Phosphate speleothems in caves developed in iron ores and laterites of the Carajás Mineral Province (Brazil) and a new occurrence of spheniscidite

The Carajás Mineral Province has one of the largest concentrations of caves in Brazil, and its iron ore is among the country’s main exports. As a result of iron ore intense extraction, new environmental policies have been implemented. In an attempt to balance economic activity and environmental conservation, an inventory and a relevance-based classification of caves were implemented in 2008 as criteria for environmental licensing of mining ventures. This implementation motivated the present study of phosphate speleothems in Carajás’ caves developed in ferriferous rocks. The objectives of this study are to describe the phosphate minerals and their formation processes based on geochemical and petrographic analysis, X-ray powder diffraction (XRD), differential scanning calorimetry coupled with thermogravimetry (DSC-TG), Fourier-transform infrared (FTIR) and scanning electron microscopy (SEM). Eight caves were selected for this study, along the slopes of the plateaus from Carajás. The phosphate speleothems usually present compositional zoning, with strengite-phosphosiderite in the core and leucophosphite-spheniscidite on the edges. This zoning cannot be related to leucophosphite-spheniscidite decomposition, since the DSC-TG curves show that dehydroxylation and loss of NH3 from leucophosphite-spheniscidite require temperatures higher than 250°C. FTIR results allowed us to identify the NH4 functional group, based on absorbance bands at 1,430 and 3,040 cm-1, which associated to other data confirm the presence of spheniscidite. Refined values of the unit cell parameters for spheniscidite (a = 9.803 Å, b = 9.722 Å, c = 9.858 Å, β = 102.78°) obtained using the Rietveld refinement, were slightly lower than those of the ICSD-39654 file. These smaller values are attributed to increasing substitutions of K+ for NH4+, and Al3+ for Fe3+. Based on the geochemical analysis, significant enrichment in P2O5, Zn, Ni, and Rb was detected in phosphate speleothems, and phosphatized laterite, suggesting that bat guano was a potential source of these elements