Alkaline Rocks and Carbonatites of the World

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Jere-Sanga

stripes

Occurrence number: 
122-00-022
Country: 
Nigeria
Location: 
Longitude: 8.9, Latitude: 10.25

This complex overlaps Saiya-Shokobo (No. 122-00-021) to the north the peripheral ring-dyke enclosing rocks of the southern end of that complex together with large areas of basement granite gneiss. The basement rocks have been eroded to basins within the complex while the younger granites form prominent peaks and hills. Initial activity involved emplacement of agglomerates, ash fall tuffs and ignimbrites small areas of which are preserved in the central and eastern parts of the complex. These rocks are essentially rhyolitic in composition and include vent rocks as well as quartz-fayalite porphyry intrusions. The volcanics appear to form a continuum with the volcanic rocks distinguished as belonging to the Saiya-Shokobo centre. There followed intrusion of syenites, notably the Limoro ring-dyke, followed by peralkaline and biotite granites which form major intrusions, particularly across the southern part of the complex. The ring-dyke is continuous in the north and west but has been mostly obliterated elsewhere by intrusion of later granites. The same rock type also forms a large central intrusion between areas of included basement gneiss and other intrusions in the northeastern part of the complex. The rock is variably porphyritic or equigranular with phenocrystal and groundmass perthite and oligoclase-andesine, interstitial quartz, aegirine-augite, often rimmed by hornblende, which also forms discrete crystals, sometimes olivine and, in one area, biotite. The earliest, and most extensive, of the granite intrusions are biotite granites which develop three major centres across the southern margin of the complex. Tin mineralization and the highest concentrations of primary columbite are associated with these granites. Three riebeckite granites were described by MacLeod (Buchanan et al. 1971), but according to Kinnaird (1981) the one located along the southern margin contains arfvedsonite rather than riebeckite. The riebeckite granite in the southwest of the complex cuts biotite granite and consists of microcline, much less albite, riebeckite and, in some facies, very abundant astrophyllite. The riebeckite granite on the eastern side contains biotite while the southern arfvedsonite granite is also variably biotitic with some variants free of amphibole. A dense swarm of microgranitic dykes cuts the basement and ring-dyke on the western side of the complex and a swarm of arcuate felsite and some basic dykes lie south of the complex. An analysis of the Limoro syenite is given in MacLeod et al. (1971).

Economic: 
Cassiterite, with minor amounts of columbite, has been mined from alluvials (Buchanan et al., 1971).
References: 
BUCHANAN, M.S., MACLEOD, W.N., TURNER, D.C., BERRIDGE, N.G. and BLACK, R. 1971. The geology of the Jos Plateau. Volume 2, Younger granite complexes. Bulletin, Geological Survey of Nigeria, 32: 1-159.KINNAIRD, J.A. 1981. Geology of the Nigerian anorogenic ring complexes. Geological map, scale 1:500 000. St. Andrews University Younger Granite Research Group.MACLEOD, W.N., TURNER, D.C. and WRIGHT, E.P. 1971. The geology of the Jos Plateau. Volume 1, General geology. Bulletin, Geological Survey of Nigeria, 32: 1-110.
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