New Investigations of Basement to the Western Athabasca Basin

Abstract

In 2002, investigations of basement to the western Athabasca Basin included reconnaissance outcrop mapping in the Careen Lake area of the Lloyd and Virgin River domains, the Clearwater Domain along the Clearwater River, and the Pine Channel/Fond-du-Lac areas of the Tantato Domain. In addition, diamond drill core was examined from the Dufferin, Carswell, and Hocking lakes areas. The Lloyd Domain contains granulite facies rocks including an old supracrustal package, the Careen Lake Group, that was intruded by ultramafic rocks and a poly-deformed quartz diorite suite. The Virgin River Domain contains dominantly high-grade felsic gneiss and a sliver of middle amphibolite facies supracrustal rocks, the Virgin Schist Group. Younger, less-deformed mafic dykes and granitoid rocks intrude the gneisses, including the equigranular and K-feldspar-phyric Clearwater granites of the Clearwater Domain. The Lloyd Domain contains an old flat-lying S1 foliation which is folded about nearly upright northeast-trending F2 folds. The D3 age Virgin River Shear Zone was subjected to multiple episodes of displacement. An old east-verging reverse shear zone was overprinted by at least two episodes of dextral shearing. West-northwest- to northwesttrending F4 folds probably formed under the same stress regime responsible for late sinistral brittle-ductile displacement along the Virgin River Shear Zone. Evidence for reverse, post-Athabasca Group faulting was observed in drill core at Dufferin Lake. Most rocks in the Lloyd Domain have been subjected to two phases of highgrade metamorphism: the first reaching granulite facies as indicated by orthopyroxene-bearing diatexites and the second at least upper amphibolite facies. The Tantato Domain is divided into upper and lower structural decks. The upper deck is dominated by high-grade supracrustal rocks and the lower deck by mylonitic rocks of plutonic origin. Most of the rocks were strongly sheared during D2 deformation which transposed a pre-existing S0/S1 composite foliation. The D2 mylonites were folded about northeast-trending F3 axial planes and again, about east-northeast trending F4 axial planes. Brittle reactivation of the ductile zone was episodic. Two phases of granulite facies metamorphism affected the Tantato Domain, but it is unclear which event was responsible for the unusually high pressures of at least 10 kbars and rarely >15 kbars. The Clearwater Magnetic High, which overprints magnetic trends of the Lloyd Domain is presumed to be underlain by the Clearwater granites, although the granites are apparently magnetite poor. Magnetite crystallised during interaction of different granite phases in the batholith-scale intrusion and is commonly found along contacts and at the contacts of xenoliths of older gneiss. The Virgin Schist Group is a tectonically bound sliver of anomalously lowgrade supracrustal rocks that was either down-dropped during early, east-verging thrusting or deposited subsequent to reverse shear but prior to later dextral shear along the Virgin River Shear Zone. Supracrustal rocks of the Tantato Domain are likely equivalent to the Careen Lake Group of the Lloyd Domain, however, the rocks that underlie the main magnetic highs in the two domains are dissimilar.