Regional isotopic fingerprinting of hydrothermal barite mineralisation and its possible association to epigenetic Fe and Mn ore formation in the Paleoproterozoic Transvaal Supergroup of South Africa

Abstract

The giant sedimentary manganese deposits of the Kalahari manganese field (KMF) in South Africa have been locally upgraded by hydrothermal alteration involving introduction of Na and K, elemental redistribution, and formation of a voluminous gangue mineralogy which includes silicates, borates and sulphates such as aegirine, andradite, gaudefroyite and barite (Beukes et al. 1995). Barite and high Ba-Na-K concentrations have been reported from the orebodies of the Postmasburg Fe–Mn Field (PMF) stretching as far as ∼150 km to the south of the KMF. However, these ores are thought to have been formed through lateritic processes and accumulation in surficial karst environments (Gutzmer and Beukes 1996; Beukes et al. 2003), and hence, barite has been regarded as evidence for localised hydrothermal fluid flow postdating ore formation. Recent work has documented barite as a widespread phase associated with ‘enriched zones’ of alkalis within the PMF ores and neighbouring lithologies. In this study, we characterise in detail Ba-bearing phases associated with barite, report new ones such as tamaite and pyrobelonite, and describe their textural relations. Complex and chemically similar barite associations exist in both PMF and KMF, include minerals such as hyalophane, banalsite, piemontite, calcite, aegirine and kentrolite, and emphasise the presence of Na, K, Ca, Pb, As and V in the mineralising fluids. Ba-rich massive iron ore in the PMF comprises barite, paragonite, apatite and berthierine. Barite forms conspicuous replacement textures such as pseudomorphs after carbonates, colloform layering along with hematite and fills hematite ooids. The initial sulphur isotopic analysis of 60 barite samples across the entire region yield high δS values, ranging between 22.1 and 29.5‰ vs. CDT, at an average of 25.6‰. δS values (n = 12) of gypsum veins within altered BIF shows an average of ∼26‰ and sulphates coexisting with barite have largely overlapping values. This large homogeneity in sulphur values implies a common source that we suggest to be that of sea water sulphate preserved within extensive evaporitic units in the Campbellrand dolomites, evidence of which were presented in Gandin et al. (Gandin et al. 2005). Preliminary results of the Sr isotopic analysis of barite reveal fluid mixing with Sr-fluids of highly radiogenic and distinct isotopic ratios for different sites, possibly reflecting interactions between evaporitic brines and various lithotypes enriched in radiogenic Sr.