gems, corundum, USA.12, Montana.12, Rock Creek, Dry Cottonwood, Missouri River

Alluvial sapphires in southwestern Montana have supported a mining industry for over 100 years, yet the source rock for the sapphires has never been identified. This study uses geological mapping, trace element analysis and inclusion identification to characterize the Montana alluvial sapphires and extrapolate their origin. The placers occur in three areas in southwestern Montana: along the Missouri River, Rock Creek area, and Dry Cottonwood Creek. Detailed mapping of the placers has established that the present distribution is due to Quaternary reworking of Tertiary alluvium, indicating that the sapphires have been in the alluvial system for a minimum of 2 Ma and that the source area may be distant or completely eroded. Trace element analyses of the alluvial sapphires and a corundum reference suite were done using the proton microprobe. Corundum contains traces of Fe, Cr, Ti, V, and Ga, for which Ga displays significantly different patterns between alkali basalt and metamorphic corundum types. Ga in alkali basalt corundum is typically >100 ppm, with wide variation for samples within one deposit. Metamorphic corundum typically has Ga < 100 ppm, and tightly clustered distributions. The Montana alluvial sapphires have chemistry consistent with a metamorphic origin. The most common inclusions in alkali basalt corundum are ilmenite and hercynite. Metamorphic corundum hosts a wider variety of inclusions. The Montana alluvial sapphires contain plagioclase, muscovite, some clinozoisite, and rarely garnet. Using 'T-P' stability fields for these inclusions, the formation conditions for the Montana sapphires is estimated at 580 to 720°C and 9 to 13 kbar (~30 km depth), consistent with mid-crustal depths. The Bitterroot lobe of the Idaho batholith is proposed as the sapphire source area. It intruded into, and re-metamorphosed, metamorphic rocks of the Bitterroot Core Complex at mid-crustal levels. Exhumation by Cretaceous thrust faulting exposed these rocks at the surface by the early Tertiary, providing a 30-million-year window to erode sapphires into the Tertiary alluvial system. Quaternary reworking of the Tertiary gravel concentrated the sapphires into the present-day placers.Ga