MONTANA — A SAPPHIRE ANOMALY

Montana has produced at least 375 million carats (75 tonnes) of sapphires, but four surrounding states and three provinces have had no significant production. With the exception of the famous Yogo deposit in central Montana, all of the major sapphire deposits are alluvial. The known or inferred bedrock sources for these deposits are Eocene volcanic rocks which range from rhyolite or dacite to trachybasalt and lamprophyre variety ouchitite for the Yogo dike. Mineral inclusions in the sapphires suggest they are metamorphic xenocrysts derived from the lower crust or lithospheric mantle. It is unlikely that the metamorphic source of the sapphires is related to the Archean Wyoming Craton exposed in southern Montana. Archean gneisses do contain corundum, but it is typically gray and not gem quality. The major sapphire deposits all lie within the Paleoproterozoic basement that is northwest of the poorly defined Great Falls Tectonic Zone. It appears that Eocene volcanism brought the sapphires to the surface from a lower crustal or lithospheric source related to the Paleoproterozoic terrane.