O DEPÓSITO DE NÍQUEL DE JACUPIRANGA (SP): EVOLUÇÃO MINERALÓGICA E GEOQUÍMICA

SÔNIA MARIA BARROS DE OLIVEIRA, JEAN-JACQUES TRESCASES

Resumo


The nickel deposit of Jacupiranga was formed by lateritic weatheríng of ultramafic rocks. The estimated reserves are 13 m. ton, 1.4% Ni. The alteration profile ís very thick and consists from bottom to top: a) fresh rock: serpentinized dunite; b) altered rock and argillaceous saprolite: serpentine and smectite, 30 to 40 m thick; c) silicified ferruginous saprolite: quartz and goethite, up to 25 m thick; d) upper horízon: kaolinite and goethite, up to 10 m thick. The weathering evolutíon of the serpentinized dunite has led to a loss of Si and Mg and a relative accumulation of Fe and Al. In the silicified horízon absolute accumulation has occurred. The upper horizon seems to be only partially derived from the evolution of the dunite (too high content of Al). Nickel ís mainly concentrated in the altered rock and in the argillaceous saprolite associated with serpentine and smectite. Furthermore ít occurs as pimelite in the garnieritic veins that cut the altered rock. The content of Ni in these horizons ranges between 0.5% and 2% (up to 9% locally). In the ferruginous saprolite there is always less Ni (< 1%), probably associated with geothite. Three factors make difficult the exploitation of the ore deposit: a) the thick overburden; b) the irregularity of the Ni distribution; c) the high sílica content of the ore.

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