Multiple syngenetic metalliferous horizons enriched in V, As, Mo, Se, Ni, Ag, Zn and Au in the Northern Carlin Trend: Implications for the genesis of world class gold-arsenic deposits

The source of the gold in the giant Carlin type deposits in Nevada is controversial; is it deeply sourced from the lower crust or mantle, from high level intrusions or from the sedimentary basin? We report new evidence that the Ordovician to Devonian carbonaceous stratigraphy that hosts the ore deposits is a viable source-rock for both gold and arsenic. Detailed studies of the host sediments away from the ore deposits has revealed at least seven metalliferous intervals of organic-rich mudstones in the stratigraphy from the base of the Roberts Mountains Formation to the top of the Rodeo Creek Formation. These intervals are enriched in syngenetic/diagenetic V, As, Mo, Se, Ni, Ag, Zn and Au and are termed the VAMSNAZ suite. These metals are preferentially concentrated in very fine grained aggregates of diagenetic pyrite, but some are also present in bedding parallel lenticles and cross cutting veins of bitumen. The metal association is typical of redox sensitive elements that are enriched in euxinic sedimentary environments by reduction and adsorption onto organic matter. Simple mass balance calculations suggest that these sediments may have contained sufficient metals to account for the entire budget of gold, arsenic and many of the other elements concentrated in the epigenetic Carlin ore deposits. Pre-enrichment of gold and arsenic in the Ordovician to Devonian slope and basin depositional facies of the Great Basin is proposed as a necessary requirement for the formation of the Carlin type ores in Nevada.