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Geologia, controle estrutural e geocronologia das rochas hospedeiras e do minério: implicações para o modelo genético do depósito aurífero do Palito, Província Tapajós, Itaituba-PA
(Universidade Federal do Pará, 2016-09-24) SERRA, Vitor Felipe Hage; TORO, Marco Antonio Galarza; http://lattes.cnpq.br/8979250766799749
The Palito gold deposit, located at east portion of the Tapajós Gold Province, southwestern Pará State, comprises a nearly vertical mineralized quartz vein system, hosted by the Palito and Rio Novo Paleoprooterozoic granites and controlled by a sinistral strike-slip brittle-ductile shear zone, trending to NW-SE direction, which belongs to the regional Tocantinzinho lineament. The host granites are two oxidized calc-alkaline monzogranite stocks of Orosirian age and magmatic arc affiliation, correlated to the Parauari Intrisive Suite. The thicker lodes of the veining system are hosted by the major shear, along the N40-50W direction, whereas the thinner veins are housed by faults and fractures of second order and oblique to the main shear direction, both at low angle (around 20), moderate angle (nearby 50) and high angle (around 80). Such a structural situation is compatible with the Riedel system, with the major lodes parallel to the main shear direction (D), associated with low-angle veins (in R and P faults), moderate-angle gash veins (in extension T fractures) and high-angle veins (in R’ an X faults). 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 are the two main hydrothermal alteration types, associated to minor potassic alteration (K- feldspar), carbonatization (calcite + sericite + quartz) and sufidation (pyrite + chalcopyrite + sphalerite). Three generations of hydrothermal veining quartz are well characterized in the Palito gold deposit. The youngest low-angle veins (R and P) are made up by quartz1, whereas the later thicker lodes (D) are composed by quartz1 and 2. Tension gash quartz veins may occur at any time of the hydrothermal system evolution and are made up by both quartz1 (youngest gash veins) and quartz2 and 3 (later gash veins). Gash veinlets of quartz3 correspond to final stages of the Palito hydrothermal process. The gold ore, hosted chiefly by quartz1 and 2 veins, is always associated to iron and copper sulfides (pyrite and chalcopyrite), besides sphalerite. Pyrrhotite, bismuthinite, galena, native bismuth and gold are minor metallic phases. Three generations of pyrite and chalcopyrite and one generation of sphalerite were recognized. Chalcopyrite1 replaces pyrite1 and it is replaced by sphalerite, which, in turn, is replaced by chalcopyrite2. Pirite2, coeval to sphalerite, occurs in mineralized veins as anhedral masses replacing tiny remnant grains (islands) of chalcopyrite1, showing concave or corroded edges. Gold is always associated or included in chalcopyrite1 and 2 and pyrite2, bismuthinite and native bismuth crystals. Veinlets of pyrite3 and chalcopyrite3, crosscutting sulfide masses in transtension zones of tiny sinistral faults, are the latest sulfide generations. Two generations of phengite and chlorite were identified in the gangue minerals, being veinlets of phengite2, chlorite2 and carbonate the last one. Fluorite, rutile, zircon and ilmenite still occur as minor gangue phases. The Pb-Pb age of 179417 Ma, obtained for the Palito ore, was interpreted as rejuvenation of the Pb-Pb system caused by Sthaterian alkaline granitic magmatism (Porquinho Intrusive Suite) related to the third extensional deformation phase. The geological features of the Palito gold deposit, as structural controlled veining style of ore bodies and hydrothermal alteration halos, predominating sericitization and cloritization and metallic association (Au-Cu-Bi-Zn) of the ore, all favor classification of the Palito deposit as intrusion-related vein gold deposits. It is a non-porphyritic intrusion-related deposit type, possible related to alkaline granites of the Maloquinha Intrusive Suite, which occur around the Palito deposit. The granitic magma should has provided the ore fluids and metals and the shearing should has controlled the fluid circulation and ore deposition of the Palito deposit.