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.
We describe the time-space evolution of a segment of the Laramide arc in east-central Arizona that is associated with porphyry copper mineralization, as constrained by U-Pb zircon geochronology conducted by laser ablation–multicollector–inductively coupled plasma–mass spectrometry. Mid-Cenozoic normal faulting dismembered and tilted many of the plutons and the associated porphyry copper deposits and produced a wide range in depths of exposure. The study area reconstructs to a 75-km-long slice along the arc, with exposures from <1 to >10 km depth. The copper deposits are related to granodioritic to granitic plutons that exhibit variable magmatic sources and locally severe degrees of zircon inheritance. U-Pb zircon ages of plutons in the study area range from 75 to 61 Ma, with dioritic rocks at the older end of the range. The age range of magmatism and mineralization in a cluster of deposits near the Schultze Granite, including the Globe-Miami, Pinto Valley, and Resolution deposits, is from ca. 69–61 Ma. To the south in the Tortilla and Dripping Spring Mountains, the porphyry systems range from ca. 74 Ma at Kelvin-Riverside to ca. 69 Ma at Ray and ca. 65 Ma at Christmas. At several localities where geologic constraints exist, mineralizing plutons were emplaced following Laramide shortening. The ages of the inherited zircon cores correspond fairly closely to the ages of basement rocks in the immediate vicinity of sample sites, implying that similar basement ages and lithologies contributed to the source areas of magmas that produced Laramide porphyry deposits. The U-Pb results on hypabyssal rocks are typically 1–5 m.y. older than previous K-Ar ages, and U-Pb ages on more deeply emplaced plutonic rocks are as much as 5–10 m.y. older. These results are consistent with predictions from thermal modeling and suggest that temporal evolution of the entire Laramide arc needs revision. For this segment of the arc, magmatism was stagnant for ~15 m.y., with minimal migration over time and mineralization occurring episodically over most of that lifespan. There is no simple geographic progression in ages along or across the strike of the arc. Thus, it is difficult to call upon time-specific far-field or plate margin triggers for magmatism or mineralization. The intrusive flux of the Laramide arc appears to be similar to that of the Sierra Nevada arc during the Mesozoic during its “background” periods, rather than during episodes of flare-up. The wide compositional diversity of the Laramide arc is more akin to northeastern Nevada during the onset of extension in the mid-Cenozoic than to the Mesozoic of the Sierra Nevada.
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