Study of zonation at the Nanisivik Zn-Pb-Ag mine, Baffin Island, Canada

Abstract

Zn-Pb-Ag mineralization at Nanisivik, northwest Baffin Island, is hosted by Proterozoic, laminated dolostone of the Society Cliffs Formation. Mineralization of the Main Orebody is highly variable in terms of texture and mineralogy exhibiting both replacement and open space filling textures. Sulfides are generally coarsely crystalline and banding, consisting of interlayered pyrite, sphalerite, galena and sparry dolomite, is common along the margins of the Upper Lens of the Main Orebody. The eastern and central portions of the Upper Lens are characterized by laterally extensive mine units, which are distinguished on the basis of texture and mineralogy. Contacts between units are generally sharp. The physical and chemical parameters responsible for the textural and mineralogical variations have been evaluated through a study of fluid inclusions, sulfur isotopes and ore mineralogy. Fluid inclusion homogenization temperatures from simple, two-phase primary and pseudo-secondary inclusions in sphalerite and sparry dolomite gangue indicate initial temperatures of ore formation from 150—210 degrees C in the eastern Upper Lens when the estimated pressure of ore formation is taken into consideration- The temperature of ore formation decreased to 100-150 degrees C in the western portion of the Upper Lens. Freezing studies indicate that the ore-forming fluid was a brine containing 20-37 equivalent weight percent CaCl[subscript] 2, The sulfur isotopic compositions of late and main stage pyrite crystals range from [symbol]S = +27.4%. to + 28.0%., suggesting relatively constant temperature, fluid source and dominant sulfur species in the ore fluid during ore formation, providing there has been no subsequent re-equilibriation of sulfur isotopes. The iron content of sphalerite varies from 14 mole % to 0 mole % from crystal centers to rims respectively, corresponding to well developed colour zonation. Sphalerite iron contents constrain the oxygen activity of the ore fluid from 10[superscript -46] to 10[superscript -41] at 200°C during sphalerite precipitation. The best developed zoning and, thus, the highest oxygen activities occur within sphalerite adjacent to carbonate wall rock. Under high oxygen activities, conditions were favorable for the generation of sulfanes considered necessary for precipitation of marcasite. X-ray diffraction studies indicate that primary marcasite has inverted completely to pyrite. The stability of the simplest sulfane, H[subscript 2] S[subscript 2] constrains the maximum allowable pH of the ore fluid at the time of marcasite precipitation to 5.0. The presence of interbanded marcasite pseudomorphs and sparry dolomite indicate that the ore fluid fluctuated around pH = 5.0. Comparison of solid organics extracted from the Society Cliffs dolostone to bitumen associated with mineralization suggests that organics within the host formation have played a role in sulfate reduction. The model of ore formation therefore proposed involves the in situ reduction of a hot, saline, metal-bearing ore fluid by hydrocarbons liberated by the replacement and dissolution of wall rock. Sulfate reduction was probably concentrated at the wall rock orebody interface along a replacement front that migrated away from the orebody. Banding was likely the result of repetitive sulfate reduction, metal precipitation and wall rock dissolution in response to the pulsatory influx of ore fluid. Gross textural and mineraiogical variations are probably a result of slight variations in the oxidation state of the ore fluid, the availability of H[subscript 2] S and, to a lesser extent, temperature.