Karin Högdahl

Reflection seismic investigations in the Dannemora area, central Sweden: Insights into the geometry of polyphase deformation zones and magnetite‐skarn deposits

The Bergslagen region is one of the most ore prospective districts in Sweden. Presented here are results from two nearly 25 km long reflection seismic profiles crossing this region in the Dannemora ...

Reactive monazite and robust zircon growth in diatexites and leucogranites from a hot, slowly cooled orogen: implications for the Palaeoproterozoic tectonic evolution of the central Fennoscandian Shield, Sweden

Monazite in melt-producing, poly-metamorphic terranes can grow, dissolve or reprecipitate at different stages during orogenic evolution particularly in hot, slowly cooling orogens such as the Svecofennian. Owing to the high heat flow in such orogens, small variations in pressure, temperature or deformation intensity may promote a mineral reaction. Monazite in diatexites and leucogranites from two Svecofennian domains yields older, coeval and younger U–Pb SIMS and EMP ages than zircon from the same rock. As zircon precipitated during the melt-bearing stage, its U–Pb ages reflect the timing of peak metamorphism, which is associated with partial melting and leucogranite formation. In one of the domains, the Granite and Diatexite Belt, zircon ages range between 1.87 and 1.86xa0Ga, whereas monazite yields two distinct double peaks at 1.87–1.86 and 1.82–1.80xa0Ga. The younger double peak is related to monazite growth or reprecipitation during subsolidus conditions associated with deformation along late-orogenic shear zones. Magmatic monazite in leucogranite records systematic variations in composition and age during growth that can be directly linked to Th/U ratios and preferential growth sites of zircon, reflecting the transition from melt to melt crystallisation of the magma. In the adjacent Ljusdal Domain, peak metamorphism in amphibolite facies occurred at 1.83–1.82xa0Ga as given by both zircon and monazite chronology. Pre-partial melting, 1.85xa0Ga contact metamorphic monazite is preserved, in spite of the high-grade overprint. By combining structural analysis, petrography and monazite and zircon geochronology, a metamorphic terrane boundary has been identified. It is concluded that the boundary formed by crustal shortening accommodated by major thrusting.

Ductile shear zones related to crustal shortening and domain boundary evolution in the central Fennoscandian Shield

The Paleoproterozoic part of the Fennoscandian Shield is composed of crustal components formed in different tectonic settings and generally separated by well-defined shear ...

Fluorapatite-monazite-allanite relations in the Grängesberg apatite-iron oxide ore district, Bergslagen, Sweden

Abstract Fluorapatite-monazite-xenotime-allanite mineralogy, petrology, and textures are described for a suite of Kiruna-type apatite-iron oxide ore bodies from the Grängesberg Mining District in the Bergslagen ore province, south central Sweden. Fluorapatite occurs in three main lithological assemblages. These include: (1) the apatite-iron oxide ore bodies, (2) breccias associated with the ore bodies, which contain fragmented fluorapatite crystals, and (3) the variably altered host rocks, which contain sporadic, isolated fluorapatite grains or aggregates that are occasionally associated with magnetite in the silicate mineral matrix. Fluorapatite associated with the ore bodies is often zoned, with the outer rim enriched in Y+REE compared to the inner core. It contains sparse monazite inclusions. In the breccia, fluorapatite is rich in monazite-(Ce) ± xenotime-(Y) inclusions, especially in its cores, along with reworked, larger monazite grains along fluorapatite and other mineral grain rims. In the host rocks, a small subset of the fluorapatite grains contain monazite ± xenotime inclusions, while the large majority are devoid of inclusions. Overall, these monazites are relatively poor in Th and U. Allanite-(Ce) is found as inclusions and crack fillings in the fluorapatite from all three assemblage types as well as in the form of independent grains in the surrounding silicate mineral matrix in the host rocks. The apatite-iron oxide ore bodies are proposed to have an igneous, sub-volcanic origin, potentially accompanied by explosive eruptions, which were responsible for the accompanying fluorapatite-rich breccias. Metasomatic alteration of the ore bodies probably began during the later stages of crystallization from residual, magmatically derived HCl- and H2SO4-bearing fluids present along grain boundaries. This was most likely followed by fluid exchange between the ore and its host rocks, both immediately after emplacement of the apatite-iron oxide body, and during subsequent phases of regional metamorphism and deformation.

Seismic characterization of the Grängesberg iron deposit and its mining-induced structures, central Sweden

AbstractWe have conducted a reflection seismic investigation over the apatite-iron deposit at Grangesberg in central Sweden. At the time of closure in 1989, the mine was operated using the sublevel caving method down to approximately a 650-m depth. This mining technique caused subsidence and generated a network of faults that propagated from excavated zones at depth up to the surface. The Grangesberg deposit is the largest iron oxide mineralization in central Sweden and is planned to be mined again in the coming years. It is therefore imperative to have a better understanding of the ore geometry and the fault network. A reconnaissance survey consisting of two seismic lines with a total length of 3.5xa0km was carried out to address these issues. The profiles intersect the Grangesberg deposit and open pit, as well as the major mining-induced fracture zone present in this area. A drop-hammer source mounted on a hydraulic truck was used to generate seismic signals; cabled and wireless receivers were used for th...

