The Bjursås ore field is located within the Bergslagen domain of central Sweden just northwest of the town of Falun. The field hosts small scale mines such as the 17th century Floberg copper mine and has been the focus of mineral exploration on several occasions during the 20th and early 21th century, with no major economic deposits being uncovered as of yet. Currently being explored by Boliden AB, this thesis focuses on alteration patterns in the southern part of the ore field, near the village of Oxberg. Several drill cores provided by Boliden AB have been logged and lithogeochemical samples were taken in intervals to aid in identifying alteration patterns and correlation.Three lithological and chemostratigraphic profiles have been created along a northeastern trend and show that the dominant lithologies are volcaniclastic silt- and sandstones with subordinate segments of pumiceous breccia-sandstones. They are primarily rhyolitic and dacitic. Pyritic argillite sediments belonging to the Oxberg formation occur sporadically and sub-intrusive and sills of amphibolites are common. Impure limestones with abundant quartz grains and limited lateral extents are encountered and suggest that high clastic input dominated during times favourable for limestone formation. The limestones do not host any major base metal mineralisation and coupled with limited lateral extents, do not appear to be a favourable future exploration target within the area.Two main alteration styles are recognised to be present in Oxberg. The first is an MgO-rich biotite-corderite-anthophyllite alteration and is associated with a weak to moderate impregnation of sphalerite and galena. The second is a FeO-rich biotite-garnet-staurolite-corderite alteration associated with magnetite-garnet-actionolite ± diopside skarn. No base metal sulphides are associated with the FeO-rich alteration. The MgO-rich style is interpreted be analogous to a metamorphosed VHMS-style hydrothermal system with a wider strong sericite-quartz alteration in the footwall and Mg-chlorite-rich near vent pipe alteration. The intercepted interval with MgO-rich alteration is located in an interpreted hinge zone of a fold in the Central profile and is interpreted to be a conduit feeder pipe for hydrothermally circulating fluids. Future drilling of this structure to learn the extent of the MgO-rich mineralised zone is recommended as it may lead to a massive sulphide lens.The Large box plot is demonstrated to be a useful tool for vectoring towards mineralisation in the area and may prove useful in future exploration of the area. Samples of sericite-quartz altered rock and MgO-rich alteration plot along trends on the Large box plot with MAI values that increase in the strongly altered sericite-quartz footwall rocks while CCPI values increase in the MgO-rich feeder zone that hosts base metal sulphides.Six chemical groups are identified around the Oxberg area. Of particular interest is the dacitic-andesitic chemical group, specifically siltstones, which display the strongest MgO-rich alteration. These are proposed to be a stratigraphic target interval for exploration and may lead to future base metal sulphide mineralisation in the area as well as the Bjurås ore field as a whole. / Bjursåsfältet ligger i Bergslagen strax nordväst om Falun. Vid Bjursåsfältet finns ett flertal hyttområden och mindre nedlagda gruvor, så som Flobergsgruvan som startades under 1600-talet. Området har varit i fokus för mineralprospektering vid flera tillfällen under 1900-talet och i början av 2000-talet men inga större mineraliseringar av ekonomiskt värde har hittats. Boliden AB undersöker för närvarande området och denna uppsats fokuserar på ett område inom Bjursåsfältet kring byn Oxberg. Omkring detta område har en stor omvandlingszon identifierats, men en detaljerad undersökning av denna zon har ännu inte gjorts. Mineraliseringar i berg beror ofta på att en fluid trängt igenom berget och av olika anledningar avsatt sitt metallinnehåll just där. Denna process leder vanligtvis också till att berget som fluiden trängt igenom omvandlas. En sådan omvandlingszon, som ofta är mycket större sett till volym än just själva mineraliseringen, kan agera som vägvisare för att nå fram till mineraliseringen.Borrkärnor som tidigare borrats inom detta område har karterats samt provtagits för kemisk analys med syfte att identifiera olika omvandlingsmönster och deras utbredning. Tre litologiska och kemiska tvärsnitt av underjorden har skapats utifrån borrkärnorna längs en nordöstlig trend. De visar att de dominerande bergarterna är utbrottsprodukter från olika vulkaner som var aktiva i området då bergarterna bildades. De är i första hand felsiska (ljusa bergarter, som exempelvis graniter) i sammansättning. Även lerrika sediment som tillhör Oxbergformationen förekommer sporadiskt och mafiska bergertar (mörka bergarter, som exempelvis basalt) samt orena kalkstenar förekommer i mindre utsträckning. Kalkstenar i Bergslagen är ofta associerade med mineralisering men detta verkar inte vara fallet kring Oxberg på grund av kalkstenarnas begränsade sidoutsträckning. Därmed föreslås kalksten inte vara ett gynnsamt mål för framtida prospektering i området.Två huvudsakliga omvandlingstyper är identifierade i Oxberg. Den första är en magnesiumrik biotit-corderit-antofyllit-omvandling som är associerad med en svag till måttlig impregnering av zinkblände och blyglans. Den andra är en järnrik biotit-granat-staurolit-corderit-omvandling i samband med magnetit-granat-aktinolit ± diopsid-omvandling. Inget zinkblände eller blyglans är förknippat med den järnrika omvandlingen. Den magnesiumrika omvandlingen gränsar till en bredare zon av stark sericit-kvarts-omvandling. Detta förhållande är utmärkande för en vis typ av malmbildningsprocess och en rekommendation är att titta närmare på just denna zon med hjälp av framtida borrningar.Utöver detta har två separata metoder identifierats vilka föreslås vara användbara för framtida prospektering av området. Metoderna baseras på kemisk provtagning av omvandlade bergarter och analysering enligt metoder beskrivna i denna uppsats.

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>