Geological Society of Nevada – September 21

Monthly meeting: September 21, 2012
Elks Lodge, Reno

RSVP by September 20 to Laura Ruud: 323-3500 or gsn@gsnv.org

Title: A magmatic-hydrothermal model for Carlin-type gold deposits
Speakers:
Adam Simon: University of Michigan
John Muntean: University of Nevada Reno (NBMG)
Jean Cline: University of Nevada Las Vegas

Geologists continue to debate the origin of Carlin-type gold deposits. Several geologically plausible hypotheses have been proposed ranging from magmatic-hydrothermal, to metamorphic, to connate, to deeply circulating meteoric fluid. In this talk, I will present new and published data that are consistent with a magmatic-hydrothermal model for the origin of CTGDs. These data include: 1) a spatial and temporal association between CTGDs and magmatism, both within individual deposits and at mid-crustal levels; 2) high-resolution electron microprobe traverses of ore-stage pyrite that demonstrate the nearly ubiquitous presence of a suite of magmatically-incompatible trace elements (As-Hg-Tl-Te-Sb) in gold-rich pyrite growth zones; 3) partitioning data from experiments and natural fluid inclusions that demonstrate the ability of magmatic aqueous fluid(s) to scavenge Au and incompatible trace elements (As-Hg-Tl-Te-Sb) and transport them into the upper crust; 4) the presence of ~0% sulfur isotope values in ore stage, gold-bearing pyrite growth zones; and, 5) O and H isotope data in ore-stage fluid inclusions that are consistent with magmatic aqueous fluid. The formative model involves: 1) degassing of the Farallon ocean plate and hydration of the overlying mantle wedge and sub-continental lithospheric mantle (SCLM); 2) the ascent of early aqueous fluids with a low Au/Cu ratio, resulting in restite having a high Au/Cu ratio and enrichment of the SCLM in As-Hg-Tl-Te-Sb; 3) flattening of the slab followed by slab detachment resulting in asthenospheric upwelling and the generation of large-scale basaltic magma at the base of the SCLM; 4) mixing, assimilation and homogenization of asthenospheric magma with the base of the crust; 5) ascent of this hydrous, fertile, Au-rich magma along deep faults that were reactivated during Basin and Range extension; 6) ponding of Au-rich, As-Hg-Tl-Te-Sb-bearing magmas at depths of ~10 km, consistent with gravity and resistivity studies; 7) degassing of the silicate melt and evolution and ascent of Au-rich, As-Hg-Tl-Te-Sb-bearing aqueous fluid; 8) vapor contraction, brine condensation and loss of Fe from the ascending aqueous fluid; 9) acidification of the aqueous fluid owing to dissociation of carbonic acid resulting in permeability enhancement of carbonate host rocks; 10) assimilation of downwelling meteoric fluid resulting in mixed H-, O- and S-isotope signatures in many deposits; 11) sulfidation of iron in the carbonate host rocks and concomitant reaction with S in the aqueous fluid to precipitate pyrite, destabilizing Au-chloride and Au-sulfide complexes and precipitating gold along with As-Hg-Tl-Te-Sb; and 12) continued ingress of meteoric water resulting in precipitation of post-ore orpiment, realgar, and stibnite. The geologically plausible model is consistent with all salient characteristics of CTGDs.