TIMING OF MINERALIZATION IN EPITHERMAL AU-AG DEPOSITS IN RELATION TO MAGMATISM ALONG THE TRACK OF THE MIOCENE YELLOWSTONE HOTSPOT

The Miocene bimodal volcanism of the Northern Great Basin has long been associated with the development of high-grade epithermal gold deposits. Geochronologic and paleomagnetic studies have shown that bimodal volcanism in the Northern Great Basin and adjacent Oregon Plateau began at ca. 16.7 Ma and subsequently migrated eastward along the Snake River Plain and trend of the Yellowstone hotspot. In detail, the duration of magmatism in any one location may be up to ca. 1-2 m.y., such that ages of ca. 16.5-14.5 Ma have been determined for a range of volcanics from early eruptive centers (see Brueseke et al., 2007, JVGR). Adularia is commonly present as an accessory mineral in the epithermal deposits, intergrown with native gold, electrum and naumanite, and is readily dated by the ⁴⁰Ar/³⁹Ar technique. We have used laser single crystal analytical methods in the Auburn Noble Isotope Mass Analysis Laboratory (ANIMAL) to determine the ⁴⁰Ar/³⁹Ar ages of adularia from approximately 40 historic Au-Ag deposits and ores that are found in the Northern Great Basin of Nevada and eastward to the Owyhee Plateau of southern Idaho. We reported earlier (Hames et al., 2009, JVGR) that adularia ages varied from ca. 16.5-15.5 Ma among many Au-Ag deposits of the region. The oldest results in our study are for the Jumbo deposit in northwestern Nevada, one of the westernmost deposits we have studied, which yielded adularia with a combined age of 16.54±0.04 Ma (the mean of 3 single-crystal plateau ages, at the 95% confidence level). Our more recent work in the Silver city district of southern Idaho indicates a shorter and generally younger duration for formation of Au-Ag deposits in this more eastern region. ⁴⁰Ar/³⁹Ar ages for adularia from several deposits and historic mines around Silver City (Oro Fino, Poorman, Cumberland Mill, Trade Dollar, Blackjack) indicate all formed within a relatively brief interval at ca. 15.5 Ma. In comparison with our earlier work and other published ages, the timing, location, and age variations of Au-Ag deposits in the region are consistent with the regional record of bimodal volcanism and eastward migration of the Yellowstone hotspot province.