On the santana serpentinized ultramafic rocks, Portel- 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

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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 (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, . 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 specimen VA-48; but for the most part, olivine is recognized through the granules of secondary magnetite outlining the boundaries of the original olivine crystals. Diallage is present in specimen VA-64. It occurs as subhedral crystals often larger than 2 mm, and alters to serpentine, tremolite and magnetite. A dark brown, pleochroic mineral occurs invariably in subordinate amounts as flakes or lamellar aggregates, irregu­ larly distributed; optical characteristics indicate an iron-rich bio- tite. Incolor tremolite occurs as fibrous aggregates of a conspicuous secondary origin, as shown by 1) veinlets cutting the serpentine minerals, and by 2) its association with pyroxene. It commonly takes a great development as asbestos fibers; in that case, optical and diffractometric features indicate actinolite (or at least an iron-rich tremolite) rather than tremolite. Some chlorite flakes, often fibro-radiated, are sometimes associated with biotite, from which they apparently derive. Interference colours are variable within a same thin section: it can be clinochlore or sheridanite, just like in some serpentinites from northern Portugal (Portu­ gal Ferreira, 1965). Talc occurs locally (e. g. in spec. VA-62A) as coarse to fine platy or fibrous aggregates, often showing a parallel arrangement perpendicular to the veinlets. It is generally associated with serpentine, tremolite-actinolite and calcite in zones of shearing. Another late mineral is calcite, which occurs as 1) euhedral crystals between massive serpentines and superficial asbestos, as 2) anhedral crystals associated with talc and amphi- bole, and as 3) long, narrow and irregular veinlets. Magnetite is relatively abundant in euhedral to anhedral crystals whose average 57 dimensions vary between 0,1 and 0,3 mm. Less abundant than magnetite is chromite, which typically occurs as as grains with rounded outlines. Chromite is sometimes partially envelopped by magnetite; a few Vickers microhardness determinations gave values from 780 to 850. Pentlandite is present in very small amounts as angular anhadral crystals of an apparent secondary origin. A comprehensive discussion of the origin of Santana serpen- tinites is beyond the scope of this note. Petrographic evidence supports, however, the idea of an history in three stages: 1) peri- dotite emplacement, 2) serpentinization, and 3) formation of asbes­ tos. A further, attractive hypothesis is to relate serpentinization and formation of asbestos respectively to regional metamorphism (but other genetical mechanisms such as weathering cannot be precluded in view of insufficency of data available) and to stress conditions, contemporaneous or posterior to metamorphism.

ENCLOSING ROCKS

As refered above, Santana serpentinites are bordered by concordant greenstones; these are grayish-green, massive to schis­ tose rocks. Typical mineral assemblages are chlorite-albite-calcite- epidote (VA-63) and adinolite-albite-epidote (SA-2). Sphene is invariably present as small rounded grains or granular aggregates. When chlorite is lacking, a blue-green amphibole seems to appear in very subordinate amounts; this could indicate a degree of meta­ morphism equivalent to the beginning of the quartz-albite-epidote- almandine subfacies of the greenschist facies in Winckler’s (1965) terminology. No study has been made of the possible relationship between these mineralogical changes and the distance from the serpentinites. These greenstones probably represent ancient basic volcanics such as diabases or basalts. They seem to stratigraphically overlay the serpentinites, but further research is obviously neces­ sary for a better understanding of their history. In any case, the close association of metamorphosed basic volcanics and the serpentinized ultramafic rocks is compatible with the assumption that both of them are genetically related. 58

SOME REMARKS ON THE SANTANA ASBESTOS DEPOSIT

Santana asbestos mine is situated in the Serra de Portel, at approximate latitude 38° 15' N and longitude 1°20' E. It is the unique asbestos occurrence at present in exploitation. Most of the asbestos veins seem to be composed of cross-fibers; however, a great deal of slip-fibers and of mass-fibers may occur 1) in con­ junction with the cross-fibers or 2) in zones of more intense shearing deformation. The length of the asbestos fibers is generally less than 1 cm in the cross-fibers, but frequently surpasses 10 cm in the slip-fibers. Both the two kinds of asbestos, when ground in a mortar, form a matted aggregate which is but hardly powered; this could indicate (Deer, Howie and Zussman, 1966, p. 248) a serpentine nature for the asbestos, but such an assumption do not seem to be confirmed by X-ray diffractometry and by differential thermal analysis. Actually, it appears that two kinds of asbestos (both chrysotile and tremolite-actinolite asbestos) exist in the Santana deposit (Figs. 2 and 3). As no adequate sampling was made, it is not possible to give here any figure concerning the relative amounts of the asbestos types; amphibole abestos are, however, apparently much more abundant than serpentine asbestos. The uses of Santana asbestos includ 1) manufacturing of heat and electrical insulators, and 2) spinning into yarns. The exploited horizon can hardly be expected to have rather conside­ rable lateral extend, and host-rocks do not seem to be widely dis­ tributed in the area. Should future exploration show that they are extensively fiberized the economic possibilities of the Santana depo­ sits could perhaps be interesting, providing markets are available.

SUMMARY AND CONCLUSIONS

Serpentinized ultramafic rocks exist in the Portel-Vidigueira region. They are closed associated with epimetamorphic basic volcanics, and apparently originated from peridotites. Yet such a close association may be genetical too. Some important questions (both academic and economical) 59 60 arise from the above description of Santana serpentinites. An academic, immediate question concerns the origin of the peri- dotites. If Brunn's (1961) sutures ophiolitiques model is accepted here, then the peridotite emplacement may be related to the preorogenic stage of the Lower Paleozoic geosyncline. Are the peridotites Hercynian rather than pre-Ordovician? If so, what is their time relationship with the post-Cambrian rocks? Here

is a very exciting problem to be solved. In fait, metavolcanics are being mapped near Odivelas, (Soares de Andrade, in press, C. R. Acad. Sci. Paris), which seem to overly a layered gabbro-anorthosite complex (= Beja gabbro-diorite com­ plex). Ultramafics are not known there, but no detailed studies have so far been made, so it is not yet possible to correlate Odi­ velas with Santana roks. From an economical point of view, the existence of deposits of both metallic and nonmetallic substances is expected in the San­ 61 tana region. Asbestos fiber will apparently be the most abundant product, but the development of talc may also be expected, mainly in marginal zones of intensely sheared serpentinites. Among the metallic substances, there seems to be good possibilities for the existence of Cr, Ni, Pt and Co concentrations: one semi-quan­ titative analysis was made which gave a platinium content of about 1000 ppn (0,1 %). Further researches are obviously neces­ sary, in order to elucidate the meaning of this unique analysis.

AKNOWLEDGMENTS

The writer is greatly indebted to Mr. Joaquim C. Cunha of the Santana asbestos mine for supplying four asbestos samples.

REFERENCES

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Thadeu, D., 1965, Notícia explicativa da Carta Mineira de Portugal na escala de 1/500 000; Serv. Geol. Portugal, p. 46. Torre de Assunção, C. and Perdigão, J. C., 1962, Sobre o prolonga­ mento filoniano dolerítico de Odemira-Vidigueira até Ouguela; Com, Serv. Geol. Portugal, t. 46, pp. 349-363. Whittaker, D. J. W. and Zussman, J., 1965, The characterization of serpentine minerals by X-ray diffraction; Min. Mag., v. xxxi, pp. 107-127. Winkler, H. J. F., 1965, Petrogenesis of metamorphic rocks; Springer Verlag, Berlin, p. 220.