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IntroductionMAGMATIC SULFIDES, when present in igneous rocks, areknown to host the major portion of chalcophile metals due tothe large partition coefficients for metals (Stimac and Hick-mott, 1996) and this controls the availability of Cu and Auduring development of porphyry copper deposits (Candelaand Holland, 1986). For example, the partitioning of Cu intomagmatic sulfides relative to silicate melts (DCu is 460–510) isorders of magnitude greater than the partitioning into silicateor oxide minerals, which have crystal and/or liquid partitioncoefficients of about one (Stimac and Hickmott, 1996). Con-sequently, the formation of magmatic sulfides has the poten-tial to deplete the silicate melt in Cu, as well as Ni, Co, Zn,As, Se, Mo, Pb, and precious metals (Stimac and Hickmott,1996). If sulfides form and persist in a magma chamber dur-ing episodes of hydrothermal fluid release (due to equilibrium

conditions with the fluid phase), no significant amount ofmetals could be collected by the fluid and incorporated intoan orebody. Thus high concentrations of magmatic sulfidesshould not be present in rocks associated with orebodies. Inorder to form a large porphyry deposit such as Bingham,Utah, Cu, Au, and S would need to be scavenged from ~250km3 of magma (assuming typical magmatic concentrations)and transported to the site of ore formation near the top ofthe magma chamber (Dilles, 2000; Hattori and Keith, 2001).

Previous workers observed high concentrations of mag-matic sulfides in some quenched dikes from the Tintic andBingham mining districts, Utah (Cannan, 1992; Hook, 1995;Waite et al., 1997; Pulsipher, 2000; Maughan, 2001). Pre-liminary data from Keith et al. (1997) showed that slowlycooled intrusions have only 1/100 of the sulfides in dikes.Hook (1995) studied the composition of magmatic sulfidesin the volcanic rocks south of the Bingham mine. He ob-served that the sulfides were mainly monosulfide solid solu-tion (Mss) or pyrrhotite and intermediate solid solution (Iss)

The Fate of Magmatic Sulfides During Intrusion or Eruption, Bingham and Tintic Districts, Utah

WILLIAM J. A. STAVAST,†,* JEFFREY D. KEITH, ERIC H. CHRISTIANSEN, MICHAEL J. DORAIS, DAVID TINGEY

Department of Geology, Brigham Young University, Provo, Utah 84602

ADRIENNE LAROCQUE,Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

AND NOREEN EVANS

CSIRO Exploration and Mining, P.O. Box 136, North Ryde NSW 1670, Australia

AbstractMagmatic sulfides in 97 samples of volcanic and intrusive rocks from the Tertiary Bingham (Cu-Au-Mo) and

Tintic (Ag-Pb-Zn-Cu-Au) districts, Utah, were examined to help better understand the fate of magmatic sul-fides during intrusion and eruption. Our findings show that shallowly emplaced dikes and sills have erratic butlocally high concentrations of sulfides. Volcanic rocks and large porphyry intrusions from these districts typi-cally have at least two orders of magnitude fewer sulfides than the dikes. Sulfide concentrations vary dramati-cally across these dikes and sills; for example, in one sill in Castro Gulch, Bingham district, sulfide abundanceincreases from 9 ppm by volume in the center to more than 2,000 ppm near the margin. Chalcophile metalsshow corresponding changes in abundance. For example, the whole-rock copper content of the sill ranges from23 ppm in the center to 35 ppm along the margins. The textures of sulfide grains (interpreted to reflect re-crystallization, resorption, and degassing) even in the most sulfide-rich samples, commonly have been modi-fied, suggesting that no sample preserves all of its original magmatic sulfide content. Immiscible liquids ofmonosulfide solid solution crystallized as pyrrhotite, pyrrhotite and chalcopyrite, or pyrite and chalcopyritewith declining temperature and pressure. These locally recrystallized to pyrite and chalcopyrite or to pyrite andan Fe oxide as they are oxidized. The alteration and preservation textures change from subspherical sulfideblebs near the margins of dikes and sills, to partially altered sulfides farther in, to complete absence of sulfidesin the vast majority of intrusions (except where small sulfides are completely enclosed by phenocrysts). Sulfideconcentrations appear to vary according to cooling rate and inferred pressure at the time of quenching or crys-tallization of the matrix. Most of the sulfides along the quenched margins of these dikes and sills are in the ma-trix. Slower cooling coupled with removal of magmatic volatiles, including sulfurous gases (e.g., H2S, SO2), al-lows the resorption or oxidation of magmatic sulfides to occur during final crystallization of a magma. Together,these processes remove greater than 90 percent of the original endowment of magmatic sulfides. This proba-bly explains the low-magmatic sulfide abundances of slowly cooled, large porphyritic intrusions, and most im-portantly, allows metals and sulfur to participate in the formation of porphyry deposits. The relative abundancesof base metals lost from the center of the sill are similar to the relative abundances of the metals in the Bing-ham deposit (production and reserves), suggesting that these processes also may have operated at a larger scale.

† Corresponding author: e-mail, bill@stavast.us*Present address: 2625 West Dove Ln, Thatcher, AZ 85552.

©2006 Society of Economic Geologists, Inc.Economic Geology, v. 101, pp. 329–345

or chalcopyrite. Two unmineralized intrusions containedpyrite as well. Hook (1995) noted that there were relativelyfew sulfide grains with degassing textures (spongy Fe oxidesformed as sulfide blebs lose sulfur; Larocque et al., 2000) inthe volcanic rocks, but such degassed grains occur more com-monly in the intrusions near the Bingham mine. In a similarstudy in the Tintic district, Cannan (1992) noted that the sul-fides in vitrophyres, latite lava flows, and dikes of the districtare pyrrhotite, pyrite, and chalcopyrite with abundances rang-ing from 0.0001 to 0.1 vol percent.

In this study, we examine the high concentrations of mag-matic sulfides in the Bingham and Tintic districts and how thedistribution and abundance of magmatic sulfides vary acrossdikes. We compare these data to comagmatic volcanic rocks

and large intrusive bodies. Castro Gulch in the Bingham dis-trict and Little Valley in the Tintic district are two areas withhigh concentrations of magmatic sulfides in dikes and werechosen for this study (Fig. 1). We hypothesize that the highmagmatic sulfide concentrations are caused by sulfide preser-vation (due to quenching under pressure) and that these sul-fide-bearing dikes are typical of preore magmatic sulfide con-centrations in ore-related porphyries in both mining districts.

Geologic SettingThe Tintic and Bingham districts lie on the eastern side of

the Basin and Range province in central Utah (Fig. 1A). TheBingham district is 25 km south-southwest of Salt Lake City.The Bingham deposit is a Cu-Au-Mo (-Ag-Pb-Zn) porphyry

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FIG. 1. A. Location of Bingham and Tintic districts. B. Regional geology of the Bingham pit area (after Phillips et al., 1997;Pulsipher, 2000). The dashed line shows the outline of high-grade ore (<0.35% Cu). C. Regional geology of the Tintic dis-trict (after Keith et al., 1991). Dashed line shows hidden porphyry Cu deposit (Morris, 1975).

deposit with reserves of 2,820 million metric tons (mt) of 0.73percent copper (Ballantyne and Smith, 1997). The area wasvolcanically active between 39 and 33 Ma; mineralization oc-curred between 39.2 and 37.7 Ma (Babcock et al., 1995). Inthe Bingham district, abundant latite dikes occur in CastroGulch, southeast of the Bingham pit (Fig. 1B). According toDeino and Keith (1997), these latite dikes are comagmaticwith the porphyry-related magma but predate the main pulseof mineralization (40Ar/39Ar age of 38.84 ± 0.19 Ma).

In Castro Gulch, there are 10 dikes and sills that range inoutcrop length from 25 to 700 m and cut quartzite of thePennsylvanian Butterfield Peaks Formation (Stavast, 2002).Block and ash flows south of Castro Gulch have similar ages tothe dikes (Maughan, 2001) and have clasts that are indistin-guishable in composition from the Castro Gulch intrusions, in-dicating that the dikes and extrusive units are comagmatic.Both units contain rare accessory sapphire crystals, approxi-mately 1 mm in diameter, of the same color and compositionin terms of included trace elements (Pulsipher, 2000).

