Pirosmalita

Revista Brasileira de Geociências

29(3):345-348, setembro de 1999

OCORRÊNCIA DE FERROPIROSMALITA NAS BRECHAS MINERALIZADAS
DO DEPÓSITO DE Au-Cu-(±ETR-U) DE IGARAPÉ BAHIA,
PROVÍNCIA MINERAL DE CARAJÁS
EDISON TAZAVA* , CLAUDINEI GOUVEIA DE OLIVEIRA** & NEWTON SOUZA GOMES*
ABSTRACT OCCURRENCE OF FERROPYROSMALITE IN THE MINERALIZATED BRECCIAS FROM IGARAPÉ BAHIA
Au-Cu-(±REE-U) DEPOSIT, CARAJÁS MINERAL PROVINCE In the last years, several works report the presence of pyrosmalite mineral series
[(Fe,Mn)8Si6O15(OH,C1)10] commonly associated with volcanic exhalative massive sulphide or Fe-Mn metamorphosed deposits. In this paper, we present the inedit occurrence of ferropyrosmalite in the Au-Cu (+REE-U) of Igarapé Bahia deposit, located in the Au-Cu district of the Carajás
Mineral Province. We consider the Igarapé Bahia mineralization as being related to the génesis of iron-oxide class deposit, like the Olympic
Dam type. Ferropyrosmalite occurs in two different contexts: i) associated with carbonate veins; ii) associated with heterolithic breccias, composed by BIF and mafic metavolcanic fragments imersed in a magnetite, chálcopyrite, bornite, pyrite, carbonates (calcite to siderite), uranium and REE minerais, and gold, -rich matrix. The growth of ferropyrosmalite is probably due to the substitution of iron rich minerais (chlorite, magnetite and siderite), controled by magmatic fluid influx rich in chlorine. The permeability of breccias and the discontinuity of veins favour fluid percolation. The mode of occurrence of ferropyrosmalite and its relation with amphibole (ferro-hornblende-actinolite) indicate metasomatic growth of the former under temperatures in the transition of greenschist/amphibolite fácies. The ferropyrosmalite of the Igarapé Bahia deposit represents an uncommom type of occurrence linked to hydrothermal/magmatic conditions.
Keywords: Carajás Mineral Province, Igarapé Bahia deposit, Au-Cu-REE-U mineralization, ferropyrosmalite.
RESUMO