Silica alteration zones in the Barberton greenstone belt: A window into subseafloor processes 3.5–3.3 Ga ago

Abstract

Silicification of volcanic rocks, ranging from komatiitic to dacitic in composition, is a common phenomenon of the 3.5 to 3.2 Ga old volcano-sedimentary succession of the Barberton greenstone belt. Silica enrichment occurs in zones several tens of metres thick at the top of volcanic sequences that are capped by sedimentary chert horizons. SiO 2 contents increase upsection, from the original igneous value up to 92%. Silicification is associated with a depletion of most elements commonly mobile during water–rock interaction, while K 2O, Rb and Ba are enriched in these zones. Some transition metals (Sc, V, and Cr) were immobile, whereas Ni and Co and some other metals (Cu, Zn) were mobile and are depleted. The δ 18O values of volcanic rocks show a positive linear relationship with silica content and range from 9.0 to 17.3‰. The element depletion-enrichment patterns and oxygen isotope data indicate low-temperature (c. 100–150 °C) hydrothermal processes for the origin of the alteration zones. The loss of most elements coupled with the addition of SiO 2 as well as REE systematics indicate high water–rock ratios and a fluid REE composition typical of Archaean seawater that had a δ 18O value of ~ 0‰. Hydrothermal activity is attributed to heat derived from cooling of volcanic rocks and a high regional heat flow in an oceanic plateau-like setting and resulted in the establishment of shallow subseafloor convection cells. Diffuse venting of hydrothermal fluids over broad areas of the ancient ocean floor provided an ideal environment for both seafloor and subsurface biota 3.5 Ga ago.