New isotopic evidence bearing on bonanza AuAg e

Authors: James A Saunders Ryan Mathur George D Kamenov Toru Shimizu Matthew E Brueseke

Publish Date: 2015/11/18

Volume: 51, Issue: 1, Pages: 1-11

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Abstract

New Cu S and Pb isotope data provide evidence for a magmatic source of metalloids and sulfur in epithermal AuAg deposits even though their oreforming solutions are composed primarily of heated meteoric ground waters The apparent isotopic discrepancy between ore metals and oreforming solutions and even between the ore and associated gangue minerals indicates two different sources of epithermal oreforming constituents 1 a shallow geothermal system that not only provides the bulk of water for the oreforming solutions but also major chemical constituents leached from host rocks silica aluminum potassium sodium calcium to make gangue minerals and 2 metals and metalloids As Te Sb etc and sulfur ±Se derived from deeper magma bodies Isotopic data are consistent with either vaporphase transport of metalloids and sulfur and their subsequent absorption by shallow geothermal waters or formation of metallic Au Ag Cu phases nanoparticles at depth from magmatic fluids prior to encountering the geothermal system The latter is most consistent with ore textures that indicate physical transport and aggregation of nanoparticles were significant oreforming processes The recognition that epithermal AuAg ores form in tectonic settings that produce magmas capable of releasing metalrich fluids necessary to form these deposits can refine exploration strategies that previously often have focused on locating fossil geothermal systemsFunding for this research was provided by grants to JAS EAR 0838208 EAR1247857 and MEB EAR0838139 from the NSF Erin Summerlin and Mike Mason conducted the S isotope work at the University of Georgia with the assistance of Doug Crowe at UGA We also thank Michelle Burke of Miami University for the SEM of a fractal dendrite from the Sleeper deposit

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