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The Hitura nickel mine is located in central Finland, ~450 kilometres north of Helsinki and 15 km SSE of the town of Nivala (#Location: 63° 50' 58"N, 25° 2' 44"E).
Mineralisation was first detected at South Hitura by the Geological Survey of Finland (GTK) when the nearby Makola deposit was discovered. A program of boulder tracing and geophysical surveys detected the presence of the ultramafic body and a few drill holes intersected mineralisation. In the 1950's, Outokumpu Oy drilled a few more holes and located pyrrhotite-rich ore. In 1961 exploration in the area was reactivated by the GTK and geophysical surveys and geological studies outlined the ultramafic body and several orebodies in North Hitura under 15 to 45 m of overburden. The mine commenced operation in 1970, with a total production of 13.6 Mt @ 0.60 % Ni and 0.22 % Cu to the end of 2005, which had grown to ~16 Mt of ore extracted to December 2012 from the open pit, and since the late 1980's, from the underground mine.
See the separate record on the Suurikuusikko deposit and Kittilä mine for an outline of the setting of the Fennoscandian/Baltic Shield and Svecofennian province of northern Finland.
The Svecofennian nickel province of Finland comprises a series of ultramafic-mafic intrusive bodies found within in a roughly circular area of supracrustal rocks around the central Finland granitoid complex. The Hitura ultramafic complex, lies in the northern segment of this supracrustal ring.
The stratigraphy of the supracrustal rocks in the Hitura region is, from the base:
• Gneissic granitoid rocks.
• Migmatitic mica gneisses with intercalations of graphite and sulphide bearing gneisses and minor amphibolites;
• Meta-greywackes with intercalations of intermediate and felsic tuffites, mafic, intermediate and felsic metavolcanic rocks; and
• Intermediate and felsic metavolcanic rocks.
This enclave of supracrustal rocks is largely surrounded by bodies of granodiorite, quartz diorite and granite gneiss.
The host nickel bearing ultramafic and mafic intrusions occur in the migmatitic mica gneisses package. Veined gneisses, migmatites and brecciated migmatites often surround the intrusions. A belt of sulphide and graphite bearing schists ("Black Schists") and mylonitic zones occurs not far from the Hitura body, and characterises the immediate environment of the ore deposit.
A number of ultramafic bodies are known in the Nivala area, including the host to the Makola nickel deposit, ~3.5 km SW of the Hitura deposit, although only these two economic deposits are known to date. A felsic dyke cutting the Hitura intrusion has been dated at 1877±2 Ma (U-Pb zircon), giving a the minimum age for the Hitura massif.The surrounding district is largely covered by glacial and glacio-fluvial deposits, and there are only sparse outcrops.
The Hitura Ultramafic Complex
The 0.3 x 1.3 km, semi-cylindrical, subvertical, NNE elongated Hitura ultramafic complex comprises three separate, but closely spaced serpentinite bodies, North Hitura, Middle Hitura and South Hitura, which occur within a suite of migmatised mica gneisses. The schistosity and bedding of the isoclinally folded mica gneisses country rocks often conform with the contacts of the plutonic rocks in the Hitura area, and the contact between the Hitura intrusion and the mica gneiss is always tectonic. The extension of the complex in depth has not been established, with mineralisation having been intersected to a depth of at least the 725 m level. Geophysical surveys indicate a continuation to at least 1000 m.
The core of the complex is serpentinite (meta-dunites), whilst the outer, or contact zones are amphibole rich ultramafics (meta-peridotites and meta-pyroxenites). Pegmatitic dykes with chloritic joints are common. The contact zone is characterised by dislocated mafic blocks, erratic wall-rock inclusions and sometimes by massive sulphide lumps in soft talcose matrix, indicating late tectonic movement and faulting. The gneiss near the contact (the "contact gneiss") is often homogenised and contains small garnet crystals and large light clots of feldspars and quartz.
Pyrrhotite disseminations are common in gneisses close to the serpentinite body, with some small, partially nickel mineralised serpentinite tongues occurring in the mica gneisses on the western margin of the Hitura intrusion. Shear zones, enclosing narrow mica gneiss tongues, generally < 100 m wide, separate, but do not offset, the North, Middle and South Hitura segments of the intrusive complex.
Several nickel ore bodies are situated in the contact zones and in the core of the North Hitura serpentinite intrusion. The Middle and South Hitura intrusions are less well known and contain no significant ore.
Five zones of nickel mineralisation are located in the contact zones and in the core of the North Hitura serpentinite intrusion forming a discontinuous ring of ore. From west to east and clockwise, the ore bodies on the contacts are, Western Ore (Länsimalmi), Northern Arc (Pohjoiskaari), the Northeast Arc (Koilliskaari), and Eastern Ore (Itämalmi). The Central Core (Keskitappi) ore body occurs in the In the centre of the North Hitura intrusion and is most likely dislocated from the outer ring mineralisation. In the intrusion, between the core and contacts, there are low-grade nickel accumulations. Low grade mineralisation also occurs on the southern contact zone of the intrusion, and has been mined as open pit ore.
