Browsing by discipline "Geologia"

Jurassic (182 Ma) Karoo flood basalt province shows great variety in geochemistry. The complexity is thought to be inherited from distinct mantle sources. Luenha River exposure in northern parts of Mozambique includes primitive picrites possibly representing the still undefined parental magma type for the North Karoo Lavas. The previously determined whole-rock data revealed chondritic to very radiogenic 87Sr/86Sr ratios and nearly chondritic eNd values. The diverse 87Sr/86Sr ratios can result from processes such as e.g. subsolidus alteration, contamination, magma mixing or source heterogeneities, which complicates assessment of petrogenetic processes. To make a contribution to this, plagioclase phenocrysts from six Luenha samples were used as tracers of magma chamber processes. In situ studies on plagioclase growth zones were performed using the CIS methods (crystal isotope stratigraphy). Cold-cathode cathodolumenescence microscopy (CL) was used to visually reveal zonation, the electron microprobe (EMPA) was utilized for major element content (core-to-rim), and laser ablation-multicollector- inductively coupled plasma- mass spectrometry (LA-MC-ICP-MS) was used for in situ (87Sr/86S)i ratio measurements. The anorthite content of plagioclase cores (n = 65) is An65 ̶ 90 and core to rim variations alternate between normal oscillatory to reverse zoning. Is situ isotope examination revealed isotopic disequilibrium in (87Sr/86Sr)i between phenocrysts (cores 0.70511–0.70671, n = 10; rims 0.70539–0.70709, n = 11) and bulk groundmass (0.70660– 0.71061, n = 12). Plagioclase cores are always less radiogenic compared to whole rock (0.70690–0.71019), but internal variation within and between lava flows exists. Core-to-rim microsampling revealed four different (87Sr/86Sr)i evolution paths reflecting heterogeneous crystallization conditions. An, open complex magma plumbing system with progressing contamination is the likely scenario. The relatively radiogenic plagioclase cores compared with the uncontaminated plume-like sample (87Sr/86Sr 0.70410) indicate that contamination was ongoing prior to plagioclase crystallization and continued until eruption. Phenocryst migration between compositionally and thermally distinct reservoirs at crustal depths could explain the heterogeneous plagioclase (An and (87Sr/86Sr)i) of Luenha picrites.

The northern peatlands are significant sources of methane into the atmosphere. As methane (CH4) is a powerful greenhouse gas, understanding the mechanics of peatland CH4 production/consumption and the controls on CH4 emissions are vital in order to calculate global CH4 budgets. The isotopic composition of CH4 can be used as a tool to identify pathways of methanogenesis, CH4 oxidation and transport. The aim was to learn how microtopography effects CH4 emission and its δ13C values at a boreal mire complex Siikaneva, located in southern Finland. The study sites were an ombrotrophic bog and a minerotrophic fen. The hypothesis of the study was that microtopography is an integrating controlling factor of CH4 emissions and that isotopic composition of the emitted CH4 would reflect the processes beneath the peat surface. Also, it was expected that at the fen there would be more acetate fermentation which would show in the isotope results. Gas samples of emitted CH4 were drawn from hummocks, lawns and hollows using a closed chamber method to analyse CH4 flux and δ13CCH4. Pore water samples for DIC analysis were collected at the depth of 0.5 m to analyse δ13CDIC. In addition, water table levels and peat temperatures were measured at each sampled plot. All samples were collected once a month from May to August to identify changes during the growing period in 2015. The results revealed that the emissions of CH4 were the lowest from the hummocks at both sites, but the differences between fluxes from the hollows and lawns were less affected by microtopography. Water table levels and peat temperatures seem not to be the controlling factors for CH4 emissions. The isotopic composition of emitted CH4 was also mostly unaffected by microtopography. Only the hummocks at the bog had distinctively higher carbon isotope values compared to the other microsites. Unlike the CH4 flux results, the δ13CCH4 results depicted a seasonal trend during the growing period with all the microsites showing the most negative values in July. Combined to the results of the DIC analyses, it appears that the fen is characterized by more acetate fermentation, though the production pathway could not be identified unambiguously. The isotopic composition of emitted CH4 proved to be a suitable, but not sufficient application to differentiate CH4 production pathways, oxidation and transport. To better understand the processes beneath the peat surface and the effects of microtopography, CH4 flux and isotope data should be combined to multi-isotopic (CH4 and DIC) pore water samples drawn from multiple depths to demonstrate the vertical changes in the isotope composition at different layers of peat.

