Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

C. Suryanarayana Rao ¹, G. R. Narayan Das ¹, B. Krishnaiah Setty ¹, N. V. A. S. Perumal ¹

Affiliations
1 Atomic Minerals Division, Department of Atomic Energy, Hyderabad, IN

Abstract

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.

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B. Krishnaiah Setty ¹, R. Dhana Raju ¹

Affiliations
1 Atomic Minerals Division, Department of Atomic Energy, Hyderabad 500 016, IN

Abstract

Taking advantage of the sympathetic variation of magnetite content with that of (i) total heavy minerals, (ii) ilmenite, and (iii) pyriboles in sand samples drawn from the Nizampatnam deposit along the south Andhra coast, regression equations relating magnetite with others are calculated. Using the wt. % of magnetite and these equations, the wt.% of total heavies, ilmenite and Pyriboles are calculated, which closely agree with the observed data, within a difference of 5%. The content of other important but minor heavy minerals like zircon, monazite and rutile need to be estimated by separation and microscopic grain-counting of only two fractions (i.e.) + 200 and -200 mesh-sizes, since these minor minerals are confined in these two fractions. By this method, consumption of heavy liquid and microscopic grain-counting can be substantially reduced by 70%, without sacrificing the accuracy for estimation of reserves of heavy minerals in this deposit.

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