Lithogeochemistry and origin of the komatiites from Mundonguara mine in the Manica greenstone belt, Mozambique

Abstract

The Manica greenstones belt in western Mozambique constitutes the eastern extension of the Odzi-Mutare greenstone belt in Zimbabwe that is one of several Archean greenstone belts within the Zimbabwe Craton. These greenstones are in Mozambique constituting the Manica Group and are subdivided in two main lithostratigraphic units: The Macequece Formation and the Vengo Formation. The former is hosting the Mundonguara Cu-Au mine and is dominated by volcanic rocks, while the younger Vengo Formation is consisting of epiclastic sedimentary rocks. This paper considers the character and origin of the ultramafic, mafic, and felsic rocks within the Macequece Formation. They include peridotitic komatiite, pyroxenitic komatiite, komatiitic cumulate rocks, gabbroic dykes, rhyolitic units, and a granitic rock intruding the komatiites. Samples of these rocks have been collected from outcrops and drill cores and are investigated through petrographic studies of thin sections and whole rock geochemistry including major and trace elements to interpret the geological environment and tectonic setting.

The supracrustal rocks are metamorphosed to greenschist facies and the komatiites consists of varying proportions of serpentine, talc, chlorite, and amphibole. Primary features are partly preserved, with spinifex, vesicular, and cumulate textures. The komatiites are variously affected by carbonate alteration and deformation and the rhyolitic rocks are mostly strongly silicified. The komatiites are of the Al-undepleted type, with a MgO content of 25–45 wt %, while the mafic intrusions are tholeiitic in character, varying from gabbronorite to diorite in composition. Trace element diagrams used for interpretation of tectonic setting gives ambiguous results that could be an effect of crustal contamination of the ultramafic and mafic magmas. Using diagrams less sensitive to crustal contamination suggests the mafic and ultramafic magma to have a mantle source Minor rhyolitic rocks are chemically similar to granitic rocks intruding the komatiites and might have a mainly crustal magma source. This suggested that the Manica greenstones belt formed from magmas generated by mantle plume activity in a continental rift setting and were deposited on older Archean continental crust. These rocks are tentatively correlated with the Bends or Brookland formations belonging to the 2.9–2.8 Ga Mtshingwe Group in the Belingwe greenstone belts in Zimbabwe.