologic. fluid inclusion. and stable Formation of coarse-grained vein alunite hy degassing or an i~otope st~dies of the tin-tung~stcn deposits of Panasqueira. epizonal stock (Abstr). Proc Chapman ConI' on crater lakes. Portugal. Econ Geol 74: 1721 - 1Hn terrestrial degassing and hyper-acid fluids in the cnvironment. Kerr PF (I96H) The Marysvale. Utah. uranium deposits. In: Ridge 5- 9 September. 1996. Crater Lake. Oregon, 27 JD (ed) Ore deposits of the United States. 1l):':' - 1967 (Graton Cunningham CG. Unruh OM. Steven TA. Rowley PD. :--Jaeser Sales vol) vol 2: New York. American I nstitute of Mining CWo Mehnert HH. Hedge CEo Ludwig KR (1997) Geochem Metallurgy Petroleum Engineers, pp 1020 · 1042 istry of volcanic rocks in the Marysvale volcanic field. west Kerr PF, Brophy GP. Dahl HM. Green J. Woolard LE (1957) central Utah. In: Friedman J D. Huffman A C (cds) Laccolith Marysvale. Utah. uranium area. Gent Soc Am Spec Pap 64: complexes of southeastern U tah-Tectonic control and time of 212 p emplacement. US Geol Surv Bull (in press) Langmuir 0 (1978) Uranium solution-mineral equilibria at low Curtis L (1981) Uranium in volcanic and volcaniclastic rocks-ex temperatures with applications to sedimentary ore deposits. amples from Canada. Australia. and Italy. In: Goodell Pc. Geochim Cosmochim Acta 42: 547 ·570 Waters AC (eds) Uranium in yolcanic and volcaniclastic rocks. Lindsey DA (19X I) Volcanism and uranium mineralization at Spor Am Assoc Petrol Geol Studies in Geology 13: pp 37 - 53 Mountain. Utah. In: Goodell PC Waters AC (eds) Uranium in Deen JA. Rye RO. Munoz JL Drexler JW (1994) The magmatic volcanic and volcaniclastic rocks. Am Assoc Petrol Geol hydrothermal system at Julcani. Peru: evidence from fluid in Studies in Geology 13: 89- 98 clusions and hydrogen and oxygen isotopes. Econ Geol H9: Morton RD. Aubut A. Ghandi SS (l97R) Fluid inclusion studies 1924-1938 and genesis of the Rexpar uranium-fluorite deposit. Birch is Dunkhase JA (1980) A comparative study of the whole rock geo land. British Columbia. In: Current Research. Part B. Canadian chemistry of the uranium mineralized Central Intrusive at Geol Surv Pap 78-IB: pp 137-140 Marysvale. Utah to nonmineralized intrusives in southwest Nash JT. Cunningham CG (1973) Fluid inclusion studies of the Utah. Unpub PhD dissertation Colorado School of Mines. fluorspar and gold deposits. Jamestown district. Colorado. Golden, 130 p Econ Geol 68: 1247-1262 EI-Mahady OR (1966) Origin of ore and alteration in the Freedom O'Rourke PJ (1975) Maureen uranium-fluorine-molybdenum No.2 and adjacent mines at Marysvale, Utah. Unpub PhD prospect. Georgetown. In: Knight CL (ed) Economic geology dissertation University of Utah, Salt Lake City. 217 p of Australia and Papua New Guinea: Aust Inst Mining Metal. Foler NK. Bethke PM, Rye RO (1989) A reinterpretation of the Monogr 5: 764-769 8 80H10 of inclusion fluids in contemporaneous quartz and Osterwald FW (1965) Structural control of uranium-bearing vein sphalerite, Creede mining district. Colorado: a generic problem deposits and districts in the conterminous United States. US for shallow ore bodies? Econ Geol 84: 1966-1977 Geol Surv Prof Pap 455-G: 23 p George-Aniel B, Leroy J, Poty B (1985) Uranium deposits of the Parks GA, Pohl DC (1988) Hydrothermal solubility of uraninite. Sierra Pena Blanca: Three examples of mechanisms of ore de Geochim Cosmochim Acta 52: 863- 875 posit formation in a volcanic environment. In: Uranium de Peiffert C. Nguyen-Trung C. Cuney M (1996) Uranium in granitic posits in volcanic rocks, Proc Technical Committee Meeting. EI magmas: part 2. Experimental determination of uranium solu Paso, Texas, 2- 5 April 1984. International Atomic Energy bility and fluid-melt partition coefficients in the uranium oxide Agency, Vienna. pp 175-186 haplogranite-H:!O-NaX (X = Cl. F) system at 770 DC. 2 kbar. Gilbert RE (1957) Notes on the relationship of uranium mineral Geochim Cosmochim Acta 60: 1515 - 1529 ization and rhyolite in the Marysvale area. Utah. US Atomic Phair G, Shimamoto KO (1952) Hvdrothermal uranothorite Energy Comm Rpt RME-2030: 29 p breccias from the Blue Jay mine, Ja~estown. Boulder County. Goddard EN (1935) The influence of Tertiary intrusive structural Colorado. Am Min 37: 659-666 features on mineral deposits at Jamestown, Colorado. Econ Potter RW II. Clynne MA, Brown DL (1978) Freezing point de Geol 30: 370-386 pression of aqueous sodium chloride solutions. Econ Geol 73: Goddard EN (1946) Fluorspar deposits of the Jamestown district. 284-285 Boulder County, Colorado. Colorado Sci Soc Proc 15( 1): 47 P Preto V A (1978) Setting and genesis of uranium mineralization at Goodell PC (1985) Chihuahua City uranium province. Chihuahua, Rexpar. Can Inst Mining Metal Bull 70: 93-100 Mexico. In: Uranium deposits in volcanic rocks, Proc Technical Rasmussen JD, Cunningham CG. Steven TA. Rye RO. Romberger Committee Meeting, EI Paso, Texas, 2-5 April 1984. Interna SB (1985) Origin of hydrothermal uranium vein deposits in the tional Atomic Energy Agency. Vienna. 97- 124 Marysvale volcanic field (Abstr). In: Uranium deposits in vol Goodell Pc. Waters AC (eds) (1981) Uranium in volcanic and canic rocks Proc Technical Commiltee Meeting. EI Paso, Texas: volcaniclastic rocks. Am Assoc Petrol Geol Studies in Geology April 2- 5. 1984. International Atomic Energy Agency. Vienna. 13.331 P Austria. 317 Green ], Kerr PF (1953) Geochemical aspects of alteration, Reed MH. Spycher NF (1985) Boiling. cooling and oxidation in Marysvale. Utah. Atomic Energy Comm Ann Rep June 30. epithermal systems: a numerical modeling approach. Rev Econ 1952-April L 1953, RME-3046: 73-99 Geol 2: 249- 272 Gruner JW. Fetzer WG. Rapaport I (1951) The uranium deposits Rich RA. Holland H D. Petersen U (1977) Hydrothermal uranium near Marysvale, Piute County. Utah. Econ Geol 46: 243-251 deposits. Developments in Econ Geol 6. Elsevier. New York: Haas JL Jr (1971) The effect of salinity on the maximum thermal 264 p gradient of a hydrothermal system at hydrostatic pressure. Romberger SB (1984) Transport and deposition of uranium in Econ Geol 66: 940-946 hydrothermal systems.at temperatures up to 300 DC: geological Henley R W (1984) Gaseous components in geothermal systems. implications. in: De Vivo B. Ippolito F. Capaldi G. Simpson Reviews in Econ Geol I: 45- 56 PR (eds) Uranium geochemistry. mineralogy, geology, explo Holland HD. Malinin SD (1979) The solubility and occurrence of ration and resources: The Institute of Mining and Metallurgy. non-ore minerals. Chapt 9. In: Barnes HL (cd) G~ochemistry of London. England. 12- 18 494 Rowley PD. Cunningham eG. Steven TA. Mehnert HH. Naeser Steven TA. Cunningham CG. Naeser CWo Menhert HH (1979) CW (1988a) Geologic map of the Marysvale quadrangle. Piute Revised stratigraphy and radiometric ages of volcanic rocks in County. Utah. Utah Geol Min Survey Map lOS the Marysvale area. west-central Utah. US Geol Surv Bull 1469: Rowley PD. Cunningham CG. Steven TA. Mehnert HH. NaeserCW ~() p (1988b) Geologic map of the Antelope Range quadrangle. Sevier Steven TA. Cunningham CG. Machette MN (1981) Integrated and Piute Counties. Utah. Utah Geol Min Survey Map 106 uranium systems ~in the Marysvale volcanic field, west-c~entral Rowley PD. Mehnert HH. Naeser CWo Snee L W. Cunningham Utah. In: Goodell Pc' Waters AC (eds) Uranium in volcanic CG. Steven TA. Anderson JJ. Sable EG. Anderson RE (1994) and volcaniclastic rocks. Am Assoc Petrol Geol Studies in Isotopic ages and stratigraphy of Cenozoic rocks of the Geology 13: 111 - [22 Marysvale volcanic field and adjacent areas. west-central Utah. Steven TA, Rowlev PO, Cunningham CG (1984) Calderas of the US Geol Surv Bull 2071: 35 p Marysvale volc~nic field, west central Utah. J Geophys Res 89 Rye RO (1993) The evolution of magmatic fluids in the epithermal (B 10): X751-8764 environment: the stable isotope perspective. Econ Geol 88: Tavlor AO. Anderson TP. OToole WL. Waddell GG. Gray AW. 733-753 . Douglas H, Cherry CL. Caywood RM (1951) Geo[ogy and Rye RO. Plumlee GS. Bethke PM. Barton PB Jr (1988) Stable uranium deposits of Marysvale. Utah. Interm report on the isotope geochemistry of the Creede. Colorado. hydrothermal producing area. US Atomic Energy Comm RMO-896: 30 p system US Geol Surv Open-File Rpt 88-356: 38 p US Geological Survey (1963) Geology of uranium-bearing veins in Shea ME (1982) Uranium migration associated with some hydro the conterminous United States. US Geol Surv Prof