DISCOVERY OF A NEW GROUP OF SAPPHIRES FROM ROCK CREEK, MONTANA

A new group of sapphires with atypical trace element chemistry has been discovered from the Rock Creek deposit in Montana. Sapphires from the three secondary deposits in Montana (Rock Creek, Missouri River, and Dry Cottonwood Creek) have a narrow range of trace element chemistry, color range, spectroscopic features, and inclusion characteristics such that sapphires from these deposits are essentially indistinguishable. This suggests a similar geological origin for these geographically distinct sources. Montana sapphires are classified gemologically as “non-classical" (neither typical metamorphic nor typical magmatic sapphires) with relatively high Fe (average of 4380 ppmw Fe, σ = 1285 ppmw) but relatively low Ga (average of 46 ppmw Ga, σ = 7 ppmw). The high Fe contents and the resulting Fe-associated absorption bands in UV-Vis spectra are used as evidence to identify Montana sapphires in the gem lab. However, this new type of Montana sapphires from Rock Creek have Fe concentrations lower by roughly an order of magnitude (average of 560 ppmw Fe, σ = 300 ppmw) while Ga is generally lower by a factor of 2 (average of 20 ppmw Ga, σ = 7 ppmw). Nonetheless, the morphology of the rough sapphires is consistent with an origin from Rock Creek with typical flattened hexagonal prismatic shapes and/or hillocky surface texture indicating partial during magmatic transport. Inclusions in this new type of Montana sapphire are identical to those seen in typical Montana sapphires including hexagonal bands of rutile silk, glassy silicate melt inclusions, (clino)zoisite, and feldspar inclusions, which suggests these two groups have a similar geological origin. Ten low-Fe, low-Ga sapphires have been identified so far by screening hundreds of rough Rock Creek sapphires using fast UV-Vis spectroscopy looking for the absence of Fe-related absorption bands. So far, these new sapphires have been found from the Missouri River or Dry Cottonwood Creek deposits. The dramatic variation in trace element chemistry despite the apparently similar geological formation conditions raises questions about the use of trace element chemistry signatures for deciphering the geological origins of gem corundum. While this new type of sapphire represents perhaps only 1% of the production from Rock Creek, at least one of these stones was cut into an exceptional golden yellow sapphire weighing over 2.5 ct, indicating that these new Montana sapphires may be seen in gemological labs.