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INTEGRATED TECHNOLOGIES FOR MINERAL EXPLORATION

PILOT PROJECT FOR NICKEL ORE DEPOSITS

Brite-EuRam BE-1117 GeoNickel

Task 1.2 Mineralogy and modelling of Ni sulfide deposits inkomatiitic/picritic extrusives

Technical Report 6.4

Geology and Mineral Depositsof the Kiannanniemi Area,

Suomussalmi

Tapio Halkoaho and Heikki Papunen

Turku University

Department of Geology

1998

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6.4 GEOLOGY AND MINERAL DEPOSITS OFTHE KIANNANNIEMI AREA, SUOMUSSALMI

6.4.1 Previous studiesH.J. Holmberg made the first attempts to map the bedrock of the Suomussalmi area in 1847-1850, but systematic bedrock mapping did not start until 1907. A preliminary map was com-pleted in 1910. The work was interrupted, however, and although H.Hausen updated the oldmap in 1931-1933, the final bedrock map did not appear until 1954, when the Suomussalmi(D5) sheet was published by A. Matisto at 1:400 000 scale. The geological explanation to themap sheet was printed in 1958 (Matisto 1958).

Ore exploration in the Kiannanniemi area got under way in autumn 1959, when a localfarmer, Gunnar Moilanen, sent Outokumpu Exploration a sample of massive sulfides assay-ing 2.37 wt.% Ni and 1.10 wt.% Cu. Outokumpu carried out 1960-1963 geological mapping,diamond drilling and geophysical surveys in the area (Pehkonen 1963). A total of 11 834 mof holes were drilled, leading to discovery of the Hietaharju and Peura-aho Ni-Cu occurrences.Hietaharju was then estimated to contain 450 000 t of ore at 0.81 wt.% Ni and 0.58 wt.% Cu(Pehkonen 1963). During that period, E. Rantala wrote his MSc thesis on the Saarijärvi area(Rantala 1963) and M. Kokkola on the Kiannanniemi area, in which he described the Kiannan-niemi greenstones in detail (Kokkola 1968). Additional holes (totalling 2523 m) were drilledat Kiannanniemi 1969-1970, mainly at the Peura-aho target, and then 238 000 t of ore at 0.86wt.% Ni, 0.43 wt.% Cu, 0.05 wt.% Co, 18.2 wt.% Fe and 8.5 wt.% S was established in theHietaharju deposit and 260 000 t of ore at 0.58 wt.% Ni, 0.20 wt.% Cu, 0.04 wt.% Co, 14.22wt.% Fe and 7.4 wt.% S in the Peura-aho deposit (Inkinen 1970).

In 1975 A. Häkli and P. Rouhunkoski launched a proposal to Outokumpu Oy Explora-tion to compare the Kiannanniemi Ni-Cu deposits with those of the other Archaean areas. Thestudy, which resulted in reports on the ore mineralogy of the deposits (Kojonen 1977) and inthe PhD Thesis of Kojonen (1981), was funded by Suomen Luonnonvarain Tutkimussäätiö.The Research Programme, “Ore Deposits in Archaean Areas” (Arkeeisten alueiden malmi-projekti) by the Department of Geology, University of Oulu, carried out new observations onthe geology of the Kiannanniemi area (Piirainen 1985, 1988, Hanski 1986) but the geology ororigin of the ore occurrences were not presented. Kurki and Papunen (1985) described thedeposits in a monograph on Fennoscandian Ni-Cu deposits.

6.4.2 Research materialsDue to the occurrence of Ni-Cu sulfides in the Hietaharju and Peura-aho deposits, the Kiannan-niemi area in Suomussalmi was selected as a research target of the GeoNickel Program. Thearea is the southern segment of the Suomussalmi Greenstone Belt (SGB) and is located ca. 40km north of the municipal center of Suomussalmi (boundaries of the area in the Finnish co-ordinate system: x = 7225.000 - 7235.000 and y = 4453.000 - 4463.000 (Fig. 6.4.1).

The geological mapping was reviewed and sampling for geochemical analyses was startedin co-operation with the Geological Survey in 1994; the diamond drill cores were re-logged

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Figure 6.4.1. Geological map of the Tipasjärvi-Kuhmo-Suomussalmi greenstone belt and location of theKiannanniemi area.

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and extended field work was conducted in 1996. A total of 179 polished thin sections, 338XRF whole-rock analyses, 20 REE + trace element and 11 PGE assays were available; 2868m of drill core were re-logged (17 out of 106 holes drilled in the area) and 316 outcrops weremapped at 1:20 000 scale, with the main emphasis on the volcanology of the greenstone beltaround the Hietaharju and Peura-aho Ni-Cu occurrences. The ideas developed in the Siivikko-vaara-Kellojärvi area were applied to the interpretation of the stratigraphy and volcanic evo-lution of the area.

