Abstract

Subduction, partial melting of mantle and magma generation, and ultimately magma degassing can lead to the preferential mobilization of volatile elements and compounds. Research indicates that devolatilization reactions in a subducted slab can cause the overlying mantle to be enriched in volatile ore-forming constituents (water, CO2, sulfur, metal[loid]s). This process of mantle enrichment plays an important role in subsequent magma generation and initiation of ore genesis. Furthermore, the relative volatility of metal(loid)s appears to play a role in the ultimate geochemistry of shallow (epithermal-epizonal) hydrothermal ores in the western United States. For example, there is a general west-to-east geographic zonation of Tertiary ores, from Se rich to Te rich. It is proposed that this geographic-geochemical zonation is the direct result of the higher geochemical volatility of Se as compared to Te. Previously, Au, Ag, Te, Se, Hg, As, Sb, and Tl have been referred to as the epithermal suite of elements. It is suggested here that their geochemical cycles are linked by their similar volatile behavior, through subduction, partial melting in the mantle, and subsequent magma degassing processes. Essentially, specific tectonomagmatic events that affected the western United States (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) were distillation events that drove devolatilization of fertile mantle after the Laramide shallow-slab event. Furthermore, the metal(loid) volatility behavior likely occurs in other regions affected by subduction and may play a fundamental role in the metal endowment of these regions.

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