Carbonate-Hosted Massive Sulfide Deposits and Hypogene Speleogenesis: A Case Study from Nanisivik Zinc/Lead Mine Baffin Island, Canada

Nanisivik zinc/lead mine is located at Lat. 73°N in northwestern Baffin Island. The host rock is a pervasively dolomitized Proterozoic platform carbonate overlain by a shale aquitard. The principal deposit, the Main Ore, is of zinc, lead, and iron sulfide plus gangue minerals, chiefly secondary dolomite. It extends for 3 km along a horst and is terminated at both ends by modern valley entrenchments. The Main Ore body is consistently ~100 m wide and five to seven m thick. The ceiling is a wide, horizontal corrosion bevel. Within the Main Ore are two or more generations of tapered fins of dolomite that extend into the cavity. The fin edges are sinuous, some meandering. Sharp, horizontal corrosion notches 20–30 cm high extend into the dolomite walls. They are filled with layered pyrites which continue into the ore body, truncating earlier, dipping mineral bodies. Sedimentary textures suggest that the ore cavity formed by paragenesis in a channel-flow mode, with ore and gangue deposition on the floor taking place in tandem with dissolutional cavity creation upward. Fluid inclusions indicate derivation of the metals from exchange reactions with metalliferous sediments (the underlying shales), indicating low water/rock ratios and moderate temperatures. The ore fluids were similar to oilfield brines. Sulfur isotope fractionations indicate temperatures of 90–150 ± 40 °C, suggesting that the Main Ore formed along a gas/brine interface at a depth of at least 1600 m. Latest studies suggest a Grenville age (~1.1 Ga) for the Main Ore.