The Sukumaland Greenstone Belt (new name), which is located in north-western Tanzania hosts a large number of gold deposits, prospects and occurrences. The Archean stratigraphy comprises an older basement of granitic gneisses (Dodoman System) overlain by mafic and ultramafic volcanics which are succeeded by felsic volcanic rocks and a banded iron formation (BIF), predominantly as oxide facies. The volcanics and BIF together make up the Nyanzian System. The uppermost part of the Archean stratigraphy (Kavirondian System) consists of conglomerates and quartzites which rest unconformably on the older units. The sequence has been intruded by both syn- and post-orogenic granitoids and by several generations of felsic and intermediate dykes and sills. Gold mineralisation is abundant in all stratigraphic units except for the granitic rocks. Six types of mineralisation can be distinguished. These are BIF-hosted, hear zone type, (quartz-) vein type, clastic sedimenthosted alluvial/eluvial, and massive sulphide type deposits. The last is represented by only one deposit in this area. The location of the BIF-hosted gold mineralisation is controlled by trachytic rocks of both dyke- and sill-like appearance and brecciated shear zones which follow the contacts between BIF and intercalations of tuff. The host rocks around these trachytes and shear zones have been pervasively mineralised with pyrite, auriferous pyrite and native gold, selectively replacing magnetite layers. The mineralisation grades from massively replaced layers at the trachyte/BIF- or tuff/BIF-contact into disseminated layers, rare specks of pyrite and eventually into barren BIF. Permeable features such as veins, fractures, breccia zones and shear zones may contain mineralisation even away from the main ore zones. Ore textures and fire assay results from Geita and Jubilee Reef indicate the introduction of ore fluids via epigenetic conduits into the adjacent wall rocks. Neither a depletion halo in the adjacent BIF (lateral secretion) nor a zone of footwall alteration or stringer ore, which might support a syngenetic/replacement concept for the mineralisation have been found. The main precipitating mechanism was probably a redox reaction of the ascending, reducing ore fluids with the magnetite of the BIF. This reaction resulted in the replacement of magnetite by pyrite, a rise in Eh, and the precipitation of gold together with the pyrite. The authors propose an epigenetic mineralisation process which produced ore-grade mineralisation of different styles in all stratigraphic units. The special physico-chemical trap of the magnetite layers led to the development of pyrite/gold mineralisation of generally stratabound appearance. The ore fluids originated probably from granitic intrusions and the underlying mafic volcanics are a possible gold source. For textural reasons the surrounding BIF is unlikely to have supplied a major portion of the gold. © 1990 Ferdinand Enke Verlag Stuttgart.
