Introduction.

Nickel and chromium, together with the typical associates cobalt and the Platinum Group Metals (PGMs- platinum, palladium, rhodium, rhenium, osmium and iridium), tend to occur in similar geological environments. These environments are: 1) layered mafic igneous intrusions such as gabbros and anorthosites (Cox and Singer models 1,2a, 2b, 5a, 5b, 7a, and 2) and ultramafic rocks such as peridotite, dunite, and serpentine (Cox and Singer models 6a, 6b, 8a, 8b, 8c, 9, 38a). Most of these models involve concentration of molten sulphide droplets or oxide crystals in a basic or ultrabasic magma chamber.

The mafic and ultramafic rocks of Tanzania warrant close scrutiny, especially for PGMs for which reliable analytical techniques have been developed only recently. Consideration should be given to reliable PGM assays for any rock samples containing significant nickel or chromium values, or any mafic or ultramafic rocks containing sulphides other than pyrite. Conversely, little or no reliance should be placed on any PGM assays made before 1975, or no any assay form other than the twenty or so qualified laboratories in the world. The history of mineral exploration is replete with wild-goose chases after erroneous PGM assays.

The metal prospects discussed in this section have been described by Temperly (1947), Harris (1961), UNDP (1979, 1980,1985, 1986) and periodic report by Western Rift Exploration Company. UNDP (1985) is especially valuable, as it contains a discussion and annotated listing of nickel deposits.

The Kabanga Nickel Deposit. Kabanga nickel deposit is located in Ngara District, Kagera Region, near the Tanzania/Burundi border. The nickel occurs as pentiandite associated with chalcopyrite and pyrrhodite in the form of either massive or matrix sulphide bodies. The host rocks consist mainly of peridotite and pyroxenidte with lesser amounts of gabbro. Gabbro is better developed away from the sulphides. The whole mafic body lies within a phyllite sequence close to the contact with a bed of steeply dipping quartzite as illustrate in Fig 42. Several mafic bodies occur but only one has so for been shown to contain nickel in quantities of commercial interest. The sediments belong to the Karagwe-Ankolean Supergroup of Protezoic age.

A regional exploration programme funded by the UNDP, carried out by the Government of Tanzania, discovered the Kabanga Nickel deposit in the late 1970s. Drilling under this programme indicated reserves of 16 Mt grading 0.7%, with some zones grading over 2.0% nickel. The Kabanga area was subdivided into three blocks. Work was concentrated in block 2 where preliminary estimates of mineral reserves indicate by drilling were 2.75Mt at 2.31% nickel, 3.0Mt at 1.12% nickel and 16.0Mt at 0.71% nickel. Geologically extrapolated reserves were 3.1 Mt at 2.69% nickel, 1.8Mt at 1.13% nickel and 10.1Mt at 0.70% nickel. Analytical results also showed significant amounts of cobalt.

In 1990, the Kabanga Nickel Company, a subsidiary of Sutton Resources and Romanex International, outlined a 144Mt resource grading 0.66% nickel and 0.06% cobalt in this area. An annual production of 24,000tonnes of nickel, 3000tonnes copper and 1400 tonnes cobalt was projected.

Total inferred resources at the Kabanga deposit, on the basis of earlier Sutton exploration and a 0.5% nickel cut-off were estimated to be 31Mt, grading 1.5% nickel 0.22% copper, and 0.13% cobalt. On the basis of a recalculation of the Kabanga resource by Anglo-American based on a cut-off grade of 1.2% nickel, Sutton reported a revised estimate of 12.7Mt, grading 2.1% nickel, 0.30% copper and 0.16% cobalt.

The deposit is being re-evaluated by Falcon Bridge with a full feasibility study to cover the whole deposit.

Anomalies West of Lake Victoria. A number of airborne magnetic and electromagnetic anomalies occur on strike to the north of Kabanga, coinciding with layered gabbro and norite bodies (fig. 42). Some of the geophysical anomalies also coincide with soil geochemical anomalies for nickel, cobalt, or chromium. The UNDP report (1980) discusses these anomalies, which have potential for further deposits similar to Kabanga.

Kapalagulu.

This prospect shows many geological similarities to Kabanga, with nickel and copper sulphides at the base of a layered norite. Kapalagulu prospect is essentially an ultramafic intrusion composed of norite, dunite and herzburgite. The rocks are sheared and intensely serpentinised along shear zones. Seams of chromite occur locally within the ultramafic intrusion. Laterite regolith up to 40m thick covers the intrusion.

Detailed geochemical exploration by Western Rift Exploration (WRE) in 1959 identified broad areas with anomalous nickel and copper. Four holes drilled over some of the anomalies showed over 0.3% and copper 0.1111%. Some zones showed over 1% nickel and 2% copper. In 1970 Nippon Mining of Japan following WRE work, estimated a total of 20.0Mt of laterite resource with an average of 0.7% nickel and 0.4% copper. A Russian geological team drilled three diamond holes in 1973 to test the WRE best soil anomalies. Promising intersections were encountered in hard rock containing 5-10% disseminated sulphides, which assayed 0.3% nickel and 0.1% copper.

Soil and stream sediment sampling conducted by BHP in showed elevated levels of copper and nickel, in soil, which are restricted to shear zones within ultramafic rocks. Conductors identified in EM survey occur also along structures within the ultramafic rocks. There is potential for nickel-copper mineralisation with PGM at Kapalagulu.

In 2001, Goldstream Mining from Australia reported PGM at Mibango ultra-mafic complex. Assays results from soil sampling showed values of platinum and palladium ranging from 0.2 to 0.77 g/t associated with gold (0.1 g/t), copper (0.2%) and nickel (0.24%).

RAB drilling confirmed these results. Two significant PGM and gold intersections include 34 m at 2.31 g/t, 10 m at 3.28 g/t and 30 m at 2.09 g/t. Preliminary determination of various PGM metal ratios gave Pt: Pd varying from O.8 TO 1.1, Pt: AU varying from 4.6 to 7.5 and Pt: Rh consistent at 12.0.Exploration is in progress to establish resources.

Zanzui.

This prospect lies about 150-km southeast of Mwanza and is described by the UNDP (1986). It is a laterite-covered ultramafic body about 5 km in diameter with a prominent airborne magnetic anomaly (Fig 43). Very poor exposures indicate an ultramafic body with nickel-cobalt enrichment in the overlying laterite. Due to the setting and shape of the anomaly, a zoned ultramafic pluton with PGMvalues in addition 6to nickel and cobalt is a possibility. Traces of were recorded on assays of selection core, but these need to be confirmed.

North Ukinga Chromite and Platinum.

Titaniferous magnetite and chromium associated with gabbroic and ultramafic bodies occur northwest of Lindengere Hill which is part of the Kipengere Range, about 125 km from Njombe. Chromite occurs as small pockets or masses in serpentine. A sample analysed by the Geological Survey of Tanganyika in the late 1940s indicated values of 33.4% Cr2O3. River concentrated samples derived from weathering of norite and ultrabasic returned platinum values of 0.047 g/t and spectrographic analysis gave 0.0001%Pt. Opportunities exist for locating commercial deposits of platinum in the region between Goliama and Liganga Hills in the south to Magoye and northern Ukinga in the northwest.

Other Prospects.

Addition mafic or ultramafic bodies with indications of metals occur at Kabulyanyele-Mwahanza Hill, Garauja-Basotu, Ngasamo-Wamangolo Hills, Mbalangeti-Beramango Hills, Keserya Hills in north Mara, Twamba, Nkenza, Itiso, Haneti and in the Uluguru Mountains.