Petrogenesis of rare-metal granites from depleted crustal sources: an example from the Cenozoic of western Utah, U.S.A.

ERIC H. CHRISTIANSEN* � Department of Geology, University of Iowa JOHN S. STUCK LESS U.S. Geological Survey . Denver, Colorado MYRA J. FUNKHOUSER-MAROLF and KRISTY H. HOWELL Department of Geology, University of Iowa . Iowa City, Iowa

ABSTRACT and the products of incremental episodes of equilibrium crys­ Cenozoic magmatism of the eastern Great Basin, western. tallization (Group 2) accumulated on the walls of the magma U.S.A., produced a number of aluminous anorogenic (A-type) chamber. rhyolites, many of which contain topaz and are rich in U, Be, The genesis of the parental magma probably involved par­ . Rb and F; such rhyolites could be called rare-metal rhyolites. tial melting of granulites in the lower crust as the result of the A small granite pluton tlu!t is chemically and temporally equi�a­ decomposition ofF-rich biotite. From trace element models of lent to these rhyolites is preserved in the Sheeprock Mountall1s batch partial melting, small degrees of melting (2.5 % to 15 %) of west-central Utah. The pluton ranges from biotite mon­ are inferred to be important in the generation of the parental zogranite to biotite-muscovite syenogranite (lUGS). Ilmenite, magma; subsequent strong fractionation occurred in the upper magnetite, apatite, zircon, monazite, fluorite, beryl, and thorite crust. A geochemically anomalous source is not required for are magmatic accessory minerals; in a few specimens topaz and the generation of granites like the Sheeprock granite that are muscovite are post-magmatic accessories. The pluton was rich in incompatible trace elements. emplaced at a shallow level. Sparse greisen-bordered fractures contain Sn and W minerals. The pluton has a Rb-Sr isochron Introduction age of 20.9 ± 1.2 Ma with an initial 87 Srl86 Sr ratio of 0.7064. Rare metal granites have been identified as those with high con­ Slight contamination of rocks collected from the pluton's mar­ centrations of normally dispersed elements such as F, Li, Rb, gins is indicated by elevated initial Sr-isotope ratios. Cs, Sn, Ta, Nb, Zr, and REE (Tauson, 1974; Kovalenko, 1978; Three geochemical groups can be identified within the tex­ the metallogenically specialized granites of Tischendorf, 1977). turally varied but geochemically zoned stock. Group 1 consists Such granites occupy a special place in petrologic thinking of highly evolved specimens from the core of the pluton with because of their unique and often extreme chemical features. high Rb/Sr (40-90) and low La/YbN (3). Relative to the other Two fundamentally different types of rare-metal granites exist two groups, Group I specimens are depleted in Th (50 ppm, - one peralkaline (agpaitic) and the other metalum nous to p r­ this and other values below are maximums), Sr (20 ppm), Ba : aluminous (plumasitic or Li-F). Each type of gramte! has a dIS­ (100 ppm), Zr (125 ppm), Hf (9 ppm), LREE (100 x chondritic), tinctive set of trace element characteristics and associated U, Ti, Fe, Mg, Ca, K, and P. Group 1 specimens are enriched mineral deposits . The peralkaline granites are extremely enriched in Na, F (0.39 wt% maximum), Sc (3 ppm), Be (50 ppm), Li in some trace elements, e.g. Zr , Nb, LREE; whereas the (260 ppm), Ta (33 ppm), Nb (110 ppm), HREE (35 x chon­ aluminous granites are enriched in Rb, Ta, U, and Sn. In dritic) and Y (60 ppm). Specimens from Groups 2 and 3 come . general, these granites appear to be part of the anorogemc group from a "collar" that partially surrounds the core but the dis­ of granites (A-type) identified by Loiselle and Wones (�979) nd tribution of these two groups overlaps with one another. � Collins et al. (1982). In addition to sharing the tectome settl g The geochemical effects of weathering, assimilation, and � in which they are produced, both types of rare-m tal gramte metasomatism are visible to varying degrees in the pluton; : are enriched in fluorine and chlorine. From a revIew of vol­ nonetheless, most of the chemical variation appears to be the canic glass analyses, Christiansen et al. (1983) hav pointed out result of fractional crystallization. The compositions of Groups � that silicic peralkaline magmas are generally CI-nch (FICI < 2 and 3 produce divergent arrays on plots of two elements with 3), whereas aluminous rare-metal granites or rhyolites are different bulk partition coefficients (e.g. Rb and Sr). This is . . enriched in fluorine (F/CI > 3). The latter are the SovIet eqUIva­ interpreted as resulting from the preservation of complemen­ lent of the "Li-F geochemical type" and contrast with the tary liquid (Group 2) and solid (Group 3) compositions. Trace agpaitic geochemical type (e.g. Kovalenko, 1978). element models suggest that in situ fractional crystallization In this study, we have examined the rare-metal Sheep rock produced these trends. The data suggest that evolved liquids granite from western Utah, U.S.A., in an effort to formulate (Group 1) were displaced inwards while cumulates (Group 3) a model for the origin and evolution of fluorine-rich aluminous magmas. We conclude that the parental magma of the Sheeprock granite appears to have been derived by small degrees *Now at Department of Geology, Brigham Young University, Provo, Utah, U.S.A. of partial melting of Rb-poor granulites of the lower continen-

Granite·Related Mineral Deposits 307 ° f 112 30' LEGEND Qat Alluvium N Tgs p-C Sheeprock Granite Tv Volcanic Rocks

Ps Sedimentary Rocks

Qat Metasedimentary Rocks p€

• o GrOUP 1 rI' �8 ". • Group 2 o Group 3 �101 •••111 �\ 1,--_...., SlC p-C Sheeprook � Mountains _ J ' GB ;

j CP o 1 2 // Utah Km

FIGURE 1. Schematic geologic map (after Cohenour 1959, and Christie-Blick 1983) of the Sheeprock granite showing the distribu­ tion of the specimens collected for this study. Symbols are used to distinguish the three geochemical groups discussed in the text. Sample localities without numbers represent unpublished analyses. Locations mentioned in text include CJ - Copper Jack mine; U - Utah mine; ED - Flying Dutchman mine; JC - Joes Canyon (samples from these localities have these labels); SLC - Salt Lake City: GB - Great Basin: CP - Colorado Plateau. tal crust. Decomposition of small amounts of F-rich biotite in middle and late Tertiary), most are Cenozoic. Mid-Tertiary high-grade metamorphic rocks controlled the degree of partial intermediate to silicic pyroclastic rocks, lavas, and plutons were melting. Most of the geochemical variation within the Sheeprock emplaced in a southward migrating volcanic belt that may have granite is the result of in situ fr actional crystallization with mar­ been part of a broad volcanic arc related to lithospheric sub­ ginal accretion of cumulate phases. These processes produced duction at the continental margin (Stewart et al., 1977; Lindsey a marked geochemical zonation in the pluton defined by three et al., 1975; Lipman, 1980). A fundamental change in the nature distinctive groups of specimens. The fractionation of accessory of Cenozoic magmatism occurred approximately 23 Ma (Best minerals played an important part in the trace-element evolu­ et al., 1980) and was marked by the onset of bimodal mafic tion of the magma. and rhyolitic volcanism in western Utah. These A-type granites and rhyolites (many of which contain topaz) are geochemically Geologic Sett,ing distinct from older I(Caledonian)-type (Pitcher, 1982) and S-type Late Cenozoic magmatism of the eastern Great Basin, western granitoids of western Utah and eastern Nevada. Even before U.S.A., produced a number of aluminous A-type rhyolites. this change in magma chemistry, tectonic extension of the Great These rhyolites are part of a bimodal (basalt-rhyolite) suite and Basin in a west-southwest direction Was underway behind a calc­ are contemporaneous with lithospheric exte'nsion in the Basin alkaline volcanic are, which by thi's time was restricted to areas and Range province. Many contain topaz and are rich in Be, near the coast (Zoback et al., 1981; Eaton, 1984). As a result U, Rb, Ta, and F and appear to be the volcanic equivalen,ts of the development and growth of a transforn boundary at the western margin of the continent, the extension direction changed of rare-metal granites (Christiansen et al., 1983, 1986). The erup" , ' tions of these rhyolites have resulted in several small uranium to approximately east-west sometime between 10 and 6 Ma ago and fluorite deposits and an important beryllium deposit within and fo rmed the present fault-controlled topography of the Great the 21 Ma Spor Mountain Formation, Utah (Lindsey, 1982; Burt Basin (Zoback et al., 1981). et al., 1982). A small pluton of chemically anq temporally The Sheeprock granite is a small pluton (ca 25 km2) exposed equivalent granite is preserved in the Sheeprock Mountains, along the southwest flank of the ,Sheeprock Mountailis horst about 60 km to the northeast (Fig. 1). in the eastern Great Basin of west-central Utah (Fig. 1). The The Sheeprock granite is only one of a number of igneous pluton intrudes Late Proterozoic quartzites, slates, phyllites and complexes exposed along the length of the east-trending Deep diamictites as well as Paleozoic sedimentary rocks . Portions of Creek-Tintic mineral belt (Shawe and Stewart, 1976; Stewart its margins are covered by Cenozoic alluvium (Christie-Blick, et al., 1977). This trend is marked by basement highs, plutonic 1982; Cohenour, 1959). Several types of geologic information and volcanic rocks. aeromagnetic highs, and by mineral deposits suggest that the pluton crystallized at a relatively shallow level; generally associated with the igneous rocks. Although the plu­ the lack of a regional metamorphic aureole, the presence of tonic rocks are of a variety of ages (Precambrian, Jurassic, miarolitic cavities, the extreme variations in texture, and the

