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Mineralogia e geoquímica da massa do Zambujal, Neves-Corvo: diferente em quê e porquê?
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O Zambujal é uma das sete massas mineralizadas do jazigo de Neves-Corvo. Este encontra-se hospedado numa das províncias metalogenéticas de referência mundial para sulfuretos maciços polimetálicos - a Faixa Piritosa Ibérica (FPI). Estudos anteriores colocaram em evidência que os minérios do Zambujal se caracterizam por vincadas singularidades no contexto do jazigo de Neves-Corvo. O presente trabalho incide sobre os diferentes tipos de minério maciço da massa do Zambujal, com vista à compreensão das características que a tornam significativamente distinta das restantes massas. Estas diferenças incluem a abundância relativa de um número significativo de metais, incluindo o In e Se, bem como a sua distribuição, determinação da fase mineral portadora e correlações intermetálicas. Importa assim caracterizá-las, compreendê-las e interpretá-las em ordem à história sin- e pós-metalogenética do sistema mineralizante. O estudo inclui o relogging de 23 sondagens e a caracterização mineralógica e textural, macro e microscópica de 58 amostras, com definição de um quadro paragenético para a massa do Zambujal. Deste conjunto de amostras foram selecionadas 10 superfícies polidas e 2 lâminas delgadas polidas para caracterização micro-analítica dos minerais associados às diferentes fases de mineralização sulfuretada, bem como do material carbonatado e/ou silicatado que constitui os exalitos representados pela unidade JC. Foi realizado igualmente um estudo geoquímico detalhado dos minérios da massa do Zambujal recorrendo à vasta base de dados de química de rocha total da Somincor-Lundin Mining, que permitiu uma avaliação da abundância, distribuição dos vários metais analisados, produção de mapas e perfis composicionais do Zambujal e a construção de matrizes de correlação intermetálica para os seus minérios. Por último, procedeu-se à determinação das fases minerais portadores de metais e integração dos resultados obtidos e interpretações preconizadas no modelo metalogenético que tem vindo a ser delineado para o jazigo de Neves-Corvo. O estudo de química mineral efetuado coloca em evidência que, dependo da tipologia de minério considerado e de acordo com as correlações intermetálicas observadas na massa do Zambujal, a esfalerite, a tetraedrite-tenantite, a estanite e a calcopirite são as principais fases portadoras de índio; a galena-claustalite e os selenetos de Bi-Pb são os principais portadores de selénio; a tetraedrite, a cosalite e os selenetos Bi-PB são os principais portadores de bismuto, e a tetraedrite, a tetraedrite-tenantite e a galeno-claustalite são os principais portadores de prata. De forma geral, é possível reconhecer que os metais de mais baixa temperatura, como o Zn, Pb, Ag e Cd estão essencialmente associados aos minérios do tipo MZ, MZP, MP e ME, ao passo que os metais de alta temperatura, como o Cu, Co, Bi, In e Se se associam essencialmente aos minérios MC e MCZ. Contrariamente ao que sucede à escala de todo o jazigo e de cada uma das restantes massas, no Zambujal, o In e o Se para além de apresentarem correlações positivas com o Cu também apresentam correlações com o Zn (especialmente no MCZ). Assim, o In e o Se (e de forma análoga, o Bi e Co) estão maioritariamente associados aos minérios cupríferos, facto que é consistente com o transporte e coprecipitação destes metais com fluidos ricos em Cu, de alta temperatura, ácidos, reduzidos e salinos (afiliação metalogenética). No entanto, o facto da esfalerite apresentar coeficiente de partição mais favorável para acomodar estes elementos favorece que, na massa do Zambujal, a forte influência dos processos de remobilização/redistribuição tectono-metamórfica, seja suficientemente vincada para justificar também uma correlação positiva secundária entre o In-Se com o Zn, circunstância que não tem expressão comparável nas restantes massas. Em termos gerais, a massa do Zambujal partilha a história metalogenética das restantes massas em Neves-Corvo, com um sistema hidrotermal convectivo sustentado e prolongado no tempo, responsável pela formação, através de processos dominantes de substituição na subsuperfície, sob temperaturas baixas a moderadas, de abundante mineralização plumbo-zincífera (minérios MZ, MZP, MP e ME). A evolução e maturação da atividade