| Popis: |
The regolith beneath the Helikian Athabasca Group sandstones in northern Saskatchewan displays up to four laterally correlatable colour zones. Their sequence, vertical extent, and chemical and mineralogical characteristics correspond closely to those of recent lateritic profiles formed in subtropical to tropical climates. Under these conditions, abundant rainfall on an unvegetated Precambrian surface would likely have resulted in rapid erosional losses from the upper hematite zone (equivalent to a modern laterite horizon), preventing the high residual concentrations of Fe and A1 characteristic of modern laterites. Post-weathering alteration from Athabasca diagenetic solutions was restricted to a relatively thin bleached zone adjacent to the unconformity. However, the pressure-temperature conditions accompanying burial caused recrystallization of illites, transformation of Mg-smectite to Mg-dioctahedral chlorite, and the possible dehydration of goethite to hematite within the pre-existing weathering profile. The presence of an oxidized upper horizon in the pre-1500 m.y. regolith is consistent with previous estimates of 1700-2000 m.y. for the beginning of an oxidizing atmosphere. Comparison with other published descriptions of Precambrian weathering shows a much closer similarity to the late Precambrian profiles than to those formed in a reducing early Precambrian environment. Within the study area, large-scale depletions of the trace elements associated with uranium mineralization have not occurred from the regolith, supporting the hypothesis that diagenetic leaching of detrital heavy minerals in the Athabasca sediments provided the main source of these elements for ore deposition. Since the bleaching alteration at the unconformity appears to be the regional equivalent of alteration observed in the ore deposits, both the extent and the trace element geochemistry of the bleached zone have potential use as exploration tools. Based on the weathering environment deduced from the study profiles, an alternative explanation for the composition of the Athabasca Group sediments can be proposed. If the tropical conditions present during weathering continued through the time of deposition, erosion of the lateritic source material would have generated a detrital assemblage of quartz, kaolinite, illite, and iron oxides - the present constituents of most of the Athabasca sediments. Thus the clay and iron oxide in the sandstone can be more simply explained by deposition of this lateritic detritus than by diagenetic alteration of detrital primary silicates produced by physical weathering in an arid climate. The role of diagenesis in the tropical model would be a much lesser one, consisting of textural and crystallinity modifications to the detrital secondary minerals. **There was no page 91 in the original thesis |
