ORIGINS OF SECONDARY MONTANA SAPPHIRES: MELTS, MELTS, AND MORE MELTS

Sapphires have been mined from secondary deposits in Montana since 1865 when the first stones were found in the Missouri River near Helena and later at Rock Creek and Dry Cottonwood Creek. These are all volcanic deposits with the sapphires having been transported to the surface roughly 50 Ma. Gemologically, Montana sapphires belong to the group of so-called non-classical sapphires in that they share some similarity with both classical metamorphic and classical magmatic sapphires, but do not belong entirely to either group. Non-classical sapphires tend to have higher iron content, lower gallium, and often fancy colors (other than blue) in lighter tone and saturation. Non-classical sapphires from Songea and Umba in Tanzania share some characteristics with secondary Montana sapphires and may occasionally be confused with them during origin determination.

One thing that does set the secondary Montana sapphires apart are the common presence of glassy, silicate melt inclusions. These melt inclusions have been documented by Palke et al. (2017) and they are generally quite silica-rich, being quartz-normative in all cases. This led to the hypothesis of these sapphires forming through a peritectic melting reacting during partial melting of some Al-rich rock, likely something like an anorthosite. Further microscopic observations since then have uncovered several new types of inclusions that further refine our understanding of the origin story of the Montana sapphires. Most importantly, there are two additional types of melt inclusions with distinct and drastically different compositions than the silica-rich glassy inclusions. The first type is a sulfide melt inclusion that has crystallized into polycrystalline assemblages of various sulfide minerals. This was likely an immiscible partial sulfide melt present during the partial melting event that may have formed the sapphires. The second type is another polycrystalline melt inclusion composed of phlogopite mica, spinel, and at least one more unidentified phase. This is interpreted to be a mafic melt inclusion, likely a secondary melt inclusion representing the mafic magma that transported some of these sapphires to the surface. The presence of multiple types of melt inclusions speaks to the igneous origin of the secondary Montana sapphires.