Atypical Cu mineralisation in the Cornwallis carbonate-hosted Zn district: Storm copper deposit, Arctic Canada

Abstract

The metallogenetically important Cornwallis Zn district, in Canada’s Arctic islands, includes the past-producing Polaris mine and numerous base-metal showings, including Storm copper. Storm is unusual because it is near the southern limit of the district, is Cu- versus Zn-dominated, is overlain and underlain by red sandstone, and is hosted by Silurian strata (rather than Ordovician strata, as at Polaris). Mineralisation styles are primarily carbonate replacement and breccia, and consist of geerite, covellite, bornite, and chalcopyrite, all of which are associated with calcite and dolomite gangue. Multiple in situ micro-analytical techniques (fluid inclusion microthermometry, evaporate mound SEM-EDS, LA ICP-MS, SIMS), plus conventional Cu isotopic analysis were integrated to characterise the nature of the mineralising fluids and events. Fluid inclusions yielded low-temperature (Th <130 °C), moderate- to low-salinity (17.0 to 0.4 wt% NaCl equiv.) fluids. Evaporate mound SEM-EDS analysis, which quantified the solute chemistry, show a change from bimodal Na- and Na+K-dominated fluid mixture to a Na+K-dominated fluid. The δ18OH2O signature of the mineralising fluid involved reservoirs reflecting involvement of meteoric (<0%) and rock-equilibrated (8%) fluids, respectively. The δ34S values are nearly homogeneous for pre-ore and second generation sulphides at 0% and 10%, respectively, whereas the third generation (recrystallised) sulphides are more variable, with values from 8.8 to 23.4%. The majority of Cu sulphides have δ65Cu values near 0%, but second-generation chalcopyrite shows elevated δ65Cu values from +1.31% in the north to +3.44% in the south, suggesting a probable southward fluid flow. Positive Eu PAAS-normalised anomalies in early dolomite cements, which indicate basement involvement, are absent in succeeding cements, suggesting a change in fluid pathways and hence the mineralogical nature of the reservoir. Negative CeSN anomalies in main-stage calcite cement indicate an oxidised fluid, but the lack of similar Ce anomalies in the dolomite cements indicates reduced fluids. These data suggest that Storm copper mineralisation is related to basement-equilibrated fluids mobilised during the Ellesmerian orogeny (Devonian), which acquired heat and leached Cu from underlying Proterozoic red sandstone (Aston Formation) and were then focussed up faults where Cu sulphides precipitated. Prolonged meteoric fluid movement from the north, with intermittent episodes of basin-equilibrated fluid influx, produced hypogene and supergene mineralisation. Fluid movement resumed in the Cenozoic, when high-latitude fluids altered Cu minerals to atacamite. Storm copper mineralisation shares characteristics with both red-bed and Irish-type Cu deposit sub-types, but may also highlight a relationship with carbonate-replacement Zn-Pb deposits. Timing of mineralisation is comparable to that of Zn ore precipitation at Polaris, suggesting that Storm may be an integral part of the Cornwallis district, in spite of its unusual composition.