Abstract
Episodes of gold mineralization in the shallow hot spring environment are related in time to hydrothermal eruption events and in space to the resulting vent breccias and peripheral stockwork zones. It is proposed that large but short-lived overpressures in a geothermal reservoir, probably triggered by sudden magmatic heat fluxes, induce hydraulic fracturing which then evolves into hydrothermal eruptions if driven through to the surface. The maximum available energy in hot, shallow reservoirs appears easily sufficient to drive such eruptions, particularly if CO 2 -rich fluids are involved. In mineralized systems, gold-bearing fluids are subsequently channeled into the outflow conduit where they flood the permeable vent breccia and peripheral stockwork. Gold is lifted into this hot spring environment above a boiling level that is elevated by high flow rates to within several hundred meters of the surface and is precipitated with abundant quartz, pyrite, and adularia, along with a distinctive and steeply zoned trace element suite. These sequential events probably occur as a continuum, which if repeated cyclically in a single vent zone, can result in ore-grade mineralization.
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