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Alteração hidrotermal ácido-sulfato associada aos jazigos de sulfuretos maciços de Lagoa Salgada, Caveira, Lousal, Aljustrel e São Domingos (Faixa Piritosa Ibérica)
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Abstract(s)
RESUMO: A Faixa Piritosa Ibérica é caracterizada por dezenas de jazigos de sulfuretos maciços formados no Famenniano e Tournaisiano em ambiente marinho pela circulação de fluidos hidrotermais através das rochas vulcânicas e sedimentares encaixantes. O seu enquadramento geológico decorre do evento hidrotermal e de processos posteriores como metamorfismo regional, deformação varisca e tardi-varisca e, em alguns casos, exumação, erosão e alteração supergénica. Nos 7 jazigos estudados - Lagoa Salgada, Caveira, Lousal, Algares, São João, São Domingos e Chança, definem-se as seguintes fases de veios de alunite: a) gerados em período sin/tardi-Varisco em deformação dúctil/semi-dúctil e associados a corredores estruturais com cisalhamento - I (paralelos a S1), IIa (oblíquos a S1) e IIb (redes deformadas), sendo a paragénese marcada por alunite/natroalunite ± pirite ± minamiite ± wavellite e acompanhada por argilização precoce (caulinite ± greenalite), apresentando estes veios estrutura em pente, dobramento e génese anterior ou coeva da clivagem S2; b) formados em fases tardi-variscas/eoalpinas/alpinas semi-frágéis/frágeis, com textura porcelanosa e matriz homogénea com pseudocubos de alunite - IIb (redes com veios sub-horizontais) e III (em falhas desligamento) e IV (fraturas irregulares). Em contexto supergénico ocorrem a jarosite e natrojarosite. Não existe uma relação direta entre a argilização supergénica e os veios de alunite I, IIa, IIb e III. A alteração ácido-sulfato foi marcada pela circulação de fluidos muito ácidos e oxidantes, de baixa temperatura, por um período de tempo geológico significativo. O modelo metalogenético infere uma evolução contínua. A caracterização dos chapéus de ferro nos jazigos estudados reflete o seu zonamento: topo com texturas terrosas e cavidades por erosão/dissolução/precipitação; zonas basais mais compactas e maciças, que transitam a muro a níveis de enriquecimento supergénico e, inferiormente, a mineralização primária. Identificaram-se também depósitos coluviais e brechas de preenchimento. A cartografia desenvolvida sobre a alteração ácidosulfato, argilização e oxidação constitui um vetor de prospeção mineral indicador das mineralizações de sulfuretos da Faixa Piritosa.
ABSTRACT: Iberian Pyrite Belt (IPB) is characterized by dozens of massive sulfide deposits formed during Famennian and Tournaisian times in a marine environment by the circulation of hydrothermal fluids through the host volcanic and sedimentary rocks. Its geological setting is complex, resulting from the hydrothermal event and subsequent processes such as regional metamorphism, deformation (Variscan to Alpine) and, in some cases, exhumation, erosion and supergenic alteration. It was possible to identify different phases of alunite veins in the seven deposits investigated - Lagoa Salgada, Caveira, Lousal, Algares, São João, São Domingos and Chança: a) formed in the sin-/tardi-Variscan period in ductile/semi-ductile deformation associated with shear structural corridors - I (parallel to S1), IIa (oblique to S1) and IIb (deformed networks) defined by alunite/natroalunite ± pyrite ± minamiite ± wavellite being accompanied by early clay alteration (kaolinite ± greenalite) and common comb structure and folding being formed before or coeval to the S2 cleavage; b) formed in late-Variscan/eo-Alpine/Alpine semi-fragile/fragile systems - IIb (networks with subhorizontal veins), III (in strike-slip faults) and IV (irregular fractures), defined by porcelanous texture and alunite pseudocubes. Jarosite and natrojarosite occur in the supergene environment. There is no direct relationship between supergene clay alteration and the alunite I, IIa, IIb and III veins. Acidsulfate alteration is defined by low temperature acidic and oxidizing fluids, for a large geological time period considering a metallogenetic continuous evolution. The characterization of the gossans at the studied deposits made it possible to understand their zoning: top with earthy textures and cavities by erosion/dissolution/precipitation; more compact and massive basal zones, which gradually pass downward to supergene enrichment levels and to the primary mineralization. Colluvial deposits and filling breccias were also identified. The mapping developed on the acid-sulfate, clay and supergene oxidation hydrothermal systems, supported by detail stratigraphy, can be used as an efficient prospecting method focused on the IPB sulphide mineralization.
ABSTRACT: Iberian Pyrite Belt (IPB) is characterized by dozens of massive sulfide deposits formed during Famennian and Tournaisian times in a marine environment by the circulation of hydrothermal fluids through the host volcanic and sedimentary rocks. Its geological setting is complex, resulting from the hydrothermal event and subsequent processes such as regional metamorphism, deformation (Variscan to Alpine) and, in some cases, exhumation, erosion and supergenic alteration. It was possible to identify different phases of alunite veins in the seven deposits investigated - Lagoa Salgada, Caveira, Lousal, Algares, São João, São Domingos and Chança: a) formed in the sin-/tardi-Variscan period in ductile/semi-ductile deformation associated with shear structural corridors - I (parallel to S1), IIa (oblique to S1) and IIb (deformed networks) defined by alunite/natroalunite ± pyrite ± minamiite ± wavellite being accompanied by early clay alteration (kaolinite ± greenalite) and common comb structure and folding being formed before or coeval to the S2 cleavage; b) formed in late-Variscan/eo-Alpine/Alpine semi-fragile/fragile systems - IIb (networks with subhorizontal veins), III (in strike-slip faults) and IV (irregular fractures), defined by porcelanous texture and alunite pseudocubes. Jarosite and natrojarosite occur in the supergene environment. There is no direct relationship between supergene clay alteration and the alunite I, IIa, IIb and III veins. Acidsulfate alteration is defined by low temperature acidic and oxidizing fluids, for a large geological time period considering a metallogenetic continuous evolution. The characterization of the gossans at the studied deposits made it possible to understand their zoning: top with earthy textures and cavities by erosion/dissolution/precipitation; more compact and massive basal zones, which gradually pass downward to supergene enrichment levels and to the primary mineralization. Colluvial deposits and filling breccias were also identified. The mapping developed on the acid-sulfate, clay and supergene oxidation hydrothermal systems, supported by detail stratigraphy, can be used as an efficient prospecting method focused on the IPB sulphide mineralization.
Description
Tese de doutoramento, Geologia (Metalogenia), Universidade de Lisboa, Faculdade de Ciências, 2021
Keywords
Alteração ácido-sulfato Oxidação e argilização Sulfuretos maciços polimetálicos Estratigrafia Faixa Piritosa Ibérica Acid-sulphate alteration Oxidation and clay alteration Polimetallic massive sulfides Stratigraphy Iberian Pyrite Belt
Citation
Matos, João Xavier - Alteração hidrotermal ácido-sulfato associada aos jazigos de sulfuretos maciços de Lagoa Salgada, Caveira, Lousal, Aljustrel e São Domingos (Faixa Piritosa Ibérica). Lisboa : Universidade de Lisboa. Faculdade de Ciências, 2021, 578 p.