Vein Textures at the Moss Low-Sulfidation Epithermal Gold Deposit, Arizona: Evidence for the Formation of Bladed Calcite Within Silica Gel

Abstract The Miocene Moss low-sulfidation epithermal deposit is a significant precious metal producer within the Oatman district in northwest Arizona. Elevated precious metal grades occur in quartz-calcite veins and breccias containing dark-gray bands that host ore minerals, including native gold, acanthite, and silver sulfosalts. The ore minerals form dendritic aggregates hosted by a matrix of quartz that formed through maturation from a noncrystalline silica precursor. The transformation of the thermodynamically unstable, noncrystalline precursor to quartz has progressed to completion, resulting in the development of characteristic quartz textures. These textures include mosaic quartz characterized by interpenetrating grain boundaries and flamboyant quartz showing radiating arrays of inclusions. Prismatic quartz grains transected by remnant recrystallization fronts containing abundant inclusions are common. The ore-bearing bands occur adjacent to bands that contain bladed calcite. The textures of the calcite and the surrounding quartz are inconsistent with calcite having formed in open spaces. Similar to the ore mineral dendrites, the textural evidence suggests that the calcite blades grew in a gel-like silica matrix. This type of calcite is texturally distinct from lattice-bladed calcite, in which polyhedral cavities between the blades are filled with late drusy quartz that grew in open space. Microthermometric investigations suggest that calcite deposition occurred at ~265°C approximately 600 m below the paleosurface. It is proposed that ore-mineral formation and calcite growth in the noncrystalline silica precursor took place during short-lived episodes of fluid flashing at far-from-equilibrium conditions. The amount of vapor generated during flashing presumably played a key control on mineral precipitation and growth.