A new online paper in Nature Geoscience offers an explanation for the origin of the Carlin-type gold deposits in Nevada, the seconds largest concentration of gold after South Africa. The authors propose that
"...upwelling asthenosphere impinged on a strongly modified subcontinental lithospheric mantle, generating magmas that released gold-bearing fluids at depths of 10 to 12 km. The rising aqueous fluids with elevated hydrogen sulphide concentrations and a high ratio of gold to copper underwent phase changes and mixed with meteoric water. Within a few kilometres of the surface, the fluids dissolved and sulphidized carbonate wall rocks, leading to deposition of gold-bearing pyrite. We conclude that the large number and size of Carlin-type deposits in Nevada is the result of an unusual convergence of a specific geologic setting, together with a tectonic trigger that led to extremely efficient transport and deposition of gold."
Ref: "Magmatic–hydrothermal origin of Nevada’s Carlin-type gold deposits," John L. Muntean, Jean S. Cline, Adam C. Simon, & Anthony A. Longo, Nature Geoscience, (2011), doi:10.1038/ngeo1064