Several stratiform sulphide deposita disseminated in argillaceous lithology rich in organíc material, show primary structural features which suggest their formation at the time of deposition of host rock. The distribution of these deposíts seems to be more noticeable in rift type basins or in structured sedímentary basins in a tectonic environment with geothermal gradient higher than normal, at least during the mineralization period. The unlínk of the mineralization mechanism to direct volcanic

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... events and the elose relatícnship between these processes with geothermically anomalous zones of the basin, are widely discussed. The formation of these deposita require the coincidence of exhalative events of connate metalliferous brínes at the bottom of a euxinic sapropelic basin in active process of biogenie generaticn of H 2S. Finally, the main parameters whích determine mineralogie zoning (frequently present in thís type of deposit) are postulated , and are discussed the mechanisms which regulate the dístribution and grades of metal sulphídes in black shales. * Petrobrás MineraçãoS.A. Petromisa, Av. Presidente Vargas, 583, 199 andar, CEP 20071, Rio de Janeiro, RJ, Brasil