THE ROLE OF METAMORPHIC AND GEOCHEMICAL FACTORS IN GEM SPINEL GENESIS

Interest in spinel (MgAl₂O₄) gemstones has increased significantly in the early 21^st century. Few areas in the world produce gem spinel, and much of this spinel is recovered from eluvial or alluvial deposits. Many spinel deposits are in tropical climates; thick vegetation overgrowth obscures the geology around these occurrences. These have been barriers to the development of a gem spinel deposit model. To address this challenge, 14 exceptionally well-exposed, in situ spinel occurrences were sampled on Baffin Island, Nunavut, Canada. These include multiple blue and purple spinel occurrences, and three cobalt-blue spinel exposures. The mineral assemblages are variable: They include calcite, dolomite, phlogopite, diopside, humite, forsterite, or amphibole and commonly include small amounts of pyrrhotite and graphite. This research seeks to understand four key aspects of gem spinel genesis: (1) The effect of whole rock geochemistry and P-T conditions on the formation of spinel-bearing mineral assemblages; (2) the origin of cobalt-enrichment in metacarbonate rocks; (3) the effect of mineral assemblage on chromophore availability to spinel; and (4) the trace element distribution in spinel and associated minerals with implications for spinel source determination. Petrography, electron probe microanalysis, and trace element analysis with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) will contribute to building the scientific framework for the trace element source determination of spinel. The trace element composition of spinel is expected to be controlled by the composition of the protolith, element distribution in different minerals, and the relative abundance of these minerals.