Estudo microtermométrico dos fluidos hidrotermais relacionados com a mineralização aurífera de Montes Áureos, Nw do Maranhão

Abstract

The Montes Aureos gold mineralization occurred within the Tentugal Shear Zone which is an approximately 100 km long, 15 km wide NW-SE- trending structure that marks the south-southwest boundary of the Sao Luis craton and corresponds to the most deformed rocks of the Gurupi mobile belt. The ore bodies are hosted by the Paleoproterozoic metavolcanic and metasedimentary rocks of the Gurupi Group, which have been metamorphosed under greenschist to low amphibolite facies conditions. A superimposed hydrothermal event generated a system of vein and veinlets both concordant and discordant to the rock foliation. Gold occurs in up to 2 cm thick, late-tectonic quartz + carbonate veins or veinlets associated with arsenopyrite, pyrite and minor chalcopyrite. The spatial distribution of the mineralized zones defines lenticular to tabular bodies with gold contents less than 2 ppm. Textural features and time relationships between hydrothermal and metamorphic assemblages indicate that the mineralization followed the metamorphic peak and that gold occurs in at least two different forms: 1) isolated grains deposited simultaneously with sulfides; and 2) in microfractures in arsenopyrite and pyrite. Gold had been most likely transported by the sulphur complex [Au(HS) 2 - ] in an aqueous-carbonic, low salinity fluid (2 - 8 wt % NaCl equiv.) at temperatures £ 450 o C. Deposition took place at a temperature range from 260 to 350 o C. For the prevailing value of 300 o C, confirmed by the chlorite geothermometer, pressure estimates fall between 1.3 and 2.8 Kbar, corresponding to depths of 4-8.5 km. Fluids related to the systems CO 2 ±CH 4 , H 2 O-CO 2 - NaCl±CH 4 ±MgCl and/or FeCl 2 and H 2 O-NaCl±MgCl and/or FeCl 2 circulated through the Montes Aureos rocks. The aqueouscarbonic fluids are considered most likely to be products of devolatization reactions of carbon-bearing sedimentary rocks at temperature probably above 500 o C. Initially homogeneous, these fluids have undergone immiscibility and were then trapped in fluid inclusions with different H 2 O/CO 2 ratios. As the metamorphic thermal regime decreased and carbonate precipitated, the aqueous-carbonic fluids became progressively impoverished in CO 2 and less saline. Mixture with cooler and low salinity surface waters may have occurred at the final stages of the Montes Aureos 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 physico-chemical characteristics of the mineralizing fluids allow the Montes Aureos gold deposit to be classified in the orogenic category, as many others that occur in deformed and metamorphosed mid-crustal blocks.