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With the advancements in acquisition and processing of seismic reflection data recorded over crystalline rocks, building three-dimensional geologic models becomes increasingly favorable. Because of little available petrophysical data, interpretations of seismic reflection data in hardrock terrains are often speculative. Potential field data modeling are sometimes performed in order to reduce the ambiguity of seismic reflection interpretations. The Kristineberg mining area in the western part of the Paleoproterozoic Skellefte Ore District was chosen to construct a pilot three-dimensional geologic model in an attempt to understand the crustal architecture in the region and how the major mineral systems operated in this architecture. To contribute to this aim, two parallel seismic reflection profiles were acquired in 2003 and processed to 20 sec with special attention to the top 4 sec of data. Several reflections were imaged and interpreted by the aid of reflector modeling, borehole data, 2.5D and 3D potential field modeling, and geological observations. Interpretations are informative at the crustal scale and help to construct a three-dimensional geologic model of the Kristineberg mining area. The three-dimensional geologic model covers an area of 30×30 km2 down to a depth of 12 km. The integrations help to interpret a structural basement to the Skellefte volcanic rocks, possibly with Bothnian Basin metasedimentary affinity. The contact is a shear-zone that separates the two units, generating large fold structures, which can be observed in the region. The interpretations help to divide the Revsund granitic rocks into two major groups based on their present shape and thickness. A large gravity low in the south is best represented by the intrusion of thick dome of Revsund granite. In the north, the low-gravity corresponds to the intrusion of sheet-like Revsund granites. In general, the structure associated with the Skellefte volcanics and the overlying metasedimentary rocks are two thrusts exposing the Skellefte volcanic rocks in the cores of hanging wall anticlinal structures. Lack of coherent reflectivity in the seismic reflection data may be due to complex faulting and folding systems observed in the Skellefte volcanics. Ultramafic sills within the metasedimentary rocks are interpreted to extend down to depths of about 5-6 km. The interpretations are helpful for targeting new VHMS deposits and areas with gold potential. For VHMS deposits, these are situated in the southern limb of a local synformal structure south of the Kristineberg mine, on the contact between the Revsund granite and the Skellefte volcanic rocks. A combination of metasedimentary and mafic-ultramafic rocks are highly gold prospective in the west, similar to observations elsewhere in the region. There are still questions that remain unanswered and need more work. New data in the study area will help to answer questions related to e.g., an enigmatic diffraction seismic signal in Profile 5 and the structural relationship between the Skellefte volcanic rocks and the Malå volcanics. Although the derived 3D geologic model is preliminary and constructed at the crustal scale, it provides useful information to better understand the tectonic evolution of the Kristineberg mining area.

<p>With the advancements in acquisition and processing of seismic reflection data recorded over crystalline rocks, building three-dimensional geologic models becomes increasingly favorable. Because of little available petrophysical data, interpretations of seismic reflection data in hardrock terrains are often speculative. Potential field data modeling are sometimes performed in order to reduce the ambiguity of seismic reflection interpretations. The Kristineberg mining area in the western part of the Paleoproterozoic Skellefte Ore District was chosen to construct a pilot three-dimensional geologic model in an attempt to understand the crustal architecture in the region and how the major mineral systems operated in this architecture. To contribute to this aim, two parallel seismic reflection profiles were acquired in 2003 and processed to 20 sec with special attention to the top 4 sec of data. Several reflections were imaged and interpreted by the aid of reflector modeling, borehole data, 2.5D and 3D potential field modeling, and geological observations. Interpretations are informative at the crustal scale and help to construct a three-dimensional geologic model of the Kristineberg mining area. The three-dimensional geologic model covers an area of 30×30 km² down to a depth of 12 km. The integrations help to interpret a structural basement to the Skellefte volcanic rocks, possibly with Bothnian Basin metasedimentary affinity. The contact is a shear-zone that separates the two units, generating large fold structures, which can be observed in the region. The interpretations help to divide the Revsund granitic rocks into two major groups based on their present shape and thickness. A large gravity low in the south is best represented by