308 Recent Advances in the Geology of presence of older (Oligocene?) volcanic rocks very near the con­ and Utah Cu-F deposits). Coarse-grained biotite monzogranite tact of the pluton (Cohenour, 1959). If no fault separates these is typical of the iron-stained collar (typical Group 2) surround­ outcrops this last bit of evidence suggests that the pluton was ing the pluton's chemically distinctive core (Group 1). The emplaced less than one kilometre from the surface; however, interior of the pluton generally consists of porphyritic granite no volcanic rocks have been identified that could have erupted with large crystals of alkali feldspar and smoky quartz set in from the Sheeprock granite itself. a finer grained iron-stained groundmass of hypidiomorphic Alteration of the wall rocks is generally weak although some plagioclase, alkali feldspar, quartz, and interstitial biotite, small roof pendants are intensely biotitized or pyritized. The fluorite, ilmenite, and magnetite. Notable accessories inclu.cte intrusive contacts are generally sharp, but gradational contacts zircon, apatite, Th-rich monazite, Nb-Ta-W oxides, thorite and with the metasedimentary rocks are found near the southeast rare uraninite (the latter four identified by autoradiography and margin. The granite within a few tens of metres of the contact microprobe analysis; Funkhouser-Marolf, 1985). Changes in the. in this area contains apparently primary muscovite; andalusite modes of the principal phases are minor in the pluton and are is not present in these rocks. Because of these obvious changes overwhelmed by the great textural variety. Sharp contacts do which resulted from assimilation of country rocks at the mar­ not separate the geochemical groups, although internal intru­ gins of the pluton, the border-phase rocks were not the subject sive contacts are present. Textures indicate the co-crystallization of detailed study. of alkali-feldspar, quartz, plagioclase, biotite and Fe-Ti oxides throughout the pluton. These phases can be found as inclusions Analytical Methods in the other. In addition, zircon and apatite .lppear to have crys­ tallized early and are present in all specimens. Monazite ha,s The specimens analyzed for this study were the freshest been identified in all specimens except SR-CJ, as inclusions in obtainable from natural outcrops, road cuts, and mine dumps silicates (generally biotite) or as discrete euhedra interstitial to within the pluton (Fig. 1). Each rock specimen consisted of at other grains. In the specimens described in this paper, monazite: least 3 kg of material. Specimens selected for chemical and iso­ is subhedral to euhedral and is not associated with minerals that topic analysis were chosen to represent the spectrum of rock are clearly the result of secondary alteration (muscovite, fluorite, compositions on the basis of preliminary XRF analyses and hematite, topaz), and we suggest that it is a primary magmatic petrography. phase. Thorite, which we also interpret as a magmatic phase, Wavelength dispersive XRF analysis of fused glass disks was is found in specimens from all three geochemical groups but used to determine Si, Ti, AI, Fe, Mn, Mg, Ca, Na, K, and P has been identified in only about 50OJo of the specimens, prob- . (Taggart et al., 1981). Energy dispersive XRF using rock pow­ ably because of its low abundance and the small area of each ders was used to determine Rb, Sr, Y, Zr and Nb. Acid diges­ thin section. Details regarding the composition and petrogra­ tion and inductively coupled plasma spectrometry were used to phy of monazite and thorite are recorded by Funkhouser-Marolf analyze for Mo (not reported because most specimens contained (1985). less than 2 ppm, the detection limit), Be, and Li (Taggart et Alkali feldspar is commonly perthitic, and rarely shows the al., 1981). Fluorine and chlorine were analyzed by a specific tartan twinning of microcline. Biotite and plagioclase grains are ion electrode technique. CI contents were at or below the 0.010/0 commonly but not universally altered in all three geochemical detection limit and are not reported. Other trace elements (Cs, groups. Biotites have narrow rims of secondary white mica and Ba, Th, U, Ta, Hf, Sb, Sc, Co, and selected REE) were ana­ the cores of some plagioclase grains have been sericitized to vary- . lyzed by instrumental neutron activation (Gordon et al., 1968). ing degrees. Sericite plus fluorite after feldspar is a common. Sr-isotope determinations were performed on 8iRb_ and 84Sr_ association that is found throughout the pluton in extremely spiked splits. SiSr/86Sr ratios have been normalized to 86Sr/88Sr minor quantities. = 0.1194. The mean 87Sr/86Sr for the Eimer and Amend Sub-spherical rosettes of beryl are also found within the core standard was 0.70800 ± 0.0002 (2 sigma) during the course of 1954). the study. Uncertainty in individual analyses (including sam­ of the pluton (Williams, These rosettes may be as much 50 pling error) is taken to be 0.035% at the 95 % confidence level. as cm in diameter and are composed of beryl crystals, radi­ Isotopic and decay constants used in all calculations are those ating from a common centre, choked with oriented inclusions recommended by the lUGS (Steiger and Jager, 1977). The of Na-plagioclase, quartz, alkali-feldspar, white mica, and fluid. isochron regression was done using a modified version of York Some rosettes are surrounded by a halo rich in alkali feldspar (1954) (1969). Analytical uncertainties for Rb and Sr concentrations or biotite. Williams suggested that these rosettes grew (Table 1) based on replicated analyses are about 1.5OJo (2 sigma). during late magmatic conditions after the magma became fluid saturated, as indicated by the coincidence of the distributions TABLE 1. Rb and Sr isotopic composition of the of rosettes and rare miarolitic cavities. Some of the miarolitic Sheeprock pluton, Utah cavities within the core of the pluton carry topaz crystals. The presence of beryl and the high whole-rock concentrations of Rb Sr 87s,/86s, Be .(up to 50 ppm) suggest a strong similarity with the Be- . Sample (ppm) (ppm) 87Rb/86s, 87s,/86s, (at 20.9 Ma) mineralized Spor Mountain rhyolite. SR CJ 172.7 327.0 1.528 0.70967 0.70922 Pegmatitic pods (the largest 50 cm wide but most less than SR 51 484.6 120.6 11.63 0.70975 10 cm across) are also present throughout the intrusion. Most SR 101 503.4 98.7 14.76 0.71034 are composed of quartz or quartz-feldpar-muscovite, but some SR 191 327.2 42.2 22.46 0.71495 0.70828 carry samarskite, wolframite , beryl, and ilmenite (Harris, 1958). SR 4 0.70759 367.4 31.2 34.13 0.71772 Sparse greisen-bordered veinlets occur along fractures and SR 61 456.5 38.7 34.15 0.71731 on the margins of some aplite dykes (Howell, 1987). These SR 161 596.2 50.0 34.53 0.71650 quartz-muscovite or quartz-muscovite-topaz alteration haloes' SR 201 538.6 30.8 50.66 0.72125 SR 131 841.9 21.0 112.0 0.73944 extend only a few centimetres away from the fractures as indi­ cated by field, geochemical, and petrographic studies. Greisen Analysts: K Futa and EH Christiansen veinlets are particularly common in the outlier of the pluton in Joes Canyon which exposes only the upper tens of metres Petrography and Mineralogy of the pluton and in the northwestern part of the pluton. Cas­ The Sheeprock pluton consists of texturally varied granite rang­ siterite is rarely present in the veinlets; it has also has been iden­ ing from biotite monzogranite to biotite-muscovite syenogranite. tified in heavy mineral concentrates from the streams that drain The marginal phase (geochemical Group 3 described below; Fig. the pluton (H. Htay and W.R. Griffitts, written comm., 1981). 1) of the pluton consists, for the most part, of fine-grained Hematite, wolframite, galena, and sphalerite also occur within nearly aplitic rocks but coarse-grained variants are also found the greisen veinlets. We have found no evidence for albitiza­ (e.g. near the southern margin in the vicinity of the Copper Jack tion in our sampling of the pluton.

Gran ite-Related Mineral Deposits 309 t = 20.9±1.2 Ma 0.74 L Sri O.7064±O.0002 U1 0.73 �...... 0.72 t"-cJ1 OO 0.71

FIGURE 2. Isochron diagram for samples of the Sheeprock granite. Symbol size corresponds to ± 2 sigma errors. Samples CJ, 191, and 4 were not used in the regression of the isochron as explained in the text.

Group o Group 1 _ 2 o Group 3 0.5

0.4

0 � 0.3 -%'l' ! - c c9 0 c o 0 0.2 0 0 - .�::> - 10. ii: 0 Group t 0 Group 2 • 0.1 Group 3 0

0.0 65 73 75 77 79 (wl'llo) 8102 FIGURE 4. An-Ab-Or (a) and Q-Ab·Or (b) ternary diagrams showing CIPW normative compositions of the Sheeprock granite fiGURE 3. fluorine and Si02 concentrations o"f samples from in relation to the polybaric minima in the granite system as deter­ the Sheeprock granite. Symbols are the same as those in Figure mined by Tuttle and Bowen (1958), Luth (1964), and Whit­ 1. Also shown is the field of compositions for other granitoids et al. ney (1975). The three geochemical groups are shown. Numbers from the Basin and Range province (Lee 1984; Christiansen and beside crosses indicate water pressure in kb at respective Lee, 1986). Many Precambrian anorogenic granites 0 from the minimum. region are also f -rich but are not as siliceous.

Electron probe analyses (E.H. Christiansen, in prep.) show tron activation analysis of Ar. Armstrong (1970) later modi­ that the hydrothermal white micas found in the greisens are fied this value using mass spectrometric determination of Ar

ferriphengite-muscovite solid solutions with molar Fe = 0.5 to on the same specimen to 17.4 ± 0.3 Ma (recalculated using new 1.0 per 24 (0 + OH + F). The white micas contain low con­ decay constants). D.A. Lindsey (written comm., 1981) obtained centrations of TiOz (typically less than 0.35 wtOJo). Calculated a fission-track (Zircon) age of 16.3 ± 0.9 Ma for the intrusion. at 350°C, log fH20/fHF ranges from 3.9 to 4.9 These micas Rb-Sr isotopic data for the Sheeprock granite (Table 1) do contain up to 2.50/0 F and have fluorine intercepts (Munoz, not fit a straight line within the limits of analytical precision 1984), that cluster around 0.7. Igneous biotites have high (MSWD = 68. 1). Regression of all the data yields an age of Fe/(Fe + Mg) ratios - 0.8 to 0.95 - and are relatively 19.8 ± 2.0 Ma and an initial 87Sr/86 of 0.7075 ± 0.0022 in a aluminous (3.4 AlI24 0 + OH + F). The biotites are also F­ Model 3 fit. Data for three specimens (Cl, 191, and 4) plot dis­ rich (1.1 to 3.5 Wt%). These values are similar to those found tinctly above the trend defined by the other six specimens. These in biotites from rhyolite vitrophyres from Spor Mountain, Utah. three specimens come from a small area near the southeastern Fluorine intercepts for the biotites range from 0.7 to 1.7, margin of the pluton, and contamination by assimilation or reac­ whereas chlorine intercepts range from - 4.4 to - 3.3. Accord­ tion with the Proterozoic wall rocks seems probable even though ing to Munoz (1984), these values are typical of those found they show no petrographic evidence of such assimilation, e.g. in F-rich magmatic and hydrothermal biotites related to por­ they have no muscovite. Exclusion of these specimens from the phyry Mo and SnoW-Be deposits. At 700°C, calculated log regression reduces the MSWD to 9.57, but a Model 3 fit to the (fHP/fHF) ranges from 4 to 2.8; the lowest values come from data is still required. The resulting age is 20.9 ± 1.2 Ma, and the core of the pluton. the initial 87Sr186Sr is 0.7064 ± 0.0002 (Fig. 2). The relatively large errors and the Model 3 fit suggest that even the remain­ Geochronology and Sr-Isotope ing six specimens were not isotopically homogenous at the time of crystallization. The effect of such contamination on elemental Geochemistry concentrations is likely to be very small. DePaolo (1981) has Armstrong (1966) initially reported a K-Ar (biotite) age for the shown that assimilation of wall-rocks by a magma may be obvi­ Sheeprock granite as 19.2 (+ 3 - 2) Ma using instrumental neu- ous from isotopic studies but will be essentially invisible in terms

