VAPOR TRANSPORT OF ORE METALS, WITH A FRESH LOOK AT THE BINGHAM PORPHYRY

New evidence from experiments and thermodynamic studies, fluid inclusion microanalyses, and active volcanic and geothermal systems show that hydrothermal vapor may be a major transporter of ore metals. Three vapor-dominated geological environments, characterised by three distinct vapor evolution paths, may overprint each other when the fluid-generating magmatic interface in the roof of a cooling upper-crustal intrusion retracts to increasing depth.

In the near-surface fumarolic environment, a low-density vapor rapidly expands and may ultimately condense and mix with surface water. This generally causes barren acid leaching, but can sometimes generate sulfide-poor gold ores.

In a shallow porphyry environment, an intermediate-density fluid may ascend from the magmatic fluid reservoir through a fractured porphyry stock. On cooling and expansion, this fluid condenses a small amount of brine. The remaining vapor phase can be the dominant ore fluid, as illustrated by a recent fluid inclusion study of the Bingham porphyry-Cu-Au deposit. Laser-ablation microanalysis of fluids combined with cathodoluminescence petrography of the stockwork veins indicate quartz re-dissolution during Cu-Fe-sulfide deposition. Retrograde quartz solubility and mass balance based on vapor and brine inclusion compositions imply that the vapor phase dominates in terms of overall mass transfer.

Epithermal gold is deposited by low- to medium salinity aqueous liquids, but stable isotope data and thermodynamic modelling indicates that the best gold-mineralising fluids may be derived from a deeper-seated process of magmatic vapor separation. Separation of brine favors depletion of the vapor in iron chloride, allowing S- and OH-complexed elements including Au, As and some Cu to stay in the vapor. This vapor phase may contract to an aqueous liquid, by cooling along a decompression path passing above the critical curve of the fluid system. Thus, extremely gold-rich liquid can cool to epithermal conditions, without dispersing the metal along the flow path. In the epithermal domain, renewed boiling, fluid mixing or rock reaction leads to high-grade gold precipitation.

References: Heinrich C.A.. (Min. Dep. 39, 864-889, 2005); Landtwing M.L. et al. (EPSL, 235, 229-243, 2005); Redmond P.B. et al. (Geology 32, 217-220, 2004); Williams-Jones A. and Heinrich C.A. (Econ. Geol. 100AV Paper, 2005).