Mineralização aurífera de Montes Áureos (Maranhão): rochas hospedeiras, controles deposicionais e fluidos mineralizantes.

Abstract

The Montes Áureos gold mineralization occurred within the Tentugal Shear Zone (ZCT) which is a + 100km long, 15km wide NW-SE trending structure that marks the south-southwest boundary of the São Luís craton and may represent a collisional suture between the Belém block and this craton. This zone is characterized by tectonites produced by compressive/transpressive and strike-slip movements, and corresponds to the most deformed rocks of the Gurupi mobile belt. The ore bodies are hosted by Proterozoic Gurupi Group metavolcanic and metasedimentary rocks which have been metamorphosed under conditions of low (chlorite + sericite), medium to high greenschist (chlorite + biotite + muscovite + epidote + actinolite + Mg- hornblende + Fe-hornblende) and low amphibolite facies (biotite + plagioclase + edenite + pargasite + ferrotchermakite). They have been also deformed under a brittle-ductile regime, leading to the formation of structures with different shapes, styles and degree of strain. A superimposed hydrothermal event generated assemblages composed of chlorite, carbonates and epidote that partially replaced metamorphic amphiboles, plagioclase and biotite. It also produced a system of vein and veinlets both concordant and discordant with respect to the rock foliation. Gold occurs in up to 2 cm thick, late-tectonic quartz + carbonates veins or veinlets associated with arsenopyrite, pyrite and minor chalcopyrite. The spatial distribution of the mineralized zones forms lenticular to tabular bodies with gold contents less than 2 ppm. Texture features and the time relationship between hydrothermal and metamorphic assemblages indicate that mineralization followed the metamorphic peak and that gold occurs in at least two different forms: 1) granular gold deposited simultaneously with sulfides; and 2) in microfractures in arsenopyrite. Gold had been most likely transported by the sulphur thio- complex [Au(HS)"] in an aqueous-carbonic, low salinity fluid (2 to 10 wt% equiv. NaCl) at temperature < 450ºC. Deposition occurred in a temperature range of 260 to 350C. For the prevalent value of 300ºC, obtained by the chlorite geothermometer, pressure estimates fall between 1,3 and 2,8 kb, corresponding to depths of 5-10 km. Desestabilization of that complex as temperature dropped and as the fluids interacted with the host rocks brought about gold precipitation. Fluids related to the systems CO; + CH,, H,0-C0O,-NaCl + CH, + MgCl e/ou FeCl, and H;0-NaCl + MgCl e/ou FeCl, circulated through Montes Áureos rocks. The aqueous-carbonic fluids are considered to be products of dehydration and decarbonization of carbon- fluids are considered to be products of dehydration and decarbonization of carbon-bearing sedimentary rocks at temperature probably above 500ºC. At first homogeneous, these fluids have undergone imiscibility and were then trapped in fluid inclusions with different H,)0/CO; ratios, some almost pure H,O or CO>. As the metamorphic thermal regime decreased and carbonates precipitated, the aqueous-carbonic fluids became progressively impoverished in CO, and less saline. Mixture with cooler and low salinity superficial waters may have occurred by the end of the evolution of the Montes Áureos hydrothermal system. The geotectonic setting, the structural control of the mineralization by shear zone, the hydrothermal alteration features, the time relations between the metamorphic peak and hydrothermal alteration, the gold ore mineral association and the physical-chemical characteristics of the mineralizing fluids allow the Montes Áureos gold deposit to be classified in the lode category, as many others that are formed at convergent plate margins.