Fluid-mineral relations in the Comstock Lode

Abstract

The Comstock lode, a world-class precious metal vein deposit in the Comstock district 15 mi (24 km) southeast of Reno, Nevada, formed 13.7 Ma in Miocene eruptive rocks of the Alta and Kate Peak Formations and in pre-Tertiary metavolcanic and intrusive rocks. Production of 8.25 million oz (311,850 kg) in gold and 192 million oz (7.26 million kg) in silver came largely from bonanzas, or vein segments containing hundreds of thousands to millions of tons and grading > or = 0.4 oz/ton ( approximately 14 g/ton) gold and > or = 10 oz/ton ( approximately 340 g/ton) silver. Within the 8-mi-(12.9 km)-long Comstock fault bonanza ores are distributed 3 mi (4.8 km) along strike and persist from the present surface to 3,300 ft (1,007 m) below the surface. During the middle Miocene andesitic to rhyodacitic Alta and Kate Peak rocks formed a stratovolcano that exceeded 7,000 ft (2,134 m) in elevation. Bonanza ores were deposited within this volcano from a +200 degrees C hydrothermal system that affected more than 37 mi 3 (150 km 3 ) of rock.At least one Cretaceous and several Miocene hydrothermal events were imposed on Comstock district rocks. The Comstock lode vein assemblage, which resulted from the youngest and only economically important event, consists of quartz, electrum, silver minerals (mainly acanthite), pyrite, base metal sulfides, and minor amounts of calcite, sericite, chlorite, K feldspar, and albite. Andesite adjacent to veins is altered to quartz, sericite, and pyrite at vein contacts and quartz, sericite, montmorillonite, chlorite, and pyrite distally. Kaolinite is present in wall rocks adjacent to vein segments in which boiling occurred. Addition of the external components water, CO 2 , and sulfur gases sufficed to produce all Comstock vein alteration assemblages.Based on paragenetic and fluid inclusion studies, multiple generations of hydrothermal fluid formed Comstock district veins, but the earliest and highest temperature fluid deposited ore minerals. It ranged in temperature from >300 degrees C at the north end of the lode (Cedar Hill) to 6 wt percent NaCl, and was composed of both meteoric and magmatic water, according to fluid isotopic analyses. Younger cospatial fluid(s) contained up to 2.1 wt percent CO 2 . The thermal center of the Comstock lode hydrothermal system, Cedar Hill, is also the site of the highest measured ore fluid salinities.Ore fluid isotherms, and to an extent, salinity isopleths, display reversals and convolutions that are spatially related to bonanzas. These sharp temperature gradients coupled with mixing of ore fluid and at least one other high-temperature fluid, indicated by enthalpy-salinity relationships and deuterium enrichment, most likely caused precipitation of ore minerals. Sphalerite and electrum compositions indicate that bonanzas were deposited over a narrow range of f (sub S 2 ) = -13.5 to -15.2, while sphalerites from low-grade and barren vein segments indicate more widely variable f (sub S 2 ) . Gold-silver ratios both in electrum and bulk ore appear to be temperature controlled as higher temperatures generally correlate with high Au/Ag ratios. Local boiling of 250 degrees to 285 degrees C fluid provides a paleosurface datum, but vein temperatures >300 degrees C require up to 3,800 ft (1,152 m) of cover over the tops of bonanzas during mineralization.The distribution of both ore-stage and late vein calcite partially reflects hydrothermal fluid circulation patterns. In veins at district margins, the abundance of late calcite marks the demise of hot water circulation and the influx of large amounts of cold Miocene ground water into vein interstices.Kate Peak intrusions between Cedar Hill and Mount Davidson share a common structural control with vein segments that comprise the Comstock lode, and their ages span vein ages. The coeval Kate Peak magmas that provided heat for hydrothermal circulation formed the core of and uplifted the Miocene stratovolcano which was centered over Cedar Hill. Partial symmetry of thermal, compositional, and kinetic gradients about Cedar Hill suggests that additional bonanzas await discovery along unexplored Comstock fault segments within declining palcothermal gradients to the north.