Petrochemical evidence on the probable origin of ferriferous metasediments in western Bushmanland |
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Authors: | Willem A. van der Westhuizen Dawid Strydom Aylva E. Schoch Eric A. W. Tordiffe Gerhard J. Beukes |
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Affiliation: | (1) Department of Geology, University of the Orange Free State, 9300 Bloemfontein, Republic of South Africa |
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Abstract: | The mineralized Proterozoic metasediments of Bushmanland are characterized by the presence of ferriferous rocks. This includes banded and unbanded iron formations and various types of gossans. These units are not laterally extensive and occur in different stratigraphic levels. The prevalent minerals in the ferriferous rocks are hematite, magnetite, quartz, garnet, muscovite, biotite and sillimanite, but less common occurrences of graphite, alunite, plumbojarosite, gahnite and dufrenite have been noted. The chemical variation (wt%) is extensive: total Fe2O3 (1.3–93.5), SiO2 (4–93), Al2O3 (0.2–14.0), CaO (0.02–20.7), MnO (0.0–14.3), MgO (0.0–5.7), TiO2 (0.0–4.4), Na2O (0.0–2.0), K2O (0.0–1.5) and P2O5 (0.1–7.0). The preliminary nature of the data set precludes, however, firm conclusions regarding stratigraphic control of the chemical composition. The trace-element contents (ppm) extend over several orders of magnitude: Zn (0–7,000), Ba (0–5,200), Cu (0–1,400), Pb (0–1,070) and Ni (6–540). Collectively, the data indicate that most of the ferriferous rocks represent highly metamorphosed sediments. |
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