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Caracterização geoquímica, mineralógica, textural e paragenética da alteração hidrotermal e mineralização na massa de Estação (Aljustrel) e sua comparação com a massa de Feitais
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O jazigo de Estação situa-se no extremo NE da vila de Aljustrel e corresponde a uma massa de sulfuretos maciços rejeitada, em cerca de 600 m, pela falha da Represa, relativamente ao depósito de Feitais, com o qual formaria um corpo mineralizado único. Este jazigo, até hoje não explorado, apresenta uma morfologia tipicamente lenticular com 600 m de comprimento, 480 m de largura e até 100 m de possança, ocorrendo no flanco normal do designado anticlinal de Feitais. A re-interpretação geoestrutural levada a cabo nesta investigação permitiu reconstituir por completo a geometria da massa de Estação, com base no estudo detalhado de três sondagens de superfície (ES13001; ES13002; ES12001), implantadas ao longo de três perfis geológicos perpendiculares à estrutura, de cujos testemunhos foi possível selecionar uma amostragem muito representativa, quer dos minérios maciços e de stockwork, quer dos vários litótipos e fácies de alteração presentes na sequência encaixante do depósito. Os vulcanitos félsicos que hospedam o corpo mineralizado de Estação encontram-se invariavelmente afetados por alteração hidrotermal de carácter regional, pré-mineralização, que resultou de regimes de circulação convectiva de água do mar pouco modificada através de uma sequência vertical de rochas vulcaniclásticas permeáveis. Apesar dos efeitos metassomáticos deste evento, o carácter riolítico e as principais características texturais destes litótipos, estabelecidas no passado por diversos autores (Munhá 1983b; Barriga, 1983), foram consubstanciados neste estudo através de sólidas evidências petrográficas e geoquímicas. A sequência litoestratigráfica que hospeda as mineralizações de Estação foi definida, da base para o topo, por riólitos dos tipos EA, EB e EC, que exprimem a evolução do quimismo magmático nas rochas da região. Estes riólitos apresentam assinaturas geoquímicas totalmente correlacionáveis com os riólitos classificados por Barrett et al. (2008) em Aljustrel, facto suportado por comparação entre razões de elementos de comportamento relativamente imóvel. Estes riólitos apresentam uma textura maioritariamente vulcaniclástica e correspondem a fácies do tipo brecha rica em fiamme e fenocristais (Rosa et al. 2010). A alteração hidrotermal mineralizante sobreposta às rochas félsicas encaixantes da massa de Estação, imprimiu um conjunto de assinaturas paragenéticas, texturais, mineralógicas e geoquímicas, amplamente conhecidas e descritas para esta tipologia de depósitos. Estas características são definidoras de padrões e zonalidade de alteração hidrotermal que foram perfeitamente identificados neste estudo e que mimetizam de forma muita aproximada, outros casos análogos em Aljustrel (e.g. Barriga, 1983; Relvas, 1991). O estudo sequencial, mas integrado, das paragéneses, texturas e variabilidade geoquímica dos litótipos de Estação, permitiu estabelecer três padrões principais de alteração e correspondente zonalidade temporal e espacial, num contexto evolutivo das suas condições físico-químicas e metalogenéticas. A zona de alteração do tipo 1 (clorítica) corresponde ao núcleo do sistema hidrotermal mineralizante e apresenta um padrão de alteração caracterizado pela associação mineralógica secundária clorite + quartzo + sulfuretos + carbonatos ± sericite, que espelha as condições termo-químicas mais intensas do sistema. A alteração nesta zona é extremamente pervasiva e evidencia uma proeminente profusão de veios que mimetizam as paleo-condutas de fluidos mineralizantes ascendentes do sistema. A lixiviação de álcalis e alcalino-terrosos (K, Na e Ca,) nesta zona revelou-se intensa e, em sentido inverso, a cloritização e sulfuretização traduziram-se num sistemático predomínio de Fe + Mg, posto em evidência pelos dados mineralógicos. A mineralogia sulfuretada nesta zona é dominada por pirite ± calcopirite + esfalerite + galena. A zona de alteração do tipo 2 constitui um halo periférico relativamente ao núcleo clorítico do sistema hidrotermal, que se traduz numa gradual intensificação da sericitização e silicificação das rochas, com evidente diminuição da cloritização e de veios mineralizados. O padrão de alteração característico desta zona é dominado pela associação mineralógica sericite + quartzo ± clorite + sulfuretos + carbonatos. A teto da massa mineralizada de Estação foi possível identificar uma sequência de rochas que, do ponto de vista mineralógico e geoquímico, nos remetem para uma alteração ultraperiférica do tipo 3. O incremento de molécula