REVISED UNDERSTANDING OF CARBONATE-HOSTED ZN-PB MINERALISATION IN THE BORDEN BASIN (NANISIVIK DISTRICT), NUNAVUT, CANADA

The Borden Basin Zn district in the Canadian Arctic islands contains the Nanisivik deposit (mined 1976-2002; 19 Mt 10% Zn+Pb) and associated regional carbonate-hosted showings, all hosted by Mesoproterozoic dolostones. Mineralisation in this district was previously understood to be hosted by peritidal dolostone that accumulated in a passive-margin succession, and constraints on the loci of metal accumulation were poorly known. New stratigraphic and structural data show, however, that base-metal bodies are predominantly hosted by an unusual deep-water, non-passive-margin dolostone, and are spatially controlled by the paleotopography of a buried unconformity surface, together with deep-seated, repeatedly reactivated faults. The depositional age of the stratigraphic succession is now known to be approximately 1.1 Ga, which is >100 m.y. younger than previously thought. New fluid inclusion data for the Nanisivik deposit, previously understood to be unusual for the high temperature of its depositional fluid (~200-250°), show that the ore formed instead from comparatively low-temperature fluids (predominantly <100°, with inclusions reset in the vicinity of a cross-cutting ca. 723 Ma mafic dyke). Previous attempts to date the mineralizing event using indirect methods yielded Mesoproterozoic to Ordovician ages; results of Re-Os dating of pyrite from the Nanisivik ore-body provide for the first time a direct date: approximately 1.1 Ga. Collectively, the stratigraphic, structural, fluid inclusion and geochronology results show that mineralisation was roughly coeval with sedimentation, exploited stratigraphic traps and structural features, and formed from fluids that were ‘normal’ in the context of MVT deposits. It is probable that the basin’s complex evolution and the nearly contemporaneous movement of mineralising fluids were both driven by tectonic events associated with the assembly of Rodinia.