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.
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Supergene Environments, Processes, and Products
At least five altered and mineralized porphyry centers related to the cooling of a polyphase Eocene intrusion occur within a 25-km2 "pampa"-type area in the southwestern sector of the Chuquicamata district in northern Chile. These deposits take place 1 to 2 km apart as discrete porphyry "columns" covered by postmineral, poorly consolidated Miocene sedimentary rocks. Such copper oxide and sulfide deposits were discovered and evaluated by drilling done by Codelco from 1996 through 2007 during a brownfield exploration program, driven by the necessity to replace and increase leacheable ore consumed by the Chuquicamata and Radomiro Tomic operations. During this program a resource of more than 20 million metric tons (Mt) Cu was discovered, including 6 Mt Cu of oxide, mixed and secondary sulfide ore, representing one of the largest supergene copper resources discovered worldwide during the last 10 years.
Despite their close location and their genetic relationship to a single, polyphase intrusion mineralization event, the five porphyry centers display contrasting host-rock and structural framework as well as different hypogene alteration and ore mineral assemblages. This picture reaches high levels of complexity because of the different levels of exposure of the mineral systems, resulting from primary emplacement processes and post-mineral faulting. These hypogene features and the effect of landscape and climate evolution controlled supergene alteration, thus generating different profiles in each specific porphyry center. The key controlling factors in the supergene overprint are discussed on the basis of their relationship to ore and gangue mineralogical abundance and occurrence, assemblage distribution, geochemical response, and the broad geologic setting.
As exploration for covered porphyry copper deposits in the southwestern sector of the Chuquicamata district progressed, numerous lessons were learned about the origin of supergene profiles and the analysis and use of supergene effects and their products as a guide for exploration. These lessons, which include geological and geochemical criteria among others, are discussed in the context of the appraisal of the mineral potential of copper oxide-mixed-secondary sulfide blankets and underlying sulfide protore.