The Diavik Diamond Mine
Location: Lac de Gras, Northwest Territories, Canada.Products: Diamonds.
Owner: Dominion Diamond Corporation and Diavik Diamond Mines Inc.
Ore Type: The mine consists of three kimberlite pipes.
Geological notes of Diamond and The Diavik Mine:
Our knowledge of the primary sources of diamonds in the lithospheric upper mantle is mainly derived from the studies of mantle xenoliths in kimberlites and of mineral inclusions in diamonds themselves. Inclusions in diamonds preserve evidence of the physical and chemical environment at the time of diamond formation, presumed to have occurred early in Earth’s history (e.g. Richardson et al. 1984). Mantle xenoliths, in contrast, integrate a more protracted history that may have involved multiple stages of melt extraction, and thermal re-equilibration in response to short lived thermal pulses or secular cooling, and metasomatic re-enrichment. Rare diamond-bearing peridotite xenoliths provide unique opportunities to study the principal source of diamonds in the Earth’s mantle directly and to obtain information on the evolution of cratonic lithosphere, spanning the time from diamond formation to kimberlite eruption. Based on inclusion studies, peridotitic diamonds largely formed in depleted harzburgitic sources (Gurney and Switzer 1973; Gurney 1984). Evidence for changes in the composition of peridotitic subcratonic lithospheric mantle over time, involving a decreasing ratio of harzburgite to lherzolite (Griffin et al. 2003), raises the possibility that diamonds are stored in mantle rocks that are compositionally quite distinct from the environment of diamond formation. This would have important implications for diamond exploration, because indicator mineral assessment, evaluating the state of mantle lithosphere at the time of kimberlite eruption, is strongly based on chemical criteria derived from inclusion studies depicting the environment of diamond formation. One of the key questions for our study of diamondiferous peridotite xenoliths from Diavik, therefore, is verifying the extent to which the originally highly depleted signature at the time of diamond formation has been preserved or modified during subsequent metasomatic events.
Based on the composition of xenoliths and garnet xenocrysts, Griffin et al. (1999a) inferred that the mantle beneath the Lac de Gras area is chemically and thermally stratified. They suggested that an ‘‘ultradepleted’’, predominantly harzburgitic layer overlies a less depleted, predominantly lherzolitic layer with the transition being located at *145 km depth. Griffin et al. (1999a) proposed the shallower ‘‘ultradepleted’’ layer to represent Mesoarchean oceanic or sub-arc mantle lithosphere and the lower layer to be the frozen head of a Neoarchean plume derived from the lower mantle. Aulbach et al. (2007) suggested that the deeper portions of the lower layer experienced secondary re-enrichment in FeO (Aulbach et al. 2007). An alternative model for the formation of subcratonic lithospheric mantle involves stacking of highly depleted Archean oceanic lithospheric mantle beneath early continents (e.g. Schulze 1986; Helmstaedt and Schulze 1989; Bulatov et al. 1991; de Wit 1998; Stachel et al. 1998). In this model, the observed increase in fertility with depth in the central Slave craton may relate to metasomatism by infiltrating fluids/melts ascending from the asthenosphere (Stachel et al. 2003).
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