DIAMONDS
Introduction.
Tanzania
has been significant diamond producer for several decades. The bulk of
diamonds are produced from the Mwadui Mine near Shinyanga where both
alluvial deposits and a kimberlite pipe are mined. The kimberlite pipe (Mwadui
pipe) is now worked near the lower economic limit. The first diamond was
discovered in Tanzania in 1911 and commercial production began in 1925.
Mining was on a small scale until 1951, when the newly discovered Mwaudi
pipe began production (Fig. 45 and
46). Tanzania�s largest diamond,
weighing 241 carats, was found at the Mwadui Mine in 1956. Williamson
Diamonds Limited (WDL) has operated the Mwadui Mine since 1961.
Between
1967 and 1997, diamond production at Mwadui mine had been affected by both
global diamond production and internal policies. As production decreased
(Fig.14) and world production increased, Tanzania�s share of the world
supply shrunk from about 1% in the 1970s to about 0.1% in the 1990s.
However, since 1998, production has increased due to the new investment
that was put into the Mwadui Diamond Mine. Nearly all production has been
from the immediate vicinity of the kimberlite pipe, with very little from
river placers. The yield of gem and industrial stones is roughly equal.
Geosurvery International GmbH (1981) undertook a detailed airborne
geophysical survey in 1979 of 9,500 square kilometers in the Shinyanga -
Mwadui area (Fig.45). This survey defined 630 magnetic anomalies, some of
which were investigated by Williamson Diamonds during the evaluation
programme in the Shinyanga region (Gobba, 1989).
Geology.
Over 300
kimberlite pipes are known, of which perhaps 20% are diamondiferous. About
half of the 300 are within 200 km of Shinyanga. Most Diamondiferous pipes
are confined to a north south zone trending southward from Mwanza through
Shinyanga and Mwadui (Fig.46).
Most of
the dated kimberlites belong to the Tertiary or Cretaceous according to
Edwards et al. (1966) and Gobba (1989). A few Pleistocene pseudo -
kimberlites show preserved tuff cones. About 90% of the diamond yield has
come from the Mwadui and the adjacent but smaller New Alamasi Mines. Six
other kimberlites have been mined with mechanized equipment on lesser
scales.
The
kimberlites occur in a number of clusters, some of which favour the centre
of the Archaean craton, south of Lake Victoria (Fig.45 and
46). The
clustering and the concentratin within the centre of an ancient carton are
typical features of kimberlite provinces. Other clusters, without
productive kimberlites, are in the Ruhuhu River Valley southwest of Dodoma
town. Isolated alluvial diamonds have been found elsewhere, suggesting
additional clusters (Fig.46).
The
Mwadui and adjacent New Alamasi diamond deposits occupy a former crater,
over 146 hectares in area, produced by the explosive eruption of a
kimberlite about 50 million years ago. Mwadui is among the largest known
kimberlite of economic interest in the world.
Mining of
the superficial black Mbuga soils, gravels, and weathered tuff, has
yielded over 17 million carats of diamonds since 1940. Average diamond
content has declined from 10 to 20 carats per 100 tonnes to a current 6
carats per 100 tonnes.Mechanical problems in the mill caused a reduction
in the tonnage of ore treated daily from 4,000 to 2,000. However, since
1995, production has increased from 44,492 carats to 273,842 carats a year
in 2004 following major rehabilition works and installation of the
alternative processing plant. As of 2004, the reserves at Mwadui have been
reported to 50.9 million carats of diamonds.
The rich
material concentrated by weathering near to surface Is now largely
exhausted. Since the 1990s, production has been from a 90 - metre deep
pit. Below 90 metres, drilling and tunneling has shown that the fresh
kimberlite is diamondiferous but it is sub - economic. However, with
current advances in technology the kimberlite could prove to be
economical.
Exploration Opportunities.
The 630
dipolar magnetic anomalies defined by Geosurvey International GmbH (1981)
have similar geophysical characteristics to known kimberlitic pipes. Other
kimberlites (e.g. Mwadui) do not have a magnetic signature and therefore
would not be identified from a magnetic survey (Gobba, 1989). A
combination of geological and geophysical methods must be applied to
kimberlite exploration in this region. Additional grass - roots
exploration opportunities exist in areas known for alluvial diamonds but
without identified kimberlites (Fig.46), such as the Eastern Rift, the
region east of Lake Rukwa and margins of the Selous Basin.
A few
occurrences of strange kimberlite - like rocks (called pseudo -
kimberlites or parakimberlites) have been recorded in young craters
associated with Rift Valley volcanism. Concerted investigation of these
could conceivably lead to discovery of diamondiferous kimberlites nearby.
One such occurrence is a tuff ring at Lashaine just west of Arusha
(Sampson, 1962), where two diamonds are reported to have been found.
Another
occurrence is the Igwisi Hills crater in central Tanzania (Fozzard, 1959).
These post Miocene rocks have affinities to both kimberlites and
carbonatites. Investigation of these rocks could be rewarding.
As
mentioned above, large deposits of transported placer diamonds have not
been found. Most of the kimberlites are believed to be pre - Miocene in
age, and most have been eroded to at least a hundred metres below their
original surface level. Mwadui and some others are less eroded exceptions.
Therefore, it is reasonable to believe that diamonds could have been
transported great distances, as in Botswana, before being deposited. Much
of the area south east of Lake Victoria is covered by alluvium in grabens
related to Rift Valley faulting, or is covered by sediments of Lake Nyanza,
the Pilo - Pleistocene ancestor to Lake Victoria.
Reconstruction of the drainage pattern, which prevailed after in principal
period of kimberlite activity, and prior to the disruption of the drainage
by Rift Valley faulting, would allow prospecting for mid - Tertiary stream
deposits beneath later alluvium. Alluvial diamonds have been recorded in
some areas, but large economic placer deposits have not yet been
discovered. Locating shallow, buried superficial deposits using airborne
infrared surveys could prove feasible.