MINERAL INCLUSIONS IN ALLUVIAL SAPPHIRES FROM BROWNS GULCH, SOUTHWESTERN MONTANA; IMPLICATIONS FOR THE ORIGIN OF MONTANA ALLUVIAL SAPPHIRES

Since their initial discovery in 1865, the original source for alluvial sapphires in gravel deposits from western Montana remain a mystery, and a continuing source of interest to mineralogists and geologists. In this study, sapphire specimens collected from Browns Gulch, located west of Butte, Mt., were studied by optical and scanning electron microscopy techniques. The goal: identify mineral inclusions hosted in the sapphires that could provide insight into the original source-rock for the sapphires. The specimens were generally clear, yellow, or purple to pale blue in color with fractured, pitted, and frosted surfaces. Shapes were generally irregular with some specimens forming rough hexagonal tablets with a distinct 0001 parting. General appearance of the Browns Gulch sapphires is similar to other alluvial sapphires from Montana (Berg and Dahy, 2002). The sapphires were examined by optical microscopy to identify specimens containing inclusions for microbeam chemical analysis by the SEM/EDS. We exposed mineral inclusions by careful grinding [on the 0001 parting when present] using the general technique developed by Berg (pers. comm.). Inclusions were mapped by Backscattered Electron Imaging (SEM/BSE) and then identified chemically using EDS. Inclusions occur as solitary phases, composite phases [i.e. lithic fragments], and multi-phase clots, and are sub-rounded to rounded with distinct contacts between inclusions and host sapphires. Mineral inclusions tentatively identified in specimens include phlogopite, albite, rutile, ilmenite, Cr-Fe spinel, apatite, and zircon. Although not definitive, the variety and composition of the inclusion assemblage-particularly the presence of zircon-supports a metamorphic origin for the Browns Gulch sapphires as suggested by Garland (2002) for other alluvial sapphires.