SYNCHRONOUS AU-ALUNITE IN THE PASCUA-LAMA HIGH-SULFIDATION SYSTEM: CONSTRAINTS ON THE ORIGIN AND CHEMISTRY OF MINERALIZING FLUIDS

The Pascua-Lama Au-Cu-Ag system, situated at the north end of the El Indio-Pascua Belt in the Main Cordillera of Chile and Argentina, hosts widespread zones of hydrothermal advanced argillic alteration and several styles of Au and Cu ± Ag mineralization. The largest mineralized zone is concentrated in and around Brecha Central, on the Chilean side of the property. In this zone, precious-metal mineralization is coeval with an assemblage of alunite-pyrite-enargite (APE). Gold occurs as inclusions in enargite and pyrite intergrown with alunite, and rarely in alunite itself. The synchronous deposition of alunite and gold is unusual, but has been documented in a few other systems (e.g., Tambo, Chile; La Mejicana, Argentina).

The co-precipitation of alunite with precious metal-bearing sulfides provides important constraints on the nature of the Brecha Central mineralizing fluids. The interpretation of the origin of these fluids and conditions of Cu-Au deposition was based on paragenetic observations, stable-isotope data (d ³⁴S, d D, d ¹⁸O_SO4, d ¹⁸O_OH), fluid-inclusion analyses, and thermochemical calculations. Mineralizing fluids were about 3 wt % NaCl equivalent with bulk sulfur concentrations (å S) greater than 0.05m and pH values of 2 to 4. Fluid-inclusion homogenization and D ³⁴S_alun-py data indicate depositional temperatures averaging 275^oC. Sulfur isotope data for alunite (+13 to +25‰) and associated sulfides (-3 to -6 ‰) are consistent with a magmatic-hydrothermal origin and indicate H₂S/SO₄ ratios of 1 to 2.5. Oxygen and hydrogen isotope systematics indicate mineralizing fluids were of magmatic origin (d D_H2O between -30 and -47‰).

Calculations indicate the solubility of gold as aqueous chloride or hydrosulfide (AuHS_(aq) or Au(HS)^-2) complexes is lowest under the conditions required for APE precipitation. Given the high concentration of sulfur in the mineralizing fluids, gold was likely transported as a bisulfide complex. Au deposition was possibly induced by H₂S_(g) loss during boiling, inferred from co-existence of vapor- and liquid-rich fluid-inclusions in the APE assemblage, and may have been further promoted by the precipitation of abundant enargite and pyrite from solution.