Rare-Earth-Element Geochemistry of Placer Monazites from Kalingapatnam Coast, Srikakulam District, Andhra Pradesh

Monazite, a rareWrth and thorium phosphate with colour varying from colourless to different shades of yellow, green and brown, is one of the major components (0.02 to 2.93%) in the heavy mineral sands of Kalingapatnam coast of Andhra Pradesh. Subtle but significant variation in colour, shape, size, intensity of micro-fractures and radiation cracks besides provenance rocks warranted (1) detail mineral chemistry at micro-domain level involving multi-elemental qualitative and quantitative analyses, (2) geo-chemical evaluation and interpretations of micro-domain chemistry data using bi-variate (LREE, REE arid REE+Y vs. Th+Ca; P+LREE, P+REE and P+LREE+Y vs. Th+Si) and ternary (LREE, Th, Ca and P; REE, Th, Ca and P) plots and (3) regression statistics, Principal Component Analysis (PCA) and R-mode Factor Analysis (RFA) aiding principal substitutions amongst major and minor elements at structural level in representative grains of different monazites.

The electron microprobe analysis data exhibit wide variation in constituent elements and compositional heterogeneity in monazite grains, especially ifl RE₂0₃ (45.40 to 63.53%), Th0₂ (5.69 to 23.17%) and UO₂(0.00 to 2.07%) indicating their derivation from different rocks occurring in the hinterland comprising khondalites, granite gneisses and charnockites. Two types of detrital monazites identified with different REE distribution pattern are of much significance and significant correlation was observed amongst constituent major and minor elements. Evaluation of mineral chemistry data on such monazites using graphical and rMti-variate statistical techniques reveal that thorium is accommodated in its structure by charge-coupled substitution as depicted by equation viz. Th⁴⁺ + Ca²⁺ 2REE³⁺ and Th⁴⁺ + Si⁴⁺ - P⁵⁺ + REE³⁺ . Th and Ca content of monazite grains &re correlated inversely with light REE and positively with HREE and Y besides their minor enrichment.