Terrestrial and extraterrestrial chemical components of early Archean impact spherule layers from Fairview Gold Mine, northern Barberton greenstone belt, South Africa

Abstract

Early Archean spherule layers, widely accepted to represent distal ejecta deposits from large-scale impact events onto the early Earth, have been described from several stratigraphic levels of the Barberton greenstone belt in South Africa. Recently, exploration drilling at the Fairview Gold Mine (25°43′53″S, 31°5′59″E) in the northern domain of the belt resulted in the discovery of a new set of spherule layer intersections. The Fairview spherule layers in drill cores BH5901, BH5907, BH5911, and BH5949 were intersected just a few meters apart, at about the same stratigraphic position within the transition from the Onverwacht Group to the Fig Tree Group. The Fairview spherule layers have petrographic and chemical similarities to at least three other well-known Barberton spherule layers (S2–S4), and multiple spherule layer bed intersections in drill cores BARB5 and CT3, all from about the same stratigraphic position. They are not uniform in composition, in particular with respect to abundances of highly siderophile elements. The highest concentrations of moderately (Cr, Co, Ni) and highly siderophile (Ir) elements are within the range of concentrations for chondrites and, thus, reinforce the impact hypothesis for the generation of the Fairview spherule layers. Iridium peak concentrations and Cr/Ir interelement ratios for spherule layer samples from drill cores BH5907, BH5911, and BH5949 suggest admixtures of 50%–60% chondritic material, whereas for the BH5901 spherule layer, only an admixture of 1% chondritic material is indicated. We discuss whether these four Fairview spherule layers represent the same impact event, and whether they can be correlated to any of the S2–S4, CT3, and BARB5 intersections.