EVALUATION OF ALTERATION ZONES IN ARCHEAN IGNEOUS ROCKS AS POSSIBLE SITES FOR CARBONACEOUS FOSSIL MICROBES

Sedimentary rocks have traditionally been the focus of the search for Archean microfossils; the Earth's oldest fossil bacteria are associated with carbonaceous matter in sedimentary cherts in greenstone belts in Australia and southern Africa. Reports of possible fossils in an igneous Martian meteorite and of modern bacteria associated with basalts have stimulated a new look at Archean igneous rocks as possible sites for fossil microbes. Many of the volcanic flow rocks of the early Archean greenstone belt of the Barberton Mountain Land, South Africa, preserve evidence of low-temperature, near-surface alteration that occurred in the intervals between eruptions. The alteration zones consist of silicified volcanic rock cut by veins of silica and carbonate, which appear to replace an earlier evaporite mineral. Samples of silicified volcanic rocks and accompanying lithologies from the Barberton greenstone belt were examined to evaluate the potential for carbonaceous fossil preservation. Preliminary results indicate that carbonaceous matter is sparse, mainly present as widely spaced clumps in silica veins that appear to be fed by overlying carbonaceous chert layers. No fossils or cohesive biofilm-like material have been located in the alteration zones. Overlying some of the altered volcanic flow rocks are laminated, slightly domed structures that have been interpreted by some investigators as stromatolites and by others as abiological silica precipitates. Previous studies suggest that the abundant tourmaline in the laminated structures contains boron that was enriched in an evaporitic environment and subsequently remobilized by hydrothermal fluids. Although the structures contain cloudy material that is probably carbonaceous, they lack well-defined, cohesive microbial mat-like laminations that characterize many carbonaceous cherts from the greenstone belt, including those that contain filamentous fossil bacteria.