Geology of Kognak River area, District of Keewatin, Northwest Territories
Authors: Eade K E 
Published: August 2015

The geology of an area in the Churchill Structural Province, west of Hudson Bay, is described and the results of three field seasons of mapping are presented in the report. The rocks are entirely Precambrian, partly Archean, and the remainder, chiefly Aphebian,
with minor Helikian. Although the Hudsonian orogeny resulted in some metamorphism and igneous intrusion, the effects are much less pronounced than in most parts of the Churchill Province. The metamorphic grade of Archean Henik Group volcanic and sedimentary rocks ranges from lower greenschist to
amphibolite facies. Schists, paragneiss, and granitoid gneiss result from metamorphism and metasomatism of some of these rocks. Plutons, chiefly of granodiorite composition and less commonly quartz diorite, monzonite or syenite, intrude the volcanic and sedimentary rocks. Gabbro or metagabbro dykes
trending north to northeast cut the Archean rocks. Areally limited and previously undifferentiated Montgomery Lake Group sedimentary rocks of early Aphebian age consist of boulder-conglomerate, quartzite, impure quartzite, greywacke, and siltstone. They unconformably overlie Archean rocks and in
turn are overlain unconformably by the Hurwitz Group. The Aphebian Hurwitz Group, which occurs widely throughout southern and central District of Keewatin, has the thickest and most varied section in this map-area. Basic sills intrude the sedimentary rocks of the Hurwitz Group in part of the area.
Sedimentary rocks occurring west of the Hurwitz Group and separated by major faults are considered to be lateral equivalents of the Hurwitz Group. An Aphebian quartz monzonite pluton related to the Hudsonian orogeny caused contact metamorphism of the Hurwitz Group in a very limited area. although no
intrusive contacts have been seen. Coarse-grained, porphyritic, fluorite-bearing granite, probably early Helikian, occurs only in one small area. The youngest rocks in the map-area are northwest-trending gabbro or diabase dykes. Metamorphism in the Montgomery Lake and Hurwitz Groups does not exceed
the greenschist facies except for limited areas of contact metamorphism in the Hurwitz Group. The earliest deformation, involving Archean rocks, caused north-trending folds but these are commonly obscured by later deformation. Two Aphebian deformations resulted in north- to northeast-trending folds
and west- to northwest-trending folds. The earlier of these deformations was responsible for pronounced cleavage in both Archean and Aphebian rocks. Prominent east-trending thrust faults are related to the deformation that produced the west- to northwest-trending folds. Normal faults of various
trends are abundant and apparent dislocation is considerable on some of them. Gold mineralization occurs in basic Archean volcanic rocks and in low-grade Archean iron-formation in some places. Minor amounts of chalcopyrite are present in shear zones in the basic volcanic rocks. Pebble-conglomerate
lenses in the orthoquartzite of the Hurwitz Group and the pyrite-bearing quartzite of the Montgomery Lake Group may be of interest in the search for radioactive minerals.