The Nanisivik deposits are Mississippi Valley-type deposits formed through both carbonate replacement and filling of karstlike solutional channelways in the Proterozoic Society Cliffs Formation. The main ore zone consists of banded ore with alternating cm-thick layers of pyrite, sphalerite, and less abundant galena, interlaminated with white sparry dolomite. Similarly laminated ore occurs as replacements that have left tapered fins and totally isolated floating remnants of host dolomite. Heavy delta 34 S values of sulfides (pyrite, sphalerite, and galena) ranging from 21.5 to 31.2 per mil suggest complete reduction of marine sulfate with no isotopic difference between sulfate and sulfides. Isotopic temperatures from intersulfide fractionations (pyrite, sphalerite and galena) range from 90 degrees to 270 degrees C. Plots of fractionation vs. delta 34 S show that the sulfur source was heavy throughout sulfide precipitation, also consistent with total in situ sulfate reduction. The reductant was organic matter, traces of which remain as pyrobitumen, although methane and volatile hydrocarbons may also have been involved. Striking evidence for the role of organic matter is found in the delta 13 C values of white sparry dolomite, which varies between +6 and -12 per mil. Oxidation of organic carbon during sulfate reduction produced CO 2 which dissolved host-rock dolostones (delta 13 C [asymp] 2ppm) and thus contributed light carbon to the dissolved carbonate pool, leading to precipitation of isotopically light dolomite. Great variation in delta 13 C values is also evident within short intervals of sulfide mineralization; delta 13 C variations > 80 percent of the total range for the deposit occur within single centimeter-thick white sparry dolomite layers, presumably due to short-term fluctuations in mixing ratios of carbon sources (organic, host rock). Heavy dolomite is most common where early hydrothermal solutions recrystallized organic-rich host dolomite, and where dissolved inorganic carbon may have equilibrated with light organic matter at relatively low temperatures, leading to high values of delta 13 C dolomite . Lack of evidence of isotopically light dolomite in other Mississippi Valley-type deposits may indicate non in situ sulfate reduction, i.e., introduction of sulfur in an H 2 S-rich brine.