Mineralizações de magnetite e sulfuretos de Monges (Santiago do Escoural, Montemor-o-Novo), Vale de Pães (Cuba-Vidigueira) e Orada (Pedrógão, Serpa) : Síntese de ensaio comparativo

As mineralizações Monges (Montemor-o-Novo), Vale de Pães (Cuba-Vidigueira) e Orada (Serpa), incluídas na Faixa Magnetítico-Zincífera da Zona de Ossa-Morena (ZOM), evidenciam características mineralógicas e geoquímicas diferentes, não obstante partilharem uma constituição dominada por magnetite ± sulfuretos (pirite, pirrotite e calcopirite). A diferença é tanto mais notória quando se compara Monges (possivelmente um sistema misto do tipo SEDEX-VMS) com Vale de Pães ou Orada, as duas últimas representando sistemas do tipo Skarn-Fe(Mg/Ca). Os dados disponíveis permitem ainda posicionar o desenvolvimento destes sistemas mineralizantes em contextos geológicos distintos, correlacionáveis com diferentes estádios da evolução geodinâmica/metalogenética da ZOM

Ocorrência de xenótimo em amostras aluvionares da região centro-leste de Portugal, Zona Centro Ibérica-Zona de Ossa Morena

Foi identificado, possivelmente pela primeira vez em Portugal, xenótimo aluvionar em concentrados de bateia colhidos numa campanha de prospecção de terras raras desenvolvida pelo ex-IGM no centro-leste deste país. O xenótimo ocorre em grãos sub-rolados de dimensão média =250um, em concentrações mais significativas em Nisa, Stº António das Areias e Marvão. A geologia regional e o cortejo mineral das amostras sugerem proveniência do xenótimo dos maciços graníticos de Penamacor e Nisa e ainda das Arcoses da Beira Baixa e níveis de cascalheiras plio-plistocénicas com intercalações argilo-arenosas

O depósito de Sn-Li associado a granito de grande tonelagem da Argemela, Portugal central

