Phosphorites of Rocinha Mine; Patos de Minas (Minas Gerais, Brazil); genesis and evolution of a middle Proterozoic deposit tectonized by the Brasiliano Orogeny

Abstract

In central Brazil (the states of Minas Gerais and Goias) the Brasiliano orogeny ([asymp]600 Ma) delineates the Sao Francisco craton and, to the west, the Brasilia fold belt. Rocks of the fold belt are thrust to the east and display increasing deformation and metamorphism westward.Almost all the phosphate deposits are found in the Brasilia fold belt, either along its boundary with the craton or in external structural units showing polycyclic deformation and greenschist metamorphism. They occur in the metasedimentary rocks of the middle Proterozoic Paranoa Group and in the upper Proterozoic Sao Francisco Group and its equivalents. Middle Proterozoic phosphorites are economically the more important. Most are spatially and temporally associated with a stromatolite reef barrier. However, the Rocinha-Patos de Minas deposit, the only presently mined, is located south of the reef barrier. It has developed on a shallow to deep ramp between a wide internal basin with reduced oxygen conditions and limited detritus and a more oxygenated marine environment to the west.In the Rocinha mine, phosphate forms grains, angular fragments, lenses, or centimeterthick beds in schists and carbonate or glauconitic schists. The P 2 O 5 content of phosphatic rocks ranges between 10 and 15 percent. Annual production is around 200,000 tons of ore containing 24 percent P 2 O 5 . The mine area comprises four tectonic slices made up of schists, quartzoschists, and phosphatic schists, separated by subhorizontal shear planes. These are defined by tectonic breccias in which secondary apatite has recrystallized. The mine is exposed in a 50- to 75-m phosphatic sequence which is repeated in a lower slice about 100 m below mine level.The phosphatic ore is a homogeneous crypto- to microcrystalline apatite in fairly continuous centimeter-thick beds which were stretched, fragmented, and boudinaged during Brasiliano isoclinal folding and subsequent subhorizontal shearing.X-ray diffraction, IR spectrometry, and chemical analysis indicate that the metasedimentary phosphate is a weakly carbonated fiuorapatite with low F and CO 2 contents. The influence of the Brasiliano orogenesis is marked by partial substitution of F by OH. Secondary prismatic apatite, kaolinite coatings, and wavellite sheaflike structures are products of weathering and are found in mine sections and bore holes to a depth of 10 to 15 m. Secondary apatites are enriched in CO 2 compared to primary metasedimentary apatites.