Primary and secondary Fe and Mn phosphates in granitic pegmatites from the Mata Azul Pegmatitic Field, Jaú do Tocantins, TO, Brazil

Abstract

In central Brazil, in the southern Tocantins and in the northern Goiás States, barren and mineralized granitic pegmatites are known and explored for a long time, but only in recent years they were properly characterized and grouped in the so called Mata Azul Pegmatitic Field. In Jaú do Tocantins, three of the pegmatite bodies explored for beryl, a Fe-Mn phosphate associations occur as centimetric nodules and aggregates. Two phosphate associations were characterized: the first, hosted in Pichorra and São Júlio pegmatites present three primary phases, identified as zwieselite, sarcopside and fluorapatite. Hydrothermal processes transform the primary phosphates in rockbridgeite, heterosite and a phase of the alluaudite subgroup. The second association was observed in the Fazenda Mesquita pegmatite, where primary phosphates are absent and orange to green rockbridgeite is the main phase, followed by phosphosiderite and strengite formed by the weathering of rockbridgeite. In both associations, the weathering of the Fe-Mn phosphates give rise to Fe and Mn oxides. Minor phases as uraninite, gahnite, almandine and columbite-(Fe) were identified as inclusions in the phosphate phases.