paragonítica (Na) nas sericites destas amostras, sobrepõese a expressões texturais condizentes com zonas mais proximais do sistema hidrotermal. Este tipo de alteração revela-se mais subtil e apresenta uma associação mineralógica dominada por sericite rica em sódio + quartzo + carbonatos. As clorites apresentam variações em Fe, Mg e Al que são função da sua proximidade ao núcleo do sistema. A razão Fe/(Fe+Mg) permitiu discriminar as clorites mais ferrosas como sendo as mais proximais ao eixo do sistema hidrotermal mineralizante e onde a influência dos fluidos mineralizantes sobre as rochas é maior, nomeadamente em zonas de extensas redes de veios (zona 1). As sericites apresentam uma variabilidade composicional do Na e K que é expressa pela razão Na/(Na+K). Esta variabilidade permitiu a discriminação de sericites fortemente potássicas (zona 2a) e sericites progressivamente mais sódicas (zona 2b) podendo chegar a paragonites quase puras, típicas de zonas mais distais. Consequentemente, o empobrecimento progressivo de Na nas sericites e alguma incorporação progressiva de Fe+Mg na respetiva estrutura cristaloquímica, constituem vetores que apontam no sentido do núcleo do sistema hidrotermal. De entre os carbonatos analisados, as anquerites tardi-metalogenéticas, com Fe e Mg na sua composição, discriminam-se composicionalmente em função do perfil considerando. Foi possível constatar a presença de anquerites com mais Fe no perfil central (mais proximal ao eixo do sistema) do que em zonas mais afastadas, nomeadamente no perfil NW. A utilização de diagramas do tipo “Alteration box-plot” (Large et al., 2001) no âmbito deste estudo, permitiu relacionar os dados da geoquímica de rocha total com a alteração hidrotermal mineralizante através da identificação de trends que definem sequências e paragéneses de alteração. Identificou-se um trend 4, de associação clorite – carbonatos – pirite, de aproximação ao eixo do sistema, que sugere a presença mais proeminente nas amostras de carbonatos de caráter ferroso, provavelmente siderite em finas frações matriciais. Em amostras de rochas pouco mineralizadas, esta ferramenta permitiu identificar dois outros trends composicionais no sentido da aproximação ao núcleo do sistema mineralizante. Um trend 3 (clorite + sericite + pirite) e um trend 2 (sericite + clorite ± pirite) que se revelaram perfeitamente consistentes com os padrões de alteração anteriormente descritos. Como vimos, a análise de microssonda das principais fases secundárias de alteração (clorites, sericites e carbonatos) complementou e reforçou os critérios paragenéticos e texturais utilizados na definição da zonalidade de alteração hidrotermal mineralizante. Em última análise, a transversalidade da presença destes minerais no contexto das várias zonas de alteração, e a sua variação, permitiram estabelecer valiosos critérios de prospeção mineral, critérios esses que foram harmoniosamente corroborados pelos dados da geoquímica de rocha total. A litogeoquímica colocou igualmente em destaque aspetos críticos relativamente aos mecanismos que promoveram a mineralização das rochas vulcânicas que hospedam o jazigo de Estação. O comportamento relativamente imóvel de vários elementos menores em rochas fortemente mineralizadas constituiu um precioso recurso neste sentido. Uma vez estabelecidas as correlações entre elementos (Ta, Nb, Zr, Hf e Y) de rochas pouco mineralizadas e suas congéneres variavelmente alteradas (stockwork e minério maciço), foi possível determinar uma correspondência entre as assinaturas dos dois grupos de rochas, o que representa uma evidência direta de um mecanismo de substituição. Por outro lado, a evolução composicional da SiO2 e do Al2O3 espelha o balanço de massa do sistema hidrotermal mineralizante durante o processo de mineralização (cf. Relvas et al., 2006a). Nesta evolução composicional, ainda que ambos os elementos sejam afetados pelos fluidos mineralizantes, fazem-no de modo diferenciado. Tanto a sílica como a alumina são subtraídas do sistema por lixiviação, mas de forma desfasada. A sílica, mais solúvel, é primeiramente lixiviada, seguindo-se-lhe a alumina sob condições físico-químicas mais extremas. Este processo conduz à substituição total da fração silicatada da rocha, por sulfuretos. Estas duas evidências demonstram claramente a existência de um estilo de mineralização por substituição na sub-superfície associado ao jazigo de Estação. A oportunidade de trabalhar com o MLA (Mineral Liberation Analyser) nos laboratórios da EPDM em Aljustrel, permitiu a obtenção de um precioso conjunto de dados de mineralogia automática. Entre estes, destacam-se os dados de análise modal expressos pela percentagem em área de cada fase mineral. Estes dados revelaram-se coerentes com as observações e resultados obtidos através da petrografia, química mineral e litogeoquímica. Na análise da alocação de elementos químicos pelas diversas fases minerais e da libertação mineral (de extrema importância para a empresa mineiras em termos geometalúrgicos), optou-se por uma abordagem essencialmente descritiva e qualitativa dos dados que se revelou absolutamente consistente com os restantes resultados obtidos, observações efetuadas e interpretações preconizadas.
