Petalite as determinant of maximum lithium content of Variscan pegmatites from NW Portugal – paragenetic and geochemical approach

Estudos petrológicos de aplito-pegmatitos Variscos, de tipo LCT, intrusivos em terrenos Silúricos do NW de Portugal, mostraram que o intercrescimento de espodumena + quartzo, em proporção modal 1:2, é persistente em dispositivos texturais circunscritos que podem ser interpretados como agregados miméticos, pseudomórficos após petalite em subsolvus, os quais depois evoluem sujeitos à deformação, metamorfismo e alteração. Cerca de 6 % dos corpos aplito-pegmatíticos que afloram naqueles terrenos, apresentam estes intercrescimentos, ou p etalite primária, a valores de Li2O situados no intervalo de 0,5 a 2,5 %. Uma sistemática químico - mineralógica das fácies analisadas sugere que o valor máximo de 2,5 % de Li2O em rocha total, pode ser considerado uma barreira geoquímica, imposta por um determinante petalítico primário cuja prevalência é balizada pelo equilíbrio com os feldspatos e pela magnitude da alteração argílica e correspondente lixiviação de LiPetrologica l studies of Variscan LCT aplite-pegmatites, hosted in Silurian metamorphic suites in Northwestern Portugal, revealed that the intergrowths of spodumene+quartz, in modal proportions 1:2, is ubiquitous in concealed pegmatite intergrowths, which can be interpreted as mimetic aggregates, pseudomorphic after petalite, that, afterwords, evolve, under changing conditions of metamorphism, deformation and alteration. 6% of the aplite –pegmatite bodies individualized in those Silurian host- rocks show this kind of intergrowths, or primary petalite, at a Li2O range between 0.5 % and 2.5 %. A chemical-mineralogical systematics of the composition data suggests that the maximum value of 2.5 % Li2O, in whole-rock analysis, should be considered a geochemical barrier imposed by a primary petalitic determinant and framed by the equilibrium with feldspars and by the magnitude of argillic alteration and corresponding lithium leaching(undefined)info:eu-repo/semantics/publishedVersio

Structure of the granitic pegmatite field of the Northern coast of Portugal—inner pegmatite structures and mineralogical fabrics

On the coastline of Northern Portugal, metamorphic formations and pegmatites were the subject of structural analysis with the main goal of understanding Variscan kinematics and related pegmatite intrusion. This study also aims to discriminate, select and characterize relevant aspects of the structure and the paragenesis of pegmatites, well exposed as a result of coastal erosion, justifying its inclusion in the geological heritage of the Northern coast of Portugal. The pegmatite bodies show distinctive internal and external structures that are attributable to different modes of emplacement and subsequent deformation. The pegmatitic implantation in the areas of Moledo and Afife occurs in an intragneissic and perigranitic environment, for the first area, and perigneissic and perigranitic environment, for the second. In Pedras Ruivas predominates the implantation into an exo-gneissic to exo-granitic domain. The Moledo veins show evidence of multiphase open/filling, revealing positions, shapes, attitudes, sizes and internal structures that change as a function of the host lithology and host structure, but mainly due to the dilation and the cycles number of local telescoping. The structural analysis of the pegmatite bodies allows the deduction of a local fulcrum of expansion that hypothetically overlaps a hidden stock of parental granite. In Afife and Pedras Ruivas, some pegmatitic lenses are specialized and mineralized in Li, Cs and Ta, with spodumene and tantalite ± cassiterite. Spodumene occurs as giant crystals, centimetric to pluri-decimetric in length, which mark very clearly the structures of in situ or in flow crystallization inside the pegmatites (primary structures) and also the secondary structures resulting from deformation. The geometric analysis of fabrics helps the individualization of well-defined stages of progressive evolution of the deformation of the pegmatites, allowing its correlation with major D2–D3 episodes of regional Variscan deformation

Typology and evolution of the new mineralogy developed at the mine waste dump of Valdarcas (Vila Nova de Cerveira - N Portugal) - Environmental Implications

