Introduction.
Carbonites are intrusions of carbonate composition. They are valuable as
potential sources of a wide range of commodities, especially the REE of
which these rocks are the primary source. Important examples of
carbonatites include Mountain Pass in California, an important producer of
REE, Palabora in South Africa, a producer of REE, Palabora in South
Africa, a producer of copper, phosphate and vermiculite. Tanzanian
carbonatites have also been considered as possible sources of
niobium,tantalum,phosphorus, uranium and thorium, together with calcium
and magnesium carbonates.
Most
carbonatites are typically of Mesozoic or Cenozoic age and occur on stable
cratons adjacent to rift faults. A typical carbonatite complex consists of
a large area of carbonate altered (fenitized) country rocks, within which
is a concentric series of alkalic silicate intrusions. A much smaller core
of intrusive carbonate may or may not be exposed. Common practice is to
refer to each occurrence as a carbonatite, even though the pure carbonate
core may not be exposed.
- In the
case of Tanzania, carbonatites are closely associated with three main
features:
- The
major African Rift (faulting) System;
- The
presence of lavas ranging from Tertiary to Recent in age; and intrusion
into Precambrian terranes (Mozambique belt in the east and Ubendian in the
west).
At least
21 carbonatites have been identified in three areas of Tanzania, as shown
in Fig.46. All the areas are near the branches of the East African Rift
System. These are:
- East -
Central; principally in the Morogoro Region;
- Near
the northern end of Lake Nyasa and the edge of the Rukwa trough; and
- Along
the northern part of the Eastern Rift Valley, associated with recent
volcanic activity.
Geological investigations have been conducted to evaluate the economic
potential of some of these carbonatites UNDP (1982), Chesworth et.al.
(!988), van straaten et al. (1992) and Mchihiyo et.al. (1992). Exploration
has indicated a significant economic potential for several Tanzanian
carbonatites. Some of these are shown in Table 5. Below is a brief review
of the better - known carbonatite complexes with their contrasting
features.
Oldoinyo Lengai.
This lies
on the Eastern Rift Fault System on the southern part of Lake Natron. The
carbonatite occurs in agglomerates as ejected blocks. It is composed
mainly of natrocarbonatite with a high content of fluoride (Sodium
fluoride). It is a rare example of an active carbonatite volcano. Volcanic
activities have been recorded since 1880. Lavas consisting mainly of soda
ash and carbon dioxide were extruded in 1960.
Gallapo.
The
Gallapo body is near the Gallapo Mission in Babati, Manyara Region. The
carbonatite dykes intrude Usagaran rocks. New consolidated Goldfields
Limited conducted estimation of reserves. It was deduced that there are 40
Mt of tuff and soil materials containng 0.12% Nb205. Niobium within the
soviet (calcium carbonate rock) is 0.06%. The soils in the area contain a
number of elements including niobium, phosphorous, nickel, titanium,
vanadium, and molybdenum,lead, copper, cobalt,beryllium, thorium and
uranium in high concentrations.
Nachendezwaya.
This
carbonatite is situated in the Itumba area, Rungwe District near Songwe
River in Mbeya Region. The carbonatite is composed mainly of soviet, which
grades into rauhaugite. Minerals of economic interests are apatite,
carbonates and niobium minerals. The Rudis Company, in 1979 estimated the
reserves to be 43,483 tonnes containing 0.03% Nb205 and 5.1 Mt at grade of
8% P205.
Wigu Hill.
This
carbonatite is in the Uluguru Mountains in the Morogoro Region. Wigu Hill
was identified as a carbonatite in 1955 and subsequently studied for its
rare earth potential by New Consolidated Gold Fields in 1956 and 1957. The
carbonatite was intruded into structurally weak zone within Usagaran
granulites. It consists of an array of carbonatite dykes, which have
brecciated and altered the host rocks.
Carbonatite dykes rich in rare earth minerals (Bastnaesite, monazite, and
florenceite) occur in the centre of the complex. The Rare - earth Oxide
(REO) content is locally as high as 16.3%, including a high component of
cerium. Phosphate content at Wigu Hill is low with most analyses
reportedly less than 1.0% P205.
Wigu Hill
has been compared to the Mountain Pass deposit in California, in terms of
its geometry and rare earth enrichment.
