Publication: Allanita-(Nd) y minerales de elementos raros en las pegmatitas graníticas de La Cabrera, Madrid (Sistema Ibérico Central)
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Publication Date
1997
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Sociedad Geológica de España
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Abstract
En el macizo granítico de La Cabrera, emplazado entre 5 y 7 Km de profundidad, son abundantes las manifestaciones pegmatíticas a todas las escalas, desde microcavidades miarolíticas hasta cuerpos de dimensiones métricas, a veces zonados, con algunas mineralizaciones de elementos raros (Be, Nb, Ta, Sn, W, D, y Y Tierras Raras). La prolongada actividad hidrotermal del granito originó, en estas pegmatitas, asociaciones minerales tardías de temperaturas decrecientes hasta la formación de ceolitas que se sobreimponen, a veces, a las asociaciones pegmatíticas s.s. Destacan, entre estas asociaciones, las de minerales cálcicos de media y baja temperatura, algunos con elementos
raros.Los minerales específicos de Be son gadolinita-(Y), helvina y bavenita. Entre los de Nb y Ta cabe destacar la vigezzita, que se encuentra en La Cabrera como -(Ce) e -(Y), así como ilmenita y titanita con contenidos notables de estos elementos. La casiterita también puede contener algo de Nb y Ta. Los minerales específicos de W son wolframita (Fe>Mn) y scheelita. El único mineral con alto contenido en D es la uraninita. El Y reside mayoritariamente en el granate; otros minerales con Y son gadolinita-(Y), thalenita-(Y), kainosita-(Y), xenotima y un carbonato, probablemente synchisita-(Y) desprovista de F. Las TR radican esencialmente en las allanitas -(Ce) y -(Nd) Y en algunas zonas de determinadas epidotas, toda vez que lamonacita-(Ce) es muy escasa. La allanita-(Nd), que se caracteriza químicamente por vez primera, parece originada a partir de la alteración de una gadolinita-(Y) anterior
In the La Cabrera granitic massif, which was formed between 5 and 7 km in depth, there are many pegmatitic signs at all scales, from miarolitic microcavities to sometimes zoned metric bodies, with some rare element mineralizations (Be, Nb, Ta, Sn, W, D, Y and REE). The long hydrothermal activity in the parental granites yielded late mineralogical associations of decreasing temperature to zeolites fOlmation in the pegmatitic bodies. These associations occasionally overprint the pegmatitic s.s. ones. The presence of calcic minerals, some of them containing rare elements, which formed at medium to low temperatures is pointed out in these associations. The specific Be minerals are gadolinite( Y), helvite and bavenite. Vigezzite, both -(Ce) and -(Y), are the main Nb-Ta minerals, along with ilmenite and Nb-Ta - rich titanite. Cassiterite may also contain sorne amount of Nb and Ta. The W specific minerals are wolframite (Fe>Mn) and scheelite. Draninite is the only D - rich mineral. Y is mostIy present in garnets; other containing Y minerals are gadolinite-(Y), thalenite-(Y), kainosite(Y), xenotime and a Y - rich carbonate, probably synchisite-(Y) without F. The REE are concentrated in the allanites -(Ce) and -(Nd), and locally into some epidote zones, because monacite-(Ce) is very scarce. Allanite-(Nd) is chemically distinguished for the first time in this work
In the La Cabrera granitic massif, which was formed between 5 and 7 km in depth, there are many pegmatitic signs at all scales, from miarolitic microcavities to sometimes zoned metric bodies, with some rare element mineralizations (Be, Nb, Ta, Sn, W, D, Y and REE). The long hydrothermal activity in the parental granites yielded late mineralogical associations of decreasing temperature to zeolites fOlmation in the pegmatitic bodies. These associations occasionally overprint the pegmatitic s.s. ones. The presence of calcic minerals, some of them containing rare elements, which formed at medium to low temperatures is pointed out in these associations. The specific Be minerals are gadolinite( Y), helvite and bavenite. Vigezzite, both -(Ce) and -(Y), are the main Nb-Ta minerals, along with ilmenite and Nb-Ta - rich titanite. Cassiterite may also contain sorne amount of Nb and Ta. The W specific minerals are wolframite (Fe>Mn) and scheelite. Draninite is the only D - rich mineral. Y is mostIy present in garnets; other containing Y minerals are gadolinite-(Y), thalenite-(Y), kainosite(Y), xenotime and a Y - rich carbonate, probably synchisite-(Y) without F. The REE are concentrated in the allanites -(Ce) and -(Nd), and locally into some epidote zones, because monacite-(Ce) is very scarce. Allanite-(Nd) is chemically distinguished for the first time in this work