SEG SP-2 GIANT DEPOSITS 285

THE GENESIS OF GIANT PORPHYRY DEPOSITS

J. D. Keith, E. H. Christiansen Department of Geology, Brigham Young University Provo, Utah U.S.A., 84604

and

R. B. Carten U. S. Geological Survey, Mackay School of Mines Reno, Nevada U.S.A., 89557-0047

ABSTRACT

Giant porphyry molybdenum deposits are best exemplified by the Climax and Henderson deposits in Colorado. The high grades of these deposits are probably inherited from magmatic molybdenum concentrations of about 4 to 5 ppm, which are high for metaluminous rhyolitic that average about 2 ppm molybdenum. High magmatic molybdenum concentrations in metaluminous rocks appear to be related to high magmatic oxygen fugacities (2 or 3 log units above QFM oxygen buffer) and are correlated with high concentrations. High oxygen fugacities are likely inherited from calc-alkaline or lamprophyric predecessors. High niobium and molybdenum are related to extreme fractionation of rhyolitic magmas. Much higher concentrations of molybdenum (> 1,000 ppm) in the ore fluid (and the cupola ) are probably achieved by crystallization in the deeper portions of a magma chamber accompanied by convection of the evolved liquid to the cupola and volatile fluxing. Exploration criteria for a giant, high-grade deposit include: 1) a tectonic setting that indicates a changeover from compressional to extensional tectonics, 2) thick continental crust at the time of deposit formation may encourage extreme differentiationand crustal contamination, 3) an isotopically zoned magma chamber indicative of a long-lived heat source, 4) a large, sub-volcanic, central-vent ash flow/dome system that erupted less than 100 km3 of rhyolite, and 5) high niobium concentrations (> 75 ppm) in a subalkaline, -bearing rhyolite.

INTRODUCTION listed in Table I are geographically located in Figure 1. Deposits considered in this paper The grade and tonnage data of each are restricted to porphyry Mo deposits which deposit cited in this paper, as well as the characteristically do not contain recoverable assignment of each deposit to a general class Cu and porphyry Mo-Cu deposits which (Table 1), is modified from the system contain coproduct or byproduct Cu and grades presented in Carten et al. (in press). Deposits of Mo which average greater than 0.05% Mo . 286 SEG SP-2 GIANT ORE DEPOSITS

Table 1. Grade and tonnage of granite-related Mo and Mo-Cu deposits (modified after Carten et al., in press).

Production + reserves % Mo % Cu metric % Mo Deposits country grade grade tons cutoff reference 6 (10 )

GRANITE-RELATED No: HIGH-SILICA RHYOLITE-ALKALINE SUITE 1) Climax USCO 0.216 769 0.120 mineable S.R. Wallace, pers. comm., 1990 0.240 907 geologic S.R. Wallace, pers. comm., 1990 2) Henderson USCO 0.294 266 0.180 geologic unpublished data 0.228 437 0.120 geologic unpublished data 0.171 727 0.060 geologic carten et al., 1988a 3) Urad USCO 0.209 12 mined Wallace et al., 1978 4) Big Ben USMT 0.090 109 0.036 mineable W.H. White, pers. comm., 1990 0.098 376 0.060 geologic W.H. White, pers. comm., 1990 5) Middle Mtn USCO ------Ranta, 1974 6) Mt Emmons USCO 0.264 141 0.120 mineable Ganster et al., 1981 7) Redwell Basin USCO 0.098 108 0.060 geologic Thomas and Galey, 1982 8) Mt Hope USNV 0.162 91 0.120 geologic W.H. White, writ. comm., 1990 0.100 510 0.035 geologic G. Westra, pers. comm., 1990 9) Mt. Pleasant CNNB 0.077 34 geologic Kooiman et al., 1986 10) Pine Grove USUT 0.170 125 0.120 geologic Sillitoe, 1980i D.E. Ranta, writ. comm., 1982 11) Questa USNM 0.144 277 0.120 mineable S.R. Wallace, pers. comm., 1990 12) Log Cabin USNM 0.090 45 0.060 geologic S.D. Olmore, writ. comm., 1979 13) Creek USCO 0.310 40 0.200 geologic Cameron et al., 1986

14) Bordvika NRWY ------Geyti and Schonwandt, 1979 15) Cave Peak USTX 0.146 0.040 26 0.100 mineable W.H. White, pers. comm., 1990 Sharp, 1979

16) Drammen NRWY ------Ihlen et al., 1982

18) Flammefjeld GRLD ------Geyti and Thomasson, 1984 17) Malmbjerg GRLD 0.138 136 0.100 geologic Geyti and Thomasson, 1984 19) Nordli NRWY 0.084 181 0.030 geologic Pedersen, 1986

20) Three Rivers USMN ------Giles and Thompson, 1972; Thompson, 1982 20a)Cone Peak USNM ______Mo_ Thompson, pers. comm., 1990

GRANITE-RELATED No: DIFFERENTIATED MONZOGRANITE 21) !danac CUBC 0.094 94 0.060 geologic W.H. White, pers. comm., 1990 0.053 270 0.030 geologic Kirkham et al., 1982 22) Anduramba AUQL 0.070 16 geologic witcher, 1975 23) Bald Butte USMT 0.100 14 geologic S.R. Wallace, pers. comm., 1990 24) Bell Moly CNBC 0.066 32 mineable Woodcock and Carter, 1976 25) Bjorntjarn SWDN 0.175 1 geologic Ohlander, 1985 26) Boss Mtn CNBC 0.074 63 geologic Kirkham et al., 1982 27) Canicanian PLPN 0.051 15 geOlogic Knittel and Burton, 1985 28) Cannivan USMT 0.096 185 geologic Worthington, 1977 29) Carmi CNBC 0.091 34 geologic Kirkham et al., 1982 30) Compaccha PERU 0.072 100 geologic Heintze, 1985; Hollister, 1978b SEG SP-2 GIANT ORE DEPOSITS 287

31) East Kounrad URKZ 0.150 30 ? Sutulov, 1978 32 ) Endako-Denak CNBC 0.087 336 0.048 mineable Kirkham et al., 1982 33) Kitsault CNBC 0.115 108 0.060 geologic W.H. White, pers. comm., 1990 34 ) Lacorne CNQU 0.210 4 mineable Kirkham et al., 1982 35 ) Logtung CNYT 0.031 162 geologic Noble et al., 1984, 1987 36) Lucky Ship CNBC 0.090 14 geologic Pilcher and McDougall, 1976 37) Mackatica YUGO 0.078 181 geologic H.T. Shassberger writ. comm., 1961 38) Mt. Haskin CNBC 0.090 12 geologic Kirkham et al., 1982 39) Munka SWDN 0.125 2 geologic Ohlander, 1985 40 ) Pidgeon Mo CNON 0.080 14 geologic Kirkham et al., 1982 41) Preissac CNQU 0.199 3 geologic Kirkham et al., 1982 42) Pine Nut USNV 0.060 181 0.030 geologic K. Roxlo & D.E. Ranta writ. comm., 1982

0.084 54 0.060 geologic K •. Roxlo & D.E. Ranta, writ. comm., 1982 43) Hill USAK 0.077 1216 0.027 geologic S.R. Wallace, pers. comm., 1990 0.091 793 0.060 mineable S.R. Wallace, pers. comm., 1990 44) Red Bird CNBC 0.108 34 0.060 geologic. Kirkham et. al., 1982 45) Red Mountain CNYT 0.100 187 0.060 geologic Brown and Kahlert, 1986

46 ) Roundy Creek CNBC 0.208 1 geologic Kirkham et al., 1982 47) storie Moly CNBC 0.078 101 0.040 geologic Bloomer, 1981 48) Thompson Creek USID 0.110 181 0.050 geologic Schmidt et al., 1982

49) Trout Lake CNBC 0.138 50 0.060 geologic Boyle and Leitch, 1983 50 ) Tyrnyauz URRS 0.065 50 ? Sutulov, 1978 51} Yorke-Hardy CNBC 0.240 21 0.120 geologic W.H. White, pers. comm., 1990 0.151 125 0.060 geologic W.H. m1ite, pers. comm., 1990

GRANITE-RELATED Mo-CU (No> 0.05 %) 52) Buckingham USNV 0.058 0.034 1297 0.036 geologic W.H. White, pers. comm., 1990 0.074 0.040 503 0.060 geologic W.H. White, pers. comm., 1990 53) Copaquire CILE 0.070 0.300 50 geologic Ambrus, 1978

54) Cumo USID 0.059 0.074 1258 0.030 geologic W.H. White, pers. comm., 1990 0.086 0.058 403 0.060 geologic W.H. White, pers. comm., 1990 55 ) Cumobabi MXCO 0.099 0.266 67 0.060 geologic W.R. White, pers. comm., 1990 56) EI Creston MXCO 0.074 0.060 181 0.030 geologic W.H. ��ite, pers. comm., 1990 0.092 0.071 126 0.060 mineable W.H. White, pers. comm., 1990 57 ) Hall USNV 0.091 0.045 181 mineable Shaver, 1986 58) Jin Dui Cheng CINA 0.100 0.030 907 geologic S.R. Wallace, pers. comm., 1990

59) Mocoa CLBA 0.062 0.400 260 geologic sillitoe et al., 1984 60) Mt Tolman USWA 0.056 0.090 793 0.036 mineable W.C. Utterback, pers. comm., 1982 0.054 0.090 2177 0.027 geologic W.C. Utterback, pers. comm., 1982 61) Rialto USNM 0.070 0.100 27 geologic Hollister, 1978b; Thompson, 1968 l Abbreviations as used in USGS Bulletin 1693 tv 00 00 <>- .v>- �? �D

(j,.27 10 /'0 ,-�:��= ���'..� ����t �.. • , . 1 3. 5 57 .. JO- i ·-�M··1l�' \? d� CI.l .2� ····· gs . CI.l AZ 15 � @ :> Western United States z Figure 1. Location map for granite-related Mo and Mo-Cu deposits. Deposit numbers � correspond to listing in Table 1. Spatially and temporally related deposits are combined as

follows: 2 = Henderson and Urad; 6 = Mount Emmons and Redwell Basin; 11 = Questa � (Sulphur Gulch, Goat Hill) and Log Cabin; 20 = Three Rivers, Cone Peak, and Rialto; 33 = Kitsault, Bell Moly, and Roundy Creek. State codes for the inset of the western United States � are AZ = Arizona, CO = Colorado, ID = Idaho, MT = Montana, NV = Nevada, NM = New CI.l - Mexico, TX = Texas, UT = Utah, and WA = Washington. cA SEG SP-2 GIANT ORE DEPOSITS 289

(Table 1; Carten et aI., in press). Porphyry porphyry deposits is derived "in toto" from Mo deposits are further subdivided into those the magma or the resulting stocks (Stein and associated with high-siliCa or alkaline rhyolites Hannah, 1985; Carten et aI., 1988a, 1988b; which are rich in potassium, fluorine, and White et al., 1981). This being the case, some incompatible trace elements and are factors that might produce a high-grade exemplified by the Climax deposit (albeit a deposit seem readily apparent. These include premier example), and those associated with the initial molybdenum content of the parent less silicic granitoids that exhibit average magma and the increase in molybdenum concentrations of fluorine and incompatible content with differentiation. The source region elements. for the magma and metal has been proposed Examination of the grade and tonnage as residing in the upper crust (Wallace et al., of 61 porphyry Mo and Mo-Cu deposits (Fig. 1978), lower crust (White et al., 1981; Stein 2) indicates thata continuum in deposit grades and Hannah, 1985), and mantle (Westra and and tonnages may exist; however, most Keith, 1981). Related factors that could deposits consist of 50 to 200 million tons of control the grade of the deposit include how ore, but only 12 deposits contain more than effectively molybdenum is sequestered by 200 million tons of ore. These 12 deposits can early magmatic (Keith and Shanks, be considered "giant" in terms of tonnage, but 1988) and how strongly it is partitioned into only two deposits - Climax and Henderson - an .evolved aqueous phase (Candela, 1989a; contain over 200 million tons of high-grade Keppler and Wyllie, 1991). Any discussion of ore (Fig. 1 and 2). Clearly, the .most what factors or processes are responsible for economically desirable molybdenum deposit variations in tonnage and grade must begin must be in this class and be "giant" in terms with the source and differentiation processes of both tonnage (-200 million tons) and grade of the magma. Processes that control the (> 0.15 % Mo). Processes and factors which location and duration of molybdenite contribute to the development of these types deposition from the aqueous phase also of deposits will be emphasized. obviously control the grade. These include The genesis of these deposits in light multiple stocks forming overlapping ore of more recent work on the partitioning bodies and steep chemical and thermal behavior . and magmatic abundances of gradients around the stock or pluton. molybdenum will also be reviewed. Inasmuch The tonnages of porphyry molybdenum as Climax-type magmas are often equated deposits are also significantly affected by the with topaz rhyolites, recent studies on the factors just mentioned. However, additional evolution of topaz rhyolites will be examined factors that are perhaps more directly linked along with the volcanic rocks that are with the tonnages are the size and water cogenetic with the Climax-type Pine Grove content of the parent magma chamber, and the and Questa deposits in southwestern Utah and size and shape of the ensuing productive stock northern New Mexico respectively. Two large or cupola. For example, if mineralization tonnage deposits that are less well occurs in a broad, shallow cupola along the characterized, but appear to not be Climax roof of a large batholith, then the total amount types are the Quartz Hill, Alaska deposit and of magmatic water and metals participating in Mt. Tolman, Washington deposit; processes mineralization may also be large, but that may have acted to produce the large unfocused. The ensuing deposit could be tonnages with moderate-grade will be large, but low grade. reviewed as well. Most of these mineralization controls Several studies in recent years have have been discussed or reviewed by other clearly demonstrated that the molybdenum in workers. However, the essential question that 290 SEG SP-2 GIANT ORE DEPOSITS

