Geochemistry, mineralogy, and isotope composition of Pb, Zn, and Cu in primary ores, gossan and barren ferruginous crust from the Perkoa base metal deposit, Burkina Faso
Abstrakt
Gossan at the Perkoa deposit of stratabound lead and zinc ores (Burkina Faso) is composed of hematite, goethite, clay minerals, minerals of the brucite group and sulfates (corkite, alunite, natroalunite, hinsdalite and hidalgonite). Residual quartz contains inclusions of chalcopyrite, sphalerite and cinnabar.
Compared with the primary ores, the gossan is especially enriched with As, Ba, Pb, P, and Sb. On the other hand, the contents of Ag, Co, Cu, Hg, Mo, Ni, and particularly Zn are significantly lower.
However, compared with the barren ferruginous crust in the same area, the gossan is significantly enriched in most of chemical elements, namely As, Pb, Sb, and Zn and to lesser extent also in Ag, Ba, Cd, Mo, S, and Sr. The isotopic composition of lead in massive primary ore and in gossan is practically identical.
However, the isotopic composition of lead in disseminated ores, the bleached gossan envelope, and in mineralized soils in the Perkoa area is very variable and reflects mixing of ore and lithogenic lead. The Pb-206/207 ratio in barren soils and ferruginous crusts is much higher compared with gossan and related altered rocks.
Compared to massive ore (delta Cu-65 = + 1.91 to + 2.17 parts per thousand and delta Zn-66 = + 0.30 to + 0.36 parts per thousand, respectively), the gossan is depleted in heavy isotopes of copper and zinc (delta Cu-65 = + 0.02 to + 0.58 and delta Zn-66 = + 0.10 to + 0.88 parts per thousand, respectively). The chemical weathering of sulfide-rich rocks thus gives rise to considerable variations in Cu isotopes (median: - 2.36 parts per thousand, Delta Cu-65(gossan-massive ore)), and minor changes in Zn isotopes (median: - 0.73 parts per thousand, Delta Zn-66 (gossan-massive ore)).
Isotopic composition of copper in gossan and lateritic soil sampled in the vicinity of gossan is similar but differs from barren laterite duricrust which is more depleted in the Cu-65 isotope.