the intrusion of thick dome of Revsund granite. In the north, the low-gravity corresponds to the intrusion of sheet-like Revsund granites. In general, the structure associated with the Skellefte volcanics and the overlying metasedimentary rocks are two thrusts exposing the Skellefte volcanic rocks in the cores of hanging wall anticlinal structures. Lack of coherent reflectivity in the seismic reflection data may be due to complex faulting and folding systems observed in the Skellefte volcanics. Ultramafic sills within the metasedimentary rocks are interpreted to extend down to depths of about 5-6 km. The interpretations are helpful for targeting new VHMS deposits and areas with gold potential. For VHMS deposits, these are situated in the southern limb of a local synformal structure south of the Kristineberg mine, on the contact between the Revsund granite and the Skellefte volcanic rocks. A combination of metasedimentary and mafic-ultramafic rocks are highly gold prospective in the west, similar to observations elsewhere in the region. There are still questions that remain unanswered and need more work. New data in the study area will help to answer questions related to e.g., an enigmatic diffraction seismic signal in Profile 5 and the structural relationship between the Skellefte volcanic rocks and the Malå volcanics. Although the derived 3D geologic model is preliminary and constructed at the crustal scale, it provides useful information to better understand the tectonic evolution of the Kristineberg mining area. </p>

The Skellefte ore district in northern Sweden is a Palaeoproterozoic volcanic arc and one of the most important ones hosting volcanogenic massive sulfide (VMS) deposits, producing mainly base metals and orogenic gold deposits. Due to high metal prices and increased difficulties in finding shallow deposits, the exploration for and exploitation of mineral resources is quickly being moved to greater depths. For this reason, a better understanding of the geological structures in 3D down to a few kilometers depth is required as a tool for ore targeting. As exploration and mining go deeper, it becomes more and more evident why a good understanding of geology in 3D at exploration depths, and even greater, is important to optimize both exploration and mining. Following a successful pilot 3D geological modeling project in the western part of the district, the Kristineberg mining area, a new project "VINNOVA 4D modeling of the Skellefte district" was launched in 2008, with the aim of improving the existing models, especially at shallow depth and extending the models to the central district. More than 100 km of reflection seismic (crooked) profiles were acquired, processed and interpreted in conjunction with geological observations and potential field data. Results were used to constrain the 3D geological model of the study area and provided new insights about the geology and mineral potential at depth. Results along the seismic profiles in the Kristineberg mining area proved the capability of the method for imaging reflections associated with mineralization zones in the area, and we could suggest that the Kristineberg mineralization and associated structures dip to the south down to at least a depth of about 2 km. In the central Skellefte area, we were able to correlate main reflections and diffractions with the major faults and shear zones. Cross-dip analysis, reflection modeling, pre-stack time migration, swath 3D processing and finite-difference seismic modeling allowed insights about the origin of some of the observed reflections and in defining the imaging challenges in the associated geological environments. / VINNOVA 4D modeling of the Skellefte district

Volcanic-hosted massive sulfide (VMS) deposits are one of the main sources for zinc, copper, lead, silver and gold in Sweden. The majority of VMS deposits in Sweden are located in the Bergslagen region and the Skellefte district (Fig. 1). The Skellefte district hosts approximately 80 VMS deposits whereas 21 deposits have been mined since 1924 and 6 mines are currently in operation. VMS deposits tend to form more often on the intersection of the normal/reverse and transfer faults since there is an increased conductivity for hydrothermal fluids and increased fluid flow, so a structural interpretation of regional and deposit scale is important for exploration. The alteration patterns and mapped structures observed in the West Maurliden coincide with major structures found in the Skellefte district. Using this data and data from previous authors a general structural evolution of the Maurliden deposit has been constructed which shows the presumed outcome from the early extensional and later compressional stages ongoing in the region. Study of the mineralization shows that there is also the possibility to find mineralized rock within possible low strain blocks which might contain preserved primary textures and structures. A schematic plan view of the structure assemblage in the Skellefte district was established which shows perspective areas for future exploration.

