Geology, Amer Lake (66H), Deep Rose Lake (66G) and parts of Pelly Lake (66F)

This open file consists of three 1:250 000 scale preliminary geological maps Amer Lake, 66H; Deep Rose Lake, 66G; Parts of Pelly Lake, 66F). Bedrock mapping was completed during the field seasons 1976 through to 1983. This compilation map incorporates additional data from topical studies carried out by several previous workers. Neoarchean granulite, garnet-biotite paragneiss, mafic schist and minor amphibolite appear to be the oldest rocks in the Amer Lake (66H) region. The oldest supracrustal rocks in the region are metamorphosed layered rocks, intermediate to mafic volcanic rocks, greywacke, chlorite schist, banded iron formation, with subordinate chert, carbonate, and quartzite. Zircon fractions from a dacite porphyry yielded a U-Pb upper concordia intercept age of 2801 +/- 20 Ma. Although considerable controversy exists as to the age of quartzites in the the Amer Lake area (66H), recent studies confirm an Archean age for these quartzites. Archean, spinifex textured, komatiitic rocks are associated with the above supracrustal rocks. Deformed supracrustal rocks of uncertain age, in part consisting of stromatolite bearing units, occur in the Deep Rose Lake map area (66G). Other rock units that range in composition from diorite to granite occur throughout the region, and collectively they represent a basement complex to the deformed and metamorphosed supracrustal rocks of the Paleoproterozoic Amer group. Zircon fractions from a granite from this basement complex yielded a U-Pb upper concordia intercept age of 2617 +/- 20 Ma. The Paleoproterozoic Amer group is dominately made up of two clastic sequences- a lower conglomerate-orthoquartzite and an upper feldspathic sandstone-arkose-siltstone, carbonate, and mudstone-siltstone-sandstone rhythmite. Stratabound uranium mineralization occurs within the upper parts of the transitional sequence. The intensity of deformation within the rocks of the Amer group increases from southwest to northeast with a corresponding increase in metamorphic grade from subgreenschist to lower amphibolite facies. Several north-northwesterly transported thrust sheets have been mapped within the lower Amer group strata. The Amer group was intruded by a younger suite of granite-syenite plutons. Zircon fractions form a syenite intrusion yielded a U-Pb upper concordia intercept age of 1849 +/- 18 Ma. Paleohelikian clastic rocks of the Thelon Formatio unconformably overlie the granitoid basement complex and the Amer group. Northeast-trending ductile high-strain zones (e.g., Amer mylonite zone), and northwest-trending brittle faults affect both the basement and the cover rocks. At least two periods of displacement occurred along the Amer mylonite zone, and that the latest movement (with a dextral sense of displacement) could be as young as ca. 1.7 Ga.