Kabanga magmatic nickel sulphide deposits, Tanzania: morphology and geochemistry of associated intrusions

Mafic magmatism is a widespread feature of the Kibaran Orogenic Belt of cen tral Africa, some of which hosts important deposits of Ni and Co. This pape r describes mafic intrusions associated with Ni sulphides in northwest Tanz ania, and attempts to define the tectonic environment of their emplacement. The Kabanga Ni deposits occur in small, layered mafic-ultramafic intrusion s, comprising olivine and orthopyroxene cumulates. These intrusions are spa tially associated with widespread gabbro-noritic sills emplaced within the enclosing Meso-proterozoic intracratonic metasedimentary rocks prior to def ormation. The marginal rocks of the sulphide-bearing intrusions comprise ga bbro-norite and melanorite, similar in texture, lithology and trace element geochemistry to the mafic sills. Marginal rocks of the mineralised intrusi ons with textures indicative of rapid cooling of largely crystal-free magma (strongly acicular pyroxene and skeletal olivine) are siliceous high Mg ba salts, with enriched LREE and negative Sr, P and Ti anomalies, suggesting a metasomatically-enriched mantle source region and/or a strong crustal comp onent. The Kabanga sulphide-bearing intrusions are inferred to have been em placed as small feeder conduits supplying magma to larger adjacent and over lying gabbro-noritic intrusions within a foreland basin, prior to or during inversion and folding. (C) 2000 Elsevier Science Limited. All rights reser ved.