stripes
This occurrence lies beneath a cover of over 100 m of Karoo sediments and was detected from the large aeromagnetic anomaly that it generates. It is an eroded volcano having an apparent extent of about 16x14 km and is the most northerly and extensive member of the Pienaars River Alkaline Complex. Direct knowledge of the rocks comes from three boreholes in the southeastern sector of the volcano (Frick and Walraven, 1985), which indicate that the volcanic rocks are a mixture of breccias, tuffs and lavas. The pyroclastic nature of many of the rocks, combined with the fine grain size and extensive alteration makes classification difficult, but analyses indicate compositions ranging from alkaline rhyolite through alkaline trachyte to phonolite and tephritic phonolite. Fresh nepheline does not occur but normative ne values up to 15 are found and some rocks are lc normative. The tuffs and breccias include airfall, lapilli and welded types. Rock fragments in the tuffs are up to 5 cm across and include trachytic and 'basaltic' rocks, similar in composition to the flows. These rocks are extensively altered and often replaced by carbonate. The welded tuffs are composed predominantly of glassy fragments amongst which shards are apparent. Lavas include trachytic and phonolitic varieties and the former include porphyritic and glassy types. Phenocrysts of sodic orthoclase are generally 1-5 cm in diameter and often partially or wholly replaced by calcite or chlorite. Other phenocrysts are of magnetite or chlorite after pyroxene. In some glassy trachytes aegirine phenocrysts are preserved. The phonolitic lavas are also extensively altered and now consist of phenocrysts of magnetite and ilmenite, porphyroblasts of pyrite and titanite, pseudomorphs of chlorite after pyroxene, with occasionally a little unaltered titanian augite, plagioclase, which is generally replaced by chlorite, epidote and albite. Chemical analyses of 28 rocks, including some trace elements, are given by Frick and Walraven (1985) and discussed in some detail.
FRICK, C. and WALRAVEN, F. 1985. The petrology and geochemistry of the pre-Karoo Elandskraal volcano, South Africa. Transactions of the Geological Society of South Africa, 88: 225-43.
