Kuhmo - Sika-aho

The Sika-aho nickel deposit is hosted by sheared felsic, mafic, ultramafic rocks and minor graphitic schist within the western edge of the north-northeast trending Tammasuo Shear Zone. Regionally, the Tammasuo Shear Zone separates a sequence of mafic and minor ultramafic units in the east from a regional antiform comprising mafic rock overlain by a thick ultramafic sill and a relatively thick komatiite sequence. To the west of Sika-aho the local geology consists of a multiply deformed synform of komatiite and chromian basalt. The nickel deposit is located a few metres west from talc-carbonate altered komatiitic cumulates and comprises a one to nine metre wide zone of massive sulphides developed over approximately 80 metres of strike length; disseminated and sheared over an additional 150 metres to the north-northeast (Figure 1). This occurrence extends at least 150 metres below surface with a dip of 85º to SE (GTK, 1998 and 2002, references 14 and 18).

The host rock is often quartz rich and partially carbonated chlorite schist and has been interpreted as originating from the highly altered and contaminated basal portion of a komatiitic lava flow of pyroxenite composition. The host rocks are strongly silicified, however the high Ni:Cu ratio (>15:1) of mineralisation is consistent with a primary magmatic origin. Nickel tenor is 5-20% for nickel mineralisation grading over 0.5% and has wider variation from 3% up to 55% for lower grade (0.3-0.5% nickel) mineralisation. The principal sulphide minerals are pyrrhotite and pentlandite.

The fact that the deposit is situated along, or near, the contact of ultramafic rocks indicates the possibility of Kambalda style mineralisation, but the strong structural correlation suggests mobilisation of the occurrence from the original position within the shear zone.

The best intercepts at Sika-aho were encountered in holes R306 and R319, drilled by the GTK in 1994 – 1997, returned 7 metres @ 1.26% nickel and 3.74% sulphur and 8 metres @ 1.16% nickel and 4.86% sulphur respectively.

GTK completed a polygonal resource estimate based on a vertical cross section in 1998 which quotes 175,000 tonnes at 0.7% nickel down to 300 metres below surface (GTK, 1998, reference 13).


Figure 1. Geological map (modified after GTK, 1998) of the Sika-aho Ni-occurrence. Ni-occurrence is shown in red. Mise-a-la-masse anomaly is shown as brown contours and drill hole collars as white dots. Map grid is 100x100m.