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Abstract

The Lake Harbour Group (LHG) metacarbonates on Baffin Island contain occurrences of the gemstones sapphire (corundum), spinel (including cobalt-blue), and lapis lazuli (blue haüyne-rich rock). This dissertation uncovers the regional geologic processes (e.g., metamorphic history, metasomatism, protolith geochemistry) that influence gemstone potential by developing genetic models for the LHG gem mineral occurrences. Both barren and gem-bearing metacarbonates were studied using field examination and sampling, petrological (optical) and scanning electron microscope petrography, and whole rock geochemistry. Boron isotope geochemistry, thermodynamic modelling, and age-dating (zircon U-Pb and mica ⁴⁰Ar-³⁹Ar) were employed for selected occurrences. Corundum formation was made possible by three equally important sequential metamorphic reactions: (1) formation of nepheline, diopside, and K-feldspar (inferred) at granulite facies peak metamorphic conditions; (2) partial retrograde replacement of the peak assemblage by phlogopite, oligoclase, calcite, and scapolite; and (3) retrograde break-down of scapolite + nepheline to form albite, muscovite, corundum, and calcite. The corundum-forming reaction only occurs in a