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Textural characteristics of high-grade low-sulfidation epithermal veins: constraints on ore-forming processes and implications to mineral exploration
Terry, Lauren R.
Terry, Lauren R.
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2025
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Low-sulfidation epithermal deposits are an important source of gold and silver, representing a significant percentage of the worldwide precious metal endowment. The deposits form in the shallow subsurface (<1.5 km depth) from near-neutral hydrothermal liquids that have temperatures of up to ~250°C from ore-forming hydrothermal liquids are rock-buffered and have low (<3-4 wt.% NaCl equiv.) salinities. Ores in low-sulfidation epithermal deposits are commonly confined to crustiform quartz veins that contain bands of differing mineralogical compositions. Based on case studies in the Omu camp in Japan and the Midas deposit in Nevada, it is shown here that mineralized bands in these veins originally largely consisted of noncrystalline silica forming microspheres. Investigations on geothermal scales from the Coso geothermal field in California confirm that noncrystalline silica can form as a result of rapid silica supersaturation. In low-sulfidation epithermal deposits, this gel-like opal-AG silica formed the matrix in which ore minerals grew by diffusion-limited processes at far-from equilibrium processes. In addition to ore minerals, it is demonstrated that gangue minerals such as adularia can also form dendrites in high-grade veins that are interpreted to have formed by similar processes. The ore minerals occur in distinct bands in the crustiform veins suggesting that the adularia and the ore minerals did not co-precipitate, which can be explained by a model that assumes that supersaturation of different solutes in the hydrothermal liquids is reached at different amounts of vapor present under two-phase flow conditions. The noncrystalline silica originally present in the veins recrystallizes to quartz over time, with mosaic quartz being the most abundant quartz texture in mineralized bands. The petrographic characteristics of mineralized bands differs from bands that are barren, which are commonly composed of chalcedony. The observed textural relationships suggest that high-grade vein formation in the epithermal environment commonly involves flashing of the hydrothermal liquids, with mineral precipitation occurring during these short-lived events of rapid depressurization along the host structures. The findings of this study have significant implications to mineral exploration as they suggest that ore zones in low-sulfidation epithermal deposits may occur at significantly larger depth than previously thought. Improved understanding of the ore deposit model and integration of geological, geochemical, and geophysical data during mineral exploration is critical to discovery as demonstrated in the case of a low-sulfidation epithermal deposit in the Omu camp.
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