Browsing by Subject "Central Lapland Greenstone Belt"

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  • Jokela, Eetu (Helsingin yliopisto, 2020)
    The Sukseton area is located in the northern part of Kittilä municipality, Central Lapland Greenstone Belt, approximately 15 km N from Suurikuusikko gold mine and 5 km NW from Iso-Kuotko orogenic gold deposit, between several large crustal scale thrust and shear zones. This area is a mix of different volcanic formations of Kittilä suite, felsic intrusion of Vuotso Complex and Paleoproterozoic intrusive rocks in the north. In addition to this, several porphyry dykes cut the Kittilä suite volcanic rocks around the area. Exploration work in this area started in the 1980’s when Outokumpu Oy found two minor gold and gold-copper mineralizations. In 2017, Agnico Eagle Finland Oy continued exploration in this area, intending to define the regional geology and the extent of the mineralizations. As a result of this exploration work, this study investigates more closely the regional geology, geochemistry, metamorphism and structural geology of the Sukseton area, as well as the geochronology of associated porphyry dykes. To understand and define the geology of the area, the following methods were used: geological bedrock and exploration trench mapping, interpretation of drill core loggings and several geophysical surveys, optical studies of polished thin sections and U–Pb dating a porphyry dyke sample. The metamorphic conditions of the area were studied through thorough petrological studies. In addition, an extensive geochemical and geotectonic classification of the rocks in the area was conducted. The Sukseton area composes mainly of different tholeiitic basalts and pyroclastic rocks with minor sulphide rich graphitic volcanic sediment and chert sections. Based on this study, these volcanic rocks originate from island arcs and mid-ocean ridges. With the help of geophysical surveys and field measurements, a couple of large fold structures were identified from the eastern part of the study area as well as a large shear zone in the middle of the area striking NE–SW. Porphyry dykes cut the volcanic rocks all around the area giving the minimum age of 1940±18 Ma for the volcanic rocks. Composition of porphyry dykes vary from rhyolites to basalt and they have similar geochemical characteristics with Nyssäkoski type felsic veins. The peak metamorphic conditions in the area represent high-P amphibolite facies metamorphism. Also, hydrothermal alteration is common in Sukseton and it can be metamorphic and magmatic in origin.
  • Riihimäki, Katariina (Helsingin yliopisto, 2021)
    The mafic-ultramafic Kevitsa intrusion, located within the Central Lapland Greenstone Belt in Northern Finland, hosts a disseminated Ni-Cu-PGE deposit. Drillhole KVX018 penetrates through the intrusion, intersecting its bottom contact at 1772 meters and is associated with relatively low resistivity at the bottom of the intrusion. The KVX018 drillhole is the deepest drilled into the intrusion so far and the observed low resistivity zone is unique for the study area. Previous studies have shown the bottom contact of the Kevitsa intrusion to be associated with seismic reflections and possible mineralization. This paper studies the characteristics of the bottom contact of the Kevitsa intrusion from the drill core KVX018 and interprets the origin of the low resistivity and its relationship with mineralogy. From geochemical and petrophysical characteristics, four layers with different characteristics were observed within the studied section: footwall, contact zone, lower cumulates and upper cumulates. The lower cumulates were found to be strongly contaminated by elements associated to hydrothermal fluids from country rocks. The contamination was observed for 125 meters upwards from the basal contact as elevated concentrations of e.g. lithium, lanthanum, rubidium and potassium, and footwall rocks close to the contact were found to be depleted in these elements. The contact zone was found to be strongly altered by silicification and albitization. Hydrothermal fluid activity at the bottom contact was also observed by epidote alteration of plagioclase feldspar. Contact zone mineralization was observed and it was found to be false ore type with Ni tenor of 2.28 %. Upwards from contact mineralization, the mineralization was found first to change into local low-grade Ni-PGE ore and then into normal ore on top part of the studied drill core section. Ultramafic intrusive rocks were observed to be pervasively altered by amphibole alteration locally into a degree where in many rocks, alteration had overprinted the primary mineralogy and textures to be undistinguishable. Alteration intensity was found to increase downwards within the lowermost part of the intrusion. Salt minerals were observed by eye on the surface of some samples and by X-Ray Diffraction in one sample. XRD studies indicated nitratine and sylvite minerals present in the studied sample. These salt minerals are presented commonly in evaporites and their presence indicates an evaporitic source. Resistivity of rocks is generally affected by e.g. sulfide content, salinity, porosity and alteration. Resistivity and chargeability were found to be correlative, indicating resistivity to correlate also with presence of sulfide minerals. However, after depth of 680 meters, resistivity decreases without a correlating trend in other petrophysical properties. This paper concludes that the observed low resistivity is resulted from a presence of salt and sulfide minerals as well as alteration intensity.
