LINKAGES BETWEEN CRUSTAL STRATIGRAPHY, MAGMATIC-HYDROTHERMAL EVOLUTION, AND SOURCES OF COMPONENTS OF GOLD-SILVER DEPOSITS IN THE ADJOINING GOLDFIELD AND TONOPAH DISTRICTS, NEVADA

The 22-19 Ma Au-Ag deposits at Goldfield and Tonopah, NV, among the highest grade quartz-alunite and quartz-adularia epithermal systems, formed in 26-16 Ma volcanic fields that are 50 km apart and straddle the inferred ⁷⁰⁶Sr boundary. Isotope (Pb, Sr, Nd, S) and element (Rb/Sr) values of host rocks and hydrothermal minerals reveal dissimilar crustal stratigraphy under each district, and support contrasting depths of origin, emplacement, and degassing of nearly coeval, subduction-related andesitic magmas.

At Goldfield, Pb, Sr, Nd, and S isotope values and data arrays of hydrothermal sulfate minerals and volcanic host rocks indicate that sub-volcanic Paleozoic-Neoproterozoic siliciclastic rocks were assimilated by magmas originating in sub-continental lithosphere. Upper crustal component sourcing is consistent with pervasive hydrolytic alteration of volcanic and older host rocks, caused by shallow emplacement and devolatilization (~1.5 km) of magmas, vapor-brine separation of magmatic fluid with H₂S>SO₂, conversion of most H₂S to SO₂, and acidification of ground water by SO₂ hydrolysis. Arrays of Pb isotope values of hydrothermal sulfide minerals suggest derivation of Pb, and possibly other chalcophile elements, from a different, relatively thorogenic source.

At Tonopah, Pb, Sr, and S isotope values of host rock and sulfide minerals are lower, Nd is higher, and all are less variable than at Goldfield, reflecting less enriched lithospheric or deeper sources and minor contributions from siliciclastic rocks. Isotope values are consistent with ore deposition from low-salinity meteoric water contaminated by magmatic volatiles with H₂S>SO₂, that were exsolved from magmas emplaced at ≥4 km. Arrays of Pb isotope values of sulfide minerals and volcanic host rocks at Tonopah, and from the large Comstock Lode Au-Ag district 240 km to the NW, also indicate relatively thorogenic sources of some Pb. Coupled with relatively constant Rb/Sr and initial Sr values of volcanic host rocks, and slightly negative S isotope values, the Tonopah and Comstock Lode Pb isotope arrays suggest that ore-related intrusions and chalcophile elements in deposits were derived primarily from deep, isotopically homogenized sources, possibly metasomatized Proterozoic crust or enriched mantle, or Mesozoic plutons derived from them.