Search

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 ôextremeö 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. [ABSTRACT FROM AUTHOR]

A new 3D crustal-scale model has been created for the Kautokeino Greenstone Belt (KKGB) in northern Norway based on 3D density modelling and aeromagnetic data, integrated with qualitative interpretation of geophysical and geological data. Detailed geophysical analyses and their integration with an existing petrophysical database allowed us to identify and interconnect shallow and deeper structures throughout the study area. To have better control on the regional tectonic setting of the KKGB, the model area has been extended to include the northern part of Finland and Sweden. The new 3D density model reveals that the KKGB is broader than supposed previously. The 3D analysis indicates that the belt reaches depths of approximately 5-6 km and is emplaced as a highly deformed structure between the Archaean Jergul Gneiss Complex (JGC) to the east and the Rommaeno Gneiss Complex (RoGC) in Finland to the west. The Raiseatnu Gneiss Complex (RaGC) in the western part of the KKGB is characterised by a dominant NNW-SSE magnetic trend that is sub-parallel to the main trend of the KKGB. Moreover, the RaGC is similar to the KKGB in terms of the presence of a large number of sub-parallel magnetic anomalies. This gneiss complex shows mostly short-wavelength anomalies and some circular anomalies that differ from the more subdued and irregular anomaly pattern of the JGC and RoGC. In the new 3D model, the RaGC is assumed to constitute a migmatised part of the Kautokeino Greenstone Belt. At a large scale, this Proterozoic greenstone belt has some similarities to Archaean counterparts, where most of the deformation is caused by gravitational tectonics. This study shows that careful integration of geological and geophysical data can strongly improve the 3D understanding of the complex, poorly exposed, Precambrian terranes of the Finnmarksvidda region in Norway. [ABSTRACT FROM AUTHOR]

Volcanic rocks in Archean and Paleoproterozoic greenstone belts are abundant and have been suggested as a potential Au source for orogenic Au deposits. The behavior of Au during metamorphism of these rocks is, however, poorly known. We present ultra-low-detection-limit Au analyses from a suite of variably metamorphosed rocks from the Archean La Grande subprovince, Canada, and the Paleoproterozoic Central Lapland greenstone belt, Finland. Both areas are well endowed in Au and have great potential for discovery of new orogenic Au deposits. The metavolcanic rocks in these belts are grouped into tholeiite and calc-alkaline magmatic series, for which the protolith Au contents are calculated using Au versus Zr/Y power-law regressions from greenschist facies samples. In the tholeiitic rocks, Au is compatible during magmatic processes and decreases with differentiation, whereas in the calc-alkaline rocks, Au is incompatible and increases with differentiation. Mass-variation calculations show that as much as 77% and 59% of the initial Au content is lost during progressive metamorphism to upper amphibolite facies conditions (>550 °C) in La Grande and Central Lapland respectively. This study highlights, first, that metavolcanic rocks release Au during metamorphism in Archean and Paleoproterozoic greenstone belts and are thus a good potential source rocks for orogenic Au deposits; second, that the Au fertility of the metavolcanic rocks is controlled by their mantle source and magmatic evolution; and third, that the metamorphic devolatilization model can be applied to Archean and Paleoproterozoic orogenic Au deposits. [ABSTRACT FROM AUTHOR]