310 Recent Advances in the Geology of G) TABLE 2. Whole-rock chemical analyses of the Sheeprock pluton, Utah iil OJ 1 2 Sample Group Group Group 3 Topaz Rhyolites' JJ Number 11 21 121 131 151 231 FD 51 61 101 111 161 171 211 4 81 191 201 CJ Spar Mtn Thomas R CD� i[ Weight Percent CD n. Si02 76.9 76.8 76.7 76.8 76.5 76.7 77.3 74.6 77.4 75.5' 76.9 76.6 76.0 76.1 76.8 77.0 77.6 77.1 72.0 74.2 75.9 :s:: Ti02 0.07 0.06 0.07 0.06 0.06 0.07 0.06 0.23 0.17 0.28 0.18 0.17 0.20 0.20 0.08 0.13 O.OB 0.14 0.26 0.05 0.10 OJ AI203 12.6 12.9 13.1 12.9 12.B 12.9 12.7 13.5 12.4 12.B 12.2 12.6 12.6 13.0 12.5 12.3 12.3 12.4 14.5 13.5 12.7 CD !il Fe203 0.89 0.87 0.88 0.91 1.00 0.88 0.84 1.35 1.26 1.46 0.99 1.21 1.36 1.19 0.87 0.86 0.83 0.89 2.01 1.29 1.07 MnO 0.03 0.02 0.02 0.02 0.03 0.02 0.05 0.05 0.02 0.03 0.02 0.03 0.03 0.02 0.02 0.02 0.01 0.02 0.04 0.06 0.06 0 CD MgO 0.10 0.10 0.10 0.11 0.10 0.12 0.11 0.26 0.19 0.31 0.20 0.20 0.24 0.21 0.12 0.13 0.12 0.15 0.69 0.11 0.14 u 0 CaO 0.49 0.51 0.37 0.43 0.40 0.39 0.06 0.77 0.43 0.73 0.60 0.26 0.58 0.46 0.45 0.47 0.39 0.49 1.83 0.61 0.80 (fI ;:;: Na20 3.37 3.75 3.62 3.58 3.86 3.32 3.59 3.48 3.04 3.34 3.08 3.10 3.01 3.30 3.13 3.11 3.15 3.01 3.14 3.95 3.78 (fI K20 5.16 4.53 4.92 4.61 4.65 4.95 4.67 5.34 4.94 4.97 5.24 5.26 5.26 5.29 5.18 5.32 5.05 5.27 4.81 4.86 4.92 P205 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.08 0.02 0.02 0.02 0.02 0.07 0.02 0.02 0.02 0.09 0.00 0.00 Total 99.63 99.56 99.80 99.44 99.42 99.55 99.42 99.60 99.87 99.50 99.43 99.45 99.33 99.79 99.17 99.36 99.55 99.66 99.36 100.00 100.00 LOI 0.56 0.51 0.64 0.66 0.59 0.75 0.42 0.40 0.69 0.45 0.55 0.69 0.79 0.56 0.43 0.56 0.51 0.75 0.56 CIPWC 0.63 0.95 1.20 1.29 0.74 1.43 1.69 0.65 1.32 0.79 0.42 1.39 0.96 1.06 1.10 0.62 1.00 0.91 1.02 0.79 0.0 A/CNK. 1.05 1.08 1.10 1.11 1.06 1.12 1.15 1.05 1.11 1.05 1.03 1.12 1.08 1.08 1.08 1.05 1.08 1.07 1.06 1.05 0.98 01 95.77 95.39 95.82 95.36 95.98 95.43 96.82 93.38 94.75 93.4 95.0 95.52 94.08 94.84 95.68 95.79 95.94 95.34 86.23 94.15 94.54 Parts Per Million F 3900 3900 3100 3400 3700 3500 2500 1800 1900 2600 3100 2900 3000 1400 100 2200 nd 2400 600 1100 5400 Rb 903 757 951 811 963 921 958 485 456 507 522 596 524 516 367 511 327 539 173 1060 585 Sr 16 18 16 20 14 11 11 121 39 99 54 50 68 65 31 36 42 31 327 5 12 Y 63 43 52 52 61 54 21 30 44 33 33 36 42 40 24 45 15 40 13 135 43 Zr 123 120 112 97' 122 109 129 147 159 186 139 163 165 163 99 125 103 131 108 120 115 Nb 113 98 96 '91 96 94 101 60 71 69 57 59 56 75 43 71 24 72 16 120 70

Li 236 223 234 153 262 269 423 40 67 98 90 182 105 121 14 66 11 94 27 80 Be 14 9 50 9 9 10 23 14 14 13 27 12. 46 14 9 14 8 13 2 52 Ta 21 18 30 26 32 23 33 11.6 14.6 14.2 10.0 11.2 10.5 12.7 12.3 10.6 8.50 8.21 1.55 25 6.4 Cs 13 13 21 12 10 18 43 6.1 5.1 7.2 4.9 13 6.8. 11 2.7 3.5 3.9 7.6 4.1 58 14.1 Sc 2.3 2.5 3.0 2.6 2.9 2.6 2.8 2.3 2.0 2.9 2.1 2.01 2.13 2.23 2.04 1.76 1.75 1.72 4.12 2.7 2.2 Sb 0.2 0.09 0.09 0.14 0.09 0.08 0.79 0.08 nd nd 0.05 0.17 0.11 0.12 0.06 0.04 0.1 0.07 0.05 HI 7.1 7.4 7.0 6.7 7.4 7.9 9.2 5.79 7.00 9.00 5.35 6.71 6.31 6.72 4.40 5.26 4.72 6.27 4.11 7.1 5.7 Th 46 45 46 46 49 49 42.2 61.0 81.8 74.5 66.1 73.6 71.3 67.7 65.2 60.1 68.4 62.4 19.4 69 56 U 44 30 12 17 12 15 8.1 17.5 28.3 24.7 11.9 6.48 9.05 8.94 32.9 14.8 15.5 8.09 6.53 38 25 La 28.5 29.0 32.4 32.9 33.0 29.9 25.4 69.6 77.1 76.9 63.4 70.5 62.9 61.6 3B.2 47.0 42.4 45.1 33.5 59 25 Ce 56.8 56.9 66.2 67.5 68.3 60.7 63.4 125 146 141 117 127 118 113 70.9 87.2 81.3 85.3 62.3 137 64 Nd 19.9 18.1 25.4 22.6 23.8 22.0 15.9 38.6 47.5 45.2 38.4 40.7 41.5 38.1 28.3 32.7 30.1 28.9 22.6 51 31 Sm 4.9 3.6 5.0 5.4 5.5 4.2 1.5 6.13 8.93 7.26 6.99 7.62 7.45 7.10 5.61 7.13 5.57 5.15 3.78 17.8 4.9 Eu 0.17 0.20 0.18 0.18 0.17 0.14 0.10 0.72 0.41 0.65 0.49 0.46 0.59 0.54 0.33 0.37 0.43 0.29 0.B1 0.04 0.06 Tb 0.68 0.49 0.68 0.73 0.82 0.60 0.36 0.66 1.21 0.74 0.80 0.93 1.03 0.88 0.86 1.09 0.64 0.71 0.39 3.4 Oy 4.2 3.2 4.5 4.5 5.7 3.8 2.1 3.82 6.42 3.59 4.90 4.84 5.85 4.78 4.B1 6.02 3.57 4.23 2.37 Yb 6.25 5.5 6.5 6.9 7.5 6.2 2.4 3.51 5.78 4.36 3.64 3.99 4.30 4.11 5.33 5.12 2.59 3.94 1.71 15.7 7.9 Lu 1.1 0.97 1.1 1.1 1.2 1.1 0.42 0.57 0.89 0.68 0.57 0.60 0.63 0.66 0.91 0.80 0.39 0.62 0.25 2.6 1.6 Cr 1.4 nd nd 2.6 2.1 nd nd 0.3 0.7 0.6 nd 0.03 nd nd nd 0.4 nd nd 3.9 3.2 nd Co 2.3 0.36 0.39 0.44 0.47 0.47 0.52 1.6 0.88 1.2 0.75 0.91 1.1 0.96 0.2 0.7 0.2 0.7 4.3 0.4 nd

CIPWC = Normative corundum calculated from fluorine·free analyses. Fe203/FeO ratios were calculated using Sack et al. (1980). DI = Differentiation index. LOI = Loss on ignition at 10000C for 4 hours. A/CNK = Molecular AI203/(CaO + Na20 + K2O). nd = Not detected. w -' = Analyses of Spor Mtn and Thomas Range, Utah, rhyolites from Christiansen et al. (1984). ,\nalysts: A. Bartel, E.H. Christiansen, E. Brandt, H. Nieman, M. Malcolm, J. Budahn, and R. Knight. 0.5 500 50 4.8 0 0 ,-..4, 4 � 400 0 0.4 o 00 • � 40 N E 0 0 0 0 N .., 4. a 0 • 0 E .., 0.3 I. 0 ;: 0 0..300 • 0..30 0 � 3. 6 0 <) 0 • ;: 0 · 3- .. 0 <» 0.. · <0 '-' � • oil 0.2 .. 03.2 0 200 20 . N QJ -'tJ- C • o • :::J 0 0 o 02.8 ::;;: 0 0 • <§> o. I Z 100 10 0 0 2.4 0 0 '!l. rrl. 60 1.4 8 0 120 0 0 0<0 1.2 0 0 SO 060 <0 100 0 6 • .0 0 0 � E 1.0 0 E 0 0 " 0.. o. 0.. 0 E 40 . 80 0 0.. 0.. • 0.. .� �0.. 0.8 . 1 . , 3-30 • '-' 60 .. 0.6 ). 0 0 0 :::J .D 0 0 >- 20 :@ 40 --1 O. 4 0 0 0 >- 2 0 0 0 0 0.2 10 20 0 o o o o

