Hematite formation by oxygenated groundwater more than 2.76 billion years ago

Geoscientific drilling in the Marble Bar area of the Pilbara Craton, Western Australia, resulted in the discovery of locally abundant hematite in Archean basalts ~ 200 m below the present land surface. The hematized basalts occurring along a bedding-parallel shear zone are cross-cut by pyrite veinlets (< 3 mm in width) and contain euhedral pyrite grains (10–500 µm in diameter) with sharp crystal edges, indicating that the hematite formed before the pyrite. We have dated the pyrite in the veinlets at 2.763 ± 0.016 Ga using the Re–Os method. Therefore, the hematite formed prior to 2.763 Ga.The basalts containing the hematite belong to the Apex Basalt of the Warrawoona Group, and were erupted onto the Archean seafloor at 3.46 Ga. Due to 2.9 Ga orogenic deformation and subsequent deep erosion, the Apex Basalt was exposed at the surface of a continental landmass prior to 2.77 Ga. Sometime in the period between ~ 2.9 Ga and 2.77 Ga, the basalt section we describe was less than 200 m below the Late Archean land surface, and within range of groundwater percolation through the shear zone in the basalts. Geological, mineralogical and geochemical lines of evidence strongly suggest that the infiltration of O2-rich groundwater through the bedding-parallel shear in the basalts formed hematite prior to 2.76 Ga, and hence oxygenated surface environments, at least localized and/or short-lived, emerged more than 300 million years before the widely accepted Great Oxidation Event during 2.45 and 2.32 Ga.