Introduction.

The Lupa Gold Field is in the southwest-central Tanzania, just north of Mbeya town (Fig. 35) where it occupies a triangular area of about 2,600 sq km. Gold production totaling about 24 tones of gold was produced from about 80 sites in placers, rubble zones and lodes. Lode mining began in 1934 at Ntumbi and in 1939 at New Saza.

The field forms a highland, reaching elevations of well over 2,500 m, and is bounded to the southeast and southwest by low lying alluvial plains separated from the highlands by Cenozoic Rift Valley faults. The field is capped by a mature erosion surface typical of the Central Plateau. Tilting has caused rejuvenation of the drainage with incision of streams and consequent exposure of mineralized zones, with would otherwise be covered by surface debris. Access to the field is by maintained gravel road from Mbeya, a town on the standard gauge TAZARA railway. The town is also served by the paved highway from Dar es Salaam to Zambia.

In the late 1990s, exploration companies such as Ormonde Mining, Princes Resources and Anmercosa Exploration (Anglogold) have carried out systematic exploration work in various localities within the Lupa Goldfield. The work included geochemical, geophysical and geological surveys. Mineralisation is confined to quartz vein and narrow shear zones. Resources have so far not yet been defined in the Lupa Goldfield

Geology.

Lupa is one of the less well understood goldfields of Tanzania. The ore deposits are hosted by Paleoproterozoic granitic and gneissic rocks. Systematic studies of the area have included regional mapping by the Geological Survey. A 1970-74 study including geophysics, drilling and alluvial testing was made by TECHNOEXPORT, A Soviet exploration group. This work was not consolidated into a formal report but maps and notes are available. Van Straaten (1984) re-appraised existing data for the ESAMRDC (no SEAMIC). In addition, the UNDP sponsored a study to identity oxide ore in the Saza Shear Zone for open-pit mining.

The gold field is underlain by gneisses, migmatites and later intrusions. Metabasite bodies and magnetite-quartzite inclusions probably represent remnants of a volcanic-sedimentary sequence, which included mafic volcanic rocks and magnetite-bearing BIF. The latter tend to stand out in topographic relief. Granodiorite and diorite intrusions cut these rocks, particularly in the northern part of the gold field. Potassic and sodic alteration of gneisses occur near the intrusions around Saza and Chunya, respectively. Metamorphic grades within the Lupa area are greenschist to lower amphibolite facies, in contrast to much higher grades of the surrounding terrain of the Ubendian Supergroup.

Mineralisation.

Gold in the Lupa Gold Field is in both lode and secondary deposits, with the bulk of historic production from secondary mineralisation. The secondary production was partly from stream placer, partly from hillside eluvial deposits and partly from the weathered rubble zones above or adjacent to quartz lodes. The Lupa and Sira rivers were largest producers of placer gold. Seasonal lack of

Figure 34: Mpanda Mineral Field. (Adapted from Nanyaro 1989)

Water is a hindrance to placer mining. Lodes in the Gold Field are typically within shear zones. The most productive of these has been the Saza Shear Zone, where ore bodies consist of steeply dipping lenticular quartz bodies. These low-sulfidesulphide lodes consist of quartz with traces of base metal sulphides, molybdenite, with sparse to abundant pyrite. The ore is free-milling with gold/silver ratios ranging from 1:1 to 2:1. Ore occurs in distinct shoots, generally up to 1.5 m wide, but rarely up to 20 m wide. The relationship of mineralisation to alteration and lithology is not clear-cut. The major shear zones, which host the larger veins, trend north-easterly. Small gold lodes have also been located in the ferruginous quartzites or BIF. The Gold Field contains literally thousands of auriferous veins, many of them too narrow for profitable working. Lupa is the best known Gold Field in Tanzania for occurrences of coarse gold, which suggest a long history of secondary enrichment of gold in the weathering profiles. This interpretation is consistent with the weakly dissected mature erosion surface. The largest recorded nugget was a mass of gold and quartz, which weighed 57.6 kg and 31.9 kg of gold. Results indicate that the highest alluvial grades occur in basal gravels.

The evolution programme by TECHNOEXPORT (1974) indicated a total of 3.3 million tones grading just over 9 g/t gold, with a content of 31.3 tones of gold, which is probably best classified as a drill-inferred resource. Scattered artisanal activity is confined along streams and outcrops of veins.

