ARCHAEAN HYDROTHERMAL SYSTEMS IN THE BARBERTON GREENSTONE BELT AND THEIR SIGNIFICANCE AS A HABITAT FOR EARLY LIFE

Hydrothermal systems have played an important role in shaping the 3.54 to 3.23 Ga volcano-sedimentary succession of the Barberton greenstone belt in South Africa and similar Eo- to Palaeoarchaean successions elsewhere. Evidence for relatively low-temperature (≤ 150 °C) seafloor hydrothermal activity is widely recorded in extensive silicification of volcanic and sedimentary rocks, leaching of elements commonly mobile during water-rock interaction, extensive hydraulic fracturing, and local barite precipitation. Evidence for the presence of high-temperature hydrothermal vents is scarce and restricted to massive sulphide deposits near the top of the succession.

The most conspicuous feature of hydrothermal activity, widespread silicification, appears characteristic of the pre-Mesoarchaean world and can provide important information on the geological processes acting on the early Earth. Silicification directly reflects on unique chemical conditions of the Archaean oceans, while extensive ocean floor hydrothermal activity may indicate high global heat flow or the presence of a long-lived crustal/mantle heat source. Hydrothermal activity resulted in the establishment of shallow subseafloor convection cells and a diffuse upflow of hydrothermal fluids over broad areas of the Archaean seafloor.

Many of the zones affected by seafloor alteration are spatially associated with traces of early life, such as putative microfossils and carbonaceous matter in cherts, and bioalteration features in pillow lavas and hyaloclastites. Diffuse venting of low-temperature hydrothermal fluids was a widespread phenomenon on the Palaeoarchaean seafloor, making it an ideal habitat for hyperthermophiles and the possible birthplace of life in earlier times. What remains to be answered more fully is to what extent hydrothermal systems and sub-seafloor environments were occupied by microbial life at that time.