Between 1999 and 2004, we undertook detailed field and petrographic studies to critically assess the biogenicity of microfossil-like structures in c. 3460 Ma chert of the Apex Basalt near Marble Bar. For almost 20 years the scientific community had generally accepted that these carbonaceous structures were Earth’s oldest known microfossils. However, the main conclusions from our study, first published about a decade ago, were that the structures interpreted to be microfossils failed to meet various key criteria required for the acceptance of a biological origin. We now present additional information on the geological origin of the host rocks, plus petrographic descriptions of the thin sections that were originally used to argue for the presence of microfossils.

Important field observations include: a) evidence that the previously reported carbonaceous microfossil-like structures are hosted not by bedded sedimentary chert (informally referred to as the ‘Apex chert’), but by a hydrothermal silica vein system 50− 100 m beneath the paleosurface; b) that hydrothermal silica veins are locally common beneath the ‘Apex chert’, and occupy upward-thickening and upward-bifurcating fissures; c) that these hydrothermal silica veins underplate and intrude the ‘Apex chert’, but do not cut through it; and d) that the silica veins are concentrated along two growth faults that divide the lower part of the Apex Basalt into three structural blocks.