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Kilvenjarvi
Finland
Main commodities: PGE PGM Cu Ni


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The high-grade Kilvenjärvi platinum-group element (PGE) bearing Cu-Fe-Ni deposit located in the basement below the Portimo Layered Igneous Complex of Finnish Lapland, 35 km south of the city of Rovaniemi, Finland.

The Portimo Complex belongs to a group of more than 24 PGE-bearing layered intrusions exposed across the Baltic Shield, from the Swedish-Finnish border in the west, across Finland into Karelia and the Kola Peninsula in the east. These intrusions, which have been dated at 2.44 Ga, are generally developed at the upper boundary of the Archean gneissic basement and are coeval with extensive metabasalts and apparently associated with the first major episode of rifting of the Archean craton. Metavolcanic rocks of the province are preserved in two NW-SE trending belts that are >600 km long, extending across the Baltic Shield. After emplacement and crystallisation, these intrusions were dislocated by faulting, eroded and overlain by Proterozoic volcanic and sedimentary rocks.

The Portimo Complex has been faulted into smaller tectonic blocks that are now dispersed within an area of 15 x 20 km. These blocks were eroded to a variable extent in the Palaeoproterozoic before 2.33 Ga. The complex is overlain above an angular unconformity by a conglomerate-pebbly schist and by a succession of alternating Proterozoic basalts and clastic sedimentary rocks, primarily quartzites and turbidites. The complex have been metamorphosed to greenshist to lower amphibolite facies at around 1.8 Ga when it was also gently folded with the closure of the rift. Contemporaneously at from 1.8 to 1.77 Ga, the voluminous Lapland granite batholith was intruded just north of the Portimo area. Subsequently, widespread alkaline, carbonatite, and kimberlite magmatism occurred in the Baltic Shield between 575 and 363 Ma).

The Portimo Complex is divided into two parts, the Narkaus and the Suhanko-Konttijärvi intrusions, believed to have crystallized in two connected magma chambers, now faulted into 9 separate blocks. The Complex comprises a Marginal and a Layered series, distinguished by their magmatic fractionation trends. The Layered series is further divided into three mega-cycle units, MCU 1, MCU 2, and MCU 3, each with a lower ultramafic and an upper gabbroic part. Where preserved the cumulate layered sequence is 850 m in thickness, while the total exposed succession amounts to about 1200 m. All thee mega-cycle units show normal igneous fractionation trends toward lower crystallisation temperatures upward, and are believed to reflect three influxes of magma into the magma chamber. The magmatic fractionation is recognised in trends of decreasing CaO/(CaO + Na2O + K2O) and MgO/(MgO + FeO) upward through each unit, reflecting primarily the crystallization of cumulus plagioclase and pyroxene. The boundaries between the three units are characterized by distinct compositional reversals in the whole rock composition major and trace elements.

The layered intrusions of the Baltic Shield, including the Portimo Complex, host a large number and variety of PGE deposits comparable in grade to the deposits in the Bushveld and the Stillwater Complexes. PGEs occur primarily in two lithological associations: i). strata-bound layers in the central parts of the complexes and ii). more irregular occurrences associated with sulphide-rich parts of their marginal zones.

Several occurrences of PGE-rich sulphides have been reported from the Marginal series of the Portimo Complex, containing up to 5 ppm total PGE. The Layered series includes two stratigraphic intervals with significant concentrations of PGE: the Siika-Kämä and the Rytikangas reefs. The former occurs at the basal contact of the MCU 3 unit and has been identified in most of the Narkaus intrusion. The Rytikangas reef occurs in the upper part of the MCU 3 unit and is only preserved in the Suhanko block of the Suhanko-Konttijärvi intrusion). These reefs have very variable total PGE grades, which locally reach 20 g/t. Sulphur concentrations in both reefs total less than 1%. These reefs represent the first of the two associations listed above.

The Kilvenjärvi deposit belongs to the second association and is located in a north-south trending fault zone that delimits the Kilvenjärvi block to the west. It covers an area of at least half a sq. km, extending to a depth of at least 120 m into the basement below the base of the intrusionand is situated between two major faults that displace the magmatic lithologic units as well as the intrusion boundary. It consists of a stockwork of massive Cu-Fe-Ni sulphide veins, up to a metre thick cutting banded gneiss, amphibolite, and granite. The veins are sandwiched by intense disseminations of sulphides and alteration of the basement mineral assemblage to chlorite, calcite, ortho-amphibole, monazite and an unidentified hydrous ferroaluminous silicate.

The veins locally carry >110 g/t PGE, while the altered basement wall rocks containing disseminated sulphfides locally assay up to 25 g/t PGE. The PGEs are primarily within the minerals michenerite (PdBiTe) and sobolevskite (Pd[Bi,Te]), with only very few grains of merenskyite, sperrylite, vysotskite-braggite, paolovite, cooperite, and isomertieite.

Two distinct mineral assemblages that reflect the addition of sulphur are recognised in the massive sulphide veins, namely: i). a primary assemblage of chalcopyrite, pentlandite and pyrrhotite which appears to represent the crystallisation products from a sulphide melt, and ii). extensive rteplacement of the primaru assemblage by a low-temperature secondary assemblage of chalcopyrite, pyrite, marcasite, Ni-bearing pyrite, violarite, and millerite.

The disseminated sulphides in the basement lithologies are wholly composed of the secondary assemblage and are notably richer in chalcopyrite than the massive sulphide veins. These sulphides are associated with chlorite and calcite alteration, indicating that they were likely introduced by carbon-rich (CO
2 or CH4) hydrothermal fluids.

The most recent source geological information used to prepare this decription was dated: 2006.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
Andersen J C O, Thalhammer O A R and Schoenberg R,  2006 - Platinum-Group Element and Re-Os Isotope Variations of the High-Grade Kilvenjarvi Platinum-Group Element Deposit, Portimo Layered Igneous Complex, Finland: in    Econ. Geol.   v101 pp 159-177


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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