Alkaline Rocks and Carbonatites of the World

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Otjisazu

stripes

Occurrence number: 
116-00-020
Country: 
Namibia
Location: 
Longitude: 17.1, Latitude: -21.92

The Otjisazu complex, located on the farm Otjisazu 53, covers about 12 km2 and is emplaced in schists of the Damara sequence. The earliest rocks to be intruded were alkali pyroxenites which dominate the southern part of the complex. They comprise aegirine-augite with subordinate K-feldspar, titanite and apatite with melanite garnet in places such that occasionally almost pure garnet rocks are present. The garnet appears to be spatially related to later sovite and is interpreted by Gunthorpe and Buerger (1986) to have been generated metasomatically under the influence of the latter. Apatite-rich patches occur throughout the pyroxenite in which the apatite may be disseminated as grains or clusters of grains or in segregations of apatite rock up to 50 cm across. An area of alkali gabbro and locally syenitic rocks has been mapped within the pyroxenites, rocks which pass gradationally into the pyroxenites by a gradual decrease in K-feldspar and hornblende; minor nepheline is present in some facies. The primary igneous assemblage of the gabbro and syenite has been replaced by garnet, diopside, wollastonite and a pale amphibole and inclusions of wollastonite-garnet rock are found within the sovites. The second major intrusive phase is a sovite which forms an elongate body more than two kilometres in length together with two smaller lenses in the southern part of the complex. These are coarse rocks of calcite and varying amounts of aegirine-augite, garnet and apatite which grade from pure sovite to calcite-bearing pyroxenite. The large body has a coarsely brecciated structure the angular to well-rounded blocks ranging from several centimetres to >20 m in diameter and lying in a banded carbonate-rich phase which appears to 'flow' around the blocks. Swarms of parallel to sub-parallel pegmatites, generally less than one metre thick, cut the sovite and pyroxenite and comprise aegirine-augite, titanite, apatite and K-feldspar. The final igneous event involved the emplacement of syenite consisting of microcline microperthite and subordinate sodic plagioclase, aegirine-augite and amphibole. There is a little fenitization along the eastern and southeastern margins, the zone reaching only 20 m in width. Scapolite, aegirine-augite rimmed by blue-green amphibole and minor biotite have developed in the schists.

Economic: 
Apatite is concentrated patchily in the alkali pyroxenite, some segregations of pure apatite rock being 50 cm in diameter, while it is also concentrated in the sovites. Geochemical sampling of soils defined areas of 2-2.9% and >3% P2O5 and 100-199 and >200 p.p.m. copper; the two areas have a high degree of coincidence (Gunthorpe and Buerger, 1986).
References: 
GUNTHORPE, R.J. and BUERGER, A.D. 1986. Geology and economic evaluation of the Otjisazu alkaline igneous complex, central South West Africa/Namibia. In. C.R. Anhaeusser and S. Maske (eds), Mineral Deposits of Southern Africa, 1: 2255-60. Geological Society of South Africa, Johannesburg.
Location: 
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith