PROBABLE SOURCES OF ALLUVIAL SAPPHIRES IN WESTERN MONTANA

In addition to blue sapphires mined from an ouachitite dike at Yogo in central Montana, sapphires have been mined from alluvial deposits along the Missouri River east of Helena, along Dry Cottonwood Creek north of Butte, and from deposits in the Rock Creek district west of Philipsburg. Although total production of sapphires from these alluvial deposits has probably far exceeded 45 short tons, the bedrock sources of these sapphires have not been recognized. Some inferences about the source bedrock can be made on the basis of examination of sapphires from these deposits. SEM examination shows delicate surface features interpreted to have formed by resorption during magmatic transport of sapphires from their original site of formation. Although pristine conchoidal fractures attributed to stream transport are present on most sapphires, they are generally not very prominent. In spite of irregular surfaces, including some deep pits, adhering rock is extremely rare. Only two sapphires were found to have adhering rock, rhyolite in one instance and volcanic glass in a protected pit on another. Tertiary volcanic rocks mainly of rhyolitic to dacitic composition including flows, breccias, and volcaniclastic units occur over large areas near all of these alluvial deposits and for some sapphire-bearing gravels no other source seems plausible. Preservation of delicate surface features formed during magmatic transport and lack of prominent fresh conchoidal fractures on many sapphires indicate a short distance of fluvial transport from a source bedrock. Rarity of adhering rock in spite of irregular surfaces indicates that sapphires must have been derived from a friable, easily eroded rock such as volcanic ash or an altered volcanic rock. Lack of recognition of a bedrock source, although mining of these sapphires began over a hundred years ago, also suggests a bedrock source readily masked by weathering products or vegetation. Most likely the sapphires were transported to the surface as xenocrysts in a magma in which they were in disequilibrium, and were subsequently liberated from their volcanic host rock by weathering.