Several lines of evidence indicate that the level of emplace-ment of the Castro Gulch intrusions was relatively shallow.First, extrapolation of the prevolcanic erosion surface that isexposed along ridge tops adjacent to Castro Gulch indicatesthat no more than 40 to 100 m of Paleozoic sedimentary rockswere present above the dikes that are most deeply incised bythe present-day ravine. If approximately 100 m of volcanicrocks was also present, at the time of dike emplacement, asindicated by the paleotopographic reconstruction of Waite etal. (1997), then perhaps 200 to 300 m of cover may have ex-isted. Other features of the rocks support this shallow level ofdike emplacement. For example, the texture and size of ma-trix crystals differ little from extrusive units, and the marginsof the dikes, where well exposed, contain glass and devitrifiedglass and retain the general appearance of vitrophyres.

The Tintic district is 60 km south of Bingham. The districthas produced Ag, Pb, Zn, Cu, and Au for more than 100 yearsfrom vein and replacement deposits related to monzonite in-trusions (Morris and Lovering, 1979). According to Moore(1993), the area was volcanically active from 35 to 32 Ma. Inthe Tintic volcanic field, both quenched latite dikes and moreslowly cooled, coarser grained monzonite dikes occur in andaround Little Valley. This area is 7 km southeast of the SilverCity stock, the ore-related intrusion (Morris, 1975), and 5 kmeast-southeast of a concealed porphyry Cu-Mo-Au prospect(Morris, 1975; Fig. 1C).

The dikes in Little Valley range from 10 to 100 m in length.There are approximately 28 dikes and six larger intrusions(Stavast, 2002). Keith et al. (1991) reported a 40Ar/39Ar age of33.0 ± 0.2 Ma for one of the larger intrusions. The dikes andintrusions cut latite lava flows, shoshonite lava flows of simi-lar composition, and intercalated Tertiary limestone and shalebeds. These dikes can be inferred to be shallowly emplacedbecause they cut coeval lava flows of the same composition aswell as lake sediments (caldera lakes?) that are interbeddedwith the lava flows.

Analytical TechniquesSeventy-four samples were collected for this study from the

major dikes in Castro Gulch (36 samples) and Little Valley(38 samples). Seven dikes were sampled near their edges and

centers to examine changes across the dike. Only dikes thatwere not cut by veins and had no noticeable alteration of min-erals were sampled. Fifteen samples of extrusive rocks andeight samples of large intrusions that were previously col-lected were also studied.

Thirty-nine samples of dike rocks were analyzed by X-rayfluorescence analysis (XRF) to determine major and trace el-ement compositions in each district and to determine differ-ences in composition across specific dikes (method inMaughan, 2001). Platinum group element and gold analyseswere performed on whole-rock samples (across one sill) atCSIRO Exploration and Mining by isotope dilution and ex-ternal calibration ICP-MS as described by Evans et al. (1993).

Phenocryst and sulfide compositions from representativedike samples were determined with a Cameca SX50 electronmicroprobe that has been upgraded to SX100 standards. Sili-cates and oxides were analyzed with a 20-nA beam and a 15-kV acceleration voltage. Sulfides were analyzed with a 20-nAbeam and a 25-kV acceleration voltage. Sulfide grains largerthan 5 µm in diameter were analyzed to minimize possiblecontamination from excitation of the surrounding silicate.Sulfides were analyzed for S, Fe, Co, Ni, Cu, Zn, As, Ag, andPb. Although the sulfide grains are not homogeneous, most ofthe individual phases were small enough that a bulk composi-tion of the grain was obtained. In larger phases, compositionsvary by less than 45 percent. Sulfides were also analyzed bysecondary ion mass spectrometry (SIMS) for Au and Pt(method in Larocque et al., 2000).

Modal sulfide abundances were obtained by automatedwavelength dispersive element mapping on the electron mi-croprobe and by petrographic examination of grain size andnumber in a 5- × 28-mm area. For samples that have highabundances of sulfides measurements were made in an area 3× 28 mm; typical numbers of sulfide grains in these samplesranged from about 600 to 1,200. Grains were classified as sul-fides or partially degassed sulfides that have lost sulfur. Grainsclassified as partially degassed sulfides contain at least 20 per-cent spongy Fe oxide (described by Larocque et al, 2000) andhave some residual sulfide and sulfur within the grain. Grainswith less than 20 percent spongy Fe oxide were counted assulfides. Spongy Fe oxide grains with no remnant sulfidewere not counted. Percent sulfide was then determined bycomparing the area of sulfides with the total area. We as-sumed that the sulfides are all spherical, they are evenly dis-tributed in three dimensions, and that the thin section cutsevery bleb down the center of each sulfide (e.g., a vol % of0.0001 would contain 44 blebs of 1-µm size, 17 blebs of 2-µmsize, and 3 blebs of 5-µm size in a 5- × 28-mm area). Modalanalyses using the electron microprobe were done by map-ping 50 random, 1-mm2 areas per thin section using a contin-uous scan and a 5-µm beam. These areas were mapped for S,Fe, and Ba. The Cameca image analysis software was used tocalculate the percent of the area that contains Fe and S, butnot Ba and S to avoid including barite. Modal analyses of fivesamples were determined twice using both the petrographiccounting method and electron microprobe element maps todetermine reproducibility (Table 1); typical uncertainties ofcounting were in the range of 0.0006 vol percent for sampleswith the lowest concentrations of sulfide and 0.012 vol per-cent for samples with the highest concentrations.

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The ability to distinguish between hydrothermal and mag-matic sulfides when both may be present in a sample is an im-portant issue. Several common characteristics of magmaticsulfides are (1) their rounded shapes, (2) they commonlyoccur as inclusions in early formed silicates and oxides, and(3) they have higher concentrations of Co, Ni, Cu, Zn, andother trace metals than hydrothermal sulfides. Hydrothermalsulfides, on the other hand, form along fractures, are notrounded, and tend to have stoichiometric compositions. Theclassification of magmatic versus hydrothermal sulfides wasbased on these criteria. Only magmatic sulfides were countedin this study.

ResultsCastro Gulch

Two types of dikes occur in Castro Gulch, fragmental andlatite (Fig. 2). The fragmental dikes are related to subverticalfractures and contain a variety of igneous lithologic units. Thelatite dikes occur as both dikes and sills. The sampledfragmental dike contains clastic material (Fig. 3), rounded

pebbles (Fig. 3B, D), silt- to coarse sand-size matrix (Fig. 3C),flow banding in the matrix (Fig. 3C), and fragments that ap-pear to have been somewhat plastic or molten at the time ofemplacement (Fig. 3A-B).

Four distinct clasts types are present, represented by sam-ples Castro-22a1, 29a, 29b, and 29c. Castro-29d is a sample ofthe matrix. The dike is of particular interest because some ofthe fragments contain distinctly higher concentrations (1 vol%) of magmatic sulfides than do the adjacent clasts or matrix(0–0.002 vol %). All clasts in this fragmental dike, except Cas-tro-22a1, contain rounded quartz, sieve-textured plagioclase,altered clinopyroxene, altered biotite, Fe-Ti oxides, and avery fine grained matrix. Clasts 29a and 29b range in sizefrom 1 to more than 20 cm across. Two of the clasts are latitesand indistinguishable from each other and from other dikes inthe Castro Gulch area. One is lavender (Castro-29a) and theother is light gray in color (Castro-29c). These clasts have asimilar mineralogy, except that 29c has less amphibole (Table2). Castro-29a was apparently plastic at the time of emplace-ment (Fig. 3A), whereas Castro-29c was brittle (Fig. 3B).Castro-29a contains 0.006 vol percent sulfide and 0.005 per-cent oxidized sulfide (Table 3). A third clast is darker gray(Castro-29b) but indistinguishable from others in its phe-nocryst proportions (Table 2). This clast contains 0.012 volpercent sulfide and 0.005 oxidized sulfide (Table 3). Thefourth clast (Castro-22a1) is a crystal-rich clot (Fig. 3D),about 3 cm across, that appears to be cumulate or glomero-porphyritic in nature. It contains plagioclase, amphibole,clinopyroxene, titanite, quartz, Fe oxide, and 1 percent sul-fide (Table 2). The sulfides appear to be magmatic based onthe presence of pyrrhotite cores; they are interstitial to othercumulate crystals, and they show no evidence of sulfidizingthe edges of mafic silicates.

