This study investigates the mineralogy and texture of gold-bearing phases in the Lappberget deposit, Garpenberg Mine, and how these characteristics affect gold recovery during mineral processing. Multiple methods such as optical microscopy, SEM-EDS, EPMA, LA-ICP-MS, and bulk chemical analysis were applied on drill core samples, and samples from the processing plant’s Knelson gravity concentrator. Electrum-type alloys were recognized as the most common gold hosts. 

By-product metals have a significant potential to bring additional economic benefits to mines. However, a detailed characterization of their distribution is generally required to fulfill this potential and should preferably be integrated into a geometallurgical assessment. This contribution presents a detailed mineralogical and textural investigation of gold-bearing phases at the footwall of the Zn–Pb–Ag–(Cu–Au) Lappberget Deposit, Garpenberg Mine, Sweden, using optical microscopy, scanning electron microscope with energy dispersive spectroscopy (SEM-EDS), electron probe microanalysis (EPMA), laser ablation-inductively coupled plasma mass spectrometer (LA-ICPMS), and bulk chemical analysis applied to drill core and Knelson concentrator samples. Gold is a by-product at the Garpenberg mine, but it is unclear how the mineralogy, occurrence, and distribution of gold-bearing phases impact on gold recoveries during mineral processing. Our results show that Au-dominant electrum is the most abundant gold-bearing phase in the footwall of the Lappberget deposit, occurring strongly associated to sulfides in a variety of textures and grain sizes. Electrum grains commonly occur within sulfide borders, as inclusions, intergrowth and overgrowths of chalcopyrite, pyrite, galena, sphalerite, and pyrrhotite. Gangue minerals may also contain disseminated electrum and inclusions. Electrum grain sizes range from ∼5 µm to 300 µm, predominantly below 100 µm. The potential of sulfide lattice-bound invisible gold in the form of solid-solution gold and colloidal gold was also investigated, showing Au depletion within the analyzed sulfide carriers. The analysis of the concentrate samples from the Knelson gravity concentrator showed 584 and 431 ppm of gold content. High degree of liberation is observed among the gold-bearing phases in the concentrate, and gold recovery is highest among fractions coarser than 106 µm mesh. Pyrite and galena are the most abundant minerals in the concentrate samples. The gold-bearing phases were categorized based on its mineralogy, texture, grain size, and association and their influence on gold processing, especially textures and grain size, which implicates its liberation in milling and recovery by the gravity separator.

The Palaeoproterozoic Skellefte mining district in northern Sweden is one of the most important mining regions in Europe and, as a result of decades of exploration in this area, there is an extensive collection of geochemical analyses from drill holes in this area. This research compiles data from over 3,000 samples from the Kristineberg area of this district to create an overview of the alteration patterns in 3D, and, for the first time ever, this data is being compared with the regional structural 3D model, which has also been developed for this area, to determine to what extent the current structure dominates the alteration pattern. Structurally-constrained 3D interpolations of calculated alteration indexes of drill core and outcrop samples from the hydrothermally altered zones in this area reveals an excellent correlation (from surface to c. 1,000 m depth) between the zones of most intense alteration and the localization of massive sulphide deposits. The results furthermore suggest that most of the alteration zones at surface are continuous with alteration zones at depth (possibly even deeper than 1,000 m). Comparison of the geometries and spatial distribution of these 3D interpolation volumes with 3D-modelled regional faults and lithological contacts in the Kristineberg area suggest that the regional distribution of alteration zones is controlled to a significant extent by the regional structure of the area, in particular by major S-dipping faults. Consequently, the structurally- constrained 3D geochemical model presents a new and exciting tool for the identification of prospective 3D volumes in the Kristineberg area for deep exploration. In addition, the 3D approach will allow quantifications of the total budget of mass gain and loss during hydrothermal alteration in the Kristineberg area, which will allow fundamental questions regarding the nature of the hydrothermal systems and the source of elements to be answered.

A 3D geochemical model of the Kristineberg area of the Skellefte District, Sweden, is currently under construction, utilizing data from more than 1600 regionally distributed whole-rock lithogeochemical samples. The model will improve our understanding of the formation the VHMS deposits in this area. The model is built by mapping geochemical variations in 3D, and using this as a basis for modelling hydrothermal alteration in the unsampled portions of the rock column. A better understanding of the geometry, intensity, vectors of transport, and zonation of the hydrothermal zones in 3D will aid deep exploration for massive sulphide deposits in the Kristineberg area, and may potentially lead to new discoveries.

This contribution presents a 3D assessment of metamorphosed and deformed, hydrothermally altered volcanic rocks, hosting the massive sulphide deposits of the Kristineberg area in the 1.9 Ga Skellefte mining district in northern Sweden, using six calculated alteration parameters: the Ishikawa alteration index, the chlorite–carbonate–pyrite index and calculated net mass changes in MgO, SiO2, Na2O and Ba. The results, which are also available as film clips in the Supplementary data, confirm inferences from geological mapping; namely that the sericite- and chlorite-rich alteration zones have complex and cross-cutting geometries and that most of these zones are semi-regional in extent and range continuously from surface to over a kilometre deep. The major known massive sulphide deposits occur proximal to zones characterised by coincidence of high values for the alteration index and chlorite–carbonate–pyrite index and large MgO gains, which corresponds to zones rich in magnesian silicates. These zones are interpreted as the original chlorite-rich, proximal parts the alteration systems, and form anomalies extending up to 400 m away from the sulphide lenses. In addition, the stratigraphically highest VHMS are hosted by rocks rich in tremolite, talc, chlorite and dolomite with lesser clinozoisite, which have high chlorite–carbonate–pyrite index and low–medium alteration index values, reflecting a greater importance of some chlorite-carbonate alteration at this stratigraphic level. Vectoring towards massive sulphide deposits in this area can be improved by combining the AI and CCPI indexes with calculated mass changes for key mobile elements. Of the ones modelled in this study, MgO and SiO2 appear to be the most useful.

Zinkgruvanite, ideally Ba₄Mn²⁺4Fe³⁺₂(Si₂O₇)₂(SO₄)₂O₂(OH)₂, is a new member of the ericssonite group, found in Ba-rich drill core samples from a sphalerite+galena- and diopside-rich metatuffite succession from the Zinkgruvan mine, Örebro county, Sweden. Zinkgruvanite is associated with massive baryte, barytocalcite, diopside and minor witherite, cerchiaraite-(Al) and sulfide minerals. It occurs as subhedral to euhedral flattened and elongated crystals up to 4 mm. It is almost black, semi-opaque with a dark brown streak. The luster is vitreous to sub-adamantine on crystal faces, resinous on fractures. The mineral is brittle with an uneven fracture. VHN100 = 539 and HMohs ~4½. In thin fragments, it is reddish-black, translucent and optically biaxial (+), 2Vz > 70°. Pleochroism is strong, deep brown-red (E ⊥ {001} cleavage) to olive-pale brown. Chemical point analyses by WDS-EPMA together with iron valencies determined from Mössbauer spectroscopy, yielded the empirical formula (based on 26 O+OH+F+Cl anions): (Ba_4.02Na_0.03)Σ_4.05(Mn_1.79Fe²⁺_1.56Fe³⁺_0.42Mg_0.14Ca_0.10Ni_0.01Zn_0.01)Σ_4.03 (Fe³⁺_1.74Ti_0.20Al_0.06)Σ_2.00Si₄(S_1.61Si_0.32P_0.07)Σ_1.99O₂₄(OH_1.63Cl_0.29F_0.08)Σ_2.00. The mineral is triclinic, space group P–1, with unit-cell parameters a = 5.3982(1) Å, b = 7.0237(1) Å, c = 14.8108(4) Å, α = 98.256(2)º, β = 93.379(2)º, γ = 89.985(2)º and V = 554.75(2) Å3 for Z = 1. The eight strongest X-ray powder diffraction lines are [d Å (I%; hkl)]: 3.508 (70; 103), 2.980(70; 11–4), 2.814 (68; 1–22), 2.777 (70; 121), 2.699 (714; 200), 2.680 (68; 20–1), 2.125 (100; 124, 204), 2.107 (96; –221). The crystal structure (R1 = 0.0379 for 3204 reflections) is an array of TS (titanium silicate) blocks alternating with intermediate blocks. The TS blocks consist of HOH sheets (H = heteropolyhedral, O = octahedral) parallel to (001). In the O sheet, the Mn²⁺-dominant MO(1,2,3) sites give ideally Mn²⁺₄ pfu. In the H sheet, the Fe³⁺-dominant MH sites and AP(1) sites give ideally Fe³⁺₂Ba₂ pfu. In the intermediate block, SO₄ oxyanions and eleven coordinated Ba atoms give ideally 2 × SO₄Ba pfu. Zinkgruvanite is related to ericssonite and ferro-ericssonite in having the same topology and type of linkage of layers in the TS block. Zinkgruvanite is also closely compositionally related to yoshimuraite, Ba₄Mn₄Ti₂(Si₂O₇)₂(PO₄)₂O₂(OH)₂, via the coupled heterovalent substitution 2 Ti⁴⁺ + 2 (PO₄)^3- →2 Fe³⁺ + 2 (SO₄)^2-, but presents a different type of linkage. The new mineral probably formed during a late stage of regional metamorphism of a Ba-enriched, syngenetic protolith, involving locally generated oxidized fluids of high salinity. 

