Lages (Fazenda Varela)

Within the Lages dome, which is some 30 km in diameter, numerous alkaline intrusions are emplaced, mainly within Permo-Carboniferous sandstones. The majority of the intrusions are sheet-like in form but dykes up to several 100s of metres in thickness also occur. Sills have also been encountered in drill holes. Apart from a few dykes of olivine melilitite, nephelinite and melteigite, and the Fazenda Varela carbonatite, the alkaline rocks are predominantly nepheline syenites, analcime trachytes and phonolites. They comprise alkali feldspar, 20-30% nepheline, aegirine, pectolite and a range of Ti and Zr minerals, including eudialyte, neptunite, murmanite, lavenite and astrophyllite. Olivine melilitites occur as narrow dykes and consist of melilite, olivine, augite, nepheline, biotite and magnetite. Carbonatites occur at Fazenda Varela in the centre of the Lages dome (Scheibe, 1978). They are found as dykes and veins cutting feldspathic breccias over a circular hill about 600 m in diameter. The breccias are highly potassic (Scheibe and Formoso, 1982, Anexo) and represent fenitized sandstones. The carbonatites are of two types; beforesites forming a 5 m thick dyke and other bodies and consisting of zoned dolomite, 5-15% microperthite, 2-5% synchisite, 2-5% baryte, pyrite and pyrochlore; and ankerite beforsites which form thin stockwork veins and consist of dolomite and/or ankerite, subsidiary calcite, goethite, synchisite (5%), quartz and baryte. Feldspathization of sandstones is observed around the feldspathic breccia body. About 30 volcanic breccia bodies have been identified, four of which have kimberlitic affinities (Scheibe et al. 1984). Analyses for major and minor elements, including REE, for a range of carbonatites and associated rocks are given by Scheibe and Formoso (1982).