Mbalizi.
The
Mbalizi carbonatite is cut by the main Tanzania - Zambia highway 10 km
west of Mbeya. It was discovered in 1972 by Soviet geologists.
The
Mbalizi and Songwe Scarp carbonatites both occur along the strike of the
eastern escarpment of the Rukwa Rift, and it is probable that they have a
common origin. Mbalizi may be the plutonic feeder for the extensive high -
level dyke system of the Songwe Scarp carbonitite.
The
Mbalizi Carbonatite is approximately 1.1 by 0.4 km in extent and consists
mainly of coarse - grained soviet. Its complex multiphase intrusive
history, accompanied by large - scale fenitisation of gneissic country
rocks, it typical of many deep - level carbonatites. The central
pyroxenite was locally altered to phlogopite - apatite rock. Coarse -
grained apatite - phlogopite - feldspar rocks to the west of the exposed
carbonatite are considered to be fenitised bedrocks.
The
centre of the exposed carbonatite is largely soviet, parts of which are
characterized by dendritic crystal growth rich in apatite, with P2O2
(Phosphate) Values up to 9%.
Weathering of the soviet (calcite) has resulted in development of a 0.5 m
thich limonitic crust, which places contain up to 30% P2O5.
Musensi.
Musensi
Hill is a prominent feature 42 km west of Mbeya. The area was explored by
New Consolidated Goldfields in 1956 - 1957 for niobium, precious metals
and base metals.
Musensi
Hill is composed mainly of weathered volcanic rocks. Plugs and vents of
feldspar - rich volcanic rocks occur, as do agglomerates and tuffs. The
Volcanic rocks are intruded by a red, ankeritic (Iron-rich) carbonatite.
Values in excess of 1% P2O5 are uncommon, while rare earth oxide values of
0.35% and barium values of 0.3% have been reported. Musensi Hill is an
iron - rich carbonatite exposed near the level of extrusion.
Panda Hill.
Also
referred to as the Mbeya carbonatite,this intrusion is 25 km west of
Mbeya. In 1952, the �crystallized limestone� was recognized as a
pyrochlore - bearing carbonatite. Between 1955 and 1963, 7,400 m of
drilling were completed along the detailed mapping and erection of a pilot
plant for recovery of niobium - bearing pyrochlore.
The
carbonatite is roughly circular,with a diameter of approximately 1.5 km.
It is surrounded and partly overlain by fenitised Proterozoic gneisses.
Its form appears to be that of a steeply dipping plug with an inner ring
of soviet. Surrounded by magnesio - carbonatite and by a swarm of ring
dykes of various compositions. Explosion vents made up of fenitised
basement rock fragments in a ferrocarbonatite matrix are exposed in the
central sovite and at the southern margin of the complex.
Work was
resumed in 1978 - 79 by a STAMICO - Yugoslav team. They estimated a
resource of 480 Mt grading 0.33 % Nb2O5 and 3.5% P2O5. The highest
concentrations of pyrochlore and apatite occur in the fenites, which form
the shallow roof cap and in the sovite zone directly beneath the cap zone.
Zizi
This
carbonatite lies near Kisaki, south of Morogoro, in an area of Usagaran
metamorphic rocks. When first found in 1931, it was considered to be an
apatite - rich bed of marble. It was evaluated several times for its
phosphate and limestone / marble potential, before its intrusive nature
was recognized.
The
deposit has the form of a 30m by 1,100m lens, concordant with the
metamorphic rocks. The apatite-carbonate rocks are coarse grained and
light grey in colour. A nepheline-bearing biotite foyal occur at the
southwest margin of the phosphatic carbonatite.
An
investigation by STAMICO and a Yugoslavian team in the late 1970s resulted
in an estimated resource of 57 million tones of 7.38% P2O5. Chemical
analyses show relatively low values for rare earth elements. Although
relatively near the TAZARA railway line, the narrow width and steep dip of
the lens would make mining expensive.-
Summary.
Carbonatites are best known for their non-metallic resources, but they are
also major contributors of high-value commodities such as rare earths,
niobium, zircon and even copper. Potential exists for discovery of these
minerals, as well as carbonites and phosphate for industrial purposes.