  • Hämäläinen, Roosa (Helsingin yliopisto, 2020)
    The Suurikuusikko orogenic Au-deposit is located in the Central Lapland Greenstone Belt within the Kittilä Group volcano-sedimentary sequence. The deposit is hosted by almost vertically dipping Kiistala Shear Zone (KiSZ). In the Suurikuusikko Au-deposit, gold occurs mainly as refractory gold within the arsenopyrite and the pyrite. The Rimpi orebody is the northernmost orebody of the Suurikuusikko deposit. Pikkurouravaara is a unit located to the northeast of the KiSZ that also has arsenopyrite and pyrite with Au as refractory in the main zone; however, in Pikkurouravaara the Au-content is not significant. The goal of this work was to examine using mineralogical tools as distribution of Au in Rimpi and Pikkurouravaara. Studying the mineralogy of the Rimpi also provides information on whether the Rimpi orebody is similar to the other orebodies in the Suurikuusikko deposit. The methods used were optical- and reflective light microscopy, FE-SEM, EPMA, and LA-SC-ICP-MS. Petrographical observations show that the main minerals, texture, and alteration are the same in Rimpi and Pikkurouravaara. The main sulfides, arsenopyrite and pyrite, are also the same, however, morphology and habit are different. In Pikkurouravaara the sulfide grains are larger and more homogenous, while in Rimpi sulfides are more frequently zoned. Also, the trace element content of the sulfides between the study areas was significantly different. In Pikkurouravaara Se, Ni, and Mo occurrence is elevated in arsenopyrite as are Se, Cr, Ni, and Co in pyrites relative to Rimpi. Most likely, Pikkurouravaara and Rimpi are of different generations and enrichment of Au in Rimpi sulfides reflects the episodic formation of the sulfides. Some of the arsenopyrite and pyrite grains in Rimpi host visible gold between the sulfide grains and in fractures. These visible Au-bearing samples were compared to those Rimpi samples where Au occurs only as refractory. In samples with visible gold, the average Au grade in arsenopyrite and pyrite is relatively higher than in samples where Au is only refractory. Also, the S-content is higher in the former. The As-grades were equally variable in both Rimpi types. The average Au grade in arsenopyrites from Rimpi is 340 - 410 ppm, which is coherent with the previously reported average values of 398 ppm. The average gold content of Rimpi pyrites is 14 – 24 ppm, which is significantly lower than the previously reported average of 235 ppm. The major- and trace element data with mineral morphology suggests that the geological history of Pikkurouravaara has been less episodic than in Rimpi.
  • McDonald, Isabel (Helsingin yliopisto, 2020)
    Talc is a problematic alteration mineral at the Kevitsa Ni-Cu-(PGE) mine in Sodankylä, Finland, and its distribution and control were assessed in this thesis. Kevitsa is a polymetallic mine hosted in an ultramafic intrusion, extracting Ni, Cu, Co, Au, Pt and Pd, which are of increasing importance in green energy technologies. Talc – a common alteration product in ultramafic rocks – detrimentally interferes with the recovery of copper in the flotation stage of ore processing when concentrations exceed 5 wt. %, thus affecting the economics of mine operations. It was found different talc concentrations had different spatial associations and controls, with three dominant styles identified, and a multi-stage genesis of talc alteration is proposed. The talc styles identified in the study are as follows: (style 1) pervasive talc-chlorite alteration, (style 2) talc-dolomite alteration haloes proximal to dolomite veins and (style 3) talc on brittle structures, associated with magnetite. Low values of talc between 0.2-0.5 wt.% (style 1) were found to have no preferential spatial distribution, occurring as background alteration throughout the intrusion. Intermediate values (between 1-5 wt. %) were associated with late brittle fractures and structures (style 3), with a notable association with the NE-flt-rv1 fault zone. Style (2) was found to have a dominant structural control, specifically being associated with north-south trending structures. Dominant structures with this association identified are NS-flt1_flt-002 and NS-flt-2_flt-009. Highest values (commonly exceeding >10 wt. %) manifest themselves as alteration haloes proximal to veins, where talc-carbonate replaces the intercumulus mineral phases. Here it is proposed that ‘low talc’ alteration, style (1), was the first talc association to occur, generated by late magmatic fluids or regional metamorphism accompanying amphibole and serpentine alteration. The association observed as style (2) was likely generated by the infilling of north-south trending structures by carbonate-talc veins through metasomatism by a CO2 rich metamorphic fluid, perhaps delivered by a deep-seated structure, often generating talc values in excess of 10 wt.%. The third stage is proposed to be talc enrichment via meteoric fluid percolation, after exhumation. This generated talc along brittle structures associated with magnetite style (3), and talc-carbonate concentrations may also be upgraded at this stage. Further enrichment of talc is observed at the surface, attributed to freeze thaw-cycles of permafrost upgrading talc values. The identification of these processes and controls on talc will not only have implications for the economics of Kevitsa as high talc zones can be avoided, but findings may have useful applications for mining of similar deposits in the Central Lapland Greenstone belt such as the nearby Sakatti Cu-Ni-(PGE) project, when it enters production.
