Mineralogy and Petrograghy of Some Tin, Lithium and Beryllium Bearing Albite-Pegmatites Near Doade, Galicia, Spain
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LEIDSE GEOLOGISCHE Vol. 249—259 1-4-1967 MEDEDELINGEN, 39, pp. published separately Mineralogy and petrograghy of some tin, lithium and beryllium bearing albite-pegmatites near Doade, Galicia, Spain BY B.J. Hensen Abstract The petrography and mineralogy of some Hercynian albite-pegmatites near Doade, Galicia, Spain is described. The mineral assemblage consists of albite, K-feldspar, quartz, muscovite, spodumene, petalite, cassiterite, beryl, columbite- tantalite, montebrasite, apatite, eosphorite-childrenite, zircon, tourmaline, garnet, pyrite, bertrandite, ferrisicklerite, heterosite and cookeite. A tentative of these minerals is From the between and paragenetical sequence given. relationships petalite spodumene approxi- mate initial temperature-pressure conditions of600°C. and 2,5—3,5 kb. can be derived. The possible cause of the high sodium content and of the fine grainsize of the bulk of the pegmatites is discussed. A genetical relationship of the pegmatites with the medium to coarse grained Hercynian two-mica granites is suggested. Resumen en español Se describe la la de hercinianas de albita de petrografia y mineralogia algunas pegmatitas cerca Doade, Galicia, Espaňa. Las contienen los minerales pegmatitas siguientes: albita, feldespato alcalino, cuarzo, muscovita, espodumena, petalita, cassiterita, berilo, columbita-tantalita, montebrasita, apatita, eosphorita-childrenita, zircon, turmalina, granate, pirita, heterosita cookeita. bertrandita, ferri-sicklerita, y Se da serie tentativa de De deducir una paragenética estos minerales. las relaciones entre petalita y espodumena se puede condiciones iniciales de de 600°C. kb. La del elevado temperatura y presion aproximadas y 2,5—3,5 causa posible porcentaje de albita del fino de la de Se los y grano mayor parte la pegmatita son discutidos. propone una relación genética con hercinianos de dos micas de medio granitos grano y grueso. Contents 600 m. wide and about 15 km. long (Fig. 1) and have thicknesses ranging between 0,5 —6 m. At approxi- Introduction 249 mately one km. to the W. of this zone an Hercynian two-mica granite out. Its contact is roughly The country rock 250 crops parallel to the schistosity of the mica schists. The Petrography of the pegmatites 251 relationship between this granite and the pegmatites is discussed below. Summary of paragenetic relationships 256 Most of the veins have been mined for cassiterite and Genesis of the pegmatite 257 some for beryl. Mining developments are mainly open but shallow Conclusions 259 cut some underground workings are present. Acknowledgements 259 of the Most the study was confined to pegmatite bodies Doade where detailed References 259 near (Fig. 2) sampling and subsequent mineralogical investigations were carried out. Two mines have been developed in this of the Parabellon in the INTRODUCTION part pegmatite zone, namely N. and Alcebedo in the S. Field-work carried In the N. vertical section reveals the was out by the author in the summer open cut a shape this of 1964 on some pegmatites occurring near the border of the main pegmatite vein (Plate I Fig. 1). In of section vertical vein its the provinces Pontevedra and Orense in Galicia, a dyke-like suddenly changes N.W. 186 and 153 of the almost horizontal and thickens and Spain. Topographical maps dip to considerably Instituto Catastral de were used further W. it losing most of its thickness Geografico y Espana steepens again scale of The is similar in the on an enlarged 1 : 25.000. (Fig. 3). pattern underground The studied veins in of the Alcebedomine to the former pegmatites occur as garnet- workings according NNW- of this mine. the W. bearing quartz-mica schists that have a vertical manager Thus, a stepwise dip to SSE striking schistosity. The veins, which run parallel seems to be general. The with the wallrock Inclusions to the schistosity, are not continuous and are of contacts are sharp. variable thickness. All veins are confined to a zone of schist in the pegmatite are rare. Some large patches 250 B. J. Hensen 2. of the Doade Thicknesses of Fig. Geological map area. the veins are exaggerated. be an axial plane to small scale folds that have sub- horizontal Numerous bands axes. quartz occur gen- erally forming boudins parallel to the schistosity and occasionally following the above mentioned small scale folding. Close to the contact with the pegmatites the schists are very rich in tourmaline. 