Gold-bearing quartz veins of the Bělčice ore district, Bohemian Massif: evidence for incursion of metamorphic fluids into a granodiorite body and for isothermal mixing between two types of metamorphic fluids
Gold-bearing quartz veins of the Bělčice ore district are hosted by I-type Variscan Blatná granodiorite of the Central Bohemian Plutonic Complex. The quartz gangue precipitated from aqueous-carbonic fluids (3-7 wt. % NaCl eq.) with significant methane admixture (up to 15 mol. % of CH4) at c. 350-400 °C and 200-400 MPa. The ore stage including gold precipitation occurred at lower temperatures (300 to 250 °C) and pressures (~ 100 MPa) from aqueous fluids. The isotope composition of the quartz and carbonate gangue indicates progressively heavier oxygen of the parental fluid (δ18Ofluid from ~ 9-10 ‰ to 10-14 ‰ SMOW). This suggests ingress of metamorphic fluids equilibrated with metasedimentary country rocks into an environment dominated by I-type granitoids. Invasion of external metamorphic fluids is also supported by nearly isothermal mixing trends in aqueous fluids as documented by salinity variations (from 5.5 to 0 wt. % NaCl eq.).
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