The South Bay Mine (Uchi) massive pyrite-sphalerite-chalcopyrite orebodies occur as a series of semiconformable lenses within a pile of Archcan acid volcanic breccia, tuff, and flow rocks. These all lie within and near the top of a domical mass of quartz-feldspar porphyry. An analysis of the major metals across selected ore intersections of five main ore lenses reflects the well-developed mineralogical banding and indicates two cycles of Cu-Zn zonation in the orebodies.Chemical analyses of rhyolite and quartz-feldspar porphyry wall rock show intense alteration in hanging-wall and footwall rocks and a well-developed alteration halo in the general mine area involving a marked variation in volatile, metal, and alkali metal content.The delta 34 S values for sphalerite and chalcopyrite from the orebodies (mean +0.5 per mil; range -0.6 to +4.2 per mil) are interpreted to indicate sulfur of igneous derivation with a possible contribution from Archean seawater. A greater total spread of approximately 16 per mil observed for the sulfur isotope composition of pyrite in both ore (mean +1.5; range -0.3 to +8.3) and wall rock (mean -2.3; range -7.3 to +0.7) implies that additional sources of sulfur may have been responsible for pyrite formation. Pyrite from either ore or wall rock showing dispersed delta 34 S values outside the delta 34 S range for ore sulfides is considered to have a genesis distinct from that of the base metal ore sulfides and represents volcanogenic or bacteriogenic sulfide deposited prior to the ore-forming episode. Dissolution and overgrowth of this early barren pyrite took place during ore deposition.Ore formation is attributed to submarine accumulation of massive sulfide lenses immediately overlying an extrusive phase of the quartz-feldspar porphyry. Orebodies are associated with discharge points of a convecting sulfur- and metal-bearing hydrothermal fluid. An estimate is made of the size of the convection cell responsible for ore formation.