Trace Elements and Inclusions in Fine-Grained Corundum from Rock Creek, Montana: Application to Exploration and Gemological Research Sampling in Volcanic Terranes

Abstract Corundum trace element composition and inclusions are characterized with LA-ICP-MS and Raman spectrometry, respectively, in a set of 165 corundum grains separated from the 0.125 to 8 mm size fractions (most grains 0.5 mm) in sapphire-producing sediment from Rock Creek, Montana. The chemistry and inclusions of fine-grained corundum are generally similar to their larger gem-quality counterparts (with exceptions), thus validating the sampling approach as a way to easily collect reliable detrital corundum research samples containing numerous individual grains. The approach is likely best applied to gem corundum deposits in volcanic terranes that have low corundum background concentrations or in deposits that experienced no or little sedimentary transport where it would be unlikely to encounter corundum from other sources. The sampling method can facilitate the generation of large datasets of corundum chemistry and inclusions from secondary sapphire deposits, where samples could even be collected in the tailings of inactive or exhausted mines. The fine-grained corundum fraction does not always completely and accurately represent the whole population of gem-quality sapphire: at Rock Creek, saturated blue corundum grains are exclusively smaller than 1 mm in size and comprise a geochemically distinct Ti-Mg-Ga-V-enriched subpopulation (a signature possibly influenced by fine Ti-oxide inclusions too small to be resolved by optical microscope). The present study reports the relative incidence of different inclusions in corundum, with the most common being plagioclase (pastel corundum subpopulation only), alkali feldspar (blue corundum primarily), glassy melt inclusions, rutile, biotite, ilmenite (blue corundum only), and zircon (blue corundum only). Glassy melt inclusions occur in ca. 15% of corundum grains and can be considered an indicator of volcanic/sub-volcanic origin if observed in detrital corundum grains in an exploration context.