Alkaline Rocks and Carbonatites of the World

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Agate Mountain


Occurrence number: 
Longitude: 12.08, Latitude: -18.45

This complex is located in a range of low hills almost totally surrounded by salt pans 6.5 km east-southeast of Cape Fria in a half graben parallel to the coast. The approximately circular, 1.1 km diameter Agate Mountain intrusion is emplaced in basalts and quartz latites of the early Cretaceous Etendeka Group. The complex has a fenite aureole with a width of up to 2 km. The intrusion comprises a discontinuous ring of sovite which is variably yellow to brown and massive to banded containing manganiferous nodules up to 10 cm across. Patches of dark grey carbonatite, often rich in bastnasite, appear to be late-stage replacements. A fluorite-bearing breccia with fragments generally <2 cm forms a central depression to the complex and extends to the eastern margin. It also forms a broad tongue that intrudes the fenites near the southern margin. The breccia consists essentially of fenitized fragments of quartz latite and basalt cemented by calcite. Two small dolerite plugs and a 100x4 m dyke cut the fenites while a small (7 m) plug of nepheline-bearing basanite also cuts the fenites and core breccia. Miller (Pers. comm., 1999) has mapped 23 carbonatitic breccia plugs within the fenite aureole the largest of which is 1.2x0.7 km. Some plugs are choked with fragments of basalt, sandstone, fenitised granite, gneiss, quartz latite and older carbonatite breccia, but in other fragments are rare. Late, fine-grained dykes of grey sovite, and less commonly of yellow beforsite, up to 30 cm thick occur within the complex and up to 4.4 km from the centre. Minor phonolite dykes and plugs with microphenocrysts of sanidine, altered plagioclase, nepheline, aegirine, biotite and altered sodalite (?) intrude the basalts and quartz latites between 1.1 and 3.9 km from the centre of the complex. In the fenite aureole quartz latites are intensively altered but basalts and dolerites are only altered within 20 m of the sovite margin. At 2 km from the sovite only patches of greenish alteration occur in the flow-top breccias of the quartz latite flows. As the sovite is approached the latites become an intense green, this green collar being between 0.4 and 0.6 km wide. The inner zone is brecciated and intruded by numerous, short, ramifying, fine-grained carbonatite dykes up to 10 cm thick. Several late, discontinuous, silicified ring fractures that carry minor botryoidal pyrolusite in places cut the fenites and carbonatites.

Post early Cretaceous.
R.McG. Miller, in prep. and pers. comm. 1999.
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith