Vein petrography and thermodynamic modeling study of a porphyry-related polymetallic Au-Ag deposit in the Central City district, Colorado, A

Abstract

The Central City district is an ambiguous polymetallic Au-Ag deposit due to the metal endowment and complexity of mineralogical relationships in veins hosting ore. The central zone of the district where significant Au production occurred hosts transitional-style pyrite-quartz and polymetallic-style base-metal veins. The goal of this study was to shed light on the paragenesis of gold in the central zone of the Central City district by studying these veins at the macro- and microscopic scales. Samples from the central zone of the district were studied petrographically using optical microscopy, field emission scanning electron microscopy (FE-SEM), and automated mineralogy (TIMA). Petrographic findings along with interpretation of mineralogical textures in visible gold specimens provided by the Denver Museum of Nature and Science concluded that Ag-bearing gold occurs in all three polymetallic-style mineral assemblages observed in the central zone of the district. Four cooling models and one isothermal inverse pH titration model at three temperatures were simulated to increase our understanding of mineral stability, metal solubility, and metal complexation for minerals and metals observed in the district. These models in conjunction with observations made for veins in this study and past studies allowed for interpretation of mechanisms controlling district scale metal zonation and ore formation. Metal zonation and ore formation in the district is explained by the evolution of one acidic, reduced fluid and another more mildly acidic, oxidized fluid where transitional-style veins and polymetallic-style veins formed as a result of the evolution of each fluid, respectively. Neutralization and equilibration of an acidic, reduced fluid along with cooling allowed for deposition of pyrite and quartz in transitional-style veins. Polymetallic-style mineral assemblages in the central zone of the district formed as a result of a mildly, acidic oxidized fluid cooling along an unbuffered pH pathway causing Au and Cu deposition and enrichment in the central zone of the district. Findings in this study clarify Au paragenesis in veins as well as fluid evolution leading to metal zonation and overall ore formation.