Mineral deposition in epithermal deposits formed through flash vaporization and implications for improved understanding of the Arista intermediate-sulfidation epithermal deposit in southern Mexico
Bonanza-type epithermal deposits are formed within hundreds of meters of the paleowater table. They typically consist of banded veins and associated breccia, with quartz being the dominant gangue mineral. Petrographic evidence suggests that much of the quartz in these veins formed through recrystallization from a thermodynamically silica precursor. Recrystallization of this originally microspherical noncrystalline silica precursor resulted in vein textures that mask original relationships complicating reconstruction of the processes of ore deposition. The noncrystalline silica originally forming the bands in the bonanza-type ores is interpreted to have formed through rapid deposition from supersaturated liquids. The presence of ore mineral dendrites within the microspherical silica suggests that mineral deposition occurred at far-from-equilibrium conditions. Mineral deposition occurred during flash vaporization of the hydrothermal liquids. Fluid inclusion evidence from the Arista intermediate-sulfidation deposit in southern Mexico suggests that flashing along the major extensional structures controlling the location of the deposit occurred over ~1 km below the paleosurface. Deposition of high-grade ores over such a large vertical extent is inconsistent with models invoking that mineralization in the epithermal environment is formed by gentle boiling. The study of the Arista intermediate-sulfidation epithermal deposit demonstrated that vein formation at this prolific polymetallic (Au-Ag-Zn-Pb-Cu) deposit was controlled by a relay ramp providing linkage between two adjacent extensional faults. The banded veins at Arista consist of quartz that formed through recrystallization of a noncrystalline precursor. The quartz hosts dendritic to massive sulfide minerals. Rare bands of quartz formed by deposition in open space are present. The veins crosscut felsic dikes and are also crosscut, themselves, by variably altered as well as unaltered dikes. The geological relationships suggest that igneous activity resulting in dike emplacement overlapped with the formation of the epithermal veins at ~18.5 Ma. Seismicity and dike emplacement may have been important in triggering rapid depressurization along the controlling structures, leading to repeated flash vaporization of the hydrothermal liquids and the formation of high-grade mineralization.
dendrite, epithermal, flashing, Mexico, quartz, recrystallization
Networked Digital Library of Theses and Dissertations
Colorado School of Mines. Arthur Lakes Library