NZEGA GREENSTONE BELT
Introduction.
The Nzega
greenstone belt (Fig.30) lies astride the Mwanza branch of the narrow
gauge Central Railway. The belt supported several small - scale gold mines
prior to the Second World War, including the Canuck Mine and Baker Mine.
Prospecting activities by the Geosurvey International GmbH, The Tanzania
Geological Survey (MADINI), Samax Ltd, BHP and Pangea have increased the
understanding of the geology and mineral potential of the Nzega greenstone
Belt.
Geology.
The Nzega
greenstone belt is a 2,800 square km belt comprising basal metasedimentary
rocks, overlain by felsic to intermediate volcanic rocks, BIFs and
tuffaceous sedimentary rocks. The latter are intruded by minor felsic and
mafic dykes. The belt differs from those described previously in that
volcanic rocks of acid composition are relatively abundant. BIF is well
represented and appears to be a strike continuation of the same rocks at
Geita. Domal features are reported to have been recognized within the
volcanic assemblage.
Mineralisation.
Gold is
reported to occur both within strata bound host rocks and in cross cutting
veins. Some base metal mineralisation has been found, and the area in
general is thought to be prospective for such deposits. Several
electromagnetic conductors were found by Geosurvey in 1979 - 1980 during
ground follow up geophysical survey. Some of these conductors were drilled
and found to be caused by massive pyrite. Several prospects close to the
Old Canuck Mine were noted during the UNDP funded prospecting work in the
area (UNDP, 1986).
Old Canuck Mine.
The Old
Canuck Mine, which lies near the Central Railway line, was found in 1944.
Mining operations were carried out until 1953, producing some 34,100
tonnes of oxide ore yielding an average of 5.8 g/t gold. The ore body is
nearly vertical quartz vein within mixed silicate and oxidefacies BIF
striking N20 Degree E. Shear zones within this rock strike parallel to the
granite margin and host the veins. At least four ore shoots have been
recognized along the structure, but only one was mined. The mineralisation
is composed of auriferous quartz and sulphides, oxidized to a depth of 65
m. The width of the vein varies from 1 to 4 m.The main shoot at 90 m depth
had a width up to 4 m containing 13 to 15 g/t gold, with the grades
irregularly distributed along a strike of 180 m. Geological reconnaissance
by the UNDP found that other quartz veins occurring along the BIF contract
ranged in width up to 2.4 m with grades up to 5.1 g/t gold.
Generally, the are has good potential for additional reserves, both below
the mine workings and along the strike extensions. The Mining operations
are reported to have stopped in sulphide mineralisation (Sulphide ore) at
a depth of 90 m. The strike length of the structure on which the shoots
lie at least 2.7 km long and corresponds with oxide - facies BIF. A
geophysical survey suggests that this length might extend to some 7 km.
Geochemical anomalies coincide with some of the geophysical anomalies.
Golden Pride Mine.
The
deposit is hosted in a 100 m wide shear zone within the volcanic rocks at
the contact with metasedimentary rocks. The shear zone and country rocks
strike north - west to south - east and dip steeply to the south.
Mineralisation consists of quartz veining accompanied by intensive quartz,
carbonate, sericite and chloride alteration. BIF hosted mineralisation is
encountered at the eastern end of the deposit. The mineralisation crops
out at surface and is oxidized to depths varying between 30 and 00 metres
below the surface. A narrow transition zone of mixed weathered and fresh
material is located below the oxidized zone. Within these zones, economic
mineralisation attains widths of up to 55 metres over a strike length of
3.5 km and is open along strike in both directions, and at depth. The
typical section of the mineralized zone is shown in Figure 31. A total
resource of 2.8 M oz of gold has been established. An average grade of the
deposit is reported to be 3.2 g/t gold.
Mwine Deposit (Chocolate Reef).
Mwine
(Chocolate Reef) lies at the western end of the Nzega greenstone belt.
Gold occurs in the Chocolate and Village Reefs in broad, irregular shears
within pyretic quartz mica schists in an area of granitic terrane
(Fig.32). The Village Reefs lies close to a mass of mafic rock. The
reported features of the deposit appear to make them unique and suggest
that gold prospecting need not necessarily be confined to the greenstone
rocks.
A
resource estimate of 1.3 M oz . of gold has been established in the year
1999 for the Chocolate Reef. The mineralized material is reported to
contain 0.1 to 0.3% copper.
Matinje Prospect.
Tancan
Gold operated a small - scale plant at Matinje in 1991 to 1993 to treat
gold ores and tailings from artisanal mine workings in the area.
Production was at the rate of about 96 oz of gold per month. Currently the
National Development Corporation (NDC) is supporting the small scale
miners to improve the processing of the ore.
Gold
occurs in steeply dipping quartz lodes carrying minor hematite or pyrite.
Grades of up to 15 g/t gold occur in narrow veins, while wider veins (up
to 2 m) tend to be lower grade. Secondary gold was recovered from
surficial rubble zones consisting of vein - quartz boulders in a matrix of
laterite. The rubble zones are typically only a metre thick, and contain
variable gold grades, normaly in the vicinity of 3 g/t gold. The rubble
zones are typically only a metre thick, and contain variable gold grades,
normally in the vicinity of 3 g/t gold. The rubble mined overlies a
mineralized vein and averages about 5 g/t gold.
Bulangamilwa Prospect (Old Baker Mine).
Bulangamiliwa prospect is located within the Nzega greenstone belt. It is
transected by the eastern extension of the Nzega shear zone hosting the
Golden Pride gold deposits. Pangea Minerals had evaluated this prospect to
the stage of pre - feasibility study.