[Abstract] The abandoned waste dump of Valdarcas tungsten skarn deposit — Vila Nova de Cerveira, N Portugal, represents a key site, appropriate to study the environmental problems related to deactivated sulfide tails. The tungsten mineralization which occurs in coexistence with metallic sulfide minerals, calcium silicates and carbonates, confers to this deposit special characteristics. The water-mineral interactions at the waste dump surface, leads to the generation of acid mine waters (AMD) with high content of metals and sulfate (ASW), related to the sulfide minerals alteration. Consequently occurs the contamination of the Coura river system Simultaneously the alteration of calcium silicates and carbonates produces neutralization reactions, and the development of a new mineralogy - soluble salts, iron crusts and clay minerals. The new paragenesis reflects the occurrence of wet-dry cycles, related to the seasonal variations of rainfall. These geochemical trends and their role in the process evolution of the waste dump surface to the stabilization are the main subject of this work.The soluble salts, iron crusts and the clay minerals have different capacities and mechanisms to fixate the chemical potential associated to the AMD/ASW. The instability of the salt minerals leads only to a temporary fixation of the contaminant elements. On the other hand, the crusts are capable of fixate efficiently these elements, specially the iron oxides, furthermore, they isolate the reactive minerals from the weathering agents. Consequently they limit the aquatic contamination, the dispersion area of the environmental impact and contribute to the chemical stabilization of the waste dum

Património mineiro da Serra de Arga - Minho

O A Região da Serra d’Arga apresenta grande diversidade litológica, de depósitos minerais e mineralizações metálicas (Au, Ag, Sn, Nb-Ta, W, Zn, Li), razão pela qual tem sido palco de vários ciclos de aproveitamento extractivo. Destes subsistem diversos vestígios mineiros na paisagem, na organização do território e na memória colectiva das comunidades. O funcionamento mineiro passado revelou ainda um importante património natural - geológico e, bem assim, se expressa num conjunto diversificado de vestígios classificáveis como património cultural e industrial mineiro. Muito embora nesta região as evidências associadas ao aproveitamento de recursos minerais se reconheçam desde o Paleolítico, e ainda com grande incidência na época romana, este trabalho analisa vestígios relativos ao séc. XX, dos quais persistam referências documentais e testemunhos da vivência mineira. No estudo usaram-se métodos multidisciplinares de análise, obtendo: 1) a discriminação de indícios de depósitos filonianos explorados e da implantação das áreas intervencionadas e/ou do edificado; 2) a dispersão espacial de vestígios da actividade extractiva, segundo os principais períodos de intervenção (pela análise documental de arquivos e por inquérito sociológico). A sistemática efectuada e a organização segundo divisões geomineiras regionais permitiu deduzir, respectivamente, faixas e campos mineiros que sustentam uma adequada fundamentação para a avaliação do interesse patrimonial mineiro e geológico associado.The Serra d‘Arga Region is characterized by a great diversity of ore and industrial minerals deposits. The overall set of metallizations includes Au, Ag, Sn, Nb-Ta, W, Zn and Li. Owing to this metalliferous diversity and potentiality the region has been the target for several exploration cycles and mining activities since pre-historical times, which remnants are still very present not only in the traditions and cultural heritage of the local resident people, but also in a lot of material remarks, natural and antropic, and ancient sites where industrial mining heritage and some of its most peculiar remains and assets, are preserved. Evidences of mining activities date from Paleolithic times. Afterwards they were strongly diversified, in what concerns the remarks of Roman activities and some post Medieval-Age thecnologies. However, the main goal of the present study is the mine exploitation occurred during de XX century. The approach follows an analytical perspective dedicated to documental references of technical and administrative nature, which are remaining in state and private-companies archives and museums. The local people collective memoir was accessible through opinion survey, planned interviews and inquiries to ancient miners and some known old, still-living, members of the ancient mining population. The material assets remaining in site or preserved in museological context were also considered and studied using analytical geology and mining archaeology procedures. This multidisciplinary approach allowed: 1) the discrimination of the technological remains and geological exposures of the affected ore deposits as well as the systematics of the mining areas and its remaining edification remarks; 2) the description of space dispersion of the mining activities according to the main mining cycles (recognized in field after documental analysis and sociologic inquiries). This systematics, and the study of the regional geological and mining organization, allowed the definition of geological corridors favorable to mineral exploitation and related mining fields, which sustain an adequate background for the evaluation of mining potentialities and the regional value of natural and industrial-archaeological heritage

