Litogeochemistry and mineral chemistry of angico dos dias metacarbonatite rocks, divise Bahia/PiauÍ, Brazil

Abstract

The rocks of Angico dos Dias Metacarbonatite Complex occur in northernmost region of Bahia and consist of metacarbonatites, several metasyenites, metapyroxenites, alkali metadiorites, tremolite rock, biotite rock, metalamprophyres, fenites and apatite rock, which are a secondary deposit of residual phosphate. The metacarbonatites yield a cumulatic layer, defined by different levels in apatite iron-magnesium minerals and magnetite, which allows separating in five petrographic types: apatite metacarbonatite; olivine-apatite metacarbonatite; phlogopite-apatite-olivine metacarbonatite; olivine-apatite-biotite/phlogopite metacarbonatite and silicified metacarbonatite. Petrographic and mineral chemistry data indicate the presence of: calcite with exsolution of dolomite; fluorapatite; olivine pseudomorphs altered to serpentine, tremolite, anthophyllite and magnetite; phlogopite; baddeleyite; exsolution of ilmenite in magnetite that alters to hematite; pyrrhotite; pyrite; chalcopyrite; sulfates as celestite, barite and baritecelestite; and oxy-hydroxides of goethite and lepidocrocite. The weathering of carbonatites is associated with ferruginous and/or silicified shells, and, besides concentrating apatite, can originate alumino-phosphates of the crandalite group, goyazite, gorceixite, plumbogumite, florencite. Anhydrous igneous and metamorphic mineral assemblages of high grade are superimposed by retrometamorphic parageneses in high greenschist facies. Geochemical data classify metacarbonatitic rocks mainly as calcium carbonatites and those intensely hydrothermalized are classified as iron carbonatites and magnesium carbonatites.