LARGE, GEM-QUALITY PRIMARY SAPPHIRES FROM THE FRENCH BAR SILL, MONTANA: IMPLICATIONS FOR THE ORIGINS OF ALLUVIAL MISSOURI RIVER SAPPHIRE

Montana has been a source of sapphires for more than 150 years. The sapphires were transported as xenocrysts during Eocene volcanism; however, most production comes from secondary deposits, leaving open questions about the sapphires’ origins. Only two primary sources (with sapphires found in-situ) are known and only one has produced commercial quantities of sapphire from a lamprophyre dike at Yogo Gulch. The other primary occurrence is a sill at French Bar along the Missouri River near Helena which Berg and Palke (2016) described as a basaltic trachyandesite. Still, to date, none of the corundum from the French Bar sill was gem quality with only small crystals seen. Recently, however, excavation at French Bar Sill has recovered higher concentrations of larger sapphires than seen before. The largest sapphire crystal found weighed 17.99 cts and an 11.15 ct crystal yielded a 3.49 ct facetted sapphire. The 19 French Bar sapphires studied here provide clues about possible bedrock sources for the Missouri River deposit.

LA-ICP-MS on the primary sapphires show almost identical ranges of trace elements as the alluvial secondary Missouri River sapphire. The exception is chromium which is enriched in the French Bar sapphire (1.6-155 ppmw Cr, average = 55.8 ppmw, median = 39.5 ppmw) compared to the secondary stones (0.6-94.9 ppmw, average = 21.0 ppmw, median = 17.9 ppmw). This is consistent with the observation of a higher prevalence of stones with pink and purple hues in the primary sapphires. Inclusions in the French Bar sapphires are also essentially identical to those seen in the alluvial stones. Surface textures are similar, especially the presence of etch features, but one important distinction exists. Around 25% of the alluvial sapphires have encrustations of spinel, likely a result of reaction during transport by a mafic magma. Primary French Bar sapphire often have an encrustation of biotite mica but never spinel. While Berg and Palke (2016) concluded that the French Bar Sill is too small and the sapphire concentrations too low to have been the source for all of the alluvial Missouri River sapphire, these new results suggest this igneous body may have been a source for at least some of these stones. However, the slight differences in trace element chemistry and the absence of spinel on their surfaces indicates that there must still be other, likely more mafic igneous bedrock sources as well.