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Geological Information

Lapin Lääni (Lapland) province forms a significant part of the Archaean Karelian craton, recording a prolonged history of sedimentation, rifting and magmatism throughout the Palaeoproterozoic.

There are four major components which form the bedrock of Northern Finland:

(i) The Archaean granite-gneiss complexes and greenstone belts

(ii) Palaeoproterozoic greenstone belts

(iii) the 1.9Ga Lapland Granulite Belt

(iv) Svecokarelian syn- to late- (1.89-1.80Ga) and post-orogenic (1.80-1.77Ga) granitoids.

The most important component is that of the Palaeoproterozoic greenstone belts, the host rock to the gold mineralisation. These belts include the Central Lapland Greenstone Belt (CLGB) and the Kuusamo and Perapohja schist belts.

Gold Mineralisation:

Structure is the most significant control for mineralisation for all epigenetic occurances in Northern Finland. Locally, structure along with host rock type can be viewed as the most significant controls. Fluid compositions suggest variable, mixed origins for volatiles and metals, with no evidence of a local source.

There are three main types of Au mineralisation evident in the Palaeoproterozoic of Northern Finland. These are the (i) orogenic gold-only type; (ii) iron-oxide-copper-gold (IOCG) type; and (iii) palaeoplacers.

Orogenic gold-only type deposits are characterised by carbonitisation with sereticisation or biotitisation, PT conditions at 300-450 degrees celsius and 1-3kbar, pyrite, pyrrhotite and arsenopyrite being the main ore minerals, consistent enrichment of Ag, Au, As, CO2, K, Rb, S, Sb and Te, and a low-salinity fluid with hydrothermal quartz showing δ18O at +11 – +13‰ and carbonate δ13C at -8 – -1‰. Orogenic gold occurrences with atypical metal association are similar to the gold-only type, except having significant chalcopyrite ± cobaltite, gersdorffite and/or uraninite contents, enrichment also in Cu and, in some cases, in Co, LREE, Ni and/or U, and intense albitisation predating the gold-related alteration.

The iron oxide-copper-gold (IOCG) occurrences are characterised by regional albitisation ± scapolitisation, multi-stage local alteration, formation T at 400–600°C, main ore minerals of magnetite, pyrite, pyrrhotite, chalcopyrite ± cobaltite, consistent enrichment in Ag, Au, Bi, Cu, Fe, S, and Te, and an aqueous high-salinity mineralising fluid with variable salinity and Eh, and δ18O at +9.6 – +17.5‰.

The palaeoplacers are Au-only, sedimentary facies-controlled, occurrences in molasse-like sediments of the uppermost stratigraphic formation of the Central Lapland greenstone belt.

Timing of gold mineralisation in northern Finland is not well-constrained. Most or all of orogenic gold mineralisation took apparently place during the continental collision epoch of the evolution of the Fennoscandian shield, at 1.85–1.79 Ga, although some orogenic mineralisation may be related to the earlier compressional stage, the microcontinent accretion, at 1.92–1.88 Ga. For the IOCG type of mineralisation, both of the extensional epochs of the Palaeoproterozoic orogenic evolution seem to be possible: the occurrences were formed during the continental extension at 1.88–1.85 Ga, or orogenic collapse and stabilisation at 1.80–1.77 Ga, or both. For the IOCG deposits in the Kolari area, the ca. 1.80 Ga timing appears to be the most probable. The probable time frame for the palaeoplacer mineralisation covers the 1.88–1.85 Ga epoch and the early parts of the 1.85–1.79 Ga epoch.