TARGETING CARBONATE-HOSTED ZN-PB ORE IN THE MACKENZIE MOUNTAINS: WHERE TO LOOK AND WHY

Numerous occurrences of carbonate-hosted (or Mississippi Valley-type) Zn±Pb mineralization are present in Neoproterozoic to Devonian rocks of the Mackenzie Mountains zinc district (Northwest Territories, Canada). Detailed stratigraphic and petrographic studies of showings in the Early Cambrian Sekwi Formation, a preferred host of mineralization, have allowed us to identify major controls on localization of sphalerite, galena, and subordinate ore minerals. Microthermometry of fluid inclusions in sphalerite and Sr, C, O, and S isotopic analyses of sphalerite, barite and dolomite have revealed the involvement of hotter fluids that rose from deeper sources than hitherto suspected.

A first-order control is proximity to faults. Rock types that are preferentially mineralized are those with mm- to cm-scale variations in mineralogy or texture, such as ooid grainstone or burrow-mottled dolostone with variable organic or siliciclastic content. This lithologic control is third-order, subordinate to a second-order stratigraphic control such that the structurally lowest susceptible rocks in the local succession are preferentially mineralized. The age of mineralization is Cretaceous-Tertiary, based on its association with late faulting.

At least two fluids participated in mineralization. Fluid 1, which may have been a saline connate fluid or seawater, equilibrated with marine carbonate rock (perhaps Sekwi Formation) at 150ºC. Fluid 2 rose along steep faults from a crystalline basement reservoir during Late Cretaceous orogeny, leaching metals from the overlying sedimentary pile, and equilibrated thermally with its surroundings at 250-350ºC as it traversed a Neoproterozoic evaporite horizon along a major detachment 7 km deep. Thermochemical reduction of evaporitic sulfate produced H₂S, leading to metal-sulfide precipitation in hydrothermally enhanced pores of the Sekwi Formation.

This deposit model can be applied throughout the Mackenzie Mountains. Carbonate-hosted Zn±Pb deposits should be sought near ramping thrusts or their splays, preferably in upper Sekwi Formation where it has been dolomitized and disrupted by multiple steep faults. The most prospective horizons are the structurally lowermost local occurrences of susceptible rock types.