Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences



Occurrence number: 
The North Nyasa Alkaline Province
Longitude: 34.45, Latitude: -15.72

The main Tambani body, which consists of nepheline syenite gneiss, is some 20x6 km with several outlying areas and is orientated parallel to the trend of the surrounding paragneisses (Cooper and Bloomfield, 1961). Contacts with the surrounding rocks are conformable and transitional but there are places which are cross-cutting, including occurrences of nepheline syenite dykes cutting the adjacent paragneisses that Cooper and Bloomfield (1961) ascribe to mobilisation of the nepheline gneiss. The nepheline gneisses are typically migmatitic with in places mobilisation apparently obliterating the gneissic structure. Concordant bands of typical country rock biotite and hornblende gneisses occur throughout the body and pegmatites up to a metre thick are also widespread. A typical example consists of microcline perthite, nepheline, biotite, muscovite and accessory zircon, magnetite, corundum and apatite; blue sodalite occurs in some areas. A range of other minerals including betafite, uraninite and monazite have been found in the area (Cooper and Bloomfield, 1961); they give analyses of two nepheline gneisses.

Corundum was formerly mined from the Tambani area and detailed maps of the corundum prospects are given by Cooper and Bloomfield(1961) who also describe in detail investigations of the radioactive minerals, of which 16 are tabulated, including betafite, uraninite and monazite.
Pb dating of zircon gave 515 to 659 Ma and monazite 542 Ma (Cooper and Bloomfield, 1961).
BLOOMFIELD, K. 1968. The pre-Karoo geology of Malawi. Memoir, Geological Survey of Malawi, 5: 1-166.COOPER, W.G.G. and BLOOMFIELD, K. 1961. The geology of the Tambani-Salambidwe area. Bulletin, Geological Survey of Nyasaland, 13: 1-63.
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