Goldschmidt Conference Abstracts 2005

Abstract

The c. 3.49 Ga Dresser Formation, a lower member of the Warrawoona Group in the North Pole Dome region, Western Australia, is a barite/black chert dominated, mafic volcanic hosted, shallow water (l100m), seafloor exhalative hydrothermal deposit. Carbonaceous matter (CM) aggregates, varying in size from 1mm to g20mm, have been isolated from black cherts in drill core from the Dresser Mine area. With total organic carbon (TOC) contents from 0.19% to 0.31%, organic petrology and SEM-EDS studies of whole rock samples reveals the CM as aggregated forms around siliceous nuclei in the majority of samples. Transmission electron microscopy (TEM) of carbonaceous concentrates show this aggregate material to be composed of sub-micron sized granular-textured particulates. The observed morphology is suggestive of the remains of microbial clusters or colonies. TEM investigation of one sample, however, showed it to be dominated by soot particles such as have been reported from the Sudbury impact and K-T boundary. The presence of soot particles supports previous evidence of meteoritic impact in the Pilbara during the early Archaean. Reflectance measurements (%Ro) of the CM have yielded three dominant groupings: 1) 0.5-1.5; 2) 2.4-2.8; and 3) 3.2-3.8. Different Ro populations in a single sample record several hydrothermal events, as well as relative proximity to a venting source. These results, combined with the bulk C-isotope values of the CM (d13C -32.1 to -38.2p PDB, n=11), are suggestive of a biogenic source, and as such, may represent remnants of some of the earliest primitive life forms on Earth.