EOCENE GOLD METALLOGENY OF NORTHERN NEVADA

Eocene Au deposits of northern Nevada have produced ~150 Moz of Au since the 1965 startup of the Carlin mine. Nearly all deposits are disseminated in either calcareous sedimentary rocks or contact-metamorphosed sedimentary rocks, and these deposits are collectively referred to here as sedimentary rock-hosted, disseminated Au deposits (SHDGs). Four distinct styles of SHDGs occur in northern Nevada, from highest to lowest T/P these include: 1) Au skarns, 2) hornfels margin, or distal disseminated deposits, 3) Carlin-type deposits, and 4) “Carlinesque” deposits, the latter closely linked to the Eocene paleosurface. Of these, Carlin-type deposits have contributed most to production, mainly from four districts (Carlin, Cortez, Getchell, and Jerritt Canyon), although important Au production has come from all styles. SHDGs formed over a brief period from ~42-34 Ma, temporally and spatially coincident with Eocene arc magmatism but likely predating large-magnitude Cenozoic extension. Individual districts mostly possess one predominant style, although more extensive mineral belts such as the Carlin trend and Battle Mountain-Eureka belt preserve a range of deposit styles that reflects along-trend variations in the Eocene erosional level.

The range of Eocene SHDGs in northern Nevada defines a distinct intrusion-related Au metallogeny that contrasts with deposits formed in many continental arc settings such as porphyry Cu and Cu-Mo, polymetallic skarns and replacements, and high- and intermediate-sulfidation epithermal deposits. The reduced mineralogy and geochemistry of ores and a Au-dominant or Au-only character of SHDGs in northern Nevada infer overall reduced ore fluids that fundamentally differ from highly oxidized fluids indicated for porphyry-related systems. Emplacement of the Eocene arc far inboard of the plate margin and into kms-thick carbonaceous slope and basinal rocks of the Neoproterozoic through Paleozoic passive margin progressively reduced and modified mid-crustal magmas from mafic to silicic compositions through assimilation. Thus, differentiated magmas may have been saturated in reduced sulfur, thereby forcing pyrrhotite (monosulfide solid solution) to precipitate, which strongly partitioned Cu over Au with consequent high Au/Cu ratios in derivative melts and ore fluids.