Structural and stratigraphic controls on carbonate-hosted base metal mineralisation in the Mesoproterozoic Borden Basin (Nanisivik district), Nunavut.

Field mapping was carried out of the carbonate-hosted base metal showings in the Milne Inlet graben of the Mesoproterozoic Borden basin to identify the main geological settings for mineralisation in the district. The district hosts the Nanisivik Pb-Zn deposit which was mined between 1976 and 2002. All known showings are associated with faults, fractures or dykes, including major graben-bounding structures with significant displacement, extensive synsedimentary and reactivated intragraben structures and comparatively minor structures with negligible displacement. The brittle structural features, together with stratigraphic factors and primary lithofacies, control the distribution of base metal showings and define four main settings for sulphide concentrations. The first is spatially constrained by erosional highs on an unconformity surface which separates host dolostone and overlying shale, and includes the most volumetrically important mineralisation such as at Nanisivik. The second and third settings are widespread but volumetrically limited fault- and fracture-controlled showings spatially associated with intragraben fracture and dyke systems. Showings in the second setting are stratigraphically limited to dolostone immediately above shale, while those in the third appear to be stratigraphically random and show a close spatial association with structural features. The fourth setting involves lithofacies-controlled mineralisation which displays replacement textures and is present in the immediate vicinity of graben-bounding faults.
Field mapping was carried out of the carbonate-hosted base metal showings in the Milne Inlet graben of the Mesoproterozoic Borden basin to identify the main geological settings for mineralisation in the district. The district hosts the Nanisivik Pb-Zn deposit which was mined between 1976 and 2002. All known showings are associated with faults, fractures or dykes, including major graben-bounding structures with significant displacement, extensive synsedimentary and reactivated intragraben structures and comparatively minor structures with negligible displacement. The brittle structural features, together with stratigraphic factors and primary lithofacies, control the distribution of base metal showings and define four main settings for sulphide concentrations. The first is spatially constrained by erosional highs on an unconformity surface which separates host dolostone and overlying shale, and includes the most volumetrically important mineralisation such as at Nanisivik. The second and third settings are widespread but volumetrically limited fault- and fracture-controlled showings spatially associated with intragraben fracture and dyke systems. Showings in the second setting are stratigraphically limited to dolostone immediately above shale, while those in the third appear to be stratigraphically random and show a close spatial association with structural features. The fourth setting involves lithofacies-controlled mineralisation which displays replacement textures and is present in the immediate vicinity of graben-bounding faults.