| Summary: | Mapping relative patterns of mineral alteration and metasomatism reveal the visible manifestation of a hydrothermal system, but the visible expression of the hydrothermal system at the distal margins of fluid circulation may be limited owing to significant kinetic barriers to mineralogical and chemical alteration. Light stable isotopes of oxygen and carbon in carbonate minerals can be sensitive indicators of the interaction between hydrothermal fluids and wall rocks that can be used to delineate fluid flow pathways, evaluate the permeability network and source of fluids, estimate integrated fluid fluxes, and define the distal extent of alteration in carbonate-bearing lithologies.
Carlin-type Au deposits in northeastern Nevada, USA are predominately sediment-hosted Au deposits formed by large hydrothermal systems. Typically, hydrothermal alteration including silicification, carbonate dissolution, clay alteration, and sulfidation are spatially restricted. The Blue Star-Goldstrike district on the Northern Carlin trend is the largest known occurrence of Carlin-type Au with a reported Au endowment of ~1,960 t. At the northern end of the Goldstrike district, the Banshee Au deposit represents a relatively small Carlin-type deposit primarily hosted within Jurassic lamprophyre dike (west Banshee) and polymict breccia units (east Banshee). Proximal alteration at west Banshee consists of silicification, illitization, and sulfidation with the addition of Au, Cs, Hg, K, Rb, Sb, Tl, and W and depletion in Ba, Ca, Mg, Mn, Na, and Sr. The breccia unit and limestone adjacent to the west Banshee lamprophyre exhibit carbonate dissolution and silicification. Outside of visible alteration, δ18O depletion in wall rock and vein calcite defines a more distal expression of alteration. Analysis of down hole δ¹⁸O in limestone from two transects across the Goldstrike district show that the isotopic alteration footprint surrounding Au mineralization may extend 2 km or more out from the main ore bodies. Reactive transport modeling of δ¹⁸O alteration at Banshee provides a tool for evaluating time integrated fluid flux associated with observed isotopic alteration. The combined datasets form the basis of an alteration model for Carlin-type Au deposits where alteration zones outward from proximal clay alteration, silicification, sulfidation, and carbonate dissolution to distal trace element metasomatism and δ¹⁸O alteration. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate
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