GLASSY MELT INCLUSIONS IN SAPPHIRES FROM THE ROCK CREEK AND MISSOURI RIVER DEPOSITS IN MONTANA, USA

Sapphires are mined in alluvial deposits in southwestern Montana including deposits along the Missouri River, Rock Creek, and the South Fork of Dry Cottonwood Creek. There is extensive evidence that sapphires from most of these deposits were transported to the surface by Eocene rhyolitic to dacitic volcanism. However, the nearly ubiquitous presence of resorption features indicates that the sapphires were not in equilibrium with the presumed volcanic host rocks, at least not at the time of emplacement. Based on mineral inclusions, trace element chemistry, oxygen isotopes, and field relationships these sapphires are thought to have originated in crustal metamorphic rocks which likely reached amphibolite-facies conditions. In order to shed light on the origin of alluvial Montana sapphires, we describe here glassy melt inclusions in sapphires from the Rock Creek and Missouri River deposits. Under microscopic observation, such inclusions are similar to ordinary two-phase fluid inclusions with negative crystal morphology. Polishing into the inclusions confirms the presence of a glassy solid phase along with one or more bubbles possibly resulting from thermal contraction of the glassy melt. The solid phase’s identification as a glass rests with its optically isotropic nature and the presence of only several broad peaks using Raman spectroscopy. Finally, Electron Probe MicroAnalysis showed the glass inclusions to be dacitic to trachydacitic in composition with wt.% oxide totals as low as 85% indicating the presence of a significant content of dissolved volatiles in the glasses. While glassy inclusions have been reported from the alkali-basalt related sapphires and rubies from Thailand, Australia, and the Central Massifs in France, this is the first time such inclusions have been described in sapphires having a metamorphic association. We hypothesize that these inclusions are the product of partial melting during high grade metamorphism of the parent rock producing a silica-rich melt and an alumina-rich (corundum-bearing) restite with corundum crystallizing as a peritectic mineral. Whether or not the specific silica-rich melt produced in this way could have ultimately transported the sapphire to the surface is a matter for future work.