Gold Mineralization in Volcanogenic Massive Sulfides: Implications of Data from Active Hydrothermal Vents on the Modern Sea Floor
M. D. Hannington, S. D. Scott, 1989. "Gold Mineralization in Volcanogenic Massive Sulfides: Implications of Data from Active Hydrothermal Vents on the Modern Sea Floor", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The occurrence and distribution of gold in volcanogenic massive sulfides on the modern sea floor is a function of the physical and chemical characteristics of the hydrothermal fluids and theil ability to become saturated with respect to gold at a high concentration. Evidence from active hydrothermal vents at midocean ridges indicates that high-temperature (350°C) hydrothermal fluids may contain 0.1 to 0.2 μg/kg Au and could transport as much as 500 to 1,000 g Au/yr in a single deposit. However, in the absence of an effective precipitation mechanism, most of the gold in high-temperature vents may be lost to a diffuse hydrothermal plume and related metalliferous sediments. Associated high-temperature, Cu-Fe sulfides and subsea-floor stockwork mineralization are typically gold poor, containing ≤0.2 ppm Au. In contrast, low-temperature (<300°C) sulfides accumulating at or near the sea floor may contain up to 6.7 ppm Au. Gold-rich assemblages are formed at elevated sulfidation states and commonly contain high levels of Zn (>10%), Pb (>0.1%), Ag (>100 ppm), As (>200 ppm), and Sb (up to 500 ppm). High concentrations of H2S (up to 8.4 mmole/kg or 285 ppm) in the vent fluids stabilize Au(HS)2− complexes down to at least 200°C and account for the enrichment of gold in late-stage, low-temperature sulfides. Gold is precipitated from Au(HS)2− by oxidation during high-level mixing of the vent fluids with ambient seawater. The locus of mixing and the extent of sulfide-sulfate reactions are important controls on the site and temperature of gold mineralization. Precipitation of gold from AuCl2 occurs in one deposit where fluids have acquired significantly elevated salinities. Similar patterns of gold enrichment in examples from ophiolite-hosted Cu pyrite, Phanerozoic Zn-Cu-Pb, and Archean Cu-Zn deposits suggest similar controls on the occurrence and distribution of gold in ancient volcanogenic massive sulfide ores.