DETERMINING THE SOURCE OF PLACER GOLD IN THE ANACONDA METAMORPHIC CORE COMPLEX SUPRADETACHMENT BASIN USING DETRITAL ZIRCON GEOCHRONOLOGY, PIONEER DISTRICT, WESTERN MONTANA

The Pioneer District, straddling the northern Flint Creek Range and the Deer Lodge Valley, was the site of the first gold discovery in Montana in 1852. Previous investigations reveal that the majority of placer gold was concentrated in Oligocene to Miocene conglomerates that were deposited within a supradetachment basin associated with the Anaconda Metamorphic Core Complex (MCC). We collected four detrital zircon (DZ) samples from Pioneer District placers to determine the source of the gold. In particular, we sought to determine whether the gold originated in hydrothermal quartz veins or as skarn gold in the metamorphic aureole of the Late Cretaceous Royal Stock. We obtained U-Pb ages for 1,058 detrital zircons and analyzed Hf isotopes on 61 zircons with U-Pb ages ≤250 Ma. Each of the four samples yielded 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 or Paleozoic-Mesozoic passive margin sedimentary rocks. 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. Hf isotopic data reveal a negative excursion (more evolved) for the ~69 Ma age-probability peak and a positive excursion (more juvenile) for the ~26 Ma age-probability peak. The mixed DZ age spectra, coupled with previous investigations of gold fineness, suggest that the gold formed in a skarn. This interpretation will be evaluated using an unmixing algorithm that compares the DZ age spectra with potential sources. We integrate our findings into a tectonic model involving exhumation of the skarn in the footwall of the Anaconda MCC during deposition of the Oligocene to Miocene strata. The maximum depositional ages calculated from our DZ data suggest that the Anaconda MCC was active until at least ~26 Ma.