Tanzania geology reflects the geologic history of the African continent as a whole. Its present appearance is a result of a series of events that began with evolution of Archean shield, followed by its modification through metamorphic reworking and accretion of other continental rocks, in turn covered by continentally derived sediments. Pre-rift magmatism followed by active rifting has also left a major mark upon the Tanzanian landscape.
Several regional geological mapping programs have been carried out across the country over the past one hundred plus years, which has led to the recognition of several major litho-structural provinces from Archean to recent age. The Archean craton covers most of the western two thirds of the country, roughly bounded to the east by the East African Rift. Archean rocks host all of the country’s kimberlite pipes and contained lode diamond deposits, and most of its lode gold deposits. The Archean basement terrain is bounded to the east and west by a series of Proterozoic mobile belts; this area, particularly that to the east, hosts most of the country’s wide variety of colored gemstone deposits. Some recent research suggests that portions of this assumed Proterozoic terrane may actually consist of Archean crust that has undergone a later phase of higher grade metamorphism.
The Phanerozoic is represented by a series of sedimentary units of Paleozoic to Mesozoic age, in turn followed by a pre-rift period of kimberlitic and related, alkalic, mantle-derived intrusive and extrusive activity that presaged active rifting. Rocks related to this event intrude up to Upper Mesozoic and Lower Cenozoic sedimentary formations. Next, came a period of rift-related intrusive and extrusive activity concentrated in the Arusha area – to the northeast and Mbeya area – to the southwest, which is responsible for mountain-sized volcanoes such as Mt. Meru and Mt.
Kilimanjaro. Finally, a wide variety of recent and largely semi- to un-consolidated wind, water, and weathering-derived recent formations are found across the country, a number of which host placer gold, diamond, and colored gemstone deposits.
TANZANIA GEOLOGY – ARCHEAN
The Archean rocks (greater than 2.5 billion years old) of Tanzania consist of granite greenstone belts in which linear belts of greenstones (volcano-sedimentary) sequences, are found within a larger region of predominantly granitic rocks; most of the granitic rocks are younger than the greenstones but a few may be older.
The Dodoman System forms an east southeast-trending spine of lowermost Archean age rocks across the lower one third of the Archean craton (in this instance, greater than 3 billion years old); it appears to be older than the greenstones and their surrounding granites. The Dodoman system is comprised of rocks of mainly sedimentary origin, along with mafic volcanics and ultra-mafic intrusives. High grade metamorphic rocks such as granulites, and garnet-amphibolite gneisses are prevalent, as well as greenschist-facies talc, chlorite, and sericite schists. The Dodoman System contains few mineral deposits of commercial interest.
The Nyanzian System comprises a series of typical Archean volcano-sedimentary sequences, or greenstone belts, within a much larger area of granite-gneiss complexes (Buganda Toro); it is between 2.6 to 3 billion years in age. Evolved volcanic complexes comprising mafic through to felsic submarine and subaerial volcanic rocks, derived volcaniclastic and sedimentary rocks, iron formations, etc., along with associated intrusives of a variety of intermediate to felsic compositions, comprise the greenstone belts. The rocks can be divided into a Lower and an Upper Series on the basis of a recognizable upward transition from mafic to felsic lavas, with minor tuffs and interbedded sediments. The Lower Series consists primarily of basalt, andesite and dacite pillow lavas. The sediments include banded iron formation (“BIF”), recrystallized cherts, and some shale and conglomerate. The Upper Series is characterized by an assemblage of felsic lavas, tuffs, ferruginous cherts, BIF and subordinate meta-pelites.
The greenstones are generally metamorphosed to greenschist-facies and are folded about steeply dipping axial planes, which define a generally east-west fabric.
The Nyanzian greenstones are of major economic importance, as they host most of Tanzania’s gold deposits, and almost all of Tanzania’s known kimberlites – diamondiferous or not, are hosted within rocks of this system.
Buganda Toro (Granite Gneiss) Terrane
The Buganda Toro comprises an intensely folded, generally east west-trending series of rocks, which surround the Nyanzian greenstone belts. Age relations are confusing: some age dates are clearly Proterozoic, others are clearly Archean.
These rocks occur in northernmost Tanzania. They consist mainly of conglomerates, coarse arkosic and feldspathic grits and quartzites, along with minor granitic (some gold-bearing) and volcanic rocks. This system lies unconformably over the Nyanzian.
TANZANIA GEOLOGY – PROTEROZOIC
Rocks of this system make up much of the central and eastern part of Tanzania. The system includes a variety of high-grade metamorphic rocks of both sedimentary and igneous origin. Amphibolite grade metamorphic assemblages predominate, and are related to granitization and migmatization that occurred during the Pan-African tectonothermal event, the same that affected the Mozambique mobile belt. Structural trends are mainly north-south. Rocks of this system host a wide variety of colored gemstone deposits, as well as a number of gold deposits along its entire extent, from Kenya south across Tanzania and into Mozambique.
As mentioned above, there is recent research that postulates a significant component of re-worked Archean basement in the Usagaran. This is proposed in the Handeni area, specifically, where it is hypothesized that gold mineralization observed therein may be a more highly metamorphosed equivalent of the more typical shear-hosted and banded iron formation related gold deposits found in the Lake Victoria gold district, a few hundred kilometers to the northwest.
