The Early Cretaceous Sung Valley Ultramafic-Alkaline-Carbonatite (SUAC) complex intruded the ProterozoicShillong Group of rocks and located in the East Khasi Hills and West Jaintia Hills districts of Meghalaya. The SUAC complex is a bowl-shaped depression covering an area of about 26 km 2 and is comprised serpentinised peridotite forming the core of the complex with pyroxenite rim. Alkaline rocks are dominantly ijolite and nepheline syenite, occur as ring-shaped bodies as well as dykes. Carbonatites are, the youngest intrusive phase in the complex, where they form oval-shaped bodies, small dykes and veins. During the course of large scale mapping in parts of the Sung Valley complex, eleven carbonatite bodies were delineated. These isolated carbonatite bodies have a general NW-SE and E-W trend and vary from 20-125 m long and 10-40 m wide. Calcite carbonatite is the dominant variety and comprises minor dolomite and apatite and accessory olivine, magnetite, pyrochlore and phlogopite. The REE-bearing minerals identified in the Sung Valley carbonatites are bastnäsite-(Ce), ancylite-(Ce), belovite-(Ce), britholite-(Ce) and pyrochlore that are associated with calcite and apatite. IntroductionCarbonatites are known to host high concentrations of rare earth elements (REE) [1]. Moreover, syenites, phonolites, alkali granite, pegmatite and secondary or supergene en- * E-mail: mohd.sadiq.gsi@gov.in richment in some bauxite, laterite, phosphorites, placers and beach sands are also potential hosts for REE's. Rare Earth Metals are becoming a critical strategic resource with an increase in demand mainly from scientific and military applications. About 95% of the REE export market is dominated by China who is a leading producer of REEs. In China, production is mainly from the Bayan Obo REE-iron ore deposits and clay deposits in southern China, and rest of the world has comparatively limited production [2]. Prior to the rise of Chinese REE production, monazite and xenotime-beach placer deposits in India, Brazil and Australia had been the main source for REEs. With the discovery of carbonatite-hosted REE deposits (mainly bastnäsite-(Ce)), carbonatite has become a primary resource of world REE production e.g. Mountain Pass, USA. In India, the first carbonatite-hosted REE deposit has been discovered at Kamthai, Barmer district, Rajasthan and contains a highest LREE grade of 17.31 wt% and a weighted average grade is 2.97 wt% LREO with a total volume of 1,38,428 tonnes [2]. The Shillong Plateau of northeastern India hosts four intrusive alkaline-carbonatite complexes, the Samchampi, Sung Valley, Jasra-Barpung and Swangkre complexes [3,4]. Sung Valley was the first carbonatite complex reported from north east India [5]. These complexes, especially the Sung Valley complex, has drawn attention of many researchers mainly for two reasons. Firstly, its petrogenesis, emplacement age and relationship with associated silicate rocks and secondly to assess mineral potential of the alkaline-carbonatite complex [3,4,[6][7][8][9][10][11]...
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