The Estação deposit is located in the NE corner of the Aljustrel village, in the Beja district (Portugal), and corresponds to a massive sulfide orebody separated, by about 600 m, by the Represa fault, from the Feitais deposit, with which it forms a unique mineralized body. This so far unexplored deposit presents a typical lenticular shape, 600m long, 480m wide and up to 100m thick, that occurs on the normal flank of the Feitais anticline. The geo-structural reinterpretation carried out in this investigation allowed for a complete reconstitution of the geometry of the Estação massive orebody, based on the detailed study of three surface drillcores (ES13001; ES13002; ES12001), implanted along three geological profiles perpendicular to the orebody structure, from which it was possible to select representative samples, both of massive and stockwork ores, and of the various lithotypes and alteration facies present in the deposit host sequence. Previous to the mineralization event, the felsic rocks that host the Estação orebody are invariably affected by regional hydrothermal alteration, which resulted from convective circulation regimes of little-modified seawater through a vertical sequence of permeable volcaniclastic rocks. Despite the metasomatic effects of this event, the rhyolitic nature and the main textural features of these lithotypes, established in the past by several researchers (e.g., Munhá 1983; Barriga, 1983), were confirmed in this study by solid petrographic and geochemical evidence. The lithostratigraphic sequence that hosts the Estação’s mineralization was defined from bottom to top as EA, EB and EC rhyolite types, which express the magmatic chemical evolution of the Aljustrel rocks. The geochemical signatures of these rhyolites fully correlate with those classified by Barrett et al. (2008) in Aljustrel, which is supported by comparing the ratios of elements with relatively immobile behaviour. Most of these rhyolites present volcaniclastic texture and correspond to a fiamme-rich facies of volcanic breccias with abundant phenocrysts (Rosa et al. 2010). The overprinting of ore-forming hydrothermal alteration to the felsic volcanic rocks that host the Estação orebody produced a set of paragenetic, textural, mineralogical and geochemical signatures, widely described for this type of ore deposits in the IPB. These features define hydrothermal alteration patterns and zoning that were perfectly identified in this study and that closely resemble other similar cases in Aljustrel (e.g. Barriga, 1983; Relvas, 1991). The sequential yet integrated study of the paragenesis, textures and geochemical variability of the Estação lithotypes allowed us to establish three main rock alteration patterns and corresponding temporal and spatial zoning, in a context of changing physicochemical and metallogenetic conditions. Type 1 alteration zone (“chloritic zone”) corresponds to the core of the mineralizing hydrothermal system and is characterized by a secondary mineralogical assemblage of chlorite + quartz + sulfides + carbonates ± sericite, which reflects the most intense thermo-chemical conditions of the system. The alteration in this particular zone is extremely pervasive and shows a prominent profusion of veins that represent the paleoconduits of ascending ore-forming fluids in the system. Leaching of alkali and alkaline-earth metals (K, Na and Ca,) in this zone proved to be intense. Chloritization and sulphidization resulted in a systematic predominance of Fe + Mg, highlighted by mineralogical data. Sulphide mineralogy in this area is dominated by pyrite ± chalcopyrite + sphalerite + galena. Type 2 alteration zone constitutes a peripheral alteration halo enveloping the chloritic core of the hydrothermal system, exhibiting a gradual intensification of rock sericitization and silicification, and evident decrease in chloritization and mineralized veins. The characteristic alteration mineralogy of this zone is dominated by sericite + quartz ± chlorite + sulfides + carbonates. On the hanging wall of the Estação orebody, it was possible to identify a sequence of rocks that, from a mineralogical and geochemical point of view, refer to an ultra-peripheral type 3 alteration zone. The increase of paragonitic component (Na) in the sericite of these