THE IMPORTANCE OF PLATE TECTONIC ARCHITECTURE FOR THE FORMATION OF CARLIN-TYPE GOLD DEPOSITS

The time-space connection between Carlin type gold deposits (CTGDs) and Eocene magmatism in north-central Nevada has stimulated the hypothesis that magmatism played a role in the formation of CTGDs. However, the exact role, either as a heat engine, a metal source, or both, remains controversial. We present a geologic model that links the formation of CTGDs with Eocene convergent and divergent plate tectonics and Paleozoic crustal sutures that formed during the Antler orogeny in western North America. Our model is consistent with all observed features of CTGDs and supports a genetic physicochemical connection between CTGDs and Eocene magmatism. We hypothesize that Au was ultimately sourced from mantle-derived silicate magma that ascended along permeability contrasts that extended from the near-surface to the base of the crust. The driving force for magma production and ascent was likely slab rollback coupled with the initiation of southward-propagating rifting in Nevada. Two-stage degassing of the Farallon slab and concomitant metasomatism of the lithospheric mantle efficiently transferred Au and the classic suite of CTGD pathfinder elements (i.e., As, Sb, Tl) from the slab to the mantle, and fractionated this metal suite from base metals. The resulting Au-As-Sb-Tl-bearing mantle-derived magma ascended along ancient plate tectonic sutures, which were inverted and opened by Eocene rifting and delamination. The crystallization and removal of pyrrhotite from the magma further fractionated CTGD metals from base metals during ascent. We hypothesize that the mantle-derived magma ultimately ponded at a depth of approximately 10 to 12 km, consistent with the brittle-ductile transition that acted as a permeability barrier. Here, the melt experienced volatile saturation and the exsolution of CTGD-metal scavenging and transporting aqueous-carbonic fluid, which continued to ascend along inverted faults until it encountered extensive near-surface silty-limestones where water-rock reaction and dilution by meteoric fluid promoted precipitation of Au and associated CTGD metals.