Espeleotemas fosfáticos em cavernas ferríferas de Carajás e uma nova ocorrência de esfeniscidita

Abstract

Iron ore mining in Brazil is important for maintaining the country’s positive trade balance. Iron ore is among the country’s main exports, and as a result of 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 iron caves in the Carajás Mineral Province. The objectives of this study were to describe the phosphate minerals and their formation processes based on data from geochemical and petrographic analysis in concert with X-ray diffraction, differential scanning calorimetry coupled with thermogravimetry, infrared and scanning electron microscopy. We studied eight caves where the development of phosphate speleothems appeared to be directly associated with the presence of guano. DSC-TG curves allowed us to identify two phosphate groups: leucophosphite-spheniscidite, which displayed two endothermic events at 323.4 and 400 °C attributed to OH and NH4 removal, respectively, and strengite-phosphosiderite, which displayed no such events. Infrared spectra allowed us to identify the NH4 functional group, related to the presence of spheniscidite, based on absorbance bands centered at 1430 and 3040 cm-1. Values of the unit cell parameters for spheniscidite, calculated by Rietveld refinement, revealed dimensions and angulations slightly lower than those reported in the literature (a = 9.803 Å, b = 9.722 Å, c = 9.858 Å, β = 102.78°). These smaller values are attributed to increased substitutions of K for NH4 and Al for Fe. Based on the geochemical analysis, significant enrichment in P2O5, Zn, Ni and Rb was detected, suggesting that bat guano was a potential source of these elements. Guano decomposition releases phosphoric acid, which percolates the lateritic substrate reacting and substituting iron hydroxides for phosphate minerals, with simultaneous assimilation of metals that had been bioaccumulated and excreted by bats.