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[en] In the Lagoa Real region of western Bahia State, Brazil, basement acid gneisses of the Sao Franciso craton (3 GA) show localized zones of uranium enrichment that in places attain ore grade. The enrichment occurs in linear shear-zones associated with retrograde metamorphism and extensive metasomatism of the amphibolite- to granulite-facies basement. Zones of uranium enrichment contain the mineral assemblage plagioclase-epidote-amphibole-aegirine-garnet typical of the epidote-amphbiolite facies. Metasomatic processes include: oxidation, sodium addition, potassium depletion, quartz removal, and addition of U-V-V and some REE. The typical product is dominated by albite and a sodic pyroxene. Oxygen-isotope data for quartz (-0.02 per thousand), albite (-1.95 per thousand), magnetite (-8.70 per thousand) and pyroxene (-4.15 per thousand) show that fluids involved were approximately -4 per thousand and that the major phase of alteration occurred near 500 degrees C. This is confirmed by fluid-inclusion studies, which also show the presence of highly saline fluids. A model is proposed in which a slice of Archean basement (approximately 15 km thick) has been thrust to the west over Proterozoic sediments. Isotopically light formation-brines carrying uranium have been expelled through hydrofractures into the hotter over-thrust plate, causing the observed mineralogical and chemical changes. The rocks provide an example of mobilization of U and some trace elements from a saline, organic-matter-rich, low-temperature environment, with precipitation in response to rising temperatures
Journal
Canadian Mineralogist; ISSN 0008-4476; 
; v. 21(4); p. 647-654
; v. 21(4); p. 647-654
[en] The Riás Schists crop out in the so-called Iberian Variscan parautochthon, surrounding the Malpica-Tui Complex (NW Iberian Massif), as part of the westernmost internal areas of the European Variscan belt. Three Variscan metamorphic events have been identified in the Riás Schists (M1, M2 and post-M2). M1 comprise unoriented microinclusions that have only been identified in garnet porphyroblast cores and inside staurolite crystals. M2, comprises the matrix foliation (S2) and the assemblage garnetRIM + staurolite + muscovite + biotite + chlorite + rutile/ilmenite + magnetite + quartz and the development of syntectonic andalusite. Finally, postM2 (post-S2) includes andalusite and plagioclase together with secondary muscovite, biotite and chlorite and accessory tourmaline, Fe/Ti oxides, apatite and carbonate. Results of multi-equilibrium thermobarometry (pressure-temperature pseudosections), for M2 indicates a medium-pressure Barrovian metamorphic event (0.5-0.6 GPa; 580-570 o C; ca. 20 km deep), followed by moderate cooling during decompression, together with the extensional collapse of the Variscan orogenic pile (post-M2; 0.3 GPa; 540 o C; ca. 10 km deep). Inferred P T conditions in the Riás Schists, and their spatial relationship with the overlying allochthonous high-pressure pelitic schists (i.e. Ceán Schists), suggest that both lithologies were part of the same continental margin during the beginning of the Variscan orogeny. However, they experienced very different ectonothermal evolutions due to their putative locations in the orogenic wedge.
[es]
Los Esquistos de Riás afloran en el parautóctono del orógeno Varisco de Iberia, en el entorno del Complejo de Malpica-Tui (NO del Macizo Ibérico), en el sector interno más occidental de la Cadena Varisca Europea. Se han identificado tres eventos metamórficos en los Esquistos de Riás (M1, M2 y post-M2). M1 incluye microinclusiones sin orientación preferente que sólo se han identificado en los núcleos de los porfiroblastos de granate y en los cristales de estaurolita. M2 comprende la foliación principal de la matriz (S2) y está constituido por la paragénesis granateBORDE + estaurolita + moscovita + biotita + clorita + rutilo/ilmenita + magnetita + cuarzo y el desarrollo de porfiroblastos sintectónicos de andalucita. El evento post-M2 (post-S2) incluye andalucita y plagioclasa, además de moscovita, biotita y clorita secundarias y cantidades accesorias de turmalina, óxidos de Fe-Ti y carbonato. Los resultados de las técnicas de termobarometría multiequilibrio (pseudosecciones presión-temperatura), caracterizan M2 como un evento metamórfico Barroviense de media presión (0.5-0.6 GPa; 580-570 o C; ca. 20 de profundidad) seguido de una exhumación con enfriamiento moderado, coetánea con el colapso extensional de la pila orogénic a Varisca (post-M2; 0.3 GPa; 540 o C; ca. 10 km de profundidad). Las estimaciones realizadas en los Esquistos de Riás, y su relación espacial con los esquistos pelíticos de alta presión del alóctono (i.e. Esquistos de Ceán), sugieren que ambas litologías formaron parte del mismo margen continental durante el comienzo de la orogenia Varisca, pero experimentaron evoluciones tectonotermales muy distintas por su posición original en la cuña orogénica.Journal
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