A Meteorite Impact Origin for the Chibougamau Formation, Québec, Canada: towards a New Interpretation of the Chibougamau-Mistassini Mining Camp.

English Abstract:

The Chibougamau Formation (CF) is an assemblage of conglomerate, arkosic sandstone, graded laminite and diamictite resting unconformably on the Archaean basement. The main outcrops of the CF occur around the Chibougamau area. They are bounded by SW-NE faults in a kind of semi-grabben. The upward movement of the southeastern compartments has been related to a subsequent tectonic event. The CF has been firstly described as a mixture of sediments with an igneous component such as diabase. Later, it has been compared to the Gowganda Formation (GF, Cobalt Group, Ontario) and interpreted as a glaciogenic rock. Because of the correlation between the GF and the CF, and the lack of carbonate fragments, the CF has been dated pre-Mistassini Group (which is rich in dolomite). Glaciogenic tenant searchers have never found a striated fragment or a specific iron-shaped one. While participating to a field trip organized by the CIM in 1984, the second author noted some sedimentary incongruities within the diamictite. Since then, the CF has been the subject of studies by our group and subsequently interpreted as a meteorite impact fallout. More recently, a master thesis work dealing with petrography on three sites of the CF has been performed by the third author in order to validate the new interpretation. Results cannot be more conclusive: shock metamorphism has affected numerous large fragments and micro-fragments in the matrix (planar deformation features (PDFs), maskelinite and glass). Further studies of the CF yielded more information: up to 3 sets of PDFs in feldspars, quartz and other minerals, ballen texture, toasting on feldspars and quartz, brown glass fragments, some of calcitic origin, and some spherules. Moreover, a large outcrop observed in the Mt Radar area shows micro-breccias associated with flowing glass in a carbon-rich fine grain matrix. The finding of well expressed black carbonate fragments in the CF is of interest. Did the CF postdated the Mistassini Group?