0.35

• 0.3 � "- • "- 0.25 0 Mt Emmons "- � --..Henderson ...... ::::::. Climax 'a. -< . '#. --.. 0.2 --...... � ...... Pine Grove a 0.15 s Big Ben - -- - X-x. __ �t,!oJ!n��� 0.05 X -- - -- 0 2177 tons o +-��4-���4-�� o 200 400 600 800 1000 1200 1400 1600 Ore (millions of metric tons)

Figure 2. Tonnage-grade diagram for granite-related Mo and Mo-Cu deposits. Deposits discussed in the text are labelled. Solid squared represent the high-silica rhyolite - alkaline suite; open circles are the differentiated monzogranite Mo suite; crosses are the Mo-Cu suite. All data are from Table 1 (Carten et al., in press).

remains unanswered is whether the processes Climax-type versus quartz monzonite type that produce giant porphyry molybdenum (White et al., 1981), granite versus deposits are inherently different from those granodiorite types (Mutschler et al., 1981), which produce normal deposits. Is there a calc-alkaline versus alkali-calcic (and alkalic) discontinuity in either sizes of parent magma types (Westra and Keith, 1981), and chambers or the mineralizing processes fluorine-enriched versus fluorine-deficient associated with them? We will discuss this types (Theodore and Menzie, 1984). One of question along with what are the best the two classes is generally typified by exploration characteristics to use in fmding a high-silica rhyolites or granites that are rich in giant deposit. potassium, fluorine, and incompatible trace elements. Characteristics of this class are CLASSIFICATION AND GENERAL exemplified by the Climax deposit. The other CHARACTERISTICS class is typified by a less silicic calc-alkaline granitoid that exhibits average concentrations Most classification schemes for of fluorine and incompatible elements. porphyry molybdenum deposits divide them Climax-type deposits are of primary concern into two main groups. These include in this paper because they exhibit both SEG SP-2 GIANT ORE DEPOSITS 291

high-grades of molybdenum and large the mantle abundance of molybdenum and tonnages (Sillitoe, 1980; White et aI., 1981). are 0.059 ppm and 0.010 ppm, However, both types will be examined in respectively (Newsom and Palme, 1984; order to understand the processes responsible Newsom et al., 1986) whereas upper crustal for formation of high-grade deposits. abundances are 1.5 ppm and 2.0 ppm, The Henderson porphyry molybdenum respectively (Taylor and McClennan, 1985). deposit is one of the premier examples of a These data suggest that tungsten is more giant molybdenum deposit in terms of both incompatible and therefore more enriched in tonnage and grade (727 millon tons of 0.17 wt the crust than is molybdenum (by a factor of % Mo; Table 1). In addition, the most recent 200 for W and only 25 for Mo). Newsom and studies (Carten et al., 1988a; Seedorff, 1988) Palme (1984) attribute the more strongly represent a culmination of 75 man years of incompatible behavior of tungsten to an geological investigation making it one of the oxidation state of 6 +, whereas the oxidation most well-studied porphyry deposits in the state of molybdenum varies from 6 + to 3 + world. under terrestrial conditions. Tungsten may exhibit an oxidation state of 4 + in some of MOLYBDENUM AND TUNGSTEN the most reduced granitoid magmas (Cygan GEOCHEMISTRY IN and Chou, 1987; Candela, 1988). Newsom IGNEOUS ROCKS and Palme (1984) suggest the molybdenum valences less than 5 + are perhaps more Inasmuch as molybdenum and tungsten common and evidence of a 4 + valence has occur in the same group of the periodic table been observed in metal-silicate partitioning and exhibit many similarities in geochemical experiments. A valence of 4 + for behavior, it is instructive to contrast and molybdenum might indicate that its compare the behavior of both elements to geochemical behavior would be similar to Ti understand the ore-forming process. For (similar charge and ionic radius). A example, both molybdenum and tungsten are correlation in the abundance of Mo and Ti has often sequestered by the same magmatic been obserVed by Kuroda and Sandell (1954) phases. Such a comparison is essential because for some igneous rocks. the partitioning behavior of 6 + molybdenum Tacker and Candela (1987) also has not been investigated experimentally; suggest that the dominant oxidation state for however, this is the valence that may be most molybdenum in silicate magmas varies from important in porphyry systems and is the 4 + to 3 + between -nickel and valence commonly exhibited by tungsten. -methane buffers. Molybdenum The concentration of both molybdenum becomes more incompatible in magnetite and and tungsten in the mantle is less than in C 1 as oxygen fugaCity is increased chondrites, indicating that a significant (Tacker and Candela, 1987; Bouton et al., fraction of Earth's original endowment of 1987). these metals now resides in the core (Newsom and Palme, 1984). Although molybdenum and Associated alkaline mafic rocks tungsten generally exhibit strongly incompatible behavior in crystallizing magmas The presence of lamprophyre dikes at due to formation of large oxy-anions, under Climax, Henderson, Quartz Hill, and Chicago sufficiently reducing conditions (Fe-FeO) Basin and of zoned magma chambers that are these metals may be strongly partitioned into floored with alkaline mafic rocks at Pine a liquid metal phase, as apparently happened Grove, Questa, and Grizzly Peak during core formation. The best estimates of (mid-Tertiary volcanic in central 292 SEG SP-2 GIANT ORE DEPOSITS

Colorado) indicate that mantle-derived mafic enough to account for the occurrence of alkaline magmas may represent a portion of porphyry molybdenum "provinces". the parental magma of Climax-type rhyolites. There is no isotopic constraint for eliminating Oxidation state and molybdenum incompat­ them as a portion of the source material. ibility in magmas Alkaline igneous rocks (minettes, lamproites, and nephelinites) from the Cenozoic volcanic Perhaps the geochemical characteristic fields in the Elkhead Mountains in Colorado, inherited from the source region that is most the Leucite Hills, Wyoming, and the western critical for the development of porphyry Mo Colorado Plateau, Utah, have epsilon Nd and magmas is the oxidation state of the magma. epsilon Sr values (Thompson et aI., 1991 ; This parameter is fundamental because a Vollmer et aI., 1984; Tingey et aI., 1991) that buffer parallel oxidation state is not easily are not far removed from those exhibited by modified during magmatic fractionation Climax-type systems. Despite the generally (Carmichael, 1991). This characteristic may low concentration of molybdenum in the affect the oxidation state of molybdenum mantle (0.059 ppm), mantle-derived alkaline which in tum affects how incompatible it may magmas formed by extremely small degrees behave during crystal fractionation. As of partial melting « 1 %) have substantially previously noted, molybdenum may exhibit higher concentrations of molybdenum that valences of 4+ and 3 + between nickel-nickel would not lower the average molybdenum oxide and graphite-methane oxygen buffers content of a largely crustal melt. For (Tacker and Candela, 1987). Most subduction example, Newsom and Palme (1984) noted related ( calc-alkaline) and lamprophyric that continental alkali have high magmas have oxidation states roughly 2-3 and contents of incompatible elements including 4-5 log units above nickel-nickel oxide molybdenum concentrations up to 4.5 ppm respectively. If the trend of increasing (well above the crustal average of 1.5 ppm) incompatibility of molybdenum with and Newsom et al. (1986) report as much as increasing f02 noted by Tacker and Candela 6.3 ppm Mo in alkaline ocean island . (1987) continues, then molybdenum may Shoshonites, minettes, and melanephelinites become even more incompatible in from central Utah contain 0.2 to 3.0 ppm Mo calc-alkaline and lamprophyric magmas with (unpublished data). high oxygen fugacities where molybdenum The source region for mafic alkaline valences greater than 4 + may exist. magmas on the continents is often proposed to Evidence of variable incompatibility be the metasomatized lithospheric mantle. for molybdenum in various rock series should Hattori et al. (1992) examined the metal be noted here. Rubidium is insensitive to content of magmatic from changes in the oxidation state of the magma metasomatized (amphibole-bearing) peridotite and can be used in variation diagrams as a xenoliths from a variety ofloealities; a monitor of magma evolution. A plot of the xenolith from Nunivak Island, Alaska abundance of tungsten versus rubidium in (underlain by continental crust) contained fresh volcanic rocks ranging in composition magmatic blebs with up to 60 ppm from mid-ocean ridge basalts to high-silica molybdenum, whereas sulfide blebs in rhyolites shows linear, strongly incompatible xenoliths from other localities contained no behavior over a range of almost four detectable molybdenum. Insufficient data are orders-of-magnitude (Fig. 3). However, the available to speculate on whether the enrichment of molybdenum in the same rock molybdenum content of lithospheric mantle of series is much more subdued increasing only different Precambrian provinces may vary two orders of magnitude; in addition, the SEG SP-2 GIANT ORE DEPOSITS 293 enrichment trend branches according to rock (1.5 ppm, Taylor and McLennan, 1985). type (Fig. 3). Ocean island basalts and More importantly, because the enrichment alkaline rocks exhibit a stronger enrichment trend of molybdenum in metaluminous rocks trend than calc-alkaline (metaluminous) is relatively flat through the granitoid region volcanic rocks; very reduced strongly (Fig. 3), an approximate magmatic fractionated rocks such as macusanites, molybdenum concentration (between 1 and 5 ongonites, and some topaz rhyolites, such as ppm with an uncertainty of only a few ppm) Spor Mountain, even show sharply lower can be inferred from the granitoid's rubidium abundances of molybdenum with increased concentration. Rhyolites or granites with fractionation, despite strong enrichment of rubidium concentrationsof over 1000 ppm are tungsten (up to 30-70 ppm; E. H. rare and commonly are reduced types that Christiansen, unpublished data; Pichavant et display molybdenum depletion with aI., 1987). The differences in the behavior of fractionation.Even if highly evolved oxidized molybdenum may be the result of variable granitic magmas existed with concentrations oxidation states with molybdenum in a lower of Rb as high· as 2000 ppm, magmatic Mo oxidation state (3 + (?» behaving more concentrations predicted by this trend would compatibly than at higher oxidation states still be less than 6 or 7 ppm. Moreover, a (6 +, similar to tungsten?) where molybdenum survey of the molybdenum contents of appears to be moderately incompatible. subalkaline volcanic rocks worldwide indicates The oxidation states of most porphyry that concentrations in excess of 10 ppm have molybdenum systems are not well constrained. not been found (Smith, 1985). This does not Keith (1982) demonstrated that the oxygen include potentially large enrichments of fugacity of the Pine Grove magma was about molybdenum that are inferred to occur at the 2 log units above nickel-nickel oxide (similar apices of Climax-typeintrusions (13,000ppm, to most calc-alkaline rocks). Christiansen et Carten et al., " 1988a) due to accumulation of al. (1986) found that the magmatic oxygen a separate aqueous phase (discussed later), but fugacity of the topaz-bearing Chalk Mountain only to molybdenum enrichment due to crystal rhyolite (comagmatic with Climax) was also fractionation. about 2-3 log units above nickel-nickel oxide. However, some topaz rhyolites from Molybdenum and niobium west-central Utah (including Spor MountitiiJ.) were found to be substantially more reduced, Another explanation of the moderately with magmatic f02 values 2 units below incompatible behavior of molybdenum is that nickel-nickel oxide. Therefore, this is its magmatic fractionation is similar to permissive evidence that Climax-typemagmas niobium. Molybdenum and niobium may would exhibit modest' enrichment of commonly exist as cations of approximately molybdenum with continued fractionation, similar size and charge in magmas. Ocean probably along the trend of other island basalts and continentalalkaline magmas metaluminous (apparently more oxidized) (excluding minettes) exhibit higher magmas (Fig. 3), rather than depletion as concentrations of niobium than do found in more reduced magmas. calc-alkaline magmas at the same rubidium From the incompatible trace element concentrations (Fig. 3). In addition, niobium systematics, we conclude that magmatic concentrations probably decrease with concentrations of Mo in metaluminous, continued evolution in macusanite, ongonites, calc-alkaline magmas are generally quite low. and the most evolved topaz rhyolites This is indicated by the average molybdenum (Christiansen et al., 1986; Pichavant et al., content of continental crust and granitoids 1987; Congdon and Nash, 1991) as does 294 SEG SP-2 GIANT ORE DEPOSITS

