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The Bingham porphyry Cu-Au-Mo deposit, Utah, may only be world-class because of substantial contributions of sulfur and metals from mafic alkaline magma to an otherwise unremarkable calc-alkaline system. Volcanic mafic alkaline rocks in the district are enriched in Cr, Ni, and Ba as well as Cu, Au, platinum group elements (PGE), and S. The bulk of the volcanic section that is co-magmatic with ore-related porphyries is dacitic to trachytic in composition, but has inherited the geochemical signature of high Cr, Ni, and Ba from magma mixing with the mafic alkaline rocks. The volcanic section that most closely correlates in time with ore-related porphyries is very heterogeneous containing clasts of scoriaceous latite, latitic, and minette, and flows of melanephelinite, shoshonite, and olivine latite in addition to volumetrically dominant dacite/trachyte. Bingham ore-related porphyries show ample evidence of prior mixing with mafic alkaline magmas. Intrusive porphyries that have not been previously well-studied have several chemical and mineralogical indications of magma mixing. These ''mixed'' lithologies include the hybrid quartz monzonite porphyry, biotite porphyry, and minette dikes. Even some of the more silicic latite and monzonite porphyries retain high Cr and Ba contents indicative of mixing and contain trace amounts of sapphire (<1 mm). The heterogeneous block and ash flow deposits also contain sapphire and are permissively correlated with the intrusions based on chemical, mineralogical, and isotopic data. Magma mixing calculations suggest about 10% of the monzonitic/latitic orerelated magma may have been derived from mafic alkaline magma similar to the melanephelinite. If the original S content of the mafic magma was about 2,000-4,000 ppm, comparable with similar magmas, then the mafic magma may have been responsible for contributing more than half of the S and a significant portion of the Cu, Au, and PGE in the Bingham deposit.

Magmatic sulfides are generally accepted as forming by segregation of an immiscible sulfide liquid from a host silicate melt. Immiscible sulfides have been observed in many types of igneous rocks; however, some types of plutonic and volcanic rocks lack sulfides. We have examined a suite of samples from Mount Pinatubo (Philippines), Volcán Popocatépetl (Mexico), Satsuma-Iwojima (Japan) and Mount St. Helens, Bingham Canyon, Tintic District, and Clear Lake (U.S.A.). The samples reflect a range of crystallization histories and compositions; they range from rhyolite to basalt to trachyandesite, with f(O 2) at the time of eruption ranging from below the fayalite-magnetite + quartz (FMQ) buffer to well above the nickel-nickel oxide (NNO) buffer. Textural and chemical evidence from our suite of samples indicate that sulfides initially were present, but were modified prior to complete cooling of the parent melt, giving rise to Fe-oxide globules. The globules formed through: (1) segregation of an immiscible Fe-SO melt, and possibly, further separation of immiscible Fe-S and Fe-O liquids, and (2) undersaturation with respect to sulfide, causing removal of S from the immiscible sulfide melt. Sulfide undersaturation may have been caused by magma degassing (passively or during eruption), or magma mixing. The recognition of modified magmatic sulfides is important because, with extensive degassing, base and precious metals (e.g., Cu, Au) could be stripped from a melt by a S-rich magmatic volatile phase and entrained into a magmatic-hydrothermal fluid, ultimately giving rise to porphyry-type or related mineralization. For a melt containing 0.01 modal % magmatic sulfides, efficient degassing of only 10 km 3 of magma could yield enough Cu to form a giant deposit.

