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Origin and evolution of Archean quartzites from the Nondweni greenstone belt (South Africa): inferences from a multidisciplinary study

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van den Kerkhof, Alfons; Riganti, A.; Scherer, T.
South African Journal of Geology
Quartzites from the Patsoana area, part of the Nondweni greenstone belt, South Africa, comprise coarse-grained (1) banded graphite-bearing black and white quartzite, (2) greenish to beige finely-banded quartzite and (3) massive sulphide-bearing quartzite. The quartzites show distinctive Ti and Al-contents (LA-ICPMS analysis) that are highest for the finely-banded quartzite varieties with intense blue cathodoluminescence (CL) and lowest structural water as measured by FT-IR analysis. The quartzites underwent contact metamorphism, but stable oxygen isotope ratios (16.7‰ to 19.2‰) suggest formation at low temperatures as chert. Complex quartz zoning in CL combined with trace element analysis points at a 2-step evolution after primary formation with (1) prograde recrystallisation of a finer-grained siliceous lithotype accompanied by extensive hydrothermal alteration resulting in high Ti-contents, and (2) retrograde reduction of trace elements by diffusion and by the final formation of pure secondary quartz. Secondary quartz veinlets formed at lower temperatures. The study testifies the local remobilisation of titanium in solution as well as diffusion in quartz at high temperatures. Laser Raman analysis of dispersed graphite shows high crystallinity in most of the quartzites and points at formation temperatures of >600°C; poorly organised graphite was mainly found in the finely-banded and greenish quartzite and formed during retrogression. Very high concentrations of paramagnetic [TiO4/Li+] and [TiO4/H+] defect centres were found in the finely-banded quartzite, whereas paramagnetic [AlO4] centres are assumed to be largely decayed with time. The perseverance of the Ti-related paramagnetic defects are explained by a long-term position of the Nondweni greenstones in a stable part of the crust.
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