NEW PB- AND CU-ISOTOPE DATA SUPPORTING A DEEP ORIGIN OF METALLIC NANOPARTICLES IN BONANZA EPITHERMAL ORES OF THE NORTHERN GREAT BASIN, NEVADA-IDAHO (Invited Presentation)

Pb-isotope data from new (previously undocumented) mining deposits and districts hosting mid-Miocene epithermal mineralization in the northern Great Basin (NGB) support our earlier published interpretation that such deposits derive precious metals from a deep primitive mafic-magmatic source. In particular, it appears that some basaltic magmas associated with the initial emergence of the Yellowstone hotspot/mantle plume led to the formation of bonanza epithermal ores. In what we believe is the first attempt of using Cu isotopes to help constrain the origin of metals in epithermal ores, we analyzed electrum, naumannite, and chalcopyrite from some of the highest grade epithermal ores in the NGB. All three minerals yielded similar values of ∂⁶⁵Cu suggesting isotopic equilibrium, and ∂⁶⁵Cu values for all three minerals clustered closely around 0‰, the meteorite standard. In general, these bonanza epithermal ores showed less variability in ∂⁶⁵Cu than porphyry copper deposits studied by one of us (Mathur) in the western USA. Previously we documented that ore textures at Sleeper, National, Buckskin National, and Midas were consistent with aggregation of precious-metal nanoparticles to make the striking self-organized “fractal” dendrite features in the ores. Together, the isotopic and textural data indicate that nanoparticles must have formed deep (close to magma chamber?) and locked in the primitive Pb-, Cu- and Re-Os isotopic signatures and were transported by hydrothermal fluids up to the (shallow) epithermal environment. In the epithermal setting, boiling-induced chaos helped nanoparticles to aggregate and form the self-organized fractal dendrites. If our hypothesis is correct, then physical transport of nanoparticles in ores should be considered as a possibility in other metallic ore deposits formed under “far-from-equilibrium” conditions.