The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

Abstract

Much research has been conducted on these banded iron formations (BIF) over the last 15–20 years. This contribution seeks to provide an overview of old and new facts and critical discussion on the latest ideas regarding the origin of these sediments in the early Proterozoic. The recently suggested stratified ocean water model and a new stratified lake water model are compared using new evidence of a stratigraphic, major and trace element, stable isotope and REE nature. It appears that any hypothesis on the genesis of these Transvaal Supergroup rocks will have to satisfactorily account for at least the following: 1. i) A constant supply of enough Fe and Si over at least several hundred thousand years. 2. ii) A macro- and a micro-cyclicity. The latter is the most basic building stone and any hypothesis that can not explain these phenomena must be considered unrealistic. 3. iii) The stratigraphic and isotopic evidence for heterogeneity of the waterbody. 4. iv) The REE and trace element evidence for contributions from different sources. 5. v) The distribution of organic carbon in rock facies and minerals. 6. vi) The difference between Proterozoic and present day atmospheres and surface waters. 7. vii) The fossil record of the early Proterozoic, and coupled to this the role that chelation, complex formation, stable colloids and co-precipitation played in weathering, transportation and deposition of Fe. 8. viii) Factors influencing or controlling cyclicity. 9. ix) The role that atmospheric and crust-mantle evolution plays in producing most large BIF-deposits over a time span of about 500 Ma from the late Archaean into the early Proterozoic. 10. x) The concomitant evidence provided by early Proterozoic paleosols. 11. xi) The fact that several large Proterozoic BIF deposits are immediately preceded by platform carbonates. Finally, the two important ore districts, Sishen in the Northern Cape Province and Thabazimbi in the Central Transvaal, are dealt with. The general geology, mineralogy and genesis of these very high-grade major deposits are presented. New information on ore morphology and new evidence on multiple epigenetic enrichment of BIF-protore are presented and discussed. The modern and specialised ore-mining, -processing and blending techniques at Sishen are explained.