The geological framework of Tanzania reflects the history of this part of the African continent and elucidates the settings of mineralization. Tanzania's present geological settings is the result of a series of events, which began, with the evolution of the ancient Archaean craton. It was then modified by metamorphic re-working and accretion of other continental matter, which was later, covered with continentally derived sediments along the coast later covered the Karoo sediments. The process of sundering the craton along the East African Rifts began during the Tertiary times.

The geological history has been reconstructed by several generations of geologists, from the pre - 1900 times of foot traverses to the 1990s world of LANDSAT, isotopes and computers. Even within the past 30 years, great strides have been made in resolving questions, which vexed the earlier generations. For example, the term Basement Systemï has been abandoned, as some of the rocks within it were eventually recognized to be younger than other units and therefore not a proper  basement. The final story yet to be written. The summary presented here is only one frame of a continuing cinema.

Tanzania lies on the African Plate, one of the Earth as larger slabs of continental crust. The African Plate contains Archaean cratons over 2.5 billion years, which preserve evidence of rock forming events shortly after solidification of the Earths Crust. Ages of 3 billion years have been determined for some rocks in the Tanzania Craton. It is one of several highly mineralized cratons in eastern Africa, and is broadly similar to the Zambian.

Zimbabwe (Rhodesian) and Kaapvaal (South Africa) cratons.

East major sequence of rocks is described below with a summary. The surface distribution of these rocks.

As a result of countrywide airborne surveys in 1977 - 1980, the geophysical features of Tanzania are better understood that those of some western countries. An unexplained gravity high close to the boarder with Kenya and east of Lake Victoria is of interest.

ARCHAEAN.

The exposed Archaean of Tanzania consists of a granite - greenstone terrain in which linear belts of greenstones are set in a field of predominantly granite rocks. Most of the granitic rocks are younger than the greenstones, but a few may be older.

Dodoman Super group.

The rocks assigned to the Dodoman Super group are of Archaean age and appear to be older than the greenstones and the granites associated with them. The Dodoman is mainly sedimentary origin and forms a band across the southern part of the craton. These rocks now include gneesses,schists, amphibolites, migmatites and quartzite's. The Dodoman has been subjected to high - grade metamorphism and regional granization. Few mineral deposits of commercial interest known in the Dodoman Supergroup rocks, which require further geological investigation.

Nyanzian Supergroup.

This supergroup comprises the sequence of dominantly mafic volcanic rocks and immature sedimentary rocks, which form the greenstone belts of the central craton. The Nyazian greenstones are of major economic importance, as they host most of Tanzania gold deposits.The rocks can be subdivided into a lower and an Upper transition from mafic to felsic lavas, with minor tuffs and interbedded sedimentary rocks. The Lower series consists primarily of basalt, andesite and dacite pillow lavas. The sedimentary rocks include banded iron formation (BIF) recrystallized cherts, and some shale and conglomerate. They comprise the entire Nyanzian sequence in the western part of the Lake Victoria greenstone belts.

A maximum thickness of 5,000 m is estimated for the lower series in the Geita and Rwamagaza greenstone belts. The upper series of the Nyanzian sequence is characterized by the assemblage of felsic lavas, tuffs, ferruginous cherts, BIF and subordinate meta - pelites. The presence of BIF within units consisting mostly of felsic tuff is a consistent relationship. Maximum thickness of the banded iron formations is probably between 100 m and 400 m, whereas the maximum thickness of felsic tuff is on the order of 2,000 m to 4000 m. The greenstones are generally metamorphosed to greenschist facies, locally to almandine amphibolite facies.They are folded about steeply dipping axial planes, which define a generally east west grain.

Kavindian Supergroup.

These rocks occur in northernmost Tanzania (Musoma - Mara greenstone belt), but are more widely distributed in adjacent Kenya. They consist mainly of conglomerates,coarse arkosic and feldspathic grits and quartzites resting unconformably on the Nyanzian rocks from which they have been partially derived.

Granite Gneiss Terrane.

These rocks form the matrix surrounding the greenstones. As in most Archaean cratons, the age relations are confusing, with some granites clearly intruding greenstones but most of indeterminate affinity. Some geochronological ages are clearly Proterozoic, others are clearly Archaean. The Archaean portions of this terrane are locally difficult to distinguish from the Proterozoic granitic gneisses.

PROTEROZOIC.

Ubendian Supergraoup. This Palaeo - Proterozoic mobile belt bounds the Archaean craton on its southwestern side. It includes a variety of high - grade metamorphic rocks of both sedimentary and igneous origin and is thought to contain a large component of reworked Archaean rocks. The dominant lithology is gneess with minor mafic and uttramafic intrusions, late granites and rare marbles. Structural trends are mainly northwesterly. Metamorphism is mainly of the almandine amphibolite facies, rarely reaching the granulite facies.