0 � 0 � 50 400 e • 3 • 30 0 E E � E 0 ¢ �40 8:: 0.. 00 0.. 0.. 300 0.. 0 0.. � � '-' 0 ... . 8 30 · gg 2 '-' 20 � 0 0 Q:l 0 . 0 200 • u o � 20 0 c � . --1 (fl l .., . 0 100 " I- 10 0 10 r:P oo<\:> .0 0 " o rD.' 80 10 24 -"'- 5.9 · 0 20 0 �S. 5 o • 8 0 ..--, 0 N o . o til. E 0 "' 0 O¢� 0.. 00 � 16 �5.1 • ' . . 0 0.. 0° 0 .' Q) · • 0.. . 0 0 ,...., � 6 4. 7 • a '-' 12 0 0 � ell • 0 8574 4- 0 o '0 8 N4.3 I 0 8 � ::<: (fl 0 a. 3. 72 0 0 9 0 0 3.5 70 2 o a 400 800 1200 o 400 800 1200 o 400 800 1200 o 400 800 1200 Rb (ppm) Rb (ppm) Rb (ppm) Rb (ppm)

FIGURE 5. Variation diagrams showing incompatihle element concentrations for the Sheeprock granite plaited versus Rh. Group I - open diamonds; Group 2 - closed squares; and Group 3 - open squares. of trace element abundances until large amounts of assimila­ in the Sheeprock granite are much higher than in most granites tion have occurred (Lee and Christiansen, 1983). For example, (Bailey, 1977), and range up to 0.39% (Fig. 3). Biotite and assimilation of approximately IOloof radiogenic Proterozoic wall fluorite are the most important mineralogical hosts of F. The rocks of the sort intruded by the Sheep rock granite could raise geochemical features of the Sheep rock granite reveal its the Sr-isotope ratio of a Sr-rieh specimen like CJ from 0.7060 similarity to topaz rhyolites from the eastern Great Basin and to 0.7070. Even less contamination could produce significant to aluminous anorogenic granites from other parts of the isotopic changes in the Sr-poor rocks. We, therefore, suggest world (Christiansen et al., 1986). All of the specimens are that the trace element concentrations of all of the specimens peraluminous and contain 0.4% to 1.7% normative corundum are petrogenetically meaningful. (Al/(K + Na + Ca) ranges from 1.03 to 1.15). CIPW norms The Rb-Sr age is somewhat older than the K-Ar and fission­ were calculated using FeO/Fe203 ratios adjusted by the track ages for the Sheeprock pluton, but it is indistinguishable method of Sack et al. (1980). from the radiometric dates obtained from the geochemically Normative feldspar compositions (Fig. 4a) plot below the 2 kb similar rhyolite lavas and tuffs of the Spor Mountain Forma­ cotectic determined by Whitney (1975), and therefore suggest tion which contain North America's largest mineable source of a low pressure of crystallization. The positions of the data points Be. Lindsey (1982) reported a K-Ar age (sanidine) of 21.2 ± 0.9 in the Q-Ab-Or ternary diagram (Fig. 4b) indicate that the plu­ Ma and a fission-track age (zircon) of 21.5 ± 1.1 Ma for this ton crystallized at a water pressure between 0.5 kb and 1 kb. distinctive volcanic unit. Vents for the Spor Mountain rhyolite This pressure estimate ignores the role of fluorine on the com­ lie 60 km west-southwest of the Sheeprock pluton. Thus, position of granitic melts. According to Manning (1981) rela­ although the Spar Mountain rhyolite did not erupt from the tively small amounts of fluorine shift the fluid-saturated mini­ Sheeprock granite magma, its similar age and geochemistry (see mum (at 1 kb) toward more albite-rich compositions; thereby below) suggest that a very similar magma developed approxi­ mimicking the effect of increased pressure. It is difficult to esti­ mately simultaneously. mate the magmatic concentration of fluorine from a non-glassy rock, but fluorine contents of the biotites indicate that the melt Geochemistry may have contained up to 1.0 wt% F. This estimate is based on a biotite/glass partition coefficient for F of 3.5 to 4 as deter­ Major Elements mined for the Spor Mountain rhyolite (E.H. Christiansen, Major element analyses and partial CIPW norms are presented unpub!. data). According to Manning's (1981) experiments, this in Table 2. Analyses of two topaz rhyolites .are also included much fluorine produces a shift in the minimum melt composi­ for comparison - one from the 21 Ma Spor Mountain rhyolite tion analogous to that produced by increasing H20 pre�sures and the other from the 6 to 7 Ma rhyolites from the Thomas up to 2 kb in the absence of fluorine. Thus although not con­ Range, Utah. The granites from the Sheeprock Mountains are clusive, all of the evidence is at least consistent with field evi­ uniformly high in Si02, ranging from 750/0 to 77.5% (recalcu­ dence for crystallization at low pressure. lated volatile-free and normalized to 1000/0). A coarse-grained variant near the southeast margin of the intrusion contains less silica (e.g. CJ has a silica content of 72.6%). In addition, the specimens all have high Fe/Mg and K/Na ratios and total alkali TraceConcentrations Element of a wideComposition variety of trace elements are shown contents, and low Ti, Mg, Ca, and P. Fluorine concentrations in Table 2. The Sheeprock granite is distinctive because of its

312 Recent Advances In the Geology of 1. a O. " ) ,

f � 0 . 8 :j J EO r . 0.3 . . • 0 . 0... r . . - 0...0. 6 ., � · � . 0 2 . 0 r N . J J 10 . 0 '" j � 4 � 0.2 ":0.I E r { 1 a. � .....a. 2. � 2. a ll---+---f--+-� 0 ;:::2.4 � 1.6 o N �2. a .., � 1. 6 f 3: 1. 2 • 81.2 ..: • � �0.8 �.

I.J...0.4 10 100 O��--�--�--� o Th (ppm) NO.6 � 4 E .., FIGURE Uranium and thorium content of the Sheeprock 3: 5:: 3 7. <> granite as compared to granitoids from the Sierra Nevada 2 batholith. California (Dodge An average l".S. grani­ o • et al., 1982). toid (Stuckless and VanTrump. 1982) with the cross represent­ ;::It '. U I " ing ± 1 S.D. is also shown. O ��--�--�--� o ' 200 400 oL 200 400 a� I 00 r-,--.,..-..,....,--,---;--r--, 150 • . Ta (Kovalenko, 1978). Figures 5 and 6 show variation diagrams � � 130 • 80 for some of the major and trace elements where incompatible . E E . •. 0... . 5::110 elements are plotted against Rb and compatible elements are 0... . � � � so plotted against Sr. Figure 7 clearly shows the uranium and 90 0 Gi 000 thorium-rich nature of the Sheeprock granite as compared to -l C'"" = U 40 0 70 Cl:<>0 I-type granitoids from the Sierra Nevada batholith (Dodge et 8� II al., 1982) and average U.S. granitic rock of Stuckless and Van 220 IDa Trump (1982). · � 80 . The major and trace element compositions of the specimens E e"0 '• .1 . 5:: EO from the Sheeprock granite define three geochemical groups and ' � indicate that the pluton is chemically zoned (Figs. 5 and 6). The 40 � L three groups produce divergent arrays on plots of two elements f- 20 that have different bulk partition coefficients (Le. an incom­ - patible vs a compatible element - Fig. 8). At a given content 80 120 °0��4�0��80�--12�O� of an incompatible element, Group 2 specimens are enriched Sr (ppm) Sr (pp::-) , in the compatible elements, relative to specimens from Group 3. Group consists of highly evolved specimens from the core FIGURE Variation diagrams showing compatible element con­ I, 6. 1 centrations for the Sheeprock granite plotted versus Sr. Group of the pluton. The area that Group specimens were collected 2 from is the exclusive host of the beryl segregations noted above. 1 - open diamonds; Group - closed squares; and Group 3 - open squares. A somewhat larger area that includes this zone has been termed the "white facies" of the Sheep rock granite (Williams, 1954; H. Htay and W.R.Griffitts, written comm., 1981). Specimens generally high concentrations of some incompatible trace ele- from Groups 2 and 3 surround the core but are not spatially ments. Rubidium contents range from less than 200 ppm to distinct from one another. These specimens were collected from nearly 1000 ppm. Likewise, the granite is enriched relative to the white facies and the outer "red facies" (iron-stained) of the Cordilleran and Caledonian I-type granitoids (Pitcher, 1982) pluton. The distribution of the red and white facies as mapped typical of the Great Basin, in Y, Nb, Ta, Li, Be, Cs, U, and by Htay and Griffitts does not correlate with the chemical Th. Zirconium and rare earth element (REE) concentrations are groups defined here. A contact between the two facies is not similar to those in many other silicic rocks, but heavy REE visible ort colour air photos or from our reconnaissance (HREE) contents are slightly elevated. Elements concentrated mapping. in feldspars (Sr, Ba, Eu) and mafic silicates (Co, Cr) are strongly Group 1 specimens have high Rb/Sr ratios (40 to 90) and low depleted. All of these features indicate that the granite is highly La/YbN (ca. 3). Group 1 specimens are enriched relative to "differentiated" or "evolved", in accord with the major ele- those of the other two groups in N a, F, Sc (3 ppm - these and ment evidence of high Si, and extreme depletions of Ti, Fe, Mg, other concentrations listed below are ma'l:imums), Be (50 ppm), " Li (260 ppm with one outlier at 420 ppm), Rb (960 ppm), Cs and Ca. In many compositional regards, the Sheeprock granite . is like the anorogenic granites described by Loiselle and Wones (20 ppm with one outlier at 43 ppm), Ta (33 ppm), Nb (130 (1979), Collins et at. (1982), Anderson (1983), Stuckless et al. ppm), Y (60 ppm), Hf (9 ppm) and HREE (Yb:-i = 35). Rela- (1983), and DuBray et al. (this volume). It also bears geochem- tive to Group 2 specimens those from Group 1 are depleted in ical similarity to the biotite-bearing Younger Granites of Nigeria Th (50 ppm - maximum), Sr (20 ppm), Zr (125 ppm), LREE (Bowden, 1966; OJade, 1980; Imeokparia, this volume; Ekwere (LaN = 100) , Ti, Fe, Mg, Ca, K, and P and have relatively and Olade, 1984), the "tin" granites of Alaska (Hudson and low Nb/Ta ratios (3 to 5). As a group, these specimens are also Anh, 1983), evolved portions of the greisenized and depleted in U relative to Groups 2 and 3 (Funkhouser-Marolf, Sn-mineralized Blue Tier Batholith (McCarthy and Groves, 1985). With the exceptions of the U and Th depletions, these 1979), the tin-specialized granites of the South Mountain are common geochemical trends produced by the differentia- Batholith, Nova Scotia (Chatterjee et al., 1983) and especially tion of aluminous high-silica magmas (Hildreth, 1981). to the Eurajoki stock of Finland (Haapala, 1977). Soviet geol- The REE patterns of specimens from the three groups are ogists would classify the Sheeprock granite as a "rare metal grossly similar, but subtly distinct (Figs. 9 and 10). Rocks from Li-F" granite based on its decided enrichment in Li, F, Rb, and the core of the pluton (Group 1) are the most depleted in Eu

Granite-Related Mineral DepOSits 313 1.0 I a)

.8 0 •

,..,. • E .6 0... 100 0... -. • '-' • 0 :J . 4 • W 0 SO w 0 I- 0 - � CJ .2 off 0 Z 0 CJ :I: 0 U ..... w 0.0 -I 0 20 40 60 80 100 120 Q. ::;: Nb ( < 10 ppm) Ul 400

b) 0 300

,..,. E 0... 0... '-' 200 I- -

L (J') Lc c. Nd Sm Eu Tb Oy Yb Lu • 100 • REE ATOMIC NUMBER

FIGUUE 10. Anomalous REE patterns of 2 samples of the o 0 Sheeprock granite which do not correlate with the geochemical 0 0 200 400 600 800 1000 1200 groups as shown in Figure 5. Their respective groups (as indi­ Rb (ppm) cated by other element concentrations) are shown beside sam­ ple labels.