Ferromanganese concretions are commonly found in the Gulf of Finland forming in different sizes and shapes. The concretions are formed in various environments, where known factors contributing to their formation are e.g. the redox properties of the local environment, quality of the seabed associated with the concretions and bottom currents. Ferromanganese concretions are formed by natural geochemical processes, catalyzed by micro-organisms, e.g. archaea and bacteria. The materials of this study were based on altogether 200 samples consisting of sample descriptions, underwater photographs and videos from the northern Gulf of Finland provided by GTK. Environmental variables used in this study were water depth, sediments associated with the concretions, seabed structure type, slope, roughness, distance to coast and distance to river, surface wave exposure, bottom wave exposure and bottom current velocity (m/s). The concretions were categorized into different groups, their distribution was illustrated on a map and environmental variables were used to determine which factors contribute to their formation and to elucidate the characteristics of the environments where each concretion type is formed. Discoidal concretions were the most common concretion type in the study area indicating that the environment of the northern Gulf of Finland is most suitable for these concretions. Discoidal concretions were most commonly found on crests which was also the dominant seabed structure type in the study area. The results show that each concretion type is formed in an environment where certain environmental factors are commonly present. Spheroidal concretions (2–7 mm) were practically missing in the study area which is likely linked to the quality of the seabed and availability of Mn. Buckshot concretions were commonly found in the proximity of the coast, in calm, sheltered and heterogenic environments. Discoidal concretions were found to form in high energy and dynamic environments. The occurrence of crust concretions is likely linked to (high) water depth, relatively high bottom currents and clayey sediments. Irregular and spheroidal concretions (20–600 mm) represent most likely transient or irregular forms of spheroidal concretions forming in unstable conditions. The economic interest towards ferromanganese concretions is likely to grow in the future. The high geodiversity of the seabed in the northern Gulf of Finland is possibly linked to the abundance and diversity of ferromanganese concretions however their ecological significance to the ecosystem is not yet fully understood.

Paleoproterotzoic evaporitic rocks in the Central Lapland Greenstone Belt (CLGB) were discovered in 2016 as a result of relatively deep diamond core drilling (400-1100m) during a Ni-Cu-PGE ore exploration project. Evaporites occur as relatively flat successions. Highly soluble evaporites were probably initially preserved by active magmatism that enveloped these layers under extrusives or/and in between subvolcanic sills. Under the deformation the plastic behaviour of evaporites may have provided a platform that may have functioned as a slip surface for the prograding thrust faults. Plastic behaviour may also have prevented folding. Evaporites in the Central Lapland Greenstone Belt formed after the Great Oxidation Event, during the late stages of Lomagundi-Jatuli carbon isotope event. The discovered evaporites belong to the Savukoski Group rocks (2.2-2.05 Ga) that resemble a closing rift and the corresponding of a shallow sea environment, where evaporites are considered to have precipitated by oversaturation induced by anomalously high terrestrial influx that was enhanced by newly introduced oxic conditions. Age constraints for these deposits are provided by their relative position in the CLGB stratigraphy: 1. They postdate the appearance of fist black shales of the Matarakoski Formation in Kitinen and Siuliunpalo Formation in Pelkosenniemi (<2.2 Ga), and 2. they predate the ultramafic volcanic rocks of the Sattasvaara Formation in Kitinen and Kummitsoiva Formation in Pelkosenniemi (>2.05Ga). These conclusions are strengthened by carbon isotope results from carbonates. Evaporitic carbonates show elevated 13C values ranging from +4 to +14 ‰, with an average at +9 ‰. These values correlate well with the timing of the Paleoproterotzoic carbon excursion (2.2-2.06 Ga). The excursion has been addressed to extensive burial of organic carbon (Karhu & Holland, 1996). 34S values form a relative tight group between +5 -+8 ‰ (averaging +6.5 ‰) and are similar with the roughly coeval Paleoproterotzoic evaporitic anhydrites of the Onega Basin. The discovery of Paleoproterozoic evaporites foremostly adds a new rocktype into the bedrock of Finland as well as gives new insight to the crustal evolution in the CLGB.

