MINERALOGY AND GEOCHEMISTRY OF PHOSPHATES AND SILICATES IN THE SAPUCAIA PEGMATITE, MINAS GERAIS, BRAZIL: GENETIC IMPLICATIONS
| Autor: | Frédéric Hatert, Maxime Baijot, Simon Philippo |
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| Rok vydání: | 2012 |
| Předmět: | |
| Zdroj: | The Canadian Mineralogist. 50:1531-1554 |
| ISSN: | 1499-1276 0008-4476 |
| DOI: | 10.3749/canmin.50.6.1531 |
| Popis: | The Sapucaia pegmatite is located in the well-known Eastern Brazilian Pegmatitic Province (EBPP), Minas Gerais, Brazil. Detailed mapping of the pegmatite body revealed five zones, showing distinct mineral assemblages: a border zone (BZ), an external wall-zone (EWZ), an internal wall-zone (IWZ), an intermediate zone (IZ), and a quartz core (Q). Phosphate masses found in the pegmatite contains two different assemblages: assemblage I is formed under oxidizing conditions, and assemblage II is formed under less strongly oxidizing conditions. In both assemblages, triphylite is the only primary phosphate, and several secondary species are produced by its alteration. In association I, three main transformation stages are observed: (i) the progressive oxidation of triphylite accompanied by the leaching of Li leads to the successive crystallization of ferrisicklerite and heterosite; (ii) a hydration stage transforms triphylite into hureaulite, barbosalite, and tavorite, and (iii) a final stage, corresponding to meteoric alteration, the latest highly hydrated phosphate and oxide species. In assemblage I, ferrisicklerite is replaced by minerals like jahnsite s.l. and frondelite s.l. during the hydration stage, and in both assemblages, a second generation of Mn-rich triphylite (triphylite II) is observed. Two unusual petrographic textures were also observed, showing reactions between triphylite and albite to form montebrasite and garnet rims. These textures formed during the albitization stage, and correspond to the reactions triphylite + albite → ferrisicklerite + montebrasite + quartz, and triphylite + albite → almandine + quartz, respectively. Finally, the geochemical trends in phosphates and silicates indicate that Sapucaia is a weakly fractionated and geochemically primitive LCT-type pegmatite. The degree of differentiation of the pegmatite increases from the border to the core, as shown by a decrease of Fe/(Fe + Mn) in olivine-type phosphates, which decreases from 0.75 in the IWZ, to 0.71 in the IZ. Muscovite and tourmaline chemical compositions also show evidence of increasing degree of differentiation; for example, the Ga and Nb contents of muscovite increases from 57 and 49 ppm in the IWZ, to 86 and 80 ppm in the IZ, respectively. |
| Databáze: | OpenAIRE |
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