NICKEL AND PGE DEPOSITS IN THE KABANGA-MUSONGATI-KAPALAGULU BELT, WESTERN TANZANIA AND BURUNDI

The ca. 1.4 Ga Kabanga-Musongati-Kapalagulu belt of mafic-ultramafic intrusions occurs within the Kibaran orogenic belt of western Tanzania and Burundi. The belt hosts Ni deposits (inferred resources ca. 36 Mt @ 2.8% Ni at Kabanga) and PGE reef deposits (at Musongati and Kapalagulu). Many of the intrusions are hosted by low- to medium-grade metamorphosed mica schists and graphitic schists which may contain abundant sedimentary sulfides. The intrusions consist of sills and sub-horizontal pipes of variable sizes. Dykes are rare. The smallest intrusions are 10s to 100s of metres wide and may host massive Ni sulfide ores (e.g Kabanga North). Mass balance considerations indicate large ratios of sulfides to silicate ratios and ultramafic to mafic lithologies in these bodies, suggesting emplacement of olivine-pyroxene-sulfide crystal mushes. The larger intrusions may be several km in thickness and host S-poor PGE reefs (Kapalagulu, Musongati). The parental magma to all intrusions were originally picritic (ca. 16.5% MgO, 52 % SiO2,), variably contaminated with the host rocks. The sulfide- and Ni rich intrusions assimilated up to ca 20% crust, resulting in a magma with ca. 14% MgO, 54% SiO2, Fo 70-90, Ni/Cu (in sulfides) ca. 8, Pd/Ir 5-20, strongly enriched incompatible trace element contents, distinct negative Nb-Ta and Ti anomalies, heavy S isotopic signatures (delta34S +10 to +24), and highly Ni depleted olivines (<100-1600 ppm Ni). The sulfide-poor, PGE enriched intrusions assimilated much less crust (ca. 5-15%), resulting in less fractionated trace element and isotope signatures, and in less depleted olivines. The key factors in Ni ore genesis are (i) Crystallization of the intrusions from relatively primitive (MgO, Cr-, and Ni-rich) mantle-derived magmas that were capable of heating the country rocks sufficiently for assimilation to occur, (ii) the presence of S-rich sedimentary country rocks that provided a source of external S, and (iii) the occurrence of relatively small and dynamic magma conduits in which magmatic sulfides could be concentrated to form economic deposits. The PGE reefs formed in less dynamic systems that assimilated less crust. The variable crustal component in the intrusions can be used to identify Ni versus PGE targets.