Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Longitude: 16.77, Latitude: -20.03

The hills of Okorusu rise to 330 m above flat, monotonous country, but much of the alkaline complex is obscured by sand. The country rocks comprise marbles and schists of the Damara System which are intruded by granite. Only the southern part and the central area of the igneous complex are exposed in which Van Zijl (1962) distinguished the following rock groups: hortonolite-bearing monzonites, which are the earliest rocks; an early syenite, which occupies about 90% of the complex; coarse-grained nepheline-bearing syenite and foyaite; fine-grained dark foyaites; pyroxene fenite and carbonatite; limonitic feldspar-carbonate metasomatic rock and iron deposits; fluorite deposits; and a suite of dykes and small plugs. The early monzonite is found as several small occurrences which are probably xenoliths within syenites and foyaites of the central hills. It consists of dominant orthoclase and microperthite, plagioclase (An70-40), olivine (Fa70), augite/hedenbergite, hornblende, scarce biotite, magnetite and apatite. The extensive early syenite occupies much of the central hills and one part displays a foliation with concentric strike and inward dips. It comprises microcline perthite, a little nepheline in areas close to the central foyaite, diopside, sometimes zoned outwards to aegirine-augite, scarce olivine and hornblende, rare biotite and abundant calcite. A core of nepheline syenite, having an estimated diameter of 1.4 km, is found in the central hills and it appears to become more undersaturated inwards. An outer 100 m-wide zone consists of exceptionally coarse microcline perthite rock in which there is a little interstitial nepheline; the nepheline increases inwards to a central core approaching urtite, in which the closely packed nephelines are up to 3 cm in diameter, with a little interstitial alkali feldspar and pyroxene. The pyroxene of these rocks is aegirine-augite and hornblende forms rims around pyroxene and sometimes fayalitic olivine; accessories include sodalite and calcite. The fine-grained dark foyaites form radial and tangential dykes and small plugs in the central hills. These rocks contain a greater proportion of pyroxene, which is more sodic, and opaques than the foyaites and much more sodalite. Carbonatite forms several plugs in the southern part of the complex. They are calcite rocks with apatite, quartz and feldspar and in one occurrence aegirine-augite. Van Zijl (1962) links pyroxene fenites with carbonatite in his description, although from the mapping they are not obviously related. These rocks are concentrated along the southern margin and were generated by metasomatism of marbles and greywackes. They comprise dominant sodic pyroxene, abundant calcite, apatite, biotite, titanite, sodic amphibole, alkali feldspar and sometimes melanite. In places the pyroxenites are brecciated with veins of calcite. Later fracturing led to replacement of pyroxenite by limonite-alkali feldspar-calcite rocks, and in places these pass into iron ores with 60-80% FeO and up to 7% TiO2. These rocks in general are highly potassic. Fluorite-rich rocks are confined to the southern margin and are present as disseminations and joint fillings in the limonite-feldspar-calcite rocks, and as replacement deposits in limestones. The most prominent of the numerous dykes at Okorusu are feldspar- and nepheline-phyric tinguaites in some of which sodalite also forms phenocrysts. A group of dark rocks forming dykes and plugs and containing mafic phenocrysts are referred to as lamprophyres by Van Zijl (1962) but are probably nephelinites. Up to 10% of olivine (Fo95-83) phenocrysts occur but pyroxene, as phenocrysts and groundmass, may comprise 20-60% of some rocks. Nepheline and analcime are the dominant felsic minerals but only occasionally constitute as much as 30%; magnetite, melanite, perovskite and apatite are accessory. Chemical analyses of both igneous and metasomatic rocks are given by Van Zijl (1962) and Prins (1981). The latter includes trace element data and, apart from discussion of igneous petrogenesis, considers at length the fenitization processes. Whole rock and Sr, Nd and Pb isotope data for two carbonatites and two dyke rocks are given by le Roex and Lanyon (1998). There are two oxygen isotope analyses in Harris (1995).

Fluorite was worked from 1949 to 1955. Van Zijl (1962) estimates reserves at 7-10 million tons at >35% CaF2.
A Rb-Sr isochron based on separated minerals gave an age of 126.6(7.3 Ma (Milner et al., 1995).
HARRIS, C. 1995. Oxygen isotope geochemistry of the Mesozoic anorogenic complexes of Damaraland, northwest Namibia: evidence for crustal contamination and its effect on silica saturation. Contributions to Mineralogy and Petrology, 122: 308-21.LE ROEX, A.P. and LANYON, R. 1998. Isotope and trace element geochemistry of Cretaceous Damaraland lamprophyres and carbonatites, northwestern Namibia: evidence for plume-lithosphere interactions. Journal of Petrology, 39: 1117-46.MILNER, S.C., LE ROEX, A.P. and O’CONNOR, J.M. 1995. Age of Mesozoic igneous rocks in northwestern Namibia, and their relationship to continental breakup. Journal of the Geological Society of London, 152: 97-104.PRINS, P. 1981. The geochemical evolution of the alkaline and carbonatite complexes of the Damaraland igneous province, South West Africa. Annale Universiteit van Stellenbosch, Serie A1, Geologie, 3: 145-278.VAN ZIJL, P.J. 1962. The geology, structure and petrology of the alkaline intrusions of Kalkfeld and Okorusu and the invaded Damara rocks. Annale Universiteit van Stellenbosch, Serie A, 37: 237-339.
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