FIGURE 8. Incompatible (Rb, Nb) versus compatible (Sr, Eu) element concentrations in the Sheeprock granite. Group 1 - open 2 circles; Group - closed squares; and Group 3 - open squares. and LREE and have elevated concentrations of Yb and Lu. Groups 2 and 3 have REE patterns that largely overlap for ele­ ments heavier than Tb, but are distinct in LREE concentrations; Group 2 specimens contain higher concentrations of LREE. Specimens from both of these groups contain higher total REE and especially Eu contents than Group I specimens. Sample CJ Group 1 D (Group 3) and FD (Group 1) are shown separately because they

Group 2 [::::t:J do not conform to the REE patterns of others from their respec­ tive groups. CJ was collected from a small Cu-Zn-F mine dump Group 3 OJ]]] (Copper Jack) but shows no petrographic signs of significant 100 hydrothermal alteration: Specimens from this small area are the most mafic of those analyzed. FD is the only specimen ana­ lyzed that contains topaz visible under a microscope. If, as we SO W I- suspect, topaz crystallized as a result of post-magmatic altera­ � tion, then modification of REE concentrations might be CJ Z expected (Alderton et al., 1980). This specimen also has lower a :I: u Y contents than others from the evolved Group 1 (21 ppm vs ..... W the 54 ppm average of the n�st of the Group 1 specimens) . -I Q. (Yttrium is geochemically similar to the heavier middle RE ele­ ::;: < 10 ments.) However, in terms of major and most trace (Rb, Nb, Ul Sr, Zr, Ta, Sc, Th, and U) element concentrations the speci­ men is .indistinguishable from the others in Group 1. The low fluorine· content of this specimen results fromits negligible bio­ tite content.

Petrogenesis of the Sheeprock Granite The Sheeprock granite is geochemically similar to rare-metal granites and to topaz rhyolites. The core of this pluton is typi­ Lc C. Nd Sm Eu Tb Oy Yb Lu fied by extremely high or low concentrations of many elements. In addition, it displays a large internal compositional variation REE ATOMIC NUMBER that defines ·a crude concentric zonation. The following section FIGURE 9. REE patterns of 17 samples of the Sheeprock granite addresses the origin of the internal variation and the genesis divided into three geochemical groups. of the magma itself and is based on models of its isotopic and trace·element composition.

314 Recent Advances In the Geology of Metasomatism Metasomatic processes may be especially effective in altering OGroup 1 • Group 2 o Group 3 the magmatic compositions of a variety of igneous rocks includ­ I I I ing rare metal granites. In fact, the greisenization of the 400 I- I Solid I Sheeprock granite is clearly the result of post-magmatic 0.7 - metasomatism. Hypothetically, the composition of the fluorine 9 and alkali-rich core of the Sheeprock granite could be attributed 300 to metasomatism. However, as shown below, metasomatism F 0.6 - E "\1.0 was not responsible for producing the major compositional Q. ..s- \ groups and trends described above. 200 "\a." Liquid ci; � Metasomatism of the Sheeprock granite produced narrow 0.5 (less than 5 cm across) envelopes of greisen about fractures in , 0.7 the pluton. These envelopes are obvious in the field and show \ 100 - 0.' �.6 anomalous elemental compositions and petrographic distinc­ - 0 �5 tions. Greisen-bordered veinlets are widely spaced and sheeted 3 � greisen veinlets are extremely rare. It is therefore unlikely that 0 � metasomatically altered rocks would be collected inadvertently. 0 200I' 400� O·600 ��p800 �10 00 No albitization is present in the rocks described here. Rb (ppm) Bulk rock analyses and petrographic studies of greisenized rocks show that metasomatism mobilized Na, K, Rb, Li, Fe, FIGURE 11. The crystallization model shown here explains the three distinctive geochemical groups as preserving liquid (Groups Zn, Pb, W, and Sn (Howell, 1987). These elements show enrich­ 1 and 2) and instantaneous solid (Group 3) compositions at differ­ ments and depletions in the greisenized rocks beyond those ent stages in the crystallization of the pluton. F is the fraction explicable by normal variation of the granite. In contrast, other of residual liquid. Rayleigh fractionation calculations were per­ elements enriched in the core of the pluton were not mobilized formed to show the composition of evolving liquid and its instan­ by greisenization and were in effect decoupled from the mobile taneous coexisting solid. The original magma was assu med to elements. Among the elements analyzed, immobile elements have 400 PPm Rb and 245 ppm Sr with bulk partition coeffi­ include Sr, Y, Zr, and Nb. For these elements, specimens of cients of ORb = 0.4 and OS. = 3.2. greisenized granite had compositions that were indistinguisha­ ble from the surrounding granite. Nonetheless, the pluton as a whole shows strong correlations between element pairs that McCarthy and Hasty (1976), ¥cCarthy and Groves (1979), i responded very differently to metasomatism. The strong posi­ Tindle and Pearce (198 ), and Lee and Christiansen (1983) in tive correlation of alkalies (Li, Na, Rb, and Cs), that were examinations of the crystallization of granitoid magmas. A mobile in this F-rich metasomatic environment with elements hypothetical parent liquid was chosen to have a Rb concentra­ (Nb, Y, Sr) that were not (Fig. 8), argues strongly against tion of 400 ppm and Sr of 245 ppm. Bulk partition coefficients metasomatic alteration as having produced the compositional (DS' = 3.2 and DRb = 0.4) were chosen by an iterative proce­ groups discussed above. Likewise, greisenized rocks from the dure to produce curves that pass through the extremes of the Sheeprock Mountains are consistently rich in Fe; ungreisenized compositional data, such that an envelope for possible compo­ specimens from the evolved core of the pluton are poor in Fe sitions is created . (Howell, 1987). In addition, Ti, Ta, and Hf (not analyzed in This differentiation model was checked by calculating the the greisens) are widely regarded as being immobile in second­ proportions of fractionating minerals by a least squares mass ary alteration environments and yet they show decided com­ balance technique using only major elements . These calcula­ positional variations in the Sheeprock granite and correlate with tions suggest that the fractionating mineral assemblage consisted "mobile" elements. On the other hand, the observed element of 46070 K-feldspar, 27% quartz, 21 % plagioclase, 5% biotite trends (Figs. 5 and 6) with enrichments of Rb, Nb, Ta, etc. , and small proportions of Fe-Ti oxides and apatite (sum of the are very similar to those seen in rhyolitic volcanic rocks whose squared residuals was 0.035). This analysis agrees with the trace glassy groundmasses show no evidence of metasomatism (Chris­ element model in suggesting that a crystallization interval of approximately 45% (Fig. 11) separates Group 2 (parent) and tiansen et at. , 1984). Instead, these elemental variations can be best explained by fractionation of observed minerals. Group 1 (residue) specimens. The calculated major element com­ position of the cumulate (Si02 = 73.5%, Alz03 = 13.9%) is broadly similar to the observed composition of sample CJ. F raet' Ion a I e rysta ll'Izat lon e I , M 0 d When combined with crystal/liquid partition coefficients for We interpret the geochemical trends and the existence of siliceous magmas (Mahood and Hildreth, 1983; Nash and geochemical groups as indicating that the Sheeprock granite Crecraft, 1985; Ewart, 1982), these mineral proportions sug­ crystallized in situ. Specimens from the core (Group 1) apPear gest that the bulk partition coefficient for Rb could range from to represent residual liquid confined to the inner and upper parts 0.38 to 0.7 and that for Sr could range from 3.4 to 4.1 - very of a magma chamber that was crystallizing by the accumula- close to our independent estimates. Consideration of the large tion of crystals (Group 3) on its walls, The chemical character changes in D's as a function of both temperature and small of the evolving liquid (Group 2) is partially preserved perhaps · changes in melt composition (Lesher, 1984; Nash and Crecraft, as a result of intermittent periods of equilibrium crystallization " 1985) implies that these two sets of estimated D's are com- of large (at the scale of a specimen) parcels of magrria. Thus, . parable. "primitive" melt compositions appear to be preserved along In spite of its inherent assumptions, the fractional crystalli- with crystal cumulates and residual "evolved" liquid. zation model appears to explain some of the major features of Elemental variations of Rb and Sr are shown in Figure 11 the evolution of the Sheep rock granite, especially the divergence on which we base a discussion of the crystallization of the of Groups 2 and 3 on compatible vs incompatible element plots. Sheeprock granite. The crystallization model shows the evolu- Specimens from Group 2 appear to be most like the original tion of the liquid and solid in a magma according to the Rayleigh liquid from which the intrusion crystallized. Specimen CJ fractionation law. The fraction of residual liquid (f)is indicated (Group 3) appears to be an early cumulate from this magma by tick marks on the curves. Thus a melt composition that plots and is poor in incompatible elements and enriched in the com- on the liquid evolution curve at f = 0.7 coexists in equilibrium patible elements. The separation of such material drove the with a mineral assemblage on the solid evolution curve at the residual liquid toward compositions typical of the core (Group point labelled 0.7. This sort of crystallization model, which 1) which is rich in Rb and other incompatible elements , nota­ emphasizes the role of granitic cumulates, has been used by bly Be. Group 1 specimens are the most evolved. If the bulk

