Mineralogy and geochemistry of a precambrian regolith in the Athabasca Basin
Date
1980
Authors
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Publisher
ORCID
Type
Degree Level
Masters
Abstract
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**
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Degree
Master of Science (M.Sc.)
Department
Geological Sciences
Program
Geological Sciences