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.