Abstract

Field mapping of carbonate-hosted base metal showings in the Milne Inlet graben of the Mesoproterozoic Borden basin, Nunavut, has identified the main geologic settings for mineralization in the district that hosts the past-producing Nanisivik deposit. All known showings are associated with faults, fractures, or dikes; these include major, graben-bounding structures with significant displacement, extensive synsedimentary and reactivated intragraben structures, and comparatively minor structures with negligible displacement. These brittle structural features, together with stratigraphic factors and primary lithofacies, control the distribution of base metal showings, and define four main settings for sulfide concentrations. The most volumetrically impressive mineralization (1) for example, Nanisivik orebody, is spatially constrained by erosional highs on an unconformity surface that separates host dolostone and overlying shale; (2) and (3) widespread but volumetrically limited fault- and fracture-controlled showings are spatially associated with intragraben fracture and dike systems—some of these showings (2) are stratigraphically limited to dolostone immediately above shale, whereas others (3) appear to be stratigraphically random and show a close spatial association with structural features; and (4) lithofacies-controlled mineralization that displays replacement textures is present in the immediate vicinity of graben-bounding faults. The most distinctive features of the district are the predominance of structural controls on the spatial distribution of sulfides, the nonplatformal, nonpassive-margin origin of the main dolostone host, the fundamental spatial control imposed by unconformity shape at the most important showings, and association of some highly prospective showings with long-lived crustal-scale faults.

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