The Svärdsjö Zn-Pb-Cu deposit is situated in the heavily mineralised Bergslagen lithotectonic unit of the Fennoscandian shield, south-central Sweden. Intense hydrothermal alteration followed by a strong overprint by deformation and metamorphism during the Svecokarelian orogeny complicate interpretation of the local geology. Integration of whole-rock lithogeochemical and petrographic methods has allowed the mainly dacitic volcanic host succession and effects of ore-related hydrothermal alteration to be characterised. Mineralisation is hosted by 2–15 m thick, commonly skarn-altered dolomitic marble interbeds. Zones of strong–intense hydrothermal chlorite-sericite alteration envelop the marble units, recording mass gains of Fe and Mg, as well as Na depletion. Minerals such as cordierite, anthophyllite and sillimanite formed in these rocks during regional metamorphism. Mineralisation via sub-seafloor replacement is suggested for the deposit based on alteration zoning and the irregular, stratabound, marble-hosted style of sulphide lenses. It is inferred that mineralisation formed via neutralisation of hot, acidic metalliferous fluids. Geochemically and lithologically distinct units adjacent to the mineralised zones can serve as marker units to aid further exploration in the area. Mass change calculations reveal that Fe and Mg enrichment, as well as Na depletion exhibit detectable changes extending up to 100 m from the mineralised lenses, providing exploration vectors.

Magnetite is a common mineral in the Paleoproterozoic Stollberg Zn–Pb–Ag plus magnetite ore field (~6.6 Mt of production), which occurs in 1.9 Ga metamorphosed felsic and mafic rocks. Mineralisation at Stollberg consists of magnetite bodies and massive to semi-massive sphalerite–galena and pyrrhotite (with subordinate pyrite, chalcopyrite, arsenopyrite and magnetite) hosted by metavolcanic rocks and skarn. Magnetite occurs in sulfides, skarn, amphibolite and altered metamorphosed rhyolitic ash–siltstone that consists of garnet–biotite, quartz–garnet–pyroxene, gedrite–albite, and sericitic rocks. Magnetite probably formed from hydrothermal ore-bearing fluids (~250–400°C) that replaced limestone and rhyolitic ash–siltstone, and subsequently recrystallised during metamorphism. The composition of magnetite from these rock types was measured using electron microprobe analysis and LA–ICP–MS. Utilisation of discrimination plots (Ca+Al+Mn vs. Ti+V, Ni/(Cr+Mn) vs. Ti+V, and trace-element variation diagrams (median concentration of Mg, Al, Ti, V, Co, Mn, Zn and Ga) suggest that the composition of magnetite in sulfides from the Stollberg ore field more closely resembles that from skarns found elsewhere rather than previously published compositions of magnetite in metamorphosed volcanogenic massive sulfide deposits. Although the variation diagrams show that magnetite compositions from various rock types have similar patterns, principal component analyses and element–element variation diagrams indicate that its composition from the same rock type in different sulfide deposits can be distinguished. This suggests that bulk-rock composition also has a strong influence on magnetite composition. Principal component analyses also show that magnetite in sulfides has a distinctive compositional signature which allows it to be a prospective pathfinder mineral for sulfide deposits in the Stollberg ore field.

The Stollberg ore field (~12 Mt), 50 km W of the giant Garpenberg Zn-Pb-Ag-(Cu-Au) district (>100 Mt) occurs in the regional Stollberg F2 syncline within 1.9 Ga bimodal felsic and mafic rocks metamorphosed to the amphibolite facies. Sulfide mineralization is hosted by volcanic rocks and skarn and consists of massive to semi-massive sphalerite-galena and pyrrhotite (with subordinate pyrite, chalcopyrite, arsenopyrite, and magnetite). The trace element composition of magnetite, which locally forms ore-grade masses and occurs as a common accessory in most rocks types at Stollberg, has previously proven to be a pathfinder in the exploration for ore deposits elsewhere and is evaluated here along with the rare earth element (REE) chemistry of altered rocks. At Stollberg, the dominant country rocks are metamorphosed rhyolitic pumice breccia and rhyolitic ash-silt-sandstone with minor amphibolite sills. On the eastern side of the Stollberg syncline, mineralization at Stollberg and Dammberget occurs as stratabound replacement of limestone/skarn that grades into iron formation spatially related to garnet-biotite and gedrite-albite alteration. At Gränsgruvan on the western side of the syncline, sulfides occur in a silicified zone along with garnet-biotite and quartz-garnet-pyroxene alteration. Although the Tvistbo and Norrgruvan deposits along the north end of the syncline are small, they show geological characteristics that are transitional to deposits found on the western and eastern side of the syncline in that the ore is hosted by skarn rock and associated with quartz-garnet-pyroxene alteration. The Gränsgruvan deposit more closely resembles deposits found at Garpenberg than those located on the eastern limb of the Stollberg syncline. Whole-rock analyses of altered and unaltered host rocks suggest that most components were derived from a felsic volcaniclastic component and that elements were immobile during alteration. These rocks (including altered rocks in the stratigraphic footwall) are light REE enriched, heavy REE depleted, and show negative Eu anomalies, whereas mineralized rocks (Fe- and base metal-rich) and altered rocks in the ore zone show the same REE pattern but with positive Eu anomalies. Trace element compositions (using LA-ICP-MS techniques) of magnetite in high-grade ore, limestone/skarn, massive magnetite, and garnet-biotite, gedrite-albite, garnet-pyroxene alteration show a range of compositions. Such ranges in composition are inconsistent with previous studies in other ore fields that suggest the composition of magnetite can be used to define compositional fields characteristic of ore deposit type (e.g., Al+Mn vs. Ti+V wt. %) or approximate temperature of the ore-forming fluid. Magnetite in garnet-biotite and gedrite-albite alteration spatially associated with Dammberget typically contains > 200 ppm Ga, > 10 ppm Sn, and Ti/V ratios of >10 whereas magnetite in garnet-biotite alteration associated the smaller Cederkreutz deposit contains < 25 ppm Ga, < 2 ppm Sn, and Ti/V ratios < 0.1. Magnetite in garnet-biotite alteration associated with the Gränsgruvan deposit contains > 10 ppm Sn, 20 to 180 ppm Ga, and Ti/V ratios of 0.1 to 2. These and other trace element compositions of magnetite as well as REE patterns of altered host rocks show potential as exploration guides to ore in the Stollberg district.

The Stollberg ore field occurs in the Bergslagen region of south-central Sweden, a polydeformed ca. 1.9 Ga igneous province dominated by bimodal felsic and mafic rocks. Sulfide mineralization is hosted by metavolcanic rocks, marble, and skarn and consists of massive to semimassive polymetallic sulfides and iron oxide in a semiregional F₂ syncline termed the Stollberg syncline. The dominant country rocks are rhyolitic pumice breccia and rhyolitic ash-siltstone with minor mafic sills metamorphosed to the amphibolite facies. On the eastern limb of the Stollberg syncline, sulfide mineralization occurs as stratabound premetamorphic replacement of volcaniclastic rocks and limestone that grades into iron formation. The development of skarn assemblages is the result of low-temperature replacement of limestone and volcaniclastic rocks rather than formation by high-temperature metasomatism or synmetamorphic or late hydrothermal replacement of marble. Metamorphosed, hydrothermally altered rocks on the eastern limb are dominated by the assemblages garnet-biotite and gedrite-albite. Silica-altered rocks are generally subordinate in the Stollberg ore field; however, sulfides at Gränsgruvan, on the western limb of the syncline, are located in a silicified zone along with metamorphosed, altered rocks dominated by sericite and the assemblage quartz-garnet-pyroxene. Although the Tvistbo and Norrgruvan prospects along the northern end of the syncline are small, they show geologic characteristics that are transitional to deposits found on the western and eastern limbs of the syncline. Ore at Tvistbo is hosted by skarn and is spatially associated with quartz-garnet-pyroxene rocks, whereas sulfides at Norrgruvan are hosted by quartz-fluorite rocks that are similar to those hosting the Brusgruvan deposit on the eastern limb of the syncline.

