A number of enclosed basins occur in Tanzania, mostly in semi-arid
areas where drainage to the sea has been disrupted by Cenozoic faulting
related to Rift Valley formation. Many of these have saline lakes or mud
flats in them. Lake Natron is a well-known example, having brines rich in
sodium carbonate and chloride. Harris (1961) mentions several others.
Trona (hydrated sodium carbonate/bi-carbonate) is harvested on commercial
scale from Lake Magadi, Kenya, close to the Tanzanian border near Lake
Natron. Salt is produced from several areas in Tanzania both from seawater
evaporation and salt spring inland as at Uvinza.
A number of other enclosed or partially enclosed basins are worth
investigation for brines or salts, either for export or for use in
domestic industries. In addition, great thickness of rock salt have been
discovered in drill holes in Jurassic strata of the Mandawa coastal basin.
The other evaporite resources are principally located in enclosed basins
of the Eastern Rift Valley, although other salt occurrences are also
known. Several are discussed in report by Harris (1961), as well as by
ESAMRDC (1982 & 1983).
Like Lake Magadi in Kenya, this sizeable has long been considered a
potential source of sodium carbonate. The lake brines contain (anhydrous
basis) 20.1% Na2CO3, 11.2% NaCl, 0.41%K, 0.25%NaF, and other constituents.
The water is surrounded by a salt crupt up to a metre thick with an
average composition (anhydrous basis) reported as 58.6% Na2CO3, with
lesser amounts of sodium bicarbonate, sodium sulphate and sodium chlorite.
The sodium fluoride content of 1.78%, high due to the volcanic origin of
most of the salts, is a potential quality problem. Investigations have
resulted in an estimate of 168million tonnes of to total salts in the
crust and brines. Problems of access, energy supply, and distance from
markets have delayed development of the resource.
The southwest part
of this enclosed lake contains salt crusts and brine pools with carbonate,
bicarbonate, chloride, and sulphate of sodium, and calcium chloride. The
total dissolved solids in some brine pools are reported to be 432g/l, with
about 80% as sodium chloride. The sodium fluoride content of 0.4% in salt
crusts is too high for human consumption.
The lake is situated in north central Tanzania, 80 km south of Lake
Eyasi in the Eastern Rift Valley. Preliminary estimates show total
reserves of 580,000 tonnes consisting of 225,000 tonnes of sodium
chloride, 132,000 tonnes of sodium sulphate, 121,000 tonnes of sodium
bicarbonate, 15,000tons of potassium chloride and 1,800 tonnes of sodium
Bahi swamp in central Tanzania covers an area of over 1000sq. km. It is
bounded to the west by a Cenozoic fault scarp. The crusts of salt are
formed by evaporation during the dry season and are gathered by local
residents for domestic consumption. Brines suitable for common salt
production have been found locally at shallow depths of about 1-3 m. These
brines contain up to 28g/l NaCl and less amounts of Na2CO3, Na2HCO3 and
Na2SO4. The brines are suitable for production of table salt. The
Geological Survey of Tanganyika drilled six diamond core holes to test the
underlying sediments for salt and other minerals in the areas (Fig 48).
The deepest hole penetrated 109 m of sediments before reaching granitic
bedrock. Further investigation of the brines is warranted.
Mandawa Rock Salt.
According to Harris (1961), test drilling by petroleum companies has
revealed the existence of great thickness of rock salt in Mesozoic
sedimentary rocks near Mandawa, about 200-km northwest of Mtwara. A
salt-bearing section, with layers of relatively pure halite up to 200m
thick was encountered at depth of about 100m below surface and continue to
300 m depth.