The Barsele area is located in the "Gold Line", southwest of the volcanogenic massive sulphide (VMS) Skellefte district which hosts more than 85 known VMS-deposits where some are gold-bearing. The Barsele area consists of the Central, Avan and Skiråsen intrusion hosted gold deposits, the Norra gold-bearing VMS-deposit and the Risberget zone. This study includes a geochronological study of two coherent volcanic rocks from the Norra zone using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry and Scanning Electron Microscope images of zircons and also includes petrographic microscopy of thin sections and lithogeochemical data used in classification diagrams.  This study presents two new LA-ICP-MS zircon U-Pb ages for VMS-hosting volcanic rocks from the Barsele area: o   1956.8 ± 6.47 Ma (n: 17, MSWD: 0.99), sample A from a hydrothermally altered volcanic rock  o   1952.2 ± 14.5 Ma (n: 6, MSWD: 1.1), sample B from a rhyolite-dacite  The new U-Pb age constraints presented in this study indicate that the volcanic rocks of the Norra zone are older than the volcanic rocks in the Skellefte Group. The results presented in this report together with the earlier dated c. 1.96 Ga metadacite suggests that the volcanic rocks of the Barsele area are located at a lower chronostratigraphic level than the Skellefte Group, i.e., at the chronostratigraphically lower Bothnian Supergroup. Additionally, the results presented here conclude that two VMS-events can be delineated by geochronological data which further opens up for the question if the Barsele area belong to an older arc system (e.g., the Knaften arc) or were part of an arc system evolving continuously between 1.95-1.88 Ga (e.g., the Knaften-Skellefte arc). / Barsele, Västerbottens län, norra Sverige, ligger i ett område som kallas för Guld linjen och är beläget sydväst om Skelleftefältet där mer än 85 kända förekomster av massiva sulfidmalmer (VMS), vissa guldförande, förekommer. Barseleområdet är uppdelat i Central, Avan och Skiråsen zonen, Norra zonen och Risberget zonen där Norra zonen är en guldförande VMS-fyndighet. Denna studie inkluderar en geokronologisk studie av zirkoner från två vulkaniter från Norra zonen som har analyserats med Laser Ablation Inductively Coupled Plasma Mass Spectrometry och avbildats med svepelektronmikroskop samt petrografisk studie av två tunnslip och klassificeringsdiagram baserade på litogeokemisk data. Denna studie presenterar två nya LA-ICP-MS zirkon U-Pb åldrar för VMS-förande vulkaniter i  Barseleområdet:  o   1956.8 ± 6.47 Ma (n: 17, MSWD: 0.99), prov A från en hydrotermalt omvandlad vulkanisk bergart o   1952.2 ± 14.5 Ma (n: 6, MSWD: 1.1), prov B från en ryolit-dacit De två nya  U-Pb zirkon åldersbestämningarna som presenteras i denna studie indikerar att de vulkaniska bergarterna i Norra zonen är äldre än de vulkaniska bergarterna som tillhör Skelleftegruppen. Vidare antyder de presenterade åldrarna i denna studie tillsammans med den tidigare genomförda dateringen av en metadacit (cirka 1.96 Ga) på att de vulkaniska bergarterna i Barseleområdet kan tillhöra suprakrustala bergarter tillhörande Bottniska bassängen. Barseleområdet föreslås därför förekomma vid en lägre kronostratigrafisk nivå jämfört med Skelleftegruppen, det vill säga vid samma kronostratigrafisk nivå som Bottniska Supergruppen. Utifrån de resultat som presenteras här kan slutsatsen att två VMS-områden kan avgränsas av geokronologiska data som ytterligare öppnar för frågan om Barsele-området tillhör ett äldre bågsystem (t.ex. Knaften-bågen) eller var en del av ett bågsystem som utvecklades mellan 1.95-1.88 Ga (t.ex. Knaften-Skelleftebågen).

Sixteen samples from 8 locations within and surrounding the Björkdal mine area in northern Sweden were chosen in order to reevaluate and characterize previous interpretations of the lithology and hydrothermal alteration. Geochemical analysis by ICP-MS was made in order to chemically classify the lithology of the area and petrological studies were made by study of thin sections and core logging. Three different sets of major alteration types with similar protolith were noted where two dominated: a) a silicified, sericitic, deformed unit; b) a felspathic altered unit with various intensity of epidote; c) amphibole and a so called green banded unit that has undergone Ca-Mg-metasomatism. The main host rock lithology could be identified as a coarse grained, equigranular, plagioclase and (Na-K)-feldspar dominated rock with slightly elongated, stubby grains that have an interlocked, igneous texture. Apatite is a reoccurring accessory mineral in all samples and remains relatively undamaged. Fragmental quartz occur in the samples and is primary to later forming alteration minerals.Amphiboleis also considered to be primary, but can also been formed during regional metamorphism. Two lithologies could be identified in addition to the main protolith. Two samples were located above the marble horizon and was classified as basaltic unit with Ca-plagioclase, biotite and amphibole. One sample mainly consisted of aligned amphibole and biotite and is considered to be an amphibolite xenolith or a dyke intruding the main protolith rock. An increase in the alteration minerals albite, amphibole, epidote and allanite, with peak intensity found north-east of the mine site. This is confirmed by the geochemistry of major elements Fe2O3, MgO, CaO and the ratio between Na2O and K2O. The zonation is explained by a suggested increase in temperaturecondition during metamorphosis. This could be explained by either an underlying intrusion or bystructurally controlled variation in metamorphism by e.g. major faults running through the area.Comparisons with previous work in Björkdal favors an intrusion-related origin for the gold deposit.

