<p>A 3000 m deep hole is being drilled in the Archean Karelian Craton in northeastern Finland in an area where the granitoids dominating the surface have yielded Neoarchean ages (2.8&#8211;2.7 Ga). Archean greenstones and Paleoproterozoic dolerites are exposed within the domain as well. The drilling site lies between ca. 2.44 Ga Koillismaa and N&#228;r&#228;nk&#228;vaara mafic layered intrusions. This site was chosen based on gravimetric, magnetic, magnetotelluric and reflection seismic studies, which have revealed a deep anomaly that seems to connect the two mafic layered intrusions. Based on modelling of the geophysical data, the upper boundary of this ca. 60 km long, roughly E-W oriented anomaly lies at approximately 1.5 km depth.</p><p>We sampled various rock types from depths of ~40&#8211;1600 m for zircon U-Pb dating. The lithologies include leucogranites, tonalite gneiss, hornblende diabase, quartz diorite and granodiorite. Based on observations from the drill core extracted so far, the source of the anomaly is likely to be ultramafic cumulates. Also, presence of Paleoproterozoic granitoids is likely. We will perform the U-Pb analyses during the winter of 2022. The results are expected to confirm the interconnection of the two layered intrusions, clarify the age distribution of the granitoids in the region, and help to decipher the detailed tectonic evolution of the Archean Koillismaa area.&#160;</p>
Structural interpretation of potential field data is important in mapping and modeling three-dimensional geologic structures. Interpretation of potential field data, however, is a difficult task with inherent ambiguity being a major problem. Recently, several new techniques have been introduced to help structural and source geometry interpretations of potential field data (e.g., Verduzco et al., 2004; Hornby et al., 1999). These methods use either straightforward calculation of derivatives of potential field anomalies or involve several calculation stages with sophisticated theoretical background.
Most of the known platinum group element (PGE) resources in Finland are in contact-type and reef-type deposits in 2.45 Ga mafic–ultramafic layered intrusions. These intrusions are also estimated to contain the majority of possibly existing, yet undiscovered PGE resources in Finland. The undiscovered Pt, Pd, Au, Ni, and Cu resources in contact-type and reef-type deposits were estimated down to 1000 m depth using a three-part quantitative assessment method. This included the delineation of 19 contact-type and 24 reef-type permissive tracts, the formulation of a grade-tonnage model for the contact-type deposits and a separate model for each reef-type tract, estimation of the number of undiscovered deposits for each permissive tract, and Monte Carlo simulation to estimate the frequency distributions of metal tonnages in the undiscovered deposits. The expected number of undiscovered contact-type deposits is 29 and that of reef-type deposits is 23. At the 50% probability level, the total undiscovered PGE resources of Finland are estimated to be at least 5600 t Pt, 12,000 t Pd, 430 t Au, 4.2 Mt Ni, and 5.7 Mt Cu. At the 90% probability level, the corresponding tonnages are 720 t Pt, 1700 t Pd, 59 t Au, 0.87 Mt Ni, and 0.91 Mt Cu. More than 80% of the Pt + Pd in the undiscovered resources are estimated to be in reef-type deposits. The known PGE resources in Finland are 259 t Pt + Pd in six contact-type deposits, plus 69 t Pt + Pd in the atypical Kevitsa deposit. These figures indicate that the presently known deposits account for approximately 2% of the total assessed PGE resources in Finland. They suggest that significant potential exists for the discovery of additional PGE resources in the country.
