Kimberlite emplacement and mantle sampling through time at A154N kimberlite volcano, Diavik Diamond Mine: lessons from the deep
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The Diavik Diamond Mine in the NWT of Canada has produced in excess of 100 million carats from 3 kimberlite pipes since mining commenced in 2002. Here, we present new findings from deep (>400 m below surface) mining, sampling and drilling work in the A154N kimberlite volcano that require a revision of previous geological and emplacement models and provide a window into how the sub-continental lithospheric mantle (SCLM) below Diavik was sampled by kimberlite magmas through time. Updated internal geological models feature two volcanic packages interpreted to represent two successive cycles of explosive eruption followed by active and passive sedimentation from a presumed crater-rim, both preceded and followed by intrusions of coherent kimberlite. Contact relationships apparent among the geological units allow for a sequential organization of as many as five temporally-discrete emplacement events. Representative populations of mantle minerals extracted from geological units corresponding to four of the emplacement events at A154N are analyzed for major and trace elements, and provide insights into the whether or not kimberlites randomly sample from the mantle. Two independent geothermometers using clinopyroxene and garnet data indicate similar source depths for clinopyroxenes and G9 garnets (130–160 km), and suggest deeper sampling with time for both clinopyroxene and garnets. Harzburgite is limited to 110–160 km, and appears more prevalent in early, low-volume events. Variable ratios of garnet parageneses from the same depth horizons suggest random sampling by passing magmas, but deeper garnet sampling through time suggests early preferential sampling of shallow/depleted SCLM. Evaluations of Ti, Zr, Y and Ga over the range of estimated depths support models of the SCLM underlying the central Slave terrane.
KeywordsSub-continental lithospheric mantle (SCLM) Kimberlite Geothermobarometry Emplacement Clinopyroxene Garnet
Thanks to the organisers of the 11th International Kimberlite Conference for the opportunity to present these findings, to the Diavik Diamond Mine geology team for ongoing collaboration, sample collection, and helpful discussions on interpretations. Some excellent laser work was completed by the Rio Tinto Exploration team in Bundoora, Australia and the Center for Ore Deposits and Earth Sciences (CODES) in Hobart, Australia. Thanks to Mineral Services Canada and the University of Stellenbosch for major element microprobe work and support. Thanks to Herman Grütter, Graham Pearson, Paolo Nimis, Maya Kopylova, Tom Nowicki, Kelly Russell, Curtis Brett, Matthew Field, Pat Hayman, and Lucy Porritt for invaluable comment, critique and discussions. Constructive comments by an anonymous reviewer and guest editor Casey M. Hetman are gratefully acknowledged.
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