Abstract

Organic petrology, electron microscopy, Transmission Electron Microscopy (TEM) and Electron Dispersive Spectrometry (EDS) of organic matter-rich sediments interlayered with silica-iron-rich units (Banded-Iron Formation – BIF) in 1.4 to 1.1Ga [1] Hamersley Group, display alternations of chemical and biological conditions in early Proterozoic seafloor hydrothermal environment. The carbonaceous shales contain organic matter with bacterial remains closely resembling hypo-thermophile methanogenic archaea found today in sea floor hydrothermal environments. These are associated with a unique mineral assemblage consisting of iron sulphide and elemental iron particles signifying strongly reducing conditions. Carbonaceous bundles of strings detached from Methanocaldococcus cell- connecting filaments abound resembling such as found in today’s hydrothermal environments associated with the Archaeon Methanocaldococcus. Relatively low organic maturity expressed by reflectance values (R₀) of 1.6-1.8% characterize these organic-rich sediments. On the other- hand organic-poor samples (less than 1% TOC) have high Ro values of OM as a result of oxidation. The fluctuations between reducing and oxidizing environments may be attributed to alternating episodes anoxic and oxygenating fluids in a turbulent hydrothermal environment inducing methane formation and methane oxidation.

Keywords:Carbonaceous shales; Iron sulphide; Electron; Silica; Hamersley basin