Large-Scale Development of Supergene Anatase-Rich Soils on Carbonatite-Associated Pyroxenites in Brazil (Abstract)
Anthony N. Mariano, Roger H. Mitchell, 2009. "Large-Scale Development of Supergene Anatase-Rich Soils on Carbonatite-Associated Pyroxenites in Brazil (Abstract)", Supergene Environments, Processes, and Products, Spencer R. Titley
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The climatic conditions in central and northern Brazil are conducive to the development of deep lateritic weathering in some carbonatite complexes, including the carbonatite rocks and some of their genetically and spatially related silicate rock units.
Lateritic weathering of bebedourite-type pyroxenites genetically related to carbonatites in Brazil has produced a spectacular accumulation of high-grade anatase derived from the decalcification of perovskite. Carbonatite occurrences that contain vast quantities of supergene anatase-rich soils overlying weathered pyroxenites include Tapira, Serra Negra, Salitre I, and Salitre II in Minas Gerais; Cataläo I, in Goiás; and Maicuru and Maraconai in Pará.
Soils overlying the weathered pyroxenites range from approximately 15 to 30 wt percent TiO2. At Serra Negra, an exploration drill hole gave an average of 20 wt percent TiO2 as anatase mineralization for a depth of 150 m. The average depth of weathering in weathered pyroxenite in the northwest quadrant of the Serra Negra circular structure is 120 m and the development of anatase is reported to be 40.5 million tons (Mt) of 28.52 wt percent TiO2. In the Bananeira area of Salitre I, the average depth of weathering is 80 to 100 m, with reported 60.34 Mt of 24.5 wt percent TiO2. In the Para carbonatite, the weathering depth may exceed 300 m.
Within the weatheredpyroxenites, diopside, perovskite, apatite, and calcite undergo total chemical breakdown. Calcium and magnesium are efficiently removed from the complexes in the groundwater. Decalcification of the perovskite produces polycrystalline aggregates of micrometer-size anatase platelets. Liberated REE from perovskite, apatite, and calcite are largely immobile, forming supergene monazite, rhabdophane,cerianite, and crandellite-group minerals that occur as crystallites that are inextricably associated with anatase.
Direct indication of the development of anatase from perovskite is clearly shown in three ways: (1) by texture with residual cores of perovskite surroundedby polycrystalline clusters of anatase, (2) by pseudomorphsof anatase after euhedral perovskite octahedral, and (3) because chondrite-normalized REE plots for anatase and perovskite show identical slopes that serve as a geochemical proof that anatase is derived from the perovskite.
Bebedourite, first described by E. Tröger in 1928, is a pyroxenite containing major amounts of perovskite. The bebedouritesof Minas Gerais and Goiás are of Cretaceous age. Similar pyroxenites occur in other carbonatites, including the Precambrian Powderhorn of Colorado, North America, and in Kovdor of the Kola Peninsula, but in these occurrences, the absence of deep-penetrating lateritic weathering produces only a very thin rock-surface coating of anatase, for which the origin from the decalicification of perovskite is virtually identical to the anatase found in the Brazil carbonatites.