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THE SILVER BOW SAPPHIRE OCCURRENCE, MONTANA: EVIDENCE FOR A VOLCANIC BEDROCK SOURCE FOR MONTANA’S ALLUVIAL SAPPHIRE DEPOSITS

Aaron L. Berger^1, and Richard B. Berg²

¹ Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, Virginia 24061
² Montana Bureau of Mines and Geology, Montana Tech of the University of Montana, 1300 W. Park Street, Butte, Montana 59701

Corresponding author: e-mail, alberger{at}vt.edu

The first confirmed bedrock source of alluvial sapphire deposits in southwest Montana has been found 9 km west of Butte, Montana. The source rock of the Silver Bow occurrence is a sapphire-bearing felsic lapilli tuff of the Eocene Lowland Creek volcanic field. Such a felsic composition is atypical of known primary sapphire deposits and may represent a previously unrecognized deposit type. Mass wasting of this volcanic unit during the Quaternary resulted in the deposition of several debris flow deposits, which constitute the majority of the sapphire deposit. Two distinct populations of sapphires were recovered from the debris flow deposits: over 5,500 pastel sapphires and 54 dark blue sapphires. The pastel sapphires exhibit surfaces indicative of resorption and in some cases spinel reaction rims. Xenoliths of amphibolite facies corundum-bearing biotite-sillimanite schist, as well as grains of garnet and spinel were also recovered from the debris-flow deposits. Based on these observations, the pastel sapphires are interpreted to be refractory xenocrysts of metamorphic origin. The second population of dark blue sapphires show little if any evidence of resorption and have no adhering reaction minerals. Due to these differences, we postulate that the dark blue sapphires are magmatic phenocrysts, the source melt for which is represented by a fragment of igneous rock recovered from the debris-flow deposits containing phenocrysts of dark blue sapphire.