THE SEARCHLIGHT DISTRICT, NEVADA; OXIDIZED, BUT WHEN?

The Searchlight district, in Clark Co., Nevada, consists of a system of Au-bearing veins that are hosted primarily by volcanic rocks (trachydacite and trachyandesite; 18.5–16.2 Ma) above the roof of the Miocene Searchlight pluton (quartz monzonite and granite in the roof, local gabbro at lower levels; 17 – 15.7 Ma; about 16 Ma at roof). The deposits produced about 250,000 ounces of gold from 1902-1962, more than half of it by 1909. The veins are in shattered volcanic rocks, with only minor drusy quartz cementing the fragments. Although Cu grades of several percent and Pb and Zn grades near one percent were common, virtually no base-metal sulfide minerals are present at the surface or at depths to 260 m. Ore minerals included gold, chrysocolla, cuprite, chalcocite, leadhillite, wulfenite, vanadinite and rarely, galena. The most common gangue assemblage is quartz-specular hematite; at deeper levels, epidote is present. Wall-rock alteration is generally weak, and the most common alteration minerals are illite, montmorillonite, and kaolinite. Pyrite occurs at only one locality. The district underwent large-magnitude east-west extension beginning around 16 Ma. Westward tilting of 70 to 90 deg or more has revealed the vein system, pluton and overlying comagmatic volcanic rocks in cross-section (> 10 km). The known veins are restricted to the conduit complex of the Searchlight stratovolcano. Structurally above the productive veins, several rhyolite domes (16.2 – 15.7 Ma) have been almost completely altered to an assemblage of quartz-alunite-hematite-jarosite. All the observed mineral assemblages indicate extremely high oxygen fugacity. In addition, igneous rocks from the Searchlight area, as well as from the Spirit Mountain batholith to the south and the Eldorado district to the north, all have high Fe³⁺/Fe²⁺ ratios. What is not clear is whether a normal sulfide-bearing assemblage has been destroyed by pervasive oxidation subsequent to deposition, or if sulfides were never widespread. The igneous rocks and hydrothermal system developed just prior to or at the inception of large-magnitude extension. We favor the interpretation that resultant massive jointing and pervasive fracturing induced by the rapid onset of extension led to a massive influx of oxidizing meteroic water while the hydrothermal system was still active.