Gran i te·Related Mineral Deposits 315 that range from 14 to 18, and specimens from Groups 2 and

o Group 1 • Group 2 o Group 3 3 vary from 20 to 28. These trends are most likely caused by 190 small variations in the amount of zircon in each specimen and can be likened to mixing lines between zircon and granite. 170 /- Nonetheless, by considering group averages, the Zr-Hf varia­ . - tions are consistent with the interpretation of Zr as a compati­ 150 - ,/ ble element and Hf as an incompatible element. This is the sort � / -/ of behaviour seen in many high-silica granitic magmas (Hildreth, E /0 0... 130 1979, 1981). o / 0... ° / 0 0 B Rare Earth Elements L 110 o 0 N 0 /0 Accepting a fractional crystallization origin for the chemical o 90 variation in the Sheeprock granite, we can examine some impli­ cations for the mineralogic controls of the trends of specific 70 REE. Other investigators have noted the difficulty of using REE for granite petrogenetic models bec�use of their occurrence in accessory phases (Fourcade and Allegre, 198 1; Gromet and Sil­ 50 6 8 10 2 ver, 1983; Lee and Christiansen, 1983). As a result, the con­ m) Hf (pp centrations of the REE are very sensitive to sampling problems FIGURE 12. Zirconium versus hafnium variation diagram show­ because of the heterogeneous distribution of minute quantities ing covariatioll within each geochemical group. of REE-rich minerals. Nonetheless, some conclusions regard­ ing the REE evolution of the Sheeprock pluton can be made. For example, the enrichment of LREE in the Group 3 (cumu­ D's and initial compositions chosen for the Rayleigh fraction­ lates) specimens and the complementary depletion of LREE in ation model are approximately correct, this group has compo­ the core (Group 1) specimens is indicative of a LREE-rich phase sitions that reflect removal of 700/0 by mass of the original in the cumulus mineral assemblage (Fig. 9). It appears that this magma. Crystal cumulates from this evolved liquid also appear phase accumulated in the margin of the crystallizing pluton and to have been preserved in the Group 3 specimens which diverge enriched the solids in LREE relative to the evolving residual from Group 1 toward low Rb concentrations at an approxi­ liquid. Conversely, the Group 1 specimens are enriched in heavy mately constant Sr content. Such a pattern is strongly indica­ REE (Fig. 9) relative to the cumulate group and these must have tive of cumulate processes in magma chambers (McCarthy and behaved as incompatible elements. Hasty, 1976). Specimens that plot between the liquid and solid When first observed in silicic igneous rocks, such depletion evolution curves can be explained readily by the incomplete sepa­ patterns for LREE and enrichment of HREE were deemed to ration of crystalline material from the evolving melt. In addi­ be inconsistent with an origin by crystal fractionation and several tion, some scatter of the data about the model curves is proba­ investigators called on "liquid-state" differentiation (Hildreth, bly the result of minor wall-rock assimilation, secondary 1979; Ludington, 1981; Whalen, 1983). Such differentiation pat­ alteration, or weathering. terns are becoming increasingly recognized as the norm for high The degree to which specimens from the Sheeprock granite silica rhyolites and granites (e.g. Bacon et al., 1981; Christiansen adhere to the trace element patterns is testimony to the applica­ et al., 1983; Crecraft et al. , 1981; Miller and Mittlefehldt, 1984; bility of the crystallization model. Plots of incompatible vs com­ Keith, 1982) and can be reasonably modelled by the fractiona­ patible elements (Fig. 8) consistently show a divergence in the tion of small amounts of several common REE-rich phases concentrations of compatible elements between Groups 2 and (Christiansen, 1983 ; Michael, 1983). For example, Christiansen 3. This suggests that the elemental compositon of the pluton et al. (1984) point out thai fractionation of 0.04 wtlllo allanite was strongly controlled by fractional crystallization and that effectively depleted La (halved after 751110 crystallization) in specimens with chemical compositions like cumulates and like rhyolites from the Thomas Range, Utah, but that this amount liquids were preserved during the solidification of the pluton. of allanite did not reduce HREE contents . Likewise, It follows from the fractional crystallization model that plots Christiansen (1983) explained the LREE-depletion of the Bishop of elements with grossly similar bulk crystal/liquid partition Tuff as resulting from fractionation of 0.06 wtlllo allanite. In coefficients should form linear arrays. This feature is shown a similar manner, monazite effectively removes LREE from in Figure 6. In these diagrams, the compositions of specimens differentiating silicic magmas (Mittlefehldt and Miller, 1983). from Groups 2 and 3 overlap substantially. This is predicted We suggest that the fractionation of monazite, which is the most from the model because for the case of similar partition coeffi­ important REE-bearing phase in the Sheeprock granite cients, solid and liquid evolution curves are not readily distin­ (Funkhouser-Marolf, 1985), was important in the REE­ guishable from one another - regardless of the initial concen­ evolution of the pluton. Mass balance calculations demonstrate trations chosen for the model. Linear variation trends such as that the presence of less than 0.01 wtlllo monazite in the frac­ these are similar to those interpreted as originating from the tionating mineral assemblage could deplete La from 80 ppm separation of restite from a granitic magma (Compston and to 30 ppm over the inferred range of crystallization (f = 0.75 Chappell, 1979) . Therefore, attempts to determine the mechan­ to 0.25) between Group 2 and Group 1 specimens. In addition, ism by which compositional variety arises in granitic plutons the La and Ce contents of the specimens from Group 3 (cumu­ should include a comparison of elements with as much geochem­ lates) relative to those of Group 1 suggest that La, Ce and Nd ical contrast as possible (i.e. elements with substantially differ­ all behave as compatible elements. However, it is important to ent bulk partition coefficients). note that no specimen analyzed here could be regarded as a LREE-rich cumulate formed from the early liquids (Group 2). AccessofY Minerals Sample CJ, which otherwise appears to be an early formed The importance of trace or accessory minerals in controlling cumulate, is poor in light REE (Fig. 10) and no discrete grains the composition of the Sheep rock granites is well-displayed by of monazite have yet been located in it with autoradiography. Zr and Hf variations. The variations in these elements are This specimen's apparent depletion in monazite and LREE may strongly controlled by zircon which (by mass balance calcula­ not be representative, or the rock may have crystallized before tions) must occur in amounts of only 0.02 to 0.04 wtlllo of the saturation with monazite occurred. Eu and HREE (as well as specimens. Figure 12 shows the covariation of Hf and Zr. The Ca, Fe, Mg, Ti) concentrations are not as sensitive to the abun­ three geochemical groups remain distinct but the compositions dance of this trace mineral so the cumulate nature of CJ is more of each group lie in semi-parallel arrays with positive slopes but apparent when considering concentrations of these elements . different Zr/Hf ratios. Group I specimens have Zr/Hf ratios Therefore it is important to note that Eu-variations, controlled

Recent Advances In the Geology of 316 as they are by the fractionation of major phases (the feldspars), lates are specimens that appear to have preserved the composi­ indicate that CJ with its high Eu content has a composition tions of relatively "primitive" liquids (Group 2 - f = 0.75 appropriate for an early cumulate from Group 2 "liquids" (Figs. to 0.5); these specimens were perhaps formed during incremental 8 and 10). In addition, other Group 3 specimens have higher episodes of equilibrium crystallization_ They are generally Eu contents than specimens from Group 1 - as expected for coarse-grained. Complementary solids (Group 3) and liquids feldspar-rich cumulates derived from "evolved liquids". Enrich­ (Group 1) continued to form while the crystalline carapace of ment of HREE is produced as a result of their more incompat­ the magma chamber grew inwards. Specimens from the core ible nature in accessory phases typical of metaluminous to per­ of the pluton (Group 1 - f = 0.40 to 0.25) represent the residual aluminous graniroids. Of the accessory minerals in the or most evolved liquids formed from this liquid line of descent. Sheeprock granite, only zircon has high partition coefficients Their porphyritic character and the presence of miarolitic cavi­ for HREE. As pointed out by Christiansen et al. (1984), the ties, pegmatites, and beryl segregations in the pluton's central low abundance of zircon (generally < 0.04 wt07o) in portion suggest that the magma became fluid-saturated during metalurninous granitic magmas is not sufficient to deplete evolv­ the crystallization of these specimens. Many of the late cumu­ ing liquids (or enrich their cumulate counterparts) in HREE. lates (Group 3 - f = 0.3 to 0.25) formed during this interval Important distinctions between the behaviour of elements con­ are aplitic, suggesting relatively large amounts of undercool­ trolled by accessory minerals and those that are controlled by ing, perhaps as a result of a pressure-quench induced by the major minerals are apparent. hydro fracturing of the country rocks upon fluid saturation. Some element mobility probably resulted from granite-fluid U, Th, and Sc Behaviour interaction - e.g. depletion of REE and Y in one specimen and The recognition of distinct geochemical groups within the pluton redistribution of U in others. However, metasomatism does not also reveals several important features regarding the behaviour appear to explain the bulk of the geochemical data. Thin greisen­ of Th and Sc. For many, indeed most, silicic rocks increasing bordered veins formed from circulating fluids after the core differentiation is marked by increases in Th and U contents (e.g. solidified and attest to local mobility of Fe, Cu, Pb, Zn, W and Stuckless et al., 1977). The lack of enrichment of these elements Rb (Howell, 1987). The isotopic heterogeneity of the pluton sug­ in the core (Group 1) of the Sheeprock granite implies that these gests that minor assimilation of wall rocks occurred. elements were either mobilized by some late- or post-magmatic process or that some early crystallizing phase removed U and Partial Melting Model Th from the residual melt. We do not entirely rule out post­ Previous investigations of granites and rhyolites from the magmatic mobility of U. Indeed, the scatter apparent on plots western United States and elsewhere have pointed toward a like those in Figure 5 suggest substantial remobilization of crustal origin for many silicic magmas (e.g. Farmer and U probably occurred during weathering as RaeU and U values DePaolo, 1983, 1984). We present a simplistic batch partial melt­ are poorly correlated (Funkhouser-Marolf, 1985). We regard ing model that is consistent with the Sr isotopic, trace element the mobilization of Th less likely under subsolidus conditions and fluorine data for the Sheep rock granite. (Zielinski et al., 1981). The presence of euhedra of thorite and The generally metaluminous nature of the rocks, the moder­ Th-rich monazite, which occur most abundantly as mineral ate Sr-isotope ratios, shallow level of emplacement, absence of inclusions and in the groundmass of specimens from the outer boron metasomatism, and only moderate water concentrations parts of the intrusion (H. Htay and W.R. GriffittS, written demonstrate that the Sheeprqck granite, as well as topaz comm. 1981), suggest that the depletion of Th in the evolved rhyolites from the western United States, are not evolved S-type rocks may have occurred due to removal of these Th-rich granites derived from partial melting of pelitic metasedimen­ minerals from the melt (Funkhouser-Marolf, 1985). Examples . tary rocks (Clemens, 1984). Such strongly peraluminous two­ of the compatible behaviour of Th from volcanic rocks are mica granites are found in the eastern Great Basin (Lee et al., provided by the Macusani glasses of Peru (Noble et al., 1984) 1981) but they are temporally, geochemically, and isotopically that have low Th/Rb ratios and Th/U ratios (near 1) implying distinct from the F-rich magmas described here (Lee and a magmatic depletion of Th content. The zoned rhyolitic Christiansen, 1983). volcanic system at Pine Grove, Utah, described by Keith (this A major constraint on the petrogenesis of the Sheep rock volume) shows a negative correlation of Th with generally granite is its relatively low initial 87Sr186Sr of 0.7064. Other late incompatible elements (Rb and Ta). Monazite and xenotime Cenozoic rhyolites from the eastern Great Basin have similar have been identified as major carriers of Th in these rhyolites. 87Sr/86Sr ratios (Wilson et al., 1983; Christiansen et al., 1986). The fractionation of small quantities of these phases could easily If the granite was derived from the crust, then it must have come deplete the residual melt in Th. from a source wi�l).a relatively low time-integrated Rb/Sr ratio On the other hand, Sc is generally depleted during the (0.05' to 0.1) because the basement under the easternGreat Basin differentiation of silidc magmas (Bacon et al., 1981; appears to be about 2 Ga (Nelson et al., 1983). This require­ Christiansen et al., 1984; Hildreth, 1981). However, its higher ment is in sharp contrast to the remarkably high Rb/Sr ratios concentration in the core of the Sheeprock granite (Fig. 5) sug­ of the Sheeprock granite itself (over 50). Lower to mid-crustal gests that it behaved here as an incompatible element, perhaps granulites commonly have low Rb/Sr ratios and only moder­ because of the paucity of biotite (DSCbiot 1O-20-Hildreth, ately elevated modern day 87Sr/86Sr ratios (Ben Othman et al., 1977; 5-20-Nash and Crecraft, 1985) and magnetite 1984; James et al., 1980; Clifford, 1974). In like fashion, the (DSCmt 9-10-Mahood and Hildreth, 1983; 1.5-16-Nash and· lower continental crust (by analogy with high-grade meta- Crecraft, 1985). Hildreth (1979) and Keith (1982) showed a simi­ . '. morphic rocks) is moderately to strongly depleted in K, Rb, Cs, lar magmatic enrichment of Sc in rhyolitic magmas as indicated U, Th, Li, Be, Ta and Y (Collerson and Fryer, 1978; Sheraton by positive correlations with decidedly incompatible trace ele­ and Black, 1983; Condie et al., 1982), all elements enriched in ments. the Sheeprock granite. We describe such a magma source as dep leted when compared to average crustal values or to the Summary of the Crystallization Model sources of strongly peraluminous two-mica granites which we We interpret the geochemical variety in the Sheep rock pluton regard as being derived from a different part of the continen­ to have arisen as the result of the in situ fractional crystalliza­ tal crust. tion of a granitic magma with A-type affinities. Early cumu­ We have used Rb , Sr, and U concentrations for granulites lates formed on the wall of the magma chamber (some Group compiled from the literature (Tarney and Windley, 1977; Bar­