samples overlaps textural imprints, which are consistent with proximal innermost zones of the hydrothermal system. This type of alteration is more subtle and has a mineralogical assemblage, dominated by sericite rich in sodium + quartz + carbonates. Variations in the Fe, Mg and Al contents of chlorite are related to the proximity to the system core: the Fe/(Fe+Mg) ratio discriminates the most ferrous chlorites as the most proximal to the oreforming hydrothermal system axis, where the influence of mineralizing fluids on the rocks is most intense, producing extensive vein networks (zone 1). The compositional variability of sericite, is expressed by their Na/ (Na + K) ratio: this variability discriminates between strongly potassic sericites (zone 2a) and progressively sodium-richer sericites (zone 2b), to almost pure paragonites in more distal zones. Consequently, the progressive depletion of Na in sericite composition and some progressive incorporation of Fe+Mg in their structure point towards the core of the hydrothermal system. Among the carbonates analysed, late-stage ore-related ankerites are compositionally discriminated according to the profile considered: it was possible to confirm the presence of Fe-richer ankerites in the central profile (more proximal to the axis of the system) than in more distant zones, namely in the NW profile. The use of “alteration box-plot”-type diagrams (Large et al., 2001) in this study has allowed us to relate whole-rock geochemistry data with ore-forming hydrothermal alteration patterns by the identification of trends that define sequences and parageneses of alteration. The chlorite – carbonate – pyrite assemblage (trend 4) identified close to the axis of the system, is in agreement with the prominent presence of iron carbonates (most likely siderite) in the fine matrix of these rocks. In less mineralized rock samples, this tool enabled two other compositional trends to be identified towards the core of the mineralizing system: trend 3 (chlorite + sericite + pyrite) and trend 2 (sericite + chlorite ± pyrite) have proved to be perfectly consistent with the alteration patterns previously described. As we have seen, the composition of the main secondary alteration phases (chlorite, sericite and carbonate) complemented and reinforced the paragenetic and textural criteria used in the definition of each hydrothermal alteration zone. The ubiquitous presence of these minerals in the context of the various alteration zones, and their variation, were invaluable to establish mineral exploration criteria, which were closely corroborated by whole-rock geochemistry. Lithogeochemistry also highlighted critical aspects regarding the mechanisms that promoted the mineralization of volcanic rocks that host the Estação deposit. The relatively immobile behaviour of several trace elements in strongly mineralized rocks are a precious resource in this regard. Once the correlations between these elements (Ta, Nb, Zr, Hf, and Y) in the least mineralized rocks and their varying altered equivalents (stockwork and massive ore) have been established, it was possible to determine a correspondence between the signatures of the two groups of rocks, which represents direct evidence of a replacement mechanism. On the other hand, the evolution of whole-rock SiO2 and Al2O3 contents reflects the mass balance of the mineralizing system during the ore-forming process (cf. Relvas et al., 2006a). Even though both elements are affected by ore-forming fluids, they are so in a different way. Both silica and alumina are removed from the system by leaching, but not at the same rate: silica, being more soluble, is leached first, followed by alumina under more extreme physical and chemical conditions. This process leads to the complete sulfide replacement of the silicate fraction of the rock. These two evidences clearly demonstrate the existence of a style of mineralization by subsurface replacement associated with the Estação deposit. The opportunity to work with MLA (Mineral Liberation Analyzer), at the EPDM laboratories in Aljustrel, produced a precious set of automatic mineralogical data. Among several operating modes, the modal analysis data, expressed by the percentage by area of each mineral phase present, stands out. These data were consistent with the petrographic observations, as well as with mineral chemistry and lithogeochemistry. For the distribution of chemical elements among the various mineral phases present and for mineral liberation analysis (extremely important for mining companies in geometallurgical terms), an essentially descriptive and qualitative approach to the data was adopted, which has proved to be consistent with the other results obtained, as well as with previous interpretations.
