Ancient sub-seafloor alteration of basaltic andesites of the Ongeluk Formation, South Africa: implications for the chemistry of Paleoproterozoic seawater

Basaltic andesites of the Ongeluk Formation, Transvaal Supergroup in Griqualand West constitute part of a large flood volcanic province that extruded at ca. 2.22 Ga onto the partly submerged Kaapvaal Craton. Pods and short veins of megaquartz, with albite, epidote and traces of calcite and Cu–Ni–Co sulphides are closely associated with beds and pods of jasper and chert that are products of low-temperature seafloor alteration. Characterization of the megaquartz pods and veins yields evidence for an origin in a “passive” seafloor alteration regime. Fluid inclusion studies suggest the involvement of two aqueous fluid end members, one NaCl-dominated with salinity similar to modern seawater, the other Ca-dominated and with distinctly elevated salinity. The chemical composition of both fluids is akin to that of seawater, modified in its composition by interaction with the volcanic host rock. The low salinity fluid appears only little affected by fluid–rock interaction processes; the composition of the more saline Ca-rich fluid is more distinctly modified. The chemical composition of the two fluids has important implications for our understanding of the composition of ocean water during the Paleoproterozoic Era. Cl/Br ratios, widely regarded as being conservative in hydrothermal solutions, are significantly below those of present-day seawater, but remarkably similar to that predicted for Archean seawater. This observation suggests that Paleoproterozoic seawater was still buffered by vent fluids, and lacked sufficient organic matter to fractionate Cl from Br.