Geochemistry, Mineralogy and Isotope Composition of Pb, Zn and Cu in Gossan from the Perkoa Lead and Zinc Deposit, Burkina Faso

The Paleoproterozoic Perkoa massive sulfide deposit (Burkina Faso) is capped with well-developed gossan. To assess changes that occur within primary ore bodies under tropical weathering conditions, the geochemical and isotopic signature of gossan was compared with that of primary ores. To aid the identification of gossan in exploration, geochemical properties of gossan were also compared with the same properties barren lateritic duricrusts which are abundant in West Africa. Compared to primary ores, the gossan is enriched with As, Ba, Pb and Sb. On the other hand, the contents of Ag, Co, Cu, Hg, Mo, Ni and particularly Zn are significantly lower. The gossan when compared with barren laterite duricrust in the same area, is enriched with Ag, As, Ba, Hg, Mn, Mo, Pb, Sr, Sb and Zn. Isotopic composition of lead bound in gossan (Pb-206/Pb-207 = 0.98+/-0.002, Pb-208/Pb-206 = 2.34+/-0.005) is practically identical with the isotopic composition of lead in the primary ore. The differences in the isotopic composition of lead in gossan and lead in the barren laterite duricrust, and in lateritic soils result from mixing of ore and lithogenic lead. Compared to primary ore (delta Cu-65 = +0.2 to +1.7 parts per thousand and delta Zn-66 = -0.6 to +0.88 parts per thousand, respectively), the gossan is depleated in heavy isotopes of copper and zinc (delta Cu-65 = -0.02 to -0.62 and delta Zn-66 = -0.1 to -0.9 parts per thousand). The chemical weathering of sulfide-rich rocks thus gives rise to considerable variations in Cu isotopes (average: -1.1 parts per thousand Delta Cu-65 gossan-ore), but minor changes in Zn isotopes were found (average: -0.3 parts per thousand Delta Zn-66 gossan-ore). Isotopic composition of copper from the barren laterite duricrust is different from copper contained in gossan. No such difference was found in the isotopic composition of zinc. The results suggest that isotope geochemistry can be an advisable tool for discrimination between mineralized and barren products of weathering.