The Estação deposit is located in the NE corner of the Aljustrel village, in the Beja district (Portugal), and corresponds to a massive sulfide orebody separated, by about 600 m, by the Represa fault, from the Feitais deposit, with which it forms a unique mineralized body. This so far unexplored deposit presents a typical lenticular shape, 600m long, 480m wide and up to 100m thick, that occurs on the normal flank of the Feitais anticline. The geo-structural reinterpretation carried out in this investigation allowed for a complete reconstitution of the geometry of the Estação massive orebody, based on the detailed study of three surface drillcores (ES13001; ES13002; ES12001), implanted along three geological profiles perpendicular to the orebody structure, from which it was possible to select representative samples, both of massive and stockwork ores, and of the various lithotypes and alteration facies present in the deposit host sequence. Previous to the mineralization event, the felsic rocks that host the Estação orebody are invariably affected by regional hydrothermal alteration, which resulted from convective circulation regimes of little-modified seawater through a vertical sequence of permeable volcaniclastic rocks. Despite the metasomatic effects of this event, the rhyolitic nature and the main textural features of these lithotypes, established in the past by several researchers (e.g., Munhá 1983; Barriga, 1983), were confirmed in this study by solid petrographic and geochemical evidence. The lithostratigraphic sequence that hosts the Estação’s mineralization was defined from bottom to top as EA, EB and EC rhyolite types, which express the magmatic chemical evolution of the Aljustrel rocks. The geochemical signatures of these rhyolites fully correlate with those classified by Barrett et al. (2008) in Aljustrel, which is supported by comparing the ratios of elements with relatively immobile behaviour. Most of these rhyolites present volcaniclastic texture and correspond to a fiamme-rich facies of volcanic breccias with abundant phenocrysts (Rosa et al. 2010). The overprinting of ore-forming hydrothermal alteration to the felsic volcanic rocks that host the Estação orebody produced a set of paragenetic, textural, mineralogical and geochemical signatures, widely described for this type of ore deposits in the IPB. These features define hydrothermal alteration patterns and zoning that were perfectly identified in this study and that closely resemble other similar cases in Aljustrel (e.g. Barriga, 1983; Relvas, 1991). The sequential yet integrated study of the paragenesis, textures and geochemical variability of the Estação lithotypes allowed us to establish three main rock alteration patterns and corresponding temporal and spatial zoning, in a context of changing physicochemical and metallogenetic conditions. Type 1 alteration zone (“chloritic zone”) corresponds to the core of the mineralizing hydrothermal system and is characterized by a secondary mineralogical assemblage of chlorite + quartz + sulfides + carbonates ± sericite, which reflects the most intense thermo-chemical conditions of the system. The alteration in this particular zone is extremely pervasive and shows a prominent profusion of veins that represent the paleoconduits of ascending ore-forming fluids in the system. Leaching of alkali and alkaline-earth metals (K, Na and Ca,) in this zone proved to be intense. Chloritization and sulphidization resulted in a systematic predominance of Fe + Mg, highlighted by mineralogical data. Sulphide mineralogy in this area is dominated by pyrite ± chalcopyrite + sphalerite + galena. Type 2 alteration zone constitutes a peripheral alteration halo enveloping the chloritic core of the hydrothermal system, exhibiting a gradual intensification of rock sericitization and silicification, and evident decrease in chloritization and mineralized veins. The characteristic alteration mineralogy of this zone is dominated by sericite + quartz ± chlorite + sulfides + carbonates. On the hanging wall of the Estação orebody, it was possible to identify a sequence of rocks that, from a mineralogical and geochemical point of view, refer to an ultra-peripheral type 3 alteration zone. The increase of paragonitic component (Na) in the sericite of these samples overlaps textural imprints, which are consistent with proximal innermost zones of the hydrothermal system. This type of alteration is more subtle and has a mineralogical assemblage, dominated by sericite rich in sodium + quartz + carbonates. Variations in the Fe, Mg and Al contents of chlorite are related to the proximity to the system core: the Fe/(Fe+Mg) ratio discriminates the most