Trace elements and inclusions in metamorphic blue sapphire (Beluga deposit, Nunavut, Canada) Updated deposit model and implications for exploration and origin determination

This study examines the trace element geochemistry and mineral inclusions of a suite of sapphire corundum samples from the Beluga deposit, Lake Harbour Group, Baffin Island, Nunavut, to (1) refine the genetic model of the sapphires (with potential implications for exploration), (2) comprehend the observed colours and colour zoning, (3) assess the validity of geochemical source rock discrimination approaches in the case of the Beluga deposit, and (4) examine whether sapphires from Beluga can be reliably differentiated from sapphires from other deposits. Sixty (60) polished cross-sections were examined, taken from two parts of the Beluga pit from a total of 54 different crystals. Inclusions were characterized with SEM-EDS and EPMA. Minor and trace elements in corundum were measured using EPMA and LA-ICP-MS. The inclusions study shows that unaltered phlogopite, scapolite, and oligoclase were incorporated into growing sapphire corundum. These relationships are hidden in the host rock due to pervasive retrograde alteration, and the new data demonstrates corundum formation during M2 retrograde metamorphism, earlier than thought previously. Beluga sapphire is intensely oscillatory colour-zoned with alternating blue and colourless bands. The blue colour is caused by Fe²⁺-Ti⁴⁺ intervalence charge transfer (IVCT). Higher Fe and Ti concentrations correlate to some degree with a darker blue colour, while colourless zones generally contain less Fe+Ti, but with significant variability and overlap. Some colourless zones contain sufficient Fe and Ti, in theory, to create a blue colour. Possible reasons for the lack of colour in these zones may be incorrect Fe oxidation state (Fe³⁺) for Fe-Ti IVCT or an abundance of rutile nano-inclusions. While no exsolved rutile is observed with optical or electron microscopes, these zones can contain high Ti concentrations, which may suggest the presence of such inclusions. Beluga sapphire is geochemically identifiable as a metamorphic-type blue sapphire but is difficult to differentiate from Sri Lankan or Madagascan metamorphic blue sapphires using physical appearance, inclusions, and geochemistry. However, rutile silk, common in other blue sapphires, is not observed in Beluga sapphire. More investigation of inclusions, but on faceted gems using Raman spectrometry, is recommended.