UNRAVELING THE GEOLOGICAL MYSTERIES OF MONTANA'S SAPPHIRE DEPOSITS

Montana has a rich history of sapphire production with mines including a primary deposit at Yogo Gulch and secondary deposits near Rock Creek and along the Missouri River near Helena and a smaller scale mine near Dry Cottonwood Creek. Sapphires from the secondary deposits generally have overlapping inclusion scenes and trace element patterns. These sapphires typically occur in pastel colors but many respond well to heat treatment which can produce saturated blue or fancy colored sapphires. Yogo sapphires are gemologically distinct with unique inclusions and trace element chemistry. Sapphires from Yogo Gulch typically have a rich cornflower blue color and do not require heat treatment. There is considerable evidence, both field and petrology based, suggesting Montana’s sapphires were transported to the surface by Eocene-age volcanic deposits. However, the sapphires show etched surfaces suggesting that at the time of eruption they were not in equilibrium with their volcanic hosts. In this study we present compositions of inclusions in these sapphires with the aim of unravelling the geological mysteries of these historical and important gem deposits. Specifically, analyses of silicate melt inclusions provide a unique glimpse into the sapphires’ formation. They are felsic melt inclusions with high dissolved volatiles with many melt inclusions being quartz-normative. This suggests the sapphires may have grown as peritectic minerals at low degrees of partial melting. Considered with the rest of the inclusions a likely protolith would be a plagioclase-feldspar rich magmatic rock such as an anorthosite, troctolite, or leucogabbro. The model proposed here of sapphire formation through partial melting an Al-rich protolith and transport to the surface by volcanic formations represents a new model that could be considered for other volcanically-associated sapphire deposits around the world.