Iw. Halbich et al., THE TRANSVAAL-GRIQUALAND WEST BANDED IRON-FORMATION - GEOLOGY, GENESIS, IRON EXPLOITATION, Journal of African earth sciences, and the Middle East, 16(1-2), 1993, pp. 63-120
Much research has been conducted on these banded iron formations (BIF)
over the last 15-20 years. This contribution seeks to provide an over
view 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 l
ake water model are compared using new evidence of a stratigraphic, ma
jor 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: i) A const
ant supply of enough Fe and Si over at least several hundred thousand
years. ii) A macro- and a micro-cyclicity. The latter is the most basi
c building stone and any hypothesis that can not explain these phenome
na must be considered unrealistic. iii) The stratigraphic and isotopic
evidence for heterogeneity of the waterbody. iv) The REE and trace el
ement evidence for contributions from different sources. v) The distri
bution of organic carbon in rock facies and minerals. vi) The differen
ce between Proterozoic and present day atmospheres and surface waters.
vii) The fossil record of the early Proterozoic, and coupled to this
the role that chelation, complex formation, stable colloids and co-pre
cipitation played in weathering, transportation and deposition of Fe.
viii) Factors influencing or controlling cyclicity. ix) The role that
atmospheric and crust-mantle evolution plays in producing most large B
IF-deposits over a time span of about 500 Ma from the late Archaean in
to the early Proterozoic. x) The concomitant evidence provided by earl
y Proterozoic paleosols. xi) The fact that several large Proterozoic B
IF 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 g
eology, 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 discus
sed. The modem and specialised ore-mining, -processing and blending te
chniques at Sishen are explained.