THE ORIGIN OF CARLIN-TYPE GOLD DEPOSITS

The Eocene in the Great Basin was a period of profuse magmatism and hydrothermal mineralization that produced the world’s second largest concentration of gold: the Carlin-type gold deposits (CTGDs) in Nevada. We use new and published data that demonstrate a closer temporal and spatial link between ore formation and magmatism than was previously apparent to develop a comprehensive magmatic-hydrothermal model for the origin of CTGDs. Geochronologic data demonstrate that formation of CTGDs tracked the southwestern sweep of Eocene magmatism in time and space across Nevada. Recent O, H, and S isotope data are consistent with mixing of magmatic fluids with evolved meteoric waters. New LAICPMS and EPMA data for ore-stage pyrite demonstrate that Au co-precipitated with As, Hg, Tl, Te, Cu, and Sb, an elemental suite that is consistent with transport by magmatic aqueous vapor. As a result, we propose a model that relates gold deposition to crustal scale processes imposed upon Nevada’s distinctive geologic setting (a deformed continental rift margin dominated by carbonates) during a change from shallow subduction to renewed magmatism contemporaneous with the onset of extension. Upwelling asthenosphere impinged on strongly modified subcontinental lithospheric mantle, generating magmas that exsolved low-salinity, Au-bearing fluids at depths of 10-15 km. These aqueous fluids with high H₂S and high Au:Cu ratios ascended from their source magma, underwent phase changes, and mixed with meteoric water. Upon cooling, the ore fluids became increasingly acidic. Within a few kilometers of the surface, these fluids dissolved and sulfidized reactive, Fe-bearing carbonate wall rocks, leading to deposition of Au-bearing pyrite. Release of ore fluids from their source magmas at mid-crustal depths explains a lack of mineral and elemental zoning in CTGDs. In contrast, porphyry and skarn deposits throughout the world are strongly zoned because of steep temperature gradients caused by release and cooling of high-temperature fluids from magmas at depths of typically <4 km. The apparent restriction of CTGDs to Nevada is the result of an unusual confluence of optimal crustal architecture, lithospheric evolution, and a tectonic trigger that led to extremely efficient extraction, transport, and deposition of Au.