LEAD-ISOTOPE PATTERNS IN PROTEROZOIC STRATIFORM MINERALIZATION IN THEBUSHMANLAND 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 co mposition, with Pb-206/Pb-204 = 16.74-16.82, Pb-207/Pb-204 = 15.54-15. 62, and Pb-208/Pb-204 = 36.63-36.71 (all 2 sigma ranges). Four separat e orebodies (Broken Hill, Black Mountain, Rig 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 gi ant Proterozoic deposits, whose Pb isotope signatures generally plot v ery close to model crustal growth curves, the Bushmanland ores have ve ry radiogenic compositions. Model ages are significantly younger than the age of mineralization, which is constrained by an Sm-Nd isochron a ge of 1649 +/- 95 Ma obtained on metabasaltic amphibolites in a strati graphically overlying formation. The extreme upper crustal signature c ould have been derived from a source bed lower in the Bushmanland stra tigraphy, characterized by abundant acid volcaniclastic material, that was a product of crustal reworking of basement rocks to the Bushmanla nd basin, 1700 to 1600 Ma.