EXAMINING POSSIBLE ORIGINS OF SAPPHIRE XENOCRYSTS on ROCK CREEK, GRANITE COUNTY, MT

One of the largest alluvial deposits of gem corundum in the United States is located in the Sapphire Mountains of western Montana, about 70km southeast of Missoula on the west fork of Rock Creek. The corundum mined there has an unusual origin, as it occurs in post-Quaternary reworked alluvium and is believed to have weathered out of nearby Tertiary volcaniclastic rhyolite flows. Globally, sapphires do not commonly occur in rhyolite flows, however based on previous mineralogy studies of the sapphires themselves, they are believed to be metamorphic xenocrysts summoned from an unidentified subterranean origin along with the eruption of the rhyolite flows. This deposit is mainly concentrated in a 15 km² area with smaller satellite deposits being reported from a broader surrounding 75 km² region. Detailed geologic field mapping of the Rock Creek volcaniclastics and the varied stratigraphy within largely undifferentiated rhyolite flows is still open to interpretation. Assessing sedimentary patterns within volcanic flows to shed light on the origins of Rock Creek sapphires should add to understanding of Tertiary paleodrainage patterns.