The Kilvenjärvi Offset Sulfide-PGE Deposit of the Portimo Complex, northern Finland

* Institute of Geological Sciences, University of Leoben, Austria, thalhamm@unileoben.ac.at,

The Kilvenjärvi offset deposit is an unusual occurrence of Cu-Ni sulfides and platinum group elements (PGE), which is hosted by the Archean granite-gneiss basement of the Portimo layered igneous complex, northern Finland (Iljina, 1994; Andersen et al., 1999).

The Portimo complex belongs to a large group of dismembered layered intrusions, which are exposed across northern Finland and into Russia (Karelia and Kola Peninsula). The intrusions are approximately 2.4 Ga in age (e.g., Alapieti, 1982) and display a large diversity of PGE deposits. The Portimo complex host two stratiform deposits, various deposits associated with marginal sulphides, PGE-rich pegmatite bodies, and an unusual sulfide-PGE deposit outside the intrusion proper (the Kilvenjärvi offset deposit) (Iljina, 1994). The Kilvenjärvi offset deposit is situated around a major N-S trending fracture system below the south-western corner of the Kilvenjärvi tectonic block of the complex. It has no surface exposures, but has been intersected by several drill holes (Outokumpu Mining Oy). Massive sulfides form up to meter-wide veins and lenses of primary chalcopyrite, pyrrhotite, pentlandite and pyrite, which are extensively replaced by a secondary assemblage of violarite, millerite, unidentified Ni-Fe-S-phases and Ni-rich pyrite. Sulfide disseminations occurs as impregnations in the host rocks to the veins. The disseminations are dominated by chalcopyrite with minor pyrrhotite, pentlandite and pyrite. Secondary sulfide phases are less common in the sulphide disseminations. PGE concentrations locally reach 110 ppm within the massive sulphides.

Platinum-group minerals (PGM) are mostly Pd-Bi-bismuthotellurides, with only minor sulphides, arsenides, and stannides as accessory phases. The PGM are closely associated with sulfides, hydrous silicates (chlorite, biotite, scapolite, and orthoamphiboles), and carbonates (Thalhammer et al., 1998). The PGE of the Kilvenjärvi offset deposit are distinct from rocks in the Portimo Complex having much higher total concentrations, higher fractionation between the PGE, and an extreme enrichment in Pd.

The host rocks of the Kilvenjärvi offset deposit comprise alternating granite- and mica-rich, strongly foliated gneisses with local bodies and dykes of amphibolite. Granite-gneiss is characterized by the assemblage garnet-biotite-muscivite-plagioclase-orthoclase-quartz-chlorite, the mica-gneiss by the assemblage orthoamphibole-cordierite-garnet-biotite-plagioclase-quartz-chlorite.

The retrograde metamorphism of the Portimo complex, the extensive alteration of the primary sulfide assemblage of the Kilvenjärvi deposit, and the association of water- and carbonate-bearing mineral phases with sulfides and PGM, indicate that the deposit was subjected to a postmagmatic fluid-flow. Preliminary textural and thermobarometrical studies of the metamorphic mineral assemblages in the basement indicate three metamorphic events: A high-grade, medium-high pressure event (probably the original formation of the high-grade gneisses) yielding conditions of around 650oC and 6kbar; followed by a high-T, low-P (around 680oC and 2.5 kbar) (probably a contact metamorphic) event; and finally a medium-T (560oC), medium-P (4.5 kbar) event – which is likely to have been associated with the fluid-flow.