Document DGZBp4Z8x5LQ2arE8DNjxe324

PRODUCED BY FORD aplnccnent of the dIohasic sills (considered forerunners of the min jshveld Intrusion) and before cooling of the main body of the Complex fillers. 1964). I Cape Aebeetoe Field. This field Is In Crlqualnnd West, Cepe ovlncc, 700 hm to the vest of the asbestos mining areas of the TransMi. It lies along a line of low hills (Asbestos Mountains, Kuruman Ills) that strctehes from 40 km south of Prleaks, northward past irunan to Pomfret near the Botswana Border, a distance of 480 km (Pig. i. The crccldollte asbestos-bearing ironstones of this area are inluded within the Pretoria Series of the Transvaal Supergroup and are iggested to be correlated vlth the Pretoria Series rocks to the east In ie Transvaal (Cllllers, 1964, p. 581). Stratigraphic correlations stvecn the Transvoal and Cape regions are subject to somewhat different iterpretntIon (Cllllers, 1964; Cllllers and Ccnls, 1964; Rcukcs, 1973, ?90). Rev nomenclature proposals by Beukes (1980) place the erocldollte?arlng banded Ironstones of the Cape within the Crlquatown and Kuruman con Formations of the Asbesheuwels Subgroup. Cllllers (1964, p. 5881) tates that It ray be accepted that the banded Ironstones In the Cape rovinee and In the Transvaal were formed contemporaneously and under Inllar conditions. The asbestos deposits of the Cape are generally seated in the monocllnnl folds. Metamorphism la generally absent. Formation of banded ironstones. The origin of the Preenmbrlan inded Ironstones of South Africa is still the subject of much debate, at it is possible to summarize some of the main Ideas on their parncnesls. The deposition of the sediments probably occurred in releIvely shallow water, in some places In saline basins. Iron and silica jre derived from normal weathering processes. Primary precipitates sueh s silica and hydrous iron oxides and silicates may have been the prejrcors to the loter-formed stHpnomclnne , mlnncsotolte, and rlcbecklte. iochcmlcat actions of organic matter also probably hod an Important nfluence on the mlnerol chemistry. The change of nmphlbole mineralogy rom smosite In the Pcnge area, mixed amoslte-crocldollte In the huntcspoort-ttallpe Drift area, to crocldollte in the Cope suggests thot here was a gradual change of conditions of sediment formation: deposllon In.a predominantly freeh water environment in the Transvaal to a 190 8005 0056 predominantly saline environment in the Cape (Cllllers, 1966), The reason why the amphlboles crystallised in an asbestlform habit rather thon the typical prismatic form is a subject of some Interest. It Is pertinent to note the contents f Prayer and Robinson (1978), who state (p. 3-6) "the greatest development of fibre occurs vherc two eete of folds Intersect to fora domes or basins. It is firmly believed that the development of fibre deposits could only have taken place in a favour able stress environment." For further contents on crystal growth of asbestos minerals in stress fields, see Zoltal (Chapter 5, this volume). Crocldollte In Ironstones of Western Australia Crocldollte asbestos Is also found In the banded Ironstones of the Itemcrsley Iron Province, Vestern Australia (Fig. 4). These formations ore remarkably similar to those of the Cape Province of South Africa ond are Included among the great iron formations of the world which formed under very similar conditions and during a single geologic epoch approxi mately 1900 to 7500 m.y. ago (James and Sims, 1973). The banded ironstones are widely distributed in the Marra Mamba, Brockman, and Boolgeeda Iron Formations of the ttamersley Group, Mount Bruce Supergroup. The ironstones are approximately 2000 m.y. old (Trendall, 1973). Although riebecklte is widely distributed In these three Iron formations, it la only abundant in fibrous, form (crocldollte) In four narrow zones. The namereley ironstones are thought to have formed In much the same way as those of South Africa, by precipitation of volcanically derived iron and silica in a shallow basin. Massive riebecklte developed during diagenesia from previously existing minerals, vlth crocldollte asbestos crystallizing later in a stress environment. The major asbestos deposits are confined to the northern limb of the broad syncllnc of the Itamersley Range (Trendall and Blockley, 1970). These authors propose a "stress-reversal" for asbestos growth; they state (p. 18) "Crocldollte grew later by splitting and dialatlon of magnetite mesobands In banded iron formation as a relief from balanced compression from two sides in particular structural situations, vhere gravitational sliding opposed 'tectonic' stress." _. -- Crocldollte mining in Western Australia began on a very small scale 291