6.4.3 General geology and rock typesThe SGB is relatively wide in the Kiannanniemi area (Fig. 6.4.2). The main rock type is tholei-itic metabasalt of the “Pahakangas type”, but felsic-intermediary volcanic rocks are locallyabundant, at the western margin of the SGB in particular. To the west of Kiannanniemi village,a zone of high magnetic and EM anomalies characterizes a north-south-trending belt composedof felsic metavolcanics, black schists, ultramafic cumulates and komatiitic and high-Cr basalts.The northern end bends southeastwards due to folding. Another separate, arch-formed exten-sion of the belt has been dislocated about 1 km northwards by a fault; this extension includesthe Ni-Cu occurrence of Peura-aho. The ultramafic rocks associated with the belt occur asseparate small lenses of metacumulates, and there are thin sheets of non-fractionated komatiiticflows (chlorite-tremolite rocks) in the area of the Hietaharju deposit.

A southeast-northwest-trending belt of magnetic and EM anomalies exists at the south-western margin of the Kiannanniemi greenstone area. This Vasonniemi belt has been inter-preted as a layer of komatiites and sulfide-bearing felsic volcanics, with the serpentinites ob-served in a few outcrops and indicated by magnetic anomalies representing cumulates of theformer lava river facies of komatiites. The largest ultramafic cumulate complex in the area isthe serpentinite body at Huutoniemi, on the eastern side of Lake Kiantajärvi.

Mafic dikes intersect he greenstone belt, some of which are composed of a boniniticmagma type and resemble the 2.44-Ga layered complexes of Koillismaa. A small mafic lay-ered body, within the greenstone belt proper, is of a similar type, as are also three separate in-trusions enclosed in granite gneiss west of the greenstone belt.

6.4.3.1 MetamorphismAll the rock types except the youngest mafic dikes are metamorphosed and recrystallized.

Metamorphism peaked in mid- to low-amphibolite facies and was coeval with D3, but retro-grade assemblages of low amphibolite to epidote-amphibolite facies were associated with D4.In the following, the term meta- should be annexed to all rock names, but it is not commonlyapplied to rock types well defined by their primary origin.

6.4.3.2 Description of rock types

6.4.3.2.1 Tonalitic gneiss

The Archaean granitoids surrounding the SGB range from reddish to gray in color and aretonalitic in composition. West of Lake Kiantajärvi, their structure is foliated, folded and veinedbut east of the lake it is more planar and banded. Due to the intense deformational structures,the granitoids are called tonalitic gneisses

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Figure 6.4.2. Geological map of the Kiannanniemi area

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6.4.3.2.2 Felsic to mafic metavolcanics and metasediments

In the Kiannanniemi area, the lowermost stratigraphic unit of the SGB is composed of felsic-intermediary metavolcanics and metasedimentary rocks. These rocks were not mapped in de-tail in the GeoNickel review 1996, and the data presented are based on the literature and in-formation distributed by GSF. This rock sequence occurs in an area of about 13 x 1.5 km2 onthe western margin of the SGB and at Keträvaara and Akonperä on the eastern margin. Theunit, which has a maximum thickness of ca. 400 m, is composed of a heterogeneous sequenceof felsic to intermediary metavolcanics, in which primary structures - tuffs, lapilli tuffs and ag-glomerates - are locally visible. Massive felsic rocks, mica schists and layered metasedimentsare also common, and a volcanic conglomerate has been found south of Hiirisuo. Sulfides arenot common in the Luoma Group of the Kiannanniemi area, but in the Palosuo area, north ofthe Juntusranta road, the EM anomaly on the airborne geophysical map and the weak anomalyat the western margin of Lehtovaara are caused by sulfidic rocks of the unit. In the 1960s, threediamond drill holes (SMS-15, 16 and 17) were drilled in the Palosuo EM anomaly.

6.4.3.2.3 Tholeiitic metabasalts

The most abundant rock types of the SGB are the tholeiitic basaltic flows overlying the felsicvolcanic unit described above. Their compositions, structure, texture and stratigraphic posi-tion are similar to those of the Pahakangas type of tholeiitic lavas in the Siivikkovaara andArola areas of the Kuhmo Greenstone Belt (KGB). With a maximum thickness of 600 m, thetholeiitic unit occurs in an area of 15 x 5 km2 in the central part of the Kiannanniemi area,surrounding the granitoid domes in places where the lowermost felsic unit of the SGB is notpresent. The tholeiites are commonly dark green in outcrops, but northwest of Peura-aho theyare light green due to the abundance of plagioclase. In texture they are often fine-grained andmassive, but banded, uralite porphyritic and medium-grained gabbroic varieties also exist.Pillow structures are present, but in intensely deformed areas the pillows have turned intobanded amphibolites. Well-preserved pillows and other volcanic structures are visible betweenSuottakangas and Tarronniemi and at Raiskionaho. Amygdaloidal basalts occur in two outcropsbetween Pahalammi and Kuikkavaara. Black schist and chert interlayers have been encoun-tered in metabasalts of Vasonniemi and southwest of Tarronkangas, but banded oxide faciesiron formations are not present.

