Desenvolvimento de uma metodologia para análise química de inclusões silicáticas em cristais de quartzo: estudo de caso em granitos estaníferos da Mina Pitinga (AM)

Abstract

Silicate inclusions (melt inclusions) are small globules of silicate melt, containing some combinations of crystals, glass and vapor, entrapped in different minerals during their growth, and can be found in volcanic and plutonic rocks. They are easily identified in volcanic rocks. On the other hand, one of the major difficulties in the study of melt inclusions in plutonic rocks is their identification, because, after being trapped, their evolution results in total or partial crystallization. Based on international literature, they provide important information about the origin, nature of magmas and their petrological evolution. In addition¸ the detection of metals in melt inclusions is a unequivocal geological evidence of the genetic association of these elements with magmatic liquids (source) and is crucial in the study of orthomagmatic or hydrothermal deposits. The techniques for studying silicate inclusions for petrological and metallogenic purposes have evolved very rapidly in the last four decades, but it is a methodology that has not yet been implemented in Brazil, both due to the absence of laboratories with adequate equipment and the inexistence of research groups engaged in this field. Recently, pioneering studies were developed at the CDTN (Centro de Desenvolvimento da Tecnologia Nuclear), in Belo Horizonte (MG), with the tin granites of the Pitinga mine (AM), through high temperature microthermometric experiments and analysis of trace elements by LA-ICP-MS, in silicate inclusions hosted in quartz crystals of these granites. However, the tests were carried out on doubly-polished sections, which made it difficult to perform chemical analyzes of major elements by electron microprobe, since the inclusions were very deep in the quartz crystals, and any attempt at polishing to expose the inclusions would damage the samples. Based on this preliminary experience, this specific work is a technique for preparing quartz crystal concentrates containing silicate inclusions, using as such tin granites from the Pitinga mine (AM), representatives of the later facies of the Madeira pluton, porphyritic hypersolvus alkali feldspar granite and albite-rich granite. Thus, the developed work at the Laboratório de Inclusões Fluidas, with the crucial support of the Oficina de Laminação, the Laboratório de Análises Químicas and the Laboratório de Microanálises of the Instituto de Geociências of the Universidade Federal do Pará (UFPA), allowed the research to establish a routine involving the following steps: detailed petrography; crushing and grinding of the samples; granulometric separation; preparation of quartz crystal concentrates; muffle furnace heating and cooling experiments; selection of crystals with appropriate inclusions; assembly of the crystals in mounts with epoxy resin and subsequent polishing; monitoring and imaging of inclusions through the SEM; particle analyzes by EDS and, finally, analysis of major element (WDS) by electron microprobe. The microanalytical data (major elements) chosen especially in those silicate inclusions containing two or more solid phases (glass, spherical globules), demonstrated that the preparation technique provided a good exposure of the inclusions. In this way, the methodology developed in this work is relevant to the study of silicate inclusions and can be applied for the preparation of concentrates of any transparent magmatic mineral (quartz, olivine, pyroxene, plagioclase, etc.), host of silicate inclusions, and that can be analyzed by any of the traditional microanalytical techniques (electron microprobe, LA-ICP-MS, Raman spectroscopy, SEM, etc.).