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Abstract

Detailed geological mapping in the Barberton Mountains of north-eastern South Africa has identified units once deposited on a river flood plain. Investigations using an optical microscope have found structures that strongly suggested they developed within a soil profile, and these units had been previously dated to an age of 3.22 billion years. Within these soil horizons we found pyrite grains (FeS2) which contained rounded cores, and these most likely resulted from abrasion during river transport. These cores are commonly overgrown by pyrite rims that have well expressed morphologies. Furthermore, trace element data indicate these rims crystallized on the grain cores at or near the time of soil formation. Using a Secondary Ion Mass Spectrometer, a highly advanced analytical technology, we found that the sulphur isotopic composition of the pyrite cores is consistent with a magmatic origin. In contrast, the rim forming pyrite has a 34S/32S isotope ratio which is some 20 ‰ lower, indicative of a biological processing of sulphur. Our results demonstrate that biological processes were already taking place outside of the marine environment at 3.22 billion years ago, which is 400 million years earlier than previously documented.