Characteristics and Models for Carlin-Type Gold Deposits

Abstract

Carlin-type gold deposits are restricted to a small part of the North American Cordillera, in northern Nevada and northwest Utah, and formed over a short interval of time (42-30 Ma) in the mid-Tertiary when the Yellowstone mantle plume is inferred to have been located below the subduction zone. They formed after a change in plate motions (43 Ma) at, or soon after, the onset of extension in an east-west-trending, subduction-related magmatic belt. The deposits do not show consistent spatial relationships to mid-Tertiary magmatic centers, rather, most are located along long-lived, deep crustal structures inherited from Late Proterozoic rifting and formation of a passive margin. These structures influenced subsequent patterns of sedimentation and deformation and localized multiple episodes of igneous and hydrothermal activity, many of which contain anomalous concentrations of gold. The mid-Tertiary surface topography was relatively flat and many systems were located below large shallow lakes. Most deposits are hosted in a Paleozoic miogeoclinal carbonate sequence that is either structurally overlain by a eugeoclinal siliciclastic sequence, the Roberts Mountains allochthon emplaced in Early Mississippian time, or stratigraphically overlain by a miogeoclinal siliciclastic sequence deposited in the resulting foredeep. These siliciclastic sequences are less permeable than underlying carbonate rocks and apparently caused fluids ascending along major structures to flow laterally into permeable and reactive rocks below them. In these areas, gold ore is localized at intersections of a complex array of structures with permeable and reactive strata. The common alteration, mineralogy, and geochemical signature of these deposits is a direct expression of the P, T, and composition of ore fluids. The deposits generally formed at depths of >2 km at temperatures of 250 degrees to 150 degrees C, from moderately acidic (pH nearly equal 5), reduced fluids containing 0.01 mole percent H (sub 2) S. The H (sub 2) S concentration was critical because it suppressed the solubility of Fe, base metals, and Ag as chloride complexes and enhanced the solubility of Au and associated trace elements (e.g., As, Sb, Tl, and Hg) as sulfide complexes. Gold was transported as AuHS degrees and/or Au(HS) (super -1) (sub 2) complexes. The main ore stage formed during cooling and neutralization of ore fluids by reactions with the host rocks. It is characterized by carbonate dissolution, argillization of silicates, sulfidation of ferroan minerals, and silicification of limestone. Gold occurs as submicron inclusions or solid solution in arsenian pyrite and precipitated as H (sub 2) S was consumed by sulfidation of Fe released from ferroan minerals. The other common trace elements (e.g., Sb, Tl, Hg) also reside in arsenian pyrite. The ideal host rock consists of permeable ferroan carbonate that is completely dissolved and its contained iron completely sulfidized such that all that remains is gold-bearing arsenian pyrite. Accordingly, large tonnage, low-grade gold deposits (e.g., Gold Quarry) are in siliceous rocks with low carbonate and reactive iron contents, and small tonnage, highgrade gold deposits (e.g., Meikle) are in carbonate rocks with high concentrations of reactive iron. Late ore-stage quartz, calcite, orpiment, realgar, stibnite, and barite occur in open fractures and pores and their abundance varies tremendously from deposit to deposit.