Evidence of anoxic to oxic atmospheric change during 2.45-2.22 Ga from lower and upper sub-Huronian paleosols, Canada

Abstract

Paleoweathered zones in four underground drill holes in Quirke II mine and one drill hole by Denison Mines in Elliot Lake area, Ontario, Canada were studied for petrography, mineralogy, and whole rock chemistry. These paleosols, formed beneath 2.45 Ga old lower Huronian formations, are characterized by upward depletion of Fe total and Fe 3+ in the whole rock, by upward depletion of Fe total, Fe/Mg ratios in chlorites and fine-grained muscovite, and by the presence of Fe 2+ minerals, e.g. pyrite, pyrrhotite, ilmenite. In contrast, the paleosols underlying upper Huronian strata (> 2.22 Ga old) exhibit an upward increase in Fe 3+ and Fe total and contain hematite and goethite. Uraninite and pyrite-bearing paleoplacers occur in lower Huronian strata, whereas the upper Huronian formations contain red beds and are devoid of detrital uraninite and pyrite. These differences between the sub-Huronian paleoweathering profiles and mineralogical characteristics of the lower Huronian strata overlying the paleosols, support the previously postulated oxyatmoversion theory and restricts the time interval for its occurrence to between 2.45 and 2.22 Ga. Assuming that few tens of millions of years were required for the deposition of lower Huronian Groups, oxyatmoversion likely occurred at about 2.4 Ga.