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.

Arc environments are locally gold-endowed and it has been long hypothesized that gold mobilization during slab dehydration could affect mantle composition and arc Au-fertility. Here, we determine gold mobility during prograde high pressure-low temperature metamorphism of both metavolcanic and metasedimentary rocks from the islands of Santorini, Ios, Naxos and Syros (Greece). These rocks experienced peak greenschist to upper-blueschist/eclogite facies metamorphism, allowing determination of large-scale gold mobility within a subduction zone. Our data shows that gold is significantly mobilized by fluids at the blueschist-eclogite facies transition. In the associated mélange zones at the slab-mantle interface gold shows heterogeneous concentration but is not enriched, implying only partial retention of gold in the mélange and its mobilization across the slab-mantle interface. Furthermore, the intrinsically complex lithologies and heterogenous gold concentrations of these mélange zones contribute to the slab component inherent to arc magmatism, and can affect magmatism gold-fertility.

Mercury (Hg), with its ultra-trace abundance in rocks and high volatility, challenges the investigation of its distribution and mobilisation in geological systems. Based on similar Hg isotope mass independent fractionation (i.e. Δ¹⁹⁹Hg) values between marine and terrestrial sediments and magmatic rocks, it is commonly suggested that Hg has been continuously exchanged between the Earth’s mantle and the atmosphere through subduction recycling. However, a review of Hg concentrations in ophiolites and orogenic peridotites, mantle xenoliths, mantle derived basalts and mafic rocks (Mg# ≥60), and variably metamorphosed collision and subduction related rocks suggests otherwise. Ultra-trace Hg concentrations in mantle rocks and mantle derived magmas are inconsistent with Hg recycling into the mantle. In fact, collision and subduction related rocks progressively lose Hg with increasing metamorphic grade, indicating that only ≤1 ng/g of Hg is transferred to the sub-arc mantle melting regions. These observations also suggest that Hg recycling was inhibited during the Archean. Here we warn about the risk of using Hg isotopes alone to interpret the Hg cycle on Earth and urge the need for new and accurate Hg concentrations in crystalline rocks from different geological settings.

Shallow-crustal magma emplacement produces characteristic host rock deformation within a “structural aureole”. Two end-member mechanisms of magma emplacement dominate in the brittle crust: roof uplift, involving upward displacement of overlying rocks, and floor subsidence, involving downward displacement of the underlying rocks—either along tectonic faults or ring faults in cauldron subsidence. While these processes are fundamental to crustal growth, heat transfer, and ore formation, their identification in the field is often based on ambiguous criteria such as intrusion geometry or internal fabrics, leading to inconsistent classifications. We propose a robust set of structural criteria for distinguishing emplacement mechanisms of upper crustal (<5 km) intrusions. These are applied to the Slaufrudalur pluton, Iceland's largest exposed granitic intrusion, using structural mapping and photogrammetry. Previous interpretations framed Slaufrudalur as a classic cauldron subsidence pluton or a sill complex. Our results reveal that the pluton formed in a transtensional setting and is bound by strike-slip faults, fundamentally revising its geotectonic interpretation. This study underscores the value of detailed structural investigation of host-rock deformation for resolving emplacement processes and provides a transferable framework to link pluton architecture and tectonic setting in shallow crustal exposures, with implications for understanding magma transport, geothermal systems, and mineralisation.

Complex breccia host rocks, quartz-topaz alumina-rich alteration, and low-sulfide ore (<3%) differentiate the Åkulla Au-Te deposit from nearby volcanogenic massive sulfide (VMS) deposits, which are the main deposit type in the Skellefte district. At Åkulla, mineralization comprises native gold and telluride minerals in disseminated, remobilized fracture-associated, quartz vein–associated, and sulfide-associated styles within a steeply dipping lens-shaped domain of strongly altered 1.9 Ga dacitic to andesitic volcanic rocks. The host rocks and mineralization are flattened and elongated subparallel to the main D₂ tectonic foliation and lineation and are metamorphosed to upper greenschist facies. The deposit has an almost concentrically zoned hydrothermal alteration pattern with a quartz-topaz–rich core that is centered on a breccia complex. Topaz occurs as discrete altered clasts in the breccias as well as pervasive to patchy alteration. The quartz-topaz-sericite-andalusite core is poor in sulfide and passes outward to sericite-andalusite ± corundum ± quartz ± chlorite associations, then to sericite and finally chlorite. Stringer veins and small semimassive to massive base metal sulfide lenses occur in the alteration zones outside the topaz core, continue ~300 m updip to the Åkulla Östra massive sulfide deposit, and resemble footwall alteration and mineralization associated with nearby VMS deposits.

