GEOLOGICAL ENVIRONMENT
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 System� 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.