Mineralizações de Sn do Maciço Granítico Serra Branca, Goiás: evolução do sistema hidrotermal e fonte dos fluidos

The Serra Branca granitic massif occurs in the Goias tin province and is part of the Paleo- to Mesoproterozoic within-plate granites suite, which is rich in F, Sn, Rb, Y, Th, Nb, Ga and REE. The Serra Branca granitic massif hosts tin mineralization and comprises: a) pink to gray, porphyritic, medium- to coarse-grained granite (g1a); b) pink to gray, heterogranular, medium- to coarse-grained granite (g1b); c) gray, heterogranular, medium- to coarse-grained granite (g1c); and d) gray, heterogranular, medium-grained granite (g1d), which is restricted to the eastern part of the massif. Greisens and mica veins also occur in the massif. Important post-magmatic processes have been identified: albitization, greisenization (the most prominent) and microclinization. A fluid inclusion study in K-feldspar from the (g1a) and (g1c) granites and in quartz, topaz and fluorite from the (g1d) granite, greisen and quartz + fluorite and quartz + topaz veins revealed the presence of two fluid inclusions types: i) type 1, interpreted as primary and modeled by the H 2 O-NaCl-CO 2 ( ± CH 4 ) system; and ii) type 2, composed of H 2 O-NaCl ± (KCl-MgCl 2 -FeCl 2 ) two-phase fluid inclusions, with petrographic characteristics of primary fluid inclusions in the hydrothermal rocks and of secondary ones in quartz from the (g1a) granite. The low T H (~240oC) and salinity (< 2 wt. % NaCl equiv.) values of fluid inclusions entrapped in K-feldspar from the granites suggest that this mineral doesn’t contain the original magmatic fluid, but fluids derived either from progressive cooling or from late hydrothermal alteration. The petrologic characteristics of the Serra Branca massif together with the fluid inclusions data are consistent with predominant magmatic source for the fluids rich in CO 2 ( ± CH 4 ). The petrographic and microthermometric data suggest that the aqueous fluids were entrapped later. It is proposed an evolution model in which an initially magmatic fluid has been released and mixed with external lower-temperature fluid. After the hydrothermal alteration and mineralization, there was entrapment of late fluids in fractures of the hydrothermal minerals, probably due to the deformation associated with the Brasilano orogenic event.