Alkaline Rocks and Carbonatites of the World

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Homa Mountain

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Occurrence number: 
085-00-044
Country: 
Kenya
Region: 
Homa Bay district
Location: 
Longitude: 34.5, Latitude: -0.38

Homa Mountain is located on the south side of the Kavirondo Gulf east of Homa Bay and consists of a central area of cone-sheets of carbonatite and breccia with numerous surrounding subsidiary centres, the whole complex being some 20 km across with the main mountain rising to 600 m above the lake. The Precambrian Nyanzian basement rocks have been sharply updomed such that much of the summit area of the Mountain is occupied by shattered Nyanzian rocks, while within the steep surrounding walls are concentrated the carbonatite cone-sheets. According to Le Bas (1977) the earliest activity involved intrusion of ijolite, updoming and brecciation of the basement and fenitization. Ijolite forms an arcuate mass of about 6x3 km on the south side of the mountain and is found as blocks in agglomerates at a number of vents. The ijolites are typically heterogeneous with angular areas of melanocratic ijolite within more leucocratic varieties and a banding is widespread consisting of layers of pegmatitic facies and comb structures of pyroxene, wollastonite and apatite. Although the pyroxene in some pyroxenites and jacupirangites is diopside or augite, the normal type is zoned to aegirine-augite rims. Besides nepheline and melanite, accessories include calcite, apatite, titanite and, in some rocks, wollastonite. Wollastonite ijolite forms late pegmatitic veins. In the marginal zones of the ijolite intrusion feldspar is more abundant, the pyroxene is aegirine and there are some nepheline syenites, which are also found as veins in the adjacent fenites. There are also several areas of phonolite immediately south of the ijolite, which probably form plugs and contain xenoliths of ijolite. The shattering and fenitization of the Nyanzian basement extends for up to 7 km from the centre of the complex and is characterised by thin veinlets of sodic pyroxene and amphibole that are most intense in the zone of cone sheets on the upper part of the Mountain. Le Bas (1977) considers the fenitization to have been produced by the ijolite and it is particularly intense in the vicinity of the ijolite body where distinctive syenitic contact rocks occur. The main carbonatite intrusive event then took place which was preceded by potassic feldspathization of the ijolites and fenites. The upper ramparts of the mountain consist of innumerable cone sheets of carbonatite and ‘ochreous breccia’ that range from 10 cm to 5 m in width. Le Bas (1977) indicates that the carbonatites, in order of emplacement, are sovites, alvikites, late stage ferrocarbonatites and further alvikites. The sovites characteristically feldspathize their wall rocks and consist of calcite, apatite, pyrochlore, phlogopitic biotite and rare sodic pyroxene. The alvikites are the most prominent cone sheets on the upper part of the mountain and also define a number of centres scattered around it. They are emplaced in fenites but do not cause feldspathization. They dip inwards at angles from 60° at the inner part to 30° at the periphery of the complex and consist essentially of calcite with accessory magnetite, apatite, pyrochlore, aegirine and biotite rarely exceeding 1%. The late stage ferrocarbonatites form veins within the alvikites and a swarm of narrow dykes cutting the earlier carbonatites. They contain monazite, baryte, fluorite and dahllite. The last phase of alvikite comprises dykes up to 15 cm wide which frequently contain lath-shaped calcite aggregates that are interpreted as being pseudomorphs after melilite. The ochreous breccias form cone sheets, radial dykes and a few pipe-like bodies. They may consist entirely of fragments of Nyanzian country rocks in a matrix of iron oxides, or fragments of fenitized Nyanzian together with some blocks of carbonatite, ijolite and phonolite. Contemporaneous with the cone sheet activity numerous small carbonatite centres were intruded around the mountain, which are described individually by Le Bas (1977). Each had a similar development of rock types to the main centre, was domed and brecciated and displays feldspathic fenitization. The last stage of igneous activity is marked by intense pyroclastic and carbonatitic volcanism and the intrusion and extrusion of melilitites and phonolites. Got Oloo is an olivine melilitite plug on the west side of Homa and Nyamatoto a large phonolite plug to the north which has several smaller plugs in its vicinity. The phonolite at Osiri, northwest of Homa, in contrast, is a remnant of lava that originated further east. Got Chiewo is a small volcano on the southeastern side of Homa Mountain consisting of a well preserved carbonatite cone composed of bedded, grey lapilli tuffs (Clarke and Roberts, 1986). The earliest rocks are breccias which were followed by intrusion and extrusion of dykes, lavas and tuffs, rectangular carbonate aggregates in which were considered to be pseudomorphs after melilite. However, Clarke and Roberts (1986) re-interpreted the pseudomorphs as being after nyerereite. Some of these rocks are illustrated by Keller (1989) who describes them as agglutinated carbonatite spatter and ash. After a period of erosion bedded rocks were deposited which now cloak the higher slopes of the Mountain. They comprise breccias consisting of angular fragments of Nyanzian rocks and some of carbonatite in a finely communited matrix, limonite and a calcareous cement, as well as crystal tuffs, sands and marly limestones. The origin of these rocks is not clear but Le Bas (1977) suggests that they represent pyroclastic rocks together with some lake deposits. Some 14 km southeast of Homa Mountain is the 15 km long Samanga area of phonolite lavas, phonolite and phonolitic nephelinite plugs and minor lapilli tuffs. There are rock analyses in Le Bas (1977, Appendix 2) and C and O isotope data for carbonatites in Deines and Gold (1973). Kalt et al. (1997) give Sr, Nd and Pb isotope data for four carbonatites and an olivine melilitite.

Age: 
Eight K-Ar whole rock and biotite ages range from 1.3 to 12 Ma (Le Bas, 1977, Appendix 1).
References: 
CLARKE, M.C.G. and ROBERTS, B. 1986. Carbonated melilitites and calcitized alkali carbonatites from Homa Mountain, western Kenya: a reinterpretation. Geological Magazine, 123: 683-92.DEINES, P. and GOLD, D.P. 1973. The isotopic composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deep-seated carbon. Geochimica et Cosmochimica Acta, 37: 1709-33.KALT, A., HEGNER, E. and SATIR, M. 1997. Nd, Sr, and Pb isotopic evidence for diverse lithospheric mantle sources of East African rift carbonatites. Tectonophysics, 278: 31-45.KELLER, J. 1989. Extrusive carbonatites and their significance. In: K. Bell (ed) Carbonatites: genesis and evolution. 70-88. Unwin Hyman, London.LE BAS, M.J. 1977. Carbonatite-nephelinite volcanism: an African case history. John Wiley, London. 347 pp.
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