Physically contiguous with the Usagaran, rocks of the Unbending System comprise Lower Proterozoic to Archean mobile belt rocks that bound the Archean craton on its southwest side. The Ubendian includes a variety of high-grade metamorphic rocks of both sedimentary and igneous origin. The dominant lithology is gneiss with minor mafic and ultramafic intrusives. Metamorphism is mainly of the garnet-amphibolite-facies, rarely reaching the granulite facies. Structural trends are mainly northwest-southeast.
Similar to the Usagaran, Ubendian rocks host a wide variety of colored gemstone deposits, as well as a number of gold and base metal occurrences/deposits.
South of the Archean craton, where the northeastern shore of Lake Nyasa forms the boundary of southern Tanzania, rocks of the Ubendian and Usagaran systems merge into a zone dominated by an east-west structural fabric, where numerous synorogenic granitic intrusive complexes are also found.
This system extends west of Lake Victoria, and is found along the northwest boundary of Tanzania with Burundi, Rwanda, and Uganda. It is younger than the Ubendian and Usagaran, consisting mainly of argillaceous and arenaceous formations. The sedimentary features of the Karagwe-Ankolean rocks reflect shallow-water deposition; low-grade metamorphism has converted many of these units to sericite schists, and quartzites. Granite complexes intrude rocks of this system, and host tin and tungsten mineralization in veins within alteration haloes which surround the intrusives. Rocks of this system are dated at 1.3-1.4 billion years.
TANZANIA GEOLOGY – PHANEROZOIC
Bukoban System rocks overlie those of the Nyanzian, mostly in northwestern Tanzania but in Kenya and Uganda, as well. They include an unmetamorphosed series of Infracambrian (spanning the Precambrian-Phanerozoic boundary) platform sediments – terrestrial and marine sedimentary formations, comprising sandstones, quartzites, shales, red beds, dolomitic limestone, and cherts, etc, along with a sequence of amygdaloidal basalts, and gabbroic to doleritic sills and dykes; several copper occurrences and deposits are associated with these mafic rocks.
The Karoo System flanks and overlies the Usgaran to the immediate east, in a broad southwest-northeast swath along Tanzania’ east coast. It comprises continental sediments ranging from Late Permian to Jurassic in age. The Karoo sediments consists predominantly of coarse sandstones, shales and siltstones with coal, deposited during a long period of uplift and continental erosion; they are entirely of continental origin, to the north, and locally marine, to the south. The Karroo lies unconformably upon the Precambrian basement and is well known for its coal resources.
In western Tanzania there are no Karoo age sedimentary rocks; however, there are a series of dolerite dyke swarms concentrated on the Iramba Plateau and the Crater Highlands, both shoulders of the Eyasi graben and in the Musoma area.
Mesozoic Kimberlite Pipes
There are six provinces of kimberlites in western Tanzania. These are the Shinyanga- Mwadui, Mabuki, Speke Gulf, Lake Eyasi and Iramba Plateau kimberlite provinces; several carbonatite intrusive complexes have been recognized and mapped, as well.
Kimberlites have been mapped only in those areas where they outcrop, usually in the form of truncated hills; elsewhere, they have been plotted on the basis aeromagnetic and/or heavy mineral data.
The overwhelming number of the more than 400 kimberlite pipes that have been identified in Tanzania, and all of the known diamondiferous ones, are found within the Archean craton; most pipes are believed to be more than 50 million years old.
However, both a number of kimberlite pipes and carbonatite complexes occur in Proterozoic belts, and at least one known kimberlite is found within an area of Cenozoic rift-related volcanics.
Upper Mesozoic Sedimentary Rocks Upper Mesozoic sedimentary rocks occur only in the coastal basins. The sediments include limestone, sandstone, shale, marls, and local evaporites (gypsum, anhydrite and salt).
The break-up of the eastern side of the African Plate during Mesozoic time greatly accelerated during Late Cenozoic time and has an important effect on the geology of Tanzania. The East African Rift system consists of a series of grabens; in Tanzania, rifting is concentrated along two arms, the Western Rift occupied by lakes Nyasa and Tanganyika and the Eastern Rift, passing through Lake Natron to Lake Nyasa. These rifts have been the focal points of sedimentation during Cenozoic time, in what were coastal plains and inland basins at the time.
Rifting was accompanied by volcanic activity and the associated development of numerous hot springs. Miocene to recent alkalic and sub-alkalic extrusive and shallow intrusive rocks are found concentrated in the Arusha area, in northeastern Tanzania, and the Mbeya area northwest of Lake Nyasa, in southwestern Tanzania. In the former, voluminous and widespread volcanic activity is manifested in numerous volcanic cones, hills, and mountains, including Mt. Kilimanjaro, Africa’s largest subaerial volcanic complex and tallest mountain, reaching a height of over 18,000 feet above sea level.
Recent A hot climate, long periods of steady rain during the monsoon period (March through May) followed by a stretch of hot, dry weather, has led to the development of deeply weathered rock formations and thick overlying lateritic and related soil horizons. The long, evolved drainage history of the country, greatly affected by relatively sudden and dramatic changes in geomorphology due to tectonic events preceding, accompanying, and following rifting have led to a complex series of fluvial, elluvial and alluvial deposits, which host a wide variety of placer deposits (gold, diamonds, colored gemstones, etc.).