Abstract

The Jocão pegmatite (or Cigana) is located in the well-known Eastern Brazilian Pegmatitic Province (EBPP), Minas Gerais, Brazil. According to their macroscopic textures, three different kinds of phosphate mineral masses were collected from the dumps of the pegmatite: association I is composed of dendritic triphylite, forming intergrowths with silicate minerals (spessartine garnet or albite); association II forms blocky nodules of triphylite-ferrisicklerite-heterosite; and association III shows exsolution lamellae of ferrisicklerite-heterosite in massive beusite. The primary textures and the Fe/(Fe+Mn) ratios of primary triphylite make it possible to establish the crystallization sequence of the primary phases; the petrogenesis of primary intergrowths between garnet and triphylite of association I is also discussed. In the three associations, these primary minerals are hydrothermally altered and secondary species are produced. In association I, the first hydrothermal alteration event was a weakly oxidizing hydroxylation stage, during which triphylite was only replaced by hureaulite (rarely associated with barbosalite) along its cleavage planes. The final stage affecting association I corresponds to meteoric processes during which ludlamite and then vivianite progressively replaced triphylite. Association II evolved under more oxidizing conditions and the first alteration stage corresponds to the progressive oxidation of triphylite accompanied by Li-leaching, leading to ferrisicklerite and heterosite. The second hydrothermal stage corresponds to a hydroxylation event, and the secondary species depend on the phosphate mineral that they replace: triphylite is only altered to Fe2+-Mn2+-bearing hydrated species (colorless hureaulite); ferrisicklerite is altered to Fe2+-, Mn2+-, and Fe3+-bearing phosphate minerals such as jahnsite s.l., frondelite s.l., and orange hureaulite; heterosite is replaced by Fe3+-bearing species such as ferristrunzite. Consequently, it appears that the secondary phosphate minerals, which crystallize during this second hydrothermal stage, strongly depend on the cations which are locally available in the sample zone. The final stage forms the meteoric species: leucophosphite and phosphosiderite directly replace heterosite, whereas vivianite, a ferrous meteoric species, only appears in the triphylite core. In association III, exsolution lamellae are formed at the expense of a high-temperature homogenous Ca-Li-bearing graftonite-beusite-like phase; when the temperature decreased, Li migrated into triphylite and Ca to the larger M1 site of beusite. During the high-temperature hydrothermal alteration processes, triphylite transforms into ferrisicklerite and heterosite, like in association II; however, beusite is not affected by any transformation process at that stage. During the low temperature hydroxylation stage, ferrisicklerite from the core remains almost unaltered, while beusite is replaced by an intimate mixture of pleochroic Ca-rich hureaulite and tavorite. After this hydroxylation stage, the meteoric stage is characterized by an increase in Ca2+ and H2O activities, responsible for the replacement of hureaulite and tavorite by mitridatite-robertsite. At the nodule border, beusite and heterosite are completely replaced by an intimate mixture mainly composed of frondelite and robertsite-mitridatite. Finally, this study shows that the small phosphate nodules from Jocão are more affected by oxidation than large nodules, thus indicating that the diffusion kinetics of hydrothermal fluids is relatively low in these phosphate nodules. As a consequence, large phosphate nodules show a typical zoning with triphylite (core)–ferrisicklerite-heterosite (rim); this zoning, which preserves the crystal structure of the phosphate minerals, was achieved during the high temperature hydrothermal transformations. The phosphate nodules consequently appear as a relatively closed system compared to the silicate matrix.

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