Whole-rock analyses of variably altered host rocks in the Stollberg ore field suggest that most components were sourced from felsic volcaniclastic rocks and that Zr, Ti, Al, Hf, Nb, Sc, Th, Ga, U, and rare-earth elements (REEs) were immobile during alteration. These rocks are enriched in light REEs, depleted in heavy REEs, and have negative Eu anomalies, whereas sulfide-bearing rocks (Fe- and base metal-rich) and altered rocks in the ore zone show the same REE pattern but with positive Eu anomalies. Indicators of proximity to sulfides in altered rocks in the Stollberg ore field include positive Eu anomalies, an increase in the concentration of Pb, Sb, As, Tl, Ba, Ba/Sr, and K₂O, as well as an increase in a modified version of the Ishikawa alteration index, which accounts for the presence of primary Ca in an original limestone component. Garnet and pyroxene enriched in either Ca or Mn are also considered to be pathfinders to ore. Cooling of an acidic, reduced hydrothermal fluid that carried sulfur and metals, which became neutralized as it reacted with limestone, is likely responsible for the formation of sulfides in the Stollberg ore field. The nature of the host rock types, the style of the alteration spatially associated with sulfide mineralization, and the spatial association with iron formation bear some resemblance to volcanogenic massive sulfide and Broken Hill-type deposits. However, the stratabound replacement of limestone by sulfides distinguishes it from these deposit types and is a so-called SVALS-type ore system, which is a class of stratabound, volcanic-hosted, limestone-skarn deposits restricted to the Bergslagen district.

This paper discusses existing methods for geological analysis of drill cores and describes the research and development directions of a machine learning framework for such a task. Drill core analysis is one of the first steps of the mining value chain. Such analysis incorporates a high complexity of input features (visual and compositional) derived from multiple sources and commonly by multiple observers. Especially the huge amount of visual information available from the drill core can provide valuable insights, but due to the complexity of many geological materials, automated data acquisition is difficult. This paper (i) describes the difficulty of drill core analysis, (ii) discusses common approaches and recent machine learning-based approaches to address the issues towards automation, and finally, (iii) proposes a machine learning-based framework for drill core analysis which is currently in development. The first major component, the registration of the drill core image for further processing, is presented in detail and evaluated on a dataset of 180 drill core images. We furthermore investigate the amount of labelled data required to automate the drill core analysis. As an interesting outcome, already a few labelled images led to an average precision (AP) of around 80%, which indicates that the manual drill core analysis can be made more efficient with the support of a Machine Learning/labeling workflow.

Sala is Sweden's most famous silver mine, with production from 1500 AD of about 450 tonnes of silver from 5 MT of ore and waste. The ores comprise vein, skarn and breccia-fill sphalerite-galena within dolomite and show similarities to other stratabound volcanic- and limestone-hosted Zn-Pb-Ag ores in Bergslagen, such as Garpenberg. The Sala dolomite contains several stromatolite occurrences, one being the type-locality for Swedish stromatolites. In this presentation, the first detailed account of stromatolites within the Sala Mine is given along with a discussion on the structural geological, stratigraphic and ore genetic significance of the stromatolites. The stromatolites display a wide range of morphologies, occurring as microbial mats, domal shapes, digitate forms as well as columns. They are best preserved in the southern part of the mine and are less conspicuous in the northern part. Stromatolitic way-up indicators reinforce earlier interpretations that the ore is hosted by a major syncline whose axis is parallel to the plunge of the mineralization. Furthermore, the stromatolites provide evidence that the planar intensely Chl-Phl-Srp-Di-Tr-Cal-Qz altered layers which transect the dolomite are concordant to bedding. These altered rocks drape stromatolitic structures, suggesting they are altered siliceous sedimentary rocks. Stromatolites also allow recognition of discordant, altered layers that are mineralogically similar to the siliceous sedimentary rocks but lack diopside and tremolite. These layers are interpreted as shear zones. The most significant is the Storgruvan Shear Zone, which parallels the strike of the deposit. Previously, these two contrasting geological features were lumped together under the loosely defined Swedish mining term 'sköl'. Way-up determinations from the stromatolites suggest that sphalerite ore mainly occurs stratigraphically below galena ore. The relationships between the shapes of workings, stromatolitic layers and altered siliceous interbeds suggest a stratigraphic control on the hydrothermal plumbing system during ore formation. Sphalerite vein-networks that occur adjacent to well-preserved stromatolitic textures suggest that ore formation was not completely texturally destructive. Skarn and sphalerite locally mimic stromatolitic laminae and form infillings in stromatolitic vugs. Although some ore is tectonically remobilized, ore displaying similar deformation patterns to the host-rock has also been observed. Thus, early to pre-orogenic ore formation is indicated. The ore may initially have formed epigenetically by sub-sea floor infiltration of metalliferous fluids into a buried stromatolite reef with volcaniclastic interbeds. Ore minerals precipitated as massive sulphide lenses and semi-massive vein-networks with long dimensions parallel to bedding. Because of later tectonic modification, ore is now concentrated in the axial plane of the Sala Syncline and in the vicinity of the Storgruvan Shear Zone.

The Bergslagen mining district of southern Sweden is one of Europe’s classic mining districts with more than 1 000 years of mining history. One of the typical features of Bergslagen is a spatial association between Zn-Pb-Ag-(Cu-Au) sulphide deposits and magnetite-rich Fe oxide deposits. The relationship between these two deposit types has been discussed intensely for more than a century, yet there are still many uncertainties to be resolved. In this thesis, the origin of Fe oxide deposits in Bergslagen and their relationship with polymetallic sulphide deposits is investigated. Detailed investigations have been undertaken at a number of Fe oxide and polymetallic sulphide deposits in the Garpenberg and Stollberg areas, where sulphides and Fe oxides are spatially associated. The deposits studied at Garpenberg include the Ryllshyttan stratabound Zn-Pb-Ag-(Cu) + magnetite deposit, the Smältarmossen calcic Fe skarn deposit, the Lappberget stratabound Zn-Pb-Ag-(Cu-Au) deposit and stratiform Fe-rich exhalites near the Ryllshyttan deposit. At Stollberg, the investigation has mainly focused on studying the regional geological framework of the ore deposits. The research project was based on detailed geological mapping and drill core logging. The ores, their host rocks and the associated hydrothermal alteration envelopes have been further studied by a combination of optic microscopy, electron microprobe mineral chemical analysis, radiometric dating and whole rock lithogeochemical analysis. The results reveal that several different types of Fe oxide deposits may be defined in the Garpenberg and Stollberg areas 1) synsedimentary-exhalative Fe oxide deposits, 2) carbonate replacement-type deposits that are locally spatially associated with polymetallic sulphide deposits, and 3) contact metasomatic Fe skarn deposits proximal to syn-volcanic intrusions. For most of the studied ore deposits, several different stages of ore formation or modification of pre-existing ores are recognized, based on textural evidence and cross-cutting relationships between hydrothermal alteration, stratigraphy, intrusive events and structures. Zoning in ore metals, mineralogy and alteration geochemistry occurs both on deposit-scale and on a regional scale in all studied areas. The zonation patterns have been studied in detail in an attempt to elucidate whether geochemical, mineralogical and mineral chemical vectors may be identi¿ed, which would aid mineral exploration where Fe oxide and polymetallic sulphide deposits co-exist. Radiometric dating indicates that the studied deposits at Garpenberg, despite being markedly different from each other in style and setting, formed during a short time span at 1892 ±4 Ma. The possibility that all studied deposit-types formed at slightly different times and/or at different depths within a large igneous system is explored. Based on stratigraphic evolution, the distribution and character of hydrothermally altered zones as well as the characteristics of the ore deposits themselves, it is inferred that the sequence of ore types 1-3 above reflects generally increasing depths of ore formation and/or proximity to causative intrusions. Documented overprinting relationships and the co-existence of all deposits at similar stratigraphic levels indicate that multiple stages of ore formation during active volcaniclastic sedimentation, burial and intrusion of magmas to shallow crustal levels in an evolving extensional basin must be considered. Continuous burial during volcaniclastic sedimentation in an extensional tectonic setting (e.g. a backarc basin on continental crust) combined with the frequent intrusion of magma to shallow crustal levels, resulted in the stratigraphic succession hosting stratiform Fe oxide mineralization (type 1) being subjected to seawater-dominated hydrothermal convection cells. This led to formation of type 2replacement-style Fe oxide and polymetallic sulphide mineralizations. During continued burial, these deposits were subsequently affected by local or widespread intrusion-associated metasomatism that formed contact metasomatic Fe skarn deposits. The ores were later subjected to polyphase ductile deformation under low P amphibolite facies metamorphic conditions during the Svecokarelian orogeny. The polymetallic sulphide ores especially, were substantially modi¿ed and remobilized in the hinge zones of folds, into ore shoots parallel with axial surfaces and locally into the ENE-trending, sheared short limbs of folds. Sulphide remobilization partly coincided with retrograde alteration of anhydrous, Fe-rich skarns to more hydrous, magnetite-rich skarns, thus locally leading to the formation of high-grade magnetite mineralization proximal to massive sulphide deposits.

