URANIUM
Introduction.
A number of uranium occurrences are
known in Tanzania. The best references to radioactive minerals in the
country are in Harris (1961), Uranerzberbau GmbH (1979) and Geosurvey
(1981, 1982) and in various unpublished reports in the Geological Survey
files in Dodoma.
Four geological environments contain
potentially significant uranium prospects:
- In sandstones, mainly of the Karoo
Supergroup and Bukoban Supergroup . These are the "sandstone uranium,
model 30c" of Cox and Singer (1986);
- In carbonatite complexes of
Mesozoic to Recent age. These are the "carbonatite, model 10" of Cox and
Singer (1986);
- In calcrete related secondary
environments in Quaternary deposits; e.g. Ndala, Itigi and Iseke; and
- At the unconformity between the
Karagwe-Ankolean and Bukoban Supergroup for vein-like type of uranium
mineralisation: also part of the Ubendian/Bukoban unconformity.
In addition, minor occurrence of
uranium and thorium are known in pegmatites, river and beach sands. None
of these is likely to become economic, although small amounts of
hand-sorted uraninite were recovered from pegmatites of the Uluguru
Mountains before 1955.
An untested environment in Tanzania
is that related to unconformities in Proterozoic rocks classified as
"Unconformity U-Au, model 37a"of Cox and Singer (1986), or "unconformity
associated with uranium model 21" of Eckstrand (1984). The most likely
locus for deposits of this type would be at the base of the Bukoban
Supergroup in the northwest. Geosurvey International (1982, p 56-58)
present a summary inventory of radioactive mineral occurrences in
Tanzania, based on group follow-up of the Geosurvey's airborne of
potential exploration interest are in sandstones of the Karoo Supergroup
in the southwest, or in carbonatites. The latter are described briefly
above in the section on carbonitites.
Geosurvey International in 1981
investigated the Karoo sandstone deposits, but details of this work were
not published due to restrictions at that time. The files are available
for review at the Geological Survey in Dodoma.
In appearances and modes of formation
sandstones of the Karoo are remainiscent of the sandstones which host
uranium deposits in South African in the Colorado Plateau area of western
U.S.A. No systematic prospecting of the Tanzanian sandstones has been
attempted, but such work could be rewarding.
Bahi Swamp.
Bahi swamp is a dry lake, or playa,
over 1,000 sq. km in extent and bounded in the west by a Cenozoic fault
scarp. During an exceptionally dry season in 1953, the Geological Survey
drilled six diamond core holes (fig. 54) to test the underlying sediments
for salt and other minerals. Fawley (1954) reported results of the work.
In 1958, the cores were scanned for
radioactivity. An anomaly was located in Hole 19/53 at 224 feet (68 m)
depth. Analysis showed 0.23% U3O8 in 6-inch (15 cm) section of indurated
sediment which had been logged in "silcrete". Recovery of core from this
apparently contained in a felted layer of strontianite (strontium
carbonate) crystals. No strontium analysis was performed.
The occurrence of uranium with
strontianite is of more than academic interest, as either may be found in
economic concentration in playa or calcrete environments. The Yeelirie
calcrete uranium deposit in Western Australia, while not quite commercial,
is often cited as an example of a mineralizing process that could create
ore under suitable circumstances. Nodules composed of strontium carbonate
and sulphide occur in playa sediments at Bristol Dry Lake in southern
California, where they were formerly harvested for their celestite
(strontium sulphate) content. Thorough examination of the drill cores is
warranted to test for the presence of uranium and other minerals missed in
the initial investigation.
Madaba Occurrence.
Located in QDS 253 and ikn the
northern part of the Selous National Park, the area is accessed by a dry
season road from Dar es Salaam via Utete/Kingupira. The Madaba area is
part of the Luwegu-Mbarang'a ndu River basin of the East African Karoo
Supergroup. The target are is about 25 km SEof Madaba hill
Uranerzbergbau GmbH drilled eleven
bore holes of which seven penetrated favourable stratigraphic sections of
low grade U-mineralisation over thickness of 0.7 m to 12 m at depth below
the group water table. The best mineralisation has a grade of 0.040% U3O8
over 11.7 m. Within the anomalous section, the richest lens contained
0.122% U3O8 over 1.6 m from 79.5 to 81.1 m. (Gross, 1982).
Gallapo Carbonatite.
Uranerzbergbau GmbH
investigated the Gallapo carbonatite in 1978. Based on geological mapping,
the carbonitite rock reserves are estimated at 65 million metric tonnes.
Group radiometry of the two Gallapo carbonatites dyke averages 1000 cps on
a 400 m long traverce across the northern dyke and 350 cps on a 1 km long
traverce on the south carbonitite dyke. The uranium content in the samples
taken along the traverce on the northern dyke ranges between 5 and 283 ppm
U3O8AND AVERANGE 100 PPM. The uranium content in the samples taken along
the traverce on the southern dyke range between 5 and 177 ppm U3O8 and
have an average of approximately 25 ppm U3O8.
Mkuju Occurrence.
The Mkuju occurrence is located in
quarter degree sheets 278/3&4, 289/1&2 and 290/1. The geology of this area
generally consist of sandstones, which are sub-divided into the Mkuju
andMbarang'andu groups of the Upper Karoo. Calcareous components are very
rare in the Mkuju Group, although commonly encountered in the Mbarang'andu
Group. Both groups show typical features of fluviatile depositional
environments.
The investigations found over ten due
to thorium and uranium, with background values ranging between 75and 120
counts per second (cps). The conclusion drawn from the survey is that all
the Mkuju proper anomalies are confined to two litostratigraphical
units-the Upper and Lower Mkuju Groups.
Further work to investigate the
stratigraphic position of the mineralized Mkuju Group is recommended.
Drilling should be done to investigate a possible control of anomalies by
redox boundaries (Seidl, 1981).