The Kevitsa mafic‐ultramafic intrusion, located within the Central Lapland Greenstone Belt in northern Finland, hosts a large, disseminated Ni–Cu–PGE sulphide deposit. A three‐dimensional seismic reflection survey was conducted over the Kevitsa intrusion in 2010 primarily for open‐pit mine planning and for deep mineral exploration purposes. In the Kevitsa three‐dimensional seismic data, laterally continuous reflections are observed within a constrained region within the intrusion. In earlier studies, it has been suggested that this internal reflectivity mainly originates from contacts between the tops and more sulphide‐rich bottoms of smaller scale, internally differentiated magma layers that represent a spectrum of olivine pyroxenites. However, this interpretation is not unequivocally supported by the borehole data. In this study, data mining, namely the Self‐Organizing Map analysis, of extensive Kevitsa borehole data is used to investigate the possible causes for the observed internal reflectivity within the Kevitsa intrusion. Modelling of the effect of mineralization and alteration on the reflectivity properties of Kevitsa rock types, based on average modal compositions of the rock types, is presented to support the results of the Self‐Organizing Map analysis. Based on the results, we suggest that the seismic reflectivity observed within the Kevitsa intrusion can possibly be attributed to alteration, and may also be linked to the presence of sulphide minerals. [ABSTRACT FROM AUTHOR]

Studying the uppermost structure of the subsurface is a necessary part of solving many practical problems (exploration of minerals, groundwater studies, geoengineering, etc.). The practical application of active seismic methods for these purposes is not always possible for different reasons, such as logistical difficulties, high cost of work, and a high level of seismic and acoustic noise. That is why developing and improving passive seismic methods is one of the important problems in applied geophysics. In our study, we describe a way of improving the quality of empirical Green's functions (EGFs), evaluated from high-frequency ambient seismic noise, by using the advanced technique of cross-correlation function stacking in the time domain (in this paper we use term "high-frequency" for frequencies higher than 1 Hz). The technique is based on the global optimization algorithm, in which the optimized objective function is a signal-to-noise ratio of an EGF, retrieved at each iteration. In comparison to existing techniques, based, for example, on weight stacking of cross-correlation functions, our technique makes it possible to significantly increase the signal-to-noise ratio and, therefore, the quality of the EGFs. The technique has been tested with the field data acquired in an area with a high level of industrial noise (Pyhäsalmi Mine, Finland) and in an area with a low level of anthropogenic noise (Kuusamo Greenstone Belt, Finland). The results show that the proposed technique can be used for the extraction of EGFs from high-frequency seismic noise in practical problems of mapping of the shallow subsurface, both in areas with high and low levels of high-frequency seismic noise. [ABSTRACT FROM AUTHOR]

• CLGB is the most prospective area for orogenic gold in northern Finland. • The scalability of the prospectivity modeling is demonstrated. • The stepwise technique effectively focuses the exploration into smaller areas. • The stepwise technique is recommended for exploration workflow. One of the key objectives for mineral exploration is to map the most prospective areas for the selected deposit type at a regional scale, subsequently narrowing these down to camp size target areas and finally to individual prospects that can be developed into economic mines. Exploration targeting, at all scales, is constrained by conceptual mineral system models and requires the integration and interpretation of a variety of geophysical, geological and geochemical data. Geographic information system (GIS) applications are commonly used to integrate spatially referenced data sets to produce mineral prospectivity maps in support of the mineral exploration targeting process. A knowledge-driven fuzzy logic method is used in this work for data integration and prospectivity modelling for orogenic gold deposits in northern Finland. The modelling workflow is done in a stepwise manner at three scales. Each step simulates the successive stage used in mineral exploration from selecting the most prospective belt scale domain to the camp size target area and individual prospects. New, higher resolution data sets are added at each stage as is typically the case in mineral exploration. Northern Finland was selected as the test area due to the regional scale coverage of publicly available geoscientific data and its proven orogenic gold potential. The regional scale prospectivity map outlines the central part of the Paleoproterozoic Central Lapland Greenstone belt (CLGB) as the most prospective area for orogenic gold in northern Finland. The belt scale model of the CLGB improves the resolution mapping of several camp sized high prospectivity target areas. The camp scale model of one of these, a past producing mining camp, further narrows down the high prospectivity zones into prospect size targets. Three models have score Area Under Curve (AUC) values between 0.839 and 0.930 from the Receiver Operating Characteristic (ROC) method validation technique, indicating that the models are robust. The stepwise approach presented confirms that prospectivity mapping using the knowledge-driven fuzzy logic method is a scalable, flexible and relatively fast method which can be used for decision making at different stages of the exploration targeting process. Adding new data and rerunning the prospectivity model with the new data is fast and easy. The most important conclusion from the study is that remodelling individual regional and camp scale domains separately is highly recommended for mapping the most prospective target areas even if the data sets used are the same. [ABSTRACT FROM AUTHOR]

