Abstract

JAQUES, Luís; BOBOS, Iuliu  and  NORONHA, Fernando. Study of fluids related to hydrothermal alteration of biotite granites from Gerês and Guarda (Portugal). Comunicações Geológicas [online]. 2010, n.97, pp.81-98. ISSN 1647-581X.

In granites, particularly from Gerês and Guarda regions, occur reddish altered rocks, commonly known as “episyenites”. These rocks resulted from a hydrothermal alteration process and involved alkali metasomatism, magmatic quartz dissolution and transformation of the primary minerals present in granites. The hydrothermal fluids, directly associated with the episyenitization process, were trapped in quartz microfractures - Fluid Inclusion Planes (FIP), consistent with the most important directions of altered structures. These fluids are low-salinity aqueous fluids from the H₂O-NaCl system and homogenization temperatures lower than 300 ºC. They have an alkaline character, possibly with a meteoric origin and circulated to a depth lower than 5 km during the final events of the Hercynian orogeny.

Keywords : Episyenitization; quartz dissolution; fluid inclusion planes.

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