EVIDENCE OF ANOXIC TO OXIC ATMOSPHERIC CHANGE DURING 2.45-2.22 GA FROM LOWER AND UPPER SUB-HURONIAN PALEOSOLS, CANADA

Paleoweathered zones in four underground drill holes in Quirke II mine and one drill hole by Denison Mines in Elliot Lake area, Ontario, Can ada were studied for petrography, mineralogy, and whole rock chemistry . These paleosols, formed beneath 2.45 Ga old lower Huronian formation s, are characterized by upward depletion of Fe-total and Fe3+ in the w hole rock, by upward depletion of Fe-total, Fe/Mg ratios in chlorites and fine-grained muscovite, and by the presence of Fe2+ minerals, e.g. pyrite, pyrrhotite, ilmenite, In contrast, the paleosols underlying u pper Huronian strata (> 2.22 Ga old) exhibit an upward increase in Fe3 + and Fe-total and contain hematite and goethite. Uraninite and pyrite -bearing paleoplacers occur in lower Huronian strata, whereas the uppe r Huronian formations contain red beds and are devoid of detrital uran inite and pyrite. These differences between the sub-Huronian paleoweat hering profiles and mineralogical characteristics of the lower Huronia n strata overlying the paleosols, support the previously postulated ox yatmoversion theory and restricts the time interval for its occurrence to between 2.45 and 2.22 Ga. Assuming that few tens of millions of ye ars were required for the deposition of lower Huronian Groups, oxyatmo version likely occurred at about 2.4 Ga.