MONTANA’S CORUNDUM CONUNDRUM: WHAT IS THE PROVENANCE AND ORIGIN OF THE MISSOURI RIVER ALLUVIAL SAPPHIRES?

Montana is home to America’s premier gem deposits, the famous Montana sapphires. Montana’s alluvial sapphires occur in three seemingly unrelated locations: Rock Creek, Dry Cottonwood Creek, and the Missouri River near Helena. The Missouri River sapphires are found in nine terraced gravel bars, of which Spokane Bar and Eldorado Bar are the most productive. Sapphires were first discovered here in 1865. Since then, more than 70 t of sapphires have been produced from alluvial locations in Montana (Palke et al, 2017), and active mining continues today. The primary origin, formation, and erosional history of the sapphires remain controversial. In this study, we use GIS mapping, mineralogic, and spectroscopic investigations to better understand the crystallization, magmatic distribution, and provenance of the alluvial sapphire deposits. We extracted ~70 sapphires, ranging from 0.19 to 5.57 carats, from ~50 kg of gravels from Spokane Bar. We polished windows on parallel faces of each gem to view the interiors because of the rounded and abraded nature of the stones. Prominent growth bands ranging up to 500 µm across were visible in many of the sapphires. The bands are defined by zones of acicular rutile inclusions and by Raman mapping of the R1 (Cr3+) peak in samples without rutile inclusions. We identified melt and/or mineral inclusions in ~45% of the crystals. Mineral inclusions include prismatic rutile, barite, pyrochlore, sanidine, and labradorite. Rutile is common in Montana sapphires but is not an indicator of origin. Barite, pyrochlore, and alkali feldspar inclusions might indicate volatile rich and/or pegmatite alteration (Zwaan et al, 2015). Contrastingly, the presence of Ca-rich plagioclase suggests a mafic igneous source. FTIR analyses of melt inclusions indicate H2O contents extending up to 3.5 wt. %. CO2 displayed two distinct populations; one exhibiting ≤ 100 ppm and the other containing 200-250 ppm. The arrangement of inclusions in sub-planar arrays suggests that these are secondary inclusions composed of the magma that transported the sapphires to the surface.