Applied Geochemistry, Vol. 6, pp. 257-266, 1991 0883-2927/91 $3.00+ .00 Printed in Great Britain © 1991 Pergamon Press plc Stable isotopic composition of the hydrothermai fluids responsible for the Nanisivik Zn-Pb deposits, Northwest Territories, Canada FEREYDOUN GHAZBAN, HENRY P. SCHWARCZ Geology Department, McMaster University, Hamilton, Ontario, Canada L8S 4M1 and DEREK C. FORD Geography Department, McMaster University, Hamilton, Ontario, Canada L8S 4K1 (Received 22 January. 1990; accepted in revised form 3 December 1990) Abstract--Direct measurements of 5180 and 6D of water in inclusion fluids in sulfide and carbonate gangue minerals from the Nanisivik deposit are used to place constraints on the origin of the fluids responsible for hydrothermal mineralization. Values of ($D of inclusion fluids in sphalerite and dolomite are near -50%0, whereas those of inclusion fluids in two galena samples are much lower at -72 ___ 3%0; ($180 of inclusion fluids in sphalerite are -2.5 + 3%0;dolomite inclusions are 2%0lighter, perhaps due to post-trapping exchange with host dolomite. Dolomite-water fractionations calculated from the inclusion data are consistent with other estimates of temperature of deposition (70°C to 250°C). Previous isotopic studies by the same authors have shown that S in the ore was derived from coeval sea water SO 4. However, water in the hydrothermal fluid cannot have been trapped, unaltered sea water (either fresh or evaporated), nor coeval meteoric water (which would have resembled sea water), ultrafiltered sea water, or meteoric water. Values of ($180and ($D of inclusions can best be explained by a scenario which includes: 1. infiltration or entrapment of either sea water, or meteoric water that has redissolved evaporitic SO4; 2. exchange of the water with shales of the Arctic Bay Formation to lower ($D, with subsequent leaching of the metals; and 3. further H isotope exchange with fluid or solid organic matter prior to entrapment. INTRODUCTION THE NANISIVIK Zn-Pb sulfide deposits are hosted by Late Proterozoic dolostones of the Society Cliffs Formation on the northern exposed margin of the Churchill structural province in northern Baffin Island (Fig. 1). These deposits have many similarities with Mississippi Valley Type (MVT) deposits. The geology of the Nanisivik area is described in detail elsewhere (OLSON, 1977, 1984; CURTIS, 1983; GHAZ- BAN, 1988) and will be only briefly summarized here. The main sulfide ore body is in the form of a fairly straight, multi-kilometer-long, sub-horizontal flat- tened tube, gently cross-cutting bedding in the weakly deformed Society Cliffs Formation. This for- mation, as well as the shales of both the overlying Victor Bay Formation and the underlying Arctic Bay Formation, is cut by E-W trending block faults that appear to define the ore zone. Extensive host rock dissolution occurred during sulfide mineralization. The host rock dissolution and sulfide replacement took place along the bedding planes and fissures, and enhanced the open space volume providing conduits for ore fluids. The tubular channels of the ore deposit were formed mainly by the attacking acidic ore fluid. They were subsequently filled with massive, banded or "coontail" ore where bedding plane cavities were filled with sulfide consisting dominantly of pyrite, sphalerite, minor galena, and white sparry dolomite (WSD) gangue. Pyrite was precipitated prior to and throughout the main phase of ore deposition. The richest ore is in the form of alternating centimeter- thick layers of sulfide (sphalerite > pyrite > galena) and WSD. Vug fillings of quartz, calcite, dolomite, sphalerite, pyrite and galena were the last deposits to form. The vugs are also filled with ice under present conditions. The mineral paragenesis is shown on Fig. 2. Two previous isotopic studies have been made of this deposit (OLSON, 1984; GHAZBAN et al., 1990). Both studies showed that the S of the ore was derived from coeval sea water SO4. GHAZBAN et al. (1990) showed that SO 4 was probably reduced in situ in the ore deposit by reaction with organic C (methane or other gaseous hydrocarbons); this was especially well shown by small-scale, intense variation in 613C of the WSD layers in the banded ore. These authors specu- lated that while the source of the S in the ore was sea water, the metals must have been derived from adjacent shales, and transported to the site of depo- sition in a heated dilute solution consisting of a mixture of sea water and meteoric water. The stable H and O isotope composition of fluids trapped in the ores can help to elucidate the origin and evolution of the ore-forming fluids. There have not been many isotopic studies of inclusion fluids in carbonate-hosted, Zn-Pb deposits. Investigations by HALL and FRIEDMAN(1963) and RICHARDSON et al. 257