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Location: 308 km NE from Yellowknife, Northwest Territories, Canada
606 Fourth Street S.W.CalgaryAlberta, CanadaT2P 1T1
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The Ekati kimberlite pipes are part of the Lac de Gras kimberlite field which is in the central Slave craton. The kimberlites intrude both granitoids and metasediments. They define several linear trends and are typically associated with dykes and lineaments. There is no dominant or unique structural association of the kimberlites.Fine-grained sediments have been preserved as xenoliths and disaggregated material in kimberlite which indicates that some sedimentary cover was present at the time of the kimberlite emplacement. None of this sedimentary cover has been preserved outside of the kimberlites. The Ekati kimberlites range in age from 45 Ma to 75 Ma. They are mostly small pipe-like bodies (surface areas are mostly <3 ha but can extend to as much as 20 ha) that typically extend to projected depths of 400–600 m below the current land surface. Kimberlite distribution is controlled by fault zones, fault intersections and dyke swarms.Pipe infill has been broadly classified into six rock types:- Coherent kimberlite (CK).- Tuffisitic kimberlite.- Primary volcaniclastic kimberlite (PVK).- Olivine-rich volcaniclastic kimberlite (VK).- Mud-rich, resedimented volcaniclastic kimberlite (RVK).- Kimberlitic sediment.With few exceptions, the kimberlites are made up almost exclusively of volcaniclastic VK, including very fine grained to medium-grained kimberlitic sediments, RVK and PVK. RVK represents pyroclastic material that has been transported (e.g. by gravitational slumping and flow processes) from its original location (likely on the crater rim) into the open pipe and has undergone varying degrees of reworking with the incorporation of surficial material (mudstone and plant material). In rare cases (e.g. Leslie), pipes are dominated by or include significant proportions of CK.While occasional peripheral kimberlite dykes are present, geological investigations undertaken to date do not provide any evidence for the presence of complex root zones or markedly flared crater zones. Kimberlites at Ekati typically contain fragments of wood that was incorporated into the pipe during deposition. The wood fragments identified are related to the redwood Sequoia and Metasequoia genera and are found relatively fresh and unmineralised. Fragments up to 1 m in size were common in several Ekati open pits, but the size and abundance decrease with depth.Depending on the lithological unit, mud can make up a reasonable percentage of a given kimberlite unit. These xenoclasts range in size from millimetres to centimetres and are usually uniformly fine-grained, dark grey to black in colour, and can have portions made up of kimberlitic minerals such as olivine and serpentine but with the majority consisting of smectite, quartz and pyrite. Economic mineralisation is mostly limited to olivine-rich resedimented volcaniclastic and primary volcaniclastic types. Approximately 10% of the 177 known kimberlites in the Ekati project are of economic interest or have exploration potential. Diamond grades are highly variable. Estimated average grades for kimberlites that have been bulk sampled range from less than 0.05 cpt to more than 4 cpt.The Ekati kimberlites are primarily steep-sided volcanic pipes that are mostly filled with volcaniclastic material interpreted to be resedimented and lesser primary volcaniclastic (pyroclastic) kimberlite (Nowicki et al., 2004). While narrow coherent kimberlite dykes are present, these are not volumetrically significant. These mostly appear to predate kimberlite volcanism and are commonly transected by the volcanic pipes. Coherent kimberlite is present in some pipes either as late-stage intrusive material emplaced into volcaniclastic kimberlite (e.g. Koala, or as large pipe-filling bodies (e.g. Leslie; Grizzly).At Ekati, the extent of mantle sampling, the degree of dilution by wall-rock and surface sediments and volcanic sorting processes are considered to be the main factors controlling variation in total diamond grade. The diamond size distribution characteristics are inherited from the original population of diamonds sampled from the mantle but can be affected by a number of secondary processes, including resorption and sorting during eruption and deposition of volcaniclastic kimberlite deposits. Diamond breakage due to geological processes are not expected to be significant enough to notice on the overall size distribution. In general, the diamonds broken by in-pipe geological processes are mostly low-quality stones (especially fibrous, boart or cheap gem categories).
