MAFIC MAGMAS AS GOLD SOURCES FOR EPITHERMAL DEPOSITS: INSIGHTS FROM HIGH-PRECISION PB ISOTOPIC MEASUREMENTS OF GOLD ORES FROM DIVERSE VOLCANO-TECTONIC SETTINGS

Magmatic contribution, if any, to low-sulfidation (LS) epithermal gold deposits is still widely debated. In this work we present high-precision Pb isotopic analyses conducted with MC-ICP-MS for two gold-producing regions in different volcano-tectonic settings. Pb isotope data for native gold and electrum from Northern Nevada LS bonanza deposits, located in continental extensional settings, show Pb isotopic disequilibrium with host rocks and gangue minerals. Pb isotopic similarity between gold and mafic lavas, possibly related to the arrival of the Columbia River Basalt plume and initiation of the Yellowstone Hot Spot track, provide evidence that the precious metal was extracted from mafic magmas. Data for the epithermal, subduction-related Ladolam deposit on Lihir Island, SW Pacific, reveal small, but significant differences between ores and associated host lavas. Gold ores show similar isotopic compositions to mafic, volatile-rich lavas in the area. Petrographic data reveal a mafic magma recharge episode accompanied by volatile exsolution that possibly was the trigger for the ore formation in the area. The isotopic disequilibrium between ore and host rocks and gangue vein minerals observed in both regions indicate that precious metals were not hydrothermally leached from local lithological units. It appears that in both cases discrete pulses of magmatic volatiles, released from deeper mafic magmas, transported the gold to the shallow epithermal system, where the precious metal precipitated to form some of the richest gold deposits in the world.