Chapter 2: The Carlin Gold System: Applications to Exploration in Nevada and Beyond
Mining of Carlin-type gold deposits in Nevada has made the United States one of the leading gold producers in the world for almost four decades. These deposits constitute an endowment of ~255 Moz (7,931 tonnes) of gold, of which 89% occurs in four main clusters of deposits: the Carlin trend, Getchell, Cortez, and Jerritt Canyon. These four clusters share many characteristics, including (1) formation during a narrow time interval (42–34 Ma), (2) lithologic and structural controls to fluid flow and ore deposition, (3) geochemical signature of the ores, (4) hydrothermal alteration and ore paragenesis, (5) relatively low temperatures and salinities of ore fluids, (6) fairly shallow depths of formation, and (7) lack of mineral and elemental zoning.
A mineral systems approach to exploring for Carlin-type gold deposits in Nevada and elsewhere is presented, in which critical processes are laid out: (1) development of source(s) for gold and other critical components of the ore fluid, (2) formation of fluid pathways, (3) water-rock interaction and gold deposition, and (4) a tectonic trigger. The critical processes are then converted into a practical targeting system for Carlin-type gold deposits within and outside of Nevada, ranging from regional to district to drill target (<~20 km2) scales. The critical processes of the Carlin mineral system are translated into targeting elements and mappable targeting criteria.
At the regional scale, targeting elements for magmatic sources of gold and ore fluid components include (1) intrusive centers with a mantle component to the magmas, (2) processes that could result in metasomatized subcontinental lithospheric mantle, (3) high-K, H2O-rich calc-alkaline magmas, and (4) evidence for fluid release. For crustal sources of gold, targeting elements include (1) carbonaceous sedimentary rocks with diagenetic/syngenetic sulfides enriched in Au-As-Hg-Tl-Sb-(Te) and sulfates and (2) a heat source to drive convection of meteoric and/or formation of metamorphic fluids. Targeting elements for fluid pathways at the regional scale include (1) basement suture zones and rifted continental margins, (2) long-lived upper crustal faults that may be linked to basement faults, and (3) a reduced crustal section to ensure long transport of gold by sulfide-rich fluids. Targeting elements at the regional scale for water-rock interaction and gold deposition include (1) passive margin dominated by carbonate rocks, (2) contractional deformation and formation of regional thrust faults and fold belts, and (3) a regional Au-As-Hg-Tl-Sb-(Te) geochemical signature. Targeting elements for tectonic triggers include (1) changes from contraction to extension, (2) periods of intense magmatism, especially related to slab rollback, and (3) plate reorganization.
At the district scale, targeting elements for fluid pathways include (1) old reactivated high-angle fault zones, (2) zones of abundant low-displacement, high-angle extensional faults, (3) fault intersections, and (4) lithologic rheology contrasts, such as preore intrusions and contact aureoles. For water-rock interaction and gold deposition, targeting elements include (1) carbonate-bearing stratigraphy, (2) low-angle features that could divert upwelling fluids out of high-angle faults and into reactive wall rocks, (3) hydrothermal system of targeted age, (4) alteration consistent with wall-rock reaction with acidic, sulfide-rich hydrothermal fluids, and (5) Fe-rich rocks in the stratigraphic section, which will drive sulfidation. At the drill target scale, the targeting elements for fluid pathways are zones of increased fault/fracture permeability. The targeting elements for water-rock interaction and gold deposition include (1) zones of increased low-angle permeability in carbonate rocks proximal to high-angle faults, (2) favorable alteration, especially hydrothermal carbonate dissolution and silicification, (3) Fe-rich rocks including ferroan carbonates and mafic volcanic rocks and intrusions, (4) favorable Au-As-Hg-Tl-Sb-(Te) geochemical signature with low base metals and Ag/Au ratios, and (5) favorable mineralization, especially arsenian pyrite with textures and chemistry consistent with Carlin-type deposits.