Lead- and Sulfur-Isotope Investigations of the Boquira Sediment-Hosted Sulfide Deposit, Brazil

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.