Alkaline Rocks and Carbonatites of the World

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Grootpenseiland And Marinkas Kwela (Grootpens Eiland, Marinkas Quellen)

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Occurrence number: 
116-00-042
Country: 
Namibia
Region: 
Kuboos-Bremen and Bremen Garub lines of intrusions
Location: 
Longitude: 17.42, Latitude: -28.17

These two complexes are in contact and define a northeast-southwest-trending 14 km long line which appears to end at the Orange River. (Smithies and Marsh, 1996). The line continues to the southwest in the form of the Tatasberg complex (see South Africa No. 116-00-037). The smaller Grootpenseiland complex, in the southwest, varies from nepheline syenite in the southwest, through syenite to a granite stock in the northeast. Similarly, the Marinkas Kwela complex comprises nepheline syenites in the southwest, various central syenites and a large granite intrusion in the northeast. The nepheline syenites vary from mesocratic to melanocratic varieties and from biotite- to amphibole-rich types. The feldspar is perthite and pyroxene diopside-hedenbergite with up to Ac35 in more leucocratic varieties. The coarse, leucocratic nepheline syenite that dominates the southwestern part of Marinkas Kwela is described as a foyaite by Smithies and Marsh (1996) and comprises amphibole- and pyroxene-rich types the amphibole varying from hastingsite to taramite and the pyroxene from hedenbergite to aegirine-augite. Nepheline may be abundant and sodalite may be present. Some syenites contain katophorite and aegirine-augite. Granite forms about half the outcrop of both complexes and varies from monzonite to peralkaline granite, which is the dominant type in Marinkas Kwela. The peralkaline granite contains aegirine-augite which is accompanied by arfvedsonite and winchite. There are bulk rock analyses and some pyroxene and amphibole analyses in Smithies and Marsh (1996).Bernasconi (1981 and 1986) indicates that a pear-shaped carbonatite body of 1.5x1 km lies north of Marinkas Kwela, together with a number of carbonatite dykes, one of which is about 2 km long. The carbonatite is described as coarse-grained, grey and dark brown, the latter with disseminated limonite.

Age: 
Rb-Sr whole rock determinations gave 529(24 Ma for the oldest phase of Grootpenseiland and 514(26 Ma for the youngest phase of Marinkas Kwela (Smithies and Marsh, 1996).
References: 
BERNASCONI, A. 1981. The Marinkas Kwela alkali-granite intrusive, a probable porphyry molybdenum system of Cambrian age, Namibia. Mineralium Deposita, 16: 447-54.BERNASCONI, A. 1986. The Marinkas Kwela alkali intrusive - a porphyry molybdenum system of Cambrian age in southern South West Africa. In: C.R. Anhaeusser and S. Maske (eds), Mineral Deposits of Southern Africa, 2: 1587-91. The Geological Society of South Africa, Johannesburg.SMITHIES, R.H. and MARSH, J.S. 1996. Alkaline rocks in the Kuboos-Bremen igneous province, southern Namibia: the Grootpenseiland and Marinkas Kwela complexes. Communications of the Geological Survey of Namiba, 11: 13-20.
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