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 is the 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 its 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.