Studies of inclusions and their host diamonds from the Gahcho Kué mine, Canada

Abstract

Gahcho Kué is a diamond mine in the Northwest Territories of Canada that opened in 2016. This is the first study into the morphology, N systematics, and stable isotope (C and N) composition of the Gahcho Kué diamonds, and the mineral chemistry of the associated inclusions. The resulting data are used to provide insight into the poorly understood and characterised mantle of the Southern Slave Craton, and the mantle conditions that gave rise to the diamond association within the area. Results from 88 inclusion-bearing diamond samples show that diamond formation beneath the Gahcho Kué mine occurred primarily in depleted peridotitic mantle (84%), with some minor contribution from eclogitic (15%) and websteritic (1%) mantle. Assessment by geothermobarometry revealed relatively cold lithospheric conditions compared to other diamondiferous localities within the Slave craton. The trace element chemistry of associated garnets revealed the standard sinusoidal pattern of depletion and re-enrichment by metasomatic fluids/melts in peridotitic source regions, and the pattern of LREE-depletion typical of eclogitic mantle sources. Temperature estimates from the assessment of N concentrations and aggregation states show largely consistent temperatures between mantle residence and formation conditions. Stable isotope data are largely within the expected mantle ranges for δ13C, whilst δ15N values are largely enriched relative to mantle values. Rare more negative δ13C and positive δ15N values were found, reflecting the influence of a subduction-related component, including a specimen with the highest δ15N value observed to date (+25.12‰) in a diamond.