Volume 38: January - December 1997

Issue 5: May 1997

Abstract

Mineralogy and petrology of the contact metamorphosed amphibole asbestos-bearing Penge Iron Formation, Eastern Transvaal, South Africa

  • Mineralogy and petrology of the contact metamorphosed amphibole asbestos-bearing Penge Iron Formation, Eastern Transvaal, South Africa

    • T. Miyano1 and NJ. Beukes2 1Institute of Geoscience, the University of Tsukuba, Ibaraki 305, Japan and 2Department of Geology, Rand Afrikaans University, PO Box 524, Auckland park, Johannesburg 2006, South Africa

    ABSTRACT

    The mineralogy and petrology of 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, eastern Transvaal, South Africa. The rocks are laterally equivalent yo the diagenetic to very low-grade metamorphic Asbeheuwels iron-formation succession in Griqualand West. Metamorphism at Penge apparently took place at 310-360oC and 2.6 +- 0.8 kbar as the result of the intrusion of the Bushveld Complex. The maximum metamorphic temperature at Mafefe was some 40-80oC 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. Couplets 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 There is also evidence, especially in the case of one specific asbestos reef, that reibeckite-crocidolite have been replaced by grunerite-amosite with increasing grade of metamorphism from Mafefe towards Penge. In contrast, small amounts of grunerite appear to hae been replaced by riebeckite during retrograde metamorphism.

    Keywords: asbestos; iron-formation; metamorphism; mineralogy; Transvaal Supergroup

    Pages: 651 - 676

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