Determining the source of placer gold in the Anaconda metamorphic core complex supradetachment basin using detrital zircon U-Pb geochronology, western Montana, USA

Abstract

Despite the widespread occurrence and economic significance of gold placer deposits, modern provenance studies of placer sediments remain largely qualitative. This study applies detrital zircon (DZ) geochronology to determine the source of zircon in placer deposits. We then evaluate the provenance of the zircon to assess whether the gold might have been derived from the same sources, thereby providing a case study of the use of DZ geochronology applied to placers. We present a new set of DZ U-Pb ages (n = 1058) and Lu-Hf (n = 61) isotopic data from four placer deposit samples collected from the Pioneer District of western Montana (USA). Each of the four samples yielded similar age spectra, with a range of U-Pb ages between 3000 and 25 Ma. We interpret that ≥250 Ma zircons were recycled from the Mesoproterozoic Belt Supergroup, Paleozoic–Mesozoic sedimentary rocks, and the Upper Cretaceous–Paleocene Beaverhead Group. Our 237 DZ U-Pb ages ≤250 Ma reveal two prominent age-probability peaks centered at ca. 69 Ma and ca. 26 Ma, which we interpret to record first-cycle derivation from the Royal stock and nearby Dillon Volcanics, respectively. We evaluate these data using an inverse Monte Carlo DZ unmixing model that calculates relative contributions from plausible source units, determining a 12% contribution from the Royal stock and a 43% contribution from the Beaverhead Group. A current absence of the Beaverhead Group in the hypothesized source region suggests complete erosion of the unit into the placer-bearing basin. Detrital zircon geochronology, Hf isotopic data, and the unmixing modeling results offer the first zircon-based support for previous interpretations that the Late Cretaceous Royal stock precipitated gold along its contact with overlying Proterozoic–Mesozoic sedimentary strata. Subsequent exhumation and erosion of the lode source led to gold deposition in the Anaconda metamorphic core complex supradetachment basin during the late Oligocene–late Miocene. The worldwide occurrence of gold placer deposits with unknown source areas provides abundant opportunity to apply these techniques elsewhere.