Locality No. 3, Bajoca Mine, Almendra, Portugal

Granitic Pegmatites: The State of the Art – International Symposium. 06th – 12th May 2007, Porto, Portugal. Locality No. 3, Bajoca Mine, Almendra, Portugal. R. VIEIRA1 & A. LIMA 1 1 GIMEF – Dpto. Geologia, Faculdade de Ciências, Universidade do Porto, Portugal, romeu.vieira@fc.up.pt; allima@fc.up.pt INTRODUCTION Located 4 Km to the W of Almendra (V. N. de Foz-Côa, Guarda, Portugal), on the limit of Alto Douro and Beira Alta regions, the Bajoca open-pit mine is actually the biggest feldspatic exploitation, for ceramic and glass industry. The mining rights belong to FELMICA Minerais Industrias, S. A. since 1996, being the mining activity older, from the past century, for cassiterite and columbo-tantalite exploitation. This region presents a high potential in geological resources. Surrounded by highly evolved granitoids, it is an area with metalogenic potential, known by its Sn, W and Li mineralization. The lithium mineralization in pegmatiteaplite veins occurs mainly as lepidolite (Charoy & Noronha, 1999) and others with spodumene (Roda et al., 2007, Vieira et al., 2007, submitted). Lima et al. (2003) and Almeida (2003), and more recently Vieira et al. (2007), state the occurrence of petalite-bearing bodies. Among the potential source granites of such mineralization, with favourable metalogenic indicators, we have to the south the Hercynian Mêda-Penedono-Lumbrales leucogranitic Complex (Figure 1) (Carnicero, 1981; Ferreira et al., 1987). A similar situation is described by Roda (1993) and Roda et al. (1999) at the Eastern part of the FregenedaAlmendra pegmatitic field, in the area of Fregeneda (Salamanca, Spain) with the occurrence of pegmatite veins enriched in rare elements, such as Li, Sn, Rb , Nb>Ta, B and P. GEOLOGICAL SETTING The Bajoca pegmatite-aplite vein is located in a region, which extends to Spain, were these bodies are abundant, denominated as Fregeneda-Almendra Pegmatitic Field (Roda et al., 2007). The region is located in the Central-Iberian Zone (Julivert et al., 1974), which host the rare element pegmatite-aplite veins within the Almendra region, in the low-grade metamorphic Precambrian to Lower Cambrian “Complexo Xisto-Grauváquico” metasediments, which comprise an alternation of quartzites, graywackes, schists and pelites, mainly in the Pinhão (Pi) and Rio Pinhão (Ri) formations, but also in the alochthonous Desejosa (De) formation, and on the autochthonous Bateiras and Ervedosa do Douro formations (Silva & Ribeiro, 1991, 1994) (Figure 1). The Almendra region is bordered by the orogenic Mêda-Penedono-Lumbrales Granitic Complex to the south (Figure 1). These granites are syn-F3 Hercynian, heterogeneous, fine- to medium-grained, two-mica leucogranites (Ferreira et al., 1987; Lopez-Plaza & Carnicero, 1987; Silva & Ribeiro, 1991 & 1994). To the Lumbrales granite, according to Rb-Sr isotopic dating, ages around 300 ± 8 M.a. were establish (Garcia Garzón & Locutura, 1981). They are high evolved granites and metalogenic specialised, namely with respect to rare-element mineralization, according to criteria defined by Černý (1991). They are peraluminous (ASI and A/CNK>1), with >70% SiO2, enriched in P2O5 (≈ 0,35), Rb, Li, Cs and Sn, and with lowest values of CaO (< 1%), FeO (t), MgO, Sr, Ba, Zr, Y and V (Gaspar, 1997; Vieira & Lima, 2005a, b). A first event of regional metamorphism took place prior to the third-Hercynian phase (F3), generating prograde assemblages with garnet-staurolite-(kyanite). A second thermal metamorphic event, related with the syn-F3 granite granite intrusion, generated an isograd overlapping, marked by minerals like andalusite-cordierite-sillimanite (Martinez et al., 1990). In the region, this metamorphism shows an isograd distribution increasing to S, parallel to the MêdaPenedono-Lumbrales Granitic Complex contact, reaching locally the sillimanite (fibrolite) isograd (Carnicero, 1982; Silva & Ribeiro, 1991, 1994). The isograds are controlled by tardi-Hercynian tectonic faults, well represented in the area by the NNE-SSW Vilariça fault, who divides this region into two unleveled blocks, with sink of the central block, generating the designated Longroiva graben. THE BAJOCA VEIN The main vein, as we can see on the geological map (Figure 2), and in the longitudinal (I-J) and transversal (C-D) cross-sections (Figures 3 & 4), exhibit an extension proximally to 700 meters. The thickness is variable, ranging between few meters to more than 35 meters, with some thinner lateral ramifications attaining several meters of extension. The main body is affected by the Barril Fault along NNE-SSW strike, and is well marked by the occurrence of clay minerals and Fe-oxides. It is clearly intrusive in the “Complexo Xisto-Grauváquico” Pinhão (Pi) metasedimentary formation, showing a general orientation N010º with dip variations between 30º and 45º W. The vein is hosted by the Pinhão (Pi) formation, that in the surrounding terrains of the vein present a turbiditic nature, constituted by meta-greywackes and green-schist alternations, with a characteristic decimetric rhythmicity. They are affected by a regional green-schist metamorphism and on the vicinity of the pegmatitic body, due to contact metamorphism, it’s possible to find spotted schist (Silva & Ribeiro, 1994). 