Several mafic-ultramafic layered intrusions were emplaced in the NE Fennoscandian Shield during a magmatic episode at 2.44 Ga. The Paleoproterozoic Näränkävaara layered intrusion, northern Finland, is one of the largest ultramafic bodies in the Fennoscandian Shield, with a surface area of 25 km x 5 km and a magmatic stratigraphic thickness of ~3 km. The intrusion comprises a 1.3 km-thick peridotitic–dioritic layered series (2436 ± 5 Ma) with two peridotitic reversals, and a 1.5–2 km thick basal dunite series mainly composed of olivine adcumulates (dated here). The intrusion has been studied since the 1960’s, but several questions regarding its structure and petrogenesis remain. The basal dunite shows several lithological features typical of komatiitic rather than intrusive olivine cumulates; namely, >1 km-thick “extreme” olivine adcumulates, some showing textures with bimodal grain sizes, oscillating variations in Mg# with stratigraphic height, and poikilitic chromite. With Archean greenstone belts nearby, it was previously hypothesized that the basal dunite series could represent an Archean komatiitic wall rock to the Paleoproterozoic layered series. However, our new U-Pb ID-TIMS baddeleyite age of 2441.7 ± 0.9 Ma for the basal dunite series shows that the basal dunite and layered series of the Näränkävaara intrusion are co-genetic. New whole-rock Sm-Nd isotope data from key stratigraphic units (initial εNd at 2440 Ma of -3.5 to -1.7) indicate that the intrusion was constructed from repeated emplacement of LREE-enriched high-MgO basaltic magmas that were mantle-derived and contaminated by crust, similarly to other Fennoscandian 2.44 Ga intrusions. The parental magmas show similar compositions regardless of stratigraphic position, suggesting that most wall rock contamination and homogenization had occurred before emplacement, with in situ contamination being a relatively minor process. The open-system features of the basal dunite suggest that it may have formed (at least partly) as a feeder channel cumulate, possibly related to the ~100 km long Koillismaa-Näränkävaara Layered Igneous Complex. The Näränkävaara parental magmas show variably depleted metal ratios and could have potential for orthomagmatic mineral deposits, given the availability of S-rich wall rocks.
Summary Koillismaa area in Finland has been captivating attention of geoscientist for decades because of it's prominent gravity and magnetic anomaly that surface geological observations do not explain. GTK drilled a 1700 m long research hole to investigate the source of the geophysical anomaly. We used the rock samples from the research drill hole to measure seismic p-wave velocity and density in laboratory. Additionally, we measured velocity in drill core using handheld ultrasonic device at 1 m interval and did sonic logging in the hole. The results suggest handheld tool is usable for cost-efficiently determining seismic velocity in the core but tool is extremely sensitive for fractures and cracks. Also sonic logging is influenced by fractures and thus laboratory sample measurements are most representative of the mineralogy. The main rock types of Koillismaa create clusters distinguishable from each other based on acoustic impedance. This encourages to plan a seismic survey to delineate the potentially ore hosting mafic intrusion in depth.
AbstractWe have discovered loveringite from two Paleoproterozoic layered intrusions at Fennoscandian Shield, the Syöte block of Koillismaa intrusion in Finland and Nyud intrusion of Monchegorsk pluton (Monchepluton) in Russia. The studied loveringite occurs in two different structural positions: 1) in non-cumulative gabbronorite of a later phase from the middle zone of the Syöte block of the Koillismaa intrusion and 2) in orthopyroxene orthocumulates from lower zone of the Nyud intrusion. The loveringite morphology is diverse. It is present both as rare euhedral and anhedral crystals and as corroded grains. Loveringite from the Syöte block, in contrast to that of the Nyud intrusion, is characterized by increased contents of Ti, Ca, V, and Zr, the presence of U and Th, and also reduced contents of REE. The loveringite composition is not controlled by the stratigraphy of the layered intrusions, but rather is determined by the composition of the residual liquid. The ilmenite reaction rims with baddeleyite and chrome-spinel are constantly present around the loveringite. Their smallest width is observed around euhedral loveringite, and the maximum is observed around corroded grains, which indicates its formation as a result of reaction with residual liquid. Loveringites were formed at the late magmatic stage, after crystallization of cumulus orthopyroxene in the Nyud intrusion and pyroxene-plagioclase assemblage in the Syöte block. And the presence of incompatible elements (Zr, U, Th, REE) in the loveringite composition indicates a decisive role in its formation of crustal contamination. Thus, loveringite is an important accessory mineral indicative of late magmatic processes in layered intrusions, and its composition may reflect variations in the chemical composition of residual liquids.
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