Early Archaean sedimentary basins: depositional environment and hydrothermal systems. Geologica Ultraiectina (244)

The topic of this thesis is the Early Archaean environment and hydrothermal systems in the Barberton Greenstone Belt (BGB) in South Africa, and the Coppin Gap Greenstone Belt (CGGB) in the Pilbara, Australia. Focus within these greenstone belts is on the ~3.45-3.42 Ga Buck Ridge volcano-sedimentary complex (BR-vsc) and the ~3.45 Ga Kittys Gap volcano-sedimentary complex (KG-vsc) respectively. Apart from their similar age, these complexes also have a very similar geological setting. Both complexes consist of a felsic to bimodal volcanic sequence that is capped by silicified sediments. The complexes were deposited in an extensional regime. The extensional regime in the BR-vsc in the upper Hooggenoeg Formation, BGB is reflected by the presence of large, syndepositional normal faults. The BR-vsc is a bimodal volcanic complex, consisting mainly of massive and pillow basalts and felsic, quartz-plagioclase porphyritic rocks. The basalts and the felsic porphyritic rocks are capped by felsic volcaniclastic deposits and other, mostly pervasively silicified, sediments. The BR-vsc was intruded by broadly contemporaneous, shallow felsic porphyritic rocks, which obscure the traces of the normal faults. The CGGB is also cut by numerous normal faults. They occur at the scale of the entire belt and, at a smaller scale, as arrays in several levels within the Warrawoona Group, which makes up the largest part of this belt. Part of these normal fault arrays was syndepositional. Syndepositional extension of Warrawoona Group deposits occurred throughout the East Pilbara, and extension is therefore interpreted to have been the dominant tectonic regime at that time. In the CGGB, the extension had a consistent orientation for a minimum period of 20 Ma. In the study area, the Warrawoona Group consists of slices of stratigraphy, bounded by deformation (mostly shear) zones. Despite the presence of such zones, new U-Pb zircon dating and geochemical analyses show that the chronostratigraphical succession in the CGGB has not been fundamentally disturbed. Precision of the U-Pb SHRIMP ages is insufficient, however, to rule out small hiatuses or duplications of stratigraphy on the order of a few million years. The sedimentary sequences of the BR-vsc (BGB, South Africa) and the KG-vsc (CGGB, Australia) were both deposited in shallow water, near base-level. Despite the similarities in geological setting and timing of the two complexes, there are small differences in the sedimentary sequences. The sediments of the BR-vsc form a regressive-transgressive sequence, which was deposited close to base level and experienced occasional emersion. The sediments of the KG-vsc have mostly been reworked and were deposited entirely subaqueously, close to base level. Deposition of the sediments of the KG-vsc was possibly influenced by tidal activity. In the top of the volcanic sequences, just below the sediments, metres-wide black chert veins occur in both the BR-vsc and the KG-vsc. They are interpreted to be remnants of early Archaean hydrothermal systems. The vein systems tend to occur preferentially in the hanging walls of the normal faults. The hydrothermal systems were active during or shortly after deposition of the sediments and caused early and multi-phase silicification, veining and brecciation of the sediments. The BR-vsc and KG-vsc hydrothermal systems are interpreted to have vented in shallow water, near base level. Fluid inclusion studies show that the hydrothermally deposited quartz in the BR-vsc contains relatively simple two-phase aqueous and mixed H2O-CO2 inclusions. The fluids are interpreted to be at least partly of magmatic hydrothermal origin. High-salinity aqueous inclusions and low-salinity mixed H2O-CO2 inclusions show unmixing. The hydrothermal fluids had a minimum temperature of ~250°C and were highly pressurised (~2.2 kb).