In October 2020, a flawless D-colour 102 carat diamond sold for $15.7 million US (read more here). Diamonds like this make headlines from time to time, but this one in particular stands out for its potential geological significance. It may be the first convincing example of a special class of sublithospheric diamonds to be found within the Superior Craton.
This gem can tentatively be labelled as a CLIPPIR diamond, which are epitomized by large, high-quality, type IIa gems such as the Cullinan diamond of the British Crown Jewels. While they have long remained enigmatic, CLIPPIR diamonds are now thought to be a distinct geological variety of diamonds that form at 360-750 km depth, based on their rare inclusions of majoritic garnet, breyite, and unusual Fe-Ni-C-S metallic melt (read more here).
Some mines, such as Karowe and Letseng in Southern Africa, have a relative abundance of these diamonds, whereas others appear to be barren of the CLIPPIR variety. In North America, there are very few examples of large type IIa diamonds, giving the impression that CLIPPIR diamonds are rare and perhaps largely absent. In the Slave Craton, the 78 carat “Ekati Spirit” and a 186 carat diamond from the Pigeon pit at the Ekati Mine are notable, presumed type IIa examples.
From the Superior Craton, however, this recently auctioned $15.7 million stunner may be the first resounding example of a CLIPPIR diamond. It was recovered in 2018 from the Victor Mine in Northern Ontario, which has since closed. This 102 carat oval diamond was cut from a 271 carat rough (read more here) that exhibited the typical irregular morphology of CLIPPIR diamonds, as well as black flake-like inclusions (which are graphitized fractures associated with smaller, less visible inclusions). If this diamond really is a CLIPPIR and originates from the sublithospheric mantle, it weaves a provocative new thread into the geological history of the Jurassic Victor kimberlite and its predominant lherzolitic diamond content from the lithosphere (read more here), which formed at ∼720 Ma following a lithosphere-scale thermal event at ~1.1 Ga (read more here).
This is the 271 ct rough diamond that was recovered at the Victor mine in 2018. Is this the first CLIPPIR diamond recognised from Canada? Image from Sotheby's.