100 A

10

- 1

0.E 0. - s: 0.1

0.01

0.001 B

10 High f0 trend : 2 I 1 ADR Lowf0 - 2 trend 0.E 0. - 0.1 0 � 0.01

Niobate - Fractionation 0.E 0. - .c Z 1

. L...... ---'-I-...... ul....-.l.-,-,-...... J.--'-....&-L.J. � 0 1 .LULL.--..J-..&.-.J,...I..Ju..&.L1---'--...... u.J �""::"...... 0.01 0.1 1 1 0 100 1 000 "'"':1 OOOO Rb (ppm) SEG SP-2 GIANT ORE DEPOSITS 295

Figure 3. Concentrations of Rb versus W, Mo, and Nb. A) Concentrations of Rb and W in fresh terrestrial volcanic rocks are strongly correlated over almost 5 orders of magnitude showing that W generally behaves as a strongly incompatible element. Mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) form distinct but nearly continuous fields (stippled). The field for , dacites, and rhyolites (AD R) extends from but overlaps the upper end of the field for ocean island basalts (OIB). For rhyolites, only data collected from glassy samples has been included. B) Concentrations of Rb and Mo in most fresh terrestrial volcanic rocks are strongly correlated showing that Mo generally behaves as a moderately incompatible element. Mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) form distinct but nearly continuous fields. At a given Rb content, the Mo enrichment in andesites, dacites, and rhyolites (AD R) from subduction zones or continental settings is less than in (OIB). Highly evolved rhyolites that crystallized under reducing conditions (macusanites and some topaz rhyolites; stippled pattern, "low f02 trend") have even lower Mo/ Rb ratios, apparently as a result of magmatic depletion of molybdenum. For rhyolites, only data collected from glassy samples has been included. C) Concentrations of Rb and Nb in fresh terrestrial volcanic rocks reveal a pattern similar to that for Rb and Mo in that andesites, dacites, and rhyolites (AD R) from continental and subduction zone settings are depleted in Nb compared to ocean island basalts (OIB) and even some mid-ocean ridge basalts (MORB). Although apparently more incompatible than Mo in most magmas, Nb behaves like Mo in that low Nb/ Rb ratios are found in highly evolved rhyolitic rocks indicated by magmatic depletion of Nb in these rocks. Data from Newsom et al. (1986), Govindaraju (1989), Christiansen et al. (1986), Kovalenko et a1. (1978, 1981), Pichavant et al. (1987), Congdon and Nash (1991), Turley and Nash (1983), Mahood (1981), Hildreth (1981) and our own unpublished analyses. molybdenum. For example, the strongly Preservation of magmatic molybdenum and reduced macusanite glasses also show a tungsten concentrations decoupling of the behavior of niobium and resulting in a very low Nb/ Ta ratio There are several reasons to believe of 1.7 (Pichavant et al., 1987). Whether the the tungsten and molybdenum concentrations behavior of niobium can be ascribed to a of many silicic igneous rocks often do not change in oxidation state from 5 + to 4 +in represent magmatic values. Perhaps most reduced magmas is unknown. But whatever importantly, both elements are substantially magmatic phases control the abundance of partitioned into an aqueous phase when one is niobium may also control that of evolved from a crystallizing granitic magma molybdenum.. (Candela, 1989a). If an aqueous phase escapes In summary, ocean island basalts and from a pluton, then the tungsten and continental alkali basalts may inherit modestly molybdenum content of a granitoid will be high molybdenum and niobium concentrations reduced whereas country rock might become from their sources. However, the high oxygen enriched by deposition of these metals from fugacitiesinherited fromthe source regions of that fluid. The very process that makes minettes and calc-alkaline magmas may be of mineralization possible, makes determination fundamental importance in allowing of original magmatic values almost molybdenum concentrationsto increase during unattainable. In this regard, Ivanova (1963) crystal fractionation of magmas related to studied the distribution of tungsten in biotite porphyry molybdenum deposits. granites and correlative greisens and concluded that tungsten content of the rocks is 296 SEG SP-2 GIANT ORE DEPOSITS

100 �------. A 10 •

- 1 c..[ - .;JI'. Granites

0.1 • • • • 0.01 •

o .00 11'----'---'---'--'-'-J...J..I..I---'---'--'-",-,-,-,-,-'----'---'---'---'-'-..L..U...l.----'---'--'--'--'-'-'-'-'----'---'-'-'--w....u.J...---'-----'---'--'--'-'-'-'-' 0.01 0.1 1 10 100 1000 10000 Rb (ppm) 100 �------���------�

B 10

1 • - • •• ••• E c.. '... ::a\" c.. - 0.1 • o .r •. Granites � •• • 0.01

o. 00 1 '----'---'---'--'-'--W-W-_--'---'--'--'--L.J..L'-'----'---'----'-'-'-'-'-'-'-----'-----'--'---'-'---'--��-'----'-'---'--'--'-'-''----"'--'--'-'--'-� 0.01 0.1 1 100 1000 10000 100 c 10 -

c..E -c.. 1 o � 0.1 F?ine Grove 0.01

0.00 1 L--�_'__'_'_�____'___'_'_...... �� ...L.- ...... L�---'--"'--'--'-'-.u.l-__'__-'---'-'...... L---- -'---'----'-'-'--'-'-'-' 0.01 0.1 1 10 100 1000 10000 Rb (ppm) SEG SP-2 GIANT ORE DEPOSITS 297

Figure 4. Concentrations of Rb versus Wand Mo for volcanic and granitoid rocks. A) Concentrations of W in fresh granitoids from western North America are generally depleted compared to fresh terrestrial rocks of the same Rb content, including the andesites, dacites, and rhyolites (ADR) commonly associated with tungsten deposits. B) Concentrations of Mo in fresh granitoids from western North America are generally depleted compared to fresh terrestrial rocks with similar Rb concentrations, including the andesites, dacites, and rhyolites (ADR) commonly associated with porphyry molybdenum deposits. C) Judging from the correlation of Rb and Mo in glassy rhyolitic rocks, the Mo concentrations in the rhyolitic magma at Pine Grove may have been as high as 3 ppm. However, concentrations of Mo in the volcanic rocks range to as low as 0.2 ppm as a result of post-magmatic losses including loss of a volatilephase during eruption and meteoric leaching (Keith and Shanks, 1988). Includes unpublished data of J. D. Keith. primarily a function of alteration. Keith et al. addition, they inferred that very fresh (1989) made the same conclusion from a comagmatic rhyolite cobbles eroded from an study of the CanTung granitoids, Northwest extrusive dome had lost -70 % of their Territories. Figure 4 illustrates that the original molybdenum content due to escape of tungsten content of. granitoids from· western magmatic volatiles (Fig. 4). Although North America (Keith, unpublished data) measured molybdenum concentrationsin fresh scatter above and mostly below the trend Pine Grove rocks range from 0.20 to 1.82 established for fresh andesites, dacites, and ppm, the pre-eruption magmatic level may rhyolites. Keith et al. (1989) also examined have been as high as -3 ppm (Fig. 4); the the molybdenum content of barren and difference may have been lost to meteoric tungsten-relatedgranitoids fromwestern North leaching or degassing of the ash flow America. No correlation of molybdenum. shortly after eruption. Consequently, it is content with degree of differentiation, size of difficult to determine the magmatic tungsten associated deposit, or other trace element and molybdenum concentrations of evolved concentrations was found. As was the case for water-rich granites and rhyolites. tungsten, the molybdenum concentrations in Molybdenum has.· been found in· fumarolic these granitoids generally are substantiallyless crusts associated with similar rhyolitic tuffs than those of fresh andesites, dacites, and (Zies, 1929). Even strongly differentiated rhyolites (Fig. 4). rhyolitic magmas such as those that formed Molybdenum concentrations of rocks the molybdenite deposits at Climax and from a porphyry molybdenum deposit would Henderson, may originally have contained always be suspect of being either enriched only about 4 ppm molybdenum; magmas with (especially within 300 m of· an ore zone, lower rubidium and niobium - such as those Mutschler et aI., 1981) or depleted by from Quartz Hill and Mount Tolman - may hydrothermal leaching or loss of a have originally had . molybdenum molybdenum-rich aqueous phase. Even fresh concentrations closer to· -2 ppm. Actual volcanic rocks may exhibit molybdenum magmatic concentrations of molybdenum for concentrations lower than pre-eruption these deposits may be unattainable for the magmatic values due to loss of a reasons just outlined. If such molybdenum­ molybdenum-bearing aqueous· phase or to niobium-rubidium correlations have some leaching by meteoric fluids. Keith and Shanks validity, then themore niobium-rubidium-rich (1988) documented that at least 90% of the magmas might be expected to produce larger molybdenum in the non-welded tuff of Pine tonnages of higher grade molybdenum ore. Grove was leached by meteoric water; in Figure 5 illustrates that deposits with higher N 1000 1.0 L Mo (wt%) Tonnes 00 Millions <> Henderson 0.294 266 500 � D PillleGrove 0.170 125

• Questa 0.144 277 300 I o Quartz Hill 0.091 793 • MtTolman 0.056 793 200

...... E 100 0. 0. '-"" -C Z 50 en tI1 0 30 f-- en �I N 20 f--Af:mm::H� -..Q- 63 :> z � � I 41t:,:�"'f:,:,:,:,',:,m:,:tj't:,t,,:,:,:,,,:,w:,:,:,t,l"'F:t:':,}ft:,',q,J,:,:,:¥"t};}:t",:,:,"t,:",tt:,V I I 10 t:::1 10 20 30 50 100 200 300 500 1000 � 0 en.... (ppm Jooo3 Rb ) en SEG SP-2 GIANT ORE DEPOSITS 299

Figure 5. Rb and Nb concentrations in silicic igneous rocks associated with porphyry molybdenum deposits in western North America are similar to other members of the , dacite, rhyolite group world wide. Granitic rocks at Urad/Henderson are the most fractionated (indicated by their high concentrations of the incompatible elements Rb and Nb) and host the highest grade Mo deposit. Rb, Nb, and Mo ore grade decrease regularly in the sequence Pine Grove, Questa, Quartz Hill, and Mt. Tolman.