This study concentrates in the Kiannanniemi area, situated in the Archaean Suomussalmi greenstone belt, the Karelia Province, Fennoscandian Shield. A zircon U-Pb geochronological study from this area shows that ages of the volcanic rocks are between ca. 2.94 and 2.82 Ga. The results indicate multiphase felsic and intermediate volcanism in three episodes at ca. 2.94, 2.84 and 2.82 Ga, of which the 2.84 Ga event has not been reported earlier from the Suomussalmi greenstone belt. The youngest zircon population in a sedimentary rock sample suggests a depositional age of ≤2.82-2.81 Ga, and the sample contains also ≥2.96 Ga old zircon grains. Based on both new and previously published geochronological data from the volcanic rocks, we propose a chronostratigraphic model for the whole Suomussalmi greenstone belt, dividing it into four units based on their age: Luoma, Tormua, Ahvenlahti, and Mesa-aho. The youngest volcanic rocks in the Suomussalmi greenstone belt are contemporaneous with some of the volcanic rocks recorded from the Kuhmo and Tipasjärvi greenstone belts of the Karelia Province, Finland. The age group ca. 2.94 Ga, however, has not been so far recorded elsewhere. Conversely, in the Suomussalmi greenstone belt, volcanic rocks with an age of ca. 2.80 Ga and sedimentary rocks with depositional ages of <2.75 Ga, frequently found from the Kuhmo and Tipasjärvi greenstone belts, are unknown. [ABSTRACT FROM AUTHOR]

The Hietaharju Ni–Cu–PGE deposit occurs in the Archean Suomussalmi greenstone belt of eastern Finland and is associated with high-Mg basalts and low-Mg komatiitic rocks. It represents a PGE-enriched type among komatiite-related Ni–Cu deposits. The Hietaharju deposit consists of several mineralized lenses in highly altered olivine–pyroxene cumulates (i.e., talc–carbonate–chlorite rocks) and contains disseminated, massive to semi-massive and brecciated sulfides. Post-magmatic processes have not significantly changed the primary magmatic sulfide assemblage (i.e., pyrrhotite–pentlandite–chalcopyrite), but has modified sulfide textures and possibly remobilized and reconcentrated the sulfides. Though the deposit is currently subeconomic, it has high PGE contents in the sulfide fraction, averaging 5.7 ppm Pd and 2 ppm Pt (average of all ore types), being highest in disseminated sulfides. The parental magma represents an intermediate type between Al-undepleted and Al-depleted komatiites and differs from most other komatiitic rocks in the Tipasjärvi–Kuhmo–Suomussalmi greenstone belt complex, which are normally of the Al-undepleted type. The relatively fractionated nature of the komatiitic basalt parental magma of the Hietaharju deposit is consistent with the low Ni/Cu (< 4) and Ni/Co (< 18) and high Pd/Ir (< 25) ratios of the sulfide fraction, similar to the values in the Paleoproterozoic Raglan belt deposits, Canada. Use of REE for constraining crustal contamination is hampered by the highly altered nature of komatiitic rocks in the Hietaharju area. Sulfides in the country rock metasediments that host the Hietaharju Ni–Cu–PGE deposit show clear mass-independent sulfur isotope fractionation (MIF-S) consistent with their age and published S isotope data from the Suomussalmi belt. In contrast, sulfides of the Hietaharju Ni–Cu–PGE deposit only possess a small degree of MIF-S, providing limited direct evidence for the role of local metasedimentary sulfur in ore formation. Archean base metal sulfide mineralization is among the potential sources of sulfur, however, and, as shown by the epithermal Taivaljärvi Au–Ag–Zn deposit, can have mass-independent sulfur isotope fractionation that is similar in magnitude and sign to that of the Hietaharju Ni–Cu–PGE deposit. [ABSTRACT FROM AUTHOR]