The matrix of the fragmental dike is tan colored and com-posed mainly of fine (0.1–1 mm) fragments of igneous rocks,quartzite, and limestone (Castro-29d). No microphenocrystsare present. The crystals and sand-sized fragments in this ma-trix are distinctly graded and flow aligned (Fig. 3C). The ma-trix contains no sulfides.

Whole-rock analyses of two of the clasts (Castro-29a and29b; Table 3) show few significant chemical differences be-tween clasts. Differences in color between clast types may bepartially related to differences in the degree of devitrification.However, the matrix (Castro-29d) has significantly lower con-centrations of TiO2, Al2O3, Fe2O3, MgO, Na2O, K2O, V, Cr,Ni, Cu, Zn, Rb, Sr, Ba, Ce, and Pb and higher amounts ofSiO2 and CaO (Table 3; Stavast, 2002). The two other clasts(sulfide-rich Castro-22a1 and Castro-29c) were not large

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TABLE 1. Volume Percent Sulfide in Samples of the Castro Gulch Sill1

Sample Count 1 Count 2 Count 3 Microprobe2 Range Std. Dev.

Castro-19 0.0003 0.0008 0.0008 0.0005 0.0003Castro-17 0.0040 0.0046 0.0006 0.0004Castro-6a 0.0133 0.0178 0.0045 0.0032Castro-11 0.0235 0.0221 0.0014 0.0010Castro-15b 0.1966 0.1846 0.0120 0.0085

1 Each count is the volume percent sulfide determined by image analysis of three different 5- × 28-mm areas of the same sample2 Volume percent determined by elemental maps of S, Fe, Ba, and Si

FIG. 2. Geologic map of Castro Gulch, Bingham district, showing loca-tions for the fragmental dike and the sill sampled in this study.

enough to allow separation of the clasts from the matrix forbulk-rock analysis.

Whole-rock compositions of the nonfragmental dikes andsills of Castro Gulch plot near the corner of the latite, an-desite, trachyte, and dacite fields (Fig. 4). These dikes arehereafter referred to as latites. Samples were taken across onesill to determine variations across the latite dikes (Fig. 5), andthese data show that S, Cu, Ni, Zn, Au, Pt, Pd, Rh, and Ir gen-erally increase toward the margins of the sill (Table 3, Fig. 6).Typical nonfragmental dike material contains approximately47 percent phenocrysts: 10 percent plagioclase (An35–52), 25percent amphibole (magnesiohastingsite), 7 percent biotite

(Phlog54 Sid27 Ann19) , 2 percent Fe-Ti oxides, <2 percentclinopyroxene (Wo44 En51 Fs5) , <1 percent quartz, and traceamounts of quartzite fragments. Some samples contain noclinopyroxene or quartz phenocrysts. Minor apatite andbarite and trace amounts of olivine and a dark blue to mottledblue and white euhedral sapphire are also present. Baritesometimes occurs as inclusions in phenocrysts. Plagioclaseand amphibole phenocrysts are generally 2 to 3 mm. Biotiteand pyroxene phenocrysts are <1 mm and oxide phenocrystsare <0.5 mm.

Resorption and magmatic-reequilibration textures are com-mon in all samples from Castro Gulch. For example, quartz

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FIG. 3. Combined brittle and ductile magma mixing shown in fragmental dike Castro-22a. Five rock types are indicatedby different colors: lavender (29a), light gray (29c), dark gray (29b), tan matrix (29d), and white (22a1) A. This sample showsthe heterogeneity of the dikes, with a plastically deformed fragment present. B. Light gray, dark gray, and white fragmentscan be seen. Lavender rock wraps around white fragment. Rounded quartzite clasts are present. C. Close-up of size-gradedmatrix, which coarsens upward. D. Close-up of sulfide rich (1 vol %) white fragment (Castro-22a1).

TABLE 2. Mineralogy of Units in Fragmental Dike and Latite Dikes and Sills from Castro Gulch

Sample no. Castro 22a1 Castro 29a Castro 29b Castro 29c Average latiteSample type clast clast clast clast dike

Amphibole (vol %) 15 15 10 25Plagioclase 79 5 4 5 10Clinopyroxene 15 1 4 1 <2Biotite 2 <1 2 7Quartz 1 <1 <1 <1 ±Fe oxides 2 <1 <1 <1 2Titanite 2Sulfide 1 0.0114 0.017 <0.2Olivine trBarite trApatite trSapphire trFine-grained matrix 75 72 80 53

phenocrysts, where present, are commonly rounded and em-bayed. Plagioclase crystals are often broken and have a sievetexture in most samples. In addition, small anhedral inclu-sions of plagioclase are commonly present within large euhe-dral to subhedral amphibole crystals.

Hydrothermal alteration is present to varying degrees inCastro Gulch, with most outcrops showing little or no effectfrom alteration. Most dike margins contain darker matrix(glass or very fine grained crystals), whereas the groundmassin the center of the dike is coarser grained. Higher degrees of

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TABLE 3. Whole-Rock XRF Analyses of Dikes

Sample Castro Castro Castro Castro Castro Castro Castro Castro Castro Castro1 2 12 13 14 10 15a 15b 29a 29b

Easting1 405334 405327 405326 405326 405326 405349 405338 405338 404999 404999Northing1 4483640 4483647 4483650 4483657 4483657 4483665 4483675 4483675 4483573 4483573Rock Type LS LS LS LS LS LS LS LS FD FDPosition Margin Center Center Center Center Center Center Margin Clast Clast

SiO2 (wt %) 59.51 61.24 61.18 60.97 62.59 62.31 59.08 55.92 57.75 56.93TiO2 0.77 0.66 0.66 0.67 0.67 0.57 0.7 0.66 0.7 0.71Al2O3 14.3 15.36 15 14.8 15.24 14.89 14.01 12.36 14.49 14.27Fe2O3 6.17 4.82 5.05 5.04 4.9 4.79 5.92 4.73 6.2 6MnO 0.09 0.08 0.08 0.07 0.08 0.07 0.09 0.08 0.09 0.09MgO 4.17 2.84 2.87 2.82 2.71 2.51 4.14 3.14 4.37 4.61CaO 5.52 4.01 4.34 4.38 3.94 4.24 5.85 6.89 4.74 4.81Na2O 3.12 3.32 3.16 3.21 3.3 3.07 2.99 2.16 4.24 3.69K2O 3.63 4 3.74 3.71 3.85 3.73 3.65 4.55 3.55 3.75P2O5 0.32 0.27 0.27 0.27 0.27 0.25 0.33 0.27 0.33 0.3LOI 1.73 2.26 2.73 2.42 1.84 2.84 1.92 8.68 2.24 2.59Total 99.34 98.87 99.08 98.37 99.37 99.27 98.68 99.43 98.7 97.74

F (ppm) 676 651 898 760 697 820 1082 725 626 634S 159 49 14 7 42 67 310 974 50 95Cl 154 146 137 142 141 120 147 99 134 124Sc 17 9 12 10 12 11 14 14 15 15V 128 93 102 99 93 94 126 120 128 127Cr 217 82 98 99 90 84 210 190 211 205Ni 50 26 30 29 28 26 47 38 52 51Cu 35 23 25 25 24 23 35 29 38 34Zn 66 61 61 60 61 58 67 56 64 66Ga 18 19 18 19 18 18 17 14 17 17As 3 2 2 2 2 3 3 2 2 3Rb 106 123 117 116 117 113 101 150 108 115Sr 773 632 695 713 717 725 789 832 769 725Y 21 21 20 22 22 18 21 16 20 20Zr 234 254 256 257 253 254 237 230 243 237Nb 9 8 7 8 8 6 9 9 8 9Mo <1 1 2 <1 0 <1 1 2 <1 <1Ba 2697 2216 2189 2117 2219 2337 3115 5957 2366 2240La 68 69 62 66 71 67 64 47 65 67Ce 113 125 121 122 126 126 119 95 121 116Nd 51 53 51 54 51 52 55 47 52 48Sm 10 10 9 10 9 10 10 8 9 9Pb 33 38 38 35 38 38 37 24 34 33Th 19 23 23 22 22 22 16 14 19 18U 4 4 5 5 4 4 4 4 4 4

