Mineralogy and genesis of rare Al-phosphate minerals in weathered itabirite and iron ore from the Quadrilátero Ferrífero, Minas Gerais, Brazil

Abstract

Phosphorus is a major deleterious element in iron ore where it occurs as primary apatite in hypogene ore, and adsorbed into Mn-, Fe-hydroxides and forming secondary phosphates in supergene ore. Secondary phosphate minerals have been formed during a long history of weathering of itabirite and hypogene iron ore in the Quadrilátero Ferrífero (QF), ongoing since the end of the Cretaceous (~70 Ma). The distribution and genesis of these secondary phosphates are poorly documented in the literature. The purpose of this study was to investigate the mineralogy, mineral chemistry and formation of rare secondary phosphate minerals found in P-bearing concretions hosted in the iron ore at the Serra do Curral, QF. Detailed mineralogical investigation revealed that three generations of secondary phosphates were formed by weathering of carbonate veins hosted within fracture zones of itabirite and iron ore. Turquoise, augelite and senegalite comprise the bulk of the secondary phosphates whereas remnants of Ca-, Ba-, Ce- and Sr-bearing minerals (crandallite, gorceixite, florencite and goyazite) are subordinated. Sulphate members of aluminium-phosphate-sulphate (APS) minerals are absent, likely due to low SO42− activities in the weathering solution. Apatite hosted within unweathered itabirite and carbonate veins was the source of P to the weathering solutions whereas Cu, Zn and Ba were released from sulphides hosted in carbonate veins and Al2O3 from country rocks. The link between the secondary phosphates to hydrothermal breccias and carbonate veins highlights the significance of hydrothermal fluids as a source of P in iron ore deposits. Furthermore, it brings attention to the structural control imposed on P-bearing minerals in these deposits. Geochemical modelling shows that the secondary phosphates were formed in response to changes in pH of the weathering fluids, providing an additional example to the literature about the application of secondary phosphate minerals as valuable trackers of paleoenvironmental weathering conditions.