THE FIRST THREE-DIMENSIONAL PETROGRAPHIC, GEOCHRONOLOGICAL, AND GEOCHEMICAL CHARACTERIZATION OF THE DELAMAR EPITHERMAL AU-AG DEPOSIT, SILVER CITY, ID

Epithermal ore deposits are formed from hydrothermal activity in the shallow crust and provide a significant amount of the world’s Au and Ag (e.g. Hedenquist and Lowenstern 1994; Einaudi et al. 2003; Simmons et al. 2005). Within this ore deposit type, low-sulfidation (LS) epithermal systems are dominated by meteoric fluids with limited to no magmatic input. DeLamar Mountain is a high-grade LS epithermal deposit located within the Northern Great Basin (NGB) in western Idaho. As one of three LS epithermal sites within the Silver City district, DeLamar is an active Au-Ag prospect that presents a unique research opportunity. Although it has been mined sporadically for over 150 years, little research has focused on this rich area. By developing the first three-dimensional geochemical perspective of a deposit within the Silver City district, this study illuminates the formation of DeLamar and improves our current understanding of the genesis of LS epithermal deposits worldwide. Complementary microscopic observations, geochronological data, and geochemical analyses shed light on the timing of the formation of DeLamar and nearby deposits as well as the potential role of the Yellowstone hotspot in the formation of the district.

Spatially-constrained samples were collected from drill core extracted from DeLamar and the nearby Florida Mountain LS epithermal deposit. Six preliminary ⁴⁰Ar/³⁹Ar ages from single adularia crystals associated with gangue range from 15.95 ±0.06 to 15.35 ±0.07 Ma, varying with site, depth, and between individual crystals within samples. These results overlap with previous studies, which reported ages for NGB deposits ranging from 16.6 to 15.2 Ma (Halsor et al. 1988; Unger 2007; Aseto 2012) and confirm that hydrothermal activity and mineralization were synchronous with the early Yellowstone hotspot. The observed age variations may reflect mineralization events and illustrate the thermal history of the DeLamar and Florida Mountain samples when interpreted within the spatial context provided by drill core data. Complementary petrographic and geochemical approaches will help construct a refined genetic model for DeLamar and the Silver City district by constraining the timing and source of mineralization.