Pt2 (ppb) 0.62 0.4 0.17 0.16 0.17 0.31 0.36 0.33Pd2 0.5 0.28 0.35 0.28 0.25 0.28 0.54 0.6Ir2 0.02 0.025 0.021 0.02 0.015 0.01 0.022 0.02Rh2 0.03 0.022 0.013 0.01 0.007 0.02 0.019 0.02Au2 1.13 0.6 0.3 0.35 0.6 0.3 0.58 0.71Sulfide

(vol %) 0.0218 0.002 0.0009 0.0024 0.0008 0.0073 0.0324 0.1966 0.0064 0.0122Oxidized

sulfide3 0.104 0.0215 0 0 0 0.0066 0.0177 0.0419 0.0049 0.0047Total 0.1258 0.0235 0.0009 0.0024 0.0008 0.0139 0.0501 0.2385 0.0114 0.017

Abbreviations: FD = fragmental dike, HAL = alkali-rich latite, HAS = alkali-rich shoshonite, LAL = alkali-poor latite, LAS = alkali-poor shoshonite, LS = latite sill, MAT = matrix

1 UTM NAD 27, zone 122 ICP-MS analyses3 Oxidized sulfide is any sulfide having greater than 20 percent spongy Fe oxide

alteration in dike interiors are indicated by some phenocrysts.For example, in one dike, pyroxenes are completely altered inthe center of the dike but are only partly altered to clays andchlorite along the edge of the dike. Amphiboles in the centerof the dike are altered along fractures and cleavage planes,

whereas those along the edges of the dike show no alteration.Plagioclase in the center of the dike is partially altered, creat-ing small cavities that have been filled with carbonate andminor sericite, whereas the plagioclase along the edges of thedike is broken but relatively unaltered. Sampling for this

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from the Bingham and Tintic Districts

Castro Tintic Tintic Tintic Tintic Tintic Tintic Tintic Tintic Tintic Tintic29d 12 14 16 21 28a 28b 28c 29a 29b 29c

404999 409235 409460 409177 409058 409279 409279 409279 409116 409116 4091164483573 4410396 4410841 4410231 4409995 4410717 4410717 4410717 4410458 4410458 4410458

FD HAL HAS LAS LAL HAL HAL HAL HAL HAL HALMAT Center Center Center Center Margin Center Margin Margin Center Margin

62.65 58.05 53.72 52.64 60.31 56.76 56.9 56.77 57.41 57.17 56.690.55 0.95 1.16 1.03 0.75 0.97 0.97 0.95 0.97 0.97 0.97

11.55 16.13 16.33 12.74 14.65 15.8 16.04 15.94 16.32 16.19 16.154.8 6.45 8.46 8.11 5.39 6.62 6.49 6.47 6.59 6.33 6.290.09 0.12 0.13 0.13 0.11 0.13 0.12 0.12 0.12 0.11 0.123.69 2.39 3.15 4.87 1.4 2.51 2.45 2.41 2.47 2.17 2.295.37 4.82 6.42 7.8 5.53 5.14 5.1 4.97 4.97 4.84 4.952.7 3.39 3.1 2.75 3.08 3.3 3.19 3.24 3.16 3.34 3.192.93 4.8 4.04 3.21 4.1 4.79 4.79 4.9 4.85 4.79 4.690.24 0.43 0.6 0.42 0.31 0.48 0.45 0.45 0.47 0.48 0.475.01 1.67 2.07 7.19 3.99 2.11 2.1 2.05 2.24 2.7 2.49

99.6 99.2 99.19 100.87 99.62 98.6 98.59 98.26 99.57 99.1 98.29

686 1671 1556 787 1294 1762 1783 1719 1590 1552 144532 150 22 0 20 45 40 69 78 196 105

102 231 112 88 155 156 153 194 204 200 19212 12 18 17 13 14 13 13 12 14 1297 151 217 194 112 172 171 170 164 165 164

152 14 8 338 69 6 6 6 7 9 740 12 13 101 20 9 7 8 10 9 1128 53 83 56 28 68 68 71 59 53 5756 75 91 84 59 83 82 82 79 79 7916 19 20 20 20 20 20 19 20 20 202 13 6 5 4 11 11 13 11 12 9

90 245 121 104 154 191 190 210 293 295 280558 785 969 752 670 899 919 890 791 809 82518 24 27 23 22 27 27 26 23 23 23

233 375 289 232 246 396 397 399 379 381 3777 7 7 10 8 5 5 5 7 7 7

<1 1 <1 <1 2 2 2 3 2 3 11658 1336 1472 1099 1192 1364 1363 1342 1305 1324 1326

50 64 57 48 48 73 72 72 66 71 66100 140 114 97 102 134 137 135 131 131 13145 59 52 49 45 59 58 57 59 57 568 9 9 8 8 10 9 10 9 10 9

24 28 23 17 25 28 30 30 28 27 2815 42 22 14 20 45 44 45 40 41 403 11 6 3 6 10 10 11 12 12 11

0 0.0315 0.0019 0.0017 0.0008 0.0116 0.0077 0.0593 0.0192 0.0047 0.0137

0 0.0069 0 0 0.0038 0.0591 0.0229 0.047 0.076 0.0016 0.00720 0.0384 0.0019 0.0017 0.0045 0.0707 0.0306 0.1064 0.0952 0.0063 0.0208

study focused on outcrops where macroscopic alteration wasminimal or negligible. Thin sections with no microscopic evi-dence of alteration were used for the study.

Sulfides from Bingham district and Castro Gulch: Very fewsamples of volcanic and intrusive rocks in the Bingham areacontain high concentrations of magmatic sulfides. Most vol-canic rocks in the Bingham district contain 0.0001 to 0.0009vol percent sulfides with an average of 0.0004 percent (Fig.7). Typical large intrusions (Last Chance stock) in the districtare generally comparable to volcanic rocks in terms of sulfideabundances and contain only 0.0008 to 0.0016 vol percentsulfides (Fig. 7; Borrok et al., 1999, Stavast, 2002). However,unusually high magmatic sulfide abundances occur in someCastro Gulch intrusions, ranging from 0.0008 to 0.21 vol per-cent (Fig. 7). Samples taken from the margins of the dikeshave the highest concentrations of magmatic sulfides (Fig. 7).The three order of magnitude range of concentration cannotbe an artifact of uncertainties in the modal analyses or hy-drothermal alteration (although some recrystallization of sul-fides and sulfidation of silicates proximal to degassed sulfidesdoes occur, as explained below). A quenched sill margin (Cas-tro-15a, located in Fig. 5) that is rich in magmatic sulfides

contains approximately 500 sulfide blebs between 1 and 5µm, 350 blebs between 5 and 20 µm, and 60 blebs that are 20µm or larger in a 5- × 28-mm area of the thin section. A typ-ical latitic volcanic rock (e.g., Tick-41, collected 12 km south-east of Castro Gulch, at UTM NAD 27 zone 12 E403820,N4418411) contains 300 sulfide blebs between 1 and 5 µm,20 blebs between 5 and 10 µm, and a few blebs larger than 10µm (Fig. 8).

Immiscible sulfides in all samples occur as (1) spherical orovoid blebs that are completely encased in silicate or oxidephenocrysts (Fig. 9F), (2) spherical or ovoid blebs that arepartly encased in silicate or oxide phenocrysts, and or (3)spherical or ovoid blebs in the matrix (Fig. 9A-E). The sulfideinclusions that are completely encased by phenocrysts consistentirely of pyrrhotite that apparently crystallized from mono-sulfide solid solution (i.e., the pyrrhotite is not homogeneousin composition) or they have exsolved to pyrrhotite withminor pyrite (~5%) and chalcopyrite (~10%; Fig. 9F).

The blebs that are only partly encased in phenocrysts or arein the matrix are mainly pyrite with traces of chalcopyrite andspongy Fe oxides (as described by Keith et al., 1991; Cannan,1992; Larocque et al., 2000). Larocque et al. (2000) identified15 chemical and textural criteria for distinguishing spongy Feoxides formed by degassing of sulfide blebs from Fe oxidesformed by other processes. The spongy Fe oxides from Castro

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FIG. 4. Total alkalis vs. silica diagram showing IUGS classification of Bing-ham and Tintic samples normalized to 100 percent on a volatile-free basis.

FIG. 5. Detailed map of Castro Gulch sill, showing sample locations. Thiswell-exposed sill has one exceptionally well preserved glassy margin and waschosen to examine changes in sulfide abundances and elemental chemistryacross the sill.