The Argemela Sn-Li quartz vein stockwork, east of the Panasqueira tungsten mine, is hosted in Cambrian slates and greywacke. An underlying Variscan granitic cupola is inferred from spotted slates and albitic microgranite dykes, the last including a thick modified albitic microgranite, enriched in F and Li, exposed some 500 m away, on the hill top. Inferred resources for shallowest deposit portion are 20.1 million tonnes (Mt) at 0.1-0.2% Sn, 0.2% Li and 0.1% (estimate) Rb, but the 650-m vertical-extent of the deposit suggests a resource of >200 Mt (with identical grades). The hydrothermal paragenetic sequence is amblygonite-montebrasite (mostly montebrasite) (Stage I)–quartz I-II–cassiterite (with columbite-tantalite inclusions)─arsenopyrite I─carbonate I-white mica I ─chlorite I–fluorite–apatite–rutile (Stage II)– white mica II–molybdenite– tourmaline– carbonate II-quartz III–arsenopyrite II–sphalerite–stannite–chalcopyrite–pyrite– pyrrhotite-chlorite II (Stage III)–covellite– vivianite–goethite/lepidocrocite (Stage IV). Amblygonite-montebrasite is the main Li carrier; Sn is evenly distributed between cassiterite and stannite; Rb is mostly in white mica (with 0.25-1.23 wt % Rb2O in the hill-top albitic microgranite). Primary aqueous, 1-3 um-wide fluid inclusions in the deposit in quartz I, carbonate I, apatite and cassiterite growth zones yield overall salinities and homogenisation temperatures of 7.2-19.1 wt % NaCl equiv. and 290-360ºC, respectively. The trace concentrations (electronic microprobe analysis) in quartz vein cassiterite reach 1.95 wt % Nb, 0.39 wt % Fe, 0.13 wt % Ti, and low/negligible values of Sb, Zn, As, Ag and Bi showing its granite-hydrothermal affiliation. Fe-poor and Fe-rich sphalerite (lower-intermediate and upper deposit parts) contain 1.0-1.6 and 7.9-9.4 wt % Fe, 64.3-66.0 and 55.9-57.2 wt % Zn, 0.4-0.5 and 0.9-1.1 wt % Cd, respectively. The sphalerite-stannite geothermometer yields temperatures of 245-297ºC. Following higher temperature amblygonite-montebrasite deposition (Stage I), hydrothermal fluids (aCl-=0.25 m), related to the hidden granitic cupola, at a mean pressure-corrected (50 MPa) temperature of 350ºC, were responsible for Stage II minerals deposition. Calculated cassiterite deposition from Sn chloride complexes occurred likewise, from probable magmatic-hydrothermal fluids, at fO2 = 10-34 -10-32 atm and pH=3.5-4. Cassiterite deposition mechanisms were oxidation, mixing, neutralisation, possible aCl- increase, and cooling. Later Fe-poor sphalerite (+kesterite/ferrokesterite) and Fe-rich sphalerite (+stannite) deposited at higher and lower fS2, respectively, the latter probably at a higher fO2 (Stage III). The uniqueness of Argemela system with abundant amblygonite-montebrasite in hydrothermal quartz tin veins may be related to an extreme fractionated F-, Li- and P-rich granitic magma. After the emplacement of the granite/albitic microgranite dykes, an emerging pegmatitic fluid was unable, possibly due to insufficient depth, to form pegmatites but only modified the hill-top albitic microgranite. As a consequence, the system, where mixing of high- and low-salinity magmatic fluids probably occurred, remained very enriched in F, Li and P and by the time the hydrothermal stockwork developed amblygonite-montebrasite (Stage I) was the first mineral to deposit abundantly before Stage II minerals deposition in those Argemela Sn-Li quartz veins.El filón de cuarzo Sn-Li de Argemela, al este de la mina de wolframio de Panasqueira, está alojado en pizarras y gravas del Cámbrico. Se infiere una cúpula granítica subyacente del Varisco a partir de pizarras moteadas y diques de microgranito albítico, este último incluye un grueso microgranito albítico modificado, enriquecido en F y Li, expuesto a unos 500 m, en la cima de la colina. Los recursos inferidos para la parte más superficial del yacimiento son de 20,1 millones de toneladas (Mt) con 0,1-0,2% de Sn, 0,2% de Li y 0,1% (estimado) de Rb, pero la extensión vertical de 650 m del yacimiento sugiere un recurso de >200 Mt (con idénticas calidades). La secuencia paragénica hidrotermal es ambligonita-montebrasita (mayoritariamente montebrasita) (etapa I)-cuarzo I-II-casiterita (con inclusiones de columbita-tantalita)─arsenopirita I─carbonato I- mica blanca I ─clorita I-fluorita-apatita-rutilo (Estadio II)-mica blanca II-molibdenita-turmalina-carbonato II-cuarzo III-arsenopirita II-esfalerita-estanita-calcopirita-pirrotita-clorita II (Estadio III)-covellita-vivianita-goethita/lepidocrocita (Estadio IV). La ambligonita-montebrasita es el principal portador de Li; el Sn está distribuido uniformemente entre la casiterita y la estannita; el Rb se encuentra mayoritariamente en la mica blanca (con 0,25-1,23 % en peso de Rb2O en el microgranito albítico de la cima). Las inclusiones fluidas primarias acuosas de 1-3 um de ancho en el yacimiento en las zonas de crecimiento de cuarzo I, carbonato I, apatito y casiterita arrojan salinidades globales y temperaturas de homogeneización de 7,2-19,1 wt % de NaCl equiv. y 290-360ºC, respectivamente. Las concentraciones de trazas (análisis por microsonda electrónica) en la casiterita de las vetas de cuarzo alcanzan el 1,95 % en peso de Nb, el 0,39 % en peso de Fe, el 0,13 % en peso de Ti, y valores bajos/negligibles de Sb, Zn, As, Ag y Bi, lo que demuestra su filiación granítica-hidrotermal. La esfalerita pobre en Fe y rica en Fe (partes inferior-intermedia y superior del yacimiento) contienen 1,0-1,6 y 7,9-9,4 % en peso de Fe, 64,3-66,0 y 55,9-57,2 % en peso de Zn, 0,4-0,5 y 0,9-1,1 % en peso de Cd, respectivamente. El geotermómetro de esfalerita-estanita arroja temperaturas de 245-297ºC. Tras la deposición de ambligonita-montebrasita a mayor temperatura (Etapa I), los fluidos hidrotermales (aCl-=0,25 m), relacionados con la cúpula granítica oculta, a una temperatura media corregida por presión (50 MPa) de 350ºC, fueron los responsables de la deposición de minerales de la Etapa II. La deposición de casiterita calculada a partir de complejos de cloruro de Sn se produjo igualmente, a partir de probables fluidos magmáticos-hidrotermales, a fO2 = 10-34 -10-32 atm y pH=3,5-4. Los mecanismos de deposición de casiterita fueron la oxidación, la mezcla, la neutralización, el posible aumento de aCl- y el enfriamiento. Más tarde, la esfalerita pobre en Fe (+kesterita/ferrokesterita) y la esfalerita rica en Fe (+estanita) se depositaron a una fS2 más alta y más baja, respectivamente, esta última probablemente a una fO2 más alta (estadio III). La singularidad del sistema de Argemela, con abundante ambligonita-montebrasita en las vetas hidrotermales de cuarzo-estaño, puede estar relacionada con un magma granítico extremadamente rico en F, Li y P. Tras el emplazamiento de los diques de granito/microgranito albítico, un fluido pegmatítico emergente fue incapaz, posiblemente debido a la insuficiente profundidad, de formar pegmatitas, sino que sólo modificó el microgranito albítico de la cima. Como consecuencia, el sistema, en el que probablemente se produjo la mezcla de fluidos magmáticos de alta y baja salinidad, permaneció muy enriquecido en F, Li y P y, en el momento en que se desarrolló el stockwork hidrotermal, la ambligonita-montebrasita (Etapa I) fue el primer mineral que se depositó abundantemente antes de la deposición de los minerales de la Etapa II en esas vetas de cuarzo con Sn-Li de Argemela

