Isotopic, Chemical, and Textural Evidence for Pervasive Calcite Dissolution and Precipitation Accompanying Hydrothermal Fluid Flow in Low-Temperature, Carbonate-Hosted, Gold Systems

Oxygen isotope ratios measured from microdrilled calcite in limestone wall rocks at the Banshee Carlin-type Au deposit show marked depletion proximal to Au mineralization, indicating isotopic exchange between wallrock calcite and hydrothermal fluids. Isotopic alteration is spatially coincident with mineralization and puts minimum constraints on hydrothermal fluid infiltration outside of visible indicators (i.e., carbonate dissolution and silicification). Additionally, O-18 depletion is nearly homogeneous at hand-specimen scale indicating near-complete alteration of limestone protolith. The primary mechanism of isotopic exchange is coupled dissolution-precipitation leading to pseudomorphic replacement of calcite during hydrothermal fluid infiltration. Surface reactions between calcite and the hydrothermal fluids are evidenced by textural and chemical variations between altered and unaltered calcite in wall-rock limestone and limestone breccia. Cathodoluminescence (CL) reveals distinct changes in the luminescence of altered calcite relative to unaltered equivalent calcite, and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) data from altered and unaltered calcite show that observed CL responses are due to changes in calcite mineral chemistry. Altered samples exhibit bright CL responses in calcite with increases in Mn and Fe. In addition to wall-rock calcite alteration, calcite veins without clear paragenetic relationships can be related to hydrothermal alteration owing to 18O depletion, CL response, and positive Eu* anomalies. Together, isotopic alteration of wall-rock calcite and evidence of hydrothermal calcite veins define the distal expression of low-temperature hydrothermal alteration in calcite-bearing rocks at the Banshee deposit.