6.4.3.2.4 Felsic schists and metavolcanics

Stratigraphically overlying the tholeiitic basalts is a heterogeneous sequence of sulfide-bear-ing felsic schists. The lithology of the felsic unit varies, and contains sericite quartzite, quartz-feldspar schists, felsic porphyry, felsic volcanic breccias and, less abundantly, carbonate bandedblack schists, phyllites and mica schists. The felsic belt bends around the area, being mostprominent in the areas west of Kiannanniemi village - at Karhusaari-Haukivaara, Hietaharju-Tarronkangas and Suottakangas - and around the Huutoniemi ultramafic body, on the easternside of Lake Kiantajärvi. The unit, which has an average thickness of 100 m, is well indicatedby EM anomalies on the geophysical map. Quartz-feldspar porphyry is massive in the Suottan-kangas and Tarronkangas areas but schistose and interlayered by massive iron sulfides in theKarhusaari-Haukivaara, Hietaharju-Tarronkangas and Peura-aho areas.

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6.4.3.2.5 Ultramafic flows and cumulates

Ultramafic chlorite-amphibole rocks have been considered metamorphic derivatives of ultra-mafic volcanics. Although not abundant in the area, their existence has been proven by dia-mond drilling around the Hietaharju and Peura-aho Ni-Cu occurrences and at Myllyaho. Ul-tramafic flow rocks crop out poorly, but single outcrops exist northeast of Pahalampi, west ofSuottakangas, on the lake shoreline at Vasonniemi, between Vasonniemi and Haukivaara, atUuransuo in Jokiniemi, and at Huutoniemi. The ultramafic flow units range from 25 to 100m in thickness, with the maximum values in the area of the Peura-aho and Hietaharju occur-rences. Geochemical signatures of the chlorite-amphibole rocks of the Hietaharju-Peura-ahobelt refer to komatiitic basalt whereas the ultramafic flows of the Vasonniemi belt are truekomatiites.

Serpentinites and related talc-magnesite rocks have been interpreted as metamorphosedchannel-facies olivine mesocumulates. They occur around the Hietaharju and Peura-aho Ni-Cu occurrences, at Huutoniemi, where the body is up to 300 m thick, and in the belt betweenVasonniemi and Haukivaara, where the serpentinite richest in MgO was encountered. TheVasonniemi-Haukivaara ultramafic belt was delineated on the basis of magnetic anomalies. Asmall outcrop of talc-carbonate rock was discovered at Uuransuo. The remaining ultramaficsare tremolite and chlorite-bearing serpentinites, representing orthocumulates of thin fraction-ated flows. The Huutoniemi serpentinite body is the largest of the ultramafic cumulates in thearea and is interpreted as being tectonically thickened. Olivine is metamorphic, and the twosamples analysed show 0.21 wt.% NiO (Fo82) and 0.14-0.17 wt.% NiO (Fo74-75.5). The gabbroicrocks at Jokiniemi display the geochemical signature of fractionated komatiitic or derivedmagma, and probably accumulated in the upper part of an ultramafic flow.

6.4.3.2.5 Komatiitic basalts (high-Mg basalts)

The ultramafic flows and related cumulates are overlain by a 100-150 m thick unit of komatiiticbasalts interbedded with felsic metasediments. Komatiitic basalts are not as abundant as theyare in the stratigraphy of the Siivikkovaara area; the best outcrops, at Tarronniemi and atSärkivaara, northeast of Huutoniemi, display well-preserved pillow structures and variolites,which are rare elsewhere in the Kiannanniemi area. At Vasonniemi, the komatiitic basaltsoverlie tholeiitic basalts with only a thin layer of intervening sulfidic felsic volcanics but nointerlayer of ultramafic volcanics. In drill core SMS-14, a 30-m-thick ultramafic chlorite-amphibole rock unit is interbedded with komatiitic basalt ca. 40 m above the upper contact ofthe ultramafic sequence.

6.4.3.2.6 Felsic interlayers in komatiitic basalts

Felsic schist and metasediment interlayers ranging in thickness from a few meters to 35 m arecommon in the lower part of the sequence of komatiitic basalts. In drill core SMS-6, there arethree 10-35 m thick interlayers of phyllite, black-schist and quartz-feldspar schist in komatiiticbasalt. In places, sericite quartzite and banded felsic metasediments have also been met with.The felsic interlayers are most abundant in the areas of Hietaharju and Peura-aho. They arealways iron sulfide-bearing and their magnetic susceptibility is high (> 6000 nT).