niobium and rubidium concentrations do have and rubidium concentrations similar to those larger tonnages of high grade ore. at Henderson contained 3.3 (+/- 0.2) ppm molybdenum. Turley and Nash (1983) found PETROGENESIS OF MAGMA from 2 to 6 (+ /- 1.5) ppm molybdenum in RELATED TO CLIMAX-TYPE vitrophyres from topaz rhyolite lavas at MOLYBDENUM DEPOSITS Smelter Knoll, Utah. There is no evidence that topaz rhyolites generally contain higher Lower and upper crustal sources concentrations of molybdenum than these values. Do the igneous rocks associated with Stein (1988) investigated the , giant porphyry molybdenum deposits inherit a strontium, , and oxygen isotope ratios sp ecial composition from their magma source and rare earth-element compositions of ores, region that distinguishes them from lesser and of volcanic and plutonic rocks from the deposits? Is there a unique tectonic setting or Colorado belt; the genesis of for their development? In this regard, Climax-type systems was of particular White et al. (1981) proposed that because the concern. Her data were interpreted to indicate Colorado Rocky Mountain region exhibits that all the Laramide-Tertiary intermediate to many deposits or mineral occurrences felsic magmas (including calc-alkaline) had a enriched in molybdenum, tungsten, and lower crustal origin; however, Climax-type fluorine, this indicates a "long-term source" granites were proposed to have originated for these elements in the underlying from a unique lower crustal source (not continental lithosphere (1.7 Ga). They related to calc-alkaline magmatism). The suggested that Climax-type magmas were magmatic characteristics critical to generated by fractional melting of the lower Climax-type mineralization acquired fromthis crust followed by fractional crystallization of unique source are not specified. each batch of magma as it diapirically rose In their review of the Henderson throughthe crust. White et al. (1981) suggest deposit, Carten et aI. (1988b) suggest that a the magmas would be enriched in unique magma that contained high molybdenum and other incompatible trace concentrations of ore components was not elements as a consequence. What levels of required to form the deposit. They argue that molybdenum enrichment would be achieved the initial concentration of molybdenum in the by this process are not specified, but they ore-related magma is not as critical to ore suggest that Climax-typegranitoids are "rather formation as the type and duration of like" the topaz rhyolites of Burt et al. (1982). fractionation processes in a high-level magma Extensive data on the molybdenum content of chamber. topaz rhyolites are not available, but Keith The average molybdenum content of and Shanks (1988) found that fresh topaz the lower and upper crust is estimated to be rhyolite fromsouthwestern Utah with niobium 0.8 and 1.5 ppm respectively (Taylor and 300 SEG SP-2 GIANT ORE DEPOSITS

McLennan, 1985). One of the ftrst attempts to the compositions of 3 samples from defme a special molybdenum-rich source for Henderson. Epsilon Nd for these samples Climax-type deposits was analysis of the ranged from -9.5 to -9.9. These values are molybdenum content of Idaho Springs indistinguishable from the high end epsilon Formation and the Silver Plume Granite; Nd of Precambrian (1.6 to 1.8 Ga) basement Zahoney (1968) found an average of 5 ppm rocks, suggesting to Farmer and DePaolo and 2-3 ppm Mo in these respective rock (1984) that the "granitic magmas were derived types. On this basis, Wallace et a1. (1978) exclusively from mid-Proterozoic crust". suggested that the tungsten and molybdenum However, it should be noted that DePaolo of the Climax and Henderson deposits were (1981) observed that Proterozoic crust in derived from these upper crustal host rocks. Colorado could have had epsilon Nd as low as However, later isotopic studies (White et aI., -14 during the middle Tertiary. If such crust 1981; Stein, 1988) have demonstrated that was involved in the genesis of Climax-type Climax-type magmas and ore deposits in the rhyolites then a signiftcant mantle component Colorado mineral belt could not have with higher epsilon Nd must have been incorporated any appreciable amount of these present even in this magma system. upper crustal rocks. Moreover, the occurrence of igneous rocks Most recent workers have proposed with epsilon Nd values as low as -13 (Johnson that the magma source of Climax-type systems and Fridrich, 1990) in the same region and and topaz rhyolites is in the lower crust broadly coeval with Henderson is consistent (White et aI., 1981; Christiansen and Wilson, with the involvement of signiftcant 1982; Christiansen et aI., 1988; Stein, 1988) mantle-derived magma at Henderson. As although enriched upper mantle has been noted below, Johnson et a1. (1989) also proposed also (Westra and Keith, 1981). propose a similar mixed (mantle and crustal) White et al. (1981) noted that the initial heritage for the magmas that led to the 87Sr/86Sr values for "unaltered" Climax and formation of the Questa deposit. Henderson rocks varies between 0.705 - 0.710. Stein (1988) found signiftcantly higher Tectonic setting and widely varying values of 0.7085 to 0.740 for Climax and Henderson rocks. Chalk Sillitoe (1980) subdivided porphyry Mountain rhyolite is a relatively unaltered molybdenum deposits into rift-related and topaz-bearing extrusive dome that is subduction-related deposits that correspond comagmatic with the Climax rhyolites (Burtet fairly well to Climax-type and the "other" aI., 1982). Stein (1988) reported a 87Sr/86Sr categories respectively. Several authors have value of 0.7085 for Chalk Mountain and recognized that few Mo deposits occur in - 0.710 - 0.711 for Climax samples. Fused young island arc settings (i.e. Canicanian, ash-flow tuffassociated with the Climax-type Table 1), and almost always in areas underlain Pine Grove deposit in Utah, which is by at least thin continental crust (Sillitoe, underlain by Proterozoic crust of the same age 1980; Westra and Keith, 1981; Christiansen as Climax, has an initial 87Sr/86Sr value of and Wilson, 1982). Although some have 0.7099 (Christiansen and Wilson, 1982). claimed that Climax -type systems correlate Although sparse, Nd isotopic data may with thicker continental crust, others suggest reveal more about the source(s) of Climax that they are independent of crustal thickness type rhyolites, because they are probably less (Westra and Keith, 1981). Perhaps the most affected by hydrothermal alteration and commonly suggested tectonic setting for wall-rock contamination than Sr isotope Climax-type systems is during the changeover ratios. Farmer and DePaolo (1984) reported from compressional to extensional tectonics SEG SP-2 GIANT ORE DEPOSITS 301

