Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 129

Fig. 5.6 Photos of banded
iron formations. (a) outcrop
of BIF in the Mt Tom Price
mine of the Pilbara, Western
Australia; (b) General view of
the mine (Photos from
K. Konhauser)
studied. The North American deposits have since been eclipsed by the much larger
deposits in the Hamersley Basin of Western Australia, the Transvaal Basin of
Brazil, the “Quadrilatero Ferrifero” in Brazil, Krivoy Rog in the Ukraine and the
Singhbhum region of India. Finally, Rapitan-type deposits, named after the type
locality in the McKenzie Mountains of northwest Canada, are a relatively minor
type associated with Neoproterozoic glacial deposits.
A parallel classification of BIF deposits is based on the mineralogy of the iron
phases. Oxides (hematite or magnetite) are the main phase in most banded iron-
formations, but in others carbonate, silicates, or sulfide predominate. The different
types of iron minerals, whose compositions are given in Table 5.4 , are stable under
contrasting conditions of Eh and pH, from relatively oxidising to highly reducing,
which suggests that they were deposited in different environments. As seen in
Fig. 5.7 , the oxides are stable in acid solutions, hematite at high Eh and magnetite
at low Eh; and pyrite and siderite are stable in near-neutral solutions at low Eh (but
these fields expand at high activities of carbonate or sulfur). In alkali solutions,
ferrous iron is soluble over a wide range of Eh but the field is broader under
reducing conditions. It can therefore be seen that an increase in Eh, oxidation,
stabilizes hematite relative to Fe 2+ , and this process is the key to formation of most
BIF deposits.
The restriction of this type of deposit to the period 2.4-2.0 Ga, and the enormous
quantity of Fe tied up in these deposits, is commonly taken as evidence that the
composition of the Earth's oceans and atmosphere changed markedly at this time.
The Archean, pre-2.7 Ga, atmosphere was a reducing, oxygen-free mixture of
nitrogen, carbon dioxide and methane, rather like the atmosphere of Jupiter; the