Mineralogy and Petrology of the Contact Metamorphosed Amphibole Asbestos-bearing Penge Iron Formation, Eastern Transvaal, South Africa

Abstract

The mineralogy and petrology of the Penge Iron Formation of the INTRODUCTION Transvaal Supergroup, situated in the contact metamorphic aureole The Penge Iron Formation of the Transvaal Supergroup of the Bushveld Complex, was studied in detail to investigate the is situated in the contact metamorphic aureole of the petrogenesis of crocidoliteand amosite-bearing rocks at Penge and Bushveld Complex in eastern Transvaal, South Africa Mafefe, in eastern Transvaal, South Africa. The rocks are laterally (Fig. 1). The iron-formation succession is unique in the equivalent to the diagenetic to very low-grade metamorphic Assense that it contains deposits of both crocidolite (asbesheuwels iron-formation succession in Griqualand West. Metabestiform riebeckite) and amosite (asbestiform grunerite), morphism at Penge apparently took place at 420–460°C and 2·6 mined extensively up to the late 1980s (Beukes & Dreyer, ±0·8 kbar as the result of the intrusion of the Bushveld Complex. The 1986; Dreyer & Sohnge, 1992), in close spatial remaximum metamorphic temperature at Mafefe was some 40–80°C lationship. It is thus the only area known in the world lower than at Penge. Bulk-rock composition was a major factor in where the relationship between the formation of crocontrolling the distribution of crocidolite and amosite in the succession. cidolite and amosite can be studied. In addition, some Riebeckite and crocidolite are absent from Al-rich rock units containing deep exploration diamond drill cores are available that biotite with Si/Al<3, but are commonly associated with ferri-annite can be used for establishing what influence the comwith Si/Al ratios of the order of three. In contrast, amosite is well position and stratigraphic setting of original iron-fordeveloped in carbonaceous Al-rich biotite–grunerite hornfels. Couplets mation lithofacies had on variations in metamorphic of amosite–crocidolite seams, developed in magnetite–grunerite banded mineral assemblages and chemical composition of certain iron-formation, may have formed from alternating mesobands of ironmetamorphic minerals in the succession. silicate–riebeckite known from the diagenetic to very low-grade metaMetamorphic assemblages in iron-formations have morphic Kuruman Iron Formation of the Asbesheuwels Subgroup. been studied very extensively [see Klein (1983) for a There is also evidence, especially in the case of one specific asbestos review] but integrated stratigraphic–metamorphic studies reef, that riebeckite–crocidolite have been replaced by grunerite–amosite have received little attention. Similarly, the geology and with increasing grade of metamorphism from Mafefe towards Penge. mineralogy of the amphibole asbestos-bearing iron-formIn contrast, small amounts of grunerite appear to have been replaced ations in the Penge area have been studied by many by riebeckite during retrograde metamorphism. workers (Peacock, 1928; Hall, 1930; Kirkman, 1930;