Jebel Dumbeir

An oval-shaped complex of about 13x4 km Dumbeir is dominated by a pink, coarse K-rich syenite that forms 90% of the exposed rock. The syenite consists of 70-80% megacrysts of orthoclase with cancrinite aggregates replacing nepheline, muscovite, ilmenite and hematite; interstitial calcite and fluorite form 10% of the rock. This rock is essentially an orthoclasite, and chemical analyses show 10.8-13.4% K2O. The country rock biotite-muscovite gneiss is gradually transformed to orthoclasite at the contacts. On the northern and western margins of the orthoclasite, and as inclusions within it, is a syenite of 40-60% orthoclase phenocrysts, often replaced by albite, nepheline, partly altered to cancrinite, 10-20% interstitial sodalite, 10-30% phlogopite, pyroxene zoned to aegirine-augite, and a blue richteritic or magnesio-arfvedsonitic amphibole. This rock type is referred to as ditroite by Harris et al. (1983a). Carbonatites and carbonatite breccias are confined to the northeastern part of the complex and define dykes and elliptical bodies cutting basement gneisses and orthoclasite. The carbonatites include aegirine-phlogopite, fluorite-apatite and phlogopite sovites (El Sharkawi, 1975) in which strontianite, pyrochlore, sodic amphibole and a range of opaque minerals are present. Some chemical data are available in El Sharkawi (1975). Northeast-trending fluorite veins, which also carry quartz, opaque minerals and carbonate, are common. To the northeast of the main complex, in an isolated outcrop, two areas of pyroxenite consisting of aegirine-augite, orthoclase, plagioclase, apatite, carbonate and pyrochlore are identified as fenite. Phlogopite, idocrase, apatite and Nb-rich rutile are concentrated at the contacts with fluorite veins. Harris et al. (1983a) give chemical analyses of sodalite, phlogopite, aegirine-augite and sodic amphibole from ditroite and a range of analyses of orthoclasite and ditroite, including trace elements with some REE data. Only trace elements are given for carbonatites.