Usagaran Supergroup.

Like the Ubendian, this supergroup consists of metamorphic rocks. It occurs south and east of the Archaean craton. Granulites and biotite gneisses of politic origin make up a large portion of the unit, with quartzites being common. The granulite facies of metamorphism is attained in a number of areas. Like the Ubendian, the Usagaran doubtless contains metamorphosed Archaean material. Structural trends are dominantly to the southwest. The usagaran has been compared to the highly metamorphosed Grenville Province of the Canadian Shield.

Rocks of the granulitic facies of the Ubendian and Usagaran Supergroups are rich in a variety of coloured gemstones.

Karagwe Ankolean Supergroup.

This supergroup forms part of the Kibaran Fold Belt, which extends from Uganda to Zambia, west of Lake Victoria, and underlies the western extremity of Tanzania. It is younger than the ubendian and Usagaran, and has obviously different lithologies and structures. The sedimentary features of the karagwe Ankolean rocks reflect shallow - water deposition, with argillites, phyllites, low grade sericite schists and quartzites. The granites have alteration haloes containing tin and tungsten mineralization in veins. The sedimentary sequence strikes to the north in general, but is deformed into ovoid domes by the granites. Resistant quartzites form ridges enclosing oval  arenas around these domes.

Bukoban Supergroup.

The bukoban which may span the Neoproterozoic - Palaeozoic boundary is weakly deformed and not metamorphosed. It includes sandstones,quartzites, shales, red beds, dolomitic, limestones, cherts and amygdaloidal lavas. The bukoban occurs mainly in the northwestern quarter of Tanzania.

Karoo Supergroup.

The karoo, named after the south African locality, reaches its northern limit in Tanzania. It consists of continental sedimentary rocks, ranging in age from Late Carboniferous to Jurassic,. At about the latitude of Dar es salaam, these rocks pass into marine rocks of the same age. The karoo sediments were deposited during a long period of denudation and erosion of crystalline highlands punctuated by episodes of glaciation, volcanism and marine incursions. Karoo sedimentary rock consist predominantly of coarse sandstones, shales and siltstones with coal. The karoo lies uncomformably upon Precambrian basement and is well known for its coal resources.

Upper Mesozoic.

Upper Mesozoic sedimentary rocks occur only in the coastal basins. The sedimentary rocks include limestone, sandstone, shale, marls and local evaporates (gypsum, anhydrite and salt). The basin in which these rocks accumulated apparently were formed during break up of Gondwana continent in Mesozoic time.

CENOZOIC.

Rift valley Faulting.

The break up of the eastern side of the African Plate during Mesozoic time was greatly accelerated during late Cenozoic time, and has an important effect on the economic geology of Tanzania. The East African Rift system consists of a series en echelon grabens or rift valleys, often with associated volcanism. In Tanzania, rifting is concentrated along two arms, the western rift occupied by lakes Nyasa and Tanganyika and the Eastern (or Gregory) rift, passing through Lake Natron of Lake Nyasa. There are also subsdiary rift grabens at Lake Rukwa, along the Indian Ocean coast in the Selous Basin of southeast Tanzania, and elsewhere.

The rifting has created Nyasa, Tanganyika, Rukwa, Eyasi, Manyara, Natron and others. It also disrupted the river pattern the extent that the four last named lakes have internal drainage. Lake Victoria is generally interpreted as being formed by gentle down warping between the Eastern and Western rifts. An alternative explanation is that it marks the site of a meteorite impact crater.

Sedimentary Rocks.

The rifts have been the sites of sedimentation during Cenozoic time. In addition, Karoo rocks are preserved only in these depressions. The coastal basins contain several kilometers of marine sedimentary rocks, mainly of Miocene and younger ages.

Igneous Rocks.

In East Africa, as elsewhere, rifting is accompanied by Volcanic activity and hot springs. There are two such areas in Tanzania: the Kilimanjaro region in the northeast and a small area of volcanics in the Rungwe area in the southwest. In both areas, the rocks are intermediate to mafic alkaline lavas, with local intrusions. Intrusion of diamond - bearing kimberlites was another, though a minor volcanic event. Some kimberlites are less than 50 million years old.

Surficial Deposits.

Extensive portions of the land surface underlain by Precambrian rocks have been exposed to weathering for tens of millions of years. The resultant ferricretes (laterites), silcretes (cement), and argillic layers are of great importance of the way they concentrate or lose metals. Some ores concentrate by residual enrichment, by secondary deposition, or by both. Examples are discussed in the sections on Gold Opportunities and Diamond Oppurtunities. These processes are important in the concentration of niobium (pyrochlore) and phosphate ( apatite) over carbonatite, or tin around granites, of nickel over ultramatic rocks, and of gemstones over metamorphic rocks.