The Sterling Hill and Franklin zinc-iron-manganese deposits, Sussex County, New Jersey, are sedimentary-exhalative in origin but were highly deformed and metamorphosed to granulite grade during the Mesoproterozoic Ottawan orogeny. They are unique because the major zinc minerals, zincite, franklinite, and willemite, are oxides and silicates rather than sulfides. However, at Sterling Hill minor granular sphalerite occurs in the central black willemite zone associated with loelling and graphite and sparsely elsewhere in the deposit. Based on textural observations and stable isotope geochemistry, sphalerite was present during metamorphism and was most likely part of the protolith mineral assemblage. It likely formed from a fluid containing seawater sulfate that circulated through volcanic rocks and reacted with igneous sulfides; some contribution from biological reduction of sulfate seems likely. This interpretation agrees with the generally accepted model that the protolith formed on the floor of an ancient rift in an environment similar to the present-day Red Sea brine pool sediments. The presence of loellingite and absence of pyrite or arsenopyrite in the black willemite zone requires that fAs2−/fS2− >1 during metamorphism. The position of the black willemite zone, occupying most of the central cross member of the deposit, and the more reduced mineral assemblages of the zone relative to the rest of the deposit are compatible with it being the feeder zone for the stratiform ores that make up the rest of the Sterling Hill deposit, indicating that the deposit is inverted relative to its original position. Pressure-corrected temperatures from fluid inclusion geothermometry and temperatures determined from carbon isotope fractionation factors between coexisting graphite and calcite suggest a postmetamorphic event that occurred at ∼360°–280°C at between ∼820 and 750 Ma. The fluid inclusion and mineral assemblage data also suggest that at least one additional postmetamorphic event affected the sphalerite and loellingite and their coexisting minerals. This event involved the development of secondary fluid inclusions in sphalerite and the formation of a secondary manganese arsenosilicate mineral and may correlate with the pervasive serpentinization-like alteration of the deposit. The complex sequence of vents indicated by the textures, isotopic ratios, and fluid inclusions is compatible with the history of prolonged cooling and uplift experienced by the New Jersey Highlands in the Neoproterozoic.