Matthew Lombardi, BSc

Layered mafic intrusions represent the solidified remnants of basaltic magmatic systems and are important repositories for chromitite, a rock containing >60 modal percent chromite. The processes responsible for the formation of chromitite layers have remained a contentious scientific problem and a variety of models have been proposed, including magma mixing, country roof rock assimilation, in-situ crystallization and variations in gravity settling models from pre-saturated chromite melts. Layered intrusion-hosted chromitite is an important host for platinum group elements (PGE), which play a key role on the development of sustainable technologies and industrial applications, so there is widespread economic interest in improving our understanding of these rocks too. To address this gap in knowledge, we examined a chromitite sample from within the 21st cyclic unit of the classic ~1270 Ma Muskox Layered Intrusion, Nunavut. The sample was collected during a 1988 mapping expedition and contains a portion of one of the two documented chromitite seams in the Muskox intrusion. In this study, we employed a combined petrographic, quantitative textural (e.g., crystal size distribution; CSD) analysis and mineral chemical (electron probe microanalysis; EPMA) approach to investigate the formation of the chromitite and its immediate footwall. The chromitite footwall comprises pegmatoidal plagioclase-bearing pyroxenite, with abundant coarse-grained (cm-sized) base-metal sulfides and disseminated chromite. The utility of the CSD method lies in the information it can provide on the nucleation and growth of the phase(s) of interest. In the case of the Muskox intrusion, where chromitite formation has traditionally been linked to magma mixing with subsequent crystal settling, such processes should be evident in the CSD data. However, our new CSD profiles generally show log linear characteristics which are more consistent with in situ nucleation and growth (crystallisation). Chromite in the chromitite seam appears to simply be the result of increased nucleation of chromite, rather than a change in the style of crystallisation. Most CSDs also show evidence for a stage of postcumulus textural coarsening. Chromite chemical compositions are consistent with previous studies, in particular exhibiting relatively elevated titanium concentrations compared to chromititite-hosted chromite in other well-known layered intrusions (e.g., the Stillwater and Bushveld Complexes). Our ongoing work is directed toward testing the classic magma mixing model for the Muskox chromitites with these approaches and observations.