Contexto metalogenético do garimpo filonar de Au ± Cu do Aguinaldo, Província Aurífera de Alta Floresta (MT)
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Data
2021-10-29
Autores
Bortolozzo, Rômulo
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Universidade Estadual Paulista (Unesp)
Resumo
O setor leste da Província Aurífera de Alta Floresta (PAAF), ao sul do Cráton Amazônico, abrange centenas de ocorrências auríferas sob a forma de veios, stockworks e disseminações em unidades graníticas, vulcânicas e vulcanossedimentares paleoproterozoicas. Essas mineralizações localizam-se ao longo de um lineamento principal denominado Peru-Trairão, de direção NW-SE. Dentre elas, está o garimpo de Au ± Cu estruturalmente controlado do Aguinaldo, localizado no município de Nova Guarita (MT). Este garimpo é hospedado por milonito de ortognaisse de textura porfiroclástica, representado por cristais de feldspato potássico avermelhados estirados, que correspondem entre 20 e 30% do volume total da rocha. Exibe bandamento gnáissico e foliação milonítica bem definidos com orientações de N10E a N38E e mergulhos de alto ângulo entre 65º e 80º para NW. Geocronologia U-Pb SHRIMP em zircão forneceu idade concordante de 1984,9 ±5,2 Ma para a hospedeira. Essa idade indica que a hospedeira corresponde ao Complexo Cuiú-Cuiú, embasamento da província. No garimpo, são reconhecidos setes estágios hidrotermais, temporalmente organizados em: (1) alteração sódica com albita; (2) alteração potássica com microclina ± quartzo ± hematita ± magnetita; (3) alteração sericítica composta de sericita + muscovita; (4) silicificação e injeções de sílica na forma de veios de quartzo; (5) alteração carbonática com ankerita ± dolomita; (6) alteração clorítica definida por clorita ± hematita ± magnetita ± apatita ± titanita ± rutilo ± monazita ± pirita ± calcopirita; (7) silicificação tardia. As zonas sulfetadas ocorrem em dois estilos: filonar e disseminado, dos quais apenas o primeiro é lavrado. Três tipologias de veios são individualizadas: (1) veio estéril, de quartzo ± carbonato; (2) veio de quartzo ± carbonato com calcopirita ± pirita ± ouro ± prata ± esfalerita ± galena ± bismutinita ± covellita ± digenita ± goethita; e (3) veio de quartzo ± carbonato com pirita ± calcopirita ± prata ± esfalerita ± galena ± bismutinita. O ouro ocorre incluso e em contato com a pirita e, subordinamente, na esfalerita. Apresenta concentrações de Ag de até 43%. A evolução paragenética do sistema hidrotermal indica um sistema formado a partir do rebaixamento de temperatura e aumento das condições de fO2 e pH. A precipitação do minério poderia estar relacionada à uma combinação de processos de descompressão adiabática, oxidação de sulfetos e reação com a hospedeira. A presença de veios de quartzo com textura em pente (comb-texture) indica percolação de fluidos em ambiente crustal raso e, portanto, a favorabilidade da contribuição de fluidos externos (e.g. fluidos meteóricos), mais frios e oxidantes. Dados de geotermobarometria sugerem que o sistema tenha se formado em nível crustal raso a médio, em profundidades entre 0,52 e 6,94 km. Nesse contexto, o garimpo do Aguinaldo se assemelharia de forma mais coerente aos sistemas epitermais do tipo low sulfidation, sobretudo se consideradas as características: (i) paragênese polimetálica (Au, Ag, Cu, Zn e Pb); (ii) elevada concentração de calcopirita; (iii) presença de quartzo com textura em pente (comb-texture); e (iv) ouro com alto conteúdo de Ag. Entretanto, o fato dos veios estarem deformados e encaixados em sistema de cisalhamento, sugere que o garimpo do Aguinaldo tenha se formado durante um evento de deformação e, portanto, deve corresponder a um sistema epitermal deformado, e possivelmente, pertencente a um evento aurífero mais antigo do que o definido em 1,78 - 1,77 Ga para a PAAF.
