Abstract
The Serra Branca pluton, of medium Proterozoic age is situated in the Tocantins sub-province of the Tin Province of Goias ; is composed by a several differenciatted grariite types: a biotite-granite, predominam in the W part of the massif, a two-mica granite and a muscovite granite. A small topaz granite is the most evolved type that outcrops in the Eastern part of the massif. The intense differentiation of the massif makes itself on an asymetrical manner due to the development of post-magmatic alteration processes that culminated with the formation of massive greisens. Ali rocks of the pluton (granite and greisens) together with the surrounding quartzites and quartz-micaschists of the Arai Groupe have been deformed and largelly recristallized during the Brasiliano tectono-metamorphic event. This deformation, as well as the intensity of the post-magmatic alterations, increases from W to E of the pluton. The relative importance of the hydrothermal alterations (s.l.) developed on the different types of the pluton manifests itself, chronologically, by located albitization, followed by greisenization and, by late microclinization that indiscriminately hits ali the granitic and greisens types of the pluton. The main minerais of economical interest related to the greisenization episode are cassiterite, topaz and beryl. This alteration have strongly modified the original geochemical signatures of the granites. Chemical variations are mainly the expressions of those superimposed modifications.
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