Ryllshyttan is a Palaeoproterozoic lower amphibolite-facies poly-metamorphosed stratabound Zn-Pb-Ag-(Cu) + magnetite deposit. It is located in the Garpenberg inlier of the Bergslagen mining district of southern Sweden and produced ~ 1 Mt sulphide ore and ~ 0.4 Mt magnetite ore from the early 16th century until 1944.The Ryllshyttan area is dominated by metamorphosed calc-alkaline rhyolitic volcanics, transitional mafic intrusions and dolomitic marble. The later hosted the mined ores and is heterogeneously altered and metamorphosed to skarns of variable composition in proximity to the ores. The ore horizon is tightly F2-folded into a series of steeply plunging synclines and anticlines. F2-folds fold an earlier S1 foliation sub-parallel to bedding. Planar S1 foliations are included in pre S2 almandine porphyroblasts, suggesting inter-tectonic regional metamorphism (M1). A second phase of regional metamorphism (M2) outlived penetrative D2 deformation as shown by post S2 almandine porphyroblasts and regional statically recrystallised S2/S1 crenulation foliations. ENE-trending sub-vertical D3 shear zones outlasted regional metamorphism as shown by protomylonites cross-cutting M2 caused static recrystallization. Brittle shallow to steeply dipping F4 faults caused small reverse displacements in northern Ryllshyttan.The limestone ore-host formed after deposition of a syn-eruptive sub-aqueous rhyolitic mass-flow deposit which constitutes Ryllshyttan's stratigraphic footwall. Limestone formation by stromatolite growth in the photic zone was followed by subsidence to deeper water conditions and deposition of fine-grained rhyolitic sediments below wave base. The rhyolitic sediments periodically co-settled with hydrothermal-exhalative calcareous-ferruginous sediments, forming sedimentary mixtures which during metamorphism formed stratiform Ca-Fe-rich aluminous skarn beds. After burial, the stratigraphic succession was intruded by syn-volcanic peperitic rhyolite porphyries. The porphyries are crosscut by shallow level pre-D1 mafic sills and dykes. Emplacement of dolerite intrusions may be coeval with a period of mafic extrusive volcanism evident stratigraphically above Ryllshyttan. The entire stratigraphy is truncated by a microgranodiorite which represents the outermost part of the GDG batholith west of the Garpenberg inlier. Epigenetic formation of sulphide and magnetite ore occurred by replacement of a limestone unit. This occurred between emplacement of the mafic intrusions and microgranodiorite and is associated with pre-D1 K-Fe-Mg +/- Si alteration proximal (< 50 m) to the ore-zone. The alteration zones developed as chlorite-sericite zones but are now metamorphosed to porphyroblastic biotite-phlogopite +/- quartz schists with elevated concentrations of Zn, Pb, Cu and Mn. Distal (> 50 m) alteration is expressed by quartz-spessartine rocks formed by alteration and metamorphism of calcareousferruginous hydrothermal sediments and epidote-calcic clinoamphibole mottling and veining of rhyolitic volcanics. Alteration in the ore-zone is expressed by sphalerite and magnetite impregnated dolomitic marble, magnesian skarns and calcic skarns in the ore-zone. A zonation with proximal Fe-Mg alteration grading outwards with decreasing Fe/Mg-ratio to more distal Mn alteration is apparent. Though epigenetic sulphides appear to slightly post-date epigenetic magnetite, no significant hiatus is observed and both may have formed during the same event.The microgranodiorite is geochemically similar to syn-volcanic dacite intrusions proximal to the currently mined sulphide ores at Garpenberg. Na-Ca alteration has affected the microgranodiorite as well as adjacent volcanics, leading to the development of diopside-oligoclase assemblages. The timing of sulphide ore formation relative to the intrusive history indicate that ore formation occurred broadly synchronously at Garpenberg and Ryllshyttan during the evolution of a large marine felsic caldera complex but at different stratigraphic levels. Ryllshyttan displays features of both regionally metamorphosed, shallow marine, sub-seafloor replacement VMS deposits and metasomatic skarn deposits. These contrasting relationships may have resulted from a prograde hydrothermal evolution starting with early K-Mg-Fe +/- Si alteration, continuing with later Na-Ca alteration following a path of increasing temperature of the hydrothermal system and eventually ending with a transition to regional metamorphism and deformation during which already existing ores were significantly modified by deformation and fluid-assisted remobilization during the Svecokarelian orogeny.

The previously mined Ryllshyttan Zn-Pb-Ag-(Cu) + magnetite deposit comprised approximately 1 Mt sphalerite-dominated massive sulphide ore and about 200 000 tons of massive to semi-massive skarn-limestone hosted magnetite ore. Ryllshyttan belongs to the Bergslagen ore district of south central Sweden, a Palaeoproterozoic igneous province renowned for its abundance and diversity of Fe-oxide and sulphide mineralisations. Ryllshyttan's ore-bodies are hosted by a carbonate horizon partially altered to skarns of tremolite, diopside-salite and andradite. The ore horizon itself is enclosed by volcanics and intrusions belonging to the earliest Svecokarelian igneous rocks (~ 1.9 Ga BP). The stratigraphy follows a first order volcanic cycle with an evolution from juvenile rhyolitic mass flow deposition to deposition of rhyolitic ash-siltstones intercalated with ferruginous-calcareous chemical and/or hydrothermal sediments. Observations suggest that iron oxides started to accumulate syngenetically at the time of host-rock formation. In contrast, sulphides are associated with discordant Mg +/- K +/- Si alteration that also overprints some intrusions. Consequently, sulphide mineralization and the earliest iron oxide mineralization are attributed to separate events.

Thin- to medium-bedded, stratiform calc-silicate deposits (banded skarns) are a peculiar, but important, component of the supracrustal successions in the Palaeoproterozoic Bergslagen mining district of central Sweden. They are referred to as “skarn-banded leptites” in the literature and are common in areas and at stratigraphic levels that contain iron oxide and base metal sulphide deposits. The stratigraphic hanging wall of the stratabound Ryllshyttan Zn–Pb–Ag + magnetite deposit at Garpenberg, contains approximately 100–150 m of interbedded aluminous skarn beds and rhyolitic ash-siltstones. The skarn beds are mineralogically variable and dominantly composed of grandite, spessartine, epidote, actinolite, quartz, clinopyroxene, and locally magnetite. Integrated field-mapping, and whole-rock lithogeochemical, microscopic and mineral chemical analyses suggest that the stratiform skarn beds are the products of at least two discrete hydrothermal events and subsequent metamorphism. The first event comprised accumulation in a quiescent subaqueous environment, below wave base, of calcareous and ferruginous sediments rich in Fe, Mn, Ca, and Mg. These chemical sediments were deposited concurrently with rhyolitic ash-silt sedimentation, thus forming a (now metamorphosed) laminated calcareous Fe formation with both a detrital rhyolitic component and rhyolitic siltstone interbeds. Positive Eu-anomalies and negative Ce-anomalies for normalized rare earth element analyses of skarn beds suggest that the iron may have been derived from exhalation of hot and reduced hydrothermal fluids, which upon mixing with more oxidized seawater, precipitated Fe oxides and/or carbonates that settled from suspension to the seafloor. The size of the positive Eu-anomalies of the chemical sediments are modified by the content of rhyolitic volcaniclastic material, which has a negative Eu anomaly, such that positive Eu-anomalies are only observed in skarn beds that possess a minor volcaniclastic component. Subsequently, the calcareous Fe formations were subjected to post-depositional alteration by hydrothermal fluids, locally yielding more manganoan and magnesian assemblages. The Mn-alteration is manifested by lateral gradations from epidote-grandite-clinopyroxene±magnetite rocks into significantly more Mn-rich quartz-spessartine rocks and massive andradite rocks over distances of less than 10 cm within individual skarn beds. Magnesian alteration is manifested by the development of discordant zones of pargasite para-amphibolites and formation of stratiform pargasite rocks texturally similar to the interlaminated grandite-epidote-ferroan diopside rocks. The latter increase in abundance towards the Ryllshyttan deposit and are associated with pre-metamorphic/pre-tectonic K–Mg–Fe±Si alteration (now biotite-phlogopite-garnet-cordierite-pargasite rocks) that is related to base metal mineralization. The zone of Mn- and Mg-altered skarn beds extends beyond the zone of pervasive K–Mg–Fe±Si alteration around Ryllshyttan. This suggests that the skarn bed progenitors, or their sedimentary contacts against rhyolitic ash-siltstones, acted as conduits to outflowing hydrothermal fluids. The chemical and mineralogical imprint, imposed on affected beds by alteration, may serve as indicators of proximity to intense K–Mg–Fe±Si alteration envelopes around other base metal sulphide deposits in Bergslagen. The last recorded event comprised syn-tectonic veining of competent massive andradite skarn beds. The veins contain quartz-albite-epidote-ferroan diopside-actinolite assemblages.

