LUPA GOLD FIELD
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.