Pap 455: thermal veins at Marysvale. Utah: a natural analog for radio 146 p active waste isolation. Unpub MS Thesis. Univ Calif Riverside: Walker GW. Adams JW (1963) Mineralogy, internal structural and [23 p textural characteristics. and paragenesis of uranium-bearing Shea M. Foland KA (1986) The Marysvale natural analog study: veins in the conterminous United States. US Geol Surv Prof preliminary oxygen isotope relations. Chem Geol 55: 281 - 295 Pap 455-0: 35 p Sherlock RL, Tosdal RM. Lehrman NJ. Graney JR, Losh S. Jo Walker GW. Osterwald FW (1956) Relation of secondary uranium wett EC, Kesler SE (1995) Origin of the McLaughlin mine minerals to pitchblende-bearing veins at Marysvale. Piute sheeted vein complex: metal zoning. fluid inclusion. and isoto County, Utah. In: Page LR, Stocking HE, Smith HB (eds) pic evidence. Econ Geo[ 90: 2156-2181 Contributions to the geology of uranium and thorium by the Staatz MHo Osterwald FW (1956) Uranium in the fluorspar de United States Geological Survey and Atomic Energy Com posits of the Thomas Range. Utah. In: Page LR. Stocking HE. mission for the United Nations international conference on the Smith HB (Compilers) Contributions to the geology of uranium peaceful uses of atomic energy, Geneva. Switzerland 1955. US and thorium by the US Geol Surv and Atomic Energy Com Geol Surv Prof Pap 300: 123-129 mission for the United Nations Int Conf on Peaceful Uses of Walker GW. Osterwald FW (1963) Introduction to the geology of Atomic Energy, Geneva, Switzerland. 1955: US Geol Surv Prof uranium-bearing veins in the conterminous United States: US Pap 300: pp 131-136 Geol Surv Prof Pap 455-A: 28 p Staatz MH, Osterwald FW (1959) Geology of the Thomas Range Willard ME, Callaghan E (1962) Geology of the Marysvale fluorite district, Juab County, Utah. US Geol Surv Bull 1069: 97 p Quadrangle, Utah. US Geo[ Surv Geol Quad Map GQ-154, Steven T A. Morris HT ( 1987) Summary mineral resource appraisal Scale 1:24.000 of the Richfield lOx 20 quadrangle. west-central Utah. US Geol Surv eirc 916: 24 p Int~rnatio~aI' J9~rnal for Geology" Mineralogy and , . ~_ vVC,,,.nij'lt. of Mineral , oe~its . .,~; !":,,[ ' } ' . # -: . : •• - ~.;' ~;: ·~~r<, .';y~t{.~~ (~ ~ I . .. " . . ~~ ' :" '. . :~~j~ .; • . • . • Official.BulJetinofthe Society for Geology Applied to. • -.,' :.~.~ ". , ,, : ;.:.:... : ,; ; ~'1-' ' \ _ • -. " • '._ ·._·: .•• ·;·. :,~. ~~?:·;f~. ::~ ~'~: Volume 33 Number 5 1998 Articles Discussion Buchholz p, Herzig P, Friedrich G, Frei R: Sikka DB, Nehru CE: Granit~hosted gold mineralization in the Midlands greenstone Comments on the article by S. C. Sarkar, S. Kabiraj, belt: a new type of low-grade gold deposit in Zimbabwe 437 S. Bhattacharya, A. B. Pal: Nature, origin and evolution of the granitoid-hosted early Proterozoic copper-molybdenum Linnen RL: mineralization at Malanjkhand, Central India Depth of emplacement, fluid provenance and metallogeny (Mineralium Deposita 31: 41~31) 524 in granitic terranes: a comparison of western Thailand with other tin belts 461 Sarkar SC, KabiraJ S, Bhattacharyya S, Pal AB: A reply to the comments by D. B. Sikka and C. E. Nehru 531 Cunningham CG , Rasmussen JD, Steven TA , Rye RO , Rowley PO , Romberger SB, Selverstone J: Book reviews 533 Hydrothermal uranium deposits containing molybdenum and fluorite in the Marysvale volcanic field, west-central Utah Forthcoming papers 536 477 Society news 537 Allibone A: Synchronous deformation and hydrothermal activity Membership application form 538 in the shear zone hosted high-sulphidation Au-Cu deposit at Peak Hill, NSW, Australia 495 Letters Gi uliani G. France-Lanord C, Coget P. Schwarz 0 , Cheilletz A. Branauet Y. Giard D. Martin-Izard A, Pavel A, Piat DH: Oxygen isotope systematics of emerald: relevance for its origin Online edition in LINK and geological significance 513 Geosciences Online Library http://link.springer.de Alderton OHM . Th irlwall MF, Baker JA: Hydrothermal alteration associated with gold mineralization Indexed in Current Contents in the southern Apuseni Mountains, Romania: 33 (5) 437-538 June 1998 preliminary Sr isotopic data 520 Printed on aCid-free paper 1111111111111111111111111111111111111111111111111 OD26 - 4598 ( 199807 ) 33 :5 ;1 - ?