Thallium Isotope Implications for the Metalliferous Source of Carlin-type Deposits in northern Nevada

Abstract

The Carlin-type deposits of northern Nevada comprise a world-class accumulation of Au deposits with a characteristic geochemical signature of elevated Tl, As, Hg, and Sb. Three possible sources of Au in Carlin-type systems have been described, and include: (1) leaching of Au from pre-enriched Paleozoic sedimentary rocks (2) metamorphic mobilization or leaching of Au by meteoric waters from deeply-buried, late Proterozoic to early Cambrian clastic sedimentary rocks, and (3) magmatic-hydrothermal fluids evolved from magmas. Traditional geochemical and isotopic tracers, including Pb, H, O, S, and C, have yielded ambiguous results, and sources of hydrothermal fluids and their components, especially gold remain enigmatic.Because there is a strong correlation between Tl and Au in these deposits, Tl can be considered as a proxy element of Au. This study describes the first-ever Tl isotope data collected for ore- and late-stage mineralization from a Carlin-type system, as well as for a suite of potential source rocks, with the goal of fingerprinting Au sources for these systems in northern Nevada. Although not pointing to an unequivocal origin for the Tl (and, therefore, Au), the Tl isotope data indicate that Paleozoic carbonate rocks are not a likely metal sources. In addition, comparison of ore- to late-stage minerals may suggest a shift in Tl isotope compositions that could be consistent with evolution of magmatic- and/or metamorphic-dominated fluid to a meteoric-dominated fluid with time.