Geology of the Fort Chipewyan District, Alberta

The geology of 432 square miles (1110 square km) of Precambrian Shield in northeastern Alberta is presented on four map sheets at a scale of 2 inches to 1 mile. Eleven main rock groups or rock types, and several sub types, are distinguished on these maps. Their characteristics, both in outcrop and in hand specimen, are described and modal and chemical analyses of representative, standard samples are presented.

The Precambrian Shield exposed in northeastern Alberta forms part of the Churchill Structural Province, the oldest (age-dated) rocks being of Archean age. The Archean rocks formed under granulite metamorphic conditions and subsequently were overprinted by amphibolite conditions imposed during the Hudsonian (orogenic) Event. Granitoid rocks were intruded during the Aphebian under amphibolite metamorphic facies conditions. A late Hudsonian retrograde greenschist facies metamorphism affected all Shield rocks exposed in Alberta. The oldest rocks consist of a basement migmatite complex of para and orthogneisses which was later intruded by several granitoid plutons. This basement complex probably represents multiple cycles of sedimentation, intrusion, deformation and metamorphism. All rock units in the basement complex and younger granitoids have been affected by either plastic or brittle deformation. A regional mylonitic fault zone cuts the basement complex parallel to the predominant northeasterly structural trend within the map area.

The gneissic basement complex and younger granitoids are overlain (unconformably?) by low-grade metasedimentary rocks of the Burntwood Group of probable late Aphebian age. The shoreline of Lake Athabasca in the Fort Chipewyan area approximately coincides with the erosional edge of unmetamorphosed sandstones of the Athabasca Formation (Helikian age), the youngest consolidated rocks to crop out in the map area.

Minor sulfide and uranium mineral showings are primarily related to bands or lenses of high-grade metasedimentary rocks within the basement complex.