GEOCHRONOLOGY OF THE SLEEPER DEPOSIT, HUMBOLDT COUNTY, NEVADA - EPITHERMAL GOLD-SILVER MINERALIZATION FOLLOWING EMPLACEMENT OF A SILICIC FLOW-DOME COMPLEX

The high-grade gold-silver deposits at the Sleeper mine are low sulfidation, quartz-adularia-type epithermal deposits, formed during the final stages of igneous hydrothermal activity of a small middle Miocene silicic flow-dome complex in north-central Nevada. Potassium-argon and 40Ar/39Ar ages of alkali feldspar indicate that the rhyolitic flow-dome complex was emplaced before about 16.5 Ma; hydrothermal alteration and mineralization followed, lasting from about 16.2 to 14+ Ma based on ages of adularia associated with electrum. There were multiple pulses of alteration and mineralization but all occurred within a period of less than 2 m.y. Later supergene alteration formed opal and alunite about 5.4 Ma but produced no Au or Ag mineralization other than some remobilization to produce locally rich pockets of secondary Au and Ag enrichment and is unrelated to the older magmatic hydrothermal system. The Sleeper deposit in the northern part of the Great Basin is genetically related to bimodal volcanism that followed a long period of arc-related andesitic volcanism in the same general region.