Caracterização do metamorfismo da Faixa Móvel Aguapeí e sua relação com a gênese dos depósitos de ouro da Região de Pontes e Lacerda - MT
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Data
2016-04-28
Autores
Melo, Rodrigo Prudente de [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
A Faixa Móvel Aguapeí compreende uma estreita faixa de dobramentos de aproximadamente 500Km de comprimento, estruturalmente confinada entre a margem leste do Craton Paraguá e o embasamento da província San-Ignacio. A reativação de estruturas de escala crustal tem sido apontado como o principal responsável pela evolução tectônica e metamórfica da faixa, que ocorreu no início do Neoproterozóico (~0.92Ga) e que causou a deformação e o metamorfismo de rochas sedimentares previamente depositadas em um Aulacógeno (Aguapeí Group), bem como seu embasamento Mezoproterozóico. A evolução da Faixa Móvel Aguapeí e sua relação com mineralizações auríferas que ocorrem em sua porção mais setentrional, foi investigado nessa pesquisa através de petrografia auxiliada pelo estudo de inclusões fluidas, geotermobarometria e geoquímica. A área de estudo está localizada a sul da cidade de Pontes e Lacerda (MT), mais ou menos na porção central da Província Aurífera Alto Guaporé que compreende uma série de depósitos de auríferos hospedados ao longo da faixa. Na parte sul da área de estudo, na região do depósito Pau-a-Pique, o arcabouço estrutural é dado por uma série de zonas de cisalhamento transcorrentes, enquanto que a porção norte, região do depósito Ernesto, é caracterizada por dobramentos e cavalgamentos. O padrão estrutural, por sua vez, sugere que a deformação foi contralada principalmente pela ação de zonas de cisalhamento com encurtamento NNE-SSW. Dados de geotermobarometria com base no conteúdo de Si em muscovita, Ti em quartzo e no fracionamento isotópico entre quartzo e magnetita, presente em veios de quartzo e em muscovita xisto ao longo de zonas de cisalhamento, sugerem que as condições metamórficas atingiram a zona de transição entre as fácies xisto verde anfibolito, com temperaturas superiores a 490ºC e pressão acima de 3.2Kbar. Essas condições estão de acordo com feições petrográficas observadas em dioritos e andesitos preservados em núcleos pouco deformados ao longo de zonas de cisalhamento anastomosadas. Também são mais ou menos coerente com presença de migração de bordas de grão (GBM) como principal mecanismo de recristalização de quartzo em zonas de cisalhamento. Dados de geoquímica de rocha total sugerem que fluidos estavam presentes durante a formação de zonas de cisalhamento transcorrente da região do depósito Pau a Pique. A percolação de fluidos associado ao cisalhamento causou importantes modificações químicas e texturais nas rochas e transformou granitóides em mica-xistos pela remobilização de elementos químicos. As condições (PT) encontradas em zonas de cisalhamento contrastam fortemente com as condições encontradas em rochas metassedimentares pouco deformadas, presentes principalmente na porção central e norte da área de estudo. A ausência de biotita em sedimentos pelíticos e a presença de Bulging (BLG) como mecanismo de recristalização do quartzo são indicativos de baixa temperatura (abaixo de 400ºC). Ao mesmo tempo o parâmetro b da célula unitária de micas brancas (sericita) em rochas metassedimentares indicam baixo conteúdo de fengita nas micas, comum em zonas de metamorfismo de pressão baixa a intermediária. Essas condições são compatíveis com metamorfismo na fácies xisto verde e sugerem que as condições metamórficas não foram uniformes ao longo da área.A percolação de fluidos redutores ricos em CO2 foram responsáveis pelo evento de mineralização aurífera, como observado nos depósitos Pau-a-Pique e Ernesto (Nível Intermediário). A relação próxima entre clorita, sulfeto e carbonato, que geralmente ocorrem ao longo de fraturas em veios de quartzo-albita foi usado para estimar a temperatura de precipitação do sulfeto através do uso de geotermômetros específicos para clorita. Os resultados sugerem que no depósito Pau-a-Pique, que clorita, carbonato e sulfeto ocorrendo em fraturas de veio de quartzo, se formaram a 325ºC durante a fase de retrometamorfismo. Os aspectos texturais de piritas revelam que se cristalizaram pela alteração de óxidos de Ti e Fe presentes nas encaixantes de veios de quartzo mineralizados do depósito Ernesto e em muscovita xisto do depósito Pau-a-Pique, devido a reação destes fluidos redutores ricos em H2S. Tal processo foi também responsável pela precipitação do ouro. As características das inclusões fluidas e da alteração hidrotermal dos depósitos estudados são comuns em depósitos do tipo ouro orogênico. Dados de isótopos estáveis (δD e δ18O) indicam fonte metamórfica para fluidos responsáveis pela cristalização dos principais minerais de ganga presente nos depósitos estudados e em outros seis depósitos da área de estudo. Além disso dados de δD em inclusões fluidas são compatíveis com valores encontrados na maioria das províncias de ouro orogênico pelo mundo. Os resultados permitem concluir que os depósitos estudados foram formados tardiamente na evolução metamórfica da Faixa Móvel Aguapeí, após o pico metamórfico da área, a partir de fluidos redutores ricos em CO2 que transportaram ouro via complexação com H2S.
