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The Irece Basin is a relatively small late Proterozoic basin within the Sao Francisco craton of northeast-central Brazil. Stratabound Zn-Pb-Ag sulfide and phosphate-rich units occur within a 50-m-thick tidal-flat sequence of dolomitic limestone and cherty dolostone within the lower Salitre Formation of the Una Group. Phosphate is concentrated in stromatolitic structures; metallic mineralization is represented by stratiform masses and disseminations of sulfide minerals in a tidal-flat sequence that has relict evaporitic structures. Zn-Pb-Ag sulfide concentrations in the Irece area are associated with parallel laminated, silty dolostone with shallow-water sedimentary structures, including laminar stromatolites, mud cracks, tepee structures, collapse breccias, length-slow quartz nodules, and pseudomorphs after evaporitic sulfates. Sulfide minerals include pyrite, sphalerite, galena, marcasite, jordanite, tetrahedrite, and covellite, in order of decreasing abundance. Nodular and bladed sulfide forms are pseud...

Lead- and sulfur-isotope investigations were performed at the Boquira sediment-hosted lead-zinc deposit in Bahia state, Brazil. This deposit, hosted by banded iron formation of metamorphosed oxide, carbonate, and silicate facies, was the most important lead mine in Brazil for nearly 40 years, until 1991, when mine operations ceased as a result of inadequate proven reserves. The lead-isotope data indicate an Archean-Paleoproterozoic age (2.5 to 2.7 Ga) for the deposit, and the lead is inferred to have been derived from crustal sources, probably from the basement rocks. The δ34S values for the layered sulfides are in the range from +8.3 to +12.8‰. The high positive values indicate that the sulfur was derived from the sediments by thermochemical reduction of Archean/Paleoproterozoic marine seawater; Boquira is a rare sediment-hosted galena-sphalerite deposit of this age. The study emphasizes the need to prospect marine sedimentary areas of Archean age for possible economic deposits of Pb and Zn.