Dorowa

The Dorowa complex is intruded into relatively uniform Archaean granitic gneisses. It consists of three ijolite and foyaite and several carbonatite intrusions all of which lie within an extensive area of fenites, which comprise nearly 90% of the complex. The fenites form a horseshoe-shaped ridge of hills that enclose the aligned and more deeply eroded ijolite-foyaite centres. A small intrusion of ijolite - the Zwibe plug -is located about a kilometre west of the main complex and the Chikomo ijolite plug is some 5 km to the south-southeast. The carbonatites form an oval intrusion of approximately 100x70 m, from which several sheet-like apophyses extend, together with a number of dykes. Most of the carbonatite is calcitic, alvikite and sovite being distinguished by Johnson (1961, 1966), but a little beforsite occurs as a dyke and a 3 metre-thick sheet in the southeastern part of the plug, which is concordant with banding of the enclosing alvikite. Beforsite also forms lenses several centimetres thick and a metre or more in length in one alvikite dyke. The alvikites contain magnetite, usually as euhedra, as the principal accessory, but coarser sovite dykes also have phlogopite, apatite and pyrite. Some of the beforsites are porphyritic with dolomite phenocrysts up to 4 cm across and with coronas of radial, fibrous dolomite set in a dolomitic matrix. The carbonatites of both the plug and dykes are in places partially replaced by microcrystalline quartz. The three areas of nepheline-rich rocks in Dorowa were originally considered by Johnson (1961) to be mobilised fenites, but later he concluded that they were igneous intrusions with sharp contacts against fenites and with included fenite blocks (Johnson, 1966). A medium-grained ijolite is the commonest rock type with the local development of foyaite, malignite and apatite-pyroxene rocks. Aegirine-augite, comprising more than 50% of malignites, is enclosed poikilitically by nepheline and in the foyaites and malignites by orthoclase. Biotite, apatite and magnetite are accessory. Three dykes, generally about 2 m wide but up to 5 m, consisting of >95% magnetite occur in the southwest of the complex. Magnetite crystals average 20 cm in diameter and there is some sugary apatite. Veins up to 10 cm thick of magnetite-apatite rock are present sporadically throughout the complex but particularly in the north. They also contain a little serpentine which in places clearly pseudomorphs olivine. Two bodies of the rock are, however, much larger, one being a tabular mass of 65x20 m in plan and about one metre thick, the other a dyke about 150 m in length. The magnetite-apatite veins are generally located within dyke-like masses of vermiculite-apatite rock. The latter rock is concentrated in dykes, veins and stringers within fenites at the northern end of the complex and forms a zone nearly one kilometre across. A second zone of vermiculite-apatite veins is distributed about the more southerly ijolite-foyaite intrusion. Vermiculite rock was also encountered in trenches at the southern edge of the carbonatite plug forming a body approximately 15 m thick between the carbonatite and fenite. Apart from the two principal minerals accessory magnetite, quartz and baddeleyite are found in the vermiculite-apatite rocks. A body mapped as magnetite-serpentine rock by Johnson (1961) within the vermiculite-apatite zone in the north of the complex, proved in thin section to consist of a little magnetite and apatite in a matrix largely of microcrystalline quartz and limonite pseudomorphing a reticulate serpentine texture. This body was probably originally peridotite. The most extensive area of rocks at Dorowa is the fenites of which fenitized granite gneiss, quartz syenitic fenite, syenitic fenite and pulaskitic fenite have been distinguished. There is a gradual increase in the degree of fenitization inwards with the pulaskitic fenites concentrated around the ijolite-foyaite intrusions. The outermost signs of fenitization, with narrow aegirine veinlets, are nearly a kilometre beyond the outer boundary of syenitic fenites delimited on the map. Inwards there is the development of aegirine-augite, a little apatite and crystallization of new alkali feldspar at the expense of biotite and quartz. The pulaskitic fenites consist of alkali feldspar, aegirine-augite, apatite, calcite, zeolite and a little nepheline. A body of dolerite at the northern margin of the complex has also been fenitized. The Zwibe plug to the west of Dorowa is approximately 110x40 m, oval in plan and cuts granitic gneiss (Johnson, 1961, Fig. 4). It consists predominantly of ijolite but with some olivine ijolite and a porphyritic nephelinite. An interstitial brown amphibole is present in the ijolite and nephelinite. There is a fenite aureole a few metres wide. The Chikomo plug to the south-southeast is also emplaced in granite gneiss and gives rise to a prominent hill. It is formed of olivine ijolite and has a narrow fenite aureole with a tongue of fenite extending from the south to the summit of the hill, which probably represents a roof fragment. Nephelinite dykes outcrop in various river courses around the complex and contain phenocrysts of nepheline, amphibole and, less commonly, biotite in a matrix largely of nepheline and sodic pyroxene; olivine has also been identified in one dyke. Analyses of fenites and an ijolite from Dorowa together with ijolite, olivine ijolite and nephelinite from the Zwibe intrusion are in Johnson (1966). Analyses, including a broad suite of trace elements and Sr, Nd and Pb isotope data, of seven ijolites from the Chikomo and Zwibe plugs, four nephelinites from dykes, and a carbonatite from Dorowa are given by Harmer et al. (1998).