MUSOMA-MARA
GREENSTONE BELT
Introduction.
The Musoma and Mara areas, which are geographically separated, show
sufficiently similar geology so as to be considered a unit (Fig. 17). Gold
occurs in a variety of geological setting, being found in Banded Iron
Formations (BIF) and shears, as well as the mafic volcanic rocks. The
important town of Musoma lies on the shore of Lake Victoria, west of the
area. Travel within the area is generally slow due to the poor condition
of the roads.
The Mara area enjoys rain throughout
much of the year. Gold was discovered at Buhemba in the early 1920s.
Mining at Buhemba, Kiabakari and elsewhere began on a small scale during
the early 1930sand later commenced at Nyasenero, Mrangi(Phoenix Mine),
Simba Sirori, Golden Glory, and many other lesser localites. At Kiabakari,
large-scale mining began in 1959 and continued until1966. Throughout the
area there is widespread
Figure 17:Musoma-Mara Greenstone Belt
Work by artisanal miners. Cumulative
production from the Musoma-Mara greenstone belt has been 9 to 10 tones of
gold and a minor amount of silver.
Knowledge of the bedrock geology comes
mostly from result of reconnaissance mapping on the scale of 1:250,000 and
1:125,000 by the Geological Survey and brief unpublished reports by
government geologists and mine inspectors. Further work was carried out by
the UNDP (UNDP, 1968) in support of an airborne geophysical survey. Some
ground geophysics was performed in selected areas and a few diamond drill
core holes were drilled.
Geology.
A wide variety of rocks
and structures occur in the Musoma-Mara greenstones. The oldest rocks,
which host the gold deposits in the south (Musoma district), are the
Nyanzian Super group, a sequence of basic and felsic flows and tuffs with
one or two well-developed horizons of magnetite-facies BIF containing
minor pyrite. To the north, in the Mara district, these rocks are
succeeded by gneisses and schists which are dubiously assigned to the
Nyanzian in the Mara district, the Nyanzian greenstones are overlain by
coarse clastic sedimentary rocks derived from them. These Kavirondian
Supergroup molassic rocks extended northward into Kenya. They are not
known to be mineralized. All these rocks have been metamorphosed to green
schist facies (higher in the Mara district) and folded along axes trending
northeast. Folds of lesser magnitude with other trends are also present.
Geophysical survey suggest that two
parallel fault zones strike 060 degrees from Musoma to the Mara areas
(Fig. 17). Many gold occurrences are closed related to these structures.
Another set of faults striking 130 degrees is suggested by prominent
linear features. The most prominent of these Suguti Shear Zone, is marked
by a topographic depression and displaces the rock units on opposite
sides, suggesting substantial movement. Other shears with the same strike
direction include several in the Golden Glory area.
Mineralization.
Most gold deposits in the Musoma-Mara greenstone belt show mineralization
styles typical of Archaean greenstones. They are characterized by flat or
steeply dipping quartz lodes carrying pyrite, gold with minor base-metal
sulphides and carbonate alteration of wall rocks, and are apparently
related to region fault. The association of some gold lodes with BIF unit
is also typical
Fracturing has been the prime are
control at most of the deposits. The 060-degree and 130 degree faulting
played an important role in controlling the distribution of gold deposits
in both the Musoma and Mara areas. Compilation of geology and mineralized
localities show these zones to be loci for most of the important mines.
The intersection the two trends near Golden Glory contain the greatest
concentration of prospects (Fig 17). It is interesting to note that the
intersection is near the crest of what could be interpreted as an
anticlinorium, at the point where the northern (Mara) and southern
(Musoma) greenstones join.
An example of regional fracture control
occurs near the former Nyasirori Mine, about 15 km northeast of the Suguti
Shear Zone. Two bodies of magnetic BIF are separated by a probable 060
degree fault with a left-handed displacement of about 8 km. The projection
of this fault passes through the Buhemba open cut and coincides with the
known shear containing gold mineralization at the Golden Glory and Mara
mines in the Mara district nearly 60 km to the northeast (Fig. 17).
The close association of gold
mineralization with these faults and their projections along a strike
distance of nearly 100 km suggests a genetic relationship, which was not
recognized during the period of prospecting and mining activity prior to
1960. Recognition of this association provides a useful exploration tool
for newcomers.
Possible exceptions to the regional fracture control are
the Kiabakari Mine and some other minor prospects. At these places, gold
mineralization occurs in beds of chert or siliceous iron formation without
obvious regional control (Van Straaten,
1984).Opportunities within the Musoma-Mara greenstone belt are described
below, from east to west. Many of the accounts are modified from Hester
(1990).
North Mara Gold Mine.
The mine incorporates the old Nyabirama and Nyabigena
mines. Afrika Mashariki Gold Mines has been exploring the area since 1996.
Reserves of 4.7M ounces of gold have been proved for both Nyabigena and
Nyabirama. The company developed an open cut mine at Nyabigena (North Mara
Gold Mine) and production started in the year 2002
with an annual production of 140,000troy ounces (4.3 tones of gold per
year). Placer Dome Inc. of Canada has acquired the mine in 2003
Buhemba Gold Mine.