The Jokisivu gold deposit is a structurally controlled orogenic gold deposit that was formed during the Svecofennian orogeny when the Pirkanmaa belt, a turbidite dominated subduction zone complex, was pushed below the Tampere schist belt in the north. The Jokisivu gold deposit formed in a brittle-ductile shear zone and gold mineralization is related to the youngest deformation stage. The main focus of this study was in fluid inclusions in quartz veins. Quartz grains host fluid inclusions that carry information of the prevailing P–T–X conditions during fluid events in the quartz veins. According to the properties of fluid inclusion assemblages (FIA), six fluid inclusion types were distinguished. Fluid inclusion types A1, A2 and A3 are aqueous and fluid inclusion types B1, B2 and B3 are aqueous–carbonic. Relative chronology of the fluid inclusion types was determined by cross cutting relationships of fluid inclusion assemblages. Relative chronology of fluid inclusion types from oldest to youngest is A1, A2 and A3, B3, B1 and B2 indicating that aqueous fluids came in first and aqueous–carbonic fluids followed them. Microthermometry was conducted for all the FIAs. The melting point of CO2 was generally below the CO2 triple point ranging from -59.15°C to -56.73°C indicating the presence of other gaseous phases that were later confirmed with Raman spectroscopy to be CH4, N2 and even H2S. Raman spectroscopy was conducted for qualitative analysis of the phases presentn the inclusions. Quartz crystallization temperatures were calculated using titanium in quartz geothermometer (TitaniQ). TitaniQ was well suited for Jokisivu samples because titanium concentrations in the hydrothermal quartz were quite constant. Combining TitaniQ P–T slopes and fluid inclusion assemblage isochores from microthermometry and Raman spectroscopy yielded a good estimate for P–T conditions of fluid entrapment of 390–470°C and 170–345 MPa. The estimated temperature range is higher than the previous studies have suggested. However, the P–T range estimated for Jokisivu is in accordance with the P–T conditions of other Svecofennian orogenic gold deposits. Judged by the mineralogy of the quartz vein - host rock contact, the most important factor triggering gold precipitation in Jokisivu has probably been fluid-rock interaction. Strong sulfidation of host rock reduced the sulfur fugacity and enabled gold precipitation.

