Epithermal deposits are an important source of precious metals that form at shallow depth by subaerial hydrothermal systems. This study aimed to unravel the processes that result in the formation of high grades in these deposits through textural investigations on epithermal veins.High-grade vein ores in low-, intermediate-, and high-sulfidation epithermal deposits are typically hosted in specific colloform bands. The ore minerals form dendritic aggregates that are hosted by a matrix that originally consisted of opal-A showing a microspherical texture. The opal-A was originally gel-like and could be shaped by the hydraulic action of the hydrothermal fluids. The opal-A in the veins was deposited broadly contemporaneously with the ore minerals although textural evidence suggests that delicate dendrites could also have grown within the silica gel. Experimental investigations confirm that the growth of mineral dendrites in silica gels is possible at far-from-equilibrium conditions. The microspherical opal-A hosting the ore mineral dendrites is thermodynamically unstable and in most deposits investigated has matured and recrystallized to mosaic quartz characterized by highly irregular and interpenetrating grain boundaries. In most deposits, ore minerals are associated with this mosaic quartz and relic microspheres may or may not be preserved in the quartz matrix. The mosaic quartz present in mineralized colloform bands is texturally distinct from quartz occurring in barren bands in epithermal veins, which includes comb quartz and quartz pseudomorphs formed after bladed calcite.
It is proposed here that ore mineral formation and deposition of opal-A within the veins occurred as a result of metal and silica supersaturation achieved during short-lived events of vigorous boiling of flashing that may have been triggered by seismic events. Ore deposition occurred in the area of two-phase liquid and vapor flow whereby the degree of vapor production varied along the vein and over time. In contrast, barren bands in epithermal veins formed during periods of gentle boiling or nonboiling. The observation that flashing is the principal mechanism resulting in high-grade ore formation in the epithermal environment has significant implications to exploration as it predicts that the boiling zone and mineralization can occur at variable depths below the paleowater table.
Copyright of the original work is retained by the author.
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