The São Jorge gold deposit lies in the easternmost part of the Tapajós Gold Province, SW portion of the State of Pará, far around 88km from Novo Progresso city. The São Jorge deposit comprises a mineralized quartz vein system hosted by the Young São Jorge Paleoproterozoic granite and it is controlled by a NW-SE trending shear zone. The Young São Jorge granite is an oxidized calc-alkaline monzogranite stock, characterized as magmatic arc type and correlated to the Tropas Intrusive Suite. The São Jorge shear zone is a brittle-ductile sinistral strike-slip shear zone which belongs to a regional structure known as Tocantinzinho Lineament. The thicker lodes, of metric width, are hosted by NW-SE main shear direction, whereas thinner veins in the NE-SW direction make middle to high angle in relation to the chief shear direction. Such a structural situation is compatible to the Riedel system, with parallel lodes hosted by main shear direction (Y/D) and gash veins in extension fractures (T). Minor stockwork veinlets complete the São Jorge vein system. The mineralized veins are always involved by hydrothermal alteration halo. Early alteration is characterized by chloritization of the ferromagnesian minerals (biotite and hornblende) which yielded chlorite+carbonate+magnetite mineral assemblage, followed by sodium metasomatism (albitization) of the K-feldspar and saussuritization of the plagioclase which generated sericite+carbonate+epidote assemblage. Intensification of that process evolved to phallic alteration (phengite+quartz+pyrite assemblage) associated to minor chloritization and sufidation (pyrite+ chalcopyrite+sphalerite assemblage). The gold-bearing ore, hosted in quartz veins, occurs in deep hydrothermal altered rocks, associated to sulfides (chiefly pyrite) and, more rarely, chalcopyrite and sphalerite. Galena, bismuthinite, native bismuth and gold are minor metallic phases. Free gold also occurs in hydrothermal quartz and magnetite. The main gangue minerals associated to ore are quartz, phengite and chlorite. Carbonate, rutile and zircon are minor gangue phases. Three types of fluids were recognized in the São Jorge deposit from fluid inclusion studies: 1) low-middle-salinity aqueous H2O-NaCl-KCl fluid, interpreted as meteoric water; 2) middle-salinity aqueous H2O-NaCl-CaCl2-MgCl2 fluid, with homogenization temperature ranging from 120 to 230°C, interpreted as magmatic brines; and 3) low-middle-salinity aquocarbonic H2O-CO2-NaCl fluid, with homogenization temperature ranging from 260 to 350°C, probably of magmatic or metamorphic origin. The magmatic saline aqueous fluid was interpreted as the ore fluid which transported the metals (mainly gold), the clorine, and the oxidized sulphur (SO2), concentrated in the residual phase of a granitic magma, whereas the aquocarbonic fluid transported the majority of CO2 and the reduced sulphur (H2S). The temperature and pressure conditions for the São Jorge gold deposit were established by combination of hydrothermal chlorite geothermometry and isochore calculated from the fluid inclusion microthermometric data. The temperature values for trapping the ore fluids range from 280°C to 360°C and the pressures vary from 1.35 to 3.6 kb. Such a temperature condition favors the gold transport as tiocomplexes. Early oxidation process brought about an increase of fO2 and magnetite formation which favored first generation of gold deposition. The mixing of ore fluid with aquocarbonic fluid and meteoric water and fluid-rock interaction, with associated phillic and sulfidation alteration reactions, caused increasing of fO2 and reduction of pH, fS2 and temperature which triggered the main gold precipitation in transtension sites of shear zone. The veining style of the São Jorge deposit, the host granitic rocks of the ore, the kinds of hydrothermal alterations, the Au(Cu-Zn-Pb-Bi) metallic association and the ore fluid of magmatic filliation, are consistent to a genetic relationship between the gold deposit and granitic magmatism (intrusion-related gold deposit). On the other hand, the structural control of the ore bodies and the occurrence of aquocarbonic fluid with higher homogenization temperature, deep-collected by the shear zone, hold up the mesozonal orogenic model. Such a characteristics support a hybrid genetic model for the São Jorge gold deposit, in which the granite supplied the ore fluid, the metals and heat to move the hydrothermal system, whereas the shear zone provided the aquocarbonic fluid, the flow paths for the fluids and the structural traps for ore deposition. Since the shear zone had affected the Yong São Jorge granite, the genetic relationship between the gold deposit and that granite, though possible, is little probable. Another more consistent possibility as magmatic source for the São Jorge ore fluid would be a younger granite, as the Maloquinha granite, 14 to 27 Ma younger, which occurs in the whole Tapajós Province and also neighborhood São Jorge area.
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