3 specimens - f = 0.7) are sparsely represented in the collec­ bey and Cuney, 1982; Sheraton and Black, 1983; Sighinolfi et tion described here. Because crystallization occurred along the al., 1981; Condie et al., 1982; Iyer et al., 1984) and estimates walls, residual liquids were displaced inwards and perhaps of bulk partition coefficients for these elements to constru�t upwards. Collected from within the outcrop area of these cumu- the trace element models of batch partial melting shown III

317 Granite-Related Min eral Deposit s Partial Melting 30 Partial Melting

E' 20 0.. 0.. �

L U1 10

400 600 Rb (ppm)

FIGURE 14. Partial melting models showing the Rb and U con­ FIGURE 13. Partial meHing models showing the Rb and Sr con­ centrations in a mel! produced by variable degrees of batch mell­ centrations in a mell produced by variable degrees of batch mell­ ing. Source concentrations and bulk partition coefficients are: ing. Source concentrations and bulk partition coefficients are: Model l Rb =30 ppm DRb = 0.02 U= 1.5 ppm OU = 0.09 Model 1 Rb = 30 ppm ORb = 0.02 Sr = 600 ppm OSr = 2.5 Model 2 50 0.1 1.5 0.09 Model 2 50 0.1 600 2.5 Model 3 50 0.02 1.5 0.09 Model 3 50 0.02 600 2.5 Fractional crystallyzation (Fe) curves for evolving liquids, shown Fractional crystallization (FC) curves of Figure 11 are also shown. as dash-dot lines emanating from the partial mel! curve at 400 The hachured area indicates the general compositions of granu­ ppm Rb, assume ORb = 0.4 and Ou = 0.1. The hachured area lites. indicates the general compositions of granulites.

Figures 13 and 14. Extending from the partial melting curves morphic rocks from a variety of terranes, but the relatively dry are Rayleigh fractionation paths for liquids and instantaneous nature of these rocks keeps the hydrous mineral content low. solids, constructed as in Figure 11. These fractionation curves Many petrologists (e.g. Burnham, 1979) suggest that decom­ pass through "parental melt" compositions appropriate for the position of hydrous minerals provides the volatiles for partial Sheeprock granite. Model l (Fig. 13) traces the composition melting. The decomposition of the small quantities of biotite of a liquid produced by variable degrees of batch partial and amphibole that persist in high-grade metamorphic rocks melting of a Rb-poor (Rb/Sr = O.OS) source with a low bulk would then of necessity only produce small degrees of melting. DRb (an alkali feldspar- and biotite-poor source). Approxi- For example, Burnham (1981) has shown that the decomposi­ mately SOlo to .6% partial melting of this source produces a tion of 1 wt% biotite would yield approximately 1.2 wt% sili­ magma that could be parental to the Sheeprock granite cate melt with 3.3 wt070 H20 plus F. Clemens (1984), using (Rb = 400 ppm Sr = 245 ppm). For Model 2, much smaller Burnham's solubility model, calculated that decomposition of degrees (2.S070 to 3%) of partial melting are required. This model 20% biotite in a felsic granulite at S kb would produce 17% assumes a higher Rb content (such that Rb/Sr = 0.083) and partial melting with 4.6% H20. A smaller amount of biotite a higher bulk DRb (simulating a source that contained approx- would proportionately reduce the amount of melting. Thus the imatety IS% alkali feldspar or with a maximum of 3% biotite small degree of melting required by the trace-element models as constrained by the crystal! liquid D's given by Nash and could occur with the complete decomposition of 2% to 12% Crecraft, 1985). A less stringent Model 3 uses the low bulk DRb biotite in the granite's protolith. The decomposition of biotite of Model l and the higher Rb concentration of Model 2. It and its replacement with pyroxene would also lower the bulk shows that about 11 % partial melting could produce a magma DRb between granite magma and restite to the values used in similar to that parental to the Sheeprock granite. Below, we Models 2 and 3. In addition, biotite-controlled partial melting justify a low DRb as the result of the decomposition of biotite is consistent with the aluminous nature of the magmas. to produce melting. However, isotopic equilibrium between melt and the residuum Similar partial melting models based on Rb and U variations is required by the moderate initial 87Sr/86Sr ratios that charac­ were constructe1:l to approximate the concentrations of these terize this pluton. elements in the Sheeprock granite. We assumed the source con- Perhaps just as important as the incompatible element-rich tained I.S ppm U (appropriate for many felsic granulites) and nature of the Sheeprock granite is its enrichment in fluorine. the Rb concentrations of Models 1, 2 and 3. The bulk Du The fluorine enrichment may also be explained by the decom­ (0.09) was chosen to simulate the effect of a small amount of position of biotite from high-grade metamorphic biotites and zircon (Du = 200 to 300; Hildreth, 1977) or another U-rich' ' . amphiboles. As shown by Holloway (1977; see also Holloway accessory in the residue. The low solubility of zircon in silica- . and Ford, 1975; Manning and Pichavant, this volume) high and alumina-saturated igneous rocks (Watson and Harrison, F/(F + OH) ratios increase the thermal stability of these 1983) and the Zr content of hIgh-grade metamorphic rocks (ca. minerals. Others have shown that F/(F + OH) ratios in hydrous 100 to 400 ppm - e.g. Sheraton and Black, 1983; Condie et mafic silicates increase with increasing metamorphic grade, al., 1982 - implying 0.02 to 0.08 wt% zircon) indicate that this extending to granulite facies (Fillipopov et at., 1974; Padovani value may be a reasonable approximation. Within the applica- pers. comm., 1984) or to the onset of melting (White, 1966). bility of the assumptions, these models also require that the Thus, although the absolute amount of biotite may decrease parental magma for Sheeprock granite was produced by low with increasing grade of metamorphism, it probably becomes degrees of partial melting (less than 12%) to explain the neces- more F-rich. The decomposition of small amo\lnts of F-rich bio- sary enrichment of uranium. tite would therefore produce small amounts of F-rich melt that, Small proportions of partial melting are a natural consequence because of the small degree of melting, would also be enriched of the water-undersaturated melting of high-grade metamorphic in incompatible elements (Rb, U, Ta, Be, Li, etc.). Extended rocks such as granulites. Small amounts (generally less than differentiation, such as that modelled in the previous section, 10%) of biotite (or amphibole) persist in granulite grade meta- would further elevate these concentrations until volatile satu-

