Download CORRELATING THE GEOLOGY OF SHINYANGA DISTRICT

CORRELATING THE GEOLOGY OF SHINYANGA DISTRICT
TANZANIA AND SIAYA DISTRICT KENYA IN DIAMOND
PROSPECTING
ABSTRACT
Diamonds are found in kimberlite pipes or lamporites which have been intruded
into the earth’s crust. They may also be found outside kimberlites where the
original material has been eroded away and deposited elsewhere. There are about
300 kimberlite pipes in Tanzania though most are not diamond bearing. One of
these kimberlites in Shinyanga district, named the Mwadui kimberlite is one of the
most productive diamondiferous kimberlite in the world. Discovered by Dr. J.T.
Williamson in 1940 it led to a diamond exploration frenzy in Tanzania leading to
the discovery of other diamond bearing kimberlites. The geology of this area
consists of Archean rocks of volcanics, sedimentary rocks and gneisses on the
stable Tanzanian craton. The kimberlites are intruded into the basement system
around the Cretaceous period.
In western Kenya, three kimberlites were found using geophysical survey methods.
They are located in Siaya district in central Nyanza. These kimberlites contained
various minerals derived from the magma but lacked mantle-derived chromian
garnet, ilmenite and spinel. The kimberlites are also poor in heavy minerals known
to be indicative of diamond bearing kimberlites. A geochemical study of their
composition showed no diamonds were present. Nevertheless in 1931 during
mining in Kakamega goldfield, south of Siaya, a few diamonds were found during
the washing of stream gravels and the source was said to be unknown. This leaves
a window of hope as to the occurrence of diamonds in the area.
Exploration of diamonds is done using a variety of methods which complement
each other such as aeromagnetic survey and soil and sediment sampling for
indicator mineral analysis. An intensive exploration for kimberlites is necessary in
Kenya to locate possible undetected kimberlite diatremes as it is known that they
occur in clusters.