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

摘要

A well-developed fracture-filling network is filled by dominantly Ca-Al-silicate minerals that can be found in thenpolymetamorphic rock body of the Baksa Gneiss Complex (SW Hungary). Detailed investigation of this veinnnetwork revealed a characteristic diopside fi epidote fi sphalerite fi albite ± kfeldspar fi chlorite1 ± preh-nnite ± adularia fi chlorite2 fi chlorite3 fi pyrite fi calcite1 fi calcite2 fi calcite3 fracture-filling mineral suc-ncession. Thermobarometric calculations (two feldspar: 230–336u0002C; chlorites: approximately 130–300u0002C) indicatenlow-temperature vein formation conditions. The relative succession of chlorites in the mineral sequence combinednwith the calculated formation temperatures reveals a cooling trend during precipitation of the different chloritenphases (Tchlorite1: 260 ± 32u0002C fi Tchlorite2: 222 ± 20u0002C fi Tchlorite3: 154 ± 13u0002C). This cooling trend can be sup-nported by the microthermometry data of primary fluid inclusions in diopside (Th: 276–362u0002C) and epidote (Th:n181–359u0002C) phases. The identical chemical character (0.2–1.5 eq. wt% NaCl) of these inclusions mean that veinnmineralization occurred in a same fluid environment. The high trace element content (e.g. As, Cu, Zn, Mn) andnCo ⁄Ni ratio approximately 1–5 of pyrite grains support the postmagmatic hydrothermal origin of the veins. Thenvein microstructure and identical fluid composition indicate that vein mineralization occurred in an interconnectednfracture system where crystals grew in fluid filled cracks. Vein system formed at approximately <200 MPa pres-nsure conditions during cooling from approximately 480u0002C to around 150u0002C. The rather different fluid characteris-ntics (Th: 75–124u0002C; 17.5–22.6 eq. wt% CaCl2) of primary inclusions of calcite1 combining with the special d18nOnsignature of fluid from which this mineral phase precipitated refer to hydrological connection between the crystal-nline basement and the sedimentary cover.