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 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 bismuth-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-situ with preexisting siderite veins and adjacent host rocks within the basement (i.e. cover rocks: carbonate, sulfate and halite dissolution; basement rocks: albitisation of plagioclase, alteration of biotite and hornblende).
Shifts of transition metal signatures from Fe- to Co-Ni- and lastly to ternary Fe-Co-Ni-dominated compositions, show a first dominance of siderite dissolution and further increased host rock mobilization during the successive evolution of the hydrothermal system. From primary to the secondary ores, stable S-isotopes and As/S-signatures of sulfides and sulfarsenides indicate an increasing sulfur mobilization from host rock sulfides (i.e. fahlbands).
Since most five-element veins spatially and temporally link to large scale rifting, information is limited about their behavior during metamorphism or the possibility of a primary formation during compressive orogenic cycles. In the present case, the comparison of complex zoned microstructures, mineral chemistries and in-situ U-Pb ages reveal the processes and timing of primary ore formation, interaction with the sulphide-bearing host rocks and secondary modifications by deformation and elemental redistribution during the Alpine Orogeny. It was possible to determine U-Pb Tera-Wasserburg ages of paragenetic mineral fractions of carbonates, magnetite and multi-mineral isochrons including Fe-Co-Ni-arsenides, -sulfarsenides, Fe-Ti-oxides, chlorite, albite and annabergite. Observed ages constrain the classification of stages I-VI, at which primary precipitation of native Bi, arsenides and sulfarsenides happened during Triassic and Jurassic times (stages II and IV) and secondary redistribution processes of these ores affected by Alpine processes (stages Va and Vb). These stages of five-element vein precipitation and corresponding mechanisms of their primary formation and remobilization are:
Stage II: Native Bi-löllingite-skutterudite-magnetite-dolomite precipitation from pristine sedimentary brines dissolving earlier siderite at 233±10 Ma;
Stage IV: native Bi-niccolite-gersdorffite-skutterudite-magnetite-ankerite ores, which indicate progressive fluid-host rock interaction and consequential influx of Ni, Co and S at 188±32 Ma;
Stage Va: in-situ recrystallization and overprint of the primary assemblages as ternary Fe-Co-Ni sulfarsenides accompanied by prograde metamorphism and further mobilization of the host rock sulfides between 71.1±1.4 and 44.6±1.5 Ma; and
Stage Vb: native Bi-safflorite-cobaltite-skutterudite stage with similar textures and mineralogies to the primary stages and common for five-element veins at 28.2±1.1 Ma.
Based on compared U-Pb ages, paleotectonic reconstructions, deformation textures, mineralogical variabilities and chemical variations, following tectonic processes caused the primary Triassic formation and its succession to post-Alpine tectonics:
Stage II-IV: Primary and pre-Alpine ore formation due to rift-induced extensive tectonics during the Meliata and Alpine Tethys Rifts;
Stage Va: syn-Alpine in-situ remobilization and deformation during European-Adriatic continent-continent collision; and
Stage Vb: late-Alpine neoformation of five-element mineralogies and native Bi remobilization due to lateral extrusion and transtensional extension of the Western Alpine arc.
The comparison of hydrochemical and mineralogical trends of the very special occurrences in the Penninic Alps to similar arsenide-dominated five-element veins in Central Europe reveals that frequently occurring mineralogical shifts between mono-, di- and triarsenides as well as the Ni→Co→Fe transition established due to simple fluid reduction of hydrothermal system. Due to differences in the fluid chemistries and pH conditions, this work suggests a further and general subdivision of arsenide-dominated five element veins into the following classes:
1. Fe-Co arsenide-dominated ores
2. Ni-Co arsenide-dominated ores
3. Ni-Co-Fe arsenide-dominate ores
Hydrothermal ore
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