FIG. 6. Variation in concentrations of S, Cu, Ni, Zn, Au, Pt, Pd, Rh, andIr across Castro Gulch sill, shown in Figure 5 (distance from top margin).Concentrations of chalcophile elements and total S are generally higher to-ward the margins of the sill.

Gulch fulfill all of these criteria, including globular forms,frothy, porous, or spongy textures, gradation from massivecores to porous, frothy rims, low concentrations of Ti, similarminor and trace element concentrations to coexisting, morepristine magmatic sulfides, and pyrite lamellae replacing theoriginal sulfide. The degassed blebs have been oxidized topyrite, and almost all have traces of chalcopyrite and orspongy Fe oxide rimming the pyrite. A trace of covellite isalso present in a few blebs. In some samples, flow layeringcurves around the degassed blebs.

Elemental compositions of pyrite in the matrix of one sam-ple, Castro-15b, are similar to hydrothermal pyrite from theore zone in the Bingham Cu deposit in that it has low or nochalcophile metals. Three out of 11 analyzed pyrites in thematrix have some traces of Cu (0.05 wt %) or Zn (0.03 wt %),suggesting that they may have been magmatic, although theyrecrystallized during cooling. All of the sulfides analyzed fromCastro Gulch samples, except those from Castro-15b, havetrace amounts of Ni, Co, Cu, Zn, As, and Ag (80% have >0.2cumulative wt % of these elements, Table 4, Fig. 10). CastroGulch sulfides also contain Au and Pt (Table 5). Sulfide blebsthat occur as inclusions in phenocrysts have the highest con-centrations of trace metals. The sulfides that occur in the ma-trix tend to have slightly lower concentrations of trace metalsbut still distinctly higher concentrations than in hydrothermalsulfides (Table 4). Average compositions of each phase wereused to calculate the initial composition of the bleb before ex-solution (Fig. 10, Table 4).

Spatial variations in Castro Gulch sulfides: Ten sampleswere taken across one well-exposed, unaltered sill to docu-ment the variations in sulfide concentrations (Fig. 4). Varia-tions in sulfide concentrations across the sill can be seen inTable 3 and Figure 11. Castro-15b is 25 cm from the top edgeof the sill and contains 0.21 vol percent sulfides and 0.03 per-cent partially degassed sulfides, whereas the center of the sill(Castro-14) contains 0.0008 percent sulfides, all of whichoccur as inclusions within silicates or magmatic oxides. Thebottom edge of the sill (Castro-1) contains 0.0218 vol percentsulfide and 0.104 percent partially degassed sulfides.

The sulfide phases that are present also vary across the sill.Samples near the margin contain pyrite + chalcopyrite + Feoxide (Castro-1, -10, -11, -15a, -15b, and -15c). Samplescloser to the center contain pyrite + Fe oxide (Castro 2), andsamples at the center contain no sulfide in the matrix (Castro-12, -13, and -14). All samples contain pyrite + chalcopyrite ±pyrrhotite ± Fe oxide as inclusions in phenocrysts.

Oxidation and degassing textures in sulfides also varyacross the sill. In the quenched margins of the sill (Castro-15b), the sulfides show little or no oxidation or degassing ef-fects other than the conversion of magmatic sulfides topyrite (Fig. 9A), which may happen simply due to equilibra-tion to lower temperatures in an open system, as discussedbelow. Grains near the margins of the sill (Castro-1), but notin the outermost sample, have preserved sulfide lamellaeand Fe oxide (Fig. 9B). Samples closer to the center of thesill contain partially oxidized and degassed sulfides with Feoxides along the edges (Fig. 9C-D). Samples near the cen-ter of the sill contain Fe oxides with a spongy texture (Fig.9E), but, as noted above, samples in the center do not con-tain remnant sulfides or spongy textured Fe oxides, except

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FIG. 7. Vol percent of sulfide, not including oxidized sulfides, in dikes, vol-canic rocks, and stocks from the Bingham and Tintic districts. Dike Margins(shown within ovals) have a significantly higher concentration of sulfides thandike interiors, volcanic rocks, and stocks. Concentrations were determined bypetrographic examination of grain size and number of blebs in a 5- × 28-mmarea (e.g., a vol % of 0.0001 would contain 44 blebs 1 µm in size, 17 blebs 2µm in size, and 3 blebs 5 µm in size in a 5- × 28-mm area, assuming sulfidesare all spherical, they are evenly distributed in three dimensions, and that thethin section cuts every bleb down the center of each sulfide).

FIG. 8. Cumulative number of sulfide blebs counted by petrographic ex-amination vs. bleb diameter, showing relative sulfide abundances for aquenched dike margin in Castro Gulch (Castro-15A) and a latitic volcanicrock (Tick-41) from the Bingham district. Blebs were measured in an area of5 × 28 mm. Both samples show a very uniform distribution of bleb size, withsmaller blebs more readily preserved than larger blebs.

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FIG. 9. Reflected light (RL) and transmitted light (TL) photomicrographs of sulfides in Castro Gulch sill. A. Round blebof pyrite in the matrix along the margin of the sill (Castro-15b) (combined RL and TL). B. Sulfide bleb on the other marginof the sill (Castro-1), with pyrite and Fe oxide lamellae and hematite and pyrite that has recrystallized from the bleb (RL). C.Two sulfide blebs near the margin of the sill (Castro-15b). The upper sulfide has exsolved to pyrite, pyrrhotite, and covelite.The curved outer boundaries indicate the original shapes and extent of the blebs (RL). D. Degassed sulfide bleb indicated bythe spongy texture of Fe oxide surrounding the remaining pyrite (Castro-15a; RL). E. Spongy Fe oxide near the center of thesill (Castro-11; RL). F. Sulfide inclusion in amphibole containing pyrrhotite, pyrite, and chalcopyrite (Castro-15a; RL). Ab-breviations: Amp = amphibole, Cov = covellite, Cp = Chalcopyrite, Hem = hematite, Mag = magnetite, Py = pyrite.

TABLE 4. Representative Microprobe Analyses of Sulfides from Castro Gulch

Inclusions1 Partly encased inclusions2 Matrix3

Castro Castro Castro Castro Castro Castro Castro Castro Castro Castro Castro CastroSample 1 29B 1 10 1 15B 10 10 1 15B 1 1

Mineral Po Po Py Cp Py Py Py Cp Py Py Cp Cp

S (wt %) 40.93 39.29 53.18 34.38 53.18 52.54 53.02 34.38 51.46 53.26 34.37 34.44Fe 56.24 58.16 45.25 30.85 45.25 45.70 45.53 30.85 46.39 45.74 29.50 29.75Co 0.07 <.02 0.10 <.02 0.10 0.06 <.02 <.02 <.02 <.02 <.02 <.02Ni 0.93 0.37 0.48 <.02 0.48 0.52 0.33 <.02 0.09 <.02 <.02 <.02Cu <.04 0.04 0.16 33.53 0.16 0.08 <.04 33.53 0.15 <.04 34.03 33.84Zn <.03 <.03 0.00 <.03 <.03 <.03 <.03 <.03 0.03 0.03 0.38 <.03As <.10 0.13 0.04 <.10 <.10 0.28 0.12 <.10 0.12 <.10 <.10 0.13Ag - 0.04 - 0.04 - - 0.07 0.04 - - - -Pb - - - - - - - - - - - -Total 98.17 98.02 99.21 98.79 99.16 99.18 99.07 98.79 98.24 99.03 98.27 98.16

Average Average Hydrothermal sulfidesinclusion matrix from Bingham Pit

Sample composition composition D393-1289 D393-1289 5090-400 5090-400

Mineral Po Py Cp bleb4 Py Cp Py Py Cp Cp

S (wt %) 39.31 51.16 34.38 39.41 51.99 34.37 53.57 54.27 35.14 35.13Fe 57.39 44.81 30.85 54.11 45.45 29.48 45.52 44.96 29.13 28.82Co 0.04 0.16 <.02 0.04 0.13 0.06 <.02 <.02 <.02 <.02Ni 0.81 0.7 <.02 0.72 0.46 0.13 0.04 <.02 <.02 <.02Cu <.04 0.12 33.53 3.38 <.04 32.66 <.04 <.04 34.20 34.12Zn <.03 <.03 <.03 <.03 <.03 0.12 <.03 <.03 <.03 <.03As <.10 0.15 <.10 <.10 0.13 <.10 <.10 <.10 <.10 <.10Ag <.04 <.04 0.04 <.04 <.04 <.04 0.05 0.05 <.04 <.04Pb - - - - - - - - - -Total 97.66 97.21 98.79 97.75 98.31 97.02 99.18 99.28 98.47 98.07

Abbreviations: Cp = Chalcopyrite, Po = Pyrrhotite, Py = Pyrite, - = not analyzed 1 Ovoid blebs completely encased by phenocryst2 Ovoid blebs partly encased by phenocryst3 Ovoid blebs in matrix4 Average bleb is 85 percent Po, 10 percent Cp, and 5 percent Py

where the original blebs were protected as inclusions in sil-icate or oxide minerals (Fig. 9F).

