Tanzanian Regional Geology
The southern part of East Africa, extending from South Africa to Kenya, is comprised of a series of Archean cratons sutured by various Proterozoic to Paleozoic age mobile belts. The geology of Tanzania is presented by all the known chronostratigraphical units of the world ranging from Archean, Proterozoic, Phanerozoic to Quarternary ages.
The Tanzanian Archean Craton is the main element of Tanzanian geological architecture. The compound structure of the craton comprises a number of granitic, gneiss-granitic and greenstone complexes. The craton is segregated into northern and southern parts by units of Dodoman System, which is composed by highly metamorphosed (from amphibolite to granulite facies) Archean sediments. The northern part represents a Greenstone Belt, the Nyazian – Kavirondian Supergroup, comprising sequences of mafic to felsic volcanics, chert/banded iron formation and clastic sediments).
The craton is framed by Paleoproterozoic mobile belts, the Karagwe-Ankolean System to the northwest, Ubendian System to the southwest and Usagaran System to the east and southeast.
The Karagwe-Ankolean System is composed of metamorphosed schists, phyllites, argillites and quartzites. The rocks of the Ubendian System are characterised by metamorphic rocks ranging from amphibolite to granulite facies, gneiss with minor mafic and ultramafic intrusives and late stage granites. The Usagaran System is characterized by metamorphic rocks, including granulites, gneisses and quartzites. The eastern part of the Usagaran System is mobilised by the Neoproterozoic Pan African Orogeny (650-580 Ma) forming the Mozambique Belt with lithological, structural and metallurgical characteristics similar to that of the Ubendian System.
Lithologies in the Mozambique Belt (MB) consist of a mixed assemblage of mafic to felsic granulites, gneisses and migmatites (interpreted to represent volcanic protoliths)interlayered with amphibolites and metasedimentary rocks, including quartzites, pelites, graphitic schists and marbles.
The Paleozoic Bukoban System is developed along the cratonic margin is presented by sandstones, quartzites, shales, red beds, dolomitic limestones, cherts and amygdaloidal lavas.
Within eastern Tanzania, Mesozoic age graben structures developed as localized linear basins extend from north to south. The key rock types present are limestones, sandstones, shales and marls. These basins are overprinted by Usagaran System structures. Further to the south from the Rufuji Trough, the Mesozoic structures form two isolated branches. One of them continues to west south (Selous Basin and Ruhuhu Basin) and another one continues along the ocean beach (Mansawa Basin).
Cenozoic sediments and volcanic rocks, presented by intermediate to basic alkalic and intrusive rocks, are developed in the north part of Tanzania.
Neogene to Quarternary continental formations infill the youngest depressions and river channels composed of clays, limestone, evaporates and sands; volcanic rocks ranging in composition from lavas (basalts, andesites, and phonolites), tuffs, ash and pumice.
Mineral Resources and Potential
The Niassa Gold belt (NGB) is the most important gold district in the area. Artisanal mining of alluvial and primary gold in quartz veins has been performed since 1990 along Lago Niassa from the Tanzanian border towards Cobuè in the south, ~90 km north-south and up to 25 km east-west. The estimated total gold production is 5-12 t/a (Lächelt 2004). The most important gold mining areas currently include Cagurué, Long Bay/Miazini, M'Papa, and M'Popo in the northern part of the belt.
Primary gold occurs in quartz veins in low-grade metasedimentary rocks and associated mafic dykes of the assumed Neoproterozoic Txitonga Group. The veins are associated with north-south-/northeast-southwest-trending shear zones of probable Ordovician age, related to the juxtaposition of the NGB as an exotic tectonic sliver against the western margin of the Unango terrane. Re-Os dating of the sulphide mineralisation gives a preliminary age of about 483 Ma. The productive zones in the alluvial fields are 0.5-5 m thick, and the gold content of the alluvium is 5-30 g/t. Minor gold panning has been reported along several rivers in other areas: Rio Chimulicamuli, Lugenda, Rio Lureco. Slabs of finely laminated metasiltstone containing abundant disseminated malachite and with extensive malachite (and locally azurite) coatings are found 50 km northwest of Marrupa on the north bank of the Rio Lureco. The secondary copper mineralisation (>1 % Cu and ~100 g/t Ag) is clearly derived from disseminated copper sulphides in the host rock. This poorly exposed area is worthy of follow-up, especially as this mineralisation is similar to that seen in the Central African Copper Belt. Minor nickel-copper mineralisation occurs in mafic intrusives in various complexes, but these are not considered to represent a major potential.
Deformational and Intrusive History
The MB comprises a predominantly north-south trending domain of highly deformed and metamorphosed rocks, which formed during oblique collision of east and west Gondwana (Fritz et al, 2005). The MB includes lithologies that have been affected by both the Usagaran (1.8-2.0 Ga) and the Pan-African (650-580 Ma) orogenic events. Throughout the MB, formation ages of around 1.8 Ga have been reported, consistent with a late- Proterozoic (i.e., Usagaran) pre-collisional basement age.
The first stages of the MB formation occurred at 1,000-700 Ma. This initial stage was marked by large-scale magmatic intrusive activity, which may include the ultramafic-mafic intrusions. This was followed with two collisional phases recorded at 640-620 Ma and 580-530 Ma, which are largely responsible for the regionally observed complexly deformed tight fold structures dominant in the project area. The 640-620 Ma phase involved the onset of deep-seated thrusts and lateral shear, while the 580-530 Ma phase of final collision involved thrust propagation and exhumation. Peak metamorphic conditions to granulite facies are dated at 640 Ma.