Numerous prospects at Lupa appear to have potential for small- to medium-scale lode, eluvial, and placer operations. With the exception of the Saza Shear Zone, there is a dearth of data on ore controls to aid in selection of targets for intensive exploration. The TECHNOEXPORT (1974) study called attention to the presence of gold in BIF, although there is n clear correlation between BIF and past mining. An analysis of the geomorphology of the gold field could lead to conclusions bearing on the locations of zones of secondary enrichment. BIF and quartz-carbonate rocks should be investigated as sources of bulk-mineable, low-grade ore. Work by TECHNOEXPORT detected values from 3g/t to 90g/t gold in 30 of 100 BIF samples analysed. Interesting targets are discussed below.

New Saza Mine (1936-1956).

From 1936 to 1956, the New Saza Mine produced 8,390 kg of gold, over one third of the district total, from ores averaging 7.5 g/t gold with a gold/silver ratio of 1:1. Ore was mined from 17 shoots and rubble zones from a 16-km strike length on the Saza Shear Zone. The maximum depth of mining was 250 m below surface, with drilling to a depth of 330 m. Escalating costs brought about closure of the mine. Targets for exploration within the mine are said to have been present. Only superficial plans remain.

Gap, Nkutano, Ruth and Maperi.

These old mines are near the east end of the Saza Shear Zone. An estimated 1.3 million tones grading 9.8 g/t gold lie in this sector. Gap is the most promising prospect in the area. Based on STAMICO drilling during 1975-1977, the Gap deposit probably contains about 1 million tones. Hole F returned 10 m of 7.9 g/t gold and Hole 5 2.5 m of 13.46 g/t gold. A drilling programme undertaken by TECHNOEXPORT included some work at Nkutano. Fifteen drill holes located an ore shoot 260-m long averaging about 10 g/t gold

Saza and Razorback (west end of Saza Shear Zone).

Diamond drilling prior to 1960 discovered blind ore shoots. Further drilling, guided by careful structural analysis, could be fruitful. The Saza Shear Zone was mined sporadically for 6 km with widths up to 20 m. A small scale mining and stamp mill of rubble zones was installed by DEMCO in the 1990s with a milling capacity of 5 tones of ore per day.

Chapa, Perseverance, Galena and Menzis.

About 20 veins in this central portion of the gold field were previously worked to shallow depth. According to TECHNOEXPORT (1974),mineralisation in shear within quartzitic rocks contains some 3 30,000 tones with 77,000 oz of gold in ore with grade of 8g/t gold.

Ntumbi Reef.

Another important producer was the Ntumbi Reef, which during 1937-1965 yielded 1,709 kg of gold from ore grading 12.1 g/t gold. Lodes within north-easterly trending shears of the shear zone were mined to 130 m depth. Fifty known veins occur within an area of 15 square kilometers. The deepest workings extend to only 130m depth, and little systematic exploration has been undertaken. Only about one quarter of the 1,200 m strike length of known shear zones has been prospected. Both TECHNOEXPORT and the UNDP (1985) reports recognized a potential for 450,000 tones of ore containing 6.000kg of gold at an average grade of 14 g/t gold.

Hodges, Niger, Black Jack, Zumbi, Siebert, and Twiga.

 These old mines all have small workings on high grade veins in the east-central part of the gold field. Reserves appear limited and suitable mainly for small mining operator. Potential exists for bulk-mineable mineralisation in shear known to be mineralized, but not extensively explored.

Andura, Chisu Zone, Kizumbe, and Kasnga Bridge Prospects.

All these prospects are narrow veins with short lengths. The BIF and some shear zones contain low-grade values, which remain to be investigated.

Sira Placers.

These deposits are the most promising of several placers, with a recorded production of some 3,200 kg of gold. The 1974 TECHNOEXPORT study recognized a potential for over 9,000kg of gold.

Nkulwisi Zone.

This zone of major displacement is 20km long and up to 1.8 km wide and is known to contain about a thousand auriferous quartz veins. The role of cross structures as a controlling agent is emphasized in the TECHNOEXPORT work.

Matundasi.

Matundasi prospect has an intense artisanal gold mining activity where

Figure 35: Lupa Gold Field. (Adapted from Kimando (1984), Van Straaten (1984), Sango (1988), Kuhn, et al (1990) and Hester (1990)

Gold is recovered from quartz reefs and veins. The Government has installed a small scale gold processing plant for demonstration and processing of ore from small scale miners.

Opportunities.

Opportunities in the Lupa Gold Field have been revealed by exploration works that have been carried out by exploration companies since 1996. These works have identified potential prospects, which qualify for further follow-up. The areas include Kasanga shear zone, Ilunga Hills, Lyaruu prospect,Makongolosi and Malambang'ombe.

The presence of arisanal working along streams and vein outcrops are also best opportunities for follow-up.