Rare Earth-Mineralogical and Petrochemical Characters of Syenite Rocks of Rairakhol Alkaline Complex, Odisha, India

Abstract

The Rairakhol alkaline complex situated in western Odisha consists of litho assemblages such as mafic-rich nepheline syenite, mafic-poor nepheline syenite and leucocratic syenite. These rocks have been investigated with respect to their mineralogy and chemistry with a special focus on their rare earth element (REE) content. Petrographic study reveals that the rocks are mainly medium to coarse grained with hypidiomorphic granular texture and composed of variable proportions of microcline/orthoclase, perthite, nepheline, albite, amphibole, biotite. The accessory minerals are calcite, sphene, zircon, thorium uranium oxide, apatite and opaques (mainly ilmenite, magnetite, pyrrhotite). Petrochemical analyses confirm that these nepheline syenites are typically miaskitic to marginally agpaitic with agpaitic coefficient [(Na + K)/Al molar prop.] varying between 0.75 and 1.01. Scanning electron microscope study shows that some REE-bearing mineral phases occur as inclusions or in interstitial spaces of the major mineral phases of nepheline syenite. The REEphases are of RE-sillicates, RE-oxides, and RE-phosphates which are generally rich in their LREE content. The identified REE-bearing mineral phases are britholite, allanite, apatite and zircon. The leucocratic syenite shows a strong positive Eu anomaly whereas the mafic-rich and mafic-poor nepheline syenites show a slightly negative Eu anomaly indicating that the rock originated from a highly fractionated magma. A rare Tm anomaly found in the leucocratic syenite indicates some influence of refractory dusts derived from carbonaceous chondritic material during their formation. This study provides an insight into the mineralogical and petrological peculiarities of this alkaline complex which might guide further exploration programs.