Geology, Lac de Gras kimberlite field, central Slave Province, Northwest Territories - Nunavut

The Eocene to Cretaceous Lac de Gras kimberlite field occurs in the central Slave Province. The bedrock geology map compilation area, which encompasses the Lac de Gras kimberlite field, includes the NTS sheets 76C (Aylmer Lake), 76D (Lac de Gras), 76E (Contwoyto Lake, south half) and 76F (Nose Lake, south west corner). The main bedrock geological elements within the map area are shown in Figure 1: 1.) Pre- to syn-Yellowknife Supergroup gneiss, migmatite and metagranitoid rocks; 2.) Yellowknife Supergroup volcanic and volcaniclastic rocks, and syn-Yellowknife Supergroup felsic porphyritic and mafic intrusive rocks; 3.) Yellowknife Supergroup sedimentary rocks; 4.) Syn-Yellowknife Supergroup granitoid rocks 5.) Post-Yellowknife Supergroup granites; 6.) Proterozoic intrusions (dominantly diabase dykes); 7.) Eocene to Cretaceous kimberlite pipes. The geology of the area is described and discussed in terms of the seven main bedrock elements, with some additional notes on structure, metamorphism, and economic geology. The map and marginal notes are compiled from previously published bedrock geology maps and accompanying marginal notes, with these sources shown in Figure 2. In addition, papers and reports on specific aspects of the geology of the compilation area were also utilized. A number of problems are inherent in creating a bedrock compilation map, in which the original individual maps were generated over a period of greater than fifty years. The map legend is based upon the mineralogical description of the pertinent rock unit, not how that unit was originally named. This was done, in part, to eliminate rock unit name conflicts at map boundaries e.g. what one geologist describes as a tonalite, on an adjacent map (within a contiguous granitoid pluton) another geologist might describe as a quartz diorite . For the Aylmer Lake map sheet (Lord and Barnes, 1954) and the southeast quadrant of the Lac de Gras map sheet (Folinsbee, 1949) mineralogical descriptions were extracted from the archived field notebooks in conjunction with the traverse maps. This information was then utilized to subdivide and place the syn- and post-Yellowknife Supergroup granitoids into a modern context. Were possible, this was confirmed by examining archived rock samples. Verification of this approach has further been confirmed from sampling of granitoid rocks in the Aylmer Lake map sheet (K. MacLachlan and B. Davis, personal communication, 2001). The surficial geology of the area (with the exception of the Nose Lake map sheet) has been mapped within the past decade (see Dredge et al., 1995a; Kerr et al., 1997; Ward et al., 1997). Additional information on till geochemistry, and gold grain counts in till can be found in Dredge et al. (1995b), Ward et al. (1996) and Dredge et al. (1997). Results from low-density kimberlite indicator mineral sampling (grain counts and chemistry) are reported in Dredge et al. (1995c) and Ward et al. (1995). McClenaghan et al. (2000) provide a detailed study of the down ice dispersion train from the Ranch Lake kimberlite (till geochemistry, grain counts and mineral chemistry). Armstrong (2001) provides a comprehensive data set of industry kimberlite indicator grain counts and mineral chemistry for the area, based on information available in public domain assessment reports.

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