Geochemical and mineralogic effects of contact metamorphism on banded iron-formation: an example from the Transvaal Basin, South Africa

Abstract

Intrusion of a diabase sill into the Paleoproterozoic Kuruman Iron Formation, Transvaal Basin, South Africa makes it possible to distinguish between effects of contact and regional metamorphism on mineralogic, elemental and isotopic compositions of banded iron-formation (BIF) lithofacies. Temperatures within the aureole of contact metamorphism likely did not significantly exceed 450°C as suggested by the presence of early diagenetic carbonates, greenalite, and stilpnomelane (formed prior to intrusion) in chert-rich facies as close as 4 m from the sill. In sediments 32 m away from the intrusion, a temperature of ∼ 280°C is indicated by the magnitude of 18O fractionation between successive chert and siderite microbands; in unmetamorphosed equivalents similar isotopic analyses on whole-rock samples yield temperature estimates between 150 and 230°C. In comparison with samples of similar lithofacies from the underlying Campbellrand-Kuruman transition zone, carbonates from within the metamorphic aureole are depleted in 13C and 18O. The abundance and C-isotopic composition of total organic C (TOC) in whole-rock samples is lower and more enriched in 13C than those in samples outside of the zone of contact metamorphism. Within 5 m of the diabase sill the abundance of Fe increases dramatically and the oxidation state of the sediments fluctuates considerably more than in sediments outside of the metamorphic aureole. Isotopic and mineralogic systematics in the zone of contact metamorphism in the Kuruman Iron Formation are similar to those in siliceous dolomites and limestones altered by similar thermal processes. As the most distinct isotopic and elemental variations occur within 10 m of the diabase sill, it is suggested that volatilization and fluid infiltration associated with the intrusion were the main processes of alteration. Consideration of the effects of metamorphism (both contact and regional) as well as early diagenesis on the mineralogy and isotopic chemistry of BIF sediments may be a key to our understanding of primary mineralogies and oxidation state of these enigmatic deposits.