VARIATIONS IN MAGMATIC SULFIDES IN DIKES, LARGE INTRUSIONS, AND VOLCANIC ROCKS, BINGHAM AND TINTIC DISTRICTS AND WASATCH PLATEAU, UTAH

The behavior of magmatic sulfides during emplacement, cooling and eruption is an important issue that has received little prior attention. To help better understand this, sulfides in 300 samples of volcanic and intrusive rocks from the Tertiary Bingham (Cu-Au-Mo) and Tintic (Ag-Pb-Zn-Cu-Au) districts and the Wasatch Plateau, Utah have been examined. Shallowly emplaced dikes have erratic, but locally high, concentrations of sulfides. Sulfide concentrations appear to vary according to cooling rate and inferred pressure at the time of crystallization. Fast inferred cooling rates (based on the abundance of glass in the sample) appear to be needed to preserve most of the original endowment of magmatic sulfides. Coeval volcanic rocks of the same composition typically have at least two orders of magnitude less sulfides than the unusual quenched portions of dikes. This may be related to a pressure dependence for sulfide preservation. Sulfide concentrations also vary across dikes; for example, sulfides in one dike in the Bingham district increase from the center (9 ppm) to the quenched margin (up to 2000 ppm). Whole rock copper concentrations also show a similar trend; one dike ranges from 23 ppm in the center to 35 ppm along the margins. Sulfide oxidation textures are very common in even the most sulfide-rich samples suggesting that no sample preserves all of its original magmatic sulfide content. The oxidation and resorption textures change toward the center of the dike from whole sulfides to partially removed sulfides to no remnant of the original sulfides, except where protected by phenocrysts. The majority of the sulfides along the quenched margins are in the matrix. There are no major compositional differences between the same type of sulfides that are inclusions and those in the matrix; for example, pyrite contains on average 1-2% minor metals (Cu, Ni, Co, As, As) in sulfide inclusions and sulfides in the matrix. Monosulfide solid solution crystallized as pyrrhotite, pyrrhotite and chalcopyrite, or pyrite and chalcopyrite. These can recrystallize to pyrite and chalcopyrite or to pyrite and an Fe-oxide. A similar removal process may explain the low magmatic sulfide abundances (1-15 ppm) in more slowly cooled large porphyritic intrusions. Thus, magmas may have higher sulfide abundances than is often apparent.