Hydrothermal palaeofluid circulation in the fracture network of the Baksa Gneiss Complex of SW Pannonian Basin,Hungary

A well‐developed fracture‐filling network is filled by dominantly Ca‐Al‐silicate minerals that can be found in the polymetamorphic rock body of the Baksa Gneiss Complex (SW Hungary). Detailed investigation of this vein network revealed a characteristic diopside→epidote→sphalerite→albite ± kfeldspar→chlorite1 ± prehnite ± adularia→chlorite2→chlorite3→pyrite→calcite1→calcite2→calcite3 fracture‐filling mineral succession. Thermobarometric calculations (two feldspar: 230–336°C; chlorites: approximately 130–300°C) indicate low‐temperature vein formation conditions. The relative succession of chlorites in the mineral sequence combined with the calculated formation temperatures reveals a cooling trend during precipitation of the different chlorite phases (T_chlorite1: 260 ± 32°C →T_chlorite2: 222 ± 20°C →T_chlorite3: 154 ± 13°C). This cooling trend can be supported by the microthermometry data of primary fluid inclusions in diopside (T_h: 276–362°C) and epidote (T_h: 181–359°C) phases. The identical chemical character (0.2–1.5 eq. wt% NaCl) of these inclusions mean that vein mineralization occurred in a same fluid environment. The high trace element content (e.g. As, Cu, Zn, Mn) and Co/Ni ratio approximately 1–5 of pyrite grains support the postmagmatic hydrothermal origin of the veins. The vein microstructure and identical fluid composition indicate that vein mineralization occurred in an interconnected fracture system where crystals grew in fluid filled cracks. Vein system formed at approximately <200 MPa pressure conditions during cooling from approximately 480°C to around 150°C. The rather different fluid characteristics (T_h: 75–124°C; 17.5–22.6 eq. wt% CaCl₂) of primary inclusions of calcite1 combining with the special δ¹⁸O signature of fluid from which this mineral phase precipitated refer to hydrological connection between the crystalline basement and the sedimentary cover.