[Display omitted] • New Re-Os isotopic data and isochron ages for the Sakatti Cu-Ni-PGE deposit. • Open-system behaviour of low-Os sulphides potentially during Svecofennian orogeny. • Os-rich sulphides yielded slightly radiogenetic initial Os composition. • In contrast, chromite and olivine show a near-chondritic γOs indicating a mantle source for Os. The Sakatti Cu-Ni-PGE sulphide deposit is located in the Central Lapland greenstone belt in northern Finland. It comprises three mineralised olivine cumulate units called the main, north-east and south-west bodies. Sulphides form massive, semi-massive, disseminated and vein-type ores in these ultramafic rocks. The ore mineral assemblage is typical for magmatic sulphide deposits, including chalcopyrite, pyrrhotite, pentlandite, pyrite and magnetite. We present Re-Os concentration and isotope data for whole-rock olivine cumulate samples and mineral separates (chromite, olivine) as well as for massive sulphides and associated oxides. Chromite separates contain 2.49–115.4 ppb Os and 0.03–2.85 ppb Re while olivine separates have 3.13–42.1 ppb Os and 0.43–3.89 ppb Re. Their host olivine cumulates show Os and Re concentrations of 1.17–23.7 ppb and 0.04–0.93 ppb, respectively. The sulphide and magnetite separates show a considerable variation and overlap in the Os and Re concentrations, with the total range being 0.69–393.5 ppb for Os and 4.44–246.9 ppb for Re. Among the sulphide separates, the highest Re and Os concentrations were obtained for pyrrhotite and pyrite, respectively. On average, Re/Os ratios increase in the following order: chromite, whole-rock olivine cumulate, olivine, pyrite, pentlandite, chalcopyrite, pyrrhotite, and magnetite. Excluding the most disturbed samples, 30 analyses yielded a Re-Os isochron age of 2063 ± 35 Ma for the Sakatti deposit, being similar to the U-Pb zircon age of 2058 ± 4 Ma determined for the Kevitsa Ni-Cu sulphide ore-bearing intrusion, which is located ~20 km NE of Sakatti. Re-Os dating of massive sulphide samples yields variable Re-Os isochron ages from 1852 ± 200 Ma to 2045 ± 300 Ma, with all sulphide samples together giving an age of 1928 ± 16 Ma. This indicates resetting and open-system behaviour of some sulphide samples, potentially related to Svecofennian orogenic events. In spite of this resetting, the low Re/Os ratios and high Os concentrations of chromite and several sulphide separates allow us to determine their initial γOs values, which turned out to be very close to chondritic for chromite and moderately radiogenic (γOs from +21 to +56) for sulphides, indicating that sulphide saturation took place after chromite crystallisation. [ABSTRACT FROM AUTHOR]

