Hydrothermal five-element veins (Ag-Co-Ni-Bi-As) are mineral successions of native metals, encapsulated by Fe-Co-Ni arsenides and carbonates. Recent studies focused on the evolution from ordinary base-metal systems (sulfide-rich) to five-element veins (sulfide-poor) and revealed the importance of hydrocarbon-dominated fluids as an essential redox agent in these fluid systems. The fluid pH is stated to subdivide this type of mineralization into arsenide-, native arsenic- and native silver or

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... uth-dominated vein types, but the mineralogical diversity upon the arsenides (MeAs, MeAs2 and MeAs3) and their transition metal variations (Me: Fe, Co and Ni) are not well understood yet. Although different studies suggested fluid reduction by oxidation of ferrous iron from sulfides and silicates, the impact of siderite dissolution as redox mechanism and its potential to diversify mineralogical features and fluid properties (such as pH) has not been considered. This is the first case study explaining mineralogical, compositional, and textural features of unique Bi-Fe-Co-Ni-As-S-U mineralized siderite-dolomite-ankerite veins from the Middle Penninic basement of the Siviez-Mischabel Nappe (Valais Alps, Switzerland). Textural relationships, mineral chemistries, fluid inclusion compositions (microthermometry and Raman spectroscopy) and stable C-O-S-isotopes were combined to thermodynamic fluid evolution models to investigate changing fluid conditions (pH, temperature, metal-, arsenic- and sulfur-activities). Ore textures, C-O-isotopes and fluid inclusion studies indicate that the dissolution of primary siderite, oxidation of ferrous iron and its precipitation as magnetite was the most important redox couple to precipitate native Bi, arsenides and sulfarsenides (Bi0, As3- and As-1) from their oxidized aqueous species Bi3+Cl4-, and As3+(OH)3 at temperatures between 200-300°C. These processes and comparative Ca-Na variations suggest that a single fluid equilibrated with cover rocks during fluid descent and later interacted in-sit [...]