Abundant iron oxide deposits including banded iron formations, apatite iron oxide ores, and enigmatic marble/skarn-hosted magnetite deposits occur in the Palaeoproterozoic Bergslagen region, southern Sweden. During the last 100 years, the latter deposit class has been interpreted as contact metasomatic skarn deposits, metamorphosed iron formations, or metamorphosed carbonate replacement deposits. Their origin is still incompletely understood. At the Smältarmossen mine, magnetite was mined from a ca. 50-m-thick calcic skarn zone at the contact between rhyolite and stratigraphically overlying limestone. A syn-volcanic dacite porphyry which intruded the footwall has numerous apophyses that extend into the mineralized zone. Whole-rock lithogeochemical and mineral chemical analyses combined with textural analysis suggests that the skarns formed by veining and replacement of the dacite porphyry and rhyolite. These rocks were added substantial Ca and Fe, minor Mg, Mn, and LREE, as well as trace Co, Sn, U, As, and Sr. In contrast, massive magnetite formed by pervasive replacement of limestone. Tectonic fabrics in magnetite and skarn are consistent with ore formation before or early during Svecokarelian ductile deformation. Whereas a syngenetic-exhalative model has previously been suggested, our results are more compatible with magnetite formation at ca. 1.89 Ga in a contact metasomatic skarn setting associated with the dacite porphyry.

Oxide- and silicate-dominated, stratiform iron formations are abundant in the northern part of the Sala inlier, Bergslagen, Sweden. The iron formations are commonly laminated and are associated with fine-grained siliciclastic and felsic volcaniclastic rocks in a 1.91-1.89 Ga succession dominated by pumiceous and lithic-bearing rhyolitic volcaniclastic rocks. Depositional features are consistent with a volcanically active, submarine environment, in which the iron formations and fine-grained host strata to sulphide mineralization accumulated during pauses in volcanism. At c. 1.87-1.81 Ga, the succession underwent polyphase folding and shearing under lower amphibolite facies conditions, followed by polyphase faulting under more brittle conditions.

The iron formations are locally directly stratigraphically overlain by  stratiform Zn-Pb-Ag sulphide mineralization. Detailed geological mapping has demonstrated that sulphide-bearing (proximal) iron formation is gradational into sulphide-poor (distal) iron formation along a strike extent of more than 7 km. Proximal iron formation is dominated by magnetite, grunerite, tremolite, quartz, almandine-rich garnet (Alm₅₄Sps₃₅Grs₈), muscovite, and chlorite, whereas distal iron formation is characterized by hematite, magnetite, epidote, actinolite, spessartine-rich garnet (Sps₅₃Adr₂₉Grs₁₅) and locally calcite. 

Elevated contents of Mn, Zn and Co are observed in both distal and proximal iron formation, whereby these elements help pinpoint the favorable horizon, but are of less use for vectoring along strike. Whole-rock lithogeochemistry samples of proximal iron formation differ from distal iron formation in: (1) Eu/Eu*>1, (2) Ce/Ce*<1, (3) suprachondritic Y/Ho, (4) elevated Tl, Cs, Cd, Sn, S, Cu, Pb, Sb and Au (5) lower volcaniclastic/siliciclastic content based on lower Al, Ti and Zr. Collectively, these features are indicative of Fe mineralization following interaction of a hot, acid and reduced hydrothermal fluid with oxidized seawater in a vent proximal position which was deprived of clastic or volcaniclastic input.

Sulphide mineralization, ranging from banded, to disseminated and fracture-hosted, is associated with chlorite-rich, locally graphitic mudstone immediately overlying proximal iron formation. Multi-grain δ³⁴S_V-CDT of sphalerite, pyrite and pyrrhotite are exclusively negative, ranging from -10.6 to -0.25 with no clear mode. The δ³⁴S_V-CDT distribution is unusual for Bergslagen deposits, and is indicative of a significant contribution of sulphur via bacteriogenic or thermochemical reduction of seawater SO₄^2-.

Stratigraphic analysis suggest that proximally, the mineralizing event followed a sudden deepening of the basin, and progressed from Fe oxide to polymetallic sulphide mineralization. The temporal zonation probably reflect a decrease in the redox potential of the basin, possibly due to venting and ponding of reduced hydrothermal fluids. Ore textures and host facies are consistent with of an exhalative mode of formation for both deposit types, albeit an importance of subseafloor mineralization processes is implied by lateral variability in both sulphide and chlorite content. In relation to the local stratigraphic evolution in the area, the mineralizing event can be directly linked to an event of basin deepening following a caldera-forming volcanic eruption. The results from stratigraphic analysis along with aforementioned proxies for redox and vent-proximity present first order vectors to stratiform Zn-Pb-Ag mineralization in the Jugansbo area, Bergslagen.

Regionally extensive, dolomitic and calcitic marble units comprise a subordinate but important component of a c. 1.91-1.89 Ga supracrustal succession in the Bergslagen lithotectonic unit, southern Sweden. These units occur as interbeds in predominantly felsic metavolcanic successions, and originated as shallow marine, stromatolitic limestone. Upon burial, the limestones were subjected to dolomitization and hydrothermal alteration, and at c. 1.87-1.80 Ga during the Svecokarelian orogeny, greenschist-amphibolite facies metamorphism and ductile deformation led to further modification.

The marble units host abundant replacement-type, syn-volcanic sulphide and iron oxide deposits and locally industrial carbonate deposits. In the Sala area, current mining targets exceptionally light dolomite marble proximal to formerly mined Zn-Pb-Ag mineralization. The geological controls on marble lightness are poorly understood, e.g. the relative importance of primary impurity caused by co-settled volcaniclastic or siliciclastic material as opposed to secondary modifications caused by hydrothermal alteration. The following study addresses these controls based on 179 geospatial dolomitic marble samples analyzed for whole-rock lithogeochemistry, spectrophotometric lightness and magnetic susceptibility.

Centered log-ratio transformed major element data suggest that lightness is positively correlated with Mg, Ca and Tot C, negatively correlated with Fe and Mn, and lack a clear relationship to Al and Si. Robust principal component analysis of major and trace element data allows representation of 81.1% of the cumulative variance by three principal components. PC1 (46.7%) mainly differentiates ‘hydrothermal’ elements such as Zn, Pb and As from elements such as Si, Mg, Ca, Tot C, Al, Zr, Ce, La and Eu. PC2 (21%) differentiates the latter into a ‘volcaniclastic-siliciclastic’ group dominated by Zr, Al, Ce, La and Eu and ‘calcite-dolomite’ group dominated by Ca, Mg and Tot C. PC3 (13.4%) is primarily controlled by Fe and Mn, with Fe+Mn-enriched samples outlining a c. 200 m wide halo around known Zn-Pb-Ag deposits. Optic microscopy and normative mineralogy suggest a negative correlation between lightness and the opaque minerals magnetite, pyrrhotite, pyrite, sphalerite and galena, which are variably enriched in this alteration halo.

Collectively, the results imply that whereas a volcaniclastic component is detrimental to lightness, this effect is negligible relative to the effect of even weak Fe-enrichments (> 1 wt.%) at SiO2 < 10 wt.% and Al2O3 < 2 wt.%. This likely reflect that in contrast to the opaque minerals, the main silicates tremolite, serpentine, chlorite and phlogopite grind into light powders during processing. Light dolomite and sulphide-mineralized samples cluster in opposite corners in RQ1-RQ3 space and form coherent geospatial clusters corresponding to known deposits. Magnetic susceptibility data effectively differentiate these clusters and is hence useful for outlining prospective marble volumes. It is speculated that light marble constitutes a peripheral member of the sulphide-associated alteration halo. Thus, light marble is in itself a vector to polymetallic sulphides, and a potential byproduct in operations targeting the latter. High lightness is tentatively attributed to leaching and recrystallization of dolomite under elevated temperatures, either driven by ‘spent’ ore-forming fluids exiting the mineralizing system, or by marine fluids in peripheral, secondary hydrothermal aleration cells.