Rhenium-Os, Pt Os, Sm Nd, Lu Hf, and Hf W isotope data, together with lithophile trace element and highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundances, are reported for 2.05 Ga Jeesiörova and Kevitsa komatiites from the Central Lapland Greenstone Belt, Fennoscandia, Finland. Both komatiites are closely genetically related, with the Kevitsa dikes having served as feeding magma conduits to the Jeesiörova pillowed and massive lavas. The parental komatiite magma is estimated to have contained ~25 wt% MgO and was, thus, derived from a mantle source at least as hot as those of some of its late Archean counterparts. A suite of Jeesiörova and Kevitsa whole-rock komatiite samples and olivine and chromite separates define an internal Re Os isochron with an age of 2049 ± 13 Ma and an initial γ187Os = −0.2 ± 0.2 (2SE), indicating long-term chondritic Re/Os in the mantle source. By contrast, Pt Os data for a set of Jeesiörova chromite separates define an average initial μ186Os = +29 ± 2 (2SE), indicating a long-term history of suprachondritic Pt/Os in the mantle source. The absolute HSE abundances in the mantle source of the Jeesiörova-Kevitsa komatiite system are estimated to have been 120 ± 5% of the present-day Bulk Silicate Earth (BSE). This is the first komatiite system for which excess HSE in the mantle source, relative to modern BSE, has been documented. The 147Sm 143Nd and 176Lu 176Hf data yield isochron ages and initial ratios of, respectively, 2046 ± 22 Ma with ε143Nd = +3.7 ± 0.3, and 2072 ± 20 Ma with ε176Hf = +8.7 ± 0.4 (2SE), indicating a long-term history of depletions of Nd relative to Sm, and Hf relative to Lu. The measured μ182W = +1.5 ± 3.3 is indistinguishable from the modern mantle value. Despite being strongly depleted in highly incompatible lithophile trace elements, the Th-Nb-La systematics of the komatiites indicate ~1% crustal contamination of the original komatiite magma, assuming the contaminant was similar in composition to the calculated Fennoscandian Tonalite Average (FTA). This level of contamination would have also significantly modified the Nd, Hf, and W isotope compositions of the original komatiitic magma, but not the Os isotope compositions or HSE abundances. The calculated original komatiite magma, corrected for the effects of crustal contamination, would have had initial ε143Nd ~ +4.9, ε176Hf ~ +10.2, and μ182W ~ −10. Our modeling indicates that the initial 186,187Os/188Os isotopic compositions and suprachondritic HSE abundances, coupled with the projected negative μ182W, are best explained by either (1) derivation from a mantle domain characterized by an excess of late accreted, differentiated planetesimal core metal, i.e., "grainy" late accretion, or (2) addition of chemically fractionated terrestrial core metal to the mantle source domain of the komatiites. The presence of these characteristics in the Jeesiörova-Kevitsa komatiite mantle source provides further evidence for the early creation and long-term survival of chemically diverse domains within the mantle. • Kevitsa komatiite source was unique in having ƩHSE BSE = 120%, μ186Os = +29, μ182W = −10. • Was derived from DMM-like mantle domain with initial ε143Nd = +4.9 and ε176Hf = +10.2 • The mantle domain experienced melt depletion within 100 Ma into Solar System history. • HSE, Os and W isotopes resulted from grainy late accretion or core-mantle interaction. • Chemically diverse early mantle domains survived for >2.5 Ga due to sluggish mixing. [ABSTRACT FROM AUTHOR]

Orogenic gold occurrences within the Central Lapland greenstone belt (CLGB), are located in shear zones within Paleoproterozoic mafic-ultramafic metavolcanic rocks and metasedimentary successions. In the mineralized zones, monazite and xenotime of hydrothermal origin are typical accessories in alteration zones and ore veins, and thus allow direct dating of the mineralization. We present U-Pb data from three different gold deposits within the CLGB: Iso-Kuotko, Levijärvi, and Saattopora. All three record several stages of hydrothermal activities along faults which were re-activated several times during the 1.92-1.8 Ga Svecofennian orogeny. The oldest, 1.91-1.90 Ga monazite occur at Levijärvi and all locations have grouping of ages at 1.87-1.85 Ga. At Saattopora, there is also a prominent monazite population with a 1.82 Ga age. However, both xenotime and monazite from the main gold ore stage are characterized by crystallization ages of 1.78-1.77 Ga. Thus the timing of gold mineralization in these deposits is connected to the late to post-orogenic stages of the Svecofennian tectonic evolution.Electron microprobe analyses of the phosphates reveal that especially monazite and to a lesser extent also xenotime have compositional variations according to age. This suggests that the hydrothermal fluid sources varied over the ~140 Ma time span of repeated hydrothermal activities, in accordance with the metamorphic-tectonic evolution of the region. [ABSTRACT FROM AUTHOR]