ORIGIN OF CARLIN-TYPE GOLD DEPOSITS

Numerous Carlin-type Au deposits located in trends or districts have made northern Nevada one of the most productive Au producing regions in the world. Similarly productive regions, however, have not been discovered elsewhere, leading to the conclusion that unique processes or unusual circumstances led to the formation of the Nevada deposits. There is currently no accepted model for these deposits and proposed models range from transport of deep crustal fluids and metals to the upper crust, to shallow level lateral secretion models. Based on new and published data we propose a comprehensive model that calls upon processes from the mantle to the upper crust and which suggests that the deposits result from a confluence of non-unique processes and ideal architecture.

Critical new data include 1) the age and location of the deposits track the sweep of late Eocene magmatism from NE to SW across northern Nevada, 2) H and O isotopes from fluid inclusions, quartz, and kaolinite, and near 0‰ S isotopes of Au-bearing pyrites are consistent with magmatic fluid, and 3) trace metals in Au-bearing pyrites, including Au, As, Cu, Hg, Tl, Sb, and Te, suggest potential metal transport by vapor. An optimal setting was established for deposit formation, whereby structural culminations of fractured, reactive carbonate rocks were capped by less permeable and less reactive siliciclastic rocks, directly above high-angle fault zones linked to underlying basement rift structures. A series of deep to upper crustal magmas produced and passed a sulfide-rich, high Au/Cu metal assemblage upwards. At >10 km fractional crystallization released a single-phase ore fluid that collected in pluton roof zones. Tapped by extension, ore fluids moved up high-angle structures, condensed minor brine, and partitioned Au, Cu, As, Sb and S to vapor. The vapor ascended, condensed, reacted with calcareous host rocks, and sulfidized host rock Fe forming pyrite that captured Au and other transported metals.