TRACE ELEMENT AND ISOTOPE MICROANALYSES SUPPORT A DEEP ORE FLUID SOURCE AT THE GETCHELL CARLIN-TYPE GOLD DEPOSIT, NORTHERN NEVADA

A consensus on the genesis of Carlin-type Au deposits remains elusive owing, in part, to the presence of superimposed hydrothermal events at most deposits and the fine-grained nature of ore minerals that provide for challenging paragenetic studies. Important conclusions from previous Carlin studies include: submicron Au occurs in trace-element rich pyrite, a wide, mostly positive, range of S isotopes is consistent with thermochemical reduction of sulfate S from sedimentary rocks, O and H isotopes indicate a deep ore fluid source at Getchell, but identified a meteoric fluid in the northern Carlin trend, and inclusion ore fluids from Getchell contain dilute magmatic/mantle He.

Recent microanalyses of ore pyrite at Getchell include quantitative electron microprobe (EMPA), LA-ICP-MS, and ion probe S isotope (SIMS) analyses, and back scatter electron imaging (BEI). BEI has revealed <0.5 micrometer ore pyrite rims, not visible under a microscope, on anhedral pre-ore pyrite; textures suggest dissolution of pre-ore pyrite prior to ore pyrite deposition.

EMPA and LA-ICP-MS analyses quantified metals, provided profiles documenting elemental changes across grains, and identified Au-bearing trace element suites. Quantitative profiles across zoned grains place some elemental suites in a paragenetic context. Collectively, analyses show that Au is consistently associated with As, Hg, and Cu, and variably associated with Sb, Tl, Te, Al, and Pb; Zn, Se, and Ag are near or below detection limits. Al occurs locally in outer rims and may be added to ore fluid in response to fluid-rock reaction near the site of Au deposition. Highest Au concentrations (~4000 ppm) occur in innermost rims and are accompanied by As, Hg, Cu, Te +/- Tl +/- Sb.

S isotopes in pre-ore pyrite are variable, whereas analyses of ore-stage pyrite cluster around 0‰ (+/- 3‰). Calculations for ore pyrite and late ore orpiment and realgar indicate an increase in the S isotopic ratio of ore fluid H2S over time. These results permit a magmatic source for S.

Microanalyses continue to support a deep ore fluid source at Getchell. Intriguingly, a fluid could acquire the high-Au metal suite through metal partitioning to, or generation of, a low-density aqueous vapor. Results are consistent with generation of an ore fluid during deep crustal magmatic and/or orogenic processes.