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Vihanti

Finland

Main commodities: Zn Cu FeS2
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The Vihanti zinc-copper-pyrite ore deposit is located some 450 km north of Helsinki, on the north-western end of Lake Ladoga, near the Gulf of Bothnia, in western Finland (#Location: 64° 24' 37"N, 25° 8' 39"E).

The zinc mineralisation is hosted by a sequence of Palaeoproterozoic (1.92 Ga) gneisses, representing metamorphosed sediments, mainly calcareous rocks, comprising dolomites and calc-silicates, felsic volcanic rocks, cordierite-bearing metasomatic rocks and black schists. The copper-pyrite ore is within felsic volcanic rocks, comprising a bimodal complex of low- to medium-K rhyolites, transitional between calc-alkaline and tholeiitic, and sub-alkaline low- to medium-K tholeiitic basalts and basaltic andesites. Trace element chemistry of the volcanic rocks indicates a mature island arc setting. These are surrounded by mafic volcanics, mica gneisses and batholithic intrusives.

The deposit occurs on the SW limb of a D2 anticline, where the host sequence dips at 40 to 45°SE and the fold axis dips at 30 to 40°SW, and is localised in the hinge zone and limbs of an overturned parasitic dragfold/monocline on one limb of the main anticline. The ore complex hosting the zinc mineralisation is an east-west trending sequence of 'arenaceous' and calcareous, skarn altered rocks located in a mica schist-dominated synform that dips at 45°S. The 'arenaceous rocks' are composed of volcanogenic material and located in a formation characterised by turbiditic mica schists, dated at 1.90 to 1.88 Ga.   The host sequence is cut by several SE-trending faults and has an estimated length of ~2 km and is 500 m wide. It extends to a depth of 800 m as a monocline of dragfolded sedimentary and volcanic rocks containing zinc, pyritic ore and disseminated copper. The ore-hosting sequence also contains a stratabound uraninite-apatite tuffite horizon on one side of the ore deposit, at the same stratigraphic position as the pyrite lodes (GTK and references cited therein).

Early (1900 Ma) and synorogenic (1875 Ma) gabbroids and granitoids surround and intrude the mica-schist area hosting the ore sequence. Mafic dykes occur in the sequence as conformable and cross-cutting bands. Felsic to intermediate porphyry dykes, parallel to the local faults, cut across both the ore and its host rocks and the mafic dykes. In addition, there are more coarse-grained granitic dykes which represent the latest intrusives in the area. The local quartz-feldspar porphyries and cordierite-sillimanite gneisses, which cut across the local volcanic and sedimentary units, are interpreted as partially altered subvolcanic intrusions which may be 5 to 60 m thick, and sub-conformable to the host sequence. Up-dip, the quartz-feldspar porphyries occupy the stratigraphic interval where the felsic metavolcanic rocks occur down dip (GTK and references cited therein).

D1 and D2 deformations produced regional scale flat-lying nappes and isoclinal folding which are not mapped locally. The regional D3 deformation, which involved NE-SE compression and peak metamorphism at ~1884 Ma, produced subvertical folding with a dominantly NW-trend, whilst D4 was associated with north-south compression, producing remobilised large-scale (>100 km long) strike-slip shear and fault zones. Post D4 brittle faulting occurred in several stages across the region, possibly at ~1784 Ma (GTK and references cited therein).

The deposit includes a number of separate orebodies that vary from 1 to 50 m in thickness, 50 to 200 m wide and 150 to 900 m long, occurring as pyrite, zinc, chalcopyrite and Pb-Ag-Au lodes. The Zn and pyrite lodes occur together throughout much of the deposit, and are 10 to 20 m apart (GTK and references cited therein).

The main zinc ore bodies are Ristonaho, Välisaari and Lampinsaari. The Lampinsaari lode is zoned, with the highest Zn grades in the central and lower parts of the orebody, higher Cu and Pb in the centre, and lower parts of the lode respectively. The contiguous Ristonaho and V√§lisaari orebodies are ~1100 m long, ~100 m wide and 10 to 60 m thick. Isoaho is a tabular body extending from 350 to 1000 m below the surface. The main pyrite ore bodies are Hautaräme and Hautakangas which contain low-grade Zn and Cu ore. The Cu ore bodies are irregular masses in the central part of the Lampinsaari Fm. The main Pb-Ag-Au ore bodies are at the western end of the mineralised complex, close to the Välisaari (Zn) and Hautaräme (pyrite) ore bodies (GTK and references cited therein).

The majority of the zinc is found on the footwall of the complex, along the folded contact between the volcanic quartz porphyry and skarn dolomite with dolomite being the immediate host.   The main ore mineral is sphalerite with lesser chalcopyrite, galena and iron sulphides, and accessory Sb and As.   The zinc ore sometimes exhibits compositional banding.

The gangue is predominantly dolomite and skarn with associated diopside, tremolite, barite, quartz, dolomite, rutile, graphite, tourmaline, fluorite.   The ore is bounded by sharp contacts, with an enveloping disseminated chalcopyrite and pyrrhotite halo which has a gradational outer margin.   The pyrite ore is associated with sericitisation and the development of sericite in the volcanic rocks and Mg skarn in the carbonates and contain small amounts of chalcopyrite and sphalerite and have pyrrhotite rich margins.   Where the host is predominantly silicic volcanics, the ore is banded and brecciated and occurs in shear zones with more copper.

The orebodies have been subjected to post-mineral regional metamorphism and polyphase deformation, resulting in significant increase in the grain size of the sulphides (from 0.05 to 1 mm) and, possibly, by deformational increases in the thickness of the Zn-lodes. Folding and multi-stage transverse faulting has also significantly modified the geometry of the lodes, as well as influencing local remobilisation of the sulphides and, especially, sulphosalts into veins. The ore and skarn rocks have, in many areas, behaved in a plastic manner during deformation, whilst the much more competent arenaceous interlayers have been and broken into small fragments within the ore-skarn matrix (GTK and references cited therein).

Total production between 1954 and 1992 amounted to - 28.1 Mt @ 5.12% Zn, 0.5% Cu, 0.36% Pb, 25 g/t Ag, 0.49 g/t Au.
Remaining resource in 1992 - 9.164 Mt @ 0.36% Zn, 0.34% Cu and
      2.59 Mt @ 284 ppm U, 3.24% P2O5 (Outokumpu Oy., 1992, internal report).

The most recent source geological information used to prepare this summary was dated: 2012.     Record last updated: 8/1/2015
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.


Vihanti

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Mikkola A K, Rouhunkoski P  1980 - The copper deposits and their metallogeny in southern Finland: in Jankovic S, Sillitoe R H (Eds.) 1980 European Copper Deposits Proceedings of an International Symposium held at Bor, Yugoslavia, 18–22 September 1979. Belgrade University,    pp 180-187
Rehtijarvi P, Aikas O, Makela M  1979 - A middle Precambrian uranium- and apatite-bearing horizon asscociated with the Vihanti zinc ore deposit, western Finland: in    Econ. Geol.   v74 pp 1102-1117


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