(Sillitoe, 1980; Westra and Keith, 1981; field at 18-23 Ma and are roughly Mutschler et aI., 1981; White et aI., 1981; contemporaneous with the inception of Bookstrom, 1981). Some authors prefer to extension in the area (Best et aI., 1987). The consider this setting as "atectonic" or back-arc Pine Grove porphyry molybdenum deposit spreading rather than simply rift-related. occurs in the eroded vent of an ash-flow/ dome The "changeover" tectonic setting has complex just east of the rim of the Indian also recently been documented for the Coulee Peak complex. Initial eruptions began Dam intrusive suite (Carlson and Moye, 1990) with high-silica rhyolite and later eruptions that hosts the Cu-rich Mt. Tolman deposit simultaneously vented rhyolitic and dacitic whose ore-related granite porphyry clearly magma. High-silica rhyolite subsequently evolved from a dominant volume of filled the vent to form domes and intruded to granodiorite (Carlson and Moye, 1990). shallow levels to form a Climax -type deposit. Additionally, a "post-orogenic" , The co-erupted dacitic magma exhibits a mode extension-related setting has been proposed for and major-element composition very similar to the Quartz Hill deposit (Hudson et al., 1981). the voluminous dacitic ash-flows that were The Quartz Hill intrusive complex was emplaced during the and to other emplaced in the Coast batholith after it had intermediate volcanic rocks of the same age been extensively uplifted and eroded. A (23-21 Ma) to the west (Best et ai., 1992). regional extension-related joint system Shortly after complete crystallization of the controls emplacement of many lamprophyre magma chamber, trachyandesitic magma dikes as well the Quartz Hill intrusions. intruded the vent and other portions of the However, the initial 87Sr/86Sr values of 0.7051 crust and covered the tuff of Pine Grove for the Quartz Hill intrusions do not differ across the region. Keith (1982) demonstrated significantly from those of the older that 60% fractionation of dacitic magma, subduction-related plutonic rocks. using observed phenocryst proportions and Additionally, the Quartz Hill intrusions show compositions, could produce the major­ minimal enrichment of K20, fluorine, and element composition of the Pine Grove other incompatible elements (Hudson et al., rhyolite. The available Sr isotopic data are 1981). consistent with this idea; no significant differences exist between the Sr-isotopic Crystal fractionation from calc-alkaline compositions of the Pine Grove magma and predecessors the older dacites of the Indian Peak volcanic field. Despite accumulation of a large Johnson and Lipman (1988) amount of data on the petrogenetic and investigated the origin of the metaluminous isotopic evolution of Climax-type rhyolites, a and alkaline volcanic rocks of the Latir number of differing source rocks are volcanic field, host of the Questa deposit (Fig. permissible. Questa and Pine Grove show 1), in the Rio Grande rift near the many petrogenetic similarities. Pine Grove Colorado-New Mexico border. Magmatism in (23-22 Ma; Fig. 1) is located in the Indian the Latir field (22 - 28.5 Ma and 11-15 Ma) Peak volcanic field that produced dominantly is approximately coeval with Climax-type high-K calc-alkaline dacitic ash flows from 32 systems (i.e. Climax, Henderson, Mt. to 27 Ma (Best et al., 1989). More than Emmons) farther north along the Rio Grande 50,000 km3 of ash flow tuff was erupted. rift. In addition, the age of magmatism spans Smaller volumes of high-K calc-alkaline the inception of rifting in the area (26 Ma). rhyolite, trachyandesite, and minor dacite Molybdenum mineralization is associated with were erupted near the Indian Peak volcanic waning stages of caldera magmatism. The 302 SEG SP-2 GIANT ORE DEPOSITS highest grades of ore are associated with the rhyolites are exposed. The published analyses Sulphur Gulch pluton (23-25 Ma) that carry all the earmarks of strongly fractionated intruded the southern rim of the Que�ta magma: low concentrations of compatible caldera (Johnson et al ., 1989). Johnson and elements (notably Sr, Mg, and Ti) and high Lipman (1988) suggested that 60% concentrations of incompatible elements like fractionation of a parental magma similar to Rb and Nb. Thus the nature of their more the Cabresto Lake monzogranite, a slightly mafic predecessors is difficult to deduce. older resurgent pluton, could produce the Isotopic, major- and trace-element major-and trace-element composition of the compositions of the rhyolitic rocks present no Sulphur Gulch pluton. They suggest' that a reason to exclude the possibility that they "base level" intermediate composition magma, evolved by crystal fractionation from high-K exemplified by the maj ority of the local calc-alkaline magma (which itself was volcanic and plutonic rocks, persisted in the partially derived from lower crustal and area for 6 m.y. The inferred monzogranite mantle sources) . parent is very similar to the dacite co-erupted One of the reasons that White et al. with the rhyolitic Pine Grove magma and the (1981) exclude magma evolution from "base level" composition of the Indian Peak calc-alkaline parents is that one of the volcanic field (Best et aI. , 1989). Climax-type deposits, Mt. Emmons, is much A critical question that should be asked younger (17 Ma) than the cessation of is whether the petrogenetic processes apparently subduction-related magmatism( � 26 exemplified by Pine Grove and Questa Ma). However, non-subduction-related magmatism operated at other Climax-type volcanic rocks with calc-alkaline porphyry Mo systems. Bookstrom (1981) characteristics are present elsewhere in the states that the Climax-type deposits in the Tertiary of the western United States. For Colorado mineral belt occur along the axes of example, a 16.5 Ma volcanic center in the slightly older calc-alkaline plutons (mostly Gold Springs and Stateline districts of Utah less than 40 -Ma) . There are about 30 and Nevada exhibits a complete spectrum of molybdenum prospects with up to �0.1 % compositions ranging from 68 to 77 % Si02• MoS2 mineralization in, the mineral belt and The lower-silica rocks have the calc-alkaline most of these are associated with broadly characteristics of steep REE patterns, calc-alkaline igneous rocks and appear to be abundant magnetite, and nonevolved trace subduction related. He notes that the element abundances. The culminating events "Climax-like" rhyolite porphyries of the of tl!e volcanic center were eruption of a Winfield prospect (35-38 Ma) formed within high-silica ash-flow tuff and emplacement of a granodioritic pluton well before initial a dome of topaz rhyolite in the vent. The rift-related basaltic volcanism C24-26 Ma) . latter hosts a gold-bearing pipe and These data would suggest that Climax-type quartz-sericite- and argillic alteration; rhyolites and calc-alkaline intermediate precious-metal veins occur adjacent to the plutons may be comagmatic. A similar vent. Both older (18-21 Ma) and younger temporal association of broadly calc-alkaline (10-12 Ma) topaz rhyolites occur nearby (Best magmas (50 to 45 Ma) , topaz-bearing granites et al., 1992; Best et aI ., 1987). and rhyolites (48 Ma) , and a Climax-type Mo Although isotopic signatures of deposit at Big Ben (48 .5 Ma), Montana, has Climax-type rhyolites are marginally been noted by Christiansen et al. (1986) . perrmsslve of purely crustal sources, The petrogenesis of the igneous rocks well-studied rhyolitic ash-flow systems that at Climax and Henderson is difficult to erupted a spectrum of cogenetic magma interpret when generally only high-silica compositions suggest that other interpretations SEG SP-2 GIANT ORE DEPOSITS 303 are also possible. For example, the 34 Ma compOSItIon volcanic rocks have relatively Grizzly Peak Tuff, located approximately 80 uniform initial 87Sr/86Sr ratios of approximately km southwest of the Climax deposit, varies in 0.7055 (identical to the intermediate lavas of composition from high-silica rhyolite at the the San Juan volcanic field; Lipman et al., base to low-silica rhyolite at the top of a 1978; Johnson et aI., 1990). Initial 87Sr/86Sr single eroded cooling unit; two heterogeneous ratio in the Amalia Tuff increases tuff layers also contain dacite to mafic latite monotonically with decreasing Sr content fiamme (Johnson and Fridrich, 1990). Initial from 0.7057 to 0.7098, the Sr-poor tuff 87Sr/86Sr ratios range from 0.7170 to as low as representing the early eruptions. Late-stage 0.7099 and epsilon Nd values vary between roofward assimilation of Proterozoic rocks is -13.0 and -11.3; similar variations are proposed as the preferred explanation of the exhibited by Pb and 0 isotopes. Johnson and increasing initial 87Sr/86Sr ratios; convincing Fridrich (1990) point out that the most mafic evidence of this model is the roofward composition (57 wt% Si�) is too mafic to be increase in the proportion of zircons with a partial melt of crustal rocks; this magmatic Proterozoic cores relative to Tertiary zircons composition is best explained by with no Proterozoic cores. The approximately 50 wt% crystal fractionation of Mo-mineralized Sulphur Gulch pluton exhibits basaltic magma, accompanied by 20 to 40 an analogous trend of decreasing initial wt% assimilation of Proterozoic crust. The 87Srf86Sr ratios (from 0.7085 to 0.7067) with more silicic portions of the magma were increasing Sr content. The inferred parent modeled to have fo rmed by crystal (Cabresto Lake pluton) has an initial 87Sr/86Sr fractionation accompanied by late-stage crustal ratio of 0.7053 (Johnson et aI., 1990) . assimilation. Evidence for late-stage Late-stage roofward contamination may also assimilation included isotopic disequilibrium cause some of the large variations in initial between whole-rock fiamme and early- and 87Sr/86Sr noted in the Climax, Henderson, and late-crystallizing phenocrysts separated from Mt. Emmons rhyolites (Stein, 1988). Stein the same fiamme. They note that without the and Crock (1990) reject that possibility fortuitous eruption of the more mafic portions because the inferred proportions of wallrock of the magma chamber, they would have needed to produce the variations are drawn the erroneous conclusion that the tuff substantially different for the Nd, Sr, and Pb was generated by 100% crustal melting. The isotopic constraints . However, Johnson et al. high-silica portion of the Grizzly Peak Tuff (1990) encountered very similar isotopic would fit all of the isotopic criteria for inconsistencies in modeling the late-stage derivation from a "Climax-type source" assimilation of the Sulphur Gulch (Questa) (Stein, 1988; Stein and Crock, 1990), in spite pluton. of its inferred mantle heritage . Not all high-level magma chambers or post-caldera plutons show evidence of Isotopically zoned magma chambers late-stage assimilation. If late-stage roofward assimilation is a common process in Plutonic and volcanic rocks related to Climax-type magma chambers, then what the Questa caldera and porphyry molybdenum process is responsible for causing deposit also show evidence of derivation from assimilation? Ultimately, the answer is related chemically and isotopically zoned magma to heat, whether by crystallization or by chambers. The Questa caldera formed upon underplating or injection of new magma. The eruption of > 500 km3 of the high-silica fact that Climax-type magmas may show peralkaline Amalia Tuff (Johnson and consistent isotopic evidence of late-stage Lipman, 1988). Precaldera intermediate assimilation of roof rocks should not be 304 SEG SP-2 GIANT ORE DEPOSITS surprising inasmuch as one of the most widely Climax and Henderson could not have been a recognized characteristics of this deposit type very significant magma modifying process. is the occurrence of multiple intrusions and However, even a trivial amount of ore bodies that are generally emplaced by assimilation (0-2% of Silver Plume Granite, assimilation and stoping. Bookstrom et aI., 1988) may explain the Chondrite-normalized rare-earth monotonic increase in initial 87Sr/86Sr values element patterns from Climax, Henderson, between comagmatic rhyolite intrusions. We Questa, and Pine Grove (discussed in a later do not propose that late-stage assimilation is section) show evidence of some crystallization adding anything "special" to the magma and fractionation of the parent magmas chamber, but it may point to a long-lived heat between intrusive units; this could be one source at the base of the system that permits source of heat for assimilation. All four of extended crystal fr actionation, volatile loss, these deposits were apparently developed and perhaps volatile fluxing without rapidly during the change from subduction-controlled solidifying the entire chamber. Isotopic to extensional tectonics. Thompson et al. zonation of the high-level magma chamber (1991) suggested that extension-related mafic may be one signature of that heat source. magma associated with the Rio Grande rift White et al. (1981) also note that played a substantial role in extending the life basaltic volcanism is not associated, on a local of "pre-rift" volcanism approximately by 3 scale, with Climax-type systems. However, m.y. For example, the Sulphur Gulch pluton felsic rocks at Climax, Henderson, and (23-25 Ma), which hosts the Questa deposit, Chicago Basin are contemporaneous with may have evolved from pre-rift magmas that minor volumes of cross-cutting biotite-rich were kept "alive" by basaltic magmatism lamprophyre dikes (Bookstrom, 1981 ; related to rifting (26 Ma). Therefore, Bookstrom et aI., 1988). Additionally, mildly underplated mafic magma at Questa may have alkaline to transitional basalts (24-20 Ma) been responsible fo r the gradual elevation in interfmger withbasin-fill sedimentary rocks of initial 87Sr/86Sr associated with roofward the Browns Park Formation west of the assimilation. Colorado mineral belt (Thompson et al., Evidence for extension-related 1991). Bookstrom (1981) suggests that the trachyandesite underplating of the Pine Grove lamprophyres represent "a different type of chamber is perhaps even more conclusive in bimodal suite." Some have suggested that substantiating a role fo r extension-related lower thermal input may result in selective magmas. Trachyandesitic magmas were partial melting of less refractory "plums" in a clearly associated with extensional tectonics "plum pudding" mantle with the resulting melt from 22 to 18 Ma across the being isotopically enriched and of alkaline Pioche-Marysvale volcanic belt. affinity (Thompson et aI., 1991; Perry et aI., Trachyandesite magma co-erupted with the 1987). Climax, Henderson, aild Chicago tuff of Pine Grove and intruded the vent just Basin are sufficiently removed from areas of after final consolidation of the magma substantial rifting that rift-related thermal chamber. Keith and Shanks (1988) suggested input may have been low, but sufficient to that the trachyandesitic magma may have extend the life of the magma chambers. provided the heat to allow the water-rich Pine In summary, when only high-silica Grove magma to ascend to slightly higher rhyolite is erupted or exposed for levels in the crust without solidifying. examination, the petrogenesis is ambiguous White et al. (1981) and Stein and (Christiansen and Wilson, 1982; Johnson and Crock (1990) point out that assimilation of Fridrich, 1990). Inasmuch as high-silica highly radiogenic country rocks around rhyolitic magmas lie near the ternary SEG SP-2 GIANT ORE DEPOSITS 305 minimum, a variety of source materials and fractionates at shallow depth or when melt petrogenetic paths may yield essentially the evolves water at great depth, he suggests that same bulk composition. We see no reason that in either case evolution of vapor during topaz and Climax-type rhyolites could not be crystallization (second boiling) is more developed by either partial melting of lower effective in removing Mo from the melt than crustal felsic granulite (Christiansen et al., any reasonable first boiling scenario. 1988; Stein, 1988; Stein and Crock, 1990; However, there is no reason to believe the Wh ite et aI., 1981), differentiation . of Climax -type rhyolitic magmas are dry C 1% subduction-related high-K calc-alkaline H20) and do not evolve an aqueous phase magmas (Keith et aI., 1986; We stra and until the last few percent of the magma Keith, 1981), or extreme fractionation of crystallizes. Keith and Shanks (1988) rift-related basaltic magma significantly documented a high water fugacity and content contaminated by lower crust. However, for the Pine Grove rhyolites; most rhyolitic rhyolites generated by these processes may not magmas are probably saturated or nearly be equally capable of generating a giant saturated with water when they erupt. In deposit. addition, rhyolites that appear to have erupted from Pine Grove, Questa, Climax, and CONCENTRATION AND Henderson have less than 35 % phenocrysts. TRANSPORTATION MECHANISMS . The alternative Candela (l989a) proposes for generating a molybdenum-rich Wa llace et al. (1968) emphasized that magma--evolving an aqueous phase at great 100-125 km3 of magma is needed to supply depth--may have some application to the molybdenum found in the Climax ore crystallization of drier, intermediate-or-mafic bodies. Carten et al. (1988a) concluded that magma at the roots of a stratified magma enrichment of ore components in tbe .cupola system. If buoyant fluids that evolved as a occurs priorto the onset of crystallization and result of crystallization at depth can extract that the initial concentration of molybdenum molybdenum from the magma they traverse in the parent magma chamber is not as and then collect efficiently in a high-level significant to ore formation as the typ e and cupola, then they may form a duration of processes that deliver molybdenum-rich magma and consequently molybdenum to the cupola. Intermingled contribute to ore formation. barren and productive stocks at Henderson Some have proposed that little have the same isotopic composition (Farmer additional crystallization or differentiation of and DePaolo, 1984) and probably represent the parent magma chamber occurred from the successive draughts from a large, persistent time of emplacement of one stock to the next magma chamber that was contiguous with (Keith and Shanks, 1988; Carten et aI., each intrusion. Carten et al. (1988a) also 1988a). For example, zirconium and niobium concluded that either variations in residence concentrations are approximately the same for time or variations in the efficiencies of all stocks at Henderson. However, relatively fractionation processes could account for the constant concentrations of these elements are intermingling of barren and productive stocks. perhaps not unusual for metaluminous magmas; crystal fractionation can occur and Crystallization maintain relatively constant values for zirconium, niobium, and molybdenum. Candela (1989a) states that partitioning of molybdenum into an aqueous phase is most efficient when an initially dry melt 306 SEG SP-2 GIANT ORE DEPOSITS

