Porphyry-epithermal Au-Ag-Mo ore formation by vapor-like fluids; new insights from geochemical modeling

Shallow magmatic-hydrothermal systems are characterized by steep gradients in temperature and pressure, and because the fluid is of low density and highly compressible, the solubility of ore minerals in these systems varies considerably as a function of both temperature and pressure. We use novel pr...

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Bibliographic Details
Published in:Geology (Boulder) 2015-07, Vol.43 (7), p.587-590
Main Authors: Hurtig, Nicole C, Williams-Jones, Anthony E
Format: Article
Language:English
Subjects:
chemical composition
copper ores
Economic geology
epithermal processes
Geochemistry
Geology
gold ores
magmatism
Mathematical models
metal ores
mineral deposits, genesis
Minerals
molybdenum ores
ore-forming fluids
polymetallic ores
porphyry copper
porphyry gold
porphyry molybdenum
Sedimentation & deposition
Thermodynamics
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Summary:Shallow magmatic-hydrothermal systems are characterized by steep gradients in temperature and pressure, and because the fluid is of low density and highly compressible, the solubility of ore minerals in these systems varies considerably as a function of both temperature and pressure. We use novel pressure- and temperature-dependent experimentally derived thermodynamic data to geochemically model the transport and deposition of Au, Ag, and Mo by vapor and low- to intermediate-density supercritical fluids in the context of Au and Mo porphyry and Au-Ag epithermal ore formation. The results show that there is a strong compositional control on the Au/Mo ratio of the parental ore fluid, which can explain Au-Mo zoning in porphyry ore deposits and the formation of Au-rich and Mo-rich subtypes. Gold solubility reaches a maximum between 320 °C and 500 °C, depending on the fluid density, whereas Mo and Ag concentrations decrease with decreasing temperature and pressure. These differences in mineral solubility help explain the fractionation of Au from Ag and Mo and the preferential mobilization of Au into sites of epithermal ore deposition. Application of this modeling of metal solubility in vapor-like fluids offers an important opportunity for understanding individual ore-forming hydrothermal systems and determining the limiting factors for metal enrichment.
ISSN:0091-7613
1943-2682
DOI:10.1130/G36685.1