Out of all igneous rocks, carbonatites are perhaps the ones most sensitive to changing chemical environments and P-T conditions. As a result, their primary chemical and textural characteristics are more often than not altered by secondary processes. Discerning between the two is essential in order to make correct petrogenetic inferences from textural and chemical data. In this study, the 1.3 Ga siderite carbonatite of the Grønnedal-IÌ ka alkaline complex of South Greenland is used as a natural laboratory to identify mineral chemical and textural fingerprints of hydrothermal alteration in iron-rich carbonatites, with a second aim of describing the paragenesis of a high-grade magnetite mineralization in the locality. Trace element chemistry of magnetite, calcite, siderite and ankerite-dolomite is analyzed in situ by electron-probe microanalysis (EPMA) and laser ablation inductively coupled mass spectrometry (LA- ICP-MS). Magnetite is shown to be a product of oxidation of siderite and is exclusively of hydrothermal origin, characterized by low Ti (1-12000 ppm) and V (1-200 ppm) concentrations. High Nb/Ta (up to 1000) and Zr/Hf (up to 300) ratios in magnetite suggest formation mediated by fluorine-rich fluids. Hydrothermally reworked siderite is enriched in Mn and light rare earth elements (LREEs) and has a depleted Y/Ho ratio. In contrast, hydrothermally reworked calcite is enriched in Y/Ho and depleted in LREEs. A secondary mineral assemblage of apatite, strontianite, barite, REE-fluorocarbonates and ankerite-dolomite is associated with the alteration, which increases toward the contact to a 55 m wide basaltic dike that cuts the carbonatite. Unusual mineral compositions are found close to the dike contact, including magnetite with up to 1 wt.% Nb and calcite with 1 wt.% REEs, both the highest reported values in the literature. Together, the data point to the dike intrusion as a heat source of a hydrothermal convection cell, driving hot F and CO2 rich fluids that mobilized P, Sr, Ba, Mn, LREE, Nb and Ta and reacted with and altered the composition of the carbonatites up to a distance of 40 m from the intrusion contact. The results underscore the necessity of a careful textural and mineral chemical assessment in studying the petrogenesis and subsolvus evolution of ferrocarbonatites.

The oldest rocks in Finland are the Archaean grey gneisses of eastern and northern Finland. The Archaean of the Karelian craton spans about 1000 Ma of crustal growth and evolution and forms the core of the Fennoscandian shield. The Karelian province is a complex patchwork of different rock types. The individual formations are of small territorial extent in accordance with often postulated small Archaean plates. Overall, the Karelian craton is a granitoid-greenstone terrain with prevailing TTGs and younger granites, which show increasing level of potassium. The craton also includes a distinct sodic variety of granites that combines features of classical Archaean TTGs and late Archaean high-K granites. A minor number of Mg-rich lithologic units, including adakites and sanukitoids, are reported as well. A small number of A-type granites, syenites and S-type granites are widely distributed and of local nature only. Peculiarly, a large number of TTGs is peraluminous. The formation of Karelian craton may be explained by accretion of small plates, perhaps during the late Archaean supercraton event in a process that at least in later stages included active plate marginal processes.

As a part of Kumpula Campus Drill Hole Project, a 370 m deep drill hole was drilled on the University of Helsinki, Kumpula campus area in December 2015. Drilling took place on an amphibolitic outcrop, which is the main rock type of the area and part of the 1.9 Ga old Svecofennian orogenic belt. In this work, the geochemistry of the campus bedrock is analyzed, focusing on the amphibolite. Granite, actinolite rock and diopside-actinolite skarn are additional rock types described from the core in this work. The geochemical methods utilized are a portable X-ray fluorescence (P-XRF) spectrometer Niton XL3t GOLDD+ by Thermo Scientific and a laboratory wavelength dispersive X-ray fluorescence (WD-XRF) spectrometer PANAlytical Axios mAX 4kW. WD-XRF device is utilized in quantitative analysis and semi-quantitative Omnian scans. In addition to geochemical interpretation of the bedrock, feasibility of the P-XRF device in outcrop and drill core related studies is evaluated by comparing the methods. The surface of the drill core was analyzed with the P-XRF device. Representative samples of each rock type were sawed of the core and analyzed with both P-XRF and WD-XRF Omnian scans. In addition to surface analyses, a fused bead was prepared from one representative amphibolite sample and analyzed with WD-XRF quantitative method. Outcrop studies focused on the feasibility of the P-XRF device in in situ analyses. Compared to nearest temporally related amphibolite units, the amphibolite of the campus bedrock seems to be more felsic on average. All described rock types are connected to former petrogenetic interpretations of the local bedrock. However, further geochemical analyses are required to verify the interpretations. WD-XRF quantitative method and Omnian scans suggest almost similar results for fused bead of the amphibolite. Changing the sample type to solid rock surface introduces heterogeneity related problems to the quantitative determination of Omnian scans and quality of the results decreases almost to the level of P-XRF. Yet, the advantage of the Omnian scans method in rock surface analyses compared to P-XRF is better detection of light elements. For example, P-XRF device detects Mg, Al and K poorly and Na is not detected at all. On the other hand, SiO2 is on average detected quite accurately from rock surface with P-XRF when compared to WD-XRF quantitative method for fused bead. WD-XRF Omnian scans and quantitative application results of fused bead do not seem to differ remarkably. Broad rock type classification can be made with P-XRF device for drill core, but results cannot be considered quantitative. It should also be noticed, that the major element oxide sum values of P-XRF drill core surface analyses are quite low on average (84.00 wt.%). In outcrop analyses, different features lower the quality of the rock surfaces, resulting in even lower major element oxide sum values in analysis. Although major oxide sum values are very low on outcrops, relatively high amounts of for example Cl, S and P are detected for unknown reasons. Major advantages of the P-XRF device are the ease of use, light weight and rather good detection of for example SiO2. Developing the quantitativeness of the device would make it more comparable to laboratory XRF devices but it already has multiple features that are highly beneficial in a wide range of scientific fields.

