Trace element and isotopic characteristics of Ga-Ge-In bearing vein and breccia systems outboards of Caledonian erosional front

Phanerozoic vein- and breccia-hosted Zn-Pb mineralisation in western Västerbotten forms a distinct mineral system which is superimposed on Palaeoproterozoic orogenic Au deposits of the Gold line and VMS deposits of the Skellefte district. Previous research on these deposits has focused on geochronological and fluid inclusion characteristics, whereas little data exist on sphalerite trace element chemistry. We present new LA-ICP-MS data from sphalerite in occurrences at Rävliden, Nyliden, Lagbäcken and Svärträsk, supplemented with SF-ICP-MS, SEM, SIMS, EPMA, SEM-CL and EBSD analyses. Sphalerite-bearing veins crosscut all ductile fabrics and are mineralogically distinct from the VMS deposits. A polyphase is suggested by evidence for repeated events of brecciation and cementation by sphalerite, galena, calcite, zeolite, fluorite and quartz. Sphalerite displays oscillatory zoning, twinning, and evidence for recrystallisation and overgrowth. EBSD reveals subtle crystallographic misorientation and twin boundaries, which correlate spatially with preferential enrichment of Ga, Ge, Sb, and Cu along growth zoning and twinning. Presence of Ga- and Ge-rich nano inclusions are suspected and will be investigated using atom probe tomography (APT) LA-ICP-MS contents in sphalerite reach 385 ppm Ge, 196 ppm Ga, 2700 ppm Cd, and 66.3 ppm In, and are among the highest reported in sphalerite from Sweden. In contrast, sphalerite from nearby VMS exhibit significantly lower values, which do not exceed 0.49, 53.7, 1644 and 113 ppm respectively. A systematic west-to-east zonation is observed, with higher Ge and Ga concentrations in sphalerite from western localities (e.g. Svärträsk and Lagbäcken) than eastern localities (Rävliden and Nyliden), coincident with an earlier identified T decrease from 150°C to 80°C based on available fluid inclusion data. Stable (δ¹³C, δ¹⁸O, δ³⁴S) and Pb isotope data indicate precipitation from low-temperature, connate fluids, possibly mixed with organic-bearing fluids. Pb isotopic compositions are distinctly more radiogenic (²⁰⁷/²⁰⁴Pb ~16.2) than Skellefte VMS ores (²⁰⁷/²⁰⁴Pb ~15.1), consistent with Phanerozoic leaching of Pb from Paleoproterozoic rocks during either the opening of the Iapetus Ocean, the Timanian orogeny, or both. The results indicate previously overlooked exploration potential for Ga-, Ge-, and In-endowed Zn mineralization. The fine-scale heterogeneity in trace element distribution in sphalerite highlights the need for high-resolution analytical workflows to inform studies assessing primary ore as well as by-product potential and tailings revalorization. This research was carried out in the Impact Innovation programme Swedish Metals & Minerals, a joint initiative by the Swedish Energy Agency, Formas, and Vinnova, and within the ANR project Critical Metals in Orogens (ANR-23-CE01-0017). We thank as well the SFVE-A program 2024 for funding travels between France and Sweden and CAMM-CRM for additional support.