OCCURRENCE OF ILVAITE IN THE IGARAPÉ BAHIA Cu-Au DEPOSIT, CARAJÁS PROVINCE, BRAZIL

Abstract

This paper describes the first known occurrence of the mineral ilvaite in Brazil. The mineral is sited in the matrix of magnetite-rich breccias and stockwork veins of the Igarape Bahia Cu-Au deposit. Electron microprobe analysis (EMPA) of the Igarape Bahia ilvaite reveals the following composition: Ca (sub 0.96) Fe (super 2+) (sub 1.96) Fe (super 3+) (sub 1.00) Si (super 2.01) O (sub 8) (OH) and X-ray diffraction (XRD) data indicate a structure compatible with the monoclinic polymorph (a (sub 0) = 13.032 Aa, b (sub 0) = 8.810 Aa, c (sub 0) = 5.850 Aa and alpha = 90.254 degrees ). It is suggested that ilvaite crystallized from hydrothermal solutions, at a temperature range of 300-350 degrees C, under conditions of high high fO (sub 2) and low sulfur activity. The formation of ilvaite is tentatively related to the reaction magnetite + calcite + quartz + H (sub 2) O = ilvaite + CO (sub 2) + O (sub 2) .