Introduction.
Several
occurrences of base metals have been well studied. However, the only
notable production of copper and lead has been from the Mukwamba Mine in
the Mpanda Mineral Field. Zinc has never been produced in Tanzania.
The well
known base metal prospects are discussed below. For most of them, little
detailed information, which would allow their complete characterization,
is available. Locations are shown on
Figures, 40, & 41. The most important
and prospective occurrences are in the Archaean greenstones, but the only
production known is from Pretorozoic rocks. Occurrences of base metals in
these rocks are in general only sketchily described and fall into a
variety of geological models. Base metals in post - Proterozoic rocks are
mainly confined to small, low grade copper showings in Karoo sandstones.
Little
exploration for base metals has been conducted away from known occurrences
for many years. This lack of new activity implies that virtually none of
the advances in geochemistry and geophysics of the past 30 years have been
applied. The sole exception appears to be the airborne surveys by BGR in
1989. Results of this work are available at Dodoma
BASE METALS IN ARCHAEN GREENSTONE BELTS.
The base
metals potential of the Archaean greenstones require a thorough, modern
evaluation based on genetic models developed for the Superior Province of
Canada. It is possible that the small number of occurrences of base metals
in Lake Victoria greenstones reflects an inherent deficiency of metals.
Alternatively, the effects of tropical weathering could have obscured any
evidence of base metals.
Based on
scanty data, these occurrences in which copper predominates appear to
follow the well documented types of the Canadian Shield, commonly referred
to as “ Volcanic - hosted massive sulphide ‘ (type 9.1 of Eckstrand,
1984), or ‘ kuroko massive sulphide’ (Model 28a of Cox and Singer, 1986).
Cyprus - type massive sulphides (model 24a of Cox and Singer, 1986) may
also occur. It is widely recognized that a number of sub - types occur,
which are characterized by their structural settings, host lithologies and
metal content. A common feature is the association with felsic (rhyolitic
to dacitic) igneous rocks. In the Nyanzian greenstones of the Lake
Victoria region, felsic rocks are much subordinate to mafic (andesitic and
basaltic) igneous rocks, except in the southern part of the greenstone
terrain.