Alkaline Rocks and Carbonatites of the World

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Doros

stripes

Occurrence number: 
116-00-015
Country: 
Namibia
Location: 
Longitude: 14.3, Latitude: -20.75

Doros is a layered gabbroic intrusion the only alkaline rocks being late aegirine bostonite dykes. The intrusion is 7x3.5 km and pear-shaped. The western intrusive margin is against Damara metasediments and Salem granite, but elsewhere is with Karoo sediments; the contacts dip inwards at 39-51°. Seven layers were mapped by Hodgson and Botha (1974) including a lower chill zone, while the third layer from the bottom is intrusive and partially transgressive. All the contacts are sharp. The layers consist essentially of plagioclase, pyroxene, olivine and magnetite differing in their proportions and compositions. Olivine is exceptionally abundant in the lower layers (>40%) and minor towards the top, while plagioclase and pyroxene increase markedly upwards. The bostonite dykes, which are up to 500 m long and 2 m wide, consist of about 85% orthoclase with the rest interstitial aegirine. Rock analyses for the layered series are given by Hodgson and Botha (1974).

Economic: 
Some interest has been shown in the intrusion by mining companies because of the presence of small amounts of native copper in the gabbroic rocks.
Age: 
K-Ar on gabbro gave 125 Ma and on plagioclase from the same rock 128 Ma (Siedner and Miller, 1968), while by the same method Siedner and Mitchell (1976) obtained 124 and 130 Ma from gabbro of the inner ring-dyke and 127 and 134 from plagioclase from the same rock.
References: 
HODGSON, F.D.I. and BOTHA, B.J.V. 1974. The Doros complex, South West Africa. Neues Jahrbuch fur Mineralogie, Monatshefte, 398-418.SIEDNER, G. and MILLER, J.A. 1968. K-Ar age determinations on basaltic rocks from South-West Africa and their bearing on continental drift. Earth and Planetary Science Letters, 4: 451-8.SIEDNER, G. and MITCHELL, J.G. 1976. Episodic Mesozoic volcanism in Namibia and Brazil: a K-Ar isochron study bearing on the opening of the South Atlantic. Earth and Planetary Science Letters, 30: 292-302.
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