Paragenetic diversity in contaminated granite pegmatites – Serra de Arga (Minho)

"VIII Congresso Nacional de Geologia"No Domo de Covas (Serra de Arga), a contaminação de magmas pegmatíticos por reacção com rochas calcossilicatadas, metacarbonatadas e metaexalíticas originam paragéneses dessilicificadas com mineralogia anómala que inclui além da plagioclase, escapolite, fosfatos de Li e minerais de epídoto. A evolução paragenética manifesta-se para uma modificação da atitude dos veios e pode resultar de processos de fraccionação e/ou evolução térmica que culminam na formação de venulações ricas em prenite e associações com oligoclase+escapolite.In Domo de Covas (Serra de Arga), the contamination of pegmatitic magmas by reactions with calcsilicate, metacarbonate and metaexalitic rocks, originate desilicated paragenesis with anomalous mineralogy, that includes in addition to plagioclase, scapolite, Li- phosphates, and epidote minerals. The paragenetic evolution is concomitant to the modification of veins direction and can result from fractionation processes and or thermal changes that culminate in the formation of prehnite veins and oligoclase + scapolite associations

Epidote populations in contaminated granitic pegmatites from Northern Portugal

Publicado em "Acta Mineralogica-Petrographica. Abstract series 6

Study of the volcano-sedimentary nature of the Serro formations – Silurian terranes of Serra de Arga – Minho (Northern Portugal)

Petrography and paragenetic analysis of some phyllitic Silurian formations from the North of Serra de Arga (Minho – Northern Portugal) revealed an abnormal modal percentage of iron sulfides in a complex set of lithologies that include amphibolic members and tourmaline-garnet rich horizons as well. Excluding the geochemical influence and expression of superposed metamorphism and metasomatism, whole rock analysis shows conspicuous high Fe and Al percentages and low contents of incompatible elements in some facies. Significant Zn, Mn, Sc, Ba and Sr may be related to the presence of accessory minerals suggesting an exhalitic affinity. Pyrrotite and pyrite are the main sulfides but sphalerite, galena and molybdenite are also present in low quantities. These petrographic and geochemical characteristics and the presence of textural aspects such as clastic and/or porphyric relict geometries support the establishment of some correspondences between the observed rock types and few possible volcanosedimentary or exhalitic ancestors. Whole rock geochemistry also suggests the manifestation of a protolithic polimodal volcanism with a main expression in the felsitic domain with several complex primordial textures indicating that the primitive rock types could be tuffaceous/volcanoclastic to porphyric

Petrogenesis of peraluminous anatectic pegmatoids

Publicado em "PEG 2013 : Contributions to The 6th International Symposium on Granitic Pegmatites

Structural and geochemical considerations about the sedex and volcanic affiliation of some metamorphic units of Arga Range – Northern Portugal: consequences to geological mapping

Dados de petrografia e geoquímica adquiridos para um conjunto de litótipos de natureza vulcanogénica, agora atribuídos à unidade cartográfica que se designa de Formações do Domo de Covas, permitem colocar em evidência um carácter de transição entre basaltos alcalinos de fonte mantélica e séries magmáticas toleíticas-transicionais a transicionais-calcoalcalinas. O seu desenvolvimento e também a génese de rochas exalativas contemporâneas enriquecidas em vanádio é mais provável em bacias euxínicas, pré-riftogénicas constituídas em retro-arco.Petrographic and geochemical data acquired for a set of volcanogenic rocks, belonging to the thrusted sequences of Domo de Covas Formations in Serra de Arga, show a transition character between mantle source alkali basalts and transitional-tholeiitic to transitional-calcalkaline magmatic series. The genesis and emplacement of this rocks as well as contemporary vanadium-rich exhalative facies is consistent with back-arc anoxic basin systems, precursor to rifting onset