Taivaljärvi - Geology
General Geology of the Fennoscandian Shield
The oldest part of Finnish bedrock was formed during Saamian orogeny 3.1-2.9 Ga. This episode produced large volumes of plutonic rocks that are presently seen as gneisses and migmatites in dominantly tonalitic-trondhjemitic-granodioritic compositions. During Lopian orogeny 2.9-2.6 Ga, subduction-related calc-alkaline volcanic rocks and granitoids, together with greenstones, were added to Archean crust nucleus. The Archean domain was split into three separate domains in Svecofennian orogeny around 1.8 Ga ago. The largest of these Archean crustal domains is the Karelian domain which exceeds 200 000 square kilometres in the area in eastern Finland and adjacent Russia. This domain is characterised by some narrow, almost north-south trending, greenstone and metasedimentary belts. The most western greenstone belt is located in the eastern part of Finland and forms the Kuhmo-Suomussalmi-Tipasjärvi greenstone belt.
Generalised distribution of crustal domains in the Fennoscandian shield.
The Tipasjärvi greenstone belt is the southernmost belt, and it is an isolated extension of the Kuhmo-Suomussalmi greenstone belt. It forms a roughly NNE-SSW oriented T-shaped belt that is approximately 25 kilometres long and a few kilometres wide, bounded by older tonalities, trondhjemites and granodiorites, which are collectively called the TTG-series. The Tipasjärvi greenstone belt has undergone multiple deformation phases and an amphibolite facies metamorphism, which generally has destroyed primary volcanic or sedimentary features. Structural evolution comprises two plastic deformational episodes producing a tight synformal structure that is later cut by a third brittle to a semi-brittle structural event. However, occasional primary features have been preserved in the northwestern part of the belt enabling the determination of depositional younging direction and reconstruction of a stratigraphic column.
Picture. Generalised regional geology for the Tipasjärvi belt.
The property's geology is dominated by siliceous quartz porphyritic rocks interpreted as felsic volcanic rocks that belong to the Koivumäki formation. The Koivumäki formation (Kof) is believed to form the lowest part of the supracrustal sequence of the Tipasjärvi belt. Rocks are interpreted to represent metamorphic derivatives of felsic volcaniclastic rocks, inclusive volcanic breccias and quartz-phyric crystal tuffs.
The Silver Mine deposit is located in the upper part of this formation. On the footwall side, or west from the known mineralisation, there is an approximately 150 metres wide zone of quartz-sericite-biotite schist with bands rich in garnet, tremolite and some chlorite plus cordierite. This zone is believed to reflect metasomatic alteration typically located below massive sulphide deposits. Continuing further west, several skarn bands and BIF's are encountered in drilling before mafic volcanic rocks are present again. Skarn bands can have elevated amounts of zinc and lead, but are barren from silver. On the hanging wall side, the dominant rock type is similar-looking quartz-sericite with minor cordierite and pyritic bands. The uppermost part of the Kof comprises sulphide-graphite schists and BIF's, and their position between the Koivumäki formation and the overlying Vuoriniemi formation could indicate a short break in volcanic activity.
The Vuoriniemi formation is dominated by amphibolites, which are interpreted to represent mafic lavas, tuffs and gabbroic sills. Primary features, like pillow basalts and amygdoidal basalts that are found in several localities, support interpretation. The Vuoriniemi formation is interpreted to be overlain by tholeiitic basalts and komatiitic lavas belonging to the Kallio formation. The top of the sequence is described as composed of greywackes reflected presently as mica schists of the Kokkoniemi formation. It is believed that the Kokkoniemi formation's provenance is underlying volcanogenic formations.
Overlying sulphide-graphite schists and mafic and ultramafic volcanics propose that the deposit is located in the hinge zone of a local anticline, which envelops the mineralized zone. Structurally, the main feature is layering which is interpreted to be of a composite texture of bedding and S1-foliation.
The Ag-Zn-Au-Pb Silver Mine mineralisation is located in series of elongated SW plunging bodies in banded quartz-sericite-biotite schist with poor to moderate pyrite dissemination. Mineralized bodies are commonly dipping towards the south-east at an approximately 60-degree angle in a similar way as observed in compositional banding. The mineralized horizon has a strike length of some hundreds of meters in an NE-SW direction, and it includes several separate higher-grade bodies. The mineralized sequence is approximately 100 meters wide and consists of several narrow bands, a few meters wide, where silver is elevated over 50 ppm. Also, spahalerite and galena are distinctive to the main mineralisation. Towards the east, or the interpreted hanging wall side, the contact of the mineralisation is sharp, although visually rocks look quite similar as in the mineralized zone. The most distinctive difference compared to the mineralized zone is the lack of quartz and quartz-ankerite veins in the barren zone.
The host of the Ag-Au-Zn mineralisation is quartz-sericite schist with quartz phenocrysts. Pyrite is the most common sulphide and can be found as disseminations and thin bands throughout the strata. Silver bearing minerals (pyrargyrite and freibergite) are associated with sulphidic quartz and quartz-ankerite veins that cross cut the main orientation of the observed banding. Previously the mineralisation has been divided into three types: silver-mineralization, zinc-silver mineralisation and low-grade footwall mineralisation.
Picture. A fairly typical sample of the Silver Mine ore. The host rock is quartz-sericite schist. One can see clearly yellowish pyrite crystals especially in the middle upper part of the picture. In the lower right-hand corner, there is a lump of galena and sphalerite crystals.
The quartz-sericite schist hosts silver mineralisation with an abundance of sulphide-bearing quartz and quartz-ankerite veins and pyrite dissemination. The major portion of galena and silver bearing minerals are concentrated to vein material, where they occur in carbonate grains and on the grain boundaries of quartz and carbonate. The grain size of galena and silver bearing minerals is usually below 100 micrometres. According to previous mineralogical studies, the major silver-carrying minerals are dyscrasite (Ag3Sb) and freibergite (Ag, Cu, Fe)12 (Sb, As)4S13 and only a minor portion of the silver are bound to pyrargyrite and native silver. Chalcopyrite is a common accessory mineral.
Picture. 3D block model of the mineralisation and the underground diamond drill holes. Brighter reddish colours represent higher silver content.
Dissemination in host rock and outside of the veins is dominated by pyrite and, in a lesser extent, by sphalerite, arsenopyrite and pyrrhotite. The total sulphide content in the silver mineralisation type is around 7% and it comprises mostly pyrite.
The Ag-Au-Zn mineralisation type has a total sulphide content of approximately 15%, with sphalerite, galena and pyrite being major sulphides. Again, host rock for this type is quartz-sericite schist with vein material. The difference from previous types is a higher overall sulphide content and, especially, a higher galena content, which also holds the larger proportion of silver than the pure silver mineralisation. It is estimated that approximately 10% of silver content is bound to galena, whereas 2% is in silver mineralisation type. Again, dyscrasite and freibergite are the main silver-carrying phases occurring as inclusions in galena or in between galena and other sulphides.
The low-grade footwall mineralisation is of low sulphide type containing less than 5% sulphides, where the average silver grade is reported to be below 40 ppm.
Table. Some selected best sections of the Taivaljärvi deposit.