Mineralogy and petrology of the contact metamorphosed amphibole asbestos-bearing Penge iron formation, eastern Transvaal, South Africa

The mineralogy and petrology of the the Penge Iron Formation of the Transvaal Supergroup, situated in the contact metamorphic aureole of the Bushveld Complex, was studied in detail to investigate the petrogenesis of crocidolite- and amosite-bearing rocks at Penge and Mafefe, in eastern Transvaal, South Africa. The rocks are laterally equivalent to the diagenetic to very low-grade metamorphic rls besheuwels iron-formation succession in Griqualand West. Metamorphism at Penge apparently took place at 420-460 degrees C and 2.6 +/- 0.8 kbar as the result of the intrusion of the Bushveld Complex. The maximum metamorphic temperature at Mafefe was some 40-80 degrees C lower than at Penge. Bulk-rock composition was a major factor in controlling the distribution of crocidolite and amosite in the succession. Riebeckite and crocidolite are absent from Al-rich rock units containing biotite with Si/Al<3, but are commonly associated with ferri-annite with Si/Al ratios of the order of three. In contrast, amosite is well developed in carbonaceous Al-rich biotite-grunerite hornfels. Couplers of amosite-crocidolite seams, developed in magnetite-grunerite banded iron-formation, may have formed from alternating mesobands of iron-silicate-riebeckite known from the diagenetic to very low-grade metamorphic Kuruman Iron Formation of the Asbesheuwels Subgroup. There is also evidence, especially in the case of one specific asbestos reef; that riebeckite-crocidolite have been replaced by grunerite-amosite with increasing grade of metamorphism from Mafefe towards Penge. In contrast, small amounts of grunerite appear to have been replaced by riebeckite during retrograde metamorphism.