Between 1995 and 1998, a detailed
geochemical, geological and geophysical investigation programme
accompanied by RAB and RC drilling by Tanganyika Gold Limited identified
new mineralized areas adjacent to the old mines of Buhemba and Nyasenero.
South of the old Buhemba mine, RC drilling led to the discovery of blind
mineralisation at Buhemba South Deposit whose resources are approximately
130,000 oz gold.
At the Mwizi deposit, located to the
southwest of the old mine and northwest of gold Nyasenero mine, resources
of 43,000 oz Au were identified. Resources at the kilimongo deposit were
estimated at 157,000 oz Au. The total resource assessment for the entire
Buhemba deposits, as of September 1997, was 11.42 m tones of ore
containing 747,000 ounces of gold. More than 80% of these resources occur
within 60 minutes below surface.
Furthermore, re-assessment of the
structural and lithological controls on gold mineralization at Buhemba by
RAB drilling identified a new zone (Hakika Prospect), of shallow, almost
horizontal, Buhemba - style mineralization. The highest grade of 3.35 g/t
gold was obtained over 3 metres from a depth of 16m to 19m., within a
length 600 m between the kilamongo and Mwizi deposits. The RAB drilling
programme also identified a possible extension of the Mwizi deposit to the
southeast , with an intersection of 2 m at 2.27 g/t gold.
So far a resource of 0.75 M oz. and a
proven reserve of 0.2 M oz have been established. An open cast mine at
Buhemba has been in operation since 2003 with an annual production of
80,000 oz. The mine is owned by Meremeta Limited of South Africa.
Kiabakari.
Kiabakari is now the site of an army
base, and permission to visit is difficult to obtain. It has little value
as a mineral prospect, but the the geology suggests a thick, well -
mineralized, metallurgically amenable horizon, which is a desirable
exploration target due to its suitability for low cost mining. It is
likely that Kiabakari is not a unique example.
Gold mineralisation occurs in vertical,
tabular bodies within a rock termed adinole (albitized amphibolite) by
the mine geologists. About 7% of the ore consisted of sulphides,
predominantly pyrite and pyrrhotite with lesser arsenopyrite and base
metal sulphides, but was recoverable by Cyanidation without recourse to
flotation. At both strike extremities; the outcrop of the ore horizon
terminates against laterite cover, which has not been prospected. The
country rock is sericite schist.
Ikungu:
The ikungu Prospect lies on the shore of
Lake Victoria 18 km southwest of Musoma, in a small greenstone area
isolated from the main Musoma Mara greenstone belt (Fig. 17). Nine holes
were drilled in 1994 by a joint venture between Sampo Resources Ltd. And
Patrician Gold Mines Ltd. Further work by JCI Ltd. Outlined a resource of
185,000 ounces of gold. Intersections of interest include one averaging
27.77 g/t gold along 4.78 m of core and another of 12.0 g/t gold along
6.00 m. Aeromagnetic survey results indicate the greenstone rocks extend
northward for a at least several kilometers under the lake. What is
interpreted as an iron formation striking eastward lies between one and
two kilometers north of the peninsular. Gold occurs in two styles within
what appear to be the same rock type, which here is a sequence of volcanic
exhalite.
The area demonstrates potential for
containing ore bodies similar to those at Kiabakari where similar rock
types are reported. The prospect warrants further investigation and has
geochemical and structural controls to guide further work. The location
near Musoma and proximity to the shore of Lake Victoria offer logistical
advantages.
KILIMAFEDHA GREENSTONE BELT.
Introduction. Gold occurs at a number of localities in the
remote Kilimafedha Silver Hill or Money Hill in Swahili) greenstone
belt (Fig.18). There has been little sustained production since mining
began during the German colonial period early in the 20th
century. Many of the greenstone exposures and over two thirds of the gold
prospects, including the Kilimafedha mining district proper, lie within
the Serengeti National Park where mining is prohibited. Additional areas
of potential interest are within two game reserves, where mining would be
by special permit only. The remaining prospective areas, to which no
restraints apply, are around Fort Ikoma in the north and around Ndutwa in
the southwest.
There
has been little work in this district for many years. The description here
is taken mainly from Harris (1961).
Geology.
Nyanzian greenstones are the productive rocks in the Kilimafedha
greenstone belt. These are intermediate to mafic tuffs and volcanic rocks,
metamorphosed to chloritic shchists. BIF occurs in an east - west band
through the centre of the district with a few small utramafic pods along
the south edge.
Mineralisation.
Gold
is reported to occur in steeply dipping lodes a few centimetres to several
metres thick, generally parallel to the regional strike (i.e.westerly).The
low - sulphide quartz veins contain pyrite and chalcopyrite along with
gold.
Opportunities.
There
is little recent information relating to the portion of the Kilimafedha
greenstone belt outside the Serengeti National Park. Exploration
opportunities may exist in the weathered zone of BIF enclaves near Fort
Ikoma and the more accessible Ndutwa area (Ngasamo prospect).