hidrotermal mineralizante no decurso de um prolongado período de tempo, conduz ao aumento da temperatura do sistema e à interação dos produtos precocemente formados com fluidos ricos em cobre, salinos, ácidos e redutores que foram responsáveis pela geração de uma mineralização cuprífera de mais alta temperatura (minérios MC e MCZ), que substituem parcialmente a mineralização zincífera previamente formada. Em termos macro- e microscópicos, este processo é evidenciado pela obliteração das características primárias mais precoces (e.g. pirite colomórfica e framboidal) e predomínio de texturas de substituição. No decurso da evolução geológica e geotectónica da região, o jazigo no seu todo e a massa do Zambujal em particular, foram sujeitos a processos tectono-metamórficos muito relevantes, que são visíveis à escala macro- e microscópica e que conduziram à formação de abundantes texturas secundárias, como brechas, bandados de segregação textural, boudinage e zonas de cominuição forte do minério. Estes processos perturbaram o zoneamento primário, que segue, em traços gerais, o zoneamento característico dos depósitos de sulfuretos maciços (Cu-Zn-Fe) e promoveram (a) a redistribuição e remobilização de metais como o As, o Sb, o Bi e o Se, que permitem sublinhar espacialmente várias zonas de falha e corredores estruturais; e (b) a formação de enriquecimentos locais de elementos económicos (Cu, Zn ou Pb) e/ou de elementos valorizantes (e.g. Ag) e/ou de elementos potencialmente valorizantes (e.g. Se, In). Estes fenómenos de enriquecimento são mais evidentes na zona centro-este da massa onde existe sobre espessamento do minério MCZ devido a empilhamento de sucessivos cavalgamentos, e em domínios mais deformados dos minérios de Zn e Zn-Pb e/ou em fraturas com remobilizações de esfalerite afetando as várias tipologias de minério. Assim, a forte deformação tectónica da massa do Zambujal, superior à das restantes massas de Neves-Corvo, justifica as dissemelhanças evidenciadas em termos de correlações intermetálicas, bem como os invulgarmente elevados efeitos de redistribuição/remobilização e enriquecimento metálico de elementos como o Se, In, Cu ou Pb. No entanto, alguns elementos penalizantes, como o Hg ou o As, acompanham igualmente este trajeto de enriquecimentos secundários por remobilização tectono-metamórfica, facto que obriga, do ponto de vista da operação mineira, a uma ponderação cuidada das estratégias de produção em cada momento e contexto de exploração da massa.
Zambujal is one of the seven orebodies of the Neves-Corvo deposit, found in one of the world's major metallogenetic provinces for polymetallic massive sulphides - the Iberian Pyrite Belt (IPB). Previous studies have shown that the Zambujal ores are characterized by sharp singularities in the context of the Neves-Corvo deposit. This work focuses on the different types of massive ore at the Zambujal orebody, in order to understand the characteristics that make it significantly different from the other orebodies. These differences include the relative abundance of a significant number of metals, such as In and Se, as well as their distribution, determination of their mineral bearing phase and intermetallic correlations. Their characterization and interpretation are thought to be crucial to the understanding of the sin- and post-metallogenetic history of this peculiar mineralizing system. The study includes the re-logging of 23 drill holes and the mineralogical, macro- and microscopic characterization of 58 samples, which led to the definition of a paragenetic sequence for the Zambujal ore-body. From this set of samples, 10 polished surfaces and 2 polished thin sections were selected for micro-analytical characterization of the minerals associated with the different stages of sulphide mineralization, as well as the carbonate and/or silicate material that constitute exhalates represented by the JC unit. A detailed geochemical study of the Zambujal ores was also carried out using the vast Somincor-Lundin Mining's whole-rock geochemistry database, which allowed an assessment of the abundance, distribution of the various metals analysed, the production of maps and compositional profiles of the Zambujal orebody, and to define intermetallic correlation matrices for their ores. Finally, the mineral bearing phase for the different metals was determined and the obtained results and interpretations were integrated in the metallogenetic model that has been