SULFUR ISOTOPES IN SURFACE AND GROUNDWATER SULFATE AS AN EXPLORATION TOOL FOR EPIZONAL GOLD DEPOSITS IN THE KUSKOKWIM BASIN, SW ALASKA

The recently defined, ca. 70 Ma Donlin Creek gold deposit is low-grade, large-tonnage resource located 450 km west-northwest of Anchorage in the Kuskokwim Mountains of southwestern Alaska. The country rock consists of interbedded Late Cretaceous greywacke and shale, and the gold-bearing veinlet systems are hosted within a 8 km x 3 km swarm of mineralized discordant dikes and sills emplaced along NE-trending strike-slip faults. Gold-related arsenopyrite, pyrite, and stibnite at the Donlin Creek deposit are mainly characterized by low d34S values that range from -16 to -10 per mil, although some sulfides are low as -27 per mil. The systematically low values likely reflect sulfur derivation from diagenetic pyrite and organic matter within the basinal sediments themselves. Seeps, streams, and groundwaters surrounding the deposit were sampled to evaluate d34S signatures of sulfate derived from sulfide weathering as a potential regional exploration tool and for baseline environmental studies. Our results show that ground and surface waters that have interacted with mineralized bedrock within the Lewis, Rochelieu, Queen, and Dome prospects, which lie along the same NE-trending zone of mineralization, have d34S values of -15 to -6 per mil, obviously reflecting weathering of hydrothermal metallic sulfide minerals. The majority of samples collected outside of the mineralized area have less depleted values ranging from -3 to +1 per mil, which suggests interaction with non-mineralized intrusions and/or greywacke and shale. However, two sampled seeps at relatively high elevation located 2 to 3 km ESE of the known mineralization have d34S values of -8 per mil, suggesting favorability for the discovery of additional new gold resources.