ACTIVE GEOTHERMAL SYSTEMS AND PLIOCENE AND YOUNGER GOLD DEPOSITS OF THE GREAT BASIN, USA: REGIONAL CONTROLS AND SELENIUM CONCENTRATIONS AS A CLUE TO ORE ENVIRONMENT

Pliocene and younger Au deposits in the Great Basin, USA contain >10 Moz Au. Many of these deposits occur near active geothermal systems with >180°C fluids, suggesting that subsurface Au deposition may be continuing today at some locations. The young Au deposits and geothermal systems can be classed as either magmatically heated or amagmatic depending on whether there is evidence of coeval silicic magmatism. The magmatically heated deposits and systems (e.g. Long Valley caldera, CA; Roosevelt, UT) are limited to the margins of the Great Basin whereas young amagmatic Au deposits (e.g. Sulphur (Crofoot-Lewis) and Florida Canyon, NV) and systems, currently producing >100 megawatts electricity, cluster in NW Nevada.

Amagmatic epithermal Au deposits and electricity-producing amagmatic geothermal systems are scarce globally, and their abundance in NW NV is attributed to a combination of high heat flow and active transtensional tectonics. The greatest concentration of such deposits/systems is found where strike-slip faulting of the Walker Lane splays outward into NE-striking extensional faults in the Great Basin interior. Individual deposits and systems formed in structurally complex zones that facilitate deep fluid circulation, such as en echelon step-overs in normal faults, pull-apart blocks, and boundaries between zones of opposite-dipping normal faults. In such environments, Au mineralization and geothermal fluid flow preferentially occur along structures oriented approximately orthogonal to the current crustal extension direction.

Compared to magmatically heated systems, the amagmatic geothermal systems have lower fluid concentrations of As, Li, B, and Cs, lower 3He/4He ratios, and lower overall vertical temperature gradients. The related young Au deposits are of the shallow hot-springs type, with steam-heated advanced argillic alteration, adularia, marcasite, cinnabar, native sulfur, and high concentrations of Sb and As. Limited data indicates that some young amagmatic gold systems have high Se concentrations, similar to many Miocene epithermal Au deposits in NV. High ratios of Se/Ag suggest that, in contrast to the Miocene deposits, Ag minerals may not be the major host of Se. Differences in the level of exposure of the Pliocene vs. Miocene Au deposits may explain host mineral differences.