Sedimentology, stratigraphy and glacial genetic model of the coromandel phosphate deposit, Vazante group, central Minas Gerais state, Brazil

Abstract

The Coromandel sedimentary phosphate deposit is inserted at the external zone of the Brasília Fold Belt geological context, more specifically in the Retiro Formation, basal unit of the Vazante Group. Compared to the world-class Rocinha Formation deposits, widely studied due to high economic interest, the Coromandel Deposit is the one that most lacks studies focused on the main rocks description, faciological characterization and proposal of a plausible metallogenetic model according to the depositional environments. Thus, from the detailed field description of outcrops, geological cartography, microscopic petrography and X-ray diffraction and fluorescence analysis, it was possible to contribute to a better understanding of the deposit. The Retiro Formation is characterized by a glaciogenic facies association that comprises polymictic diamictite, conglomeratic sandstone, fine whitish arkosean, massive and laminated siltstone, besides phosphate facies, including phosphate siltstone, laminated phosphorite and brecciated phosphorite. The phosphatic facies occur in straight relation to the glaciogenic sediments and not rare directly over diamictite, which matrix can be phosphated as well. These rocks P2O5 contents range from 2% to 34% and the succession suggest an establishment of a glaciotransitional environment responsible to constrain favorable alkalinity, temperature and oxi-reduction conditions to directly chemical precipitation of fluorapatite in a mechanism analogue as the cap dolomites, supporting an idea of “cap phosphorites”. The phosphate source is related to the glacial sedimentary supply of continental origin (granite, carbonatite and other alkaline bearing rocks). In addition, the input of cold water, as result of ice melting episodes, became a limiting development factor for phosphorus metabolizing organism which improves the P2O5 system enrichment. For the unique genetic conditions, this phosphatic deposit has been called as the Coromandel Deposit Type.