Paleoarchean record of the detrital pyrite-bearing, Jacobina Au–U deposits, Bahia, Brazil

Abstract

U–Pb and Lu–Hf isotope analyses were performed on detrital zircon grains from eight samples distributed along a 1000-m-thick section of the Au–U pyrite deposits of Jacobina to identify the primary source areas and define the maximum age for the basin sedimentation. These deposits contain detrital pyrite similar to that observed in the Witwatersrand gold province. Although the sequence was deformed by Paleoproterozoic orogeny, all of the detrital zircon grains (n=785) yielded Paleoarchean ages between 3.2 and 3.5Ga with primary peaks of approximately 3.3–3.4Ga. Most of the grains have nearly chondritic compositions with ɛHf(t) values between −5.6 and −0.1 and Hf model ages between 3.5 and 3.8Ga. The presence of detrital Au–U pyrite in the SiO2-rich metasedimentary rocks provide unequivocal evidence that the basin formed in an O2-poor atmosphere. The U–Pb data set and lithogeochemistry suggest that its infill material was derived from an exclusively Paleoarchean source, specifically sediments derived from the TTG-greenstone assembly of the Gavião Block (3.2–3.5Ga). These source rocks were strongly hydrothermally altered and weathered before deposition. The geochemical composition of the sedimentary rocks and the isotopic signature of Hf indicate that the sediments were derived from a juvenile source with minor crustal contamination and a short duration of crustal residence (between 150 and 350myr).