ORIGIN OF THE AMAGMATIC LOW-SULFIDATION EPITHERMAL FLORIDA CANYON AU-AG DEPOSIT, PERSHING COUNTY, NEVADA

The Florida Canyon is a large, low-sulfidation (LS), epithermal gold deposit characterized by low-grade, disseminated mineralization. It is localized along and in the footwall of an active range-front fault system that at the surface separates Mesozoic rocks of the Humboldt Range from late Tertiary and Quaternary basin fill deposits of the adjacent Humboldt River valley. Mineralizing fluids were guided by the range-front fault and a northeast-trending shear zone. Florida Canyon is unusual for LS epithermal Au-Ag deposits in that it is hosted by weakly metamorphosed mudstones, siltstones and sandstones (Triassic Grass Valley Formation) and apparently lacks an association with coeval igneous rocks. The Humboldt House geothermal system immediately west of the deposit in basin sediments has a reservoir temperature of ~250 ºC and is marked by auriferous quartz-cemented gravels, sinter and travertine.

An investigation of the origin of the deposit revealed that the deposit formed in three main stages. Early quartz-adularia-sulfide-gold stockwork bodies formed at 5.0 to 4.5 Ma from meteoric water that had isotopically exchanged with and leached metals and sulfur from sedimentary and volcanic rocks hosting the deposit. Mineral deposition occurred at 150 to 200 ºC from episodically boiling fluids. Oxidizing, acid-sulfate steam-heated alteration consisting of friable alunite-quartz-kaolinite±sulfur developed at 3.4 to 1.8 Ma and overprinted the stockwork bodies during a relative drop in the water table. Within the last ~1 Ma weathering overprinted the deposit producing widespread hematite-goethite alteration. The origin of the deposit is closely related to the kinematic history of the range front fault system, regional high heat flow and possibly to paleoclimate changes. Moreover, it apparently represents an older expression of a long-lived (~5 Ma) extensional-type hydrothermal system that is presently manifested as the Humboldt House geothermal field.