ALBERT

Notes: Abstract The Lengenbach Pb-Zn-As-Tl-Ba mineralisation is located in Triassic dolostones of the Pennínic zone in the Swiss Alps where Alpine metamorphism reached upper greenschist to lower amphibolite grade. Geochemical data are used to constrain the origin of this unique occurrence. Two metamorphic redox environments are present: the As(III)-rich zone is controlled by barite-pyrite while the reduced zone contains graphite or pyrrhotitepyrite and formally zerovalent As. The As(III)-rich zone is characterised by a mineral assemblage consistent withfO2 in the stability field of barite + pyrite. An As-(Pb, Tl)-rich sulphide melt coexisted with a hydrothermal fluid at 〉 300 °C in this zone. Mineralised dolostones are anomalous in As, Pb, Ag, Tl, Hg, Zn, Ba, Cd, Fe, Cu, Mo, U, V, B, Ga, Cr and possibly Sri and Au (in order of decreasing enrichment). As, Pb and Zn are present in the 0.1 to 1% range, Tl and Ag reach several hundred ppm. Uraninite is concentrated in silicate-rich bands and yields a late Alpine U-Pb age of 18.5 ± 0.5 Ma. Pb- and S isotopic variations are interpreted by metamorphic overprinting and re-equilibration within an isochemically metamorphosed mineralisation. Hydrothermal sulphides are more strongly affected by uranogenic Pb than massive Pb-As-sulphides representing a former sulphide melt. The least overprinted mineralisation is characterised by206Pb/204Pb = 18.44 – 18.56,207Pb/204Pb = 15.60 – 15.75,208Pb/204Pb = 38.44 – 38.84 and δ34S (sulphide) = − 25 ± 2%. S isotopic variations are largely a result of sulphide-sulphate re-equilibration yielding temperatures of 450 ±30 °C.87Sr/86Sr ratios of mineralised samples are lower than or equal to host dolostones, precluding major infiltration of basement-derived fluids during Alpine metamorphism. The Sr source (87Sr/86Sr close to 0.708) probably was seawater with a radiogenic, detrital mineral component. The genesis of the unique Lengenbach mineralisation is interpreted as the result of isochemical metamorphic overprinting of a carbonate hosted stratiform sulphide mineralisation. Well-crystallised sulphide minerals in fissures and druses formed during retrograde cooling of a sulphide melt in equilibrium with a hydrothermal fluid. The primary mineralisation was probably formed at or close below the sea floor and fed by sulphide-poor hydrothermal fluids. Sulphide was largely derived from seawater by open system bacterial sulphate reduction. U, V and Mo may be seawater-derived.

Notes: Abstract Massive and well-crystallized Sulfides of Fe, Pb, As, Tl, Ag, Cu and Zn occur in the Lengenbach mineral deposit (Valais, Swiss Alps), located in a low- to medium-grade metamorphosed Triassic dolomite. Inclusions arranged along healed fractures in quartz consist of complex intergrowths of Tl-rich sartorite+tennantite+orpiment and are interpreted as relics of a sulfide melt generated during metamorphism and trapped in a similar way to associated aqueous fluid inclusions. Microprobe analyses of melt inclusions homogenized at 500°C demonstrate a strong enrichment of Tl in the melt phase, consistent with the observed late-stage Tl-enrichment in the deposit. The presence of a sulfide melt is compatible with Alpine metamorphism of the Lengenbach deposit under relatively closed conditions. The unique sulfide mineral assemblage of the Lengenbach deposit can be ascribed to fractional crystallization in a slowly cooling melt-aqueous fluid system during Alpine uplift.

Notes: Summary The mineralogy and geochemistry of reduction spheroids from continental and marine red beds of Europe, North America and Oman were investigated. Reduction spheroids are spheroidal, isolated reduction sites in hematitic rocks. They consist of a mineralized core (0.1 to 5 cm diameter) and a hematite-dissolution halo (1 to 20 cm diameter). Irrespective of origin and age of host rocks, all reduction spheroid cores show a very similar mineralogy dominated by the vanadian mica roscoelite and a similar pattern of element enrichment relative to their host rocks dominated by V and U. Element enrichments in most reduction spheroids are very similar to those of sandstone-hosted vanadium-uranium deposits except for a lack of a molybdenum enrichment. Isotopically light sulfide of in-situ, low-temperature origin is an indication for the involvement of bacterial sulfate reduction during reduction spheroid formation.

Notes: Abstract.  The Lengenbach Pb-Zn-As-Tl-Ba mineralisation is located in Triassic dolostones of the Penninic zone in the Swiss Alps where Alpine metamorphism reached upper greenschist to lower amphibolite grade. Geochemical data are used to constrain the origin of this unique occurrence. Two metamorphic redox environments are present: the As(III)-rich zone is controlled by barite-pyrite while the reduced zone contains graphite or pyrrhotite-pyrite and formally zerovalent As. The As(III)-rich zone is characterised by a mineral assemblage consistent with fO2 in the stability field of barite+pyrite. An As-(Pb, Tl)-rich sulphide melt coexisted with a hydrothermal fluid at 〉kk300 °C in this zone. Mineralised dolostones are anomalous in As, Pb, Ag, Tl, Hg, Zn, Ba, Cd, Fe, Cu, Mo, U, V, B, Ga, Cr and possibly Sn and Au (in order of decreasing enrichment). As, Pb and Zn are present in the 0.1 to 1% range, Tl and Ag reach several hundred ppm. Uraninite is concentrated in silicate-rich bands and yields a late Alpine U-Pb age of 18.5±0.5 Ma. Pb- and S isotopic variations are interpreted by metamorphic overprinting and re-equilibration within an isochemically metamorphosed mineralisation. Hydrothermal sulphides are more strongly affected by uranogenic Pb than massive Pb-As-sulphides representing a former sulphide melt. The least overprinted mineralisation is characterised by 206Pb/204Pb U003U=18.44−18.56, 207Pb/204Pb=15.60−15.75, 208Pb/204Pb =38.44−38.84 and δ34S (sulphide)=−25±2‰. S isotopic variations are largely a result of sulphide-sulphate re-equilibration yielding temperatures of 450± 30 °C. 87Sr/86Sr ratios of mineralised samples are lower than or equal to host dolostones, precluding major infiltration of basement-derived fluids during Alpine metamorphism. The Sr source (87Sr/86Sr close to 0.708) probably was seawater with a radiogenic, detrital mineral component. The genesis of the unique Lengenbach mineralisation is interpreted as the result of isochemical metamorphic overprinting of a carbonate hosted stratiform sulphide mineralisation. Well-crystallised sulphide minerals in fissures and druses formed during retrograde cooling of a sulphide melt in equilibrium with a hydrothermal fluid. The primary mineralisation was probably formed at or close below the sea floor and fed by sulphide-poor hydrothermal fluids. Sulphide was largely derived from seawater by open system bacterial sulphate reduction. U, V and Mo may be seawater-derived.