Geologia, geometalurgia, controles e gênese dos depósitos de fósforo, terras raras e titânio do complexo carbonatítico Catalão i , GO

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

This work describes the geological and geometallurgical characteristics of the phosphate, titanium, niobium and rare-earth elements deposits in the northeast part of the Catalão I carbonatite complex, Central Brazil. The mineralization controls were determined for magmatic, metasomatic and weathering processes. The ore types were defined on the basis of geometallurgical concepts. Study methods comprised geological mapping, drill-core description, petrography, mineral chemistry, statistics, and technological characterization of the ore. Field and petrographic observations indicate the existence of three petrogenetic series: bebedouritic, phoscoritic, and carbonatitic. The Bebedouritic Series derived from a silicate magma generated by liquid immiscibility from a carbonated silicate parent, and evolved through crystal fractionation, producing dunites, pyroxenites, and residual veinlets of carbonatite. During a second immiscibility episode, the initial carbonatite magma separated into phoscorite and carbonatite components. The phoscorite magma thus formed was initially olivine-rich, and evolved to apatite cumulates. The carbonatite branch of this immiscible pair underwent a third, and last, immiscibility event, again generating phoscorite and carbonatite branches. This second phoscorite generation is olivine-free, but pyrochlore- and magnetite-rich. The following protoliths are recognized for the studied deposits: in the phosphate deposits, protoliths are late-stage pyroxenites from the Bebedourite Series, and early-formed olivine-phoscorites; protoliths for the titanium mineralization are early pyroxenites from the Bebedourite Series; for the niobium mineralization, the second generation of phoscorites, formed in the third immiscibility event; for the REE, the protoliths are metasomatic phlogopitites, Nb-rich phoscorites and dolomite carbonatites subjected to carbo-hydrothermal alteration. Magmatism played an important role in the Genesis of the phosphate, titanium, and niobium deposits, respectively concentrating apatite, perovskite and pyrochlore trough fractional crystallization. It also conditioned the dyke-swarm, stockwork-like structure of the primary ore. Metasomatism played only a subordinate role in the genesis of the phosphate deposits, forming hydrothermal apatite in phlogopitites and Nb-rich phoscorites. On the other hand, it was the main ore-forming agent for the monazite deposits. Carbo-hydrothermal fluids strongly enriched in REE, possibly as a consequence of the perovskite-anatase transformation, reacted with apatite and dolomite from bebedourites, foscorites and carbonatites, replacing them with monazite. The interaction of carbo-hydrothermal fluids with rocks of the Phoscorite Series produced two new petrographic types: nelsonites mineralized with apatite, ilmenite, pyrochlore and monazite, called C2a unit, and a rock essentially composed of monazite, with subordinate apatite and carbonates, classified as monazitite. Both units contain explosion pipes filled with breccia, indicating magma fragmentation and turbulent flow of particulate material within the magma chamber. Regarding the anatase mineralization, carbo-hydrothermal fluids interacted with perovskite-bearing pyroxenites, transforming the rock into a metasomatic phlogopitite and replacing perovskite with anatase. The role of weathering on the formation of the phosphate deposit is the residual concentration of apatite, mostly in the isalterite level. In the titanium deposit, weathering concentrated the anatase formed by carbo-hydrothermal processes, and produced new (supergenic) anatase from perovskite remnants. Regarding the REE deposit, weathering promoted substitution of the carbonates by silica in magnesite-bearing dolomite carbonatites, with residual concentration of monazite as part of the carbonate was eliminated from the system. Phlogopitites were transformed into highly REE-enriched saprolite, whereas in phoscoritic rocks from the C2a and C2b units, weathering promoted carbonate and apatite dissolution, with consequent residual concentration of monazite. In the niobium deposit, weathering concentrated and transformed the original Ca-pyrochlore from phoscorites into Ba-pyrocholore, in the alloteric saprolite. The phosphate deposit is composed of the following ore types, on the basis of geometallurgical criteria: oxidized ore and micaceous-oxidized ore, located at the top of the isalteritic saprolite; upper, intermediate, and basal micaceous ore, located at the base of the isalteritic saprolite; siliceous-carbonated ore, located at the base of the altered rock level. All the above were further subdivided in to phoscoritic and phlogopititic. Descriptive statistical analysis revealed the presence of two phoscorite populations, and Principal Component Analysis allowed establishing geometallurgical signatures, represented in map as zones with different geometallurgical characteristics. Geostatistical analysis showed the presence of zonal anomalies, related with the two different phoscorite types, adding further support to the conclusion that these rocks evolved separately. The REE deposit consists of four ore types: saprolite ore, derived from the weathering of phlogopitite with monazite veins; nelsonite ore, derived from the metasomatism of Nb-rich phoscorites; carbonatite ore, derived from metasomatic transformation of dolomite carbonatites; siliceous ore, derived from the weathering of the carbonatite ore. Statistical analysis of geochemical data confirms the presence of different populations within the monazite deposit, suggesting distinct genesis or distinct protoliths. The titanium deposit consists of four ore types: Type I, located between the lower part of the alloteritic saprolite and the upper part of the isalteritic saprolite, is characterized by the best recovery during flotation, lacks apatite and perovskite, and has moderate iron content; Type II, located at the top of the alloteritic saprolite, is very iron-rich and characterized by lower recovery than Type I; Type III, located in the lower part of the isalteritic saprolite is characterized by the presence of mica and apatite, and yields low recovery; Type IV, located between the upper part of the altered rock level and the lower part of the isalteritic saprolite level, is characterized by low recovery and by the presence of perovskite remnants associated with anatase. Statistical analysis of geochemical data confirms the presence of more than one population within the titanium deposit.

ASSUNTO(S)

terras raras titanium fosfato rare-earths gênese titânio ore genesis geometalurgia geologia ore-deposit controls controle geometallurgy phosphate

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