Reconstruction of a >200 Ma multi-stage “five element” Bi-Co-Ni-Fe-As-S system in the Penninic Alps, Switzerland

Abstract

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 essential redox agent in these systems. Although mineral successions reveal the natural subdivision into native As-, native Ag/Bi- or arsenide-dominated vein types and suggested processes explain the mineralogical variation to a certain degree, the Ni-Co-Fe-variations among the arsenides are not well understood yet.This is the first case study explaining compositional, mineralogical, and textural features of five-element veins by changing metal contents, arsenic/sulfur activities, pH and temperatures, applied to a multi-stage vein mineralization in the Penninic Alps, Switzerland. Textural relationships, mineral chemistry, fluid inclusion compositions (microthermometry and Raman spectroscopy), stable S-isotopes and in-situ U-Pb age dating of carbonates, magnetite and multi-mineral isochrons were investigated.U-Pb ages of paragenetic mineral fractions constrain a primary formation of löllingite-skutterudite-dolomite-dominated ores at 233 ± 10 Ma and niccolite-gersdorffite-skutterudite-ankerite-dominated ores at 188 ± 32 Ma, which links their formation to crustal thinning caused by the breakup of the Meliata ocean and Alpine Tethys. As secondary processes during the Alpine Orogeny, a in-situ remobilization of the ores occurred as mostly ternary Fe-Co-Ni sulfarsenides at ∼73–24 Ma due to continent-continent collision and neoformations of safflorite-cobaltite-skutterudite-dominated ores at ∼29–16 Ma due to transtensional strike-slip tectonics.Ore textures indicate that the dissolution of primary siderite, oxidation of ferrous iron and its precipitation as magnetite was the 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 and 300 °C. Ni and Co signatures of the arsenides/sulfarsenides show increasing mobilization from the host rocks during the successive evolution of the hydrothermal system in the Triassic, Jurassic and during the Alpine Orogeny. Stable S-isotopes and As/S signatures of the sulfarsenides indicate an increasing mobilization of host rock sulfides (i.e. fahlbands) from the primary to the secondary ores. Fluid inclusions and stable S-isotopes suggest involvement of fluid modification by water-rock interactions (i.e. cover rocks: carbonate, sulfate and halite dissolution; basement rocks: albitisation of plagioclase) during fluid descent.