The Central Finland Granitoid Complex (CFGC) is a large (44,000 km2) plutonic core of a Svecofennian (Paleoproterozoic, 1.91–1.82 Ga) arc complex, formed from collisions of several volcanic arcs and their accretion over the Karelian craton. The CFGC consists mostly of granitic to granodioritic rock types. Mafic-ultramafic plutonic rock types are not common, and they consist of mostly small gabbro-diorite intrusions, which may have ultramafic parts. There are two distinct belts around the CFGC, where Ni-Cu potential mafic-ultramafic intrusions are situated – Vammala and Kotalahti. The intrusions within these belts were formed during the height of magmatism within the CFGC (1.89–1.87 Ga). They host Ni-Cu mineralizations, some of which have been economically exploited. The mineralizations are hosted by olivine-rich ultramafic cumulates. The intrusions formed from hydrous tholeiitic basalts (10–12 wt-% MgO) with arc-type trace element chemistry. The difference between Vammala and Kotalahti type intrusions (clinopyroxene and orthopyroxene-dominated, respectively) are attributed to the rock type of the assimilated country rock. In this thesis, three previously unknown or poorly studied mafic-ultramafic intrusions (Matokulma, Palojärvi, and Hongonniittu) within the CFGC are studied in detail. The petrology, similarity to Vammala-Kotalahti type intrusions, parental magma compositions, ore potential, and petrogenesis of the intrusions are described. Rock samples and field observations were gathered during the summer of 2017. Whole-rock geochemistry, mineral geochemistry, isotope geochemistry, and geophysics are used to describe the petrology of the intrusions. Matokulma and Palojärvi intrusions are studied in detail, compared to Hongonniittu intrusion, which was not studied as intricately. The Matokulma intrusion is the least evolved (whole-rock median Mg#=72) of the studied intrusions and consists of tholeiitic melagabbros where clinopyroxene±orthopyroxene and plagioclase are the main cumulus phases within interstitial, magmatic amphibole (magnesiohastingsite to pargasite in composition). Orthopyroxene and plagioclase are intercumulus phases in some samples. There are also mafic dikes that intrude the tonalitic country rock that surrounds the gabbro. The dikes are similar to the gabbros in geochemistry although they are generally more evolved. Trace element geochemistry suggests that the gabbros and dikes are genetically connected, and the dikes possibly represent the residual magmas of the gabbros. The Palojärvi intrusion is noticeably more evolved than the Matokulma intrusion (median Mg#=49), which is apparent in the iron and titanium rich mineral and whole-rock geochemistry. The strongly tholeiitic melagabbros are composed of both orthopyroxene and clinopyroxene as cumulus phases with plagioclase and common Fe-Ti oxide, often within interstitial magmatic amphibole (magnesio-hastingsite to magnesioferri-hornblende in composition). The Fe-Ti oxides are mostly ilmenomagnetite but both magnetite and ilmenite grains are present in same samples. Based on a few mineral analyzes, the ilmenomagnetite contains up to 1.4 wt-% V2O3. U-Pb age determination samples from a leucogabbro dike within the intrusion and granite that crosscuts the intrusion yielded weighted average 206Pb/207Pb ages of 1883.4±4.8 Ma and 1893.8±7.1 Ma, respectively. The age results are in contrast to the intrusive relationship observed in the field. However, considering the margin of error of the results, the granite