The Heavy-Metal Enrichment in Fault Zones of the South Alpine Basement: Implications for Excavation Waste Valorisation

The study area is located in southern Switzerland and forms part of the pre-Alpine basement of the Southern Alps. It is situated within a tectonically and structurally complex setting at the frontal zone of the south-verging thrust of the Upper Orobic Nappe over the Varesotto slices. The area is bounded to the west by the Variscan or post-Variscan Val Colla Fault and to the east by the Lugano Fault, which formed during Jurassic extension and was later reactivated during Alpine compression. The exposed lithologies are dominated by paragneisses, schists, and orthogneisses affected by Variscan amphibolite-facies metamorphism, locally preserving mafic relics recording eclogite-facies conditions. During the Alpine cycle, including Jurassic extension and Cenozoic collision, these rocks likely remained at temperatures below ca. 180 °C. Heavy-metal mineralisation, including gold occurrences, has traditionally been associated with the Val Colla Fault; however, its age remains poorly constrained and may be Permian, Jurassic, or Cenozoic.

Here, we present new geological and structural maps and cross-sections, combined with petrological investigations, whole-rock geochemical analyses (major and trace elements), and electron-microprobe data, to characterise heavy-metal mineralisation along a transect between the Val Colla and Lugano faults, where a new motorway tunnel is planned. Our results show that heavy-metal enrichment along the future tunnel trace is not pervasive, but rather localised and strongly structurally controlled. Mineralisation is preferentially associated with (i) NE–SW-striking faults with left-lateral strike-slip kinematics and a normal component, and (ii) NW–SE-striking normal faults. Enrichment is concentrated within ductile–brittle fault zones, particularly in dark cataclasites, locally graphite-rich, and in mylonitic gneisses, whereas light-coloured gneisses and porphyrites are largely barren.

Elevated concentrations of As, Sb, and Zn are linked to fine-grained sulfides (<100–200 µm), including arsenopyrite, bournonite, boulangerite, and sphalerite, hosted in intensely deformed rocks and carbonate veins formed during the circulation of alkaline, carbonate-rich fluids. Microprobe analyses indicate that vein carbonates are commonly iron-rich dolomite–ankerite and, locally, magnesium-rich siderite. Overall, our findings highlight deformation zones as the primary pathways and traps for heavy metals. Finally, we provide first-order estimates of heavy-metal concentrations in tunnel excavation waste to evaluate its potential as a source of sub-economic raw materials after selective treatment and enrichment aimed at increasing metal concentrations and reducing environmental risk.