MAGMATIC VAPOR PLUMES, MIGRATED EXTENSION, AND THE FORMATION OF EPITHERMAL HIGH-SULFIDATION GOLD-COPPER DEPOSITS

Epithermal, high-sulfidation gold-copper deposits have two distinct styles: low-grade, bulk-tonnage deposits (e.g., Yanacocha, Peru), and high-grade, low-tonnage, structually-controlled deposits (e.g., Goldfield, Nevada). The formation of both styles is consistent with degassing phenomena within composite volcanoes and domes (e.g., Hedenquist, 1991, 1993).

A singularity, particularly of the bonanza deposits, is a uni-directional temporal fluid progression that has occurred at all high-grade deposits, irrespective of age (Precambrian to Neogene) or location, from pre-ore advanced argillic quartz-alunite-kaolinite alteration to superposed gold-pyrite-enargite mineralization. Typically, a sequencing of two independent fluids has been invoked to explain this singularity (e.g., Arribas, 1995; Cooke and Simmons, 2000). However, we suggest that both "stages" derive from a single, magmatic "vapor" plume (Henley and McNabb, 1978) in response to migration of the locus of extension and consequent effect on permeability in the flow-controlling fault system. The initial leaching and silica-alunite-kaolinite-pyrite alteration results from phase separation generating an acidic liquid phase in veins and wallrocks. Subsequent reorientation of local tectonic stresses results in enhanced, focussed flow of supercritical fluid in hot, acid-altered rocks. Localized dilation and irreversible adiabatic expansion of supercritical fluid promote rapid metal-sulfide deposition from solvated gas-phase metal complexes (e.g., Williams-Jones, 2002).

We suggest that the bulk-tonnage deposits, with localized breccias, develop within the high-level discharge regime of magmatic-gas systems through highly efficient condensation into surficial ground-water regimes resulting in secondary dispersive acidic steam-heated-water plumes. For high grades, a second stage of structural development allows localized grade enhancement in response to higher gas discharge rates.