“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 howsome epithermal ores in the northern
Great Basin form. These techniques include: 1) plotting the relative abundances ofAu,
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 fromthe mantle as opposed
to representing a shalloworigin; and 2) Preliminary Pb and Re-Os isotopic analyses of
Au-Ag minerals indicate that they are derived fromthe mafic that were part of the bimodal
volcanic suite associated with the initial emergence of the Yellowstone Hotspot
(YHS) in the northernGreat Basin at about 16-15 Ma. Epithermal ore formation associated
with theYHSwhich may suggest the source region of the mantle was rapidly depleted.