IF, GIF, and two BIF in northwest Canada: time-space relationships and depositional environments
Luke Ootes1, Kurt Konhauser
2, Rasmus Haugaard II
2, Andrey Bekker
3, Ernesto Pecoits
2 and Natalie Aubet
2, (1)Northwest Territories Geoscience Office, Yellowknife, NT, Canada, (2)University of Alberta, Earth and Atmospheric Sciences, Edmonton, AB, Canada, (3)University of California, Earth Sciences, Riverside, United States
Contact First Author: Luke Ootes; luke_ootes@gov.nt.ca
English Abstract:
Bedrock exposures in northwest Canada preserve many relevant features critical to understanding Precambrian Earth history. One of these is iron formation (IF), whose physical and chemical attributes uniquely provide details regarding the composition of Precambrian seawater and environmental change. Two distinct types of banded IFs are preserved in the Slave craton. The older was deposited at ca. 2.85 Ga and precipitated on top of fuchsitic quartzite, unconformably overlying basement gneisses. The BIFs are up to 10 m thick and consist of micro- and meso-bands of chert and magnetite. They are generally overlain by massive to pillowed basalt flows or, locally, by pebble conglomerates. The younger BIF type occurs as 5 to 10 m thick beds of interlaminated chert-magnetite-carbonate, with local grunerite and sandstone beds, within ca. 2.62 Ga turbidite packages. These BIFs are preserved across the craton in a southwest to northeast fashion and represent a significant hiatus in turbidite deposition to allow for the chemical precipitation and preservation. The deep-water nature of these Neoarchean BIFs is in stark contrast to the more shallow nature of the Mesoarchean BIFs.
In the East Arm basin, south of the Slave craton, up to 9 m of ca. 1.88 Ga granular IF is preserved within green, grey, and red shales of the Gibraltar Formation in the Great Slave Supergroup. This GIF contains hematite-jasper, and its granular nature, along with ripple marks and local stromatolites, attest to the shallow-water deposition of this iron formation on a marine shelf. The age is similar to that of the Gunflint Iron Formation in the Superior craton, which is also granular and was deposited in a shelf environment. In the northern Mackenzie Mountains, up to 120 m of ca. 700 Ma IF is preserved in the Rapitan Group. It consists of hematite with chert-carbonate nodules and less common individual jasper and hematite beds up to 10 cm thick. The upper exposure of the IF contains granular hematite, perhaps indicating an upward-shallowing depositional environment. Unlike the older IFs, the Rapitan Iron Formation was deposited under glacial conditions of the Sturtian Snowball Earth and represents a return to IF deposition after a 1.15 Ga hiatus.
