GSC Open File D3942 presents the mineral chemistry and other chemical characteristics of kimberlite indicator minerals from the Drybones Bay area, N.W.T. (Fig. 1). As part of the ongoing Yellowknife EXTECH program, a GSC surficial geology study in 1999 undertook regional till sampling in the Yellowknife and Drybones Bay areas. Detailed till sampling was also undertaken in the Drybones Bay area in view of the presence of the diamondiferous Drybones kimberlite. This is part of a glacial dispersal study which is currently focussing on an area approximately 4 km southeast of Drybones Bay, in a region informally called Mud Lake (Fig. 2). Five regional till samples weighing approximately 10 kg each were collected, in addition to 25 till samples weighing approximately 20 to 30 kg each, as part of the detailed till sampling program in the (Fig. 3). This report supercedes Open File D3861 (Kerr et al., 2000) which presented the preliminary counts of suspected kimberlite indicator minerals, as well as counts of gold grains and gold geochemical data for till samples in the Yellowknife and Drybones Bay area, N.W.T.

The majority of the 1700 analyzed grains listed are ilmenites, both low-Mg, i.e regional ilmenites (n=590), and high-Mg or low-Mg/high Cr kimberlitic ilmenites, including Mn-ilmenites (n=474). There are only 31 chromites. The analyzed silicates are dominantly garnets, most of which are mantle-derived Cr-pyropes (n=504); the others are crustal almandines and spessartines (n=14). Only three Cr-diopsides were found.

Electron microprobe studies have shown that indicator minerals of at least two distinct kimberlites appear in the heavy mineral concentrates recovered from till samples in the Drybones Bay area. Clearly, the sample from site 6058K located immediately down-ice (west) of the Drybones Bay kimberlite relates to this diamondiferous pipe. The MLT sample series, more specifically MLT4, 5, 6 and 6059K in the Mud Lake area, 4 km southeast of Drybones Bay, relate to an undiscovered kimberlite(s).

The relatively large number of indicator minerals recovered in the Mud Lake till samples is indicative of close proximity to their kimberlitic source, because of the regional background of 0 grains. Ice flow patterns suggest a source likely a short distance up-ice (east/northeast) of these samples because indicator grains originating from the kimberlite(s) would be transported southwestward in till. However, additional till sampling is required to further define the source in view of the possibility that the till may have been reworked briefly by the waters of glacial Lake McConnell, accounting for the lack of a typical linear dispersal train.

An edited version of the report accompanying Open File D3942 (in Adobe Acrobat pdf format), complete with explanatory notes, figures and references, can be downloaded here [PDF, 2.4 Mb, viewer].

The complete Open File D3942, with digital data files for the geochemistry of kimberlite indicator minerals, can be purchased from The Geological of Canada Bookstore.

Author: Daniel Kerr