Avery Dobson, BSc

The Stillwater Complex is a mafic-ultramafic layered intrusion in south-central Montana (USA) that hosts significant stratiform chromitite deposits and platinum-group element (PGE) mineralization, the latter notably within the J-M Reef. Although PGE mineralization in layered intrusions is typically found in chromitites, the chromitites in the lower part of the Stillwater Complex have relatively low abundances of the PGE. Conversely, the J-M Reef, with the highest PGE grades of any ore deposit of its kind, mainly comprises plagioclase-rich gabbros and troctolites, without any chromitite evident. This study describes a coarse-grained chromitite autolith found within the J-M Reef package, and which contains relatively abundant sulfide and evidence of platinum-group minerals (PGM). This project aims to elucidate the origin and crystallization history of this autolith, in part by comparing its mineral chemistry and textures to the massive chromitite lower in the Stillwater Complex stratigraphy (i.e., in the Peridotite Zone). Thin sections from both samples were analyzed using crystal size distribution and microprobe analysis to assess mineral compositions, grain size distributions and associated sulfide assemblages. The autolith comprises a chromite adcumulate with interstitial serpentine and disseminated sulfides. By contrast, the Peridotite Zone G seam Chromitite shows complex interlayering of chromite and peridotite, with much greater variability in chromite grain size. Early results suggest that the autolith does not share mineral chemical similarities with the G Chromitite (e.g., chromite in the autolith has much higher Al2O3 contents), which could mean that the autolith originated from a massive chromitite seam excised during the erosional processes preexisting that formed the J-M reef. More broadly, this project will help understand the mechanisms behind chromitite formation, providing insight into magmatic processes within the Stillwater Complex.