Northwest Territories Geological Survey

The Ptarmigan and Tom deposits comprise a series of echelon vein-type gold deposits hosted within deformed turbiditic rocks of the Archean Burwash Formation of the Slave Structural Province and are located approximately 10 km east of the Giant Deposit, which produced over 7 million oz. of gold. The Ptarmigan and Tom deposits are located within a 4 km wide metamorphic aureole of the Prosperous Granite to the west. The host rocks contain corderite porphyroblasts and several pegmatitic dykes.
Groundwork assessment of the structural controls of gold-bearing quartz veins of the Ptarmigan and Tom deposits was completed over the course of 15 days in the summer of 2018. Three generations of structures were identified, (D1) A bedding-parallel, weak spaced cleavage interpreted as S1, (D2) A dominant foliation with variable intensity which ranges from spaced cleavage in greywacke to a schistosity within slate horizons, (D3) crenulation cleavage. In addition to the recognized structural generations, a sequence in style of quartz veins was observed. The sequence displays progressive deformation as follows; (A) Bedding parallel veins, ptygmatically folded and overprinted by porphyroblasts and refolded by outcrop-scale asymmetric folding. (B) Stratabound echelon veins folded by asymmetric F2 folds. (C) Ptarmigan style veins, sub-parallel to cleavage that locally cross-cut bedding, locally boudinaged, asymmetrically folded and crosscut by straight shear veins. (C*) Flat veins (extensional veins) that occur on both sides of the Ptarmigan and Tom Veins locally. (D) Cleavage parallel straight shear veins.

Based on field observations, a preliminary model involving oblique sinistral shearing along a high strain zone is proposed in order to explain relative timing of auriferous vein emplacement. (1) A syn-D2 emplacement of a series of echelon shear veins (preferentially along F2 fold axis); (2) Continued oblique-sinistral transpression generating curved-shaped geometry of the echelon veins.