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Magmatic 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 sulfides during intrusion and eruption. Our findings show that shallowly emplaced dikes and sills have erratic but locally high concentrations of sulfides. Volcanic rocks and large porphyry intrusions from these districts typically have at least two orders of magnitude fewer sulfides than the dikes. Sulfide concentrations vary dramatically across these dikes and sills; for example, in one sill in Castro Gulch, Bingham district, sulfide abundance increases from 9 ppm by volume in the center to more than 2,000 ppm near the margin. Chalcophile metals show corresponding changes in abundance. For example, the whole-rock copper content of the sill ranges from 23 ppm in the center to 35 ppm along the margins. The textures of sulfide grains (interpreted to reflect recrystallization, resorption, and degassing) even in the most sulfide-rich samples, commonly have been modified, suggesting that no sample preserves all of its original magmatic sulfide content. Immiscible liquids of monosulfide solid solution crystallized as pyrrhotite, pyrrhotite and chalcopyrite, or pyrite and chalcopyrite with declining temperature and pressure. These locally recrystallized to pyrite and chalcopyrite or to pyrite and an Fe oxide as they are oxidized. The alteration and preservation textures change from subspherical sulfide blebs near the margins of dikes and sills, to partially altered sulfides farther in, to complete absence of sulfides in the vast majority of intrusions (except where small sulfides are completely enclosed by phenocrysts). Sulfide concentrations appear to vary according to cooling rate and inferred pressure at the time of quenching or crystallization of the matrix. Most of the sulfides along the quenched margins of these dikes and sills are in the matrix. Slower cooling coupled with removal of magmatic volatiles, including sulfurous gases (e.g., H2S, SO2), allows 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 probably explains the low-magmatic sulfide abundances of slowly cooled, large porphyritic intrusions, and most importantly, allows metals and sulfur to participate in the formation of porphyry deposits. The relative abundances of base metals lost from the center of the sill are similar to the relative abundances of the metals in the Bingham deposit (production and reserves), suggesting that these processes also may have operated at a larger scale.

Widespread base-and precious-metal anomalies, oxidized sulfi de veins, silicifi ed calcareous shales and carbonates, and altered porphyry intrusions occur in the northeastern Sulphur Spring Range, Nevada, 80 km south of important gold deposits in the Carlin trend. The small historic mines and prospects in the area are spatially and perhaps genetically related to a suite of variably altered dikes, small lava fl ows, silicic domes, and related pyroclastic rocks. New major-and trace-element data and U-Pb zircon ages show that the East Sulphur Spring volcanic suite is Eo-Oligocene in age (36-31 Ma) and ranges in composition from high MgO-basaltic andesite to peraluminous rhyolite. The major-and trace-element compositions of the volcanic rocks are characteristic of continental margin subduction zone magmas and form a high-K, calc-alkaline suite with low Fe/Mg ratios. In addition, the rocks have negative Nb and Ti anomalies and elevated Ba, K, and Pb on normalized trace-element diagrams. Crustal melting is indicated by the eruption of a peraluminous garnet-bearing ignimbrite and as a component in hybridized andesite.The nature of this suite and its potential for mineralization is elucidated via comparisons to other Eocene age volcanic rocks associated with much larger gold and copper deposits in the Great Basin. The East Sulphur Spring suite is more similar to Eocene igneous rocks found along and near the Carlin trend than it is to those erupted while the Bingham porphyry copper deposit developed 300 km farther to the east. For example, the East Sulphur Spring suite and the Eocene magmatic rocks along the Carlin trend are less alkaline than the Bingham suite and lack its unusual enrichment of Cr, Ni, and Ba in intermediate composition rocks (58-68 wt% silica). Nonetheless, the Bingham and East Sulphur Spring volcanic suites both preserve evidence of mixing that created intermediate compositions. For example, an andesite has obvious mineral disequilibrium with plagioclase, biotite, clinopyroxene, orthopyroxene, olivine, and amphibole coexisting with extensively resorbed megacrysts of quartz, K-feldspar, and garnet-indicative of mixing basaltic andesite or andesite and largely crystallized garnet-bearing rhyolite. On the other hand, we found no evidence for mixing with a mafi c alkaline magma like that in the Bingham Canyon magma-ore system. We conclude that: (1) an unusual tectonic setting prevailed during the Eocene and Oligocene of the western United States that promoted the production of oxidized mafi c magma in an arclike setting, but far inland as a result of the rollback of the Farallon slab;(2) the mafi c magmas intruded or erupted separately, or mixed with more silicic magma generated by fractional crystallization and assimilation of crustal materials; and (3) these mafi c magmas may have delivered signifi cant amounts of sulfur and chalcophile metals to upper crustal magma chambers and eventually to Paleogene ore deposits in the eastern Great Basin.

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