Lead isotope patterns in Proterozoic stratiform mineralization in the Bushmanland Group, Namaqua Province, South Africa

Galena in giant Proterozoic sediment-hosted Pb-Zn sulfide deposits in the Bushmanland Group of South Africa has a very uniform Pb isotope composition, with 206 Pb/ 204 Pb = 16.74-16.82, 207 Pb/ 204 Pb = 15.54-15.62, and 208 Pb/ 204 Pb = 36.63-36.71 (all 2sigma ranges). Four separate orebodies (Broken Hill, Black Mountain, Big Syncline, and Gamsberg) show very little departure from these mean data, suggesting that they were all formed from a common mineralizing fluid. Compared to other giant Proterozoic deposits, whose Pb isotope signatures generally plot very close to model crustal growth curves, the Bushmanland ores have very radiogenic compositions. Model ages are significantly younger than the age of mineralization, which is constrained by an Sm-Nd isochron age of 1649+ or -95 Ma obtained on metabasaltic amphibolites in a stratigraphically overlying formation. The extreme upper crustal signature could have been derived from a source bed lower in the Bushmanland stratigraphy, characterized by abundant acid volcaniclastic material, that was a product of crustal reworking of basement rocks to the Bushmandland basin, 1700 to 1600 Ma.