Beyer and four co-authors reported in 2011 that the Paleoproterozoic Thelon Formation in the western Thelon Basin at Boomerang Lake contains four lithofacies and several sub-facies that were grouped into four facies associations, representing deposition in alluvial fan and braided fluvial environments.
A sequence stratigraphic interpretation for the Thelon Formation is proposed, based on a relative hierarchy of widespread surfaces exposed in numerous drill holes. The surfaces mark significant subaerial exposure or change in depositional environment, and bound three composite sequences and a nested series of high-frequency sequences. Low-accommodation and high-accommodation systems tracts are recognized in most high-frequency sequences, and substitute for traditional systems tracts that are linked to changes in eustatic sea level, proxies for which were not observed in the Thelon Formation.
A depositional model based on the sequence stratigraphic interpretation was derived to explain the evolution of the western Thelon Basin. The oldest composite sequence was deposited as talus and lacustrine sediments in local fault-related depressions that may have been initiated by far-field stresses during the nascent stages of Thelon Basin formation.
The next composite sequence represents the first widespread deposition in the area, and comprises strata deposited in braided alluvial plains that drained uplifted basement at the western basin margin. Nested high-frequency sequences are presumed to be individual plains that formed in response to basin-bounding fault displacement and changes in the rate of creation of accommodation.
The youngest composite sequence was deposited in regionally widespread braid plains during a period of post-orogenic lithospheric cooling, represented by aggradationally-stacked, consistently coarse-grained strata. The composite sequence that overlies the Boomerang Lake U prospect on the F-trend is dominated by diagenetic aquicludes that did not conduct U-bearing basinal fluids during the hydrothermal alteration phase.
Finer, muddier diagenetic aquicludes accumulated in distal portions of braided alluvial plains, and became isolated from basinal fluid flow during diagenesis due to stylolite-catalyzed quartz cementation and pervasive occlusion of secondary porosity by K1 kaolinite +M1 muscovite, and partial chloritization. Low basinal fluid flux at the Boomerang Lake prospect resulted in restricted, uneconomic U-phosphate mineralization despite previous reports of “fine-grained pitchblende” mineralization (Davidson and Gandhi, 1989).
Diagenetic aquifers dominate the G-trend to the north, and likely conducted U-bearing basinal fluids throughout diagenesis, including the hydrothermal alteration phase. These coarse-grained, poorly-sorted strata contained minor primary detrital clay, which inhibited early diagenetic stylolitization and associated quartz cementation, prevented secondary porosity from being highly occluded by peak diagenetic phyllosilicates. Prolonged interaction with basinal fluids resulted in the K1 kaolinite +M1 muscovite + K2 dickite assemblage in diagenetic aquifers. Intersections of re-activated basement faults with
the thick diagenetic aquifers of G-trend are the preferred exploration
target at Boomerang Lake.
The western Thelon Basin evolved distinctly from the eastern Thelon Basin, and each is considered as a separate sub-basin. The eastern Thelon Basin contains stratigraphically significant, aquitard-prone shoreface and aeolian facies, and displays an illite/muscovite-dominated diagenetic mineral assemblage. In contrast, the western Thelon Basin evolved in a similar fashion to the eastern Athabasca Basin represented by the Manitou Falls Formation based on the predominance of braided fluvial depositional environments and a kaolin-dominated diagenetic mineral assemblage.
Successful exploration methods in the Athabasca Basin may be applicable to the western Thelon Basin based on their similarities, specifically in geological environments as in the case of the G-trend where thick diagenetic aquifers overlie the unconformity.
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