Copper isotopic evidence of microbial gold fixation in the Mesoarchean Witwatersrand Basin

Abstract

A comparison of the copper isotope composition of sedimentary rock strata spanning in age across the Great Oxidation Event (GOE) and that of pyrite, carbon, and native gold from the Mesoarchean Witwatersrand Basin reveal that an oxidative gradient triggered gold precipitation on biologic material, now present in the form of gold-rich carbon seams in the latter. Oxidative redox reactions are known to favor the heavier ⁶⁵Cu isotope. A sequence of sedimentary rock strata on the Kaapvaal Craton records higher Cu isotope values before the start of the oxidation of the Proterozoic atmosphere in comparison to that of younger transitional-GOE strata. A similar increase in Cu isotope ratios was found in gold within carbon seams in comparison to the Cu isotope ratios in rounded pyrite in auriferous metaconglomerates of the ca. 2.9 Ga basal Central Rand Group. However, the Cu isotope ratios in the post-GOE sedimentary rocks of the Kaapvaal Craton and the pre-GOE native gold in the ca. 2.9 Ga carbon seams record different causes of oxidation. The gold in the carbon seams was not uniformly deposited across the lower Central Rand Basin but in isolated depositional environments. Changes in large-scale atmospheric oxidation, evident by the increase in δ⁶⁵Cu in the pre and transitional-GOE sedimentary strata in the Kaapvaal Craton, cannot be tied to the elevated Cu isotope values in the carbon seams. Instead, the latter, which are higher than all other measured values (δ⁶⁵Cu = +2.41 ± 0.24 ‰), mimics Cu isotope fractionation associated with acidophilic bacterial activity. Therefore, the significant difference in Cu isotope values of the carbon seams compared to those of detrital pyrite in the Witwatersrand metaconglomerates could be interpreted as a fingerprint of a microbial redox gradient that might have induced the precipitation of gold, thus having formed the richest known gold province in the world.