Partial resorption is readily apparent among crystalline sul-fides. The observed embayments and irregular rims aroundsulfides are similar to those on resorbed quartz and otherminerals (Fig 9E). Sulfides in the matrix near the edge of thesill tend to have more spherical shapes than those closer tothe center, which have been resorbed.

Little Valley

Four types of dikes occur in Little Valley (Fig. 12): an al-kali-poor and an alkali-rich shoshonite and an alkali-poor andan alkali-rich latite (Fig. 4).

The alkali-poor shoshonite dikes (with K2O + Na2O <6.4 wt%) contain approximately 12 percent plagioclase (An41–61), 12percent clinopyroxene (Wo46 En44 Fs10), 2 percent biotite(Phlog58 Sid18 Ann24), 2 percent oxide, a trace of orthopyrox-ene (Wo2 En66 Fs32), and 72 percent matrix (Table 6). Theyare poorer in P, Ba, K, Th, Zr, Cu, Pb, and Sr and richer in Cr,Ni, and Nb than the alkali-rich shoshonite dikes (Table 3).These dikes do not contain glass, and the matrix is composedof larger microlites than in other dike samples from the area,indicating slower cooling. The dikes contain small vesicles(1–5 mm) that are lined with limonite. The dikes contain onlytraces of magmatic sulfide.

The alkali-rich shoshonite dikes (with K2O + Na2O >7.1 wt%) have a texture and composition similar to the shoshonitelava flows on the west side of Little Valley. They contain ap-proximately 15 percent plagioclase (An40–59), 7 percentclinopyroxene (Wo45 En44 Fs11), 7 percent biotite (Phlog56

Sid21 Ann23), 2 percent oxide, trace orthopyroxene (Wo2 En70

Fs28), and 69 percent matrix (Table 6). In contrast to the al-kali-poor shoshonite dikes, these dikes have a glassy to veryfine grained matrix and higher concentrations of magmaticsulfides (0.002–0.003 vol %). Crumbly perlitic dike marginsprevented collection of the outermost margins and completedefinition of sulfide abundances across these dikes.

The alkali-poor latites (with K2O + Na2O <7.5 wt %) con-tain approximately 20 percent plagioclase (An34–51), 8 percentbiotite (Phlog58 Sid17 Ann24), 5 percent clinopyroxene (Wo45

En43 Fs12), 2 percent oxide, a trace of orthopyroxene (Wo2

En66 Fs31), and 65 percent matrix (Stavast, 2002). Thesedikes are poorer in Cu, Rb, Sr, Zr, Ce, Nd, Pb, and Th, andricher in Ni, Cr, and Nb than the alkali-rich latite dikes (Table3). They have a coarser grained matrix (with crystals about0.25 mm and no apparent glass) and low sulfide abundances.One sample (Tintic-1), however, has high concentrations of

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FIG. 10. Compositions of end-member sulfides (open squares), sulfideswithin blebs (diamonds), the average of sulfide blebs as inclusions in phe-nocrysts (open circle), and the average of sulfide blebs in the matrix (closedcircle) from Castro Gulch, plotted in a Cu-Fe-S diagram. Sulfide phaseswithin the blebs are pyrite, chalcopyrite, and sulfur-rich pyrrhotite.

TABLE 5. Au and Pt Analyses for Castro Gulch Sulfides

Sample no. Castro-1 Castro-11 Castro-15a Castro-15b

Location B A B D C A A B CMineral Py Py Cp Po Py Py Py Py PyAu (ppb) 179 131 156 110 53 295 352 388 18Total Pt (ppb) 69 97

Sample no. Castro-16 Castro-22a1

Location A C C D A B C C D E E EMineral Py Py Py Py Py Py Py Py Py Py Py PyAu (ppb) 122 42 64 96 56 71 97 90 188 27 25Total Pt (ppb) 83 97 125

Notes: Abbreviations: Cp = Chalcopyrite, Po = Pyrrhotite, Py = Pyrite; SIMS analyses on sulfide grains bigger than 50 µm

FIG. 11. Variations in the abundance of sulfide and total sulfide (includ-ing degassed sulfide with >20% spongy Fe oxides) across Castro Gulch sillfrom Figure 5 (distance from upper margin). Sample numbers are includedfor reference. Concentrations increase toward the margins of the sill.

sulfides (0.22 vol %) as grains of pyrite that occur along frac-tures and do not have spherical shapes.

The alkali-rich latite dikes (with K2O + Na2O >8.3 wt %)contain 10 percent plagioclase (An41-62), 10 percent biotite(Phlog59 Sid20 Ann21), 7 percent clinopyroxene (Wo47 En43

Fs10), 2 percent Fe-Ti oxide, and 71 percent matrix (Table 6).No orthopyroxene was present. The matrix is glassy or very

fine grained. These dikes are similar in chemical compositionand texture to the latite lava flows that cover most of LittleValley (Moore, 1993). They tend to weather more rapidlythan the dikes that have a coarser-grained matrix, which maybe due to perlitization and fracturing of glass. These dikescontain the highest abundances (up to 0.11 vol %) of mag-matic sulfides. Two dikes (Tintic-28 and -29, Fig. 12) weresampled to determine variations in chemistry across thedikes, and these data show higher concentrations of S, Cu, Ni,and Zn along the margins of the dikes (Table 3, Fig. 13).However, the concentration of sulfur is higher in the centerof dike Tintic-29 (Table 3, Fig. 13).

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TABLE 6. Mineralogy of Dikes from Little Valley

Rock type Alkali-poor Alkali-rich Alkali-poor Alkali-richshoshonite shoshonite latite latite

Plagioclase (vol %) 12 15 20 10Clinopyroxene 12 7 5 7Orthopyroxene tr tr trBiotite 2 7 8 10Fe oxides 2 2 2 2Fine-grained matrix 72 69 65 71

FIG. 12. Geologic map of Little Valley, Tintic district, showing the distri-bution of dikes and dikes sampled in this study.

FIG. 13. Sulfide and total sulfide (including degassed sulfide with >20%spongy Fe oxide), S, Cu, Ni, and Zn concentrations across dikes Tintic-28 andTintic-29 (distance from west margin). Sulfides and elements generally in-crease toward the margin of the dikes.

Sulfides from Tintic district and Little Valley: The dikesfrom Little Valley contain 0.0002 to 0.11 vol percent mag-matic sulfides, whereas volcanic rocks and a large ore-relatedintrusion (Silver City stock) contain 0.0001 to 0.0045 and0.0001 to 0.0006 vol percent, respectively (Fig. 7). The orderof magnitude difference cannot be an artifact of uncertaintiesin the modal analyses. A dike margin (Tintic-28c, Fig. 14)contains approximately 300 sulfide blebs between 1 and 5µm, 220 blebs between 5 and 20 µm, and 25 blebs that are 20µm or larger; a typical large intrusion (Silver City stock, lo-cated at UTM NAD27 zone 12 E412824, N4474208 in Fig. 1)contains 60 sulfide blebs between 1 and 5 µm and 5 blebs be-tween 5 and 10 µm (Fig. 14).

Immiscible sulfides in Little Valley dikes are present in thesame three textural sites noted for the Castro Gulch dikes andinclude variable proportions of pyrrhotite, pyrite, and chal-copyrite (Fig. 15). The sulfide inclusions completely enclosedby phenocrysts are pyrrhotite (or monosulfide solid solution)or pyrrhotite with pyrite and chalcopyrite. Blebs that are onlypartly encased in phenocrysts are composed of pyrite withchalcopyrite, with only some containing pyrrhotite. The sul-fide blebs in the matrix are either pyrite, pyrite with rimmingchalcopyrite, pyrrhotite with rimming pyrite, or rarelypyrrhotite (all of the blebs have spongy Fe oxide rims). Chal-copyrite, when present normally makes up 10 to 20 percent ofthe sulfide bleb in all bleb types. Traces of covellite are alsopresent in some of the blebs. The fracture-hosted hydrother-mal sulfides (in Tintic-1) are distinct from magmatic sulfidesin that they contain very low concentrations of trace metals(Table 7).

