Abstract:

The Borden Basin Project is a multiyear endeavour that encompasses a suite of investigations aimed at understanding the geological evolution and economic potential of Mesoproterozoic basins of eastern Nunavut. Part of the Borden Basin is a proven but poorly understood Zn district that includes the Nanisivik mine (in operation from 1976 to 2002). Since the end of the 2011 field season, field and analytical advances have resulted in publication of results from five thematic studies focused on the basin’s economic potential:<br/>1) A field-based study mapped and characterized the many known carbonate-hosted Zn-Pb±Cu showings in the 250 by 100 km Milne Inlet graben (main zone of known mineralization), identified the structural and stratigraphic controls on the spatial distribution of three showing types, and highlighted two geographic zones of elevated economic potential.<br/>2) Afield-based and analytical study of a thick (>100 m) and unexplored black shale unit showed that geochemical and geological conditions in the basin during shale accumulation would have been appropriate for the deposition of sedimentary exhalative (SEDEX)-type sulphides, if local vent sources of dissolved metals had been present (currently unknown). This work also provided a first direct date of 1092±59 Ma (U-Th-Pb whole-rock geochemistry on black shale) for the deposition of part of the Bylot Supergroup, which is considerably younger than the previous, assumed depositional age, and coincides with the Grenvillian orogeny and assembly of Rodinia.<br/>3) A fluid inclusion study of the Nanisivik orebody showed that ore-forming fluids were Na-rich and comparatively low temperature (<100 °C), in striking contrast to results of earlier studies (200–250 °C). The fluid inclusion homogenization temperatures increase toward the ‘mine dike’, a Franklin-aged (ca. 720 Ma) intrusion that crosscut the orebody and locally modified the fluid inclusions (i.e., to higher homogenization-temperatures).<br/>4) Rhenium-osmium dating of pyrite from the Nanisivik orebody and the Hawker Creek group of showings provided for the first time a direct date of ca. 1100 Ma for the mineralizing event. This date refutes recent work that indicated a Phanerozoic age for the mineralizing event, and refocuses the economic potential of the event onto Proterozoic host rocks only. The results also suggest that fluid movement was roughly coeval with sediment deposition, and may have been driven by tectonic events associated with the amalgamation of Rodinia.<br/>5) Study of the siliciclastic basal strata of the Mesoproterozoic succession showed that early phases of the basin’s depositional history were not aulacogenic, as previously assumed, and that the potential for unconformity-type U deposits in this basin is low. <br/>Fieldwork in 2012 focused on understanding vent-related deep-water carbonate mounds that were deposited during the accumulation of a black shale succession, and on outlining the tectonic evolution of the basin using sediment provenance.