Structural Style and Regional Comparison of the Paleoproterozoic Ketyet River Group in the Region North-Northwest of Baker Lake, Nunavut

Abstract

Archean to Paleoproterozoic rocks of the Rae Craton, Western Churchill Province, have been affected by polyphase deformation and metamorphism causing structural complexity and confusion regarding the age and affiliation of rock units. This study improves the stratigraphic and structural understanding of the Paleoproterozoic Ketyet River group and immediately subjacent Neoarchean rocks through detailed mapping of four areas north and west of Baker Lake: “Nipterk Lake”, “Ukalik Lake”, “Bar Lake” and Kiggavik, north of the uranium deposits. This improves knowledge of the basement rocks marginal to the late Paleoproterozoic Thelon Basin for unconformity-related uranium exploration. In 2010 and 2011, detailed mapping in the first three areas revealed that the Ketyet River group comprises thin metaconglomerate gradationally overlain by orthoquartzite and grey pelitic schist. At Bar Lake, sills of metagabbro within the latter may be equivalent to the Five Mile Lake basalts, substantiating correlation with the Amer Group. The metaconglomerate and orthoquartzite unconformably overlie 2.6 Ga quartz-K-feldspar porphyritic schist (QFP schist) and parts of the Woodburn Lake group ranging from feldspathic metagreywacke to komatiite. Where the metaconglomerate is absent, the base of the orthoquartzite contains “quartz eyes” resembling those of the QFP schist. Cross-beds at the base and top of the orthoquartzite respectively face away from the QFP schist and toward the pelitic schist, providing control on the younging direction. The quartzite-pelitic schist contact is gradational; approaching the contact, decimetre-scale granule- to pebble-metaconglomerate and centimetre-scale pelitic schist interbeds are common, whereas above the contact, the pelitic schist contains graded granule metaconglomerate interbeds. Five ductile deformational events (D1 – D5) affected all of these rocks, although D4 was not observed in this study. The first two strongly controlled the map pattern, whereas D3 and D5 are recorded mainly at the outcrop scale as strong domainal crenulation cleavages defined by micas in the QFP and pelitic schists. D1 recumbent isoclinal folds and thrust faults caused multiple structural repetitions of the Neoarchean and Paleoproterozoic strata. D2 coaxially refolded D1 structures, producing type 3 “hook” interference patterns. D1 structures were generally transposed sub-parallel to the inclined axial planes of second generation open to closed folds (F2) cut by northwesterly directed thrust faults. At Kiggavik, relationships are similar except that F2 axial planes are moderately north-dipping rather than steeply south-dipping. Brittle faulting related to Thelon Basin development dextrally offset basement rocks at Bar Lake and Kiggavik, allowing definition of three structural domains at Bar Lake that are displaced and rotated relative to one another.