The Aguapeí belt comprises a narrow and long (~500Km) NNW-SSE zone of deformed and metamorphosed rocks structurally confined between the eastern margin of the Paraguá Craton and the reactivated basement of Rondonian San-Ignacio. Neoproterozoic (~0.92 Ga) reactivation of crustal scale structures has been pointed out as responsible for the tectonic and metamorphic evolution of the belt. It has caused the deformation and metamorphism of previously deposited sedimentary rocks in an aulacogen basin and its Mesoproterozoic basement as well. The metamorphic evolution of Aguapeí belt, and its relationship with gold mineralization occurring on its southernmost portion was investigated in this research through petrography coupled with fluid inclusions studies, geochemistry and geothermobarometry. The studied area is located to the south of Pontes e Lacerda city (MT), more or less on central portion of Alto Guaporé Gold Province (AGGP), which in turn comprises a series of gold deposits hosted along the belt. The structural architecture of the south portion of the studied area (Pau-a-Pique region) is given by a series of strike-slip shear zones, whereas trusting and folding are dominant on central and northern (Ernesto area). The structural pattern suggest that the deformation was controlled by strike-slip shearing due to oblique shortening. Geothermobarometry based on Si content in muscovite, Ti in quartz and oxygen isotopes fractionation between the pair quartz-magnetite occurring in quartz veins and in muscovite schist present in shear zones suggest that the conditions reached the greenschist-amphibolite facies transition with temperature above 490ºC and minimum pressure of 3.2Kbars. These P-T condition are in agreement with the petrographic features found in mafic diorite and andesite preserved in cores of low deformation along anastomosed shear zones in Pau-a-Pique area and are also more or less consistent with the presence GBM quartz recrystallization mechanism occurring within shear zones. Whole rock geochemistry suggest that fluid were present during strike-slip shearing in Pau-a-Pique area. Percolating fluids during shearing has caused major chemical transformation of the mylonite during the prograde conditions. In the extremely modified rocks, metagranitoids were transformed to micas schist due elemental remobilization. The P-T conditions found in shear zones contrast strongly with those recorded in most of low deformed metasedimentary rocks of Aguapeí Group, mainly present on central and northern portion of the studied area. The presence of biotite-absent pelite and bulging recrystallization mechanism of quartz suggest low temperature conditions (below 400ºC). Meanwhile b-parameter in white micas of metasedimentary rocks records low content of phengite generally found in low to intermediate pressure regions. These conditions are compatible with greenschist facies metamorphism and suggest that the metamorphic conditions was not uniform through the area. Percolation of reduced aqueous-CO2 fluids were responsible for gold mineralization as recorded in Pau a Pique and Ernesto (intermediate Level) fluid inclusions. The close relationship between chlorite, sulfide and carbonate occurring along fractures in early formed quartz albite veins in Pau-a-Pique was used to estimate the temperature of sulfide precipitation using chlorite thermometry. The results suggest that chlorite, carbonate and sulfide found in fractures of early formed quartz, has been crystallized at 325ºC at the retrograde path in Pau-a-Pique deposit. The textural aspects of pyrite suggest that previously formed Fe-Ti oxides occurring in the wall rock of mineralized veins in the Ernesto deposit (Intermediate level) and in mica schist in shear zones of the Pau a Pique deposit, were replaced by pyrite due to reaction of Fe-Ti oxides with reducing H2S rich fluids. Such process was the main mechanism responsible for gold precipitation. The characteristics of fluid inclusions and the hydrothermal alteration of the studied deposits are similar to those found in most of orogenic gold deposit around the world. δD and δ18O stable isotope geochemistry are in agreement with metamorphic source for the fluid-forming of the main guange phases present in Pau-a-Pique, Ernesto (intermediate level) and other six gold occurrences in the central portion of AGGP. Meanwhile δD signature of fluid inclusions are compatible with those observed in major orogenic gold provinces worldwide. The results allows to conclude that the studied gold deposits were formed latter on metamorphic evolution of Aguapeí belt, during retrograde conditions, and from mixed aqueos-CO2 fluids that has transported gold by complexation with H2S.
