USING DETRITAL ZIRCON U-PB GEOCHRONOLOGY TO DETERMINE THE SOURCE OF PLACER GOLD: A CASE-STUDY FROM THE PIONEER DISTRICT, WESTERN MONTANA

Despite the widespread occurrence and economic significance of gold placer deposits, modern provenance studies of placer sediments are scarce and remain largely qualitative. This study is an attempt to test the plausibility—and better refine the current approach—of using detrital zircon (DZ) geochronology to trace the source of economically valuable minerals in placer deposits.

The Pioneer District placer deposits were the site of the first gold discovery in Montana in 1852, and their hypothesized proximity to the lode source and complex surrounding structural framework makes them an ideal candidate to rigorously test the utility of DZ geochronology in placer provenance analysis. We present a new set of zircon U-Pb (n=1,058) and Lu-Hf (n=61) isotopic data from four samples within the Pioneer District placer deposits. Each of the four samples yielded strikingly similar age spectra with a range of U-Pb ages between 2200 and 25 Ma. We interpret that ≥250 Ma zircons were recycled from Mesoproterozoic Belt Supergroup, Paleozoic-Mesozoic passive margin sedimentary rocks, and the synorogenic Beaverhead Conglomerate. A composite age-probability plot of 237 DZ U-Pb ages ≤250 Ma reveals two prominent age-probability peaks centered at ~69 Ma and ~26 Ma, which we interpret records first-cycle derivation from the Royal Stock and nearby Dillon Volcanics, respectively. We evaluate these data further using an inverse Monte Carlo DZ unmixing model that compares the DZ age spectra with potential sources. Geochronology and unmixing modeling suggest that gold in the Pioneer District placer deposits was derived from vein and skarn lode sources in northern footwall of the Anaconda metamorphic core complex. Our data offers the first quantitative support for previous interpretations that the Late Cretaceous Royal Stock pluton precipitated gold along its contact with overlying Proterozoic through Mesozoic supracrustal rock, and was subsequently weathered, transported, and deposited in the AMCC supradetachment basin during the Late Oligocene. These results serve as confirmation of the utility of using DZ zircon geochronology and emerging modeling techniques to determine the source area of dense, economically-valuable minerals. The worldwide occurrence of gold placer deposits with unknown source areas provides abundant opportunity to apply these techniques.