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Variscan granites in the Catalan Coastal Ranges (CCR) show hydrothermal alteration due to fluid circulation through fractures. This caused primary quartz (i.e., formed by crystallisation of granitic magma) dissolution and replacement of primary feldspars by albite and /or microcline, which implies sodium and/or potassium enrichment, respectively. Other secondary minerals are also frequent. The altered rock is called episyenite and shows a reddish colour in contrast with the original white-grey ones of unaltered granites (Fig. 1). Episyenites can be related to different types of mineralisations, even of rare earth elements, which are very demanded at present for their technological interest.

Reddish episyenite in contrast with grey unaltered granodiorite in the Costa Brava outcrops (Sa Conca beach, Girona).

Two researchers from the Geology Department of the UAB, within the frame of the project MICIU/AEI/10.13039/501100011033 on the Origin, mobility and concentration of Rare Earth Elements in Variscan materials of the Iberian Central System and the Catalan Coastal Ranges, have studied the alteration of two types of Variscan granites in the latter area: leucogranites from Les Guilleries Massif (305-299 Ma age) and granodiorites from the Gavarres-Costa Brava Massif (288 Ma). They also differ in their chemical composition: the leucogranites are alkali-calcic and the granodiorites are calc-alkali. Our aims were: to constrain the age of the alteration, to characterise the mineralogical and geochemical changes in the granites, to estimate temperatures and pressures of the process, assess the nature of the involved fluids and to compare the case studies with coeval ones in the Iberian Massif and the Pyrenees.

From field data, characterisation of granites, their alterations and minerals under the optical and electronic microscope and chemical analyses of their major and trace elements, the results achieved are the following: there were several post-Variscan hydrothermal alteration episodes, from Lower Permian to Middle Triassic. The two examples follow the same alteration trend, characterised by both albitisation and dequartzification. However, the alteration is less severe in Les Guilleries leucogranites than in the Costa Brava granodiorite. This is related to lower rock/fluid ratio and starting temperature and pressure of the process in the former example (380 ºC y 30 MPa) than in the latter (500 ºC y 50 MPa). 

Trace elements such as uranium behaved differently. Uranium concentration is high in Les Guilleries leucogranites and decreased during the alteration, whereas this element shows normal concentration in the Costa Brava granodiorite and increased slightly during the alteration. By contrast, rare earth element concentrations did not change significantly. We have inferred that the involved fluids were deep aqueous ones, with low rare earth element ligands, and with possible participation in the process of shallower evaporitic water. The case studies and coeval examples in the Iberian Massif are alike but differ from younger alterations affecting Variscan granites in the Pyrenees.