A TALE OF TWO ARC MAGMAS, AND TWO METALLOGENIC PATHS? (Invited Presentation)

Porphyry copper and related deposits are the world’s primary sources of Cu and Mo, and major sources of Au, Ag, Pb, and Zn. These well-studied magmatic-hydrothermal deposits result from oxidized arc magmas, which promoted high contents of sulfur and copper through concomitant increases in their solubilities. Not all continental magmatic arcs are characterized by porphyry-type mineralization, however, and some enigmatic arcs contain Au-dominant, Cu-poor, reduced intrusion-related deposits that sharply contrast from a classic porphyry Cu metallogeny. We examine this contrast in metals from subduction-related arcs in continental settings by focusing on the Eocene arc in the Great Basin, U.S. The east-west Eocene arc, which orthogonally cuts the structural and lithofacies grain of the U.S. Cordillera, generated major porphyry Cu systems in a more inland location in Utah and Earth’s second largest concentration of gold in adjacent Nevada, allowing a unique opportunity to understand metallogenic variability within a single arc. Redox-sensitive parameters in zircon and apatite from Eocene intrusions associated with ores in major districts along the arc reveal that arc magmas in Utah were more oxidized by 0.5 to 3 log units fO₂ (FMQ buffer) and had >3X higher sulfur in apatite (e.g., fO₂ mostly log +0.5 to 2.5; avg. 0.24% SO₃) than those in Nevada (-1 to +1 log fO₂; avg. <0.07% SO₃). This resulted in porphyry copper deposits including supergiant Bingham Canyon that have relatively low Au/Cu of ≤0.6 (where Au/Cu = Au ppm x 10⁴/Cu ppm) in Utah and Au skarns and distal disseminated deposits, and low-T Carlin-type Au deposits in Nevada with high Au/Cu ≥7. Evolved magmas in Nevada were likely reduced through assimilation of Proterozoic to Paleozoic basin and slope sedimentary strata of the Cordilleran passive margin, largely absent in the more easterly platform lithofacies setting of Utah. Reduction of magmas in Nevada increased S^2-, sharply increasing both gold solubility and the Au/Cu ratio in magmatically exsolved fluids through to low-T fluids typical of Carlin-type systems. Thus, carbonaceous crust reduced initially oxidized arc magmas, leading to a gold-rich, reduced metallogeny. A reduced path for arc magmatism provides a unifying link between porphyry copper, intrusion-related gold, and Carlin-type gold deposits.