The Aurora zone is an ore zone which was recently discovered in the Björkdal gold mine, northern Sweden, and it has been the main focus of mining and exploration activities for the past few years (Pressacco et al., 2020). The purpose of this project is to determine how gold occurs in the Aurora zone. A three-day long campaign was therefore done at the processing plant at the Björkdal mine where 11 000 tonnes of ore from the drive Aurora 370/1650 E+W were processed. The issue regarding the ore from the Aurora zone is that it has a lower recovery compared to the rest of the mine.  Six chip samples, 12 samples from the ingoing plant feed and two tailing samples were analyzed using optical microscopy, scanning electron microscopy (SEM), and automated mineralogy (QEMSCAN). This was done to determine the mineralogy, grain size distribution, mineral associations, textures and modal mineralogy which all are factors that could influence the metal recovery at the processing plant. The samples were prepared using the cold mounting method and epoxy mounts were created. After cutting, grinding, and polishing, the sections were ready to be examined. Optical microscopy was performed using a Nikon ECLIPSE E600 POL microscope. Ten epoxy mounts were carbon coated and automated mineralogy was performed on nine of them in a ZEISS Sigma 300 VP using a recipe (analysis mode) for “bright phase search”. Manual point-ID analysis was done using a ZEISS MERLIN SEM. Fifty gold grains were identified in this study, 48 of them in the chip samples and two of them in the ingoing-feed samples. 64% of them were associated with silicates, 22% were quartz associated, 12% were associated with bismuth minerals and 2% of them were associated with sulfides. The grain size distribution has a range between 0.7 and 19 μm and the median grain size is 4.8 μm. The gold grains identified from the Aurora zone have a significantly smaller median grain size than gold from other parts of the mine. The majority of the gold grains identified in this study, have a very fine grain size, are mainly associated with silicates and most prominently occur as inclusions. Gold that occurs in this way is typically difficult to recover in the processing plant and it seems like this is the main reason for the lower gold recovery from the Aurora ore zone.  No gold was found in the tailings, suggesting that the mineral process is performing well although no thorough conclusion can be made in regards of the processing. The lack of data for the different sample types in this project is an issue. Gold from the tailings must be identified and examined to draw any clear conclusions regarding the processing. For future work, it is therefore recommended to analyze more tailing samples and to implement hydroseparation at the sample preparation stage, to separate the heavier gold particles from lighter minerals. Then more gold will most likely get detected in the tailing samples.

La plupart des gisements de Ni-Cu sont issus de magmas komatiitique ou tholéiitique associés à des panaches mantelliques. Leur genèse fait intervenir l’exsolution d’un liquide sulfuré immiscible, l’interaction entre les liquides silicaté et sulfuré afin de concentrer ce-dernier en éléments chalcophiles, et l’accumulation du liquide sulfuré en quantités économiques. La saturation en sulfure est généralement atteinte en réduisant la solubilité des sulfures. Celle-ci se fait par assimilation de roches encaissantes siliceuses et/ou sulfurés.Le dyke d’Älgliden de la ceinture de Skellefte, en Suède, contient des sulfures de Cu et Ni dont les quantités ne sont actuellement pas économiques. La minéralisation d’Älgliden est atypique dans la mesure où elle contient d’importantes teneurs en Au, elle a un faible rapport Ni/Cu et enfin est formé dans un contexte de subduction. Le dyke recoupe un gisement porphyrique à Cu-Au contenant des sulfures ce qui laisse suggérer que la minéralisation d’Älgliden est formée par assimilation.Les objectifs de ce projet de recherche étaient d’examiner les processus de formation de la minéralisation d’Älgliden et son potentiel minier ainsi que de mieux comprendre la formation des gisements à Ni-Cu en contexte de subduction. Ce travail inclut l’étude pétrologique du minerai et de ses roches hôtes, la détermination de compositions minérales, l’analyse des éléments majeurs et traces sur roche totale et enfin des analyses des isotopes du soufre. Ce projet a été financé par la compagnie Boliden qui détient le gisement d’Älgliden.Le dyke est composé en majeure partie de norite à olivines, et minoritairement de leucogabbros. Les compositions sur roches totales, les textures magmatiques et les compositions minérales suggèrent que les norites à olivine se sont formées par accumulation d’olivine tandis que les leucogabbros représentent des liquides résiduels avec ou sans cristaux cumulus de plagioclase ± orthopyroxene. Les norites sont interprétées comme étant formées par une ou deux injections de bouillie cristalline suivie de cristallisation fractionnée. Le magma parent des roches d’Älgliden était un basalte hydraté et évolué dont la teneur en MgO est estimé à 6%.Le minerai sulfuré est principalement disséminé à travers l’ensemble de