VOLATILITY OF METAL(LOIDS) AND THE GEOGRAPHIC-GEOCHEMICAL ZONATION OF TERTIARY EPITHERMAL/EPIZONAL ORES IN WESTERN USA

Subduction, partial melting of mantle and magma generation, and ultimately magma “degassing” can lead to the preferential mobilization of volatile elements and compounds. Recent research indicates that devolatilization reactions in a subducted slab can cause the overlying wedge of lithospheric mantle to be enriched in volatile ore-forming constituents (water, CO₂, sulfur, metal(loids)). This process of enriching the lithospheric mantle plays an important role in subsequent magma generation and initiation of orogenesis in the wedge. The relative volatility of metals and metalloids appears to play a role in the ultimate geochemistry of shallow (epithermal/epizonal) hydrothermal ores in the western USA. For example, there is a general west-to-east geographic zonation of Tertiary ores, going from Hg-rich, to Se-rich, and ultimately Te-rich. This trend parallels increasing depth to the subducted Farallon plate during the Laramide Orogeny. We propose that this geographic-geochemical zonation is the direct result of progressive volatility of Hg->Se->Te. The volatility of gold, silver, and copper (and As, Sb, Tl, Pb) generally overlap with Se and Te. Thus, these shallow hydrothermal ores typically have been exploited for their valuable Au, and Ag, and Hg. Previously, these elements (Au, Ag, Te, Se, Hg, As, Sb, and Tl) have often been referred to as the “epithermal suite” of elements. We suggest that their geochemical cycles are linked by their similar volatile behavior, through subduction, partial melting in mantle, and subsequent magma degassing processes. Essentially, specific teconomagmatic events that affected the western USA (e.g. Oligocene-Eocene magmatism in the Great Basin and Southern Rocky Mountains, mid-Miocene magmatism associated with the inception of the Yellowstone hotspot in the Pacific Northwest, magmatism associated with the San Andreas fault and transform boundary, etc.) were distillation events that drove devolatilization of fertile lithospheric mantle after the Laramide. Furthermore, the presence of many very young ores (<~7 Ma) in the western USA suggests that the underlying mantle is still “fertile” and future devolatilization and ore-deposit formation is likely.