TIN
Introduction.
Tin has
been an important mineral product since its discovery in 1924, except
during period of low prices when production slowed, and since 1974 when
the power line from Uganda to the mines in Karagwe was cut. Cumulative
production from 1924-1974 is on the order of 6,000 tones of cassiterte
concentrates. Stockely et al. (1938) and Harris (1961) give useful
descriptions of the tin deposits and geology of the principle areas of
production. The following discussion is largely taken from Harris (1961).
The reader should consult the references for details.
Geology.
All
recorded production of tin has come from the Karagwe Tinfield in the
northwestern tip of the country and especially from the Kyerwa deposits, (Fig 44). Similar geology occur in the Rulkenge area 150 km to the south
(Fig 44) where the little-studied mineralisation is probably similar to
Karagwe Tinfield. The FIELD IS WITHIN ROCKS OF THE Karagwe-Ankolean
Supergroup of Neoproterozoic low-grade metasedimentary rocks
MIneralisation.
Mineralisation occur in steep veins and flat tension fractures, which in
some cases are preferentially developed in phyllited near granite
contacts. Many veins pinch and swell along strike for up to 1,000 m and
contain erratic coarse-grained cassiterite mineralisation, controlledin
some cases by intersections with cross veins. Veins are composed of white
quartz and are typically 1 to 6 m wide. Mineralisation consists of
cassiterite, tourmaline and muscovite, while wolframite occurs locally.
Cassiterite values are erratic but grades tend to be less than 5 kg/tones
wherever mined. Only a fraction of the quartz was considered workable.
Little tantalum occur with the cassiterite, although some is reported at
Rulenge. The mineralisation could be classified within the "Sn vein model"
no. 15b, with some gradation into the "Sn greisen model", no. 15c of Cox
and Singer (1986).
A
significant portion of production from Karagwe was derived from eluvial
and alluvial deposits overlying or downhill from the lodes. Most of the
alluvial deposits are shallow and did not sustain major operations.
Eluvial deposits over veins were invariably worked prior to the lodes and
proved richer in most cases.
The grade
of ore varied widely from vein to vein and between lode, eluvial, and
alluvial ares. An average is difficult to calculate, but a study by
Colonial Development Corporation in 1953 (after the peak of production in
the district) outlined 250,000 tones of remaining proven reserves, which
graded 0.3% tin (3 kg/tone), plus a much larger inferred tonnage.
Opportunities.
Like all
medium to higher investment cost tinfield mining here has been plagued by
the erratic tin price. In addition, its remote inland location has been a
handicap improved road and rail transport in Tanzania and Uganda would
address the second problem; the first might be ameliorated by a systematic
study of the geologic control on higher-grade lode and eluvial cassiterite
mineralisation.
The open
pit Kyerwa Mine in the Karagwe district was operated by the Kyerwa
Syndicate until 1973, when the power line from Uganda ceased to function.
The mine had been operating profitably until then. The possibility of
re-opening the mine warrant attention now that the power line has been
re-installed.