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Geotermometria no sector norte da Faixa Piritosa Ibérica
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A abandonada mina da Herdade da Corte Carrasco, parte do Campo Mineiro de Albernoa e
concessionada para exploração de minério de manganês durante mais de um século entre os anos de
1866 e 1976, situa-se no sector NE da Faixa Piritosa Ibérica.
Sendo a primeira abordagem a esta mineralização, o presente relatório baseou-se em amostras recolhidas
na escombreira da antiga mina, com objectivo de caracterizar mineralogicamente e geoquimicamente a
mineralização da Herdade da Corte Carrasco.
Em bolsadas, sub-horizontais e de direcção E-W, encaixadas em metajaspes e metassedimentos do CVS
(XJM), a mineralogia primária é constituída por rodonite e rodocrosite (SRM), com predomínio desta
última. Os minerais primários são posteriormente afectados por metamorfismo regional, sendo
retrabalhadas e enriquecidas em Mn - substituindo-as fazendo precipitar espessartinas e braunite
(MMM). Segue-se a substituição das massas de braunite por, pelo menos, dois (hidr)óxidos de Mn,
associados a processos de enriquecimento supergénico, entre eles pirolusite (MNS). Por fim, a
albitização intensa afecta todos os domínios anteriores, deixando ocasionalmente relíquias e fazendo-se
acompanhar de ocasionais veios de quartzo, tardios, num dos quais se observam grãos de ardennite, o
primeiro reporte deste mineral na FPI.
Sem geotermómetros indicados para o cálculo expedito das temperaturas associadas à génese dos
principais minerais de Mn, recorreu-se a dados de estabilidade da espessartina para definir um limite
mínimo de 400º C, provavelmente coincidente com o pico metamórfico local.
Apesar da bibliografia sobre esta mineralização ser praticamente inexistente, o estudo mineralógico
realizado neste relatório corrobora, em parte, observações que vários autores preconizaram para jazigos
de Mn nesta região. Não obstante, considera-se crucial reforçar o conhecimento geológico deste
depósito, nomeadamente na identificação precisa dos (hidr)óxidos de Mn e utilização de métodos
alternativos para constranger as temperaturas do episódio mineralizante.
Herdade da Corte Carrasco, an abandoned mine part of the Albernoa Mining Field and concessioned for the exploitation of manganese ore for more than a century between 1866 and 1976, is located in the northern branch of the Iberian Pyrite Belt (IPB). Being the first report regarding this mineralization, this study was based on samples collected in the old mine’s tailings, with the aim to mineralogically and geochemically characterize the mineralization of Herdade da Corte Carrasco. The Mn orebody occur as lenses and is hosted in CVS’s jasper and metasediments (XJM). Primary paragenesis consists of rhodonite and rhodochrosite (SRM), with a predominance of the latter. These are subsequently affected by regional metamorphism, being recrystallized and enriched in Mn – partially replacing them with both spessartine and braunite (MMM). This is followed by the replacement of the braunite masses by at least two Mn (hydr)oxides minerals, most probably associated to supergene enrichment, including pyrolusite (MNS). Finally, intense albitization affects all previous geological domains, occasionally leaving relics and being accompanied by rare late quartz veins, in one of which grains of ardennite are observed - the first finding of this mineral in the IPB. Lacking suitable geothermometers to calculate the range of temperatures during the main Mn minerals’ genesis, spessartine stability data was used to set a minimum temperature of 400º C, certainly matching the local metamorphic peak. Although the references on this mineralization are practically non-existent, the mineralogical study carried out in this report corroborates, in part, the observations that several authors denote for Mn orebodies in the region. However, it is crucial to strengthen the geological knowledge of this deposit, to accurately identify the Mn (hydr)oxides and to use alternative methods to determine the range of temperatures over the mineralizing episode.
Herdade da Corte Carrasco, an abandoned mine part of the Albernoa Mining Field and concessioned for the exploitation of manganese ore for more than a century between 1866 and 1976, is located in the northern branch of the Iberian Pyrite Belt (IPB). Being the first report regarding this mineralization, this study was based on samples collected in the old mine’s tailings, with the aim to mineralogically and geochemically characterize the mineralization of Herdade da Corte Carrasco. The Mn orebody occur as lenses and is hosted in CVS’s jasper and metasediments (XJM). Primary paragenesis consists of rhodonite and rhodochrosite (SRM), with a predominance of the latter. These are subsequently affected by regional metamorphism, being recrystallized and enriched in Mn – partially replacing them with both spessartine and braunite (MMM). This is followed by the replacement of the braunite masses by at least two Mn (hydr)oxides minerals, most probably associated to supergene enrichment, including pyrolusite (MNS). Finally, intense albitization affects all previous geological domains, occasionally leaving relics and being accompanied by rare late quartz veins, in one of which grains of ardennite are observed - the first finding of this mineral in the IPB. Lacking suitable geothermometers to calculate the range of temperatures during the main Mn minerals’ genesis, spessartine stability data was used to set a minimum temperature of 400º C, certainly matching the local metamorphic peak. Although the references on this mineralization are practically non-existent, the mineralogical study carried out in this report corroborates, in part, the observations that several authors denote for Mn orebodies in the region. However, it is crucial to strengthen the geological knowledge of this deposit, to accurately identify the Mn (hydr)oxides and to use alternative methods to determine the range of temperatures over the mineralizing episode.
Descrição
Relatório de estágio de mestrado, Geologia Económica , 2024, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
Geotermometria Faixa Piritosa Ibérica Manganês Espessartina Herdade da Corte Carrasco Relatórios de estágio - 2024