A significant gem corundum deposit in rhyolitic ignimbrite: the enigmatic Rock Creek sapphire deposit, Montana, USA

Abstract

Detrital sapphire (gem corundum) in the Rock Creek deposit (Montana, USA) is identified as having originated from Eocene peraluminous rhyolitic ignimbrite, which occurs as clasts in sapphire-producing colluvium. Fine-grained corundum and other heavy minerals (allanite, various garnets, rare chromite, and others) occur in this rhyolite tuff in similar concentrations to the sapphire ore gravels, indicating derivation of the ore gravels by weathering of the tuff. Fine ash and biotite in tuff clasts show signs of subaerial alteration. The corundum-bearing tuff is predominantly composed of vitric ash particles with subordinate lithic fragments (Belt Supergroup quartzite), crystals (plagioclase, biotite, quartz), carbonised wood, and trace concentrations of various heavy minerals. A gem quality sapphire crystal (recovered by mining) hosts a secondary assemblage of plagioclase, Ti–rich/Al-poor biotite (identical to biotite phenocrysts in tuff), and vesicular rhyolitic glass filling a thin fracture through the grain, which indicates that sapphire was present in the rhyolitic magma before extrusion. The peraluminous composition of Rock Creek rhyolite, together with the presence of corundum, garnet of variable composition (some comparable to garnet in amphibolite xenoliths), and chromite suggest the assimilation of varied crustal rocks. The high alumina saturation index and rapid quenching of the rhyolite magma may promote corundum survival. Sapphire distribution at Rock Creek is expected to be controlled by Eocene paleotopography, the structure of the eruptive volcanic center(s), and the Quaternary weathering, erosion, and deposition of unconsolidated sapphire-bearing gravels.