INTERNAL ZONATION IN A CARBONATE-HOSTED ZN-PB-AG DEPOSIT - NANISIVIK, BAFFIN ISLAND, CANADA

Zn-Pb-Ag ores at Nanisivik, northwest Baffin Island, are hosted by upper Proterozoic laminated dolostone of the Society Cliffs Formation. Sulfide orebodies containing pyrite, sphalerite, galena, sparry dolomite, and pyrite pseudomorphous after marcasite are characterized by well-banded ore textures considered to have resulted from the progressive replacement of carbonate wall rock. Textural and mineralogical variations enable the Main orebody to be divided into three vertical and four horizontal ore zones. The upper lens of the Main orebody may be further subdivided into six texturally distinct, laterally extensive mine units. In the eastern upper lens, where textural variations are best developed, fluid inclusions indicate that most ore mineralization took place over the temperature range of 165-degrees to 210-degrees-C and involved a brine containing 20 to 38 wt percent NaCl equiv. The sulfur isotope compositions of main- and late-stage pyrite crystals range from delta-S-34 = 27.4 to 28.0 per mil. Iron contents of sphalerite vary from 14 to 0 mole percent FeS, corresponding to well-developed color zoning, and constrain the oxygen activity of the ore fluid to 10(-46) to 10(-41) at 200-degrees-C during sphalerite precipitation. Interbanded primary marcasite and sparry dolomite gangue indicates that the ore fluid fluctuated around pH = 5. Ore formation is modeled based on processes involving in situ reduction of sulfate by hydrocarbons and minor H2S encountered at the site of ore deposition. The location and configuration of the orebodies was probably controlled in part by the former position of a horizontal fluid interface. Banding is likely the result of repetitive sulfate reduction, metal precipitation, wall-rock dissolution, and dolomite recrystallization in response to the pulsatory influx of ore fluid. Textural and mineralogic variations probably resulted from slight variations in the availability of reduced sulfur.