Geochemistry and genesis of the Lengenbach Pb-Zn-As-Tl-Ba-mineralisation, Binn Valley, Switzerland

Abstract

Abstract. The Lengenbach Pb-Zn-As-TI-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 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, T1)-rich sulphide melt coexisted with a hydrothermal fluid at > 300 °C in this zone. Mineralised dolostones are anomalous in As, Pb, Ag, T1, 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, T1 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 2°6pb/2°*pb = 18.44 -18.56, 2°TPb/2°4Pb = 15.60 -15.75, 2°Spb/2°4pb = 38.44 -38.84 and ~534S (sulphide) = -25 + 2%0. S isotopic variations are largely a result of sulphide-sulphate re-equilibration yielding temperatures of 450 + 30°C. SVSr/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 Correspondence to: B.A. Hofmann 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.

Thallium Toxicity in Humans

“…Cosmic dust, although insignificant compared to other fl uxes, is another potential source of dissolved oceanic thallium with a maximum concentration of 150 ng g -1 (13). In the world there are a few areas with naturally high thallium concentrations such as the southwestern Guizhou Province in China (16), Allchar mine in Kavadarci, Macedonia (17), Lengenbach in Switzerland (18), and Rotokawa in New Zealand (19).…”

The chemistry of barium anomalies in the Berisal Complex, Simplon Region, Switzerland

“…Fortey and Beddoe-Stephens 1982). There is also a distinct lack in the zoisite-celsian gneiss of Fe, Mn and other base metals, which are commonly associated with such processes (Hofmann and Knill 1996 and references therein).…”

“…Barium anomalies in sedimentary rocks are common in black shales, in which up to 6,000 ppm Ba have been reported (Morand 1990;Vine and Tourtelot 1970), or in syn-sedimentary exhalative-type deposits associated with white smokers (Hofmann and Knill 1996). Anomalies in igneous rocks are lower than those in sedimentary environments, and are typically associated with intermediate to granitic rocks (i.e.…”

Lead Isotopes as Tracers of Metal Sources and Timing of the Carbonate-Hosted Pb-Zn Deposits in the Nappes Zone, Northern Tunisia

“…The plausibility of basement rocks as the deep-seated source of Pb in the studied ore deposits is supported by the similarity of Pb isotopic ratios between the Late Miocene igneous rocks and the abovementioned deposits, as well as the presence of jordanite, orpiment and realgar minerals. Jordanite is also present in the Lengenbach Pb-Zn-As-Tl-Ba deposits, which are hosted in Triassic dolostones in the Swiss Alps, for which it was proven that Pb and other metals were leached from basement rocks [24]. Another support for the plausibility of basement rocks as the deep-seated source of radiogenic Pb in the studied deposits is the presence of inherited faults connected to deep-seated faults cutting the Mesozoic-Cenozoic cover and the Triassic salts (Figure 3) and Pb for the Pb-Zn deposits in Jalta, Jebel Ghozlane, Jebel Hallouf, Oued Maden and Fedj Hassene compared to the Late Miocene igneous rocks and sedimentary country rocks from the Nefza area [21].…”