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Vein textures at the Moss low-sulfidation epithermal gold deposit, Arizona: constraints on the processes of mineral deposition
Seitter, Gregory Charles
Seitter, Gregory Charles
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2025
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The Moss low-sulfidation epithermal deposit in the northern Oatman district in northwest Arizona formed as a result of the Miocene extension in the Colorado River extensional corridor. The main vein zone can be traced over a strike length of ~6.2 km although elevated precious metal grades occur primarily along a central, ~1.9 km-long segment. The vein zone crosscuts the medium-grained Moss monzonite and the Peach Springs Tuff, which was deposited during the 18.78 ± 0.02 Ma eruption of the Silver Creek Caldera. The highest precious metals grades occur where vein intersections contain dark gray bands of ore minerals, including native gold, acanthite, and silver sulfosalts. The ore minerals occur as dendritic aggregates hosted by a matrix of quartz that formed through recrystallization of a noncrystalline silica precursor originally deposited along vein walls and cementing wall rock clasts in breccias. Recrystallization of the noncrystalline silica matrix has progressed to completeness resulting in the development of characteristic quartz textures such as mosaic quartz characterized by interpenetrating grain boundaries and flamboyant quartz showing radiating arrays of inclusions. Prismatic quartz transected by recrystallization fronts containing abundant inclusions is common. The ore-bearing bands alternate with bands that contain bladed calcite. The texture of the calcite is inconsistent with calcite growth in open space. Similar to the ore mineral dendrites, the textural evidence suggests that the calcite blades grew in the gel-like silica matrix. This type of calcite is texturally distinct from lattice-bladed calcite in which polyhedral cavities separate blades. Microthermometric investigations suggest that calcite deposition occurred at ~265°C at approximately 600 m below the paleosurface. It is proposed here that ore mineral formation and the growth of calcite in the noncrystalline silica precursor took place during short-lived events of fluid flashing at far-from-equilibrium conditions. The amount of vapor present during flashing presumably played a key control on mineral precipitation and growth.
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