Intra-orogenic Svecofennian magmatism in SW Finland constrained by LA-MC-ICP-MS zircon dating and geochemistry

We have studied plutonic rocks from the Korpo and Rauma areas of south-western Finland which can be categorized as intra-orogenic, i.e. they were intruded during a proposed extensional period between the two main Svecofennian orogenic cycles: the Fennian and Svecobaltic orogenies. The diorite from Rauma yielded an age of 1865 ± 9 Ma and the diorite from Korpo an age of 1852 ± 4 Ma. The adjacent garnet-bearing Korpo granite was 1849 ± 8 Ma in age. Zircons from the granite also included inherited Archaean and older Palaeoproterozic zircons, as well as metamorphic c. 1820 Ma rims. The diorites are high-K to shoshonitic, mantle-derived magmas, rich in Fe, P, F and light rare earth elements. The Korpo granites show typical features of crustal-derived melts and form hybrids with the diorites in contact zones. Both the mantle-derived and crustal-derived intra-orogenic magmatism are considered to have had a causal effect on the subsequent late Svecofennian (Svecobaltic) thermal evolution in southern Finland which culminated in granulite facies metamorphism and large-scale crustal melting.

Metamorphism and deformation of a Palaeoproterozoic polymetallic sulphide-oxide mineralisation : Hornkullen, Bergslagen, Sweden

Abstract The Hornkullen mineralisation is situated in the westernmost part of the Bergslagen ore province, south-central Sweden. Here, polymetallic sulphides and oxides are hosted by an inlier of Svecofennian, c. 1.9 Ga skarn-bearing metavolcanic units, enclosed in the c. 1.8 Ga Filipstad granite belonging to the Transscandinavian Igneous Belt. The Ag- and Au-bearing mineralisation is dominated by veins and impregnations of magnetite, pyrrhotite, galena, chalcopyrite and arsenopyrite with subordinate pyrite, sphalerite, ilmenite, löllingite, Pb–Fe–Ag–Cu–Sb sulphosalts and rare gudmundite, pentlandite and molybdenite. Overall, a detailed textural and mineralogical study of the ore assemblages suggests significant deformation and remobilisation at high temperature, which is corroborated by sulphide geothermobarometry. The arsenopyrite geothermometer yields an average temperature of c. 525 °C, which is likely to be the result of metamorphic re-equilibration. Sphalerite geobarometry gives peak pressures of c. 300–400 MPa, albeit with caveats. The combined observations suggest that the present mineralogical and textural nature of the ore assemblages at Hornkullen is primarily related to remobilisation during Svecokarelian regional metamorphism of a pre-existing, most likely syn-volcanic mineralisation. This scenario is likely to be applicable to many other Svecofennian metasupracrustal-hosted deposits in the Bergslagen ore province.

Deep drilling in a Palaeoproterozoic hot orogen – potential for deciphering the orogenic accretion and physical properties of a tectonically layered crust

The Svecofennian is a large hot orogen composed of different accreted crustal units. The boundaries between these units are often characterised by major, steeply dipping shear zones with post-accretionary signatures. However, some of these shear zones have recorded an earlier and long-lived activity related to the accretionary episode, so have moderately eastward dipping shear zones identified in the eastern part of the Ljusdal Domain in the central part of the orogen. These shear zones, repeated at multiple lithostratigraphical levels, are associated with west verging asymmetric F2-folds indicating thrusting with imbricate slices in thickness comparable to those in the Caledonides. In the Ljusdal Domain these structures have been recognised in rock of significantly different metamorphic grade indicating thick-skin thrusting on the crustal scale possibly accompanied by channel flow. Information from deep drilling through these stacked units would shed light on the tectonostratigraphy and consequently the accretionary to post-accretionary evolution of hot orogens. In addition, information about groundwater circulation, geothermal energy potential and reservoir quality of tectonically layered rocks for e.g. CO2 sequestration experiments in crystalline rocks would be gained.

On the occurrence of gallium and germanium in the Bergslagen ore province, Sweden

ABSTRACT The presence of the critical and sought-after (semi-)metals gallium (Ga) and germanium (Ge) has previously been reported from mineralisations in the Bergslagen ore province, south central Sweden. Some of these reports were however recently shown to be questionable or erroneous. Here we summarise early analytical work on these metals in mineral deposits of the Bergslagen province, as well as briefly report new analytical data for Ga and Ge from recent, in part on-going work on different mineralisation types. The new data show that the sampled sulphide and iron oxide mineralisations in the Bergslagen province are overall not particularly enriched in Ga, and even less so with regards to Ge. One major exception is the significant Ga enrichment observed in skarn-hosted Fe-REE(-polymetallic) deposits of Bastnäs type. Notably, these mineralisations also host increased contents of Ge. Based on this broader suite of sampled deposits, the suggested correlation between Ga and Al contents in previously studied material with relatively increased Ga grades, is in part contradicted, indicating that Ga is only in part sequestered through straightforward Al-substitution into aluminium silicate and oxide minerals. The mineralisations that do exhibit significantly increased Ge contents, in addition to the Bastnäs-type deposits, are represented by both sulphide-dominated ones and Fe (-Mn) oxide-rich systems.

Reflection Seismic Characterization of the Grängesberg Iron Deposit and Its Mining-induced Structures, Central Sweden

Reflection seismic investigation has been conducted on the Grangesberg apatite iron deposit. At the time of closure in 1989, the mine was operated at about 650 m below the surface. Mining activities might be resumed in the next years, which require better understanding of (1) the ore geometry and (2) the fault network which has developed up to the surface from excavated zones at depth. Two E-W oriented reflection lines with a total length of 3.5 km were acquired. The seismic lines intersect the Grangesberg ore body and open pit, as well as several of the mining-induced faults. A weight drop mounted on an hydraulic bobcat truck was used as a seismic source; both cabled and wireless receivers were used for the data recording. Preprocessing of the data first required the cable- and wireless- recorded datasets to be merged before stacking all data available at each shot point. The dataset exhibits several shallow reflections which are likely to occur on steep lithologic or tectonic structures. Other deeper reflections are recorded; careful processing will be carried out in order to preserve such events in final stacked sections and help with refining the geological model of the area.