Stable isotope and geochemical studies in the role of early diagenesis in ore formation, Konkola Basin, Zambian copper belt

Univ. Aston at Birmingham, Dep. Geol. Sci., Birmingham, United Kingdom

Studies have been undertaken on a series of underground borehole samples of ores and associated metasediments from the Konkola area of the Zambian copper belt. The samples, which extend through the ore shale of the Kirila Bomwe South orebody and the associated overlying (hanging wall) and underlying (footwall) strata, were subject to petrographic and mineralogical examination along with chemical analysis and the determination of stable isotope ratios in selected minerals. Chemical analyses included electron microprobe analysis of carrollites and dolomites, and whole-rock analysis for total copper, total carbon, and organic carbon content. Stable isotope studies involved determination of delta ³⁴ S values in chalcopyrites and carrollites, and both delta ¹³ C and delta ¹⁸ O values in calcites and dolomites. The delta ¹⁸ O values of present-day mine waters are also presented.A strong correlation is observed between the concentration of copper and of carbonate carbon in the rocks examined. This observation, along with petrographic evidence, supports the theory that the sulfides in the ore shale at Konkola were formed by the bacterial reduction of seawater sulfate, probably during early diagenesis, with carbonates formed as a by-product of the reaction. The delta ³⁴ S values of chalcopyrites and carrollite fall within the range of -7.0 to +1.2 per mil (relative to Canyon Diablo troilite) with a mean value of -2.8 per mil. The delta ³⁴ S values of chalcopyrite and carrollite also show a systematic variation through the stratigraphic sequence. When this variation is considered in relation to the sedimentological interpretation of the sequence, it can be seen that there is a clear correlation of delta ³⁴ S values with transgressive and regressive events; rocks associated with transgressions are accompanied by increasingly lighter delta ³⁴ S and those associated with regressions with increasingly heavier delta ³⁴ S values. Determinations of sulfur isotope fractionation between coexisting sulfides yielded some temperatures ( approximately 330 degrees C) in line with those expected from the later metamorphism of these rocks. However, many samples gave anomalous results suggesting that widespread reequilibration did not take place during metamorphism.Petrographic evidence that dolomites and calcites at Konkola were also formed during early diagenesis is supported by the delta ¹⁸ O and delta ¹³ C determinations. Values of delta ¹⁸ O (SMOW) for dolomites from the ore shale are found to be in the range of 14.85 to 16.16 per mil, whereas consistent values of delta ¹⁸ O for the mine waters of -6.2 per mil (SMOW) effectively rule out recent dolomite formation. Dolomite samples from the footwall rocks gave delta ¹⁸ O values of 20.82 to 26.38 per mil (SMOW) which suggests that they derived their oxygen from seawater, whereas dolomite samples from the ore shale have values intermediate between those expected for Precambrian seawater and possible Precambrian ground water. In considering delta ¹³ C values, the dolomites from the ore shale exhibit values of -8.77 to -20.52 per mil (PDB) compatible with a dominantly organic source. The samples of dolomite from the footwall rocks are noticeably enriched in ¹³ C compared to those from the ore shale (-4.42 to -9.37 per mil, PDB), indicating the influence of marine-derived carbon. These oxygen and carbon isotope values could again be related to early diagenetic processes influenced by the sequence of transgressive and regressive events leading to the formation of the original sediments.Dolomites that formed as a pore-lining cement in footwall clastic rocks contain up to 4.9 wt percent CoCO ₃ and are isotopically shown to be of marine origin. This suggests a basinward source of the cobalt in these ores, possibly via submarine hydrothermal activity. Copper, on the other hand, may have been derived from the basement which is known to contain copper porphyry-type deposits and mineralized gneisses.

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