Ulanerge

The alkaline rocks of this complex cut Upper Proterozoic marbles and Palaeozoic granite dykes. It comprises predominantly hastingsite-nepheline syenites which are cut by pegmatite veins of nepheline syenite and dyke-like and stock-like bodies of quartz syenite and granosyenite. The nepheline syenites have a banded structure as indicated by the distribution of mafic minerals. The principal rock-forming minerals are nepheline (28-56%), microcline (25-55%) and hastingsite (6-25%) with lesser biotite, albite, calcite, possibly fluorite, titanite, apatite, titanomagnetite, pyrrhotite, andradite, graphite, pyrochlore and britholite. Nepheline syenite pegmatites are 0.5-3 m thick, up to 150 m long and are zoned. The centres are composed of giant crystal aggregates of microcline, nepheline, hastingsite, lepidomelane, cancrinite and cleavelandite (variety of albite), which forms nests and pockets. The nepheline syenites are sometimes replaced by albite and there is accessory zircon, biotite, pyrochlore and thorite.