Química mineral em xenotímio e minerais associados as fases pegmatóides da facies albita-granito, Suíte Madeira, mina Pitinga, Amazonas.

Abstract

The Pitinga Mine is located in the northern state of Amazonas. In this area, the plutons Pitinga, Madeira, Água Boa and Europa, grouped in Madeira Suite, were dated in the Paleoproterozoic. The Madeira Granite contains four distinct facies: amphibolebiotitesyenogranite, biotite alkali feldspar granite, porphyritic hypersolvus alkali feldspar and last phase is the albite-granite, a rock strongly mineralized in rare metals and associated with pegmatites. The pegmatites have well developed crystals of quartz, potassic feldspar, albite, dark mica, cryolite, opaque minerals and xenotime, this last subject of this study. The geochemistry of the rocks belong to Madeira Granite showed that the albite-granite equigranular core (ABGN) has REE concentrations ranging from 187.43 to 328.03 ppm, reaching 1276.63 in the fluidal ABGN, and 6916.07 ppm in pegmatites. The yttrium concentrations ranging from 48.80 to 128.80 at ABGN, from 83.30 to 240.60 ppm in the fluidal ABGN and 1193.40 ppm in pegmatites. The xenotime, main HREE and Y mineral ore presented HREE content of 32% to 36% and Y of 21 to 25%. The distribution of ETR on xenotime is different from that observed in the equigranular ABGN, fluidal ABGN and in the pegmatite with strong enrichment in HREE, which suggests their late crystallization from highly differentiated and fractionated fluids, however, all patterns exhibit tetrad distribution type with high levels in HREE compared to LREE. Studies by SEM / EDS in the xenotime crystals revealed mineralogical associations and unusual textural patterns, not observed in optical microscopy, such as: exsolution features of thorite (ThSiO4) and gagarinita [NaCaY (F, Cl) 6], fractures filled by galena, cassiterite, micas and cryolite. Chemical analysis by WDS in highly altered areas and less altered areas in xenotime crystals revealed significant chemical differences in the gagarinite and thoriteexsolutions belong high altered portions when compared to the less altered and unchanged portions. The altered areas showed lower levels of SiO2 and ThO2 (average 0.05% and 0.06%, respectively) compared to those portions with incipient alteration (average 0.40% SiO2 and 0.62% of ThO2), reaching a maximum of 1.50% ThO2. The exsolution of thorite in xenotime, and differences in the compositional contents of Th and Si, in altered and unaltered portions of the crystal, show remobilization of these chemical elements in the altered areas of the crystal, where Th incorporated into the xenotime is exsolved, precipitating as thorite. The association xenotime-gagarinite is unprecedent in in the context of mineralization of Pitinga mine and it is first documented in this work, indicating that the gagarinite is not limited cryolite veins and associated to fluocerite. In the xenotime, the gagarinite occurs as oriented exsolved granules (altered areas) and as inclusions (subhedral to euhedral crystals) and thorite as oriented exsolved granules. The Pitinga Province is responsible for 60% of tin Brazilian production, with reserves associated to Sn Nb, Ta and F, beyond anomalous concentrations of Zr, Y, Rb and Th Adding to these reserves, the HREE oxides, also present in the pegmatite with xenotime, gagarinite and cryolite, enable the exploitation of these minerals as byproducts.