MINERALIZATION AND WALL-ROCK ALTERATION AT THE BOGALA GRAPHITE DEPOSIT, BULATHKOHUPITIYA, SRI LANKA.

Vein-type mineralization occurs in the zones of deep-seated fractures in supracrustal stratiform metasedimentary rocks resulting from the differential rise of subcrustal migamatites. The mineralization at Bogala comprises several veins of graphite and a large vein of syenite pegmatite. Mineralogical and geochemical study of the wall rocks reveals a well-zoned alteration halo about the deposit. Chemical zoning in the altered wall rock and the relict mineral replacement textures indicate deep-seated hydrothermal activity. Formation of vein graphite by decarbonation of carbonates and reduction of CO//2 formed during the decarbonation reaction is indicated by the available carbon isotope data. Chemical zoning in the alteration halo may be a useful aid in the exploitation of known graphite deposits.