Variations in vapor/melt partitioning molybdenum in natural systems may be significantly higher than experimentally In orthomagmatichydrothermal fluids, determined values. For instance, porphyry Mo does not appear to be complexed and molybdenum systems have f02 conditions of transported with either CI or F; in fact, calc-alkaline rocks (about 2 log units above Keppler and Wy llie (1991) found that NNO) that are well above the f02's molybdenum is most strongly partitioned into investigated experimentally; the trends of the an aqueous phase (KD = 5 .5) when CI and F experimental data suggest that molybdenum are absent. Consequently, if our conclusions should be even more strongly partitioned into regarding the maximum enrichment, -5 ppm, the aqueous phase under such high f02 of molybdenum in magmas are correct, none conditions. Candela (1989b) also speculates of the experimentally determined vapor/melt that if the oxidation state of molybdenum in partition coefficients for molybdenum would the aqueous phase is higher than in the melt, produce an extremely molybdenum-rich ore then the tendency of molybdenum to enter the fluid (one with thousands of ppm Mo) from a vapor phase may be compounded. water-rich rhyolitic melt at shallow depths Keith and Shanks (1988) reviewed the (Keppler and Wy llie, 1991; Tacker and evidence that extension-related trachyandesitic Candela, 1987). magmas were episodically injected into the Lowenstem et al. (1991) investigated base of the Pine Grove magma chamber. If the content of melt inclusions in quartz such magmas are injected into the base of a phenocrysts from the peralkaline rhyolites dacitic or rhyolitic magma chamber, they from Pantelleria. They found 3 ppm copper in would quench or crystallize and release matrix glass and in melt inclusions with volatiles that could be fluxed through the fractures to the crystal surface. Non-degassed magma chamber. Lamprophyric magmas or melt inclusions contained 20 ppm copper and less primitive K -rich mafic magmas that are melt inclusions that also contained large vapor generally volatile-rich, may have interacted bubbles contained up to 300 ppm copper. with Climax, Henderson, Quartz Hill, and They calculated that the vapor/melt partition other porphyry molybdenum systems to coefficients for copper may be well over produce similar volatile fluxing. 1000; this value is much higher than those If such a molybdenum-transporting determined experimentally (-50; Candela and magmatic fluid phase existed and was Holland, 1984). Lowenstem et a1. (1991) CO2-rich, then evidence of CO2 in propose that the stronger partitioning of high-temperature fluid inclusions should copper into the fluid phase could be due to perhaps be found. Linnen and Wi lliams-Jones more "available CI" in the fluid phase than in (1990) note that aqueous-carbonic fluid the experiments. This might have been caused inclusions are common in fluorine-deficient by a high CO2/H20 ratio, rendering He! more molybdenum deposits, but are scarce in dissociated. They also emphasize that fluorine-rich deposits. However, Wh ite et a1. crystallization-induced volatile saturation (1981) note that a carbonate daughter mineral (second-boiling) may not be necessary for the is present in some fluid inclusions from production of Cu-rich fluids. Exsolved Henderson. Seedorff (1988) found similar volatiles were present in pantellerite magma daughter minerals in fluid inclusions from a with only 10% crystals and in rhyolite from rhodochrosite-bearing alteration assemblage at the Valley of Ten Thousand Smokes with only Henderson. Thus, CO2-rich fluid inclusions 2 % phenocrysts (Lowenstem et aI., 1991). have been observed at Climax and Henderson, Similarly, there are several reasons but are not abundant (Seedorff, 1988; Hall et why the vapor/melt partitioning coefficients of al. 1974). Hypersaline fluid inclusions are SEG SP-2 GIANT ORE DEPOSITS 307 rare in the F-deficient deposits, but common that of the rhyolite is by exsolution of in Climax-type systems (Linnen and volatiles in the dacitic magma. Crystalliiation, Williams-Jones, 1990). These data suggest magma mixing, and/or arrival of mafic that volatile fluxing by a CO2-rich fluid phase magma at the base of the chamber may have in Climax-type systems may not be a helped create a flux of components with low significant process. solubility such as CO2 or S02 ; exsolved Analysis of volcanic gas emissions bubbles of these gases would contain some from active volcanoes indicates that sulfurous water and other ore constituents such as gases are emitted upon the inferred arrival of molybdenum. Evidence of similarly rafted new magma at the base of a magma chamber. blocks in other porphyry systems may often At the f02 conditions of calc-alkaline magmas, go unrecognized. As a possible example, S02 and H2S would be present with about White et al. (1981) describe a textural variant sub-equal fugacities (Whitney, 1988) . The low of the Late rhyolite at Climax wherein blocks solubility of S02 in magmas may allow a flux of biotite-rich (3-5%) rock, 2 m to 1 cm in of S02-bearing volatiles to rise through a size, occur as inclusions within the mam zoned magma chamber and perhaps scavenge phase. Carten et al. (1988a) propose that molybdenum. Molybdenum is not complexed convection could augment the transport of ore with sulfur; but the partitioning behavior of components as dissolved or undissolved molybdenum relative to a fluid phase that is volatiles. These data indicate that a stream of rich in an oxidized sulfur species has not been undissolved volatiles could begin at the investigated. Indeed, fluxing of S-rich fluids deepest levels of the magma chamber. through a magma is probably required to account for large amount of magmatic sulfur Central vent architecture associated with a giant porphyry molybdenum deposit. The low solubility of sulfur in Magmatic volatile collectionassocia ted high-silica rhyolitic melts argues for such a with both porphyry copper and molybdenum process, but mass balance calculations to test deposits often appears to be channelled to a this scenario would involve much uncertainty. vent associated with prior eruptions; such is clearly the case with Henderson and Climax. Convection and volatilefluxing Although many authors lament the loss of volatiles and ore constituents associated with There are several reasons to suspect opening a vent, it may be a critical positive that the process of metal enrichment involves feedback mechanism. Sub�equent volatiles convection and volatile fluxing through a could be collected from the roof zone by a large volume of the chamber. Keith et al. single, flaring funnel, eroded into the wall (1986) document that large blocks of rocks during eruption. Volatiles collected in biotite-rich (5-7%) dacitic magma from the the sub-volcanic pipe could experience a base of the Pine Grove magma chamber were pressure drop to hydrostatic values more rafted to the top of the cupola just prior to the readily than any other portion of the roof main episode of ore formation. Individual zone. If the pipe geometry is lacking, then the blocks are surrounded by rhyolitic magma and ore zone may be more sheetlike extending successive intrusions are rhyolitic. Because of along the roof of the chamber as seen at Mt the higher density of dacitic magma relative to Tolman (W. Utterback, unpublished report) . rhyolitic magma, emplacement of dacitic The same volume of magma that could be blocks at the top of a rhyolitic intrusion would "seen" by the vent during eruption may also not be expected to occur. One mechanism for be seen during collection of volatiles. The reducing the density of dacitic magma below larger the volume of magma seen by one vent 308 SEG SP-2 GIANT ORE DEPOSITS during fluid collection, then the larger, or of molybdenum (White et al., 1981). higher grade, the deposit could be. Certainly However, on a fm e scale, molybdenite a single, well-situated vent would be optimal. crystals are preferentially deposited along Smith (1979) pointed out that central K-feldspar crystals relative to quartz or other vent volcanoes do not erupt more than about minerals. Sericite separates molybdenite from 100 km3 of ash flow tuff; larger eruption the K-feldspar. The high modal abundance of volumes are related to where multiple K-feldspar in some Climax-type stocks may vents are probably involved, as is the case of be, in part, a depositional control to their the 500 km3 Amalia Tuff expelled from the grade. Questa caldera. Several resurgent (or slightly Climax-type deposits may also be younger) plutons occur along the ring fracture high-grade due to deposition from a purely and elsewhere; all of the intrusions are magmatic aqueous phase that begins variably mineralized with molybdenite deposition at the magma-wallrock interface. (Johnson et aI., 1989). In addition, the The ore-fluid for F-deficient deposits may be molybdenite-bearing Sulphur Gulch and Bear a mixture of magmatic and meteoric water Canyon plutons connect at depth (Leonardson and, at deposits such as Trout Lake, must et aI., 1983), but only the Sulphur Gulch have cooled at least a few hundred degrees pluton is ore-grade. Multiple vents or prior to mineralization (Linnen and caldera-related systems may not be as likely to Williams-Jones, 1990). This amount of produce a single giant deposit. cooling and mixing may result in deposition The 10:1 ratio of chamber volume to over a larger volume of rock resulting in eruption volume (Smith, 1979) suggests that lower grades. Deposition of molybdenite from a central vent volcano that erupts the the ore fluid in both Climax- and maximum 100 km3 could "see" no more than F-deficient-type deposits is related to -1000 km3, but much less may be required decreasing temperature and/or pH (Linnen and considering that 150 km3 of magma stripped Williams-Jones, 1990). of 3 ppm of molybdenum would supply all the Processes that control the location and metal needed to form the Henderson deposit. duration of molybdenite deposition from the aqueous phase would also obviously control CHARACTERISTICS OF THE the grade. These would include multiple DEPOSITIONAL SITE stocks forming overlapping ore bodies and high chemical and thermal gradients around There is no evidence to indicate that the stock or pluton. Climax-type systems more effi ciently deposit molybdenum from a magmatic ore fluid than SUMMARY MODEL AND other types; but they may do it in a smaller EXPLORATION CRITERIA volume of rock t() produce a higher grade. Carten et al. (l988a) identified one of the From the foregoing discussion it is primary controls for the deposition of apparent that the most significant process (or molybdenite at Henderson as the presence of problem) in the formation of Climax-type K-feldspar along vein margins. Westra and deposits is scavenging a few ppm Keith (1981) noted a linear correlation molybdenum from a large chamber eventually between KzO content of hydrothermally to yield an ore fluid with thousands of ppm altered rock and grade, K20-rich stocks may molybdenum. A Mo-rich magma would be have equilibrated with a larger volume of formed in the cupola during this process due magmatic water prior to consolidation, and to equilibration with the fluid after its arrival thereby originally contained a greater amount and prior to release during mineralization. SEG SP-2 GIANT ORE DEPOSITS 309

The following processes appear to have acted crystallization of modestly water-rich magma together to produce a molybdenum-rich cupola near the surface produced an ore deposit. In of magma: terms of increased complexity beyond simple crystallization of cupola magma, this model 1. Crystallization along the walls or only suggests that convection aids the floor of the magma chamber would transport of volatiles produced by account for the gradually reduced REB crystallization along the walls and floor and content of the magma between from underplated mafic magma. The successive intrusions. The successively processes of convection and volatile fluxing deeper U-shaped REE patterns are not may have helped transform Mo-poor magma unique to Climax-type systems, but (3 ppm) into Mo-rich magma (> 1000 ppm), have been noted in comagmatic an enrichment that otherwise would be intrusions from other non-mineralized impossible by crystal fractionation alone. granitoids (Johnson et al., 1989). Similar processes may have operated in the Gradual accumulation of water and parent magma chambers of many porphyry fluorine along the chamber roof may Mo systems, although the evidence for these lower the solidus, prevent processes often cannot be found in crystallization, and permit mineralized cupolas alone. Geochemical data assimilation. reviewed here suggest that initially high incompatible traceelement abundances may be 2. Underplated or injected mafic much less a "cause" of mineralization thanthe magmas may play several roles. They above processes. However, fluorine and water may melt the lower crust and mix with may be important positive feedback lower crustal melts to provide an parameters; as their concentrations increase, oxidized magma in which they reduce the viscosity of the magma and molybdenum would behave allow more rapid convection and accumulation incompatibly during fractionation. of ore fluid components in the cupola. They may provide heat for: a) Exploration criteria for a giant, convective transport of dissolved and high-grade deposit include: undissolved volatiles to the cupola, b) assimilation of roof rocks and the 1. a tectonic setting that indicates a cores of older stocks, c) extension of changeover from compressional to the life of the chamber to allow extensional tectonics, generation of a large volume of rhyolitic magma and emplacement of 2. thick continental crust at the time of multiple intrusions. They may also deposit formation may encourage supply some flux of less soluble, extreme differentiation and crustal especially sulfurous, gases. contamination,

Examination of the histories of most 3. an isotopically zoned magma volcanoes reveals that theytend to be complex chamber indicative of a long-lived heat rather than simple, with the common source, occurrence of repeated injections of magma, storage of magma at different levels, and 4. a large, sub-volcanic, central-vent magmatic degassing occurring during periods ash flow/dome system that erupted less of relative quiescence or activity. Ore deposits than 100 km3 of rhyolite, and would be far more common if simple passive 310 SEG SP-2 GIANT ORE DEPOSITS

5. high niobium concentrations (>75 porphyry molybdenum deposits. in ppm) in a subalkaline, Dickinson, W.R. , and Payne, W.D., magnetite-bearing rhyolite. eds. , Relations of tectonics to ore deposits in the southern Cordillera. ACKNOWLEDGEMENTS Arizona Geol . Soc. Digest, vol. 14, p. 215-226. Reviews of this paper by J. F. H. Bookstrom, A.A., Carten, R.B., Shannon, Thompson and A. H. Clark are gratefully J.R., and Smith, R.P., 1988, Origins acknowledged. We appreciate the patience and of bimodal leucogranite-Iamprophyre helpfulness of the editors - especially Ben suites, Climax and Red Mountain Whiting. This work was supported by NSF porphyry molybdenum systems, grant EAR-8904774 to JDK. Colorado: Petrologic and strontium isotopic evidence. Colorado School of REFERENCES Mines Quarterly, vol. 83, no. 2, p. 1- 22. Ambrus, J., 1978, Chilean molybdenum Bouton, S.L., Candela, P.A., Weidner, J.R., resources, in Sutulov, A. , ed. , and Joseph, J., 1987, Experimental International molybdenum determination of the partitioning of encyclopaedia 1778-1978, Volume 1. tungsten and molybdenum between a Santiago, Chile, Intermet Publications, high silica melt and Ti-bearing p. 59-85. minerals. Geol. Soc. America, Best, M.G., Christiansen, E.H., and Blank, Abstracts with Programs, vol. 19, p. H.R., Jr., 1989, Oligocene caldera 596. complex and calc-alkaline tuffs and Boyle, H., and Leitch, H.B., 1983, Geology lavas of the Indian Peak volcanic field, of the Trout Lake molybdenum Nevada and Utah. Geol. Soc. deposit, B.C .. Canadian Inst. Min. America, Bull., vol. 101, p. 1076- Metall. Bull. vol. 76, no. 849, p. 115- 1090. 124. Best, M.G., Keith, J.D., and Williams, V.S., Brown, P., and Kahlert, B., 1986, Geology 1992, Preliminary geologic map of the and mineralization of the Red Ursine and Deer Lodge Canyon quad­ Mountain porphyry molybdenum rangles, Lincoln County, Nevada and deposit south-central Yukon. Canadian

