BASE METALS IN PROTEROZOIC ROCKS.
Introduction.
Disseminated low
grade copper occurrences are widespread in the Protrusion rocks, normally
in mafic gneisses where less than 1% copper minerals occur in
metasedimentary rocks. The only mine developed at Songoa in the Pare
Mountains is described below. Lead minerals occur mainly in the Mpanda
area, although minor occurrences have been reported in other regions. In
the Mukwamba Mine at Mpanda, lead occurs with polymetallic ores in vein
fractures containing copper, gold and silver.
The Mukwamba
occurrence is unusual because of the almost complete absence of any of the
zinc minerals, which are normally associated with lead. Old Mkwamba Mine.
This mine, in the Mpanda District, was the only significant base metal
producer. Production from 1950 to 1961 was, 1,948,000 tonnes averaging
2.4% lead, 0.6% copper, 1.45 g/t gold, and 78 g/t silver. Additional
production from 1937 to 1947 was 247.8 kg of gold and a minor amount of
lead ore. An underground vein mining operation of this type is type is
unlikely to be of interest to investors unless significant better gold
grades can be established. Thus, Mkwamba is better considered as a gold
prospect. Pare Copper Mine. The Pare copper deposit, discovered in 1955,
is located on the western flank of the North Pare Mountains. Prospecting
and development continued until 1959. A total of 247 tonnes of ore was
produced with an average grade of 10.18% copper.
Mineralization
occurs in sheared horizons and sheeted fault zones in quartz – pyroxene
granulites. The mine and other miner copper prospects occur near the lower
boundary of a major thrust zone extending 15 km parallel to the Pare
Mountains at the locality of Songoa in Usangi. Mineralization includes
chalcopyrite, cubanite, covellite, malachite, and azurite localized in a
thrust zone. Copper sulpphides occur as disseminations in the sheared
granulites and as small pockets of massive sulphide. The Pare Mine may
represent a metamorphosed stratiform copper occurrence, but there is
insufficient data to speculate further. Regional and local mapping might
delimit regions in which the same strata and structural zones occur within
which syntectonic concentration of sulphides might present. Kigugwe Copper
Prospect. Kigugwe is in the Njombe area about 100 km east of Mbeya. Copper
mineralization has been known since the German colonial time. A thorough
investigation, including drilling, was undertaken by GEOMIN in 1972.
The prospect
occurs in sedimentary rocks of the Bukoban Super group, mainly in shale's
overlying quartzite sandstones. The shale's are in turn overlain by beds
of dolomitic limestone and andesitic lava. Three cupriferous horizons
occur in the shales above the quartzite with the middle horizon having the
highest copper content. Copper occurs as irregular nodules of chalcocite
and bornite with a thick coating of malachite and rare stains of azurite.
The cupriferous shale's bands are generally about 30 cm thick.
The nodules
contain approximately 40% copper but samples of the cupriferous shale
including the nodules show not more than 3% and normally less than 1%
copper. Metatorbernite, a uranium mineral, occurs locally. The Kigugwe
prospect is the best studied of a number of other possibly related copper
occurrences described below. Other Stratiform Copper – Silver Prospects.
Many other copper occurrences are known in the Bukoban Supergroup of
northwest Tanzania. Harris (1961) mentions the Nyamori Hills just east of
kigoma and the Sindi River area south of the Malaragasi River.
The Nyamori Hills
occurrence consists of oxidized copper minerals in cherts within andesitic
lavas and dolomites. Values up to 3.5 % copper and about 60 g/t silver has
been reported. The general nature of the showing is broadly similar to
that at kigugwe (above) and may be genetically related. Some of the
prospects described in the next section as the ‘Sango Trend’ may also be
of this type.
Nyamori Hills,
Sindi River and a host of other copper showings in the region, as well as
the Kigugwe prospect, may be indicative of syngenetic stratiform copper –
silver – uranium mineralisation well documented from other parts of the
world. This type is classified as the ‘Sediment – hosted copper, model 30b
by Cox and Singer (1986), or ‘sedimentary copper, type 6.3’ by Eckstrand
(1984). Variants of this type are commonly referred to as Copper Belt,
Kupferschiefer, or Spar Lake deposits, based on analogous deposits in
Zaire – Zambia, Germany , and Montana, USA.
Ore commonly
occurs in sandstones or shales in which copper and associated metals
present in unusually high syngenetic concentrations are further
concentrated by digenesis. Interestingly, two of the anaologies are also
in the Proterozoic rocks.
An alternative
origin mentioned by Harris (1961) is that the copper is derived from
copper filled amygdules in the volcanic rocks. While this origin is
possible, it is also consistent with a sedimentary deposit type, some of
which (e.g. White Pine, USA) are spatially associated with copper –
bearing flows. Further field examination is necessary to settle the
question in the Bukoban.
Given the very
large size of some deposits in this class, further investigation of the
Bukoban and karoo occurrences seems warranted. Sango Trend.