The Stollberg Zn-Pb-Ag and magnetite mining field is located in the Bergslagen region of the Fenno -scandian Shield. The main Stollberg ore deposits comprise a chain of orebodies that occur discontinuously for5 km along a prominent marble and skarn horizon. Orebodies mainly contain magnetite and combinations ofsphalerite, galena, pyrrhotite, and lesser pyrite and chalcopyrite within marble and skarn. Previously, the twomain limestone (marble) units in the Stollberg area were regarded as structural repetitions of one single horizon.Based on sedimentary and volcanic facies and structural analysis, the mineralized Stollberg limestone isnow shown to be the uppermost of two different limestone units within a ca. 3-km-thick Paleoproterozoic (∼1.9Ga) volcanosedimentary succession. Approximately 2 km of preserved footwall stratigraphy is recognized belowthe Stollberg limestone, as opposed to ca. 500 m in previous structural models. This new interpretation hasallowed the stratigraphic evolution prior to the mineralizing event and extent of the Stollberg hydrothermal systemto be investigated in detail.After formation of the Staren limestone ca. 1 km below Stollberg, the depositional basin subsided to belowwave base, while adjacent areas were uplifted and eroded. This led to the deposition of a ca. 600-m-thick, shallowing-upward sedimentary sequence in which normal-graded subaqueous mass flow deposits pass upward topolymict limestone-volcanic breccia-conglomerates. This sequence is attributed to progradation of a fan deltadepositional system. The breccia-conglomerates are overlain by ca. 500 m of juvenile rhyolitic pumice brecciathat is interpreted as a major pyroclastic deposit. Conformably above is the Stollberg ore host, which comprisesplanar-stratified, rhyolitic ash-siltstone interbedded with Fe-Mn-rich hydrothermal sedimentary rocks andlimestone, all deposited below wave base. This ore host package is extensively altered to skarn and mica schist.The thickness, extent, and homogeneous composition of the rhyolitic pumice breccia below the ore host suggestthat volcanism was accompanied by caldera subsidence and that the Stollberg ore deposits formed withinthe caldera structure. The ore host is overlain by planar-stratified, rhyolitic ash-siltstone and subordinate sedimentarybreccias deposited below wave base from turbidity currents and suspension.Skarns in the Stollberg ore host unit are interpreted as metamorphosed mixtures of variably altered rhyolite,limestone, and hydrothermal sediments. Whole-rock contents of Al, Ti, Zr, Hf, Nb, Sc, Th, Ta, U, and heavyrare-earth elements are highly correlated in skarns, limestone, magnetite mineralization, and variably alteredrhyolites in the Stollberg succession, suggesting that these elements were supplied by a felsic volcaniclasticcomponent and were immobile during alteration. The felsic volcaniclastic component is calc-alkaline and characterizedby negative Eu anomalies and light rare-earth element enrichment. Strong positive Eu anomalies areonly observed in limestone, skarn, and iron ore in the Stollberg ore host, i.e., in samples rich in Mn, Ca, andFe.The Stollberg ore deposits are interpreted as metamorphosed, hydrothermal-exhalative and carbonate replacement-type mineralization. The hydrothermal-exhalative component formed first by accumulation of sedimentsrich in Mn and Fe, coeval with limestone formation during waning volcanism. Burial of the hydrothermal systemby sediments of the stratigraphic hanging wall led to a gradual shift to more reducing conditions. At thisstage, the Stollberg limestone interacted with more sulfur rich hydrothermal fluids below the sea floor, producingstrata-bound, replacement-type Zn-Pb-Ag sulfide and additional iron oxide mineralization

A structural investigation of the Sala Zn–Pb–Ag deposit in the Bergslagen mining district of southern Sweden shows that it is associated with two tectonic structures: the N–NW-trending Storgruveskölen shear zone (SSZ), which is parallel to the strike of the mined ore bodies, and the F1 Sala syncline with a fold hinge plunging c. 35° towards NNW, which is parallel to the plunge of the entire mineralised system. The Sala syncline was refolded by F2 folds, leading to flattening and local reversals in the plunge of F1 folds and the ore bodies. Field evidence suggests that the SSZ represents both a phase of D3 reverse dip-slip shearing and a later (D4) phase of dextral strike-slip reactivation. However, a high concentration of pre- to syn-D1skarn- and sulphide-bearing vein networks and breccias adjacent to the SSZ, which are gradational into the mined massive sulphide ore bodies, suggest that stages in the formation of the SSZ predated D3 and D4. It may consequently constitute a reactivated pre- to syn-D1 structure. The distribution of breccia and hydrothermal alteration together with the highly discordant nature of the deposit are consistent with a pre- to syn-D1 timing of ore formation, involving of cross-stratal fluid flow along the proto-SSZ and subordinate fluid flow parallel to volcanic interbeds in the host carbonate rock. Three δ34S determinations on sphalerite (2.1–2.4‰) and galena (1.2‰), respectively, are consistent with a magmatic source for ore sulphur, as has been suggested for many other sulphide deposits in Bergslagen.

Numerous magnetite skarn deposits and marble- and skarn-hosted base metal sulphide deposits occur in polydeformed and metamorphosed, felsic-dominated metavolcanic inliers in the Palaeoproterozoic Bergslagen region of south-central Sweden, including the Ryllshyttan magnetite and Zn-Pb-Ag-(Cu) sulphide deposit, approximately 2.5 km SW of the large Garpenberg Zn-Pb-Ag-(Cu-Au) deposit. The Ryllshyttan deposit, from which approximately 1 Mt of Zn-rich massive sulphide ore and 0.2 Mt of semi-massive magnetite were extracted, is located near a transition between magnesian skarn and dolomitic marble. The host unit consists of a 10-20 m-thick former calcitic limestone of likely stromatolitic origin that is commonly pervasively altered to skarn, locally hosting magnetite skarn deposits. The ore-bearing unit is one of several mineralised marble units within a more than 1 km-thick, felsic-dominated metavolcanic succession that includes a metamorphosed, large caldera-fill pyroclastic deposit, 800 m stratigraphically above the Ryllshyttan host succession. The Garpenberg stratabound Zn-Pb-Ag-(Cu)-(Au) deposit is located higher in the stratigraphy, just below the caldera fill deposits. The metavolcanic succession is bounded to the NW by a large granitoid batholith and intruded by a microgranodiorite pluton less than a 100 m from the Ryllshyttan deposit. Magnetite laminae in bedded skarns and metavolcanic rocks in the hanging wall of Ryllshyttan indicate an early (syngenetic) accumulation of Fe-rich exhalites. In contrast, the sulphide mineralisation consists of stratabound replacement-style ore associated with dolomitisation of the host and with discordant K-Mg-Fe±Si alteration of volcanic rocks and early porphyritic intrusions in the footwall and hanging wall. The microgranodiorite that intrudes the host succession crosscuts the K-Mg-Fe±Si alteration envelope and is overprinted by Na-Ca alteration (diopside and plagioclase-bearing mineral associations) that also overprints K-Mg-Fe±Si-altered rocks. The Na-Ca alteration is interpreted to be associated with the formation of calcic and magnesian iron skarn deposits semi-regionally at a similar stratigraphic position. Despite superimposed amphibolite facies regional metamorphism and substantial syn-D2-D3 remobilisation of sulphides concurrent with retrograde alteration of skarn assemblages, cross-cutting field relationships indicate that the Ryllshyttan magnetite and Zn-Pb-Ag-(Cu) sulphide deposit results from protracted VMS-style hydrothermal activity including early seafloor mineralisation (Fe-rich exhalites), closely followed by sub-seafloor carbonate-replacement-style mineralisation (base metal-bearing massive sulphides). Both mineralisation styles were overprinted by contact metasomatism associated with the formation of abundant magnetite skarn deposits during the emplacement of granitoid intrusions. As for other deposits in the Bergslagen region, the ore-forming system at Ryllshyttan thus has similarities to both metamorphosed VMS deposits and metasomatic Fe and Zn skarn deposits. Our results suggest that the sequence of volcanic, intrusive and hydrothermal events in this region is compatible with prograde heating of a long-lived hydrothermal system, wherein a shift from a convective seawater-dominated system to a contact metamorphic and/or metasomatic environment occurred during the early stage of the 1.9-1.8 Ga Svecokarelian orogeny. This model partly resolves the controversy regarding genesis of the iron oxide and base metal sulphide deposits in Bergslagen, as we recognise that these deposits have a complex history of alteration, metamorphism, deformation and (re)mobilisation, and no unique established genetic model can account for all their features.

The timing of Palaeoproterozoic magmatism in the Garpenberg area in the Bergslagen region of the Fennoscandian shield has been constrained by secondary ion mass spectrometry (SIMS) U-Pb zircon dating of metamorphosed igneous rocks. Volcanism is constrained by igneous crystallisation ages of 1895 ± 4Ma for a syn-volcanic rhyolite porphyry intrusion and 1893 ± 3Ma for a rhyolitic pumice breccia. Granite and microgranodiorite, which intruded into the stratigraphy, are dated at 1895 ± 3 and 1894 ± 4 Ma, respectively. The identical U-Pb ages suggest rapid geological evolution from the emplacement of volcanics, their burial and subsidence to 2-5 km depths and intrusion by granitoids. The timing of metamorphism and the extent of metamorphic resetting of titanite have been evaluated. SIMS titanite 207Pb-206Pb ages from the same samples as the zircon yield younger ages. Although errors are large in individual analyses and fractions, a weighted average of 59 analyses from four samples yields a 207Pb-206Pb age of 1858 ± 14 Ma, interpreted as the age of regional metamorphism. The results add constraints to the timing of sulphide and iron oxide mineralisation at Garpenberg. The rhyolite porphyry is intruded into a syngenetic iron formation. Its crystallisation age provides a minimum age for syngenetic iron oxide deposits at Garpenberg. The major Zn-Pb sulphide deposits are accompanied by alteration envelopes. Units formed before alteration yield similar igneous crystallisation ages as intrusions post-dating alteration. It is concluded that both iron oxide and sulphide mineralisation formed within the same age-span as the dated units.

