SULFIDE SATURATION IN MAGMAS ASSOCIATED WITH PRECIOUS METAL DEPOSITION AND QUARTZ-ALUNITE ALTERATION AT GOLDFIELD AND OTHER DISTRICTS IN THE ANCESTRAL CASCADES MAGMATIC ARC, CA-NV

In the ancestral Cascades magmatic arc, unaltered 23-5 Ma andesites and dacites that slightly predate or are coeval with Au+Ag deposits and quartz-alunite alteration zones, contain phenocrysts with sulfide melt inclusions (smi). Amphibole and Fe-Ti-O thermobarometry indicate that sulfide saturation occurred at magmatic temperatures, depths, and oxidation states of ~1000-800° C, 27-3 km, and NNO 1±1, respectively. Greater magma depths and temperatures broadly correlate with higher Au+Ag production.

The Au+Ag+Cu deposits at Goldfield, NV, are partly in ~22.3-21.2 Ma andesites with 5-25%, 1-10 mm plagioclase, pyroxene, amphibole, and biotite phenocrysts. Most smi are in plagioclase and pyroxene, few are in amphibole and magnetite, and none is in biotite. The smi contain 10s-1000s of ppm chalcophile elements, and have δ³⁴S = 3.3-4.7‰. Apatite inclusions in biotite have δ³⁴S = 6-14 ‰. These relationships, together with S concentrations of phenocrysts, silicate melt inclusions, and matrices, and whole rock δ³⁴S = 4.1-8.2 ‰, support sulfide saturation during crystallization of anhydrous minerals (NNO ≤1). Higher S⁶⁺/S^2-during biotite crystallization (NNO 1-1.5) reflects increased αH₂O and/or degassing. Large poikilitic plagioclase phenocrysts with amphibole and biotite microlites suggest that devolatilization accompanied mingling of andesite and coeval rhyolite (~22.1-21.6 Ma) magmas.

At Goldfield, paleosurfaces, coeval vapor and brine fluid inclusions, alteration mineral assemblages, and igneous and hydrothermal mineral isotopic compositions, indicate that SO₂-rich fluids, with δ³⁴S ≈ 3.5‰, similar to smi, were exsolved from andesite magmas at ≤3 km depth. These fluids condensed in meteoric water-saturated transtensional fault zones and altered host rocks to quartz-alunite (22.1-21.0 Ma)-pyrite assemblages. Overpressured, H₂S-rich fluids that accumulated In ductile carapaces of deeper magmas episodically transported chalcophile elements, possibly derived from decomposed smi, upward into fault zones, and encrusted breccia clasts with sulfide, telluride, and selenide mineral aggregates and Au alloys, rendering some faults minable. Generation of large, subduction-related metal deposits in the ancestral Cascades arc minimally spans the middle to upper crust.