Four main breccia facies are identified based on textural characteristics: monomict jigsaw-fit breccia; poorly sorted, dominantly polymict breccia with rotated clasts; moderately sorted matrix-supported polymict breccia; and well-sorted breccias with graded bedding. These breccias are attributed to quench and hydrothermal brecciation in a sea-floor to subseafloor setting during emplacement of a dacitic-andesitic volcanic complex. Clast size grading within breccia facies with topaz-rich clasts is dominated by fining to the northeast, the same direction as in nearby volcaniclastic sedimentary rocks, indicating that these breccias were emplaced while the strata were flat lying or gently dipping. This constrains the formation of the topaz-rich breccias and Au-Te mineralization to the early synvolcanic evolution of the succession, possibly prior to or during rotation of strata in extensional fault blocks. The topaz- and alumina-rich alteration of the breccias is attributed to acidic, F-rich, probably magmatic-hydrothermal fluids. The Åkulla Au-Te deposit represents unusual, F-rich, subseafloor synvolcanic mineralization within a larger VMS mineral system. The relationship we demonstrate between VMS mineralization, sulfide-poor Au-Te mineralization, complex breccias, and magmatic ore fluids at Åkulla will modify exploration models for VMS mineralization in the Skellefte district and should be considered in other VMS districts.

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.

The southern Superior Province (Canada) comprises large metasedimentary belts, such as the Pontiac and Quetico subprovinces, adjacent to Au-endowed greenstone belts. The Pontiac sedimentary rocks have been proposed as a source of Au for the highly endowed southern Abitibi greenstone belt. However, a comparison with sedimentary rocks adjacent to poorly endowed greenstone belts is lacking. Here we examine a suite of metasedimentary and minor volcanic rocks collected from three transects in the Pontiac and Quetico metasedimentary belts adjacent to greenstone belts with variable Au endowments. We combine in situ Au data of sulfides in metasedimentary rocks with whole-rock ultra-low-detection Au data. Gold concentrations decrease across metamorphic isograds in the well- and moderately endowed transects, while limited Au mobility is observed in the poorly endowed transect. The release of Au from metasedimentary rocks is linked to the pyrite-pyrrhotite transition. This reaction is incomplete in the poorly endowed transect, explaining the limited Au mobility observed. Our data reveal a spatial correlation between Au mobility in metasedimentary belts and Au endowment in the adjacent greenstone belts.

Magma reservoirs typically form through the incremental emplacement of smaller magma pulses over extended timescales. Pulsed reservoir growth significantly impacts a magma body's temperature evolution, chemical differentiation potential, and the probability, scale, and timing of volcanic eruptions. Moreover, the addition of thermal energy and magmatic fluids reheat and hydrothermally alter previously emplaced magma. Consequently, it may be difficult to distinguish individual magma pulses in exposed solidified intrusions (plutons), obscuring evidence of magma body construction and evolution. In this study, we employ geological mapping combined with petrofabric and Anisotropy of Magnetic Susceptibility (AMS), Anisotropy of Anhysteretic Remanent Magnetization (AARM), hysteresis, First-Order Reversal Curves (FORCs) and susceptibility versus temperature analyses to investigate pulsed magma emplacement and its consequences in terms of fabric overprinting and hydrothermal alteration within the Slaufrudalur pluton in Southeast Iceland. The field mapping documents distinct emplacement styles, including magma ascent in marginal zones, subhorizontal sheet emplacement, and bulk intrusion below the sheets. The AMS fabrics show high K_m values (∼1 × 10⁻² SI), but overall weak degrees of anisotropy (P_j < 2%). The weak magnetic fabrics reflect the destructive interference between the magnetite fabric and the fabric of hematite and iron hydroxides. Later, pulses of magma are less oxidized, which indicates that the alteration was caused by volatile release from magma that intruded below already emplaced magma. Our results demonstrate that rock magnetic data provide a novel approach to detecting magma pulse interactions and associated alteration in plutons, offering insights into magma body dynamics.

In response to the rising needs for long-term research and innovation in the field of critical raw material exploration, the Smart Exploration Research Centre was established in 2024 in Sweden. Funded by the Swedish Foundation for Strategic Research (SSF), this initiative involves collaboration among academic institutions, industry, and the public sector. Building on the H2020-funded Smart Exploration project, which involved 27 European organisations including the European Association of Geoscientists and Engineers (EAGE), the centre aims to advance the global standing of Sweden's exploration. It seeks to gather skills and create a network that will leave a lasting legacy in the field of mineral exploration. The multidisciplinary centre aims to be a fast-track hub for addressing exploration challenges in the mining industry through synergistic efforts. It connects exploration with mineral processing and nanotechnology to enhance environmental studies and develop effective extraction and beneficiation methods.

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.