VOLATILITY OF Se AND Te DURING SUBDUCTION-RELATED DISTILLATION AND THE GEOCHEAHSTRY OF EPITHERMAL ORES OF THE WESTERN UNITED STATES

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 si ibdiicted slab can cause die overlying mantle to be enriched in volatile ore-forming constituents (water, CO2, sulfur, inetailloidls). This process of mantle enrichment plays an important role in subsequemt magma generation and initiation of ore genesis. Furthermore, the reatve volatility 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 pi-oposed that this geogniphic-geochemical zoiNition is the direct result of the higher geochemical vokitility of Se as compared to 'Fe. Previously, Ali, Ag, Te, Sc. rig, As St), Old TI has Ne 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 eVelits that affected the western United States (e.g., Oligocene-Eocene magmatism in the Great Basin and southern Rocky Mountains, mid-Miocene diagmatisin associated with the inception of the Yellowstone hotspot in the Pacific Northwest) were distillation events that drove devolatilization of fertile mantle after the Laramide shallw-slab event. Furthermore, the inetal(loid) volatility behavior likely occurs tm other regions affected by slibdiiction and may play a lUndamental role in the metal endowment of these regions.