Characteristics of source rocks of xenotime-bearing Riverine placers in Central India

In central India, economic concentrations of xenotime occur in riverine placers mostly in Chhotanagpur granite gneiss complex (CGGC) terrain. Its limited resources are also known from parts of Bastar craton. The source rocks for xenotime in these areas are felsic igneous bodies, especially specific granites and related pegmatitic bodies. Riverine placers occur as bars, islands and flood plain deposits. In the Kunkuri area in Jashpur district, where economic concentrations of xenotime occur in placers along the stream beds, texturally four different types of granites, forming southwestern part of the CGGC, have been identified. They are (i) Porphyritic Granite (PRG), (ii) Fine-grained grey granite (FGGG), (iii) Pink granite (PG) and (iv) Medium Grained Grey Granite (MGGG). All the variants of granitic rocks are peraluminous (A/CNK >1.0) with normative corundum (0.25 – 5.26%) and have a restricted range in major element composition with high SiO₂, Al₂O₃ and low CaO, MgO. Among four types, xenotime is hosted mostly in PG and FGGG and pegmatitic veins genetically related with them. Only in a few places xenotime occurs in MGGG also. LIL and HFS elements and their ratios show wide and systematic variations in different types pointing out the role of fractional crystallization in formation of these granites. In all types, there is a preponderance of LREE over HREE. The chondrite-normalised patterns of these granites also show enriched LREE and more or less flat HREE pattern with moderate to pronounced negative Eu-anomaly (Eu/Eu* = 0.19 – 0.64) indicating higher degree of plagioclase fractionation. Kunkuri granites are dated to be 816-1000 Ma with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7046-0.7538. Mineralogical, chemical and isotopic characteristics together with REE patterns of granites indicate them to be 'S-type granite' derived from a crustal source. In Bastar Craton, concentrations of xenotime-bearing placers are limited, and their source rocks (granites) are also peraluminous, two-mica type, and have formed from crustal sources. However, relative to source granites of xenotime-bearing placers of CGGC terrain, source granites of Bastar craton are older. The data suggests that the younger granitic rocks carry relatively more concentrations of xenotime, and hence areas around such bodies may be looked for xenotime-bearing placers in their vicinity. (author)