Near vein metasomatism along propylitic veins in the Baksa Gneiss Complex, Pannonian Basin, Hungary

Abstract

In many parts of the metapelitic (gneiss, mica schist) rock section of the Baksa Complex, significant wall-rock alteration is observable along the Ca-Al silicate veins, which show a di → ep ± czo →sp → ab ± kfs → chl → adu → prh → py → cal mineral sequence (FINTOR et al., 2009). These alterations appear as narrow (few cm thick) bleached margins beside thin veins, and broad alteration bands along thick veins where detailed epidotization and chloritization of the adjacent rock are recognizable. Based on petrographic and mineralogical examination of the altered wallrocks, metasomatic zones with characteristic mineral paragenesis can be distinguished: Zone 1 (ab + ttn ± ep), Zone 2 (ep + chl+ ttn + ab ± ser), Zone 3 (chl + ep + ser + rt ± ttn), Zone 4 (ser ± chl). Bulk rock chemical analyses were made from the different metasomatic zones. The results show that fluid circulated in the propylitic veins caused metasomatic alteration of the wall-rock, with transport of considerable amount of Ca²⁺ toward the adjacent rocks. The hydrothermal leaching almost totally removed the K, Fe, Mg, and Mn ions from the wall rock. The main alteration processes are the epidotization and chloritization of biotite, and albitization of micas (muscovite + biotite) content of metapelites. Based on mobilization of different cations alteration was due to? to a near neutral fl uid (~pH 5–7). The pervasive hydrothermal leaching caused significant secondary porosity (cavities) in the altered domains, which were partially filled by epidote. Fluid inclusions of cavity filling epidote indicate a similar character (Th: 180–360 °C; Salinity: 0.2–1.6 mass% eq. NaCl) to that can be found in Ca-Al silicate veins. The alteration most probably occurred in the 360–480 °C temperature range as products of 'near vein metasomatism' and the altered rock can be related to the propylite metasomatic family.