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The role of metamorphic and geochemical factors in the formation of gem corundum, spinel, and haüyne in metacarbonates of the Lake Harbour Group, Baffin Island, Canada Belley, Philippe Maxime


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 <100 °C window. Spinel and haüyne formed at granulite facies peak metamorphic conditions, and spinel remained stable through upper amphibolite facies retrogression. The corundum, spinel, and haüyne occurrences are interpreted to have different sedimentary protoliths with the exception of a few metasomatic spinel occurrences. The protoliths of the occurrences are interpreted to be: [A] impure dolomitic limestone (spinel); [B] dolomitic marl (spinel, corundum); [C] magnesite-rich evaporitic marl (spinel); and [D] evaporite (halite and anhydrite)-bearing dolomite-rich marl (haüyne). The meta-marls have Al/Si abundances comparable to that expected in a siliciclastic mud. Calc-silicate rocks barren of gem minerals have lower Al/Si (sandier protoliths). Spinel-bearing calc-silicates have higher Al/Si relative to that of sapphire- and haüyne-bearing rocks. High abundances of Al to Si are crucial to spinel formation in Mg-rich calc-silicates. Spinel formation in impure dolomitic marbles requires low K activity and does not require high Al/Si. Cobalt enrichment at two spinel occurrences is localized and interpreted to represent geochemical features of the protolith. The genetic models provide broad gemstone exploration criteria for carbonate-bearing metasedimentary sequences such as the LHG.

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