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Caracterização mineralógica de rochas alcalinas da Serra da Neve, Angola
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O Complexo da Serra da Neve, em Angola, é maioritariamente composto por rochas ígneas
alcalinas, em estruturas anelares dispostas segundo WNW-ESE. Foram estudadas as estruturas de
Lutala, Nejoio, Chitucubero, Babaque, cortejo filoniano e Chiuerinde.
A petrografia revelou várias litologias: sienito nefelínico (amostras GR201, GR214, D-05),
sienito feldspatóidico (A-56, A-59), sienito com feldspato alcalino (B-15), micro-sienito nefelínico (D49, A-48, A-37), fonólito nefelínico (GR200, D-28), traquito (A-43), “lamprófiro” (D-59) e brecha
vulcânica (B-10).
Nestas rochas foram estudados diversos minerais: feldspato alcalino, nefelina, sodalite,
piroxenas (diópsido a aegirina), anfíbolas (e.g., Mg-horneblenda a Mg-arfvedsonite), biotite e flúorflogopite, titanite, zeólitos (aparentemente do grupo da natrolite, analcite e heulandite), quartzo,
plagioclase, cancrinite a cancrissilite, epídoto, clorite, flúor-apatite, titanomagnetite, ilmenite, zircão,
vesuvianite (?), fluorite, moscovite, sericite e calcite. Além da mineralogia típica destas rochas,
essencialmente alcalinas, foram identificados minerais mais raros, de carácter agpaítico, como
mangano-eudialite, kupletskite, låvenite (?), carbonatos e fosfatos de Ce-La, e também piroxenas e
anfíbolas com cores particulares, atribuídas à possível incorporação de Mn3+ na sua estrutura. Nalgumas
das amostras analisadas por micro-sonda electrónica, sobressaem minerais particularmente ricos em
certos elementos interessantes (LREE, HFSE e Mn), nomeadamente, mangano-eudialite, apatite e
fosfato de Ce-La (em Babaque) e kupletskite e carbonato de Ce-La (no cortejo filoniano); estas amostras,
de natureza vulcânica e sub-vulcânica, podem vir a ser foco de interesse a nível económico.
Parece existir alguma afinidade entre as litologias e a mineralogia das rochas do Complexo
Alcalino da Serra da Neve e as de outros complexos alcalinos: nomeadamente os complexos do
alinhamento magmático Cabo Frio – Poços de Caldas, no Brasil, bem como os complexos de Pilansberg
(África do Sul), Tamazeght (Marrocos), Lovozero (Sibéria) e Monte Saint-Hilaire (Canadá). A
correlação com as rochas alcalinas do Brasil e da Namíbia é particularmente relevante, dada a
localização destes complexos na Província Magmática de Paraná-Angola-Etendeka.
The Serra da Neve Complex, in Angola, is mainly composed of alkaline igneous rocks, forming ring structures, with WNW-ESE orientation. The structures under study were Lutala, Nejoio, Chitucubero, Babaque, some dike swarms and Chiuerinde. Detailed petrography revealed various lithologies: nepheline syenite (samples GR201, GR214, D-05), foid syenite (A-56, A-59), alkali feldspar syenite (B-15), nepheline micro-syenite (D-49, A-48, A-37), nepheline phonolite (GR200, D-28), alkali feldspar trachyte (A-43), ‘lamprophyre’ (D-59) and volcanic breccia (B-10). Various minerals were studied: alkali feldspar, nepheline, sodalite, pyroxenes (diopside to aegirine), amphiboles (e.g., Mg-horneblende to Mg-arfvedsonite), biotite and fluor-phlogopite, titanite, zeolites (apparently from the natrolite, analcite and heulandite groups), quartz, plagioclase, cancrinite to cancrisilite, epidote, chlorite, fluor-apatite, titanomagnetite, ilmenite, zircon, vesuvianite (?), fluorite, muscovite, sericite and calcite. In addition to the typical mineralogy of these mostly alkaline rocks, minerals of agpaitic nature were identified, such as mangano-eudialyte, kupletskite, låvenite (?), Ce-La carbonates and phosphates, as well as unusually coloured pyroxenes and amphiboles, attributed to the possible incorporation of Mn3+. In some of the samples analysed by electron microprobe, a few minerals stood out in terms of their enrichment in interesting elements (LREE, HFSE, and Mn), namely, mangano-eudialyte, apatite and Ce-La phosphate (in Babaque) and kupletskite and Ce-La carbonate (in the dike swarm); these volcanic and subvolcanic samples might constitute a focus of economic interest in the future. There seems to be some affinity between the lithologies and mineralogy of the Serra da Neve rocks and those from other alkaline complexes, namely those from the Cabo Frio - Poços de Caldas Magmatic Lineament in Brazil, as well as other complexes, such as Pilansberg (South Africa), Tamazeght (Marocco), Lovozero (Siberia) and Mont Saint-Hilaire (Canada). Correlation between the Serra da Neve Complex and alkaline complexes of Brazil and Namibia is of particular relevance, as they all belong to the Paraná-Angola-Etendeka Magmatic Province.
The Serra da Neve Complex, in Angola, is mainly composed of alkaline igneous rocks, forming ring structures, with WNW-ESE orientation. The structures under study were Lutala, Nejoio, Chitucubero, Babaque, some dike swarms and Chiuerinde. Detailed petrography revealed various lithologies: nepheline syenite (samples GR201, GR214, D-05), foid syenite (A-56, A-59), alkali feldspar syenite (B-15), nepheline micro-syenite (D-49, A-48, A-37), nepheline phonolite (GR200, D-28), alkali feldspar trachyte (A-43), ‘lamprophyre’ (D-59) and volcanic breccia (B-10). Various minerals were studied: alkali feldspar, nepheline, sodalite, pyroxenes (diopside to aegirine), amphiboles (e.g., Mg-horneblende to Mg-arfvedsonite), biotite and fluor-phlogopite, titanite, zeolites (apparently from the natrolite, analcite and heulandite groups), quartz, plagioclase, cancrinite to cancrisilite, epidote, chlorite, fluor-apatite, titanomagnetite, ilmenite, zircon, vesuvianite (?), fluorite, muscovite, sericite and calcite. In addition to the typical mineralogy of these mostly alkaline rocks, minerals of agpaitic nature were identified, such as mangano-eudialyte, kupletskite, låvenite (?), Ce-La carbonates and phosphates, as well as unusually coloured pyroxenes and amphiboles, attributed to the possible incorporation of Mn3+. In some of the samples analysed by electron microprobe, a few minerals stood out in terms of their enrichment in interesting elements (LREE, HFSE, and Mn), namely, mangano-eudialyte, apatite and Ce-La phosphate (in Babaque) and kupletskite and Ce-La carbonate (in the dike swarm); these volcanic and subvolcanic samples might constitute a focus of economic interest in the future. There seems to be some affinity between the lithologies and mineralogy of the Serra da Neve rocks and those from other alkaline complexes, namely those from the Cabo Frio - Poços de Caldas Magmatic Lineament in Brazil, as well as other complexes, such as Pilansberg (South Africa), Tamazeght (Marocco), Lovozero (Siberia) and Mont Saint-Hilaire (Canada). Correlation between the Serra da Neve Complex and alkaline complexes of Brazil and Namibia is of particular relevance, as they all belong to the Paraná-Angola-Etendeka Magmatic Province.
Descrição
Tese de mestrado, Geologia (Geodinâmica e Recursos Geológicos), 2024, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
Mineralogia Rochas alcalinas Serra da Neve Angola Cretácico superior Teses de mestrado - 2024