Composição química e isótopos de oxigênio em cassiterita e wolframita nos greisens do albita granito Palanqueta, depósito de estanho de Bom Futuro (RO)

Abstract

The Rondonia Ste hosts Meso- to Neoproterozoic granitic suites associated with important Sn, W, Nb, Ta, F, topaz and beryl mineralization confined in greisens, veins, breccia pipes and pegmatites. However, in these deposits, the chemical and isotopic composition, as well as fluid inclusions data, is still discrete. We provide in this paper new data on chemical and oxygen isotope composition of cassiterite and wolframite from greisens of the Palanqueta albite granite, in the Bom Futuro tin deposit. In these greisens the cassiterite crystals are typically zoned, with nearly pure SnO 2 light zones and high Fe, Ti, Ta, Nb e W dark zones, while the wolframite crystals are relatively Fe-rich, indicative of the specimen ferberite (Fe 0,52-0,86 -Mn 0,10-0,38 ) WO 4 , with low contents of Nb, Ta and Sn. Both cassiterite and wolframite host micro-inclusions of Nb-Ta minerals, probably as exsolution product during the lowering of temperature. The oxygen isotope ( δ 18 O ‰) composition of cassiterite (1.8 - 2.0), wolframite (1.4 – 1.6) and quartz (9.6) exhibits discrepant values, indicative of isotope fractionation during rise of the hydrothermal fluids responsible for mineralization. This is because quartz is less susceptible to isotopic fractionation, better observed in cassiterite and wolframite. The oxygen isotope composition of cassiterite, wolframite and quartz to geotermometry, as well as to calculation of oxygen isotope composition in coexisten “fossil” hydrothermal fluids, suggests that cassiterite and wolframite were precipitated at the temperature range of 460 o to 420 o C from the same hydrothermal fluid of magmatic derivation.