'Forensic' geochemical approaches to constrain the source of Au-Ag in low-sulfidation epithermal ores

In order to better constrain genetic processes involved in forming mineral deposits (and ultimately exploration models), it helps to know from where the metals of interest are derived. How the metals arrived at their point of deposition, and why they were deposited there, are separate issues. We are using three different techniques in an attempt to better understand these processes for epithermal ores. All have some ambiguity inherent to them, but we think that based on our preliminary investigations, together they point to a new understanding of how some epithermal ores in the northern Great Basin form. These techniques include: 1) plotting the relative abundances of Au, Ag, Pb, As, Sb, Se, Te of the ores; 2) Pb-isotope abundances in Au-Ag minerals; and 3) Re-Os isotope analyses of Au-Ag minerals in the ores. Results to date suggest: 1) the “epithermal suite” geochemical association is likely related to the similar volatility of these metal(loid)s, and thus we conclude they are derived from the mantle as opposed to representing a shallow origin; and 2) Preliminary Pb and Re-Os isotopic analyses of Au-Ag minerals indicate that they are derived from the mafic that were part of the bimodal volcanic suite associated with the initial emergence of the Yellowstone Hotspot (YHS) in the northern Great Basin at about 16-15 Ma. Epithermal ore formation associated with the YHS which may suggest the source region of the mantle was rapidly depleted.