Iron County, Utah . U.S. Geol. _ Inst. Min. Metall., Special Volume Survey, Open-File Report No. 92-341, 37, p. 288-297. 9 p. Burt, D.M., Sheridan, M.F., Bikun, J.V., Best, M.G., Mehnert, H.H., Keith, J.D., and and Christiansen E.H., 1982, Topaz Naeser, C.W., 1987, Miocene rhyolites--Distribution, origin, and magmatism and tectonism in and near significance for exploration. Econ. the southern Wah Wah Mountains, Geol., vol. 77, p. 1818-1836. southwestern Utah. U.S. Geol. Cameron, D.E., Barrett, L.F., and Wilson, Survey, Prof. Paper 1433-B, p. 29-47. J.C., 1986, Discovery of the Silver Bloomer, C., 1981, The Casmo (Storie) Creek molybdenite deposit, Rico, molybdenite deposit, Cassiar, B.C. Colorado. American Inst. Min. [abs.]. Canadian Inst. Min. Metall. Metall. Petrol. Eng., Trans., vol. 280, Bull., vol. 74, no. 833, p. 64-65. p. 2099-2105. Bookstrom, A.A., 1981, Tectonic setting and Candela, P.A., 1989a, Magmatic ore-forming generation of Rocky Mountain fluids, Thermodynamic and mass SEG SP-2 GIANT ORE DEPOSITS 311

transfer calculations . of metal and Gunow, A.J., 1988b, Comparison concentrations. in Whitney, J.A., and of field based studies of the Henderson Naldrett, A.J., eds., Ore deposition porphyry molybdenum deposit, associated with magmas. Reviews in Colorado, with experimental and Econ. Geol., vol. 4, p. 203-221. theoretical models of porphyry Candela, P.A., 1989b, Felsic magmas, systems. in Taylor, R.P, and Strong, volatiles, and metallogenesis. in D. F . , eds. , Recent advances in the Whitney, J.A., and Naldrett, A.J., geology of granite-related mineral eds. , Ore deposition associated with deposits. Canadian Inst. Min. Metall., magmas. Reviews in Econ. Geol., vol. Special Volume 39, p. 351-366. 4, p. 223-233. Carten, R.B., White, W.H., and Stein, H.l., Candela, P.A. and Holland, H.D., 1984, The in press, High-grade granite-related partitioning of copper and Mo system: Classification and origin. molybdenum between silicate melts Geol. Assoc. Canada, Special Paper, and aqueous fluids. Geochim. IAGOD Conference--Deposit Cosmochim. Acta, vol. 48, p. 373- Modeling Program. ·380. Christiansen, E.H., and Wilson, R.T., 1982, Carlson, D.H., and Moye, F.J., 1990, The The classification and genesis of Colville igneous complex: Paleogene stockwork molybdenum deposits--A volcanism, plutonism, andext ension in discussion. Econ. Geol., vol. 77. p. northeasternWashin gton . in Anderson, 1250-1252. J.L., ed. , The nature and origin of Christiansen, E.H., Burt, D.M., and Sheridan Cordilleran magmatism. Geol. Soc. M.F., 1986, The geology of topaz America, Memoir 174, p. 375-394. rhyolites from the western United Carmichael, I.S.E., 1991, The redox states of States. Geol. Soc. America, Special basic and silicic magmas: a reflection Paper 205 , 82 p. of their source regions? . Contrib . Christiansen, E.H., Stuckless, 1.S., Mineral. Petrol., vol. 106, p. Funkhouser, M.J., and Howell, K.A., 129-141. 1988, Petrogenesis of rare-metal Carten, R.B., Geraghty, E.P., Walker, B.M., granites from depleted crustal and Shannon, J.R., 1988a, Cyclic sources--an example from the development of igneous fe atures and Cenozoic of western Utah, USA. in their relationship to high-temperature Taylor, R.P, and Strong, D.F., eds., hydrothermal features in the Recent advances in the geology of Henderson porphyry molybdenum granite-related mineral deposits. deposit; Colorado. Econ. Geol., vol. Canadian lnst. Min. Metall., Special 83, p. 266-296. Volume 39, p. 307-321. Carten, R.B., Geraghty, E.P., Walker, B.M., Congdon, R.D., and Nash, W.P., 1991, and Shannon, J.R., 1988a, Cyclic Eruptive pegmatite magma: Rhyolite generation of weakly and stongly of the Honeycomb Hills, Utah. mineralizing intrusions in the American Mineral., vol. 76, p. Henderson porphyry molybdenum 1261-1278. deposit, Colorado: Correlation of Cygan, G.L., and Chou, I-Ming, 1987, igneous features with high-temperature Calibration of the W02-W03 buffer. hydrothermal alteration. Econ. Geol., EOS, vol. 68, p. 45 1. vol. 83, p. 266-296. DePaolo, D.J., 1981, Nd in the Colorado Carten, R.B., Walker, B.M., Geraghty, E.P., Front Range and implications for crust 312 SEG SP-2 GIANT ORE DEPOSITS

formation and mantle evolution in the sulphides detected by in-situ Proterozoic. Nature, vol. 291, p. micro-PIXEanal ysis. EOS, vol. 37, p. 193-196. 344. Farmer, G.L., and DePaolo, D.J., 1984, Heintze, L., 1985, Geology and geochemistry Origin of Mesozoic and Tertiary of the porphyry stockwork granite in the western United States molybdenum deposit at Tamboras, La and implications for pre-Mesozoic Negra Zone (Peru). Econ. Geol., vol. crustal structure 2. N d and Sr isotopic 80, p. 2019-2027. studies of unmineralized and Cu- and Hildreth, W., 1981, Gradients in silicic Mo-mineralized granite in the magma chambers: Implications for Precambrian craton. Jour. Geophys. lithospheric magmatism. Jour. Res., vol. 89, p. 10,141-10,160. Geophys. Res., vol. 86, p. 10,153- Ganster, M.W., Dowsett, F.R., and Ranta, 10,192. D.E., 1981, Geology of the Mount Hollister, V.F., 1978b, Porphyry Emmons deposit [abs.]. American molybdenum deposits. in Sutulov, A., Inst. Min. Metall. Pet. Eng., Annual ed., International molybdenum Meeting Program, p. 21. encyclopaedia 1778-1978, Volume 1. Geyti, A., and Schonwandt, H.K., 1979, Santiago, Chile, Intermet Publications, Bordvika--a possible porphyry p. 270-288. molybdenum occurrencewith the Oslo Hudson, T., Arth, J.G., and Muth, K.G., rift, Norway. Econ. Geol., vol. 74, p. 1981, Geochemistry of intrusive rocks 1211-1220. associated with molybdenite deposits, Geyti, A., and Thomasson, B., 1984, Ketchikan Quadrangle, southeastern Molybdenum and precious metal Alaska. Econ. Geol., vol. 76, p. mineralizaiton at Flammefjeld, South­ 1225-1232. East Greenland. Econ. Geol., vol. 79, Ihlen, P.M., Tronnes, R., and Vokes, F.M., p. 1921-1929. 1982, Mineralization, wall rock Giles, D.L. , and Thompson, T.B., 1972, alteration and zonation of ore deposits Petrology and mineralizaion of a associated with the Drammen Granite molybdenum- bearing alkalic stock, in the Oslo Region, Norway. in Sierra Blanca, New Mexico. Geol. Evans, A.M., ed., Metallization Soc. America Bull., vol. 83, p. 2129- associated with acid magmatism. 2148. Chichester, John Wiley, p. 111-136. Govindaraju, K., 1989, 1989 compilation of Ivanova, G.F., 1963, Content of , working values and sample description tungsten, molybdenum, in granites for 272 geostandards. Geostandards enclosing tin-tungsten deposits. Newsletter, vol. 13, Special Issue, p. Geochemistry, vol. 8, p. 492-500. 1-113. Johnson, C.M., and Fridrich, C.J., 1990, Hall, W.E., Friedman, 1., and Nash, J.T., Non-monotonic chemical and 0, Sr, 1974, Fluid inclusion and light stable Nd, and Pb isotope zonations and isotope study of the Climax hetrogeniety in the mafic- to molybdenum deposits, Colorado. silicic-composition magma chamber of Econ. Geol., vol. 69, p. 884-901. the Grizzly Peak Tuff, Colorado. Hattori, K., Arai, S., Francis, D.M., and Contrib. Mineral. Petrol., vol. 105, p. Clarke, D.B., 1992, Variations of 677-690. siderophile and chalcophile trace Johnson, C.M., and Lipman, P.W., 1988, elements among mantle-derived Origin of metaluminous and alkaline SEG SP-2 GIANT ORE DEPOSITS 313

volcanic rocks of the Latif volcanic Naldrett, A.J., eds., Ore deposition field, northern Rio Grande rift, New associated with magmas. Reviews in Mexico. Contrib. Mineral. Petrol., Econ. Geol., vol. 4, p. 235-250. vol. 100, p. 107-128. Keppler, H., and Wy llie, P.I., 1991, Johnson, C.M., Czamanske, G.K., and Partitioning of Cu, Sn, Mo, W, U, Lipman P. W., 1989, Geochemistry of and Th between melt and aqueous intrusive rocks associated with the fluid in the system haplogranite-H20- Latirvolcanic field, NewMexico, and HCl and haplogranite-H20-HF. contrastsbetwee n evolution of plutonic Contrib. Mineral. Petrol., vol. 109, p. and volcanic rocks. Contrib. Mineral. 139-150. Petrol., vol. 103, p. 90-109. Kirkham, R.V., McCarn, C., Prasad, N., Johnson, C.M., Lipman, P.W., and Soregaroli, A.E., Vokes, F.M., and Czamanske, 1990, H, 0, Sr, Nd, and Wi ne, G., 1982, Molybdenum in Pb isotope geochemistry of the Latif Canada, Part 2: MOLYFILE--An volcanic fieldand cogenetic intrusions, index-level computer file of New Mexico, and relations between molybdenum deposits and occurrences evolution of a continental magmatic in Canada. Geol. Surv. Canada, Econ. center and modifications of the Geol. Report 33, 208p. lithosphere. Contrib. Mineral. Petrol., Knittel, D., and Burton, C.K., 1985, PololIo vol. 104, p. 99-124. Island (Philippines): Molybdenum Keith, J.D., 1982, Magmatic evolution of the mineralization in an island arc. Econ. Pine Grove porphyry molybdenum Geol., vol. 80, p. 2013-2018. system, southwestern Utah. unpubl. Kooiman, G.J.A., McLeod, M.J., and Ph.D. thesis, Dniv. Wi sconsin, Sinclair, W. D. , 1986, Porphyry Madison, Wi sconsin, 246 p. tungsten-molybdenum orebodies, Keith, J.D., Shanks, W. e., III, Archibald, polymetallic veins and replacement D.A., and Farrar, E., 1986, Volcanic bodies, and tin-bearing greisen zones and intrusive history of the Pine Grove in the Fire Tower Zone, Mount porphyry molybdenum system, Pleasant, New Brunswick. Econ. southwestern Utah. Econ. Geol., vol. Geol., vol. 81, p. 1356-1373. 81, p. 553-577. Kovalenko, V.I., Antipin, V.S., Konusova, Keith, J.D., and Shanks, W. C., III, 1988, V.V., Smirnova, Ye.V., Petrov,L.L. , Chemical evolution and volatile Vladykin, N. V. , Kuznetsova, A.I., fugacities of the Pine Grove porphyry Kostyukova, Ye.S., and Pisarskaya, molybdenum and ash-flowtuff system, V.A., 1978, Partition coefficients of southwestern Utah. in Taylor, R.P, fluorine, niobium, tantalum, and Strong, D.F., eds., Recent lanthanum, ytterbium, yttrium, tinand advances in the geology of tungsten in ongonite. Geochemistry granite-related mineral deposits. Inti., vol. 15, p. 203-205. Canadian Inst. Min. Metall., Special Kovalenko, V.I., Samoylov, V.S., and Volume 39, p. 402-423. Goreglyad, A.V., 1981, Volcanic Keith, I.D., van Middlelaar, W. , Clark, ongonites enriched in rare elements. A.H., and Hodgson, C.I., 1989, Doklady Akademia N auk SSSR, vol. Granitoid textures, compositions, and 246, p. 58-61. volatile fugacities associated with the Kuroda, P.K., and Sandell, E.B., 1954, formation of tungsten-dominated skarn Geochemistry of molybdenum. deposits. in Wh itney, I.A., and Geochim. Cosmochim. Acta, vol. 6, 314 SEG SP-2 GIANT ORE DEPOSITS

