SOAS Research Online

Abstract

Copper and cobalt mineralisation at Baluba are hosted in tremolite schists and argillites. The principal copper bearing minerals are chalcopyrite, bornite and chalcocite and cobalt occurs as carrollite in the schists and as cobaltiferous pyrite in the argillites. The primary mineralisation is envisaged to be synsedimentary. The 1% copper assay footwall is found to be coincidental with the geological footwall defined by the contact between conglomerates and schists, but the assay hangingwall does not have a corresponding geological marker. A vertical zonation of the orebody into ore mineral groups is noted in the Baluba orebody. A mathematical model that possibly describes the vertical copper grade distribution prior to diagenetic and metamorphic effects is presented. Petrographic examinations of the Baluba Centre Limb rocks indicate that the rocks have undergone metamorphism of the greenschist facies. The recrystallisation and remobilisation of the metallic minerals has affected the distribution of the copper and cobalt grades within the Baluba Centre Limb rocks. The geochemistry of the Basement suggests that the Basement schists and granites are possible sources of the rock forming and metallic minerals of the Baluba Centre Limb sediments. Whereas the copper and iron geochemical patterns in the Baluba Centre Limb can be adequately explained by the Basement geochemistry there is a notable deficiency of cobalt in the Basement. The geochemical patterns displayed in the Baluba Centre Limb rocks appear complex when all the samples are treated together without applying any geological controls to separate them. Clearer geochemical relationships are obtained when the samples are grouped according to lithological and mineralogical classes. The close correlation between cobalt and iron in the pyrites in the hangingwall region signifies the cobaltiferous nature of the pyrite. Statistically it has been observed that the distributions of copper grades are positively skewed and the histograms exhibit frequency distributions which deviate from the normal (gaussian) model. The skewness is, however, not strong enough to fit the log-normal model. The deviation from the normal distribution is attributed to the co-existence of several copper-bearing minerals through the Baluba Centre Limb orebody. The analysis of variance indicates that the chip sampling method practised at Baluba gives rise to higher sampling variances compared to diamond drill sampling. It is shown that the sampling variance due to chip sampling accounts for about 20-40% of the total variance whereas the sampling variance contribution to total variance in diamond drill sampling is about 10-20%. Evaluation of the precision of copper and cobalt analysis by the AAS procedure at the Baluba mine laboratory indicate detection limits of 0.111% copper and 0.062% cobalt. Variograms have been constructed in the down-hole direction and in the horizontal plane. The spherical model with a nugget effect is used to fit the experimental variogram data. The fitted variogram models indicate a zonal anisotropy (between the down-hole and horizontal planes), but a geometric isotropy within the horizontal plane. In the down-hole direction the range of influence has been evaluated to be 1.6m in the schist and 8m in the argillites. In the horizontal plane the modelled variograms are isotropic with a range of 100m. The optimisation of the position of the hangingwall has been evaluated by assessing the influence of internal and hangingwall dilution. Using the effects of internal dilution only, a cutoff grade of 0.75% copper is found to optimise the hangingwall location. If hangingwall dilution is allowed for, in addition to the internal dilution, then a higher cutoff grade of 1.25% copper optimises the hangingwall position. A density of 2.779t/m.;3is obtained for the mineralised sediments of Baluba.