Radioactive Carbonatites of Pakkanadu and Mulakkadu, Salem District, Tamil Nadu

Precambrian carbonatites associated with pyroxenites occur as discontinuous lenticular bodies intruding the syenites west of Pakkanadu (11°40'15'': 77°50'1'') and Mulakkadu (11°44'3'' : 77°50'2''). They strike northeast to southwest with easterly dips and have been emplaced along a major northeasterly trending deep crustal fault zone. Large outcrops of serpentinised dunites carrying accessory chromite, cut by asbestos and magnesite veins are also found along the same fault zone to the east and northeast of Pakkanadu.

The carbonatites are of the pure calcite-rich sovite type, with biotitic and ankeritic variants. Apatite, magnetite, allanite, barite, monazite, zircon and cerianite, are the accessory minerals.

Spectrographic analysis has revealed Ba, Sr, Ce and La in major amounts (more than 10,000 ppm) and Th, Nb, P, Zr, Y and Sc in minor amounts (< 10,000 ppm). Spectrographic data suggest that the carbonatite-pyroxenite complex is Ce-La rich.

Niobium is not found in economic concentrations. The general radioactivity recorded by the carbonatite-pyroxenites ranges from 5 to 20 x BG. Representative whole rock samples assay from 0.02% to 0.054% _eU₃O₈ (Carbonatites) and 0.02% to 0.06% _eU₃O₈ (pyroxenites). Allanites assay from 0.35% to 0.4% _eU₃O₈ with chemical U₃O₈ values from 0.007% to 0.034%, thereby indicating the predominance of thorium over uranium. Monazite is mostly concentrated in the biotite-rich portions of the carbonatites to the west of Pakkanadu, whereas allanites predominate in the pyroxenites west of Mulakkadu.