ferrous chlorites as the most proximal to the oreforming hydrothermal system axis, where the influence of mineralizing fluids on the rocks is most intense, producing extensive vein networks (zone 1). The compositional variability of sericite, is expressed by their Na/ (Na + K) ratio: this variability discriminates between strongly potassic sericites (zone 2a) and progressively sodium-richer sericites (zone 2b), to almost pure paragonites in more distal zones. Consequently, the progressive depletion of Na in sericite composition and some progressive incorporation of Fe+Mg in their structure point towards the core of the hydrothermal system. Among the carbonates analysed, late-stage ore-related ankerites are compositionally discriminated according to the profile considered: it was possible to confirm the presence of Fe-richer ankerites in the central profile (more proximal to the axis of the system) than in more distant zones, namely in the NW profile. The use of “alteration box-plot”-type diagrams (Large et al., 2001) in this study has allowed us to relate whole-rock geochemistry data with ore-forming hydrothermal alteration patterns by the identification of trends that define sequences and parageneses of alteration. The chlorite – carbonate – pyrite assemblage (trend 4) identified close to the axis of the system, is in agreement with the prominent presence of iron carbonates (most likely siderite) in the fine matrix of these rocks. In less mineralized rock samples, this tool enabled two other compositional trends to be identified towards the core of the mineralizing system: trend 3 (chlorite + sericite + pyrite) and trend 2 (sericite + chlorite ± pyrite) have proved to be perfectly consistent with the alteration patterns previously described. As we have seen, the composition of the main secondary alteration phases (chlorite, sericite and carbonate) complemented and reinforced the paragenetic and textural criteria used in the definition of each hydrothermal alteration zone. The ubiquitous presence of these minerals in the context of the various alteration zones, and their variation, were invaluable to establish mineral exploration criteria, which were closely corroborated by whole-rock geochemistry. Lithogeochemistry also highlighted critical aspects regarding the mechanisms that promoted the mineralization of volcanic rocks that host the Estação deposit. The relatively immobile behaviour of several trace elements in strongly mineralized rocks are a precious resource in this regard. Once the correlations between these elements (Ta, Nb, Zr, Hf, and Y) in the least mineralized rocks and their varying altered equivalents (stockwork and massive ore) have been established, it was possible to determine a correspondence between the signatures of the two groups of rocks, which represents direct evidence of a replacement mechanism. On the other hand, the evolution of whole-rock SiO2 and Al2O3 contents reflects the mass balance of the mineralizing system during the ore-forming process (cf. Relvas et al., 2006a). Even though both elements are affected by ore-forming fluids, they are so in a different way. Both silica and alumina are removed from the system by leaching, but not at the same rate: silica, being more soluble, is leached first, followed by alumina under more extreme physical and chemical conditions. This process leads to the complete sulfide replacement of the silicate fraction of the rock. These two evidences clearly demonstrate the existence of a style of mineralization by subsurface replacement associated with the Estação deposit. The opportunity to work with MLA (Mineral Liberation Analyzer), at the EPDM laboratories in Aljustrel, produced a precious set of automatic mineralogical data. Among several operating modes, the modal analysis data, expressed by the percentage by area of each mineral phase present, stands out. These data were consistent with the petrographic observations, as well as with mineral chemistry and lithogeochemistry. For the distribution of chemical elements among the various mineral phases present and for mineral liberation analysis (extremely important for mining companies in geometallurgical terms), an essentially descriptive and qualitative approach to the data was adopted, which has proved to be consistent with the other results obtained, as well as with previous interpretations.
Descrição
Relatório de estágio de mestrado, Geologia Económica (Prospeção Mineral) Universidade de Lisboa, Faculdade de Ciências, 2020
Palavras-chave
Aljustrel Estação Alteração mineralizante Vetores de prospeção Metalogénese Mineralização por substituição na sub-superfície Relatórios de estágio de mestrado - 2020