Hydrothermal eruption mechanisms and hot spring gold deposits

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 CO2-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.

Mineralización de oro en la franja aurífera de Veraguas, Panamá

“…Los depósitos piroclásticos asociados a estos eventos contienen únicamente material magmático juvenil como fragmentos de vidrio volcánico y pómez. Las erupciones hidrotermales (incluyendo geysers) son mucho menos poderosas y son desatadas por la energía contenida en la roca y el reservorio de agua (Nelson & Giles, 1985). El producto de su explosión no contiene material magmático juvenil.…”

Hydrothermal breccia pipe structures: general features and genetic criteria - II Phreatic breccia

“…There is a unanimous opinion (Sillitoe, 1985;Nelson & Giles, 1985;Hedenquist & Henley, 1985;Baker et al, 1986;Corbett & Leach, 1996 etc. ) according to which phreatic breccia pipes formed at shallow levels in volcanic environments and geothermal fields.…”

“…The contacts of the phreatic breccia pipes with their host rocks are usually sharp, but they have an irregular shape, with multiples interpenetrations, which form finger-like contacts (Baker et al, 1986;Nelson & Giles, 1985). Sometimes there is a transition zone between the real breccia body and the host rocks, the latter often being intensely fractured and transformed into a stockworked area close to the breccia margins (Baker et al, 1986;Nelson & Giles, 1985;Izawa & Cunningham, 1989).…”

“…Sometimes there is a transition zone between the real breccia body and the host rocks, the latter often being intensely fractured and transformed into a stockworked area close to the breccia margins (Baker et al, 1986;Nelson & Giles, 1985;Izawa & Cunningham, 1989). The breccia walls are often sheeted fractured (Sillitoe, 1985;Nelson & Giles, 1985), giving rise, towards the edges of the breccia body, to different breccia facies with angular rock fragments, sometimes tabular (mosaic breccias, shingle breccias, crackle breccias, etc.). There are cases when phreatic breccia bodies are hosted by unfractured rocks, their limits being extremely sharp.…”

The geology and hydrothermal alteration of the Bear Creek Butte area, Crook County, central Oregon

“…Recent studies by Hedenquist and Henley (1985) and Nelson and Giles (1985) indicate that substantial fluid overpressures in the upper silicif ied portion of the hydrothermal system are not required to develop hydrothermal breccias. Instead these studies indicate that only local sealing of near-surface discharge channels and the transmission of deeper reservoir pressures to the sealed area by the evolution of a compressible cap of exsolved gas, predominantly CO , that is released by the 2 violent boiling of the fluid can be the mechanism that creates hydrothermal breccias.…”