A GIANT GEM CORUNDUM DEPOSIT IN RHYOLITIC IGNIMBRITE: THE ENIGMATIC ROCK CREEK SAPPHIRE DEPOSIT, MONTANA, USA

We present new data on samples from Potentate Mining LLC’s Rock Creek sapphire deposit which include sediment, tuff clasts, heavy mineral concentrates, sapphires with adhering materials, and metamorphic xenoliths. The rhyolite tuff is composed of Si-rich moderately to strongly peraluminous rhyolitic glass with subordinate plagioclase, quartz and altered biotite phenocrysts, quartzite lithic fragments, carbonised wood, and trace amounts of corundum, allanite, and garnets. Some garnets in tuff closely resemble garnet in an amphibolite xenolith. Heavy mineral concentrates from sapphire-bearing colluvium suggest that the sediment is largely derived from pyroclastic rock. A sapphire crystal contains a fracture lined with plagioclase, very weakly peraluminous rhyolite glass, and Ti-rich/Al-poor biotite identical to phenocrysts in tuff, which provides additional evidence for the transport of sapphires by rhyolitic magma. We evaluate these new data in the context of previous research on SW Montana sapphires (Berg 2014, Palke et al. 2017, Turnier 2022). Corundum formed from the partial melting of varied crustal protoliths (metaigneous, including mafic, and metasedimentary). The rhyolitic magma originates, at least partly, from crustal melting driven by heat input from the intrusion of mantle-derived magmas during Eocene Challis-Kamloops magmatism. The high ASI of peraluminous rhyolite and rapid quenching of the magma likely facilitated corundum preservation. Sapphire distribution at Rock Creek is expected to be controlled by (A) ignimbrite distribution in the context of Eocene paleotopography and the structure of the eruptive volcanic center(s), and (B) the Quaternary weathering, erosion, and deposition of unconsolidated sapphire-bearing gravels derived from the ignimbrite.