| Summary: | A study has been made of tin and beryllium mineralisation in South-West Ankole, including beryl-bearing granite pegmatites, quartz-mica cassiteritebearing veins and hydrothermal quartz cassiterite-bearing veins. The mineralisation belongs to one of several tectono-metallogenic provinces which occur along the Kibaran orogenic belt, and the mineral deposits of s.w. Ankole are situated at the northern end of this belt. The mineral deposits are shown to occur in the exo- andendo~ontact zones of updomed ,pre- syn- and late-tectonic 'arena' granites which were emplaced during the Kibaran orogeny, and the mineralisation which post-dates emplacement of the granitic rocks, is shown to exhibit marked regional zoning with respect to the granite boundaries. The nature, mode of occurrence, time of formation and relationship of the mineral deposits to each other and to the granitic and metasedimentary rocks of S.W. Ankole are described and the granite pegmatites in particular are classified and placed into a conventional classification scheme. The results of a geochemical study of the mineralisation are presented and show the distribution of tin and beryllium in all the Jnain rock types of S.Y. Ankole. The geological and geochcmical data are applied to prove that that mineral deposits are epigenetic in origin and Were formed as a result of post-tectonic magmatic and hydrothermal processes which operated at the end of the Kibaran orogeny. Details of the mode of occurrence, and distribution of tin in tin-bearing rocks and minerals are presented and hypotheses are advanced to explain the mechanisms of transport and concentration of tin and beryllium during formation of the mineral deposits. It 1s established that the tin and beryllium were probably derived from pre-existing granitic rocks containing average crustal abundances or these elements, and that concentration occurred as a result of the magmatic and hydrothermal processes that operated at the end of the Kibaran Orogeny. Transport of tin is suggested to have been in the forln of alkali-hydroxyfluo- stannate complexes. Data on the alkali, fluorine and tin content of Inuscovites from all the mineral deposits are presented, and it is shown that muscovites from the hydrothermal quartz-cassitcrite-bearing veins can be distinguished from those from the other mineral deposit types on their alkali and tin contents and the use of muscovite as a prospecting indicator in searching for new hydrothermal tin deposits (in S.W. Ankole) is advocated.
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