p. 35-63. Siderophile and chalcophile element Leonardson, R.W., Dunlap, G., Starquist, abundances in oceanic basalts, Pb V.L., Bratton, G.P., Meyer, J.W., isotope evolution and growth of the Osborn, L.W., Atkin, S.A., Molling, Earth's core. Earth Planet. Sci. Lett., P.A., Moore, R.F., 0lmore, S.D., vol. 80, p. 299-313. 1983, Preliminary geology and Noble, S.R., Spooner, E.T.C., and Harris, molybdenum deposits at Questa, New F.R., 1984, The Logtung large Mexico. in The genesis of Rocky tonnage, low-grade W ()-Mo Mountain ore deposits: Changes with porphyry deposit, south-centralYukon time and tectonics. Proceedings of Territory. Econ. Geol., vol. 79, p. Denver Region ExplorationGeologists 848-868. Society Symposium, p. 151-155. Noble, S.R., Spooner, E.T.C., and Harris, Lipman, P.W., Doe, B.R., Hedge, C.E., and F.R., 1987, Logtung: porphyry W-Mo Steven, T.A., 1978, Petrologic deposit in the southern Yukon. evolution of the San Juan volcanic Canadian Inst. Min. Metall., Special field, southwestern Colorado: Pb and Volume 37, p. 274-287. Sr isotope evidence. Geol. Soc. Ohlander, B., 1985, Geochemistry of America, Bull., vol. 89, p. 59-82. Proterozoic molybdenite-mineralized Lowenstern, J.B., Mahood, G.A., Rivers, aplites in Northern Sweden. M.L., and Sutton, S.R., 1991, Mineralium Deposita, vol. 20, p. 241- Evidence for extreme partitioning of 248. copper into a magmatic vapor phase. Pedersen, F.D., 1986, An outline of the Science, vol. 252, p. 1405-1409. geology of the Hurdal area and Nordli Linnen, Robert L., and Williams-Jones, granite molybdenite deposit. in Anthony E., 1990, Evolution of Olerud, S., and Ihlen, P.M., eds., aqueous-carbonic fluids during contact Metallogeny associated with the Oslo metamorphism, wall-rock alteration, paleorift. Geol. Surv. Sweden, Serial and molybdenite deposition at Trout Ca. 59, p. 18-23. Lake, British Columbia. Econ. Geol., Perry, F.V., Baldridge, W.S., and DePaolo, vol. 85, p. 1840-1856. D.J., 1987, Role of asthenosphere and Mahood, G.A., 1981, Chemical evolution of lithosphere in the genesis of late a Pleistocene rhyolitic center, Sierra Cenozoic basaltic rocks from the Rio La Primavera, Jalisco, Mexico. Grande rift and adjacent regions of the Contrib. Mineral. Petrol., vol. 77, p. southwestern United States. Jour. 129-149. Geophys. Res., vol. 92, p. 9193-9213 . Mutschler, F.E., Wright, E.G., Ludington, Pichavant, M., Herrera, J.V., Boulmier, S., S., and Abbott, J.T. , 1981, Granite Briqueu, L., Joron, J.-L., Juteau, M., molybdenite systems. Econ. Geol., Marin, L., Michard, A., Sheppard, vol. 76, p. 874-897. S.M.F., Treuil, M., and Vernet, M., Newsom, H.E., and Palme, H., 1984, The 1987, The Macusani glasses, SE Peru: depletion of siderophile elements in evidence of chemical fractionation in the Earth's mantle: new evidence from peraluminous magmas. in Mysen, molybdenum and tungsten. Earth B.O., ed., Magmatic processes: Planet. Sci. Lett., vol. 69, p. Physicochemical principles. Geochem. 354-364. Soc., Special Publ. 1, p. 359-373. Newsom, H.E., White, W.M., Jochum, Pilcher, S.H., and McDougall, I.J., 1976, K.P., and Hofman, A.W., 1986, Characteristics of some Canadian SEG SP-2 GIANT ORE DEPOSITS 315

Cordilleran porphyry prospects. Geol. Soc. America, Special Paper Canadian Inst, Min. Metall., Special 180, p. 5-27. Volume 15, p. 79-82. Smith, R.L., 1985, Tin, molybdenum, and Ranta, D.E., 1974, Geology, alteration, and other metal enrichments in silicic mineralization of the Winfield (La magmas and their relation to fluorine Plata) district, Chaffee County, and chlorine distribution. in Krafft, Colorado . unpubl. Ph.D. thesis, K., ed., USGS research on mineral Colorado School of Mines, Golden, resources -- 1985 program and Colorado, 261 p. abstracts. u.S. Geol. Survey, Circular Schmidt, E.A., Broch, M.I. , and White, 949, p. 52-53 . R.O., 1982, Geology of the Thompson Stein, H.I., 1988, Genetic traits of Creek molybdenum deposit, Custer Climax-typegranites and molybdenum County, Idaho. in The genesis of mineralization, Colorado mineral belt. Rocky Mountain ore deposits: Changes in Taylor, R.P, and Strong, D.F., with time and tectonics. Proceedings eds., Recent advances in the geology of the Denver Region Exploration of granite-related mineral deposits. Geologists Society Symposium, Canadian Inst. Min. Metall., Special Denver, Colorado, p. 79-84. Volume 39, p. 394-40l. Seedorff, E., 1988, Cyclic development of Stein, H.J., Hannah, J.L., 1985, Movement hydrothermal mineral assemblages and origin of ore fluids in Climax-type related to multiple intrusions at the systems. Geology, vol. 13, p. Henderson porphyry molybdenum 469-474. deposit, Colorado. Canadian Inst. Stein, H.I., and Crock, J.G., 1990, Late Min. Metall., Special Volume 39, p. -Tertiary magmatism in the 367-393. Colorado Mineral Belt; Rare earth Sharp, J.E., 1979, Cave Peak, a moly­ element and samarium-neodymium bdenum-mineralized pipe isotopic studies. in Anderson, J.L., complex in Cluberson Country, Texas. ed. � The nature and origin of Econ. Geol., vol. 74, p. 517-534. Cordilleran magmatism. Geol. Soc. Shaver, S.A., 1986, Elemental dispersion America, Memoir 174, p. 195-223 . associated with alteration and Sutulov, A., 1978, International molybdenum mineralization at the Hall (Nevada encyclopaedia 1778-1978, Volume 1. Moly) quartz-monzonite typeporphyry Santiago, Chile, Intermet Publications, molybdenum deposit, with a section 402 p. on comparison of dispersion patterns Tacker, R.C., and Candela, P.A., 1987, with those from Climax-type deposits. Partitioning of molybdenum between Jour. Geochem. Explor., vol. 25, p. magnetite and melt: A prliminary 81-98. experimental study of partitioning of Sillitoe, R.H., 1980, Types of porphyry ore metals between silicic magmas and molybdenum deposits. Mining crystalline phases. Econ. Geol., vol. Magazine, vol. 142, p. 550-553. 82, p. 1827-1838. Sillitoe, R.B., Jarmillo, L., and Castro, H., Taylor, S.R., and McClennan, S.M., 1985, 1984, Geologic exploration of a The continental crust: Its composition molybdenum-rich porphyry copper and evolution. Blackwell Scientific deposit at Mocoa, Columbia. Econ. Publications, Oxford, 312 p. Geol., vo1.79, p. 106-123. Theodore, T.G., and Menzie, W.D., 1984, Smith, R.L., 1979, Ash-flow magmatism. Fluorine-deficient porphyry 316 SEG SP-2 GIANT ORE DEPOSITS

molybdenum depositli in the western Geology of the Urad and Henderson North America Cordillera. in molybdenite deposits, Clear Creek Janc1idze, T.W. , and Tvalchrelidze, County, Colorado, with a section on a A.G. , eds., Prodeedings of the sixth comparison of these deposits with quadrenneal IAGOD symposium, those at Climax, Colorado. Econ. Tbilisis, USSR, p. 463-470. Geol. , vol. 73, p. 325-368. Thomas, J.A. , and Galey, J.T. , Jr. , 1982, Wallace, S.R. , Muncaster, N.K., Jonson, Exploration and geology of the Mt. D.C., MacKenzie, W.B. , Bookstrom, Emmons molybdenite deposits, A.A., and Surface, V.E., 1968, Gunnison County , Colorado. Econ. Multiple intrusion and mineralization Geol., vol. 77, p. 1085-1104. at Climax, Colorado. in Ridge, J.D., Thompson, R.A., Johnson, C.M. , and ed. , Ore deposits of the United States, Mehnert, H.H., 1991, Oligocene 1933-1967 . (Graton-Sales voL), New basaltic volcanism of the northern Rio York, American Inst. Mining Metan . Grande Rift: San Luis Hills, Colorado . Pet. Eng. , p. 605-640. Jour. Geophys. Res. , vol. 96, p. Westra, G. , and Keith, S.B., 1981, 13,577-13,592. Classification and genesis of Thompson, T.B. , 1968, Hydrothermal stockwork molybdenum deposits. alteration and mineralization of the Econ. Oeol. , vol. 76, p. 844-873. Rialto stock, Lincoln County, New White, W.H., Bookstrom, A. A., Kamilli, Mexico. Econ. Geol. , vol. 63, p. 943- R.J., Ganster, M.W. , Smith, R.P. , 949. Ranta, D.E. , and Steininger, R.C., Thompson, T.B., 1982, Classification and 1981, Character and origin of genesis of stockwork molybdenum Climax-type molybdenum deposits. deposits--A discussion. Econ. Geol. , Econ. Geol . 75th Anniversary vol. 77, p. 709-714. Volume., p. 270-3 16. Tingey, D.G., Christiansen, E. H., Best, Whitney, J.A. , 1988, Composition and M.G., Ruiz, J., and Lux, D.R., 1991, activity of sulfurous species in Tertiary minette and melanephelinite quenched magmatic gases associated dikes, Wasatch Plateau, Utah: Records withpyrrhotit e-bearingsilicic magmas. of mantle heterogeneities and changing Econ. Geol. , vol. 83 , p. 86-92. tectonics. Contrib. Mineral. Petrol. , Witcher , I.G., 1975, Anduramba vol. 87, p. 13,529-13,544. molybdenum prospect. in Knight, Turley, C.H., and Nash, W.P. , 1983, C.L., ed. , of Petrology of late Tertiary and Australia and Papua New Guinea 1. Quaternary volcanism in western Juab Metals, Monograph 5, Victoria, and Millard Counties, Utah. Utah Australasian Inst. Min. Metall. , p. Geol. Mineral Survey , Special Studies 793-794. 52, p. 1-33. Woodcock, J.R. , and Carter, N.C., 1976, Vollmer, R. , Ogden, P. Shilling, J.G., Geology and geochemistry of theAlice Kingsley, R.H. , and Waggoner, D.G., Arm molybdenum deposits. Canadian 1984, Nd and Sr isotopes in ultra­ Inst. Min. Metall ., Special Volume potassic volcanic rocks fr om the 15, p. 462-475 . Leucite Hills, Wyoming. Contrib. Worthington, J., 1977, Molybdenum mineral­ Mineral . Petrol. , vol. 87, p. 359-368. ization at Cannivan Gulch, Montana Wallace, S.R., Mackenzie, W.B. , Blair, [abs.], Geol. Assoc. Canada, Program R.G., and Muncaster, N.K., 1978, with Abstracts, vol. 2, p. 172. SEG SP-2 GIANT ORE DEPOSITS 317

Zahoney, S., 1968, Chemical controls of molybdenum ore formation. unpubl. report, Climax Molybdenum Co., 20 p. Zies, E.G. , 1929, The Valley of Ten Thousand Smokes, the fumarolic incrustations and their bearing on ore deposits. National Geographic, Contributed Technical Papers, vol. 1, no. 4, p. 1-61. 318 SEG SP-2 GIANT ORE DEPOSITS