Abstract:

The Sekwi Formation, a Lower Cambrian carbonate unit, is a preferred base metal sulfide host in the Mackenzie Mountains zinc district (Northwest Territories, Canada), but controls on the mineralization are poorly understood. Stratigraphic work in three mineralized areas together with petrographic, fluid inclusion and isotopic studies, demonstrate a first-order structural control (proximity to Cretaceous-Tertiary faults), a second-order lithological control (dolograinstone/rudstone and dark, mottled dolostone), and a local stratigraphic control on the distribution of mineralization. Structural relationships indicate a Late Cretaceous or Tertiary age for much of the mineralization, but some evidence implies that that there was also an earlier (Late Devonian or Early Carboniferous) mineralizing event. At least two fluids mixed to form the showings. Fluid 1 originated as Cretaceous seawater, or formation water containing dissolved Neoproterozoic evaporite rock, and had thermally equilibrated with the Sekwi Formation at 4-6 km depth by the time of mineralization. Fluid 2 originated in felsic crystalline basement, and carried the most radiogenic Sr yet recorded from the northern Canadian Cordillera. Fluid 2 scavenged metals from the sedimentary pile as it moved upward through a network of faults under pressure caused by Cretaceous-Tertiary compressional orogeny. Fluid 2 was 250-350°C and hydrothermal when it penetrated susceptible strata of the Sekwi Formation by hydraulic fracturing and dissolution, and catalyzed thermochemical reduction of the sulfate in fluid 1. This reaction consumed local organic matter. The reduced-sulfur species then reacted with the metals brought by fluid 2, precipitating metal sulfides in the lowest susceptible units. Preferred sites of mineralization were in the hangingwall and immediate footwall of regional thrusts where steep faults intersected each other and susceptible units. This model will be a useful tool in exploration for new deposits or deposit extensions, but further work is needed to refine it.