previously outlined for the Neves-Corvo deposit. The study of mineral chemistry data shows that, depending on the type of considered ore and according to the intermetallic correlations observed in the Zambujal orebody, sphalerite, tetrahedrite-tennantite, stannite and chalcopyrite are the main carriers of indium ; galena-claustalite and several selenides of Bi-Pb are the main carriers of selenium; tetrahedrite, cosalite and Bi-Pb selenides are the main carriers of bismuth, and tetrahedrite, tetrahedrite-tennantite and galena-claustalite are the main carriers of silver. In general, it is possible to recognize that lower temperature metals such as Zn, Pb, Ag and Cd are essentially associated with the MZ, MZP, MP and ME ores, while higher temperature metals such as Cu, Co, Bi and In are mainly associated with the MC and MCZ ores. Contrary to what happens at the deposit scale and in each of the remaining orebodies, in the Zambujal orebody, In and Se positively correlated with Cu and also Zn, especially in the MCZ ores. Thus, In and Se (and similarly Bi and Co) are mostly associated with the cupriferous ores, which is consistent with the transport and co-precipitation of these metals with high temperature, acidic fluids, rich in Cu (metallogenetic affiliation). Nevertheless, the fact that sphalerite has a greater crystallochemical aptness to accommodate those trace metals favours that, in the Zambujal orebody, the strong influence of tectono-metamorphic remobilization/redistribution processes, is strong enough to also justify a secondary positive correlation between In-Se and Zn, which has no similar expression in the remaining orebodies. In general terms, the Zambujal orebody shares a similar metallogenic history of the remaining orebodies in Neves-Corvo, with a sustained and long-lived convective hydrothermal system responsible for the formation of abundant lead-zinc mineralization (MZ, MZP, MP and ME), through dominant processes of subsurface replacement, under low to moderate temperatures. The evolution and development of the mineralizing hydrothermal activity over a long period of time led to an increase in the temperature of the system and to the interaction of the early-formed products with copper-rich, saline, acid and reducing fluids that were responsible for the generation of the cupriferous mineralization (MC and MCZ ores), which partially replaced the previously formed zinciferous mineralization. At macro- and microscopic scales, this process is recorded by the obliteration of the earliest and pristine primary characteristics (e.g., collomorphic and framboidal pyrite) and predominance of replacement textures. In the course of the geological and geotectonic evolution of the region, the entire deposit, and the Zambujal orebody in particular, underwent very relevant tectono-metamorphic processes, which are visible at the macro- and microscopic scale leading to the formation of several deformation textures such as breccias, segregation bands, boudinage and domains of strong mineral comminution. These processes disrupt primary zoning, which broadly follows the typical zoning of massive sulphide deposits (Cu-Zn-Fe), and promoted (a) the redistribution and remobilization of metals such as As, Sb, Bi and Se, which allow spatial underlining of various fault zones and structural corridors; and (b) the formation of local enrichments of economic metals (Cu, Zn or Pb), and/or by-products (e.g. Ag), and/or potentially by-products (e.g. Se, In). These enrichment phenomena are more evident in the centre-east part of the orebody, where the MCZ ore was thickened due to overthrust stacking, as well as in particularly deformed domains of Zn and Zn-Pb ores, and/or in fractures with remobilization of sphalerite affecting the various of ore types. Thus, the strong tectonic deformation of the Zambujal orebody, which is much more pronounced than in the remaining orebodies of the Neves-Corvo deposit, explains the dissimilarities with the other ore-bodied evidenced by distinct intermetallic correlations, as well as the unusually high redistribution/re-mobilization effects and the local enrichment of metals such as Se, In, Cu or Pb. However, some penalizing elements, such as Hg or As, also accompany this path of secondary enrichment by tectono-meta-morphic remobilization, which forces, from a mining operation point of view, a careful production strategy in each moment and context of the orebody exploitation.
