Precious metal vein systems in the National District, Humboldt County, Nevada

Abstract

The National district, 20 miles (33 km) southeast of McDermitt, Nevada, produced 200,000 ounces of gold and 750,000 ounces of silver during the period 1906-1941. Precious metal quartz-sulfide-silicate veins near the townsite of National and on Buckskin Mountain occur within a 3,000-ft-thick (910 m) sequence of Miocene volcanic and intrusive rocks which vary in composition from basalt to rhyolite. Veins formed about 15.6 million years ago, and associated rhyolites are approximately 18 to 15 m. y. old. The volcanic pile at National is highly faulted and deposits are localized within orthogonal fault sets that coincide with major upper crustal structures in the northern Great Basin. Intrusive rhyolite and eruptive centers spatially related to veins at National partly define the Goosey Lake depression, a shallow basin situated on the western edge of the Owyhee volcanic plateau.Wall rock in the district is progressively hydrolyzed and sulfidized to assemblages dominated by chlorite + calcite, illite-montmorillonite + quartz + pyrite, and kaolinite + quartz + pyrite as veins are approached. Alteration assemblages exposed by exploration on Buckskin Mountain include the paleosurface. At Buckskin planar, open-space-filled veins disperse upward into structurally unconfined, flat-lying zones of kaolinite + quartz, quartz + alunite, and chalcedonic silica (sinter) + quartz-altered rhyolite, the latter being the paleosurface assemblage. Metallic minerals in four nested and coalescing vein stages include cinnabar, stibnite, electrum, silver sulfosalts and selenides, and base metal sulfides. Vein silicates are quartz, kaolinite, illite, and, at depth, muscovite.The spatial distribution of hydrothermal sulfides and silicates throughout Buckskin Mountain is strongly influenced by physical properties of premineralization volcanic rocks. Porous and permeable rhyolite tuff capping Buckskin Mountain is pervasively hydrolyzed to laterally extensive silica, alunite, and kaolinite-bearing assemblages which lack tabular zones of open-space filling. Underlying dense and brittle rhyolite flows confine hydrothermal assemblages to a relatively few planar fractures flanked by parallel selvedges of progressively altered wall rock.At Buckskin Mountain two distinct but synchronous thermal events are documented by fluid inclusion studies. The earlier event involved temperatures ranging from approximately 295 degrees to 250 degrees C and culminated in rupture of overlying rhyolite, producing an inverted conical mass of bilithic breccia. Fluids associated with the second, postbrecciation event boiled at 100 degrees to 255 degrees C from the paleosurface to depths exceeding 2,000 ft (606 m). Near-surface hydrothermal assemblages were deposited from low-salinity (1-2 wt % NaCl) solutions containing <2 weight percent dissolved gases. Precious metal precipitation occurred at a temperature of about 250 degrees C and a pressure of about 40 bars, largely from boiling solution. Periodic restrictions to convective heat loss resulting from silica deposition and episodic fluid circulation cause observed temperature and pressure data to deviate somewhat from an ideal hydrostatic model. The vertical temperature gradient is consistent with a water-saturated melt of granitic composition about 10,000 ft (3,030 m) below the paleosurface during mineralization. Close spacing of isotherms below 200 degrees C reflects paleosurface proximity (cooling by conduction and steam loss) as well as accelerated convective cooling of fluid circulating through permeable rhyolite tuff.Integration of hydrothermal assemblages, thermal measurements, fluid-mineral equilibria, and active hot springs data suggests that as temperatures in Buckskin Mountain decreased from 250 degrees C at depth to 100 degrees C at the surface, pH increased from approximately 3.7 to near neutral, Sigma S increased from 10 (super -4.5) m to approximately 10 (super -2.6) m, and f (sub O 2 ) decreased from 10 (super -39) to <10 (super -48) . The distribution of alteration assemblages, precious metal minerals, stibnite, and cinnabar was apparently controlled by boiling, a sulfldation gradient, temperature decreases, and near-surface hydrology.