MAFIC MAGMAS AS SOURCES FOR GOLD IN MIDDLE MIOCENE EPITHERMAL DEPOSITS OF THE NORTHERN GREAT BASIN, UNITED STATES: EVIDENCE FROM Pb ISOTOPE COMPOSITIONS OF NATIVE GOLD

Abstract

We measured the Pb isotope compositions of traces of lead alloyed by gold, electrum and naumannite, and gangue vein minerals in bonanza epithermal gold deposits of the Northern Great Basin, and compared them to possible sources of lead in the region. Gangue minerals have Pb isotope compositions consistent with hydrothermal leaching of lead from local host rocks. Lead isotope ratios of gold ores, however, have signatures that are distinct from the gangue minerals and common host rocks in the area, such as metasediments and Miocene rhyolites. The gold ores are isotopically similar to local mafic rocks of the middle Miocene bimodal volcanic assemblage, indicating that the latter are the likely source of the precious metals. The local mafic rocks are also similar to the middle Miocene Columbia River Basalts, suggesting that both were derived from a mantle source with similar Pb isotope composition. Our preliminary results indicate that the precious metals were derived from mafic magmas likely related to northern Nevada rifting processes and/or to the arrival of the Columbia River Basalt plume and the emergence of the Yellowstone hot spot. Exsolving volatiles from the mafic magmas most likely transported gold upward to the shallow, meteoric water-dominated epithermal systems, where the precious metals precipitated together with quartz and adularia to form bonanza ores that typify these deposits.