Abstract

Precious metal veins of the National district, Humboldt County, Nevada, occur in strongly altered Miocene volcanic rocks. Hydrothermal assemblages in Buckskin Mountain display a marked zonation from the paleosurface downward that correlates with the elevation and physical properties of enclosing rhyolites. Light stable isotope analyses indicate that meteoric water from two distinct sources comprised the ore fluid. The delta D values in fluid inclusions from deep-level quartz vein segments, >1,250 ft (>379 m) below the palcosurface, range from -115 to -129 per mil. Fluid inclusion waters from near-surface quartz vein segments, 0 to 1,250 ft (0-379 m) below the paleosurface, have delta D = -86 to -125 per mil and are generally more depleted than local Miocene ground water whose delta D is calculated at -85 per mil. The delta D range in inclusion waters from near-surface quartz veins has resulted from mixing of the fluids present in deep quartz veins with local ground water and condensed steam derived from boiling of both fluids. The fluids present in deep quartz veins probably originated as precipitation on distal highlands with elevations approaching 6,000 ft (1,818 m). Oxygen isotope data from vein and wall-rock minerals record increasing depletion as veins are approached, indicating that quartz veins were the major conduits of fluid flow. The degree of oxygen exchange between hydrothermal fluid and wall rocks, water/rock mass ratios, and the abundance of hydrothermal phases increase toward the paleosurface. The progressive influx of local ground water with increasing elevation is also indicated by chloride dilution.The estimated duration of the Buckskin Mountain hydrothermal system, 50,000 to 200,000 yrs, is limited by the volume of high-temperature alteration assemblages and the heat budget of a proposed intrusion. Thermal constraints and isotopic data provide an internally consistent range in lifetimes which is several times shorter than that proposed for some active hydrothermal systems. These long-lived modern systems apparently require multiple thermal and/or intrusive events.Based on geologic, isotopic, and floral data, the palcogeography of ancestral Buckskin Mountain was dominated by forested volcanic terrane having considerable relief. An intermontane basin, remnants of which comprise the summit of Buckskin Mountain today, contained scattered thermal pools and patches of steaming ground. The basin covered several square miles and stood at 3,000 ft above sea level, while adjacent uplands were several thousand feet higher in elevation. Floras included subalpine spruce and fir, mixed conifer-hardwood slope forests, and basin-floor hydrophytes (willow and marsh grasses). Buckskin Mountain during the Miocene closely resembled present landforms in Yellowstone National Park, Wyoming.

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