Abstract

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 orebodies are present at Nampundwe, although only two (orebodies 1 and 2) are currently being exploited (for pyrite and chalcopyrite). The orebodies are strata bound, steeply dipping (from 70 degrees E-10 degrees overturned), 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 yet been encountered). The ores typically consist of massive, bedded, fine-grained pyritic "sulfidite" horizons (i.e., >30% sulfide) up to 1.5 m thick and laterally extensive. Variable amounts of pyrrhotite, chalcopyrite, 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 intercalations. The region has been metamorphosed to amphibolite facies and deformed to form tight to isoclinal folding.Data on the copper and sulfur grades indicate that ore grades were controlled by paleotopography, the ores being concentrated in paleodepressions. Analysis of acid-leached bulk-rock samples by inductively coupled plasma atomic emission spectroscopy indicates three episodes of base metal mineralization (one in orebody 1, two in orebody 2) with maximum Cu and Co concentrations reaching 8 percent and 3,400 ppm, respectively.Sulfur isotope determinations on pyrite, pyrrhotite, chalcopyrite, and sphalerite show delta 34 S values ranging from -10 to +9.8 per mil. Orebody 1 displays distinct linear relationships between delta 34 S and stratigraphic height, there being a positive correlation in the south of the mine (from -10 at the base to +5ppm at the top of the orebody), whereas 1.2 km to the north (along strike) a negative correlation (from +9.8 at the base to -3.5ppm at the top of the orebody) is present. The opposing trends in delta 34 S 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 physical and/or chemical conditions at the time of precipitation) between the fluid and precipitated mineral being dominant to the north. A general 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 mixing.A large spread in the Pb isotope composition of sulfides from the ores is present ( 206 Pb/ 204 Pb = 17.89-19.34; 207 Pb/ 204 Pb = 15.69-15.74; 208 Pb/ 204 Pb = 37.62-38.97) and thought to be due to mixing of at least two upper crustal reservoirs.

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