A note on the occurrence and formation of magnetite in the carbonatites ofSevvattur, North Arcot district, Tamil Nadu, Southern India

The important accessory mineral magnetite is ubiquitously present within al l constituent members of the Proterozoic zoned carbonatite body of the Sevv attur complex, North Arcot District, Tamil Nadu in southern India. The carb onatite appears to have been mainly emplaced as an arcuate or crescent-shap ed, zoned intrusion. The carbonatite contain two generations of magnetite t hat occur as (1) opaque dust-like inclusions in the carbonate minerals, and (2) large euhedral phenocrysts. The latter measures up to 10 cm in length, are primary in nature and found mainly associated with essential minerals within the ferro-carbonatites (para-ankeritic carbonatites). In this paper, we present the detailed ore petrographic features of magnetite occurring w ithin carbonatites of the Sevvattur complex. It is suggested that fine dust -like inclusions in the carbonate minerals may have formed at a late stage through dissociation of ankerite to calcite and magnetite. This occurred du ring upward migration of melts from a deep magma chamber that subsequently suffered secondary oxidation. The phenocryst variety, however, crystallized directly as primary minerals in the magma. (C) 2001 Elsevier Science Ltd. All rights reserved.