Paper No. 1
Presentation Time: 8:00 AM-6:00 PM
SULFUR ISOTOPES OF BASE-METAL SULFIDE AND BARITE PAIRS FROM THE AGGENEYS-GAMSBERG BHT DISTRICT, SOUTH AFRICA
MCCLUNG, Craig R.1, GUTZMER, Jens
2, GERTLOFF, Ellen
1, BEUKES, Nicholas J.
3 and STRAUSS, Harald
4, (1)Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, 2006, South Africa, (2)Department of Mineralogy, TU Bergakademie, Freiberg, D-09596, Germany, (3)Department of Geology, University of Johannesburg, Auckland Park, 2006, South Africa, (4)Geologisch-Palaontologisches Institut, Westfalische Wilhelms-Universitat Munster, Corrensstrabe, Munster, D-48149, Germany, crmcclung@uj.ac.za
Stratiform and stratabound base metal deposits of the Aggeneys-Gamsberg BHT district of South Africa, occur in amphibolite-facies metasedimentary rocks of the Mesoproterozoic Bushmanland Group. Sulfur isotope ratios from the district are distinctly enriched in
34S when compared to contemporaneous Mesoproterozoic seawater. All deposits exhibit a bimodal distribution in δ
34S
sulfide values and a systematic increase in δ
34S from the west to the east across the district. At the Swartberg deposit δ
34S
sulfide values average -0.6‰ and +16‰, while δ
34S
barite values average +23.8‰. The central portion of the district is occupied by the Broken Hill and Big Syncline deposits. The δ
34S
sulfide values for the Broken Hill deposit average +1.9-10.4‰ and +17.5‰, while the Big Syncline deposit exhibits average δ
34S
sulfide values of +11.1‰ and +19.7‰, with barites averaging +29.8‰.
The heavy δ34S values of primary sulfides are consistent with the values of the stratabound barites from throughout the district. In contrast, petrographic evaluations of the samples that display lower values indicate remobilization and reflect the influx of isotopically lighter magmatic sulfur. The heavier values fall within the range of coeval seawater, indicating a seawater sulfur source; however, the heavier values of the Big Syncline and Gamsberg can only be explained by deposition in a restricted basin. We suggest that the high δ34S values at Gamsberg and Big Syncline resulted from the preferential removal of 32S from the initial S-reservoir, via BSR, and precipitated as early diagenetic pyrite thereby enriching the sulfur reservoir in 34S.
