Geology, Ranji-Cotterill lakes, District of Mackenzie, Northwest Territories

This contribution presents preliminary results of a three year study of the gold-rich Indin Lake area of the western Archean Slave Province (Figure 1; Mineral Initiatives project C4.111). The aim of the project is to update understanding of the structure and stratigraphy of the Indin Lake supracrustal belt, the nature of its contact with high grade gneisses to the east, and the setting of mineral occurrences through a combination of 1:50,000 to 1:5000 scale mapping. Emphasis has been placed on 1) the detailed structural study of well-exposed lake shore outcrops and areas affected by recent forest fires to investigate the geometry of structures in metasedimentary rocks, 2) 1:50,000 and locally more detailed mapping of metavolcanic rocks to test and refine previous lithostratigraphic subdivisions, 3) the detailed study of metavolcanic/metasedimentary rock contacts to investigate their nature, and 4) the definition of the macroscopic structural geometry of metavolcanic units. Nine months of field work in the region (parts of 86B/2,3,6,7, and 86B10/11) have been completed to date (Pehrsson and Beaumont-Smith, 1994; Pehrsson et al., 1995). The adjacent map is one of a pair of preliminary bedrock geological maps that present the results of that field work (Open File 3395 and 3396). Previous mapping in the area includes 1"= 4 mile scale mapping by Lord (west of 115°; 1942); 1"= 2 mile scale mapping by Fortier (east of 115°; 1949) and Frith (86B Indin Lake; 1992); and 1"= 1 mile scale mapping by Tremblay et al. (86B3 Ranji Lake, 1953) and Stanton et al. (86B6 Chalco Lake, 1954). Morgan (1988, 1990) produced maps showing the distribution of mineral occurrences in parts of the Indin Lake belt. This Open File map incorporates data from previous workers as cited in Figure 2. The centre of the combined map area is located 220 km north-northwest of Yellowknife, Northwest Territories and is accessible by float plane. Bedrock exposure is moderate to poor away from lakeshore, particularly east of Indin Lake and north of Spider Lake where Quaternary glacial cover is heavy. The bedrock geology is well exposed in areas of recent burns north and west of Indin Lake and north of Cotterill Lake. The area is underlain by metamorphosed Archean volcano-sedimentary rocks of the Indin Lake supracrustal belt, an eastern gneissic domain and a variety of younger tonalite to syenite intrusions. A summary of the geologic units is presented below, along with a description of structures, metamorphism and mineral occurrences. No stratigraphic succession is implied by the order of Archean supracrustal units in the legend. All the supracrustal rock units have been metamorphosed but for simplicity the prefix 'meta' is omitted from the legend and following descriptions. As noted above, the adjacent map is one of a pair of preliminary, bedrock geological maps that present new data for the Indin Lake greenstone belt and environs. Open File 3395 covers NTS map areas 86B6, 7 and parts of 10/11; principally the north half of the Indin Lake supracrustal belt. Open File 3396 adjoins Open File 3395 to the south and covers NTS map areas 86B2 and 3; principally the south half of the Indin Lake supracrustal belt and adjacent gneiss complex (Figure 2). Both open file maps have a common legend and marginal notes. Consequently, not all units present in the legend and not all features, units and relationships discussed in the marginal notes will appear on a given map. Mineral occurrences in Table 1 are grouped by 1;50,000 scale NTS map areas. Readers are requested to refer to Figure 3 (Mineral occurrence location map) in order to locate specific mineral occurrences pertaining to the map area of interest.