The deformational history of the Black Bay structure near Uranium City, Northern Saskatchewan

The Black Bay Shear Zone, northwestern Saskatchewan, lies within the Rae Province of the Canadian Shield. It is one of several major, northeast trending, steeply dipping shear zones that were active during the Paleoproterozoic Trans-­Hudson or Thelon Orogenies. For part of its exposed length of approximately 50 km it separates Archean supracrustal gneisses of the Tazin Group to the northwest, from the unconformably overlying Paleoproterozoic, red-bed sequence of the Martin Group to the southeast. Deformation fabrics along the Black Bay Shear Zone indicate the sequential development of early ductile to late brittle episodes of movement. The early ductile episode (D1, pre-Martin Gp) is characterized by a 1 km-wide mylonite belt in the Tazin gneisses, with mylonitic lamination (C-fabric), dextral ä-asymmetric winged porphyroclasts, stretch lineation (L1) and rare sheath folds. This was followed by a ductile to brittle transition (D2, also pre-Martin Gp) characterized by small asymmetric folds in C, and small post-C compressional and extensional shear bands (SB) ranging from thin ductile shear zones to brittle fractures commonly vein filled. The late brittle phase (D3, post-Martin Gp), to which uranium vein mineralization is related, included the formation of a major brittle fault zone along the southeastern side of the mylonitic shear zone, and several sets of vein filled joints. Movement directions during D1 to D3 are reflected by various shear sense indicators. The D1 dextral ä-asymmetrical winged porphyroclasts, combined with the gently NE and SW plunging L1, stretch lineation, indicate sub-horizontal dextral displacement during the ductile phase of deformation. Predominantly NE-verging and steeply dipping SW plunging minor D2 folds, along with the predominantly dextral-verging, post-C shear bands, indicate that oblique dextral SE-side-up slip occurred during the ductile to brittle transition. Brittle movement (D3)resulted in the preservation of an approximately 8-km thick succession of Martin Group on the SE-side of the fault, indicating a SE-side-down vertical throw of at least 8 km. The local presence of poorly preserved, down-dip slickenlineations suggests that at least some of this movement was dip slip. Time constraints on deformation are poor, tentatively all of the Black Bay Structure deformational history took place between 2300 Ma and 1700 Ma