Abstract
Abstract The Carlin-type deposits in Nevada (western USA) constitute the world’s second-largest gold ore province. These structurally and stratigraphically controlled, sediment-hosted ore bodies are characterized by carbonate dissolution attending hydrothermal precipitation of gold-rich arsenian pyrite. The origin of the mineralizing fluids and the source of the gold remain debated. Conceptual models, favoring either sedimentary, metamorphic, or magmatic fluid sources, are based on isotopic tracers, giving ambiguous results. Here we use the trace element compositions of fluid inclusions to separate geochemical signals of the large-scale fluid source from effects of deposit-scale fluid interaction with the sedimentary host rocks. Specifically, we compare the ratios of Rb, K, B, As, Sr, and Ba between clearly magmatic-hydrothermal Cu-Au ores at Copper Canyon in the Battle Mountain–Eureka trend with the Gold Quarry and Chukar Footwall deposits on the Carlin trend that contain high-grade gold in similar sedimentary host rocks. Results indicate that both ore districts can be related to upper crustal hydrous magmatic intrusions, but are now exposed at different levels of erosion and formed at different distances from their magmatic fluid source. Fluid compositions are best explained by separation of a deep magmatic fluid into Rb-K–enriched brine and B-As-Au–enriched vapor, followed by cooling and contraction of the magmatic vapor phase to an epithermal liquid, which reacted with Sr-Ba–bearing sedimentary rocks during ascent and eventual precipitation of Au-rich arsenian pyrite.
Highlights
The Eocene Au deposits of Nevada collectively represent the second-largest Au concentration on Earth, accounting for ~6% of total world production (Muntean et al, 2011)
For this study we identified ore-stage fluid inclusions hosted in coarser quartz grains in veinlets and breccia zones by relating them to arsenian pyrite grains that are hosted in the same quartz generation (Figs. 1F and 1G)
Chemical Composition of Fluids at Battle Mountain At Copper Canyon, three fluid inclusion types were recognized by petrography and microthermometry: (1) vapor inclusions (Fig. 1B) with high vapor/liquid ratios and low salinities (
Summary
The Eocene Au deposits of Nevada (western USA) collectively represent the second-largest Au concentration on Earth (the Archean Wit watersrand Basin, South Africa, is the largest), accounting for ~6% of total world production (Muntean et al, 2011). Most Carlin-type Au deposits are aligned on three trends in the Basin and Range Province of Nevada (Fig. 1A). Despite their economic attraction, only two comparatively minor districts of similar type have been found outside Nevada (Su et al, 2009; Tucker et al, 2012). Only two comparatively minor districts of similar type have been found outside Nevada (Su et al, 2009; Tucker et al, 2012) Because they are geologically subtle and lack particles of free gold, Carlin-type deposits are difficult to discover without a process-based exploration model. Deposition of gold in solid solution within arsenian pyrite is thought to be controlled by reduction and desulfidation of an Au-As-S–rich fluid attending dissolution of ferroan calcite (Hofstra and Cline, 2000)
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