Zambujal is one of the seven orebodies of the Neves-Corvo deposit, found in one of the world's major metallogenetic provinces for polymetallic massive sulphides - the Iberian Pyrite Belt (IPB). Previous studies have shown that the Zambujal ores are characterized by sharp singularities in the context of the Neves-Corvo deposit. This work focuses on the different types of massive ore at the Zambujal orebody, in order to understand the characteristics that make it significantly different from the other orebodies. These differences include the relative abundance of a significant number of metals, such as In and Se, as well as their distribution, determination of their mineral bearing phase and intermetallic correlations. Their characterization and interpretation are thought to be crucial to the understanding of the sin- and post-metallogenetic history of this peculiar mineralizing system. The study includes the re-logging of 23 drill holes and the mineralogical, macro- and microscopic characterization of 58 samples, which led to the definition of a paragenetic sequence for the Zambujal ore-body. From this set of samples, 10 polished surfaces and 2 polished thin sections were selected for micro-analytical characterization of the minerals associated with the different stages of sulphide mineralization, as well as the carbonate and/or silicate material that constitute exhalates represented by the JC unit. A detailed geochemical study of the Zambujal ores was also carried out using the vast Somincor-Lundin Mining's whole-rock geochemistry database, which allowed an assessment of the abundance, distribution of the various metals analysed, the production of maps and compositional profiles of the Zambujal orebody, and to define intermetallic correlation matrices for their ores. Finally, the mineral bearing phase for the different metals was determined and the obtained results and interpretations were integrated in the metallogenetic model that has been previously outlined for the Neves-Corvo deposit. The study of mineral chemistry data shows that, depending on the type of considered ore and according to the intermetallic correlations observed in the Zambujal orebody, sphalerite, tetrahedrite-tennantite, stannite and chalcopyrite are the main carriers of indium ; galena-claustalite and several selenides of Bi-Pb are the main carriers of selenium; tetrahedrite, cosalite and Bi-Pb selenides are the main carriers of bismuth, and tetrahedrite, tetrahedrite-tennantite and galena-claustalite are the main carriers of silver. In general, it is possible to recognize that lower temperature metals such as Zn, Pb, Ag and Cd are essentially associated with the MZ, MZP, MP and ME ores, while higher temperature metals such as Cu, Co, Bi and In are mainly associated with the MC and MCZ ores. Contrary to what happens at the deposit scale and in each of the remaining orebodies, in the Zambujal orebody, In and Se positively correlated with Cu and also Zn, especially in the MCZ ores. Thus, In and Se (and similarly Bi and Co) are mostly associated with the cupriferous ores, which is consistent with the transport and co-precipitation of these metals with high temperature, acidic fluids, rich in Cu (metallogenetic affiliation). Nevertheless, the fact that sphalerite has a greater crystallochemical aptness to accommodate those trace metals favours that, in the Zambujal orebody, the strong influence of tectono-metamorphic remobilization/redistribution processes, is strong enough to also justify a secondary positive correlation between In-Se and Zn, which has no similar expression in the remaining orebodies. In general terms, the Zambujal orebody shares a similar metallogenic history of the remaining orebodies in Neves-Corvo, with a sustained and long-lived convective hydrothermal system responsible for the formation of abundant lead-zinc mineralization (MZ, MZP, MP and ME), through dominant processes of subsurface replacement, under low to moderate temperatures. The evolution and development of the mineralizing hydrothermal activity over a long period of time led to an increase in the temperature of the system and to the interaction of the early-formed products with copper-rich, saline, acid and reducing fluids that were responsible for the generation of the cupriferous mineralization (MC and MCZ ores), which partially replaced the previously formed zinciferous mineralization. At macro- and microscopic scales, this process is recorded by the obliteration of the earliest and pristine primary characteristics (e.g., collomorphic and framboidal pyrite) and predominance of replacement textures. In the course of the geological and geotectonic evolution of the region, the entire deposit, and the Zambujal orebody in particular, underwent very relevant tectono-metamorphic processes, which are visible at the macro- and microscopic scale leading to the formation of several deformation textures such as breccias, segregation bands, boudinage and domains of strong mineral comminution. These processes disrupt primary zoning, which broadly follows the typical zoning of massive sulphide deposits (Cu-Zn-Fe), and promoted (a) the redistribution and remobilization of metals such as As, Sb, Bi and Se, which allow spatial underlining of various fault zones and structural corridors; and (b) the formation of local enrichments of economic metals (Cu, Zn or Pb), and/or by-products (e.g. Ag), and/or potentially by-products (e.g. Se, In). These enrichment phenomena are more evident in the centre-east part of the orebody, where the MCZ ore was thickened due to overthrust stacking, as well as in particularly deformed domains of Zn and Zn-Pb ores, and/or in fractures with remobilization of sphalerite affecting the various of ore types. Thus, the strong tectonic deformation of the Zambujal orebody, which is much more pronounced than in the remaining orebodies of the Neves-Corvo deposit, explains the dissimilarities with the other ore-bodied evidenced by distinct intermetallic correlations, as well as the unusually high redistribution/re-mobilization effects and the local enrichment of metals such as Se, In, Cu or Pb. However, some penalizing elements, such as Hg or As, also accompany this path of secondary enrichment by tectono-meta-morphic remobilization, which forces, from a mining operation point of view, a careful production strategy in each moment and context of the orebody exploitation.
Descrição
Relatório de estágio de mestrado, Geologia Económica (Prospeção Mineral), Universidade de Lisboa, Faculdade de Ciências, 2019
Palavras-chave
Neves-Corvo Zambujal Química Mineral Correlações intermetálicas Redistribuição e remobilização metalífera Enriquecimentos secundários Processos tectono-metamórficos Teses de mestrado - 2019