39 FIGURE 1. Almendra Geological Setting Granitic Pegmatites: The State of the Art – International Symposium. 06th – 12th May 2007, Porto, Portugal. 40 Granitic Pegmatites: The State of the Art – International Symposium. 06th – 12th May 2007, Porto, Portugal. FIGURE 2. Geological Map of the Bajoca- Mine main body (Drawings gently ceded by Felmica – Minerais Industriais, S. A.). FIGURE 3 & 4. I-J longitudinal and C-D transversal cross-sections of the Bajoca Mine main body (Drawings gently ceded by Felmica – Minerais Industriais, S. A.). 41 Granitic Pegmatites: The State of the Art – International Symposium. 06th – 12th May 2007, Porto, Portugal. Mineralogy According to Almeida (2003) and Lima et al. (2003), the main mineralogical association is simple and corresponds to a granitic composition. The pegmatitic facies is basically: 1) big euhedral to subhedral K-feldspar and Albite crystals, forming occasionally crystalline aggregates; 2) subhedral to anhedral petalite crystalline aggregates; and, 3) small rounded quartz grains, several times as crystalline aggregates. The muscovite, sometimes centimetric, is scarce. As accessory minerals it’s possible to find montebrasite, Fe-Mn phosphates and apatite appears as accessory minerals. Mineralization of Sn as cassiterite occurs in the greisens zones. The aplitic mineralogy correspond mainly to small grains of albite with minor quantities of quartz and muscovite. Petalite, largely microcrystalline and very fresh, appears within big white masses. However, it’s possible to distinguish centimetric crystals, with perfect {001} cleavage. On optical microscopy petalite shows characteristic polarization “silverplated” colours, being also common lamellar twin planes (001). Petalite occurs as: i) subhedral to anhedral centimetric crystals; and, ii) irregular millimetric crystals, with rounded inclusions of quartz. Until the moment the isochemical passage Petalite to Spodumene+Quartz described by London (1984) was not observed. Relatively to the zonal distribution of lithium, petalite assumes an important role, because the vein is clearly barren on the base, with progressive enrichment to the top (Almeida, 2003; Lima et al., 2003; Bobos et al., 2004). Geochemistry Channel and drill-core samples geochemistry results from the Bajoca Mine main body (Almeida, 2003; Lima et al., 2004; Vieira & Lima, 2005a, b), shows that it’s clearly peraluminous (A/CNK>1), with low value of SiO2, and clear domination of Na2O over K2O, reflecting the albite dominance above K-feldspar, mostly in the aplitic facies. Almeida (2003) describes an inverse correlation between the Na2O and Li values, evident macro and microscopically in the Bajoca vein. These geochemistry characteristics are also referred by Charoy & Noronha (1999) about the lepidolite veins outcrouping North of the Bajoca Mine, and according to the K/Rb ratio criteria defined by Černý (1992), Lima et al., (2004) consider these ones most evolved then the Bajoca Mine petalite-bearing vein (Table 1). TABLE 1. Bulk analysis average values for major and minor elements from the lepidolite-bearing1 and petalite-bearing2,3 Almendra veins. 1 Lepidolite SiO2 (%) TiO2 Al2O3 FeO (total) MnO MgO CaO Na2O K2O P2O5 F Total ASI A/CNK Na2O/K2O Li (ppm) Rb (ppm) K/Rb 69,57 nd 17,35 0,16 0,05 nd 0,3 5,05 3,25 0,73 1,33 97,79 1,47 1,4 1,55 4960 2570 10,51 2 Petalite 69,56 0,01 16,09 0,13 0 0,04 0,63 5,78 2,93 0,91 0,07 96,15 1,27 1,16 1,97 2050 800 30,38 3 Petalite 70,95 0,01 16,53 0,14 0 0,06 0,53 7 2,95 0,68 0,07 98,92 1,12 1,05 2,37 nd nd - (1Charoy & Noronha, 1999; 2,3 Almeida, 2003); (2 channel sampling; 3 drill-core); [(ASI=Al2O3/(Na2O+K2O) e A/CNK=Al2O3/ (CaO+Na2O+K2O)] PETROGENETIC CONSIDERATIONS According to Černý & Ercit (2005), the rare-element Bajoca Mine vein can be classified as belonging to the LCT (Li, Cs, Ta) family, Complex-type, Petalite Sub-type. These kind of pegmatite-aplite veins point to P-T stability fields around ≈ 200-400 MPa and ≈ 500-600ºC, with the petalite equilibrium conditions ranging within high temperatures, but low-P ≈ 200-300 MPa (London, 1984). REFERENCES CITED Almeida, C., (2003) Estudo do filão aplitopegmatítico da mina da Bajoca, Almendra. Contribuição científico – tecnológica, p. 148. Master Thesis, Porto University, Portugal. Bobos, I., Lima, A., Almeida, C., Vide, R. & Noronha F., (2004) Geochemistry of the lithiniferous veins of the pegmatite-aplite field of AlmendraBarca de Alva (Northern Portugal). CD-Rom of the Geoscience in a Changing World 2004, GSA Annual Meeting, Denver, USA. Carnicero, M. A., (1981) Granitoides del Centro Oeste de la Provincia de Salamanca. Clasificación y correlación. Cuad. Lab. Xeol. Laxe. 2: 45-49. Carnicero, M. A., (1982) Estudio del metamorfismo existente en torno al granito de Lumbrales (Salamanca).Stvd. Geol., 17, 7-20. Černý, P., (1991) Rare-element granitic pegmatites. 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