The Aguapeí belt comprises a narrow and long (~500Km) NNW-SSE zone of deformed and metamorphosed rocks structurally confined between the eastern margin of the Paraguá Craton and the reactivated basement of Rondonian San-Ignacio. Neoproterozoic (~0.92 Ga) reactivation of crustal scale structures has been pointed out as responsible for the tectonic and metamorphic evolution of the belt. It has caused the deformation and metamorphism of previously deposited sedimentary rocks in an aulacogen basin and its Mesoproterozoic basement as well. The metamorphic evolution of Aguapeí belt, and its relationship with gold mineralization occurring on its southernmost portion was investigated in this research through petrography coupled with fluid inclusions studies, geochemistry and geothermobarometry. The studied area is located to the south of Pontes e Lacerda city (MT), more or less on central portion of Alto Guaporé Gold Province (AGGP), which in turn comprises a series of gold deposits hosted along the belt. The structural architecture of the south portion of the studied area (Pau-a-Pique region) is given by a series of strike-slip shear zones, whereas trusting and folding are dominant on central and northern (Ernesto area). The structural pattern suggest that the deformation was controlled by strike-slip shearing due to oblique shortening. Geothermobarometry based on Si content in muscovite, Ti in quartz and oxygen isotopes fractionation between the pair quartz-magnetite occurring in quartz veins and in muscovite schist present in shear zones suggest that the conditions reached the greenschist-amphibolite facies transition with temperature above 490ºC and minimum pressure of 3.2Kbars. These P-T condition are in agreement with the petrographic features found in mafic diorite and andesite preserved in cores of low deformation along anastomosed shear zones in Pau-a-Pique area and are also more or less consistent with the presence GBM quartz recrystallization mechanism occurring within shear zones. Whole rock geochemistry suggest that fluid were present during strike-slip shearing in Pau-a-Pique area. Percolating fluids during shearing has caused major chemical transformation of the mylonite during the prograde conditions. In the extremely modified rocks, metagranitoids were transformed to micas schist due elemental remobilization. The P-T conditions found in shear zones contrast strongly with those recorded in most of low deformed metasedimentary rocks of Aguapeí Group, mainly present on central and northern portion of the studied area. The presence of biotite-absent pelite and bulging recrystallization mechanism of quartz suggest low temperature conditions (below 400ºC). Meanwhile b-parameter in white micas of metasedimentary rocks records low content of phengite generally found in low to intermediate pressure regions. These conditions are compatible with greenschist facies metamorphism and suggest that the metamorphic conditions was not uniform through the area. Percolation of reduced aqueous-CO2 fluids were responsible for gold mineralization as recorded in Pau a Pique and Ernesto (intermediate Level) fluid inclusions. The close relationship between chlorite, sulfide and carbonate occurring along fractures in early formed quartz albite veins in Pau-a-Pique was used to estimate the temperature of sulfide precipitation using chlorite thermometry. The results suggest that chlorite, carbonate and sulfide found in fractures of early formed quartz, has been crystallized at 325ºC at the retrograde path in Pau-a-Pique deposit. The textural aspects of pyrite suggest that previously formed Fe-Ti oxides occurring in the wall rock of mineralized veins in the Ernesto deposit (Intermediate level) and in mica schist in shear zones of the Pau a Pique deposit, were replaced by pyrite due to reaction of Fe-Ti oxides with reducing H2S rich fluids. Such process was the main mechanism responsible for gold precipitation. The characteristics of fluid inclusions and the hydrothermal alteration of the studied deposits are similar to those found in most of orogenic gold deposit around the world. δD and δ18O stable isotope geochemistry are in agreement with metamorphic source for the fluid-forming of the main guange phases present in Pau-a-Pique, Ernesto (intermediate level) and other six gold occurrences in the central portion of AGGP. Meanwhile δD signature of fluid inclusions are compatible with those observed in major orogenic gold provinces worldwide. The results allows to conclude that the studied gold deposits were formed latter on metamorphic evolution of Aguapeí belt, during retrograde conditions, and from mixed aqueos-CO2 fluids that has transported gold by complexation with H2S.
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Palavras-chave
Ouro orogênico, Depósito Pau-a-Pique, Depósito Ernesto, Província Aurífera Alto Guaporé, Orogenic gold, Pau-a-Pique deposit, Ernesto deposit, Alto Guaporé gold province