Lithogeochemistry of Carlin-type gold mineralization in the Gold Bar district, Battle Mountain-Eureka trend, Nevada

Abstract

The Gold Bar district contains five Carlin-type gold deposits and four resources for a combined gold endowment of 1.6 M oz [50 t]. The gold deposits are hosted in Devonian carbonate rocks below parautochthonous and allochthonous Paleozoic siliciclastic rocks emplaced during the Early Mississippian Antler orogeny. The district is in the Battle Mountain-Eureka trend, a long-lived structural feature that localized intrusions and ore deposits of different types and ages. The whole-rock geochemistry of four different mineralized and unmineralized Devonian carbonate rock units (two favorable and two unfavorable) were determined and interpreted in the context of the regional geology. A combination of basic statistics, R-mode factor analysis, isocon plots, and alteration diagrams were utilized to (1) identify favorable geochemical attributes of the host rocks, (2) characterize alteration and associated element enrichments and depletions, and (3) identify the mechanism of gold precipitation. This approach also led to the recognition of other types of alteration and mineralization in host rocks previously thought to be solely affected by Carlin-type mineralization. Unit 2 of the Upper Member of the Denay Formation, with the highest Al 2O 3, Fe 2O 3 and SiO 2 contents and the lowest CaO content, is the most favorable host rock. Based on the high regression coefficients of data arrays on X– Y plots that project toward the origin, Al 2O 3 and TiO 2 were immobile and K 2O and Fe 2O 3 were relatively immobile during alteration and mineralization. Specific element associations identified by factor analysis are also prominent on isocon diagrams that compare the composition of fresh and altered equivalents of the same rock units. The most prominent associations are: Au, As, Sb, SiO 2, Tl, –CaO and –LOI, the main gold mineralizing event and related silicification and decalcification; Cd, Zn, Ag, P, Ni and Tl, an early base metal event; and MgO, early dolomitization. Alteration diagrams, consisting of X– Y plots of SiO 2/Al 2O 3, K 2O/Al 2O 3, CO 2/Al 2O 3, [S/Al 2O 3]/[Fe 2O 3/Al 2O 3], provide evidence for progressive silicification, decarbonation (decalcification and dedolomitization), argillization (illite), and sulfidation as a function of gold mineralization. The latter process is identified as the principal mechanism of gold precipitation. The lithogeochemistry of the ores in the Gold Bar district is typical of that documented in classic Carlin-type gold deposits in the region, but the size of the deposits and the intensity of alteration and mineralization are less. The presence of other types of mineralization in the Gold Bar district is also common to most of the other Carlin-type districts located in major mineral belts. The approach used in this study is well suited to the interpretation of multi-element geochemical data from other study areas with superimposed alteration and mineralization.