The Hitura ore can be divided into six different types:
• High grade disseminated interstitial sulphides and massive accumulations in the amphibole rock of the contact zones (Western Ore, Northern Arc, Northeast Arc and East Ore ore bodies);
• Medium to coarse-grained moderate dissemination in the marginal serpentinite and amphibole rock (Western Ore, Northeast Arc and East Ore ore bodies);
• Scattered fine-grained sulphides disseminated in the serpentinite core (Central Core or Keskitappi ore body);
• Medium to coarse-grained disseminated sulphides in serpentinite (Eastern and Western contact of the Open pit extension);
• Very fine-grained disseminated sulphides in serpentinite (Central parts of the Open Pit extension);
• Sulphide disseminations and veinlets in mica gneisses outside the ultramafic body (mainly in the western side of the North Hitura intrusion.
In the contact ores, the nickel grade in the sulphide phase is low (~5%), whilst in the serpentinite it is higher (7 to 9%). The core has the highest nickel grade in the sulphide phase (10 to 14%).
The dominant ore minerals are pentlandite and chalcopyrite but in places mackinawite, cubanite and valleriite are abundant. Pentlandite is the main nickel bearing mineral but mackinawite containing up to 6% Ni can also be important. Pentlandite occurs as euhedral grains in the marginal amphibole-bearing serpentinite. Small exsolution flakes are also found, mainly in troilite flames. In the core of the serpentinite, pentlandite has only been preserved as small fractured grains, commonly encapsulated in magnetite, but otherwise has been altered to mackinawite. Cubanite is common in the marginal amphibole-bearing serpentinite, whilst valleriite is most commonly in the core of the serpentinite.
Copper is in chalcopyrite but also in cubanite and valleriite. In the core of the serpentinite, chalcopyrite has been totally obliterated, whilst magnetite is common (7 to 8 vol.%).
Numerous accessory minerals have been identified, e.g., pyrrhotite, violarite, maucherite, nikkolite, gersdoffite and millerite. Pyrrhotite occurs in the marginal amphibole-bearing serpentinite, mainly as hexagonal, exsolved troilite flames common. Monoclinic pyrrhotite is associated with alteration processes and typically occurs along the cleavage and margins of the hexagonal pyrrhotite grains. In the core of the serpentinite, pyrrhotite has been totally obliterated.
Pyrite only occurs in joints with carbonates. Platinum minerals such as sperrylite, michenerite, irarsite, iridararsenite and hollingworthite have also been known.
Alteration during serpentinisation and faulting/shearing of the intrusive complex caused remobilisation of the sulphides, with sub-microscopic networks of sulphide veins developed in the contact zone. Massive accumulation of sulphides, up to several cubic metres, may also occur close to the contact shear zones. Chalcopyrite impregnations are common in the contact zone.
The ore averages 0.6 to 0.7 wt.% Ni, 0.2 to 3 wt.% Cu, 0.1 ppm Pt and 0.1 ppm Pd. The average PGEs in the serpentinite are 0.010 ppm Pt, 0.015 ppm Pd and 0.005 ppm Rh (GTK).
The original total mineral resources is calculated to have been 39 Mt @ 0.52 % Ni and 0.18 % Cu. In 2005, the remaining mineral resources were ~5 Mt @ 0.7 % Ni and 0.2 % Cu (GTK).
Ore reserve and mineral resource estimates at 1 December, 2012, at a cut-off of 0.54% Ni (Belvedere Resources Limited, 2012) were:
North Hitura measured resource - 0.072 Mt @ 0.7% Ni, 0.29% Cu, 3.00% S;
North Hitura indicated resource - 0.749 Mt @ 0.64% Ni, 0.23% Cu, 3.54% S;
Middle Hitura indicated resource - 0.182 Mt @ 0.68% Ni, 0.21% Cu, 2.33% S;
Open pit extension indicated resource - 1.604 Mt @ 0.42% Ni, 0.13% Cu, 1.33% S;
TOTAL measured + indicated resource - 2.607 Mt @ 0.55% Ni, 0.18% Cu, 2.21% S;
TOTAL North, Middle, South and West Offset inferred resource - 0.457 Mt @ 0.70% Ni, 0.29% Cu, 4.56% S;
This summary is drawn from "Meriläinen, M., Lovén, P., Seppä, V-M. and Strauss, T., 2012 - Updated Reserve and Resource Estimate of the Hitura Nickel Mine in Central Finland, an NI 43-101 Technical Report prepared for Belvedere Resources Ltd, 71p", and the Geological Survey of Finland (GTK) online database.
The most recent source geological information used to prepare this summary was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
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