LEAD-ISOTOPE VARIATIONS WITHIN THE BUSHVELD COMPLEX, SOUTHERN AFRICA - A RECONNAISSANCE STUDY

Variations in initial Sr-87/Sr-86 through the mafic cumulates of the B ushveld complex have long been used to trace the nature and relative i nfluence of different parental liquids in the development of the magma chamber. The authors report the results of the first systematic inves tigation of the lead isotopic variation in the complex and provide inf ormation on both silicate and sulphide phases through the cumulate suc cession. Marginal and cumulate lithologies indicate that the initial l ead isotopic composition (represented by model source U-238/Pb-204 rat ios) varies systematically in a manner analogous to that detected with strontium isotopic ratios. Low mu(2) ratios are evident in the Lower Zone and increase up the cumulate pile to the Main Zone, which apparen tly had homogeneous initial lead isotopic ratios. Unlike Sr-87/Sr-86, however, considerable mu(2) variation is detected in the Upper Zone. C umulate lithologies also exhibit a greater range of mu(2) values than those noted in the Marginal suite representatives, comparable to the s ituation for strontium isotopes. Most of the model mu(2) results are h igher than values expected for mantle-derived magmas, suggesting the i ncorporation of material with a crustal pre-history. Analyses of a det ailed sampling of the Merensky and Bastard cyclic units at the Atok mi ne, eastern Bushveld are also presented and these reveal extreme value s of mu(2) while the variations in mu 2 do not mirror the Sr-87/Sr-86 variations reported for the same samples by Lee and Butcher (1990). El evated mu(2) in some cumulate samples are impossibly high for a magmat ic origin and presumably reflect the introduction of extra-magmatic co mponents during sub-solidus hydrothermal alteration.