All of the magmatic sulfides analyzed from Little Valleyhad trace amounts (>0.3 cumulative wt %) of Ni, Co, Cu, Zn,As, and Ag (Table 7). Pyrrhotite is sulfur rich (Fig. 15). Sul-fides within blebs in the matrix have slightly lower concen-trations of these trace metals than sulfides that are sealed in

phenocrysts (Table 7). The distribution of trace elements inindividual sulfide phases is not homogeneous. Average com-positions of each phase were used to calculate the initial com-position of the bleb before exsolution (Fig. 15, Table 7).

Spatial variations in Little Valley sulfides: Two of the glassyalkali-rich latite dikes (Tintic-28 and -29, Fig. 12) were sam-pled at three points across their widths, at each margin and inthe center. They show an increase in sulfide abundance andmetal content toward the dike margins (Fig. 13). In Tintic-280.06 vol percent sulfide and 0.05 percent oxidized sulfide oc-curs along one margin and 0.01 percent sulfide and 0.07 per-cent oxidized sulfide occurs at the other, whereas 0.008 per-cent sulfide and 0.022 percent oxidized sulfide occurs at thecenter (Fig. 13). Sulfide concentrations along the two marginsof Tintic-29 are 0.02 vol percent sulfide and 0.07 percent ox-idized sulfide and 0.01 percent sulfide and 0.02 percent oxi-dized sulfide, respectively, whereas 0.005 percent sulfide and0.002 percent oxidized sulfide occurs at the center (Fig. 13).Depletions in sulfide abundances in dike cores are not asgreat as at Castro Gulch. This may reflect that the dikes aremuch narrower (1.5 and 2 m vs. 34 m) in Little Valley and/orthe inability to collect samples of the extreme margins due topoor preservation of perlitic glass.

Discussion

High concentrations of sulfides in dikes

The most significant observation of this study is the orderof magnitude difference in sulfide abundance betweenquenched dikes and more slowly cooled equivalent igneousrocks. This result was not unanticipated, based on the similarfindings of Moore and Schilling (1973) in a survey of sulfideabundances in midocean ridge basalts. They showed thatmagmatic sulfides are well preserved below a water depth of200 m and that a 2- to 3-cm-thick quenched rind on the pil-lows had the highest concentrations of sulfur and sulfides. Intheir interpretation, quenching under pressure inhibits volatileloss and magmatic sulfides are preserved. An implication

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FIG. 14. Cumulative number of sulfide blebs counted by petrographic ex-amination vs. bleb diameter, showing relative sulfide abundances and sizesfor a quenched dike margin in Little Valley, Tintic district (Tintic-28C), andin the Silver City monzonite (SCID). Blebs were measured in an area of 5 ×28 mm. A total of 586 blebs were present in Tintic-28c and only 64 blebs inSCID.

FIG. 15. Compositions of end-member sulfides (open squares), sulfideswithin blebs (diamonds), the average of sulfide blebs as inclusions in phe-nocrysts (open circle), and the average of sulfide blebs in the matrix (closedcircle) from Little Valley (Tintic district) plotted in a Cu-Fe-S diagram. Sul-fide phases within the blebs occur as pyrite, chalcopyrite, and sulfur-richpyrrhotite. These data confirm the empirical observation that blebs hosted asinclusions in phenocrysts are generally richer in pyrrhotite due to less exten-sive oxidation (see Figs. 16 and 17).

drawn from both studies is that most unquenched magmaticrocks rarely preserve a significant fraction of the magmaticsulfides that were present prior to eruption or intrusion. AtCastro Gulch and Little Valley, the outermost margins of thedikes (with a glassy to very fine grained groundmass) have thehighest concentrations of sulfides, analogous to the glassyrims of pillow basalts. Dikes that have a coarser groundmasshave lower concentrations of sulfides, similar to large intru-sions. Sulfide accumulation may occur in some instances andmay have occurred in the cumulate clast in the fragmentaldike in Castro Gulch that contains 1 vol percent sulfide.

Thus, the removal of magmatic sulfides from normal dikecores can be related to the slow cooling and loss of volatilesthat also occurs in large intrusions. All three of the dikes pro-filed in this study have lower concentrations of sulfides intheir centers. Furthermore, the larger sill from Castro Gulchhas lower concentrations (0.001 vol %) of sulfide in its centerthan the narrower dikes (0.01 vol %). A slower rate of coolingmay allow time for the removal of sulfides and the escape ofsulfur-rich fluids containing chalcophile elements. Based onthe comparison of magmatic sulfides in dike margins to largeintrusions, dike interiors, and volcanic rocks, 90 to 99 percent

of magmatic sulfides may be destroyed and removed from in-trusions and volcanic rocks in this manner.

Sulfide crystallization

The types, abundances, and compositions of magmatic sul-fides that eventually crystallize from high-temperature Fe-S-O immiscible liquids depend largely on the sulfur and oxygenfugacities of the system. Whitney (1984) has shown that inorder to achieve sulfide saturation in highly oxidized magmas,high sulfur fugacities are required. Reduced magma (1 logunit < NNO) crystallizes pyrrhotite and magnetite from themonosulfide solid solution. Oxidized magma (3 log units >NNO) crystallizes pyrite and an Fe oxide from monosulfidesolid solution. Pyrite would crystallize at about 600° to 700°C.The Bingham and Tintic dikes are examples of oxidized sys-tems. The oxygen fugacities of the Bingham and Tintic intru-sions were 2 to 4 log units > NNO (Kim, 1992; Tomlinson,1997). The fact that the magmas were sulfide saturated indi-cates that sulfur fugacities were also high. This is consistentwith the presence of 0.2 vol percent sulfide and 1,000 ppmwhole-rock sulfur (Castro-15b). A consequence of high sul-fur fugacities is the formation of S-rich monosulfide solid

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TABLE 7. Representative Microprobe Analyses of Sulfides from Little Valley

Partly encasedInclusions1 inclusions2 Matrix3

Sample Tintic Tintic Tintic Tintic Tintic Tintic Tintic Tintic Tintic Tintic Tintic Tintic12 14 2 12 12 2 12 12 12 2 12 12

Mineral Po Po Py Py Cp Py Py Cp Po Py Py Cp

S (wt %) 40.53 38.95 52.74 51.88 34.56 53.20 51.88 34.27 39.46 52.73 53.07 34.66Fe 57.24 58.38 44.84 40.42 30.16 45.92 40.42 30.14 57.80 46.13 45.87 30.37Co 0.04 <0.02 0.09 0.51 0.25 0.08 0.51 0.06 0.05 0.10 0.07 0.10Ni 0.13 0.19 0.10 1.21 0.13 0.08 1.21 0.05 0.09 0.05 0.25 0.06Cu 0.19 0.14 0.38 2.94 32.82 <0.04 2.94 32.74 0.32 0.04 0.08 32.62Zn <0.03 <0.03 0.06 <0.03 0.09 <0.03 <0.03 0.13 <0.03 <0.03 <0.03 0.14As 0.08 0.12 <0.10 0.13 0.14 0.11 0.13 0.14 0.11 0.11 <0.10 0.11Ag <0.04 <0.04 <0.04 0.05 0.07 <0.04 0.05 0.07 <0.04 <0.04 <0.04 0.05Pb - - - - - - - - - - - -Total 98.22 97.79 98.21 97.13 98.20 99.39 97.13 97.59 97.83 99.15 99.34 98.10

Average Average Hydrothermal sulfidesinclusion matrix from Tintic district

Sample composition composition Tintic Tintic1 1

Mineral Po Py Cp bleb4 Po Py Cp 4bleb Py Py

S (wt %) 39.07 51.07 34.56 38.99 40.07 51.43 34.18 40.62 53.5 53.69Fe 57.23 44.33 30.32 52.55 55.88 44.62 28.6 52.03 45.91 45.97Co 0.02 0.23 0.11 0.04 0.06 0.12 0.81 0.14 <0.02 <0.02Ni 0.21 0.55 0.16 0.22 0.12 0.20 0.27 0.14 <0.02 <0.02Cu 0.25 0.37 31.43 4.93 0.61 0.25 32.1 3.72 <0.04 <0.04Zn <0.03 <0.03 0.03 <0.03 <0.03 <0.03 0.06 <0.03 <0.03 <0.03As <0.10 0.62 0.11 <0.10 0.11 0.52 0.12 0.15 0.10 0.10Ag <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04Pb - - - - - - - - - -Total 96.90 97.20 96.75 96.89 96.87 97.17 96.16 96.83 99.52 99.75

Abbreviations: Cp = Chalcopyrite, Po = Pyrrhotite, Py = Pyrite, - = not analyzed1 Ovoid blebs completely encased by phenocryst2 Ovoid blebs partly encased by phenocryst3 Ovoid blebs in matrix4 A verage bleb is 85 percent Po, 10 percent Cp, and 5 percent Py

solution (Whitney, 1984) that reequilibrates to S-richpyrrhotite, intermediate solid solution), pyrite, and Fe oxide,all of which are found in the Bingham and Tintic systems(Figs. 10, 15).

