Abstract
Este trabajo se centra en el estudio de los minerales fosfato-sulfato alumínicos (APS) que se producenen una secuencia clástica del Triásico (Buntsandstein) de la Cordillera Ibérica. El depósito está constituido por areniscas, lutitas y conglomerados con matriz arenosa, que fueron depositados en ambientes de fluviala marino somero. En la secuencia clástica además de los APS están presentes los siguientes minerales dia-genéticos: cuarzo,feldespato potásico, minerales del grupo de la caolinita, illita, óxidos-hidróxidos de Fe,cementos-reemplazamientos de carbonatos y sulfatos y minerales secundarios uraníferos. Los mineralesAPS fueron identificados y caracterizados por microscopía óptica, difracción de rayos X, microscopía elec-trónica de barrido, y microsonda electrónica. Los microcristales de APS reemplazan a minerales uraníferos,asociados con caolinita-dickita, mica y rellenando la porosidad de arcosas-subarcosas fluviales a marealesdistales. Dado su contenido en Ca, Sr y Ba, los minerales APS se pueden definir como una solución sólida de crandallita-goyacita-gorceixita (0,53 Ca, 0,46 Sr y 0,01 Ba). La baja concentración en LREE y elcontenido de Sr>S, sugieren que los minerales APS se originaron durante la alteración supergénica de lasareniscas del Buntsandstein, debido a la presencia de fluidos mineralizantes que provocaron la formaciónde areniscas uraníferas en una zona de alteración distal. Además, la aparición de dickita asociada a losminerales APS, indica que éstos precipitaron a temperatura superior a 80ºC, relacionada con el levanta-miento que tuvo lugar durante la etapa post-rift del Cretácico Superior.
Highlights
Aluminium phosphate-sulphate (APS) minerals occur in a wide range of geological environments, ranging from superficial weathering through sedimentary, diagenetic, hydrothermal and metamorphic environments, to post-magmatic systems (Dill, 2001)
APS minerals are abundant in the clay mineral host-rock alteration assemblages associated with some uranium areas in the middle Proterozoic basins (Wilson 1985; Beaufort et al, 2005; Gaboreau et al, 2005; Gaboreau et al, 2007) and complex APS minerals were identified by Spötl (1990) as an early diagenetic precipitation in the Late Permian sandstones of the Northern Calcareous Alps (Austria)
The geochemical conditions in the Triassic basin during the APS mineral precipitation were oxidising and slightly acid, as indicated by the presence of the paragenesis formed by kaolinite-illite-quartz and hematite
Summary
Aluminium phosphate-sulphate (APS) minerals occur in a wide range of geological environments, ranging from superficial weathering through sedimentary, diagenetic, hydrothermal and metamorphic environments, to post-magmatic systems (Dill, 2001). In an extensive review, Dill (2001) remarked that the precipitation of these minerals indicates extreme acid and oxidising conditions at shallow depths, and describes that these are common occurrences in different geological settings. This origin was proposed by Benito et al, (2005) and Galán Abellán et al, (2008) who studied the APS minerals group in palaeosols from the southeastern continental Late Permian of the Iberian Range. APS mineral formation occurred shortly after sedimentation, due to the circulation of acid meteoric groundwaters, and concluded that the lack of carbonates and the presence of APS minerals at the base of the Triassic are a clear indication of extremely acid conditions during the Permian-Triassic transition in the studied area
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