On the santana serpentinized ultramafic rocks, Portel-Vidigueira region Autor(es): Andrade, A. A. Soares de Museu e Laboratório mineralógico e geológico; Centro de Estudos Publicado por: Geológicos URL persistente: http://hdl.handle.net/10316.2/36510 Accessed : 5-Oct-2021 16:50:07 A navegação consulta e descarregamento dos títulos inseridos nas Bibliotecas Digitais UC Digitalis, UC Pombalina e UC Impactum, pressupõem a aceitação plena e sem reservas dos Termos e Condições de Uso destas Bibliotecas Digitais, disponíveis em https://digitalis.uc.pt/pt-pt/termos. Conforme exposto nos referidos Termos e Condições de Uso, o descarregamento de títulos de acesso restrito requer uma licença válida de autorização devendo o utilizador aceder ao(s) documento(s) a partir de um endereço de IP da instituição detentora da supramencionada licença. Ao utilizador é apenas permitido o descarregamento para uso pessoal, pelo que o emprego do(s) título(s) descarregado(s) para outro fim, designadamente comercial, carece de autorização do respetivo autor ou editor da obra. Na medida em que todas as obras da UC Digitalis se encontram protegidas pelo Código do Direito de Autor e Direitos Conexos e demais legislação aplicável, toda a cópia, parcial ou total, deste documento, nos casos em que é legalmente admitida, deverá conter ou fazer-se acompanhar por este aviso. impactum.uc.pt digitalis.uc.pt PUBLICAÇÕES DO MUSEU E LABORATÓRIO MINERALÓGICO E GEOLÓGICO DA UNIVERSIDADE DE COIMBRA E DO CENTRO DE ESTUDOS GEOLÓGICOS N.° 66 Memórias e Notícias SUMÁRIO A. Ferreira Soares — Contribution à Vétude de la distribution des echi- nidés du crétacé supérieur du Portugal (les echinidés de la région entre Sargento-Mor et Montemor-o-Velho. M. R. Portugal V. Ferreira — Late tectonic stresses and jointing of rocks. C. S. Figueiredo Gomes — On a Sr and Al basic phosphate-sulphate close to svanbergite, occuring in a Portuguese bauxitic clay. C. S. Figueiredo Gomes — Tridimite e cris- tobalite (formas de baixa temperatura) no espangolito de S. Martinho do Bispo, Coimbra. A. A. Soares de Andrade — On the Santana serpen- tinized ultramafic rocks, Portel-Vidigueira region. A. A. Soares de Andrade — Ocurrence of algal balls-like structures in the pre-ordovician limestones of Portei, Alentejo. A. F. Ferreira Pinto — Dolerito com hor- neblenda e biotite do Alto Lindoso. A. Ferreira Pinto — Lamprófiro com biotite e actinolite de Vilarinho das Furnas. 1968 ON THE SANTANA SERPENTINIZED ULTRAMAFIC ROCKS, PORTEL-VIDIGUEIRA REGION by A. A. Soares de Andrade ABSTRACT: Serpentinized ultramafic rocks occur along a northwesterly trending zone of pre-Ordovician schistose rocks in the Portel-Vidigueira region of the Évora district. Mineral assemblages are complex: lizardite, chrysotile, antigorite, olivine, diallage, biotite, tremolite-actinolite, chlorite, talc, calcite, magnetite, chromite and pentlandite were identified. A few asbestos deposits are shown to be genetically related to the serpentinites. The possibility of existence of Cr, Ni, Co and Pt deposits is considered, and some palaeogeographical aspects are examined. RÉSUMÉ: Sur les péridotites serpentinisés de Santana (région Portel-Vidi- gueira). Des péridotites serpentinisés affleurent dans la région Portel- -Vidigueira, suivant des corps lenticulaires allongés NW-SE; le plus sou­ vent, une auréole de roches vertes les sépare des formations schisteuses d’âge hypothétique, probablement cambrien, où des aspectes de métamor­ phisme de contact font apparemment défaut. La composition minéralogique des serpentinites s’avère assez com­ plexe: lizardite, chrysotile, antigorite, olivine, diallage, biotite, trémolite- -actinote, chlorite, talc, calcite, magnétite, chromite et pentlandite furent identifiés. Des concentrations d’amiante (trémolite-actinote surtout, chrysotile subordonné), en cours d’exploitation, seraient génétiquement liées aux péridotites serpentinisés. On discute égalemente la possibilité d’existence de dépôts de Cr, Ni, Co et Pt: une détermination semi-quantitative a fourni une teneur en Pt d’environ 0,1 %. L’origine des serpentinites pourrait être à rattacher au métamor­ phisme hercynien d’un corps ophiolitique cambrien (?). RESUMO : Sobre os peridotitos serpentinizados de Santana (região de Portei, Alentejo). Os peridotitos serpentinizados descritos nesta nota afloram na região de Portel-Vidigueira, segundo corpos lenticulares alongados numadirec- 52 ção NW-SE. No caso mais frequente, uma faixa concordante de rochas verdes separa-os de formações xistosas de idade provavelmente câmbrica, sem que se notem aparentemente aspectos de metamorfismo de contacto. A composição mineralógica destes serpentinitos é relativamente com­ plexa, tendo sido identificados os seguintes minerais: lizardite, crisótito, antigorite, olivina, diálage, biotite, tremolite-actinolite, clorite, talco, cal­ cite, magnetite, cromite e pentlandite. Mostra-se que concentrações de amianto (serpentínico e sobretudo anfibólico), actualmente em exploração, estão geneticamente associados aos peridotitos serpentinizados e discute-se a possibilidade de existência de depósitos de Cr, Ni, Co e Pt; de notar que uma análise semiquanti­ tativa forneceu um teor em platina de cerca de 0,1 %. Parece lógico supor que a origem dos serpentinitos esteja ligada ao metamorfismo hercínico de um corpo ofiolítico câmbrico ( ? ). Within the province of Alto Alentejo, more or less serpenti- nized ultramafic rocks have been reported (Cotelo Neiva, 1947) along a northwesterly trending zone of Cambrian rocks in the Cabeço de Vide region. Serpentinite occurrences outside the Cabeço de Vide region were but recently refered near Portel (Soares de Andrade, 1966) and Orada (Carvalhosa, 1965). In the Portel area, a few small asbestos deposits are long known (Thadeu, 1965); they have been interpreted as geneticallv related to 1) amphibolitic rocks (Thadeu, 1965) or to 2) a nor­ theasterly trending dolerite dyke (Torre de Assunção and C. Per­ digão, 1962). This paper constitutes a preliminary note on the Santana serpentinites and related asbestos deposits, in the Por- tel-Vidigueira region. The location of all specimens from San­ tana serpentinites described in this paper is shown in Fig. 1. GEOLOGICAL SETTING The Portel region is underlain by epimetamorphic sediments and pyroclastic rocks interbedded with lava flows (Carvalhosa, 1965, 1966, 1967; Soares de Andrade, 1966). It belongs to the vast Aracena-Évora Complex (Fabriès, 1963) which is assumed to have had a geosynclinal character during the Lower Paleozoic sedimentary cycle (Fabriès, 1963; Carvalhosa, 1965). In spite of the absence of fossils, a Cambrian (and Infracambrian ?) age was assigned by Carvalhosa to these rocks which are intensively fol­ ded and faulted. Emphasis has been made on a great NE-SW 53 4 — Memórias e Notícias — N0 66 54 wrench-fault filled up by an Hercynian (?) pigeonite dolerite (Torre de Assunção and C. Perdigão, 1962). The ultramafic bodies crop out as elongated lenses, striking generally N. 45. W (Hercynian trend); they are concordant with the regional structure and show a tendency to form disconti­ nuous, parallel trains suggesting folding and/or faulting. They are typically bordered by concordant greenstones which separate them from the widespread muscovite-chlorite-quartz pelitic micas- chists. Contact metamorphic phenomena are conspicuously lacking, but no detailed observations have yet been made. PETROGRAPHIC AND MINERALOGICAL FEATURES The serpentinites are dark-coloured (greenish black), massive rocks in fresh samples. Shearing is noticeably intense, and ser- pentinization has been almost complet in the rocks under study. Veins of both cross-fibers and slip-fibers are conspicuously abun­ dant. Mineral assemblages are rather complex, as shown by DTA and X-ray diffraction methods, and by polarizing and metallo- graphic microscopy. The serpentine minerals are mainly lizardite and chrysotile, with subordinate amounts of antigorite; among the nonserpentine minerals, olivine, diallage, biotite, tremolite- -actinolite, chlorite, talc, calcite, magnetite, chromite and pentlan- dite have been identified. Owing to the small amount of spe­ cimens studied, this list is probably not exhaustive. Serpentine minerals Lizardite and chrysotile were found to be the main consti- tuints. Under the microscope, the fibrous chrysotile shows a tendency to exhibit a leaf-shaped arrangements of the fibers, whereas the fine-grained lizardite often forms a felt-like dense mass of interwoven fibrolamellar platelets with positive elonga­ tion ; but it is possible too that such a distinction reflects textural arrangements rather than accounts for different mineralogical varieties. A near isotropic material is present in minor amounts, as well as a fibrous serpentine outlining single pseudomorphs; frequently cited in the litterature as respectively serpophite and 55 56 bastite, they seem to constitute varieties of lizardite or chrysotile (Whittaker and Zussman, 1956; Portugal Ferreira, 1965, p. 191). The existence of antigorite is suggested by an endothermic peak between 750° and 800° in DTA curves (cf. Faust and Fahey, 1962). Its microscopic identification is obviously a diffi­ cult thing, as most «antigorite» described in the litterature may probably be lizardite (cf. Deer, Howie and Zussman, 1966). Elon­ gated subrectangular flakes sometimes showing an interwoven arrangement may probably be ascribed to antigorite. Nonserpentine minerals. A few euhedral crystals of olivine, more or less altered to serpentine, were found in