The H.Y.C. lead-zinc mineralization at McArthur River has long been regarded as a classic example of the sediment-hosted massive sulfide deposit category. Its pristine and fine-grained nature, however, has made it difficult to study as well as mine. A key issue concerning the genesis of this deposit has been to define the role of the host sediment pyrites (Py 1 and its overgrowths of Py 2 ) in relation to the localization of the base metal mineralization. Utilization of the SHRIMP ion microprobe for in situ analyses of the sulfur isotope compositions of Py 1 Py 2 , and base metal sulfides in the orebodies has shown that growth of Py 1 and Py 2 , with delta 34 S values ranging from -13 to +15 and -5 to +45 per mil, respectively, likely involved H 2 S formed by biogenic processes. On average, the delta 34 S values of Py 2 are about 15 per mil higher than those of Py 1 , suggesting that the pyrite types may often have formed sequentially from single batches of sulfate in an environment which became closed with respect to sulfate supply through time. In detail, the isotopic systematics of the pyrites can be quite complex with as much as 50 per mil or so variation in a distance of millimeters so that multiple events may be required to account for their formation. Such an origin for pyrite is entirely consistent with a geologic setting offering a restricted sulfate supply such as a semiemergent lacustrine or coastal lagoonal environment as described by Williams and Logan (1986).The sulfur isotope variability exhibited by the main base metal sulfides (sphalerite, galena, and chalcopyrite) is found to be more restricted than of either pyrite type, ranging from -5 to +14 per mil and excluding the few late-stage veins, the range is limited to -5 to +8 per mil. The delta 34 S values of the main-stage base metal sulfides may be lower, the same as, or higher than nearby Py 1 though they are generally lower than adjacent Py 2 . These data would suggest that formation of the base metal sulfides was distinct from that of Py 1 or Py 2 precipitation and that localization of the base metal sulfides did not involve biogenic sulfide either directly through use of residual microbially generated H 2 S( (sub (aq)) )or by replacement of Py 1 or Py 2 . The lack of isotopic communication between pyrite and base metal sulfides combined with the fundamental textural constraints of overgrowth and crosscutting relationships, preclude the onset of base metal mineralization prior to the cessation of pyrite growth. However, soft-sediment deformation features imply that in many cases mineralization occurred before the sediment was completely lithified.Linkage of the isotopic and textural characteristics with the geologic setting (Williams, 1979; Williams and Logan, 1986) produces a model where the base metal sulfide mineralization formed epigenetically (or diagenetically, i.e., Gustafson and Williams, 1981) from permeability-controlled migration of basinal brines along bedding. The presence of marcasite in the main-stage mineralization suggests that the brines were relatively low in pH and temperature. Occurrence of euhedral ankerite and prismatic quartz with the base metal sulfides implies the dolomitic sediment underwent extensive recrystallization during hydrothermal mineralization which included addition of Fe, CO 2 , and Si as well as the more obvious Zn, Pb, Cu, and possibly H 2 S. It is possible that early fluids were dominated by Fe, CO 2 , and SO 4 whereas later, warmer fluids carried more Zn, Pb, Cu, CO 2 , as well as H 2 S. In light of these new data and interpretations, all earlier models for the formation of the H.Y.C. deposit which involve syngenetic accumulation of fallout from hydrothermal brine pools and/or those which incorporate biogenically produced reduced sulfur in base metal mineralization must be rejected.