This contribution presents an application of principal component analysis (PCA) and K-means clustering as tools for data dimension reduction and grouping of multivariate, whole-rock lithogeochemical data. The study dataset consists of 64 geochemical variables and measurements of spectrophotometric brightness determined from 181 dolomite marble samples, collected at various distance from two contrasting types of mineral deposits, 1) stratabound, dolomite marble- and skarn-hosted Zn-Pb-Ag sulphide deposits and 2) industrial dolomite deposits. Clustering and PCA outputs are assessed based on spatial distribution relative to known mineral deposits and interpretability using geological domain knowledge, to test if the methods can provide a non-biased classification of dolomite samples which is useful for exploration vectoring. The PCA illustrate that three principle components derived from centered log-ratio transformed data can account for 79.69% of the dataset variance. K-means clustering provide unsupervised division of samples into different groups reflecting relative contents of detrital (siliciclastic-volcaniclastic), biogenic and hydrothermal components in the marble protoliths. Spatial analysis of principal components and K-means clusters reveal systematic distribution patterns relative to known deposits, thus providing an exploration guide. The samples most prospective for Zn-Pb-Ag deposits are divided into groups of ‘halo dolomite’ exhibiting elevated Fe and Mn, and an ‘ore dolomite’ also showing elevated Zn, Pb, Ag, Sb, Hg. This can be reconciled with magnetite and Mn-bearing Mg-silicates and carbonates in hydrothermal alteration haloes, and proximal enrichment in hydrothermal sulphides (galena, pyrrhotite, pyrite, sphalerite). Samples in these groups returned low spectrophotometric brightness, resulting from sulphides and Fe oxides grinding to dark powders during sample preparation, significantly lowering the brightness of powdered dolomite marble, even when occurring in low concentrations. Conversely, a ‘clean dolomite’ group is characterized by low contents of the elements above, high contents of Ca, Mg, Sr and total carbon, low magnetic susceptibility and high spectrophotometric brightness, and spatially coincide with known industrial dolomite deposits. An additional group of ‘detrital-rich dolomite’ is distinct from the other groups in an elevated content of high field strength elements and Al, and intermediate spectrophotometric brightness. This variety represent samples containing a higher content of co-settled volcaniclastic-siliciclastic material in the marble precursor. Assessment of the clustered data in relation to magnetic susceptibility measurements from the same samples show that Halo and Ore dolomite can be differentiated from other dolomite types by geomagnetic methods, hence providing a proxy for their indirect detection during geophysical surveys.

Unravelling the genesis of metamorphosed mineral deposits can be complicated due to difficulties in separating between primary features and features that formed during the metamorphic overprint. Such uncertainty exists for stratabound and dolomite- and skarn-hosted Zn-Pb-Ag sulfide deposits in 1.89 Ga rocks in the Bergslagen lithotectonic unit (BLU) of Sweden, where a metasomatic vs. regional metamorphic origin for skarns has long been discussed. By integrating geological mapping with new lithogeochemical, mineralogical, and stable isotope data (C, O, S), we show that complexly zoned garnet and clinopyroxene skarns in the Sala area of the central BLU predate mineralization. Sphalerite-galena mineralization formed after the deposition of a younger, more Mn-rich ferroan diopside and andradite-grossular garnet, and is associated with phlogopite, tremolite-actinolite, chlorite, serpentine, and calcite. Mineralization in conjunction with a transition from high-T metasomatism to hydrolytic alteration is inferred. An average δ³⁴S_V-CDT of 1.6 ± 1.9‰ in sulfides is consistent with a primordial sulfur source. Trends defined by negative shifts in δ¹⁸O_V-SMOW and δ¹³C_V-PDB in dolomite and calcite are consistent with fluid infiltration at 300–500 °C. The alteration system is sharply truncated by unaltered, c. 1.89 Ga calc-alkaline granite and porphyritic intrusions, which along with F₁ folding of the alteration zones and mineralization suggest that mineralization predate regional metamorphism. The Sala deposits are interpreted as Zn skarn deposits formed in conjunction with the emplacement of intrusions into penecontemporaneous marine volcanic and dolomitized limestone strata. The unusually Mg-rich mineralogy in relation to Zn skarns worldwide most likely reflects the dolomitic precursor.

This contribution presents an updated view on the genesis of stratabound Zn-Pb-Ag mineralization in the Sala area, Bergslagen, Sweden. Integrated legacy and new geological, geochemical and geophysical data reveal that the deposits are hosted by a complex array of magnesian skarn-altered zones in dolomitic marble. These mineralized zones parallel early faults and metavolcanic interbeds in the host marble, and converge downwards in the stratigraphy adjacent to a 1.89 Ga calc-alkaline granite-granodiorite batholith. Prograde alteration involved formation of early barren ferroan diopside- and forsterite-bearing skarns. Mineralization is mainly associated with subsequent alteration to tremolite, chlorite, serpentine, magnetite and calcite. The hydrous associations overlap mineralogically with assemblages formed during subsequent greenschist facies regional metamorphism between 1.87 Ga and 1.8 Ga. However, ferroan diopside and forsterite are unique to the alteration system, and indicate mineralization in conjunction with an early, high T, metasomatic alteration event at 1.89 Ga. The Sala deposits can be classified as Zn skarn deposits, albeit atypical in the magnesian nature of the skarns and the lack of minerals with essential Mn. The Fe and Mn content in magnesian silicates and carbonates is however sufficient to induce clear enrichment haloes of these elements around the deposits. The magnesian nature of the skarns probably reflect formation in a shallow marine continental backarc tectonic setting, and an importance of seawater in early pre-skarn alteration stages, such as dolomitization.

Metamorphosed polymetallic sulfide deposits in Bergslagen, Sweden, are currently divided into 1: Strata bound volcanic-associated limestone-skarn Zn-Pb-Ag-CuAu sulfide deposits (SVALS) and 2: Stratiform ash-siltstonehosted Zn-Pb-Ag sulfide deposits (SAS). It has not been completely resolved if these deposit types formed from similar hydrothermal fluids. Recent investigations at the Falun SVALS deposit and the Zinkgruvan SAS deposit suggest that fluids of contrasting pH, fO(2), salinity and T were involved in their origin. Whereas Falun formed by cooling and neutralization of acidic (pH<4), hot (300-400 C) and reducing fluids carrying metals and sulfur together, Zinkgruvan formed by reduction of oxidized brines at a near-neutral pH. Falun is a vent-proximal, synvolcanic carbonate-replacement deposit with similarities to VMS and skarn deposits, whereas Zinkgruvan is a post-volcanic, exhalative deposit with similarities to some SEDEX deposits. Our results suggest that the different character of SVALS and SAS deposits in part are functions of fundamental differences in fluid chemistry, controls on sulfide precipitation and relationship to volcanism.

We present improved age constraints for the world-class Zinkgruvan Zn-Pb-Ag and Cu deposit: one of the largest Zn deposits of the Fennoscandian shield, and one of the earliest large, basin-hosted Zn deposits that formed from oxidized saline brines. Secondary Ion Mass Spectrometry (SIMS) U-Pb dating on zircon is used to constrain at least two phases of c. 1.9 Ga volcanism in the Zinkgruvan area, separated by a period of fluvial sedimentation, all of which predated formation of the stratiform Zn-Pb-Ag mineralization. A 1908 ± 4 Ma age for a rhyolitic rock of the first volcanic phase is the oldest recorded U-Pb zircon age of a volcanic rock in the Bergslagen lithotectonic unit (BLU) where Zinkgruvan is located. Similarly, two identical ages of 1898 ± 5 Ma for volcanic rocks belonging to the second volcanic phase indicate that the local volcanic activity, which formed the stratigraphic footwall, ended earlier in the Zinkgruvan area than in other parts of the BLU, where intense explosive felsic volcanic and intrusive activity until c. 1891 Ma has been demonstrated. This, along with a hybrid siliciclastic-volcaniclastic (tuffitic) character of the Zinkgruvan ore host, confirms earlier interpretations that the Zinkgruvan deposit formed in an actively subsiding basin, distal to active volcanic centers in the BLU in the time range 1.90–1.89 Ga. Our results support models suggesting that basinal brine-driven hydrothermal systems in sedimentary basins distal to volcanic centers could form world-class Zn deposits as early as c. 1.90 Ga.

The metamorphosed, stratiform, c. 1.9 Ga Zinkgruvan Zn-Pb-Ag deposit is one of Europe’s largest producers of Zn. Since 2010, disseminated Cu mineralization is also mined from dolomite marble in a hydrothermal vent-proximal position in the stratigraphic footwall. Local enrichments of Co and REE exist in the vent-proximal mineralization types, albeit their distribution is poorly known. This contribution provides new data on the distribution of Co and REE within the Zinkgruvan deposit.

