Petrogenesis of Ni-Cu ore bodies, their host rocks and country rocks at Selebi-Phikwe, eastern Botswana

Abstract

The Phikwe, Selebi and Selebi North Ni-Cu sulphide deposits occur in highly deformed and metamorphosed Archaean gneisses near the north margin of the Central Zone of the Limpopo Belt. The ores and host rocks have suffered all the phases of deformation that have affected the enclosing gneisses. Nappes and thrusts formed during the first deformation period, during which granulite facies conditions of 800^oC and 10 kbar (accompanied by minor partial melting) were attained. Sheets of anorthosite were thrust into the sequence during late Dl. The main components of the Selebi-Phikwe sequence are hornblende gneiss, grey leucocratic gneiss and anorthositic gneiss. Granitic gneiss represents post-Dl granitic intrusions, not a basement or a sandstone unit as previously proposed. The hornblende gneiss has the geochemistry of a suite of variably altered fractionated tholeiitic basalts and Ti-rich ferrobasalts. Low-Ca biotite gneiss associated with hornblende gneiss represents a variety of basalt-derived rocks ranging from in situ alteration products to volcaniclastic and pelitic sediments. Lithological associations indicate that the hornblende gneiss represents both extrusive and high-level intrusive basalt. The grey gneiss has the geochemistry of a calc-alkaline suite. The mineralogical and chemical variation shown by the grey gneiss indicate a volcano-sedimentary rather than an intrusive origin. The protoliths of the anorthositic gneiss were gabbroic plagioclase-clinopyroxene cumulates. Ultramafic lenses within the sequence were olivine and clinopyroxene-rich cumulates. Minor components of the gneiss sequence are magnetite quartzite representing chemically precipitated silica-iron oxide sediment and associated cummingtonite-actinolite amphibolites representing mixtures of Mg-clays and carbonate. The three Ni-Cu ore bodies are intimately associated with a layer of hornblende-plagioclase amphibolite plus minor pyroxenite within grey gneiss. The ore bodies consist of Ni-rich massive sulphides and Cu-rich disseminations. Metamorphism of the host rocks was mainly isochemical. Localised shearing resulted in higher contents of Ti, K, P, Nb, Rb, Y and Zr and lower contents of Ca. Fe-rich garnet amphibolite resulted from reaction between sulphide and silicate. The variation in host rock chemistry between the three ore bodies can be modelled in terms of mixtures of cumulus olivine and plagioclase, basaltic liquid and syngenetic immiscible massive and disseminated sulphide. The host amphibolite represents a tholeiitic intrusive mush. Chemical variation within the host body was due to a combination of flow differentiation during much emplacement and later gravitational settling. Deformation resulted in physical remobilisation of massive sulphides and chemical mobilisation of Cu, Ni and S relative to Fe. The subeconomic Ni-Cu sulphide-bearing ultramafic rocks at Dikoloti and Lentswe were tholeiitic cumulates, which were intruded or thrust into the sequence. The Selebi-Phikwe and Dikoloti-Lentswe host rocks and sulphides were comagmatic with the country-rock hornblende gneiss (tholeiitic basalts and ferrobasalts). The concentration of S in the ore bodies was due to magmatic processes in periodically replenished tholeiitic magma systems. The rocks at Selebi-Phikwe are more like those of a greenstone belt than a high-grade quartzite-marble-anorthosite association. They have affinities with oceanic crust. (D76773)

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