Native Metals and Alloys in Trachytes and Shoshonite from the Continental United States and High-K Dacite from the Bolivian Andes: Magmatic Origins of Ore Metals in Convergent and Within-Plate Tectonic Settings

摘要

Nb rich trachytes from Virginia, shoshonite from Colorado and high-K dacite from the Bolivian Andes contain diverse suites of native metals and alloys. Virginia trachytes are differentiates of mafic withinplate magmas derived from oceanic island basalt (OIB)-type mantle sources in the post-orogenic extensional regime of the Appalachian mobile belt. Colorado shoshonite and Bolivian dacite were produced via crystal fractionation of Fe-Mg silicates from trachybasalt and basalt magma, respectively, derived from subductionrelated mantle modified by either crustal contamination (both regions are underlain by thick, mature continental-type crust) or recycling of marine sediment. Metal assemblages include refractory metals (Pt with minor Fe and Cr, native W, Co-W and Fe-W alloys), Cu-Ag-Au alloys, chalcophile metals (Cu-Sn alloy, native Cu and Zn) and associated sulfides (galena). Textural evidence (metals occur as inclusions in phenocrysts, high temperatures of their formation, etc.) and compositional features of metallic phases suggest their magmatic origin. Metal-rich magmas (both mafic and differentiated) transport metals in convergent and within-plate tectonic settings and deliver ore metals to the oxidized porphyry and epithermal environments in the upper crust.