Montana Sapphires

Numerous pastel colored sapphires recovered from the SBO Example of the garnets recovered from the SBO Representative picture of the Montana sapphires recovered from the SOB.	THE SILVER BOW CREEK SAPPHIRE OCCURRENCE, MONTANA – EVIDENCE FOR A VOLCANIC BEDROCK SOURCE FOR MONTANA’S ALLUVIAL SAPPHIRE DEPOSITS AARON L. BERGER* Department of Geosciences, 4044 Derring Hall - Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 AND RICHARD B. BERG* Montana Bureau of Mines and Geology, Montana Tech of the University of Montana, 1300 W. Park ST., Butte, MT 59701 Abstract: The first confirmed bedrock source for southwest Montana 's many alluvial sapphire deposits has been found 9 km west of Butte , Montana for the Silver Bow Creek sapphire occurrence. The host rock is a sapphire-bearing, weathered, nonresistant felsic lapilli tuff of the Eocene Lowland Creek Volcanic field. In the Quaternary, mass wasting of this volcanic unit resulted in the deposition of several debris flow deposits which constitute the majority of the sapphire deposit. During the course of the study over 5,500 typical Montana sapphires (numerous pastel colors, but usually light green to light blue-green stones) and 25 dark blue sapphires were recovered from the Silver Bow Creek sapphire occurrence. The typical Montana sapphires exhibit partially resorbed morphologies and often have spinel reaction rims implying disequilibrium with the felsic melt that brought them to the surface. Intact xenoliths of corundum-bearing biotite-sillimanite schist, as well as xenocrysts of garnet and spinel were also recovered along with the sapphires from the debris flow deposits. Based on this assemblage, the resorbed surface morphologies, and the spinel reaction rims, the typical Montana sapphires weathering out of the sapphire-bearing felsic lapilli tuff are interpreted to be metamorphic xenocrysts which are the remnants of otherwise completely assimilated amphibolite facies corundum-bearing biotite-sillimanite schist. The second population of dark blue sapphires found within the Silver Bow sapphire occurrence exhibit little if any evidence of resorption (often euhedral shaped) and do not contain any adhering reaction minerals. Because of these differences, it is postulated that the dark blue sapphires are magmatic phenocrysts whose source melt is represented by a pebble of plumasite (corundum bearing igneous rock) which contains roughly 25% by volume dark blue sapphire phenocrysts also recovered from the debris flows at the Silver Bow Creek sapphire occurrence. The plumasite possibly represents a melt which formed as a result of the host magma becoming locally under saturated with respect to silica proximal to the assimilating aluminous schist. *Corresponding e-mail addresses: alberger at. vt.edu (A. Berger) and dberg at. mtech.edu (D. Berg)

Numerous pastel colored sapphires recovered from the SBO
Example of the garnets recovered from the SBO

Representative picture of the Montana sapphires recovered from the SOB.

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

AARON L. BERGER*
Department of Geosciences, 4044 Derring Hall - Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

AND RICHARD B. BERG*
Montana Bureau of Mines and Geology, Montana Tech of the University of Montana, 1300 W. Park ST., Butte, MT 59701

Abstract:
The first confirmed bedrock source for southwest Montana 's many alluvial sapphire deposits has been found 9 km west of Butte , Montana for the Silver Bow Creek sapphire occurrence. The host rock is a sapphire-bearing, weathered, nonresistant felsic lapilli tuff of the Eocene Lowland Creek Volcanic field. In the Quaternary, mass wasting of this volcanic unit resulted in the deposition of several debris flow deposits which constitute the majority of the sapphire deposit. During the course of the study over 5,500 typical Montana sapphires (numerous pastel colors, but usually light green to light blue-green stones) and 25 dark blue sapphires were recovered from the Silver Bow Creek sapphire occurrence. The typical Montana sapphires exhibit partially resorbed morphologies and often have spinel reaction rims implying disequilibrium with the felsic melt that brought them to the surface. Intact xenoliths of corundum-bearing biotite-sillimanite schist, as well as xenocrysts of garnet and spinel were also recovered along with the sapphires from the debris flow deposits. Based on this assemblage, the resorbed surface morphologies, and the spinel reaction rims, the typical Montana sapphires weathering out of the sapphire-bearing felsic lapilli tuff are interpreted to be metamorphic xenocrysts which are the remnants of otherwise completely assimilated amphibolite facies corundum-bearing biotite-sillimanite schist. The second population of dark blue sapphires found within the Silver Bow sapphire occurrence exhibit little if any evidence of resorption (often euhedral shaped) and do not contain any adhering reaction minerals. Because of these differences, it is postulated that the dark blue sapphires are magmatic phenocrysts whose source melt is represented by a pebble of plumasite (corundum bearing igneous rock) which contains roughly 25% by volume dark blue sapphire phenocrysts also recovered from the debris flows at the Silver Bow Creek sapphire occurrence. The plumasite possibly represents a melt which formed as a result of the host magma becoming locally under saturated with respect to silica proximal to the assimilating aluminous schist.

*Corresponding e-mail addresses: alberger at. vt.edu (A. Berger) and dberg at. mtech.edu (D. Berg)

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