SAPPHIRES FROM AN EOCENE SILL NEAR HELENA, MONTANA

Sapphires have been recognized in Montana in only two igneous bodies: the lamprophyre dike at Yogo famous for blue sapphires and the less studied basaltic trachyandesite sill at French Bar 23 km northeast of Helena along the Missouri River. An Ar/Ar plateau age of 50.8 ± 0.1 Ma (Irving and Hearn, 2003) for this sill is close to that of other inferred igneous bedrock sources for Montana alluvial sapphires. Biotite and augite phenocrysts are set in a partly altered groundmass with plagioclase microlites (An₆₃) and biotite autoliths. Corundum-bearing metamorphic xenoliths contain calcic plagioclase, amphibole, garnet, margarite, and spinel. Xenocrysts in order of decreasing abundance are pyrope, quartz, and corundum (variety sapphire). Very pale green to colorless sapphire xenocrysts from 0.4 to 2 mm are generally tabular and rimmed by a very thin layer of tan clay and an outer thicker layer of biotite. Ten LA/ICPMS analyses of sapphires in the sill average 26 ppmw Mg, 38 ppmw Ga, and 3180 ppmw Fe with a Ga/Mg ratio of 1.5 within the range for alluvial sapphires from western Montana (Zwaan, et al., 2015). This sill is <2 m thick with sparse sapphires and so it is clearly not the source for the more than 4 tonnes of sapphires that have been mined from the Pleistocense strath terraces along the Missouri River. However this sill does provide a clue to the possible bedrock source and also suggests on the basis of sapphire-bearing xenoliths that these sapphires have been derived from crustal metamorphic rocks and carried to the surface by Eocene magma.