Abstract

Extensive stratiform copper mineralization occurs in three upper Proterozoic embayments along a 250-km-long arcuate belt in the Mackenzie Mountains Supergroup, Northwest Territories, Canada. Mineralization is found in transgressive strata between the continental red beds of the Redstone River Formation and shallow restricted marine carbonates of the overlying Coppercap Formation. A mafic volcanic succession lying stratigraphically beneath the red beds possibly provided copper-bearing detritus to the red beds.The Coates Lake deposit, the object of this study, contains cupriferous sulfides (principally chalcocite, bornite, and chalcopyrite) hosted by microbial-laminated limestones and dolostones. These carbonates are intercalated with red siltstones to form as many as seven red bed-carbonate couplets within the 110-m-thick transition zone. Regional and local studies indicate that the host sediments were deposited in a peritidal setting associated with a sabkha environment.Paragenetic studies reveal a consistent order of precipitation for gangue and ore-forming minerals. These minerals can be related to the diagenetic evolution of the host sediments. Their presence at the Coates Lake deposit may be divided chronologically into four syngenetic to early diagenetic stages: syndiagenesis--precipitation of gypsum, anhydrite after gypsum, dolomitization; diagenesis 1--deposition of calcite with partial replacement of anhydrite, preore pyrite, authigenic silica with partial replacement of anhydrite and calcite, and authigenic feldspar; diagenesis 2--precipitation of copper-bearing sulfides in the presence of reduced sulfur, sulfate-reducing bacteria, and organic matter, and partial replacement of preore pyrite, anhydrite, calcite and quartz; and diagenesis 3--neomorphism of calcite. Mineralization, identified with the second stage of diagenesis, would have preceded the marked reduction in permeability associated with the formation of neomorphic calcite.

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