A STABLE ISOTOPE AND RE-OS STUDY OF ORGANIC-RICH MUDSTONES ACROSS A PROTEROZOIC GLACIAL CYCLE IN BRAZIL

The Poço Verde Formation of the Vazante Group of southeastern Brazil is interpreted to have been deposited during a Neoproterozoic low-latitude glacial event. Evidence of a glaciogenic origin includes dropstone-laden diamictites, associated iron-formations and characteristic negative d¹³C_carb values in overlying carbonates. Furthermore, the Vazante is deposited atop the Sao Francisco craton which was conjugate to the Congo craton during the Neoproterozoic and where the Neoproterozoic “Snowball Earth” hypothesis was resurrected. Within the Vazante Group exist several discrete layers of well-preserved organic-rich mudstones. Here, we present elemental, stable isotope, and rhenium-osmium isotopic data from the pre-glacial Garrote Formation, the synglacial Poço Verde Formation, and the post-glacial Lapa Formation. These mudstones have up to 4 wt % organic carbon, with d¹³C_TOC ranging from -20 to -30‰. Sulfur isotopes from sulfides range widely (total range of 55‰) through the succession, but show an inverse relationship with organic carbon d¹³C within the glaciogenic interval. Rhenium and osmium concentrations within the mudstones range from ~0.2 to 16 ppb and ~0.04 to 1 ppb, respectively. The Re-Os data suggest this series of formations are temporally resolvable from each other and significantly older (i.e., one billion years) than previously considered. Successive isochron ages for the Garrote and Poço Verde formations are ca. 2153±130 Ma and 1831± 83 Ma, respectively. Insofar as the succession has been extensively mineralized in some areas and houses world-class Pb-Zn deposits, we consider whether metals extracted from the basement could have altered the Re/Os data. This is unlikely as Re and Os concentrations are low, and isotopic data spread along a wide range of values and produce single cohesive isochrons. If correct, the diamictites and mudstones of the Poço Verde Formation may not be associated with a global Neoproterozoic glaciation, but possibly reflect an older, regional glacial event that is not recognized elsewhere in late Paleoproterozoic time.