The eastern sector of the Alta Floresta Gold Province (AFGP), Amazon Craton, comprises several of significant vein-type, stockwork and disseminated gold deposits and occurrences associated to granitic, volcanic and volcanic-sedimentary rocks. These mineralizations lie along the NW-SE Peru-Trairão lineament. Among them is the structurally controlled Au ± Cu occurrence at the Aguinaldo mine, located in the Nova Guarita county. It’s hosted by orthogneiss mylonite with porphyroblastic texture defined by stretched reddish potassic feldspar crystals that encompass 20 to 30% of the rock vol. The host rock has a well-defined gneiss banding and mylonitic foliation striking N10E to N38E and dipping 65º to 80º to NW. The U-Pb SHRIMP IIe zircon analysis provided a concordia age of 1984.9 ±5.2 Ma, which indicates that it corresponds to the basement unit (Cuiú-Cuiú Complex). The hydrothermal alteration temporal sequence is defined as: (1) albite-bearing alteration; (2) potassic alteration with orthoclase ± quartz ± hematite ± magnetite; (3) sericitic-muscovite alteration (4) silicification and silica injections as quartz veins; (5) alteration with ankerite ± dolomite; (6) chloritic alteration with chlorite ± hematite ± magnetite ± apatite ± titanite ± rutile ± monazite ± pyrite ± chalcopyrite; (7) late silicification. Sulfide zones occur in two styles: vein-type and disseminated, in which only the former is mined. Three types of veins were classified: (1) barren quartz ± carbonate vein; (2) quartz ± carbonate vein with chalcopyrite ± pyrite ± gold ± silver ± sphalerite ± galena ± bismuthinite ± covellite ± digenite ± goethite; (3) quartz ± carbonate vein with pyrite ± chalcopyrite ± silver ± sphalerite ± galena ± bismuthinite. Gold occurs as inclusions or in contact with pyrite and, subordinately, in sphalerite. Moreover, it has Ag concentrations of up to 43 w%. The hydrothermal system evolution indicates a deposition caused by lowering of temperature and increasing of fO2 and pH conditions. Ore precipitation could have been related to the combination of different processes, such as adiabatic decompression, fluid oxidation and reaction with the host rock. The presence of comb-textured quartz veins indicates fluid percolation in a shallow crustal environment and, therefore, the possible contribution of colder and oxidizing external fluids. Geothermobarometry data suggest the Aguinaldo gold occurrence have formed at a shallow to medium crustal level, at dephs from 0.52 to 6.94 km. Therefore, the Aguinaldo mine might represent a low sulfidation epithermal systems, especially if considered its (i) polymetallic paragenesis (Au, Ag, Cu, Zn and Pb); (ii) high chalcopyrite concentration; (iii) the presence of comb-textured quartz; and (iv) gold with high Ag content. However, the fact that the veins are deformed and housed by a shear zone system, suggests the Aguinaldo mine might perform a deformed epithermal system, but possibly belonging to a gold-mineralizing event older than that defined at ca. 1.78 - 1.77 Ga for the AFGP.
The eastern sector of the Alta Floresta Gold Province (AFGP), Amazon Craton, comprises several of significant vein-type, stockwork and disseminated gold deposits and occurrences associated to granitic, volcanic and volcanic-sedimentary rocks. These mineralizations lie along the NW-SE Peru-Trairão lineament. Among them is the structurally controlled Au ± Cu occurrence at the Aguinaldo mine, located in the Nova Guarita county. It’s hosted by orthogneiss mylonite with porphyroblastic texture defined by stretched reddish potassic feldspar crystals that encompass 20 to 30% of the rock vol. The host rock has a well-defined gneiss banding and mylonitic foliation striking N10E to N38E and dipping 65º to 80º to NW. The U-Pb SHRIMP IIe zircon analysis provided a concordia age of 1984.9 ±5.2 Ma, which indicates that it corresponds to the basement unit (Cuiú-Cuiú Complex). The hydrothermal alteration temporal sequence is defined as: (1) albite-bearing alteration; (2) potassic alteration with orthoclase ± quartz ± hematite ± magnetite; (3) sericitic-muscovite alteration (4) silicification and silica injections as quartz veins; (5) alteration with ankerite ± dolomite; (6) chloritic alteration with chlorite ± hematite ± magnetite ± apatite ± titanite ± rutile ± monazite ± pyrite ± chalcopyrite; (7) late silicification. Sulfide zones occur in two styles: vein-type and disseminated, in which only the former is mined. Three types of veins were classified: (1) barren quartz ± carbonate vein; (2) quartz ± carbonate vein with chalcopyrite ± pyrite ± gold ± silver ± sphalerite ± galena ± bismuthinite ± covellite ± digenite ± goethite; (3) quartz ± carbonate vein with pyrite ± chalcopyrite ± silver ± sphalerite ± galena ± bismuthinite. Gold occurs as inclusions or in contact with pyrite and, subordinately, in sphalerite. Moreover, it has Ag concentrations of up to 43 w%. The hydrothermal system evolution indicates a deposition caused by lowering of temperature and increasing of fO2 and pH conditions. Ore precipitation could have been related to the combination of different processes, such as adiabatic decompression, fluid oxidation and reaction with the host rock. The presence of comb-textured quartz veins indicates fluid percolation in a shallow crustal environment and, therefore, the possible contribution of colder and oxidizing external fluids. Geothermobarometry data suggest the Aguinaldo gold occurrence have formed at a shallow to medium crustal level, at dephs from 0.52 to 6.94 km. Therefore, the Aguinaldo mine might represent a low sulfidation epithermal systems, especially if considered its (i) polymetallic paragenesis (Au, Ag, Cu, Zn and Pb); (ii) high chalcopyrite concentration; (iii) the presence of comb-textured quartz; and (iv) gold with high Ag content. However, the fact that the veins are deformed and housed by a shear zone system, suggests the Aguinaldo mine might perform a deformed epithermal system, but possibly belonging to a gold-mineralizing event older than that defined at ca. 1.78 - 1.77 Ga for the AFGP.
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Metalogenia, Minérios de ouro, Geocronologia