l’intrusion d’Älgliden. Quelques concentrations modérées de minerai se présentent sous la forme de sulfures en réseaux, de veines de sulfures et de sulfures massifs, lesquelles sont spatialement associées aux leucogabbros et aux xénolites de l’encaissant. L’association entre les leucogabbros et les concentrations en sulfures, leur faible teneur en métaux et leur faible rapport Ni/Cu suggèrent que la phase sulfuré s’est exsolvée tardivement au cours de la différentiation magmatique. Ce timing semble défavorable pour la minéralisation d’Älgliden puisqu’il inhibe à la fois l’interaction entre les liquides silicaté et sulfuré et l’accumulation du liquide sulfuré.La contamination du magma d’Älgliden par son encaissant n’est pas corroboré par les concentrations en élément trace et les compositions isotopiques du soufre. En revanche, ces données indiquent que le magma d’Älgliden s’est mis en place dans une zone de subduction où l’on pense que la saturation en sulfure a été atteinte par réduction d’un magma oxydé et riche en élément volatiles, via la cristallisation de magnétite et/ou dégazage. Les valeurs positives de δ34S suggèrent que l’apport de matériel dérivé du slab est responsable du caractère oxydé du magma d’Älgliden.L’état d’oxydation des magmas d’arc leur permet de dissoudre de grandes quantités de S et d’Au. Leur caractère évolué est responsable de leur fortes concentrations relatives en Au et leur faible rapport Ni/Cu. Ainsi, en contexte subduction les sulfures magmatiques sont susceptibles d’avoir ces caractéristiques, et si l’exsolution du liquide sulfuré a lieu plus tôt que dans le cas d’Älgliden cela pourrait conduire à la formation de gisements économiques. / Most major sulfide Ni-Cu deposits originated from komatiitic or tholeiitic magmas that formed in association with mantle plumes. Their genesis involves the segregation of a immiscible sulfide liquid, reaction of the sulfide liquid with silicate melt to upgrade the sulfide in chalcophile elements, and the concentration of the sulfide liquid in economic amounts. Saturation in sulfide is commonly achieved by lowering the sulfide solubility via assimilation of siliceous wall rock or by increasing the S content by adding S-bearing materials.The Älgliden dike in the Skellefte Belt in Sweden contains currently uneconomic Ni-Cu sulfide mineralization. The Älgliden mineralization is atypical insofar as it contains a significant amount of Au, has a low Ni/Cu ratio and formed in a subduction-related geodynamic setting. The host intrusion intrudes sulfide-bearing Cu-Au porphyry mineralization which led to the suggestion that the Älgliden Ni-Cu-Au mineralization was linked to the assimilation of sulfide-bearing wall rocks.The goals of this research project were to investigate the ore forming processes of the Älgliden mineralization and its ore potential, as well as to improve our understanding of the genesis of Ni-Cu deposits in subduction zones. The work is based on a petrological study of the ore and its host rocks, determination of mineral compositions, analyses of major and trace elements in bulk rocks, and sulfur isotope analyses. This was supported by the Boliden company which owns the deposit.The dike is composed mainly of olivine norites with minor leucogabbros. Bulk rock compositions, magmatic textures and mineral compositions suggest that the olivine norites formed by accumulation of olivine and that the leucogabbros represent residual melts with or without cumulus plagioclase ± orthopyroxene. The norites are interpreted to form by one or two injections of an olivine-rich crystal mush and subsequent fractional crystallization. The parental melt of the Älgliden rocks was a hydrous and evolved basalt estimated to contain ≈6 wt.% MgO.The sulfide ore is mainly disseminated throughout the whole Älgliden intrusion. Some weak ore concentrations occur as network to vein and massive sulfides that are spatially associated with the leucogabbros and wallrock xenoliths. The association between the leucogabbros and the concentrations of sulfide, their low ore grade and Ni/Cu ratio suggest that the sulfide segregated late in the differentiation process. This timing appears unfavorable for the Älgliden mineralization because it inhibited both sulfide-silicate liquid interaction and the accumulation of sulfide.Contamination of the Älgliden magma by its wall rocks is not supported by trace element data and S isotope compositions. Instead these data indicate that the Älgliden magma was emplaced above a subduction zone where the sulfide saturation is thought to occur by reduction of the oxidized and volatile-rich magma by magnetite fractionation and/or by degassing. Positive δ34S values suggest addition of slab-derived material which is thought to be responsible for the oxidized character of the Älgliden magma.The oxidation state of arc magmas allows them to carry large amounts of S and Au. Their evolved character is also responsible for their relatively high Au contents and low Ni/Cu. Such characteristics are likely to occur in magmatic sulfide mineralization in subduction zone settings, and if sulfide liquid segregation had occurred earlier than at Älgliden the process may have produced economic sulfide deposits.