142-5 Ore Fluid Source and Timing Using In Situ Pb Isotopes on Nevada County, California Gold in the Sierra Nevada Foothills

Session: Undergraduate and Graduate Geoscience Student Lightning Talk Showcase (Posters)

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Abstract:

Approximately 10 million ounces of bedrock lode gold were historically mined between 1849 to the 1940s in Nevada County, California, located in the Sierra Nevada Foothills Metamorphic Belt, NW of the Sierra Nevada batholith. Some of the most productive gold-quartz vein mines were North Star, Empire, and Idaho-Maryland near Grass Valley, CA. Numerous historic mines, prospects, and former underground workings are also found scattered across Nevada County, with the historic Mother Lode vein districts to the south. Bedrock gold mineralization is found in the central and western assemblages of the belt, consisting of Paleozoic to Mesozoic accreted and faulted subduction-related rocks, such as ophiolite sequences, island arc volcanics, mélange, and broad igneous intrusive activity.

Current literature on the source and timing of mineralizing fluids in the Foothills districts utilizes isotopic analysis or radiometric dating of minerals found in the gold-quartz veins, such as K-Ar, Rb-Sr, Ar-Ar on Cr-mica and U-Pb on REE-phosphate. Here we go further by analyzing phases with cogenetic textures (growth in and along fractures, zoning, inclusions, etc.) with the ore mineral of interest (Au), which was not considered in previous studies. Utilizing in situ LA-ICP-MS of Pb-rich minerals with and without cogenetic textures with Au-phases (electrum (Au,Ag), petzite (Ag₃AuTe₂), and native Au) will provide unique isotopic data for interpretations of ore fluid source and timing, and to determine unique mineralizing episodes.

Nevada County ore material gathered from historical mines and collections have been prepared for microscopic analysis by mounting in epoxy and polishing. Reflected light and scanning electron microscopy of samples has shown that Au-phases are commonly found in cogenetic textures with base metal sulfides (galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS₂)), other sulfide-phases (pyrite (FeS₂), arsenopyrite (FeAsS)), and tellurides (altaite (PbTe), hessite (Ag₂Te), coloradoite (HgTe)). Importantly, Pb-rich minerals, galena and altaite, are found in multiple unique textures coincident and non-coincident with gold. For reference, Pb isotope data is also being collected from regional Pb mineralization, such as from massive sulfide deposits also found in the Foothills. If shown to be useful in interpreting Au-source(s) and timing, this approach can be expanded to other orogenic gold districts globally, and to critical-metal bearing hydrothermal ore deposits more broadly.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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