Ore-distal hydrothermal alteration zones are commonly suggested as a source of metals to ore-forming fluids. The Bergslagen ore district, Sweden exhibits extensive ore-proximal and ore-distal alterations and has been used as a typical locality for establishing the hydrothermal leaching model for volcanogenic massive sulphide (VMS) deposits. The ore-distal alteration in the region has been reported as depleted in ore-forming metals but robust mass change evaluations are lacking. Defining least-altered reference compositions is a major hurdle in Bergslagen due to compositional variation in the stratigraphy, extensive alteration, and high-grade metamorphic overprint. This study presents mass balance calculations for Na- and Mg-altered rocks in the Hällefors area using a set of systematically defined least-altered samples. Results show systematic mobility of light rare earth elements (LREE, here La-Eu; e.g., 80% of the Ce is mobilised during alteration which equates to 60 µg/g Ce), but no mobility of base metals. Precursor rock compositions have conspicuously low base metal concentrations (median: Zn 10 µg/g, Pb 2.5 µg/g; n = 13) compared to other volcanic centres in Bergslagen. Major base metal deposits occur in areas where least-altered volcanic rocks have higher base metal concentrations (e.g., Garpenberg; median: Zn 31.50 µg/g; Pb 11.75 µg/g; n = 10). The REE contents in least-altered rocks are relatively elevated in areas that host REE mineralisation such as the Riddarhyttan area. The results indicate that regional differences in metal fertility of the volcanic host succession may be a primary control on the metal enrichments, including REEs, occurring in the ore deposits throughout Bergslagen.

Hydrothermal alteration systems in ancient polymetallic terranes are commonly overprinted by deformation and metamorphism, obscuring genetic links between alteration and mineralisation and limiting their use as exploration vectors. The Riddarhyttan area hosts one of thelargest alteration halos in the Bergslagen lithotectonic unit. This Mg-enriched halo is spatially associated with Bastnäs-type Fe–REE-(Cu–Mo–Bi–Au), stratiform Fe-oxide, and sheet-like Fe-sulphide mineralisation, yet its metal fertility remains poorly constrained. This study uses whole-rock geochemistry from rhyolitic metavolcanics, principal component analysis and k-means clustering to objectively classify alteration into four groups validated by field, drill-core and petrographic observations (i.e. weakly altered, K-altered, Mg-altered, and Mg–OH-altered). Results show that both K- and Mg-alteration are dominated by feldspar-destructive processes, while Mg-alteration can be subdivided into a widespread cordierite-dominated assemblage reflecting mass loss and a structurally controlled Mg–OH assemblage characterised by significant mass gain through precipitation of hydrous Mg-rich minerals. Immobile-element constraints indicate systematic depletion of Fe, Pb, Zn, LREEs, Bi, and In in Mg-altered rocks, suggesting that this extensive alteration halo acted as an important regional metal source. Spatial distribution patterns and stratigraphic sequence indicate that Mg-alteration postdates earlier Na- and K-alteration and is broadly  ontemporaneous with the first phase of polymetallic mineralisation (Andersson et al. 2025). However, sulphide and REE mineralisation were subject to substantial post-ore remobilisation. These results highlight the exploration significance of large Mg-rich alteration systems in polymetallic Precambrian terranes.

Utö is a key reference locality in the Bergslagen mining district, preserving a well-exposed Palaeoproterozoic volcanosedimentary succession associated with stratiform Fe-oxide and polymetallic sulphide mineralisation. Despite its importance for regional ore genetic models, the mineralisation and associated hydrothermal alteration at Utö have not previously been documented using modern field-based and geochemical methods. This study presents new stratigraphic, petrographic, whole-rock geochemical, and sulphur isotope data from northeastern Utö, based on detailed field mapping and drill-core logging of felsic volcanic footwall rocks. The succession records a transition from detrital marine sedimentation to felsic volcaniclastic deposition, followed by volcano-distal ash-siltstone and carbonate sedimentation during waning volcanism in a shallow-marine basin. Stratiform magnetite–hematite and Zn–Pb–Ag sulphide mineralisation occur at the same narrow stratigraphic level within interbedded volcanic ash siltstones and limestones and preserve features consistent with submarine exhalative deposition. Sulphur isotope compositions (δ³⁴S = −8.9 to +7.2 ‰, V-CDT) overlap the range reported for stratiform sulphide deposits elsewhere in Bergslagen and indicate dominantly magmatic sulphur with variable seawater sulphate contribution. Hydrothermal alteration of the felsic volcanic succession is spatially heterogeneous and dominated by potassium enrichment, with locally developed stratabound and structurally controlled magnesium-rich alteration zones now expressed as cordierite due to amphibolite-facies metamorphism. Whole-rock geochemical data indicate depletion of iron and base metals in Mg-altered rocks, whereas K-altered rocks show no systematical metal depletion. Utö represents a small-scale, shallow-marine exhalative system that shares characteristics with exhalative iron oxide and sulphide deposit, as well as carbonate replacement deposits elsewhere in Bergslagen. The study of this deposit and its hydrothermal envelope help to define the range of hydrothermal ore forming systems and their environmental control in Bergslagen.

The Bergslagen Region in central Sweden hosts over 6000 metallic mineral deposits, most of them hosted by a Paleoproterozoic metavolcanic to metasedimentary succession. Iron oxide- and polymetallic sulfide-depositsare the most abundant and economically the most important deposits. Many deposits are enriched in Rare Earth Elements (REEs) and there is an ongoing drive to evaluate the REE resource in the region. The polymetallic sulfide deposits are thought to have formed by seawater-derived fluids which leached metals from the metavolcanic host rocks. This study investigates the trace metal mobility with specific focus on the critical metals (CMs), during this hydrothermal alteration in western Bergslagen. Preliminary results show that a broad range of trace elements and especially light rare earth elements (LREEs) were mobilized.

The Bastnäs ore field, in central Sweden, is the cradle of the rare earth elements (REE). It is the place of the discovery of several REE and important REE-minerals (e.g., Bastnäsite one of the primary REE-ore minerals). In recent years there has been an increased interest due to rising demand of REE for technological applications. Several recent studies have focused on the mineralogy and geochemistry but a lack of fresh in situ samples has meant that textural and stratigraphic relationships are not as well described. Recent exploration in the area has produced drill core traverses across the host stratigraphy of the Bastnäs deposit, allowing the collection of relatively fresh in situ samples which can be placed in lithological context. Here we present new mineralogical and textural information linked to the lithology indicating that the REE-mineralisation in Bastnäs is commonly associated with magnetite skarn and that it occurs over a wide range of stratigraphic levels.

Hydrothermal alteration halos are commonly used as vectors in mineral exploration, as they are often genetically linked to mineralization. The Riddarhyttan area in central Bergslagen is an example of a mining area situated in a strong alteration halo in felsic volcanic rocks which have undergone polyphase metamorphism and deformation. The area hosts historically important Bastnäs style REE deposits besides several iron oxides and polymetallic sulfide deposits. Mg-alteration has affected large parts of the host stratigraphy. However, because of metamorphism, it can be difficult to differentiate between alteration and protolith-related geochemical variations. To “see through” metamorphism, we focus on alteration in rhyolites using whole-rock geochemistry and data-driven classification to define and map out different alteration styles and investigate the mobility of ore related elements. The results show that grouping assisted by PCA and k- means clustering work well in a dataset limited to one precursor rock type, and that Mg-alteration surrounding the ore deposits has mobilized LREE’s from the volcanic rocks.