Alluvial nodular monazite in Monfortinho (Idanha-a-Nova, Portugal): Regional distribution and genesis

ABSTRACT: This work constitutes a contribution to the knowledge on the occurrence of nodular monazite in the Monfortinho (Idanha-a-Nova) alluviums and its genesis. A new edition of the alluvial nodular monazite regional distribution map is presented, underlining its wide occurrence and preferential concentration in the north-western and central zones of this region. The assessment of the geological and environmental evolution during Neoproterozoic-Palaeozoic and Caenozoic times and alluvial nodular monazite accumulation seems to provide a direct association between the presence of the Ordovician rocks, in particular the radioactive (carrying radioactive heavy minerals) quartzite (one of the most probable original sources) and Caenozoic sedimentary rocks (most probably the secondary source). Nevertheless, the Slate-Greywacke Complex cannot be excluded as a host rock for nodular monazite as well. Alluvial heavy mineral concentrates include: iron oxide/hydroxide, ilmenite, tourmaline, nodular monazite, monazite, xenotime, zircon, rutile, anatase, brookite, apatite, andalusite, gold, cassiterite and topaz. There is a significant REE enrichment in these concentrates (up to> 32,000 mg/kg), mainly in LREE-MREE. The contents in Ce and Th, Th and REE and Ce and La showed good correlation (0.97, 0.96 and 0.99, respectively), reflecting mainly the striking presence of nodular monazite, as can be proved by the similarity among NASCN patterns of this mineral and concentrates. In Monfortinho there are two distinct alluvial nodular monazite populations: 1) distal pre-deformation nodules generation, from the central western area, mostly ellipsoidal to discoidal, with irregular not orientated probable detrital mineral inclusions; and 2) proximal generation, in the northern area near the Ordovician rocks, with smaller grains, with mostly irregular surfaces. Diagenetic/low metamorphic pre-deformation distal Monfortinho nodules population growth can be recognised and characterised by the encompassing of irregular unoriented mineral inclusions of the host matrix rock; preferential incorporation of MREE over other REE in the core nodules, consistent with diagenetic MREE-rich environmental/mineralized fluids supplied by the dehydration of Variscan sedimentary marine sequences with phosphatic rocks, at the start of nodule formation. The slight increase in Ca towards the nodule rims denotes a relative increase in fluid salinity during nodule growth, consistent with the ineffectiveness of metamorphic dewatering in dissolving the significant salt content of those marine sequence(s); Th increment in nodule rims points to the temperature increment at this stage; its moderately to pronouncedly negative Eu anomalies and the general involvement of the cheralitic substitution mechanism are characteristic of metamorphic monazites. Monfortinho and other published data suggest that in the beginning of nodular monazite formation the fractionation (La/Sm) N tended to be lower than that of the original source (detrital relic mineral/seawater), very similar to primary synsedimentary apatite or to monazite nodules interpreted to have precipitated directly from seawater. At the end the fractionation values can be substantially higher than those and may be dependent on the conditions established during the different geologic environments.info:eu-repo/semantics/publishedVersio