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.

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