ABSTRACT. Sulphur isotope ratios in sulphides and baryte from stratabound and stratiform orebodies in a metavolcanic-sedimentary sequence in Namaqualand were found, in part, to be extreme for Precambrian sulphur. Black Mountain, Aggeneys, in the west gave an average for the sulphides of 634S = + 8.9_+ 3.7 %0 (9 samples), an average for barytes of +20.6+4.3%0 (3 samples). Broken Hill, Aggeneys, in the centre gave an average for the sulphides of ~34S = +19.8+3.1%o (19 samples). Gamsberg, in the east, gave an average for the sulphides of 634S = + 29.2 + 1.8 %o (24 samples), and an average for barytes of + 35.4 • 0.2 %0 (2 samples). The 634S values increase eastward. Their range is strongly on the positive side and does not centre around zero. The Gamsberg barytes and most Gamsberg sulphides have more positive 634S values than those reported for other Precambrian sulphides and sulphates. We interpret the above sulphur isotope range as being mainly due to the varying contributions of submarine-exhalative sulphide sulphur with 634S close to zero and bacterially(?) reduced sulphate with strongly positive 6a4S, apparently from evaporites in the east. Metamorphism of amphibolite facies grade has partly isotopically re-equilibrated the ore minerals, as indicated by galena pyrrhotine and sulphidebaryte isotope temperatures from single specimens, but has not destroyed the primary sulphur isotope range indicating pre-existing sulphate concentrations.IN the early 1970s, four major Pb-Zn-sulphide deposits were discovered in the Bushmanland area
Copyrioht the Mineralogical Societywest of Pofadder, NW Cape Province ( fig. 1). They are stratiform deposits and belong to the highly metamorphosed Proterozoic volcano-sedimentary Bushmanland Sequence in the Namaqua Mobile Belt. The aim of this sulphur isotope study has been to obtain information on the source of the sulphur and on the primary conditions of ore formation. The present paper deals with the first results obtained from a series of fifty-seven mineral samples, which already permit some interesting conclusions. Chadwick (1981) listed the ore reserves of the four deposits as follows (from west to east): Black Mountain, more than 80 million tons with 2.7 % Pb, 0.6 % Zn, 0.7 % Cu; Broken Hill, 38 million tons with 6.35 % Pb, 2.87 % Zn, 0.45 % Cu, 85 g/t Ag; Big Syncline (Aggeneys Mountain), about 100 million tons with 1.0% Pb, 2.5~ Zn; Gamsberg, 152.55 million tons with 0.55 % Pb, 7.11% Zn.So far, only Broken Hill is in production. Its dominant economic constituent is galena, and pyrrhotine is the most abundant iron sulphide. The massive sulphide ore varies from 5 to 55 % Pb, with local zinc contents as high as 20% (Chadwick, 1981). Wilson (1981) gave the average of the well mineralized massive units within the Broken Hill deposit as 0.98 % Cu, 22.3 % Pb, 3.8 % Zn, and 300 ppm Ag.
