Geologia e metalogênese do depósito aurífero do Palito, Província Tapajós, Itaituba - Pa

Abstract

The Palito Gold Deposit lies in the Jamanxim river region, easternmost part of the Tapajós Gold Province, sonthern Pará State. The Palito deposit comprises a mineralized quartz vein system hosted by Palito and Rio Novo Paleoproterozoic granites and is controlled by a NW-SE brittleductile stike-slip shear zone which belongs to a regional structure so called Tocantinzinho Lineament. The Palito Granite crosscuts the Rio Novo Granite and is the main host rock for the gold mineralization. The Rio Novo and Palito granites are two oxidized calc-alkaline monzogranitic stocks correlated respectively to Creporizão and Tropas Suites, characterized as magmatic arc granites. The thicker lodes, of metric width, are hosted by the NW-SE main fractures, whereas thinner veins, of centimetric width, associated with the thicker lodes, make both low angle (around E-W direction) and middle to high angle (NE-SW direction) in relation to the main shear direction. Such a structural situation is compatible to the Riedel system, with parallel lodes (D), low-angle veins (R and P), middle-angle veins (gash) and high-angle veins (R’ and X). Stockwork veinlets also occur as a minor ore type. The veins are always involved by a well developed, normally brecciated, hydrothermal alteration halo. Phyllic alteration ( quartz + phengite + pyrite) and chloritization (chamosite) are the two main hydrothermal alteration types which are followed by potassic alteration (K- feldspar + biotite), carbonatization (calcite + siderite + sericite) and subordinated sufidation (pyrite + chalcopyrite + sphalerite). The gold ore, hosted by quartz veins, is always associated to iron and copper sulfides (chiefly pyrite and chalcopyrite), besides sphalerite. Pyrrhotite, bismuthinite, native bismuth and gold are minor metallic phases. Three generations of pyrite, two generation of chalcopyrite and one generation of sphalerite were recognized. Substitution of chalcopirite1 for pyrite1 and pyrite2 for chalcopirite1 are widespread, whereas sphalerite crystallization is controlled by temperature and Cu/Zn ratio of the system, that is, substitution of sphalerite for chalcopirite1 ocurrs when temperature and Cu/Zn ratio are low and, vice versa, substitution of chalcopirite1 for sphalerite occurs in higher temperature and Cu/Zn ratio conditions. The gold is always associated or included in chalcopyrite, bismuthinite and native bismuth. Late sulfide generations are represented by veinlets of pyrite3 and chalcopirite2. The main gangue minerals associated to gold ore are quartz, phengite and chlorite, besides minor carbonate, fluorite, rutile, zircon and ilmenite Three kinds of fluids were recognized in the fluid inclusion study: 1) low salinity H2O-NaCl (up to 2,0 wt.% NaCl equiv.), interpreted as meteoric water; 2) high salinity H2O-NaCl-FeCl2-MgCl2 (45-50 wt.% NaCl equiv.) interpreted as magmatic brines, 3) low salinity H2O-CO2-NaCl (1,0-1,7 wt.% NaCl equiv.), probably of magmatic origin. The temperature and pressure conditions for the Palito gold deposit formation were estimated by combination of hydrothermal chlorite geothermometry and isochores calculated from the fluid inclusion microthermometric data. P-T values for trapping of the brines range from 340 to 405oC and 2 to 4,7 kb. The magmatic brines were interpreted as the main mineralized fluid, that is, sulphur, chlorine and metals (chiefly Au and Cu) were originated and concentrated from the residual phase of the granitic magma which gave rise the Palito Granite. The brines transported the metals as chloride complexes at high temperature (over 400oC). Fluid-rock interaction and mixing between mineralized fluid and low-T and low salinity fluids (probably meteoric water) promote a raising of pH and lowering of fS2 through hydrolysis and sulfidation reactions which have triggered ore deposition at lower temperature conditions (below 400oC) in transtension sites of the shear zone. The geological and hydrothermal alteration features and the mineralizing fluid characteristics are consistent to a structurally controlled intrusion related gold deposit model for the genesis of the Palito deposit. The host granitic rocks of the gold deposit, the vein style, the hydrothermal alteration (seritization and chloritization with minor potassic alteration), the Au-Cu-Zn (Pb-Bi) metallic association and the mineralizing fluid (H2O-NaCl-FaCl2-MgCl2) are consistent to a genetic relationship between the Palito Granite and the gold deposit. The granite supplied fluids, metals and heat to move the hydrothermal system, whereas the shear zone provided the structural traps for the ore deposition.