can be younger than the gabbro, 1887 Ma and 1888 Ma, respectively. The age of ca. 1.89 Ga is at the early stage of the most voluminous mafic-ultramafic magmatism in the Svecofennian terrane. The parental magmas of the Matokulma and Palojärvi intrusions were evolved and contained approximately 5 wt-% and 2 wt-% MgO, respectively. The presence of magmatic amphiboles in most samples indicate that the parental magmas were hydrous. Samples from all intrusions plot similarly in primitive mantle normalized Rare Earth Element (REE) and Normal-Mid-Ocean Ridge Basalt (NMORB) normalized spider diagrams. Similar patterns indicate a similar source for the parental magmas. The trace element geochemistry has signatures of subduction related fluid metasomatism. The rocks are enriched in large ion lithophile elements (LILE) and depleted in High Field Strength Elements (HFSE). These geochemical characteristics indicate that the studied intrusions crystallized from a hydrous, NMORB-like evolved basaltic magma, which has experienced fluid metasomatism. The studied intrusions differ from olivine-rich ultramafic cumulates of Vammala and Kotalahti type intrusions based on their more evolved, gabbroic composition and because of this, they are not Ni-Cu ore potential. Palojärvi may host a Fe-Ti-V mineralization, if there are magnetite rich layers within the intrusion.

The geology of the Eastern Lapland Archean domain (the sudy area of this study) is vaguely known due to high metamorphism and poorly exposed lithologic assemblages. Three new U-Pb age determinations from key locations aim for more profound understanding of the geochronology of the area. Cu-Fe-S stable isotope data for copper sulfides from the Eastern Lapland Archean domain are presented for the first time. The stable isotope analytical method for Cu-Fe-S isotopes is refined and its applicability as a proxy for magmatic Ni-Cu-PGE sulfide deposits is evaluated. It has been suggested that, within an individual mineralized intrusion, negative correlation of δ34S and δ65Cu might be a useful indicator of the economic potential of Ni-Cu-PGE sulfide deposits. Combined S and Fe isotope systematics have also been used to fingerprint for the origin of sulfur. Three new samples were dated using the LA-ICP-MS method on zircon. Most of the Pultoselkä granite data are concordant and yield an age of 2795 ± 6 Ma. The data from the Kontioselkä pegmatite are mostly discordant and define maximum age for the pegmatite at 1766 ± 36 Ma. The Janesselkä gabbro yields an minimum age of 2425 ± 17 Ma, consistent with the 2,4–2,5 Ga age group of mafic magmatism observed widely across the Fennoscandian shield. The established U-Pb data imply distinct metamorphic effects during the Svecofennian and Caledonian orogenies. 28 stable isotope (Cu-Fe-S) data sets were measured from dissolved samples utilizing MC ICP-MS. A micro drilling method was used to extract sulfide cores for the solution work. Collectively the δ65Cu values of analysed sulfides (n = 28) range from -4,65 to 0,29‰, δ56Fe values from -0,21 to 1,44‰, and δ34S values from -1,38 to 11,77‰. Isotope values from magmatic sulfides (n = 7, 2σ) have a restricted range and distinct mean δ65Cu value of -0,99 ± 0,21‰, mean δ56Fe value of 0,16 ± 0,23‰, and mean δ34S value of 2,08 ± 0,3‰, consistent with chondritic compositions. The variation of isotopic values reflects the composition of the sulfide. Overall, they reflect varying crystallization histories and subsequent modification of the samples examined.