LA-ICP-MS analysis suggest that lattice-bound cobalt in sphalerite range between 44 ppm and 1372 ppm, with the lowest and highest values occurring in distal and proximal mineralization, respectively. Proximal Co-rich sphalerite is always Fe-rich. Lattice-bound Co also occur in pyrrhotite; ranging from 52 ppm in distal ore to 1608 ppm in proximal ore. There is a concurrent increase in lattice-bound Ni from 3 ppm to 529 ppm. In proximal ore, Co is also hosted by cobalt minerals such as costibite (27.37 wt.% Co), safflorite (16.21 wt.% Co), nickeline (7.54 wt.% Co), cobaltite (32.74 wt.% Co) and cobaltpentlandite (25.49 wt.% Co). Automated quantitative mineralogy suggest that these minerals are highly subordinate to sphalerite (<70.11%) and pyrrhotite (<14.69%), amounting to <2.88% cobalt minerals with safflorite being most common (up to 2.67%). Cobalt deportment calculations suggest that the proportion of whole-rock Co that is lattice-bound to sphalerite and pyrrhotite ranges from 7.80% to 100%, with sphalerite being the main host. Whole-rock As and Ni contents pose a strong control on whether Co occurs lattice-bound or as Co minerals.

LA-ICP-MS analysis show that accessory apatite in proximal, marble-hosted Cu mineralization carries a few thousand ppm ∑REE, but locally up to c. 1.6 wt.% ∑REE. The apatite can be subdivided into two types. Type 1 apatite is characterized by dumbbell-shaped chondrite-normalized REE profiles with relative enrichment of in particular Sm-Tb, depletion of Yb-Lu relative to La-Pr, local positive Gd anomalies, and weak positive to negative Eu anomalies. Type 2 apatite is characterized by flat to negatively sloping REE profiles from La to Gd and relative HREE depletion. Additional REE is hosted by monazite. Type 1 apatite was only found as a gangue to Cu mineralization. The Type 1 apatite REE signature is characteristic of hydrothermal apatite, and a direct genetic association with vent-proximal Cu mineralization can be inferred.

Comparison with published REE contents in apatite suggest that vent-proximal Zinkgruvan apatite is locally as REE-rich as apatite from Kiruna-type apatite iron oxide deposits, and more REE-rich than apatite in other metamorphosed sediment-hosted sulphide deposits in the world, such as the Gamsberg deposit (RSA).

Metamorphosed polymetallic sulfide deposits in Bergslagen, Sweden, are currently divided into 1: Stratabound volcanic-associated limestone-skarn Zn-Pb-Ag-Cu-Au sulfide deposits (SVALS) and 2: Stratiform ash-siltstone hostedZn-Pb-Ag sulfide deposits (SAS). It has not been completely resolved if these deposit types formed from similar hydrothermal fluids. Recent investigations at the Falun SVALS deposit and the Zinkgruvan SAS deposit suggest that fluids of contrasting pH, ƒO2, salinity and Twere involved in their origin. Whereas Falun formed by cooling and neutralization of acidic (pH<4), hot (300-400ºC) and reducing fluids carrying metals and sulfur together, Zinkgruvan formed by reduction of oxidized brines at a near-neutral pH. Falun is a vent-proximal, synvolcanic carbonate-replacement deposit with similarities to VMS and skarn deposits, whereas Zinkgruvan is a post-volcanic, exhalative deposit with similarities to some SEDEX deposits. Our results suggest that the different character of SVALS and SAS deposits in part are functions of fundamental differences in fluid chemistry, controls on sulfide precipitation and relationship to volcanism.

We address controls on Co and Ni distribution, based on their solubility in hydrothermal fluids as functions of pH, ƒO₂ and T, in two end-member types of sulphide deposits in Bergslagen, Sweden. Oxidized hydrothermal fluids, as have been suggested for the formation of the Zinkgruvan deposit, would efficiently transport Co and Ni in solution, even at 150 ^oC. Formation of Co and Ni sulphides and sulphosalts supersedes or overlaps the precipitation of other sulphides along a reduction or H₂S-mixing path. This is consistent with the presence of Co and Ni sulphides and sulphosalts in vent-proximal Cu-Zn mineralization at Zinkgruvan.

Reduced, acidic and hot (≥250 ^oC) hydrothermal fluids that have been invoked for the formation of deposits like Falun and Stollberg could also transport Co and Ni in solution. However, their solubility is strongly dependent on high T and low pH. Cooling and neutralization are here proposed as likely key triggers for the deposition of Co and Ni, yet, unlike in the Zinkgruvan scenario, saturation will occur within the pyrite stability field, whereby these metals may be sequestered as stoichiometric lattice substitutions in pyrite and other sulphides rather than forming minerals of their own.

We conclude that at any T or realistic pH, hydrothermal systems involving oxidized brines have a greater ability to traverse and leach large rock volumes of Co and Ni. Consequently, areas hosting deposits that formed from such brines have a significant exploration potential for these metals, even in areas where Co-enriched source rocks are lacking or subordinate.

The Zinkgruvan deposit has been included in compilations of exceptionally Ga- and Ge-endowed deposits in Sweden. Available published data sets do however not support a substantial enrichment in Ga and Ge. In this contribution, we investigate the Ga- and Ge-endowment based on a whole-rock lithogeochemical data and ore grade analyses from the deposit. Based on our results, we find it highly unlikely that a Ga-endowment exists in the ore. A Ge-endowment may exist, but we find no evidence of Ge grades at the 1,000 ppm level that have been reported previously.

Recent disruptions of raw material value chains during the COVID-19 pandemic have highlighted Europe’s dependency on imports of metals and minerals. Meanwhile, the European Commission is establishing ambitious  policy  initiatives,  aiming  at  making Europe climate neutral in 2050. In this contribution, we emphasise the critical role of geosciences education in this energy transition, in forming the next generation of  mining  professionals.  In  the  Nordic  countries, active industry–university collaboration in one of the most active mining hubs in Europe has allowed frequent student–industry interaction, access to real-life learning environments, and development of specialised educational modules. These have been made accessible to exchange students from other European countries via exchange programmes and innovative digipedagogical learning tools.

Medium- to high-grade metamorphosed, 1.9 Ga, stratiform, syngenetic Zn-Pb±Ag sulfide deposits comprise an economically important type of ore deposit in the Bergslagen lithotectonic unit of the Fennoscandian shield. The Lovisa Zn-Pb deposit occurs in a metamorphosed succession of rhyolitic ash-siltstone, rhyolitic mass flow deposits, limestone and iron formation, deposited at a stage of waning volcanism in Bergslagen.

Accessory graphite, absence of Ce anomalies in shale-normalized rare-earth element (REE) data, and absence of hematite in Mn-rich iron formations stratigraphically below the Lovisa Zn-Pb deposit indicate a suboxic-anoxic depositional environment. The uppermost Mn-rich iron formation contains disseminated, inferred syngenetic Pb-Ag mineralization with mainly negative δ³⁴S values in sphalerite and galena (-6.1 to -1.9‰).

Deposition of this iron formation terminated during a pulse of explosive felsic volcanism. The Lovisa Zn-Pb deposit is interpreted to have formed in an alkali-rich brine pool developed immediately after this volcanic event, based on lithogeochemical and stratigraphic evidence. The first stage of mineralization deposited stratiform sphalerite mineralization with mainly positive δ³⁴S values (-0.9 to +4.7‰). This was succeeded by deposition of more sphalerite-galena stratiform mineralization with δ³⁴S values close to 0‰ (-2.1 to +1.5‰). The more galena-rich mineralization partitioned strain and was partly remobilized during later ductile deformation.

The stratigraphic context, sulfide mineralogy, sulfur isotopes and alteration geochemistry suggest that the metalliferous fluids and the depositional environment were H₂S-deficient (S-poor or SO₄^2--dominant). The source of sulfur is interpreted to have been a mixture of H₂S derived from bacterial and thermochemical seawater sulfate reduction, and sulfur derived from leaching of volcanic rocks, with the latter becoming more important over time.

Lovisa formed in a setting where basin subsidence was periodically punctuated by the deposition of thick, syn-eruptive felsic volcaniclastic massflow deposits. Coeval volcanism was likely important for driving hydrothermal activity and supplying a reservoir of metals and sulfur. However, the high rate of deposition of volcaniclastic sediment in Bergslagen also precluded the establishment of long-lived, deep and anoxic environments favorable for accumulation of organic matter and H₂S. This stratigraphic pattern is common in Bergslagen and may explain why large stratiform Zn-Pb deposits are uncommon in the region and restricted to the uppermost part of the metavolcanic succession, directly stratigraphically beneath post-volcanic pelitic rocks.