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.