Mutanen Tapani. Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara layered complex, northern Finland

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Geological Survey of Finland, Bulletin 395

Tapani Mutanen

Fig. 61. Plot of Ni vs Co, Keivitsa rocks and ore types. a – all samples; b – zoomed on Ni 100–800 ppm and Co 30–80 ppm; note

bimodal distribution of Ni for each band of Co concentrations. The Ni-poor cluster represents lower marginal part of the ultramafic

zone, also characterized by very low PGE.

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Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara...

Fig. 62. Plot of Ni(100S) vs Ni/Co, all cases, Keivitsa intrusion. Note the sharp lower boundary of the plot cluster; this seems to

represent the mixing line between pelitic and komatiitic sulphides. The halo extending into high Ni(100S) values represents

sulphide-olivine mixtures.

sulphides of the false ore type make up the

bulk of the total mass of sulphides in the intru-

sion.

The regular ore type forms the main mass of

the Keivitsansarvi deposit. Its sulphide content

is 2–6%, with Ni(100S) 4–7%, Ni/Co 15–25

(Fig. 63) and Ni/Cu 0.6–0.8. A typical fair ore

contains 0.5–1 ppm PGE+Au. The ore mass

consists of homogeneous dissemination over

hundreds of metres, almost without a break.

There are, however, invisible lateral trends in

the Main Ore from the proximal to the distal

part. For example, the Ni/Cu ratio has been

found to change within a short distance from

1.5 to 0.4, while the Pt/Rh ratio decreases from

40 to 10.

In the regular ore type the PGE+Au sum

generally correlates positively with Ni+Cu

(Figs 70–71), but among successive drill core

samples there are numerous aberrations normal

to the linear trend (i.e., Ni+Cu decreases as

PGE increases). This behaviour indicates ad-

mixtures of transitional and Ni-PGE-type ma-

terial.

The transitional ore type occurs in the upper

part of the deposit, between the regular ore and

the Ni-PGE-type ore. Gradations into both are

common. A thick transitional ore was inter-

sected in a deep DDH below regular ore in the

Main Ore (see Appendix 5). Rare pegmatoid

pockets with massive to semimassive sul-

phides occur in this type. The gradual contacts,

presence of euhedral olivine and clinopyrox-

ene in massive sulphide matrix and high

Ni(100S) contents indicate that the pegmatoids

represent intercumulus sulphide-silicate emul-

sion, accumulated in contraction voids.

This type has a lower sulphide content,

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Geological Survey of Finland, Bulletin 395

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Fig. 63. Plots of Ni(100S) vs Ni/Co, Keivitsa intrusion. a – samples with S > 0.5%,

b – S > 1%, c – S > 2%. Note the parting of the cluster into regular and false ore with

the increase in sulphur.

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Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara...

Fig. 64. Histograms of Ni/Co, Keivitsa. a – for various classes of S concentrations. Note the gradual parting of the histogram into

regular and false ore with the increase in sulphur; b – samples with Ni+Cu > 0.8%. The peak around 21 represents the regular ore.

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Fig. 65 a–d.

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Geological Survey of Finland, Bulletin 395

Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara...

Fig. 65. Plots of S/Te vs S/Se, rocks and ore types of the Keivitsa-Satovaara complex. Ni(S), = Ni% in 100% sulphides.

a – regular ore type, Keivitsa; b – Ni-PGE ore type and transitional (between regular and Ni-PGE type) ore type; c – false ore types;

d – massive sulphide veins; e – barren olivine lherzolites and partially weathered (oxidized) olivine lherzolites, gabbros and

ferrogabbros; f – metasedimentary floor and roof rocks, xenoliths in the Keivitsa intrusion (pelitic, komatiitic and mixed komatiitic-

sedimentary xenoliths, komatiitic dunite xenoliths), quartz-carbonate rocks at the base of the big serpentinite-dunite xenolith, and

granophyre; g (next page) – fields of various rocks classes, Keivitsa-Satovaara complex.

This type differs from the Ni-PGE type in

having a higher sulphide content, lower Ni/Cu

and slightly higher S/Se and S/Te (Fig. 65).

The Ni content in olivine is equal to or lower

than in the Ni-PGE type. The REE(CN) distri-

bution is similar to that of the Ni-PGE type

(Fig. 87).

The volume of the Ni-PGE type ores is

small. The ores occur in the upper part of the

deposit, at about the level of the Upper Ore,

higher Ni/Cu (1.5–2.5 or more) and Ni(100S)

(15–23%), higher total PGE and higher Pt/Au

(6–10) than regular ore. In contrast to regular

ore, there is no correlation whatsoever be-

tween PGE and S (see Fig. 72). Moreover, the

Cu-S correlation is weak, particularly in sam-

ples poor in S (Fig. 74). Transitional ores in

the lower part of the Main Ore differ from the

upper transitional ores only in their higher Pt/

Rh ratios.

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Tapani Mutanen

Fig. 65 g.

Fig. 66. Histogram of Pt/Rh ratios, Keivitsa intrusion.

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Geological Survey of Finland, Bulletin 395

Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara...

Fig. 67. Pt-Au relationships, Keivitsa intrusion. a – plot of Pt vs Au; b – plot of Pt vs Au

for samples with Pt < 1000 ppb and Au < 400 ppb; c – histogram of Pt/Au ratios.

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Fig. 68. Pt-Rh-Au relationships, Keivitsa rocks and ore types. a – plot of Pt/Rh vs Pt/Au; b – ore types in the Pt/Rh vs Pt/Au plot.

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Fig. 69. Plot of Re/Ir vs Ir, Keivitsa.

rocks of the transitional and Ni-PGE ore types.

Massive sulphide veins are common but vol-

umetrically insignificant. They are simple or

consist of branching stockworks or breccias.

The contacts are sharp. Contact effects in fresh

olivine peridotites (hydration, carbonate alter-

ation) are conspicuously weak, although por-

tions of metaperidotite usually contain net-

works of sulphide veins. Compositionally mas-

sive sulphides are of the false ore type, poor in

Ni but occasionally rich in Cu. The PGE-Au is

low (Figs 71–76). The record low is a massive

sulphide vein with a mere 4 ppb Pt.

Ultramafic xenoliths within the ore contain

fine-grained disseminated sulphides of the

same composition as sulphides in the cumulate

matrix. This suggests, as discussed above, that

and are characterized by a very low abundance

of sulphides, high Ni/Cu (50–90) and Ni/Co

(25–80), high Ni(100S) (40–60%), PGE(100S)

(25–300 ppm) and low Au (max. 0.13 ppm). In

general, the highest PGE contents are found in

this type (Fig. 76). There is no correlation be-

tween PGE and S (Fig. 72) and only very weak

correlation between Cu and S, particularly

when compared with other ore types (Fig. 74).

The sulphides are high in Ni (millerite, heazle-

woodite, Ni-rich pentlandite). Because of the

high Ni content in silicates (in olivine up to

1.74% NiO, in clinopyroxene up to 0.26%

NiO), all calculated Ni(100S) values are far

too high. Occasional high As values are char-

acteristic.

Komatiitic xenoliths are very rare in host