The Midas Pond gold prospect, Victoria Lake Group: geology, alternation and mineralization

Abstract

The Midas Pond gold prospect, discovered by BP Canada Inc. in 1985, is located at the southwest end of the Cambro-Ordovician Tulks Hill volcanics, Victoria Lake Group. Host rocks include variably deformed, waterlain felsic and mafic pyroclastic rocks of island arc origin. -- Alteration and mineralization are confined to a 200 m wide, brittle, ductile-shear zone. This shear zone formed in response to regional D₁ deformation and the shear zone fabric parallels the regional S₁ foliation. D₂ deformation resulted in broad Z-shaped flexuring of the shear zone. -- Advanced argillic alteration and an extensive Fe-carbonate and pyrite halo surrounds the gold mineralization. Alteration assemblages include pyrophyllite, paragonite, quartz, plagioclase, chlorite, fluorite, Fe-carbonate and pyrite. -- The gold, which is intimately associated with pyrite, occurs in three structurally-controlled vein sets. These veins are confined to the contact between a highly deformed breccia termed the Banded Mafic unit and the structurally overlying, altered felsic tuffaceous rocks. The vein sets include: V₁ boudinaged veins developed parallel to the shear zone fabric (C-shear veins) , and V₂ and V₃ extensional fracture veins which are controlled by an S₂ fracture cleavage. V₁ veins are the earliest vein set and contain the lowest concentrations of gold. V₂ and V₃ veins appear to be concentrated within the hinges of the D₂ flexures and selected assays from these veins include 7.9 g/t Au over 0.9 m and 14.74 g/t Au over 1.15 m. -- The mineralizing fluids are interpreted to have been CO₃ ²⁻ and F⁻ rich, slightly acidic and v/ith a temperature range of 250 to 300°C. These fluids are interpreted to have originated through metamorphic dehydration and decarbonation of mixed island arc and continental crustal rocks. Gold precipitation resulted from the reaction of these fluids with the Fe-rich Banded Mafic unit. Pre-existing and simultaneously crystallizing pyrite served as nuclei for gold precipitation.