Petrografia das rochas do complexo metacarbonatítico de Angico dos Dias, Divisa Bahia/piauí, Brasil.

Abstract

The Angico dos Dias Metacarbonatite Complex rocks are mined for mineral phosphate by GALVANI Mining Unit, in the extreme northwest region of Bahia State. These are intrusive Paleoproterozoic rocks occurring in Archean-Paleoproterozoic rocks of the Sobradinho-Remanso gneissic-migmatitic Complex and consist of primary metacarbonatites, varied metasyenites, metapyroxenites, metassyenogranites, tremolitite, biotite schist, metalamprophyres and fenites. Secondary and/or residual deposits are formed by supergene processes, generating weathering mantle installed on the carbonatite which gave rise to residual phosphate deposit, called apatite-rock. These deposits are associated with ferruginous and/or silicified discontinuous shell. The carbonatite rocks are mainly classified as calcium-carbonatite and/or sovite and consist dominantly by calcite and apatite and subordinate dolomite, olivine, phlogopite and magnetite. These arise by fractional crystallization, gravitational segregation, main mechanism responsible for magma evolution, generating a cumulatic layering, which is defined by different levels, more enriched in apatite, iron-magnesium minerals and magnetite. The process of differentiation allowed the generation of various lithologies of ultrabasic rocks and enables to individualize mainly five petrographic types of carbonatites in the mining: the apatite metacarbonatite; olivine-apatite metacarbonatite; phlogopite-apatite-olivine metacarbonatite; olivine-apatite biotite/phlogopite metacarbonatite and silicified metacarbonatite. The mineral assemblages identify high amphibolite facies metamorphism, with superimposed parageneses by retrograde metamorphism over medium to high green schist.