318 Recent Advances in the Geology of ration and consequent quenching of the magma prevented fur­ REFERENCES ther crystallization. ALDERTON, D.H.M., PEARCE, J.A., and POTTS, P.J., 1980, Rare An alternative mechanism for the generation of rhyolitic melts earth mobility during granite alteration: Evidence from southwest with the typically high F I CI ratios of aluminous anorogenic England; Earth Pl anet. Sci. Lett., V. 49, pp . 149-165. magmas calls on the depletion of magmatic CI as the result of ANDERSON, J.L., 1983, Proterozoic an orogenic granite plutonism the loss of a fluid phase (Christiansen et al., 1983, 1986; of North America: Geol. Soc. Am. Mem oir., V. 161, pp. 133-154. Burnham, 1979). Fluids could be lost as the result of satura­ ARMSTRONG, R.L., 1966, K-Ar dating using neutron activation for tion and subsequent escape to the wallrocks or as the result of Ar analysis: Granitic plutons of the eastern Great Basin, Nevada V. 30, 565-600. eruption. In the case of the Sheep rock granite neither of these and Utah: Geochim. Cosmochim. Acta, pp. possibilities appears likely. Judging from biotite compositions ARMSTRONG, R.L., 1970, K-Ar dating using neutron activation for Ar analysis: Comparison with isotope dilution Ar analyses: the magma wasrelatively rich in fluorine, long before fluidsatu­ Geochfm. Cosmochim. Acta, V. 34, pp. 233-236. ration occurred as indicated by the presence of miarolitic cavi­ BACON, C.R., MACDONALD, R., SMITH, R.L., and ties. In addition, no volcanic equivalents of the distinctive BAEDECKER, P.A., 1981, Pleistocene high-silica rhyolites of the Sheeprock magma have been identified in spite of the preser­ Coso volcanic field, Inyo County, California: J. Geophys. Res., vation of older volcanic rocks in the immediate vicinity of the V. 86, pp. 10223-10241. pluton. The absence of such volcanic equivalents suggests vent­ BAILEY, J.C., 1977, Fluorine in granitic rocks and melts: A review: ing did not occur. Chem. Geol. , V. 19, pp. 1-42. BARBEY, P., and CUNEY, M., 1982, K, Rb, Sr, Ba, U and Th Conclusions geochemistry of the Lapland granulites (Fennoscandia). LILE frac­ tionation controlling factors: Contrib. Min eral. Petrol., V. 81, In spite of recent advances in analytical and theoretical tech- pp. 304-316. niques, the construction of unique geochemical models for the BEN OTH:v1AN D., POLVE, M., and ALLEGRE, C.J., 1984, Nd-Sr origin and evolution of granitic magmas is extremely difficult, isotopic composition of granulites and constraints on the evolu- if not impossible. In many cases we can at best demonstrate tion of the lower continental crust: Na ture, V. 307, pp. 510-515. that it is possible that a specific rock evolved by a certain set BEST, M.G., McKEE, E.H., and DAMON, P.E., 1980, Space-time- of geologic processes. In this paper, we have attempted to erect composition patterns of late Cenozoic mafic volcanism, south- V. 280, one such possible model that shows how the internal variation western Utah and adjoining areas:. Am. J. Sci. , pp. 1035-1050. of the Sheeprock granite arose and ow the parental magma h BOWDEN, P., 1966, Lithium in Younger Granites of north em Nigeria: was generated. To do so we have made explicit assumptions Geochim. Cosmochim. Acta, V. 30, pp. 555-564. regarding the elemental composition of parental materials and · BURNHA.V1, C.W., 1979, Magmas and hydrothermal fluids:In, Barnes, the bulk partition coefficients that govern their distributions H.L., ed., Geochemistry of hydrothermal ore deposits, 2nd ed., between crystals and liquids. John Wiley & Sons: 71-136. Within the limits imposed by the uncertainties in these BURT, D.M., SHERIDAN, M.F., BIKUN, J.V., and CHRISTIAN- assumptions, we have shown that relatively simple models of SEN, E.H., 1982, Topaz rhyolites-Distribution, origin, and sig- batch partial melting and fractional crystallization can explain nificance for exploration: Econ. Geol., V. 77, pp. 1818-1836. . CHATTERJEE, A.K., STRONG, D.F., and MUECKE, G.K., 1983, th e geoch· emlstry f t h e Sh eeproc k gramte andb y ana ogy oth er 0 I A multivariate approach to geochemical distinction between tin- aluminous rare-metal granites. As a source for the Sheeprock specialized and uranium-specialized granites of southern Nova granite, we have called upon partial melting of a depleted, Scotia: Can. J. Earth Sci. , V. 20, pp. 420-430. "granulitic" proto lith in the lower crust. An important require- CHRISTIANSEN, E.H., 1983, The Bishop Tuff revisited: Composi- ment of the model is that the amount of partial melting remains tional zonation by double-diffusive fractional crystallization small (2.5070 to 11%). The decomposition of small amounts of (DDFC): Geol. Soc. Am. Abstr. Programs, V. 15 p. 390. refractory F-rich biotite will yield smallquantities of F-rich melt. CHRISTIANSEN, E.H. , BIKUN, J.V., SHERIDAN, M.F., and The extensional tectonic setting of the eastern Great Basin (and BURT, D.M., 198.. , Geochemical evolution of topazrhyo lites from V. 69, in other areas where A-type granites occur) may have been a the Thomas Range and Spor Mountain, Utah: Am. Min ., 223-236. prerequisite to the separation of small melt fractions from crys- pp. CHRISTIANSEN, E.H., BURT, D.M., SHERIDAN, M.F., and talIine restite. This sort of tectonic setting may have a I so set WILSON, R.T., 1983, The petrogenesis of topaz rhyolites from the stage for the intrusion of hot mafic magmas into the crust. the western United States: Contrib. Mineral. Petrol., V. 83, pp. Mafic magmas may have acted as a heat source for partial melt- 16-30. ing. Subsequent high level fractional crystallization enhanced CHRISTIANSEN, E.H., and LEE, D.E., 1986, Fluorine and chlo- the depletions and enrichments of some elements. The preser- rine in granitoids fr om the Basin and Range province, western vation of cumulates on the walls of the pluton carries imp or- United States: Econ. Geo/. , V. 81, pp. 1484-1494. tant consequences for interpretations of the mechanism. of crys- CHRISTIANSEN, E.H., SHERIDAN, M.F., and BURT, D.M., 1986, tallization of granitic magmas. The geology and geochemistry oftopaz rhyolites fr om the western . ' ' United States: Geological Society of America Special Paper 205, M agmas nch fl'uonne an d mcompa t'bl e tirace e eme nts (e - III J .t;. 82 p. Rb, Li, U) can be derived from high-grade metamorphic rocks CHRISTIE-BLICK, N., 1982, Upper Proterozoic and lower Cambrian that have relatively low concentrations of these elements and rocks of the Sheep rock Mountains, Utah: Regional correlation and low RblSr ratios. The ratios of element concentration in the significance: Geol. Soc. Am. BuII., V. 93, pp. 735-750. probable source to that in the evolved magma are in the order CHRISTIE-BLICK, N., 1983, Structural geology of the southern of 20 to 30 for incompatible elements. If these enrichments also " Sheep rock Mountains, Utah: Regional significance: Geol. Soc. '. Am. Mem., V. 157, pp. 102-124. hold for economically important elements (Be, Ta, Sn, W, Mo, . . . . . CLEMENS, J .D , 198 , Water contents of to IntermedIate U, etc.) then efficient magmatic accumulations of such elements : .. SIIJCIC magmas: Luhos, 17 pp. 273 287. to pro tore status are within reason. Geochemically "enriched" V. : � . . r anoma ous crusta s urces are not reqUIre d f or h t e g nera- CLIFFORD, T.N., 1974, RevIew of Afncan granuhtes and related rocks: � I I � � Geol. Soc. Am. Spec. Paper 156, 48 p. tlO such as the rock gra te. n of rare-metal gramtes Sheep m COHENOUR, R.E., 1959, Sheeprock Mountains, Tooele and Juab Counties: Precambrian and Paleozoic stratigraphy, igneous rocks, Acknowledgments structure, geomorphology and economic geology: Utah Geol. This work was initiated while EHC was a National Research Mineral. Survey Bull. 63 , 20 1 p. Council-U .S. Geological Survey Postdoctoral Research Associ­ COLLERSON, K.D., and FRYER, B.J., 1978, The role of fluids in the formation and subsequent development of early continental ate. Institutional support was provided by the University of Iowa cruSt: Contrib . .\;fineral. Pelrol. , V. 67, pp. 151-167. and the U.S. Geological Survey. In addition to those formally COLLINS, W.J., BEA:"!S, S.D., WHITE, A.J.R., and CHAPPELL, acknowledged in the text, we thank C.E. Hedge, D. Lambert, B.W., 1982, Nature and origin of A-type granites with particular Z. Peterman, and J. Edie for assistance in some of the analyti­ reference to southeastern Australia: Contrib. lvlineral. Pelrol., V. 80, 189-200. cal work. Discussions with W. Griffitts were extremely valuable. pp.

Gran ite·Related Min eral Deposits 319 COMPSTON, W., and CHAPPELL, B.W., 1979, Sr-isotope evolu­ IMEOKPARIA; E.G.; 1982, Geochemistry and relationships to miner­ tion of granitoid source rocks: In, M.W.·McELHINNEY, ed., alization of granitic rocks from the Afu Younger Granite Com­ The Earth, its Origin, Structure and Evolution: New York, Aca­ plex, Central Nigeria: Geol. Mag., V. 1l9, pp. 39-56. demic Press, pp. 377-426. IYER, S.S., CHIYNDHURI, A., VASCONCELLOS, M.B.A., and CONDIE, K.C., ALLEN, P., and NARAYANA, B.L., 1982, CORDANI, U.G., 1984, Radioactive element distribution in the Geochemistry of the Archean low to high-grade transition zone, Archean granulite terrane of Jequi-Bahia, Brazil: Contrib. V. 85, 95- 101. southern India: Contrib. Mineral Petrol., V. 81, pp. 157-167. Mi'leral. 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EKWERE, S.J., and OLADE, M.A., 1984, Geochemistry of the tin­ LEE, D.E., KISTLER, R.W., FREIDMAN, I., and VAN LOENEN, niobium-bearing granites of the Liruei (Ririwai) complex, Youn­ R.E., 1981, Two-mica granites of northeastern Nevada: ger granite province. Nigeria: Chem. Geol., V. 45, pp. 225-243. J. Geophys. Res., V. 86, pp. 10607-10616. FARMER, G.L.. and DEPAOLO, D.J., 1983, Origin of Mesozoic and LESHER, C.E., 1984, Soret fractionation of silicate melts: Bulk com­ Tertiary granite in the western United States and implications for positional control 'on minor and trace element partitioning; Proc. pre-Mesozoic crustal structure: 1. Nd and Sr isotopic studies in Conf. Open Magmatic Systems, Dungan, M.A., Grove, T.L., Hil­ V. the geocline of the northern Great Basin: J. Geophys. Res., dreth, W., eds., pp. 96-98. 88, pp. 3379-3401. LINDSEY, D.A.. 1982, Tertiary volcanic rocks and uranium in the FARMER, G.L.. and DEPAOLO, D.J., 1984. 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