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Mineralizações de cobre da Mina de Miguel Vacas: caracterização petrográfica e geoquímica
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O depósito de Cobre da Mina de Miguel Vacas localizado na Zona de Ossa-Morena (ZOM), a Sudeste de Vila Viçosa, foi alvo de várias campanhas de prospecção e de exploração desde o princípio do século XX até ao ano de 2007, no entanto, não existe na literatura nenhum estudo detalhado da sua mineralogia. O presente trabalho, pretende colmatar esta lacuna através de uma caracterização mineralógica e química mineral detalhada das paragéneses do minério primário e supergénico, juntamente com uma interpretação dos processos evolutivos do depósito. O jazigo de Miguel Vacas corresponde a uma estrutura filoniana, preenchida por quartzo e carbonatos, de atitude NNW-SSE sub-vertical e é sub-concordante com a estrutura regional. O jazigo encontra-se encaixado em formações metassedimentares (xistos negros e cinzentos, com intercalações de liditos), estende-se ao longo de mais de 2 km e. com uma possança variável (10 a 20 m). Em Miguel Vacas é possível observar duas tipologias de minério completamente distintas: minério primário e minério supergénico. O minério primário é composto por calcopirite, pirite, arsenopirite, gersdorffite. O minério supergénico é constituído por malaquite, azurite, pseudomalaquite, libethenite, brochantite, cuprite, óxido de cobre1, calcocite, digenite, anilite, djurleite, covelite, bornite, cobre nativo, wittichenite, cuprobismutite, emplectite, bismuthinite e bismuto nativo. É ainda possível observar rútilo nos halos de alteração proximal. Com base na petrografia e química mineral foi possível por em evidência o carácter episódico da mineralização primária associado a fenómenos de remobilização de metais e reprecipitação de sulfuretos. Na mineralização supergénica é possível diferenciar três zonas distintas (zona de lixiviação, zona supergénica oxidada e supergénica redutora) que reflectem o perfil típico e os processos característicos deste tipo de alteração. Na zona oxidada destaca-se a presença de carbonatos Cu (malaquite ± azurite) e fosfatos de Cu (pseudomalaquite ± libethenite) que sugerem variações significativas dos parâmetros fisico-químicos do meio, em especial do pH, ǷCO2, e aPO43-. O Au, que está sempre presente embora de forma errática ao longo de todo o perfil, ocorre provavelmente na forma de microinclusões, quer nas fases primárias quer nas fases secundárias de Cu, o que sugere uma mobilidade à escala local.
The copper deposit from Miguel Vacas Mine is located in the Ossa-Morena Zone (OMZ), Southeast of Vila Viçosa, was the target of various exploration campaigns since the begin of the 20th century to 2007, however, there isn't any literature about the detailed study of their mineralogy. The present work intend to fill this gap through a mineralogical and chemical characterization of the primary and supergenic ore, together with an interpretation of the evolutionary processes of the deposit. The Miguel Vacas mine deposit corresponds to a lode structure system filled with quartz and carbonates, sub-vertical with an attitude NNW-SSE and is sub-concordant with the regional structure. The deposit is embedded in metasedimentary formations (black and grey shales, with interbbeded lyddites) extending over more than 2 km long and with a variable thickness (10 to 20 m). In Miguel Vacas it's possible to observe two completely different ore tipology, primary ore and supergene ore. The primary ore is composed by chalcopyrite, pyrite, arsenopyrite and gersdorffite. The supergene ore consists in malachite, azurite, pseudomalachite, libethenite, brochantite, cuprite, copper oxide2, chalcocite, digenite, anilite, djurleite, covellite, bornite, native copper, wittichenite, cuprobismutite, emplectite, bismuthinite and native bismut. It's still possible to observe rutile in proximal alteration haloes. Based in petrography and mineral chemistry it's possible to highlight the episodic character of the primary mineralization with the phenomenon of metal remobilization and sulfide reprecipitation. In the supergene mineralization is possible to distinguish three different disctinct zones (the leached zone, the oxidized and reduced supergene zones) that reflects the typical profile and the characteristic processes of this kind of alteration. In the oxidized zone stands out the presence of copper carbonates (malachite ± azurite) and copper phosphates (pseudomalchite ± libethenite) that suggests significant phisical-chemistry changes of the environment parameters, in particular pH, ǷCO2 and aPO43-. The Au, is always present in the entire profile but in an erratic form, probably occurs in the microinclusions form in the primary or in the secondary copper phases, which suggests a regional scale mobility.
The copper deposit from Miguel Vacas Mine is located in the Ossa-Morena Zone (OMZ), Southeast of Vila Viçosa, was the target of various exploration campaigns since the begin of the 20th century to 2007, however, there isn't any literature about the detailed study of their mineralogy. The present work intend to fill this gap through a mineralogical and chemical characterization of the primary and supergenic ore, together with an interpretation of the evolutionary processes of the deposit. The Miguel Vacas mine deposit corresponds to a lode structure system filled with quartz and carbonates, sub-vertical with an attitude NNW-SSE and is sub-concordant with the regional structure. The deposit is embedded in metasedimentary formations (black and grey shales, with interbbeded lyddites) extending over more than 2 km long and with a variable thickness (10 to 20 m). In Miguel Vacas it's possible to observe two completely different ore tipology, primary ore and supergene ore. The primary ore is composed by chalcopyrite, pyrite, arsenopyrite and gersdorffite. The supergene ore consists in malachite, azurite, pseudomalachite, libethenite, brochantite, cuprite, copper oxide2, chalcocite, digenite, anilite, djurleite, covellite, bornite, native copper, wittichenite, cuprobismutite, emplectite, bismuthinite and native bismut. It's still possible to observe rutile in proximal alteration haloes. Based in petrography and mineral chemistry it's possible to highlight the episodic character of the primary mineralization with the phenomenon of metal remobilization and sulfide reprecipitation. In the supergene mineralization is possible to distinguish three different disctinct zones (the leached zone, the oxidized and reduced supergene zones) that reflects the typical profile and the characteristic processes of this kind of alteration. In the oxidized zone stands out the presence of copper carbonates (malachite ± azurite) and copper phosphates (pseudomalchite ± libethenite) that suggests significant phisical-chemistry changes of the environment parameters, in particular pH, ǷCO2 and aPO43-. The Au, is always present in the entire profile but in an erratic form, probably occurs in the microinclusions form in the primary or in the secondary copper phases, which suggests a regional scale mobility.
Descrição
Tese de mestrado em Geologia Económica (Prospecção Mineral), apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2012
Palavras-chave
Zona de Ossa-Morena Mina de Miguel Vacas Minério primário Minério secundário Mineralização de Cu Teses de mestado - 2012