1. after Mineralogy Fig. Simplified geological map Parga-Pondal (1963) constituent. Quartz. Quartz is always a major and unpublisheddata ofthe Department ofPetrology it shows undulose extinction. Different and Mineralogy, Leiden University. Encircled Invariably of of them to numbers refer to the locations of handspecimens of types quartz, some probably belonging different have been found. Small which the micas were analysed for trace elements. generations, quartzes (Read late-Hercynian instead of post-Hercynian) with their long dimension oriented parallel to the while in the schistosity occur in the mica-rich bands of schist enclosed in which are completely or partly bands quartz bigger equidimensional grains occur. pegmatite have not been rotated or otherwise dis- However, in some thin sections of the mica schists placed to any extent. Irregular apophyses ofpegmatite the these grains have an elongated habit, elongation in the schist are relatively rare but seem to be slightly being parallel or at a small angle to that of the smaller more abundant in the thick, irregular flat-lying parts grains in the mica-rich bands, and in others they are of the veins pegmatite (Plate I, fig. 2). strongly undulose, showing amoeboid intergrowth In both the N. and the S. a vein of milky is quartz produced by recrystallisation ofcrushed zonesbetween found in the immediatevicinity ofthe main pegmatite the grains. vein. The dimensions of this vein 2 quartz are m. Muscovite. This mineral is relatively fine-grained thick and several tens of metres long (Fig. 2). to and has a preferred orientation parallel the schisto- crenulation sity. Some recrystallised cleavages are THE COUNTRY ROCK the foliation observed, showing that present was The schists that constitute the earlier A small quartz-mica country preceded by an schistosity. proportion rock third ill-defined which is have a vertical NNW-SSE striking schistosity. of the micas indicate a plane This of amoeboid schistosity has been observed in some places to parallel to the direction elongated quartz Mineralogy and Petrography of some Albite-Pegmatites 251 PETROGRAPHY OF THE PEGMATITES The pegmatites are leucocratic. Albite is the main constituent. and muscovite K-feldspar, quartz are found in varying amounts. Petalite, spodumene and ofthe minerals difficult most accessory are to recognize macroscopically because of their white colour and fine grainsize. Exceptions are cassiterite, columbite- tantalite and beryl. The in pegmatites vary grainsize. Only K-feldspar in 5 None occurs relatively large crystals up to cm. of the other minerals exceeds the grainsize of 2 cm. Furthermore, the grainsize distribution is inhomo- In the wall is geneous. places a layering parallel to marked by variations in grainsize and changes in mineralogical composition (Plate I, fig. 3). The for layering, however, cannot be traced any great distance. Irregular variations in grainsize occur even within a thin section. Fine fractions have grainsizes of have 0,05—0,15 cm., coarser parts 0,3—1,5 cm. while the K-feldspar-rich bands and patches vary from 1,5—4 cm. A of 2 —10 thick is contact zone cm. generally present. It consists of fine-grained muscovite and quartz. Very fine grains of cassiterite and apatite have been found in this zone. The first albite crystals adjoining the contact zone are often oriented with their longest axis normal to the wall. Mineralogy Fig. 3. Sketch of the pegmatite vein at Parrabellon. profile Albite. Albite is the most abundant constituent of Outcrop of the vein is inserted. map the which contain 80 pegmatites, occasionally up to % ofthis mineral. It is white to colourless and the longest dimensions of the generally lath shaped crystals do described above. Late cross-muscovites occur. These not exceed 1 cm. also show deformation in the form of bending and Microscopically, several varieties of albite may be kinking, similar to that observed in all of the above recognised. Crystals of all varieties have large negative mentioned varieties. extinction angles (in sections perpendicular to X the Biotite. This mineral is less abundant than between Y and the traces of between musco- angle (010) vary 14 and albite vite. Its pleochroism is red-brown (Z, Y) to yellow 17°, indicating a nearly pure to pure The fabric is the for Albite is but Carlsbad- (X). arrangement same as composition). twinning common albite and muscovite with which it is frequently intergrown. pericline twinning are relatively rare. Cross-biotite has not been observed. The varieties are as follows: Small albite orientation Garnet. Small poikiloblastic crystals (1 —3 mm.) of i) crystals with random are contain small included in and show corroded garnet are present. They elongated K-feldspar out- quartzes that are mainly parallel to the external lines. These probably crystallised at a very early foliation. Mica-rich bands bend around the crystals. stage. Tourmaline. Small, euhedral, needle shaped crystals ii) Relatively big crystals that often display graphic with of (1 mm.) have a pleochroism of yellow-brown (O) to intergrowth non-perthitic K-feldspar are colourless (E). When zoned the pleochroism of the later origin. The K-feldspar is usually about but in cores is blue-green (O) to colourless (E). The c-axes 5—10 % rare cases may reach 50 % of the crystals are parallel to the schistosity, being (Plate I, fig. 4). As indicated below this albite randomly oriented in this plane. In thin sections in variety commonly accompanies petalite and which the show amoeboid The subhedral euhedral quartzes intergrowth, slight spodumene. to crystals deformation of the tourmaline has occurred in the are lath shaped and exhibit irregular twinning, form of bending accompanied by fracturing at right various patches having lamellae of different angles to the c-axis.