SULFUR AND LEAD ISOTOPE CONSTRAINTS ON THE GENESIS OF A SOUTHERN ZAMBIAN MASSIVE SULFIDE DEPOSIT

The Nampundwe deposit is situated 45 km west of Lusaka (on 27-degrees 24'E, 15-degrees 33'S), in the southern province of Zambia. Eight oreb odies are present at Nampundwe, although only two (orebodies 1 and 2) are currently being exploited (for pyrite and chalcopyrite). The orebo dies are strata bound, steeply dipping (from 70-degrees E-10-degrees o verturned), laterally extensive (typically 2-3,000 m along strike), 10 to 15 m thick, and over 300 m deep (the base of the deposit has not y et been encountered). The ores typically consist of massive, bedded, f ine-grained pyritic ''sulfidite'' horizons (i.e., >30% sulfide) up to 1.5 m thick and laterally extensive. Variable amounts of pyrrhotite, c halcopyrite, and sphalerite are present, typically forming veinlets or blebs, although massive pyrrhotite occurs in orebody 1. The ores are hosted in massive, fine-grained dolomites with occasional pelitic inte rcalations. The region has been metamorphosed to amphibolite facies an d deformed to form tight to isoclinal folding. Data on the copper and sulfur grades indicate that ore grades were controlled by paleotopogra phy, the ores being concentrated in paleodepressions. Analysis of acid -leached bulk-rock samples by inductively coupled plasma atomic emissi on spectroscopy indicates three episodes of base metal mineralization (one in orebody 1, two in orebody 2) with maximum Cu and Co concentrat ions reaching 8 percent and 3,400 ppm, respectively. Sulfur isotope de terminations on pyrite, pyrrhotite, chalcopyrite, and sphalerite show deltaS-34 values ranging from -10 to +9.8 per mil. Orebody 1 displays distinct linear relationships between deltaS-34 and stratigraphic heig ht, there being a positive correlation in the south of the mine (from -10 at the base to +5 parts per thousand at the top of the orebody), w hereas 1.2 km to the north (along strike) a negative correlation (from +9.8 at the base to -3.5 parts per thousand at the top of the orebody ) is present. The opposing trends in deltaS-34 values through the same stratigraphy are thought to be due to mixing processes being dominant to the south, and to changes in fractionation (related to changing ph ysical and/or chemical conditions at the time of precipitation) betwee n the fluid and precipitated mineral being dominant to the north. A ge neral trend to increasingly heavy sulfur with stratigraphy is present in orebody 2 (both for the north and the south of the mine); this has been interpreted as being due to closed-system fractionation and/or mi xing. A large spread in the Pb isotope composition of sulfides from th e ores is present (Pb-206/Pb-204 = 17.89-19.34; Pb-207/Pb-204 = 15.69- 15.74; Pb-208/Pb-204 = 37.62-38.97) and thought to be due to mixing of at least two upper crustal reservoirs.