Mineralogy, fluid inclusions, C, O, S, Sr isotopes and trace elements have been studied in amygdule and vein mineralization hosted by the Lower Cretaceous effusive picrite at Honcova hurka (Silesian Unit, Flysch Belt of the Outer Western Carpathians). Besides dominating dolomite, magnesite, siderite, quartz, calcite, chlorite, glauconite, fluorite, barite, pyrite and millerite were also identified. The parent fluids are characterized by low temperatures (<50–170 °C), low salinities (0.4 to 3.7 wt. % NaCl eq.), low content of strong REE-complexing ligands, δ ¹⁸O, δ ¹³C and δ³⁴S ranges of 0/+14 ‰ SMOW, 0/–9 ‰ PDB and ~0 ‰ CDT, respectively, and initial 87Sr/86Sr ratios much more radiogenic (0.7060 to 0.7068) than those of host picrite (0.7042 and 0.7046). The fluids are interpreted to be predominantly of external origin, derived from mixing of seawater with diagenetic waters produced by dewatering of clay minerals in the associated flysch sediments. The isotope and REE signatures indicate interaction of at least a part of fluids with sedimentary carbonates. The interaction of fluids with host picrite led to strong alteration of rock-forming minerals and leaching of some elements (Mg, Ni, S, partly also REE) that widely participated during precipitation of vein- and amygdule-hosted mineral phases.