Chemical discrimination of magmatic vs. metamorphic blue corundum: The problem of corundum formed in partial melt and its implications for sapphire exploration and deposit modeling

Blue and brown corundum from the Blue Jay sapphire occurrence, British Columbia, Canada, which occurs in restite-rich pods suspended in voluminous leucosome formed from the partial melting of a metapelitic protolith, plots as “magmatic” in well-established chemical discrimination diagrams for blue sapphire based on trace element concentrations and ratios (Mg, Fe, Ti, Cr, Ga). The “magmatic” classification in these diagrams was previously known to apply only to blue corundum from syenites and some alkali basalts, where corundum is interpreted to have formed from magmatic (sensu stricto) crystallization and not as a restitic phase. Blue Jay corundum δ¹⁸O_VSMOW values are in the range of ca. 4–6 ‰, which is commonly interpreted to reflect a mantle-like signature, inconsistent with the petrogenesis of the Blue Jay corundum-bearing restite in leucosome. Blue Jay corundum represents an important exception to the common rules of geological origin estimation using trace elements and oxygen isotopes. These new data must be taken into consideration when developing genetic models for xenocrystic sapphire or identifying possible source rocks for detrital sapphire.