Work by Davis et al. (1991) and Nilsson and Peach (1993)indicated that intermediate magmas normally have low sulfurconcentrations (<500 ppm). However, the latite magmas atBingham have high sulfur concentrations (1,000 ppm) as a re-sult of magma mixing or volatile transfer from underplatedmafic magmas (Keith et al., 1997; Waite et al., 1997; Hattoriand Keith, 2001; Maughan, 2001).

Competing processes during dike emplacement

During dike emplacement, four processes may simultane-ously affect the immiscible sulfide blebs: crystallization of sul-fide melt, resorption (accompanying degassing of magma),exsolution during cooling, and oxidation and/or degassing.For those sulfide blebs that were not resorbed before the sil-icate melt quenched, a common sequence is noted that in-volves crystallization, exsolution, and reequilibration (Fig.16). Our observations suggest that immiscible monosulfidesolid solution liquid (Fig. 16A) crystallized initially to formpyrrhotite or pyrrhotite + intermediate solid solution + pyrite

(Fig. 16B). These phases then exsolved to pyrrhotite + chal-copyrite + pyrite (seen in inclusions and in some matrix sul-fide from Little Valley) as they cool (Fig. 16C). This sequenceis predicted in the phase diagram of Barton and Skinner(1979; Fig. 17).

Oxidation of sulfides occurs in several steps but may occurbefore or after exsolution of discrete sulfide phases (Fig. 16).Iron in pyrrhotite is oxidized to Fe oxide as S and some chal-cophile metals escape. Some S may combine with the re-maining pyrrhotite to form pyrite (Fig. 16D-E). When oxida-tion effects are more severe, pyrite may then be oxidized toFe oxide, releasing additional sulfur and chalcophile metals tothe magmatic fluid (Fig. 16E-F). Chalcopyrite is often the lastmajor sulfide component to be removed by oxidation (Fig16F-G). Sulfides in the matrix or those hosted by fracturedphenocrysts are the most vulnerable to oxidation.

Resorption of crystallized sulfides is evidenced by the com-mon embayments and irregular rims in the sulfides (Figs. 9H,16E, H). As noted, sulfides near the edge of the dikes tend tohave more spherical shapes than those nearer the dike inte-rior. The dramatic decrease in the number of sulfides and lackof spongy textured Fe oxides toward the center of the dikessuggests that most resorption of sulfides occurred while sili-cate melt was present and sulfides were immiscible melts(Fig. 16I). Resorption appears to be the major process for re-moval of sulfide melt. Destabilization of sulfides could havebeen caused by decompression and removal of sulfurousgases from the melt during dike emplacement and open-sys-tem behavior.

Whole-rock composition and metal ratios

The majority of chalcophile elements are hosted by sulfides(Candela and Holland, 1986; Halter et al., 2002). If sulfidesare resorbed, oxidized, and removed by an aqueous fluid inresponse to open-system behavior, then whole-rock S, Cu,Zn, Ni, and PGE should dramatically decrease as well. This isseen in the samples from Castro Gulch and Little Valley. In

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FIG. 16. Schematic drawing illustrating one possible sequence of crystal-istallization (A-C) and destruction of sulfides (D-G) by resorption and oxida-tion. The immiscible monosulfide solid solution (Mss) segregates from thesilicate melt (A). It crystallizes as pyrrhotite (po), pyrite (py), and intermedi-ate solid solution (Iss) (B). Iss crystallizes to chalcopyrite (cp) and pyrrhotite(po) (C). Some resorption of the bleb back into the melt also occurs at thisstage. Pyrrhotite is subsequently oxidized to pyrite and Fe oxide (D). Re-sorption continues and pyrite is oxidized to Fe oxide (E). Pyrite and chal-copyrite are oxidized to Fe oxide (F), leaving spongy Fe oxide remnants (G).Fe oxide also is resorbed (H). If the sulfide bleb is completely resorbed, notrace of sulfide or Fe oxide is present (I). Complete resorption of sulfide intomelt could occur at any point along the sequence and is not necessarily thelast stage.

FIG. 17. Plot of logƒS2 vs. temperature for the system Cu-Fe-S-O (afterBarton and Skinner, 1979), showing the approximate path of crystallization ofthe sulfides in Castro Gulch and Little Valley (arrow).

Castro Gulch, sulfides, S, Cu, Ni, Zn, Au, Pt, Pd, Rh and Irgenerally increase toward the margins of the sill (Figs. 6, 11),and in Little Valley the abundance of sulfides, Cu, Ni, and Znincrease toward the margins of the dikes (Fig. 13) with lessclear patterns for the other chalcophile elements. The loss ofmetals from the center of the dikes and sill may be an indica-tion of the proportions of metals that could be lost by largerintrusions and incorporated into an ore deposit.

The loss of S, Cu, Au, and Zn from the dikes can be esti-mated by comparing whole-rock concentrations at the mar-gins with concentrations near the center of the dike. For theCastro Gulch sill, the losses are 1,000 ppm S, 15 ppm Cu, 0.8ppb Au, 8 ppm Zn, and 3 ppm Pb if the highest concentrationfrom either margin is used as the original concentration. Moconcentrations are near detection limits, so an assumptionwas made that half of the Mo (0.5 ppm) was lost. The ratiosof metals removed from the centers of Castro Gulch dikes es-timated in this way are comparable to the metal ratios of thetotal production and reserves of the entire Bingham district(Krahulec, 1997; Fig. 18). An exception is zinc, which may below in the production and reserve estimates because it mainlyoccurs outside of the high-grade ore zone or it has been re-moved by erosion.

ConclusionsVery few samples of magmatic rocks from the Bingham and

Tintic areas contain high concentrations of magmatic sulfides.These data suggest that the original endowment of magmaticsulfides is rarely preserved in intrusions or volcanic rocks dueto the escape of sulfurous gasses and the destruction of thesulfides by magmatic fluids. However, quenched dike mar-gins may provide clues as to the original metal and sulfur con-tent of the magma. Original magmatic sulfide concentrationsin the latitic magma at Castro Gulch, Bingham district, wereat least 0.12 vol percent and could have been as high as 0.24percent. Original magmatic sulfide concentrations in latitic toshoshonitic dikes in Little Valley in the Tintic district were atleast 0.11 vol percent. Based on a comparison of magmatic

sulfides in dike margins to large intrusions, dike interiors, andvolcanic rocks, 90 to 99 percent of magmatic sulfides maycommonly be destroyed and removed from intrusions andvolcanic rocks. This occurs by a combination of resorption(during degassing of magma), oxidation, and/or interactionwith a hydrothermal fluid. Destruction of magmatic sulfidesand incorporation of base metals from these sulfides into hy-drothermal fluids in the Bingham and Tintic magmas mayhave been a key process in producing the ore deposits in eachdistrict. This hypothesis is supported by the similarity of met-als lost from the sill in Castro Gulch and the ratios of metalsin the entire Bingham deposit.

AcknowledgmentsWe would like to acknowledge Kennecott Copper Inc. for

their cooperation in the research done on their property. Wewould like to thank Eric Seedorff, Werner Halter, and SteveKesler for reviewing the manuscript. This project was fundedby National Science Foundation grant EAR-97-25728.

February 19, 2003; February 14, 2006

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