  • Mueller, Matthias; Peltonen, Petri; Eilu, Pasi; Goldfarb, Richard; Hanski, Eero (2020)
    The Mustajarvi gold occurrence lies in the southern part of the Paleoproterozoic Central Lapland Greenstone Belt, in proximity to the first-order transcrustal Venejoki thrust fault system. The gold occurrence is structurally controlled by the second-order Mustajarvi shear zone, which is located at the contact between siliciclastic metasedimentary and mafic to ultramafic metavolcanic rocks. The main mineralization comprises a set of parallel veins and sulfidized rocks that are slightly oblique to the shear zone and are hosted by third-order structures likely representing Riedel R-type shears. The gold-mineralized rock at Mustajarvi is associated with pyrite that is present in 0.15- to 1-m-wide quartz-pyrite-tourmaline veins and in zones of massive pyrite in the host rocks with thicknesses ranging from 1.15 to 2 m. In unweathered rock, hypogene gold is hosted by Au- and Au-Bi-telluride micro-inclusions in pyrite, whereas strong weathering at near surface levels has caused a remobilization of gold, resulting in free gold deposited mainly in the cracks of oxidized pyrite. The geochemistry of both mineralization styles is typical of orogenic gold systems with strong enrichments comprising Au, B, Bi, CO2, Te, and Se; and less consistent anomalous amounts of Ag, As, Sb, and W. Unusual for orogenic gold deposits is the strong enrichment of Ni and Co, which leads to the classification of Mustajarvi as orogenic gold occurrence with atypical metal association.
  • Harjama, Noora (Helsingin yliopisto, 2020)
    The seismic reflection methods produce high-resolution images from the subsurface, which are useful in structural studies of geology. Northern Finland features a complex Precambrian geological history, including massive extension and compression stages, which has been extensively studied. The xSoDEx survey is the most recent seismic survey carried out in northern Finland by the Geological Survey of Finland (GTK). The XSoDEx concluded four survey lines, which are located in Central Lapland Greenstone Belt (CLGB) in Sodankylä, Lapland. This thesis aims to find out whether the strong reflections shown in the xSoDEx Alaliesintie reflection profile, underneath the outcropping Archaean basement indicate a lithological contact or a fault zone. The Alaliesintie profile is characterized by Koitelainen intrusion, Archaean outcrops, and layers of younger Paleoproterozoic group rocks. The work was carried out in stages, with the use of the SKUA - GOCAD 3D modeling software. The four stages are: 1. Create a 3D geological model based on the Alaliesintie reflection section and geological bedrock observations. 2. Use gravity and magnetic geophysical data from the study area to improve model reliability. 3. Use the geological 3D model and petrophysical data to build a synthetic seismic forward mode. 4. Analyze and evaluate the modeling result for understanding the possible origins of the reflections. In the geological 3D model, I presented that the reflection would present lithological contacts and that the Archean bedrock would have folded and partly overthrust on top of the younger Proterozoic rocks. The seismic forward model is used as an experiment to test the geological 3D model’s lithological contact respondence to the synthetic seismic signal and to discover the possible reflector underneath the Archaean basement. The results present that the seismic forward model can be used to perform the reflections and that the geological 3D model presented similar reflections in the seismic forward model comparing to the original Alaliesintie reflection data.