Earth Sciences Report 1984-06

Author(s) Date 1986-12-31

The study area comprises essentially two major rock groups, the granite gneisses and the granitoids. Arch Lake and Slave Granitoids form a major pluton and are by far the most abundant group in the map area. The Archean Granite Gneiss belt, situated in the east, consists of biotite and hornblende gneisses with subordinate amounts of high-grade metasediments and very minor amphibolite. Minor granitoid bodies of various sizes are interspersed throughout the Granite Gneiss belt. Most of these granitoids are related to the major granitoid rock units of the pluton either directly or as intermediate lithological phases. Gradational phases are evident both between and within the granitoids, and to some extent between the granitoids and the granite gneisses. The latter are regarded as the protolithic material from which the granitoids were formed in the course of ultrametamorphism and partial melting.

The crystalline rocks have recorded metamorphic conditions ranging from high pressure granulite to low pressure amphibolite facies, followed by a ubiquitous regional retrogressive greenschist facies metamorphism.

The metamorphic foliation in the mantling granite gneisses shows complex patterns of ductile flow folds, but overall it has a general regional northerly trend. By contrast, the metamorphic foliation in the granitoids, although deformed, generally shows much more open folds on a local scale. The Hooker Lake synform in Arch Lake Granitoids, almost 40 km across, is the principal structure in the map area.

The bedrock is cut by two regional shear zones. Both major shear zones are characterized by wide mylonitic bands indicative of deep-seated ductile deformation. Both generally parallel to the regional foliation, but are locally transgressive. The Allan Fault, in the east, trends northerly and is almost confined to the Granite Gneiss belt. The arcuate Warren Fault, in the west, trends northeasterly and its wide shear zone coincides with a complex, interfingered, ductile deformation contact zone between major bodies of the Slave and Arch Lake Granitoids. Other arcuate faults follow metamorphic foliation within the Arch Lake Granitoids synform. Biotite and hornblende K-Ar age dates show that the region was subjected to an intense thermal event connected with the Hudsonian Orogeny, resetting the K-Ar isotopic ratios at approximately 1900 Ma.

Continental Pleistocene glaciation scoured the region, leaving a Precambrian Shield rocky landscape with abundant evidence of glacial advance and retreat. The major ice advance was from the east, and glacial retreat appears to have been in stages, with ice front positions being delineated by the accumulation of ice contact outwash deposits. Aeolian reworking of the typically sandy glacial deposits by storm winds has led to the formation of dunes and associated wind polish, facetting and grooves on bedrock outcrop.

Minor mineralization is found in two geological settings. Scattered mineralization consisting of uranium, copper, molybdenite and arsenopyrite is found in high-grade metasedimentary rocks and granite gneisses. In the regolith at the base of Devonian cover rocks seated on the crystalline Precambrian Shield, secondary copper mineralization is found in the highly oxidized Shield rocks. This contact between Devonian cover and the Precambrian Shield is exposed in places along the Slave River. The granitoids are virtually devoid of any signs of mineralization.

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Godfrey, J.D. and Langenberg, C.W. (1987): Geology of the Myers-Daly Lakes district, Alberta; Alberta Research Council, ARC/AGS Earth Sciences Report 1984-06, 44 p.