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6.4.3.2.7 Medium-Cr basalt

A rock type called medium-Cr basalt in the Siivikkovaara area was also encountered in theKiannanniemi area, at Pahalammi, Tarronkangas, west of Suottakangas and Suottaniemi. Itoverlies the komatiitic basalt as a 100-150 m thick layer and forms the uppermost stratigraphicunit of the volcanic sequence. In outcrop, the rock resembles tholeiitic basalt, but pillow struc-tures are rare and variolitic structures occur only in the area west of Suottakangas.

6.4.3.2.8 Diorite and granodiorite

A medium-grained, locally porphyritic intrusive rock ranging in composition from diorite togranodiorite forms a 12 x 9 km2 dome structure in the area extending from Lake Kiantajärvinorthwards to the Juntusranta road. On Varpasaari Island, Kuivikkoselkä, diorite forms intru-sive breccias with numerous fragments of mafic volcanics, and mafic gabbroic enclaves (5 x5 cm) are common throughout the intrusive complex. Two other intrusive bodies of about thesame composition occur at Lehtovaara, northwest of Kiannanniemi (3 x 1.5 km2) and atNuottivaara, north of Kiannanniemi (2 x 1 km2). The idea of the dome structures derives fromthe fact that the oldest supracrustal rocks surround the intrusives as if they had been pushedupwards by the rising felsic pluton. The schistosity of the supracrustals, related to D3 defor-mation, follows the granitoids concordantly, suggesting that the intrusions are younger than D3.

6.4.3.2.9 Mafic-ultramafic dikes and intrusions

All the stratigraphic units described above are cut by three dike swarms, from oldest to young-est: 1) mafic boninitic dikes, 2) Ti-rich Fe-tholeiitic dikes and 3) Mg-tholeiitic dikes.

6.4.3.2.9.1. Boninitic dikes and intrusionsMagnetic anomalies of five roundish fractionated mafic intrusive bodies occur in the granitoidarea to the west of the SGB. They are mainly pyroxenitic in composition but minor gabbroiccumulates have also been encountered. The northernmost intrusion includes disseminated ironsulfides. Two similar mafic bodies, one of them 600 x 300 m2 in size and bounded by faultssouth of Suottakangas, are enclosed by the SGB. The north-south-trending layering dips 75-50° to the east, with the younging direction likewise to the east. The chemical composition andstructure of the intrusive bodies resemble those of the 2.44 Ga Koillismaa layered mafic com-plexes. It has been suggested that they derive from boninitic magma. Northwest-southeast -trending boninitic mafic dikes crop out at Lehtovaara, Tarronkangas, Tarronniemi and Vehka-aho and occur in drill core SMS-13. Vuollo (1994) has reported similar noritic dikes and datedthem to 2440 Ma.

6.4.3.2.9.2 Ti-rich Fe-tholeiitic dikesMedium-grained mafic dikes, Fe-tholeiitic in composition and trending northwest to southeast,are more abundant to the east than to the west of the SGB. They are heavily magnetized andcan be readily traced on magnetic maps. According to Vuollo (1994), this type of dike swarmis 2100 Ma old.

6.4.3.2.9.3 Mg-tholeiitic dikes

Most of the Mg-tholeiitic dikes trend approximately east-west, but some of them trend south-west-northeast as do those observed at Peura-aho and Katiskalampi, northeast of Kuivikko-

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selkä. They are less magnetized and abundant than the Fe-tholeiitic dikes. The rock type is amedium-grained gabbro, and Vuollo (1994) has dated the dikes of this composition to 1970Ma.

6.4.3.3 Stratigraphy and structureSome of the primary volcanic structures are well preserved, but indications of stratigraphicyounging directions are rare. Correlation with observations from the Kuhmo Belt and theSaarijärvi area, north of Kiannanniemi, suggests the following succession (Fig. 6.4.3).

The lowermost unit is composed of felsic-intermediate-mafic volcanic lavas, tuffs,pyroclasts and sediments derived from volcanic material. We correlate this with the LuomaGroup. They are overlain by tholeiitic basalts of the Pahakangas type that are locally inter-layered with cherts and black schists. A felsic volcanic unit with a bed of sulfidic black schistoverlies the tholeiitic basalts. The next unit is composed of rare komatiitic flows, the morecommon komatiitic basalts, and the related ultramafic cumulates, which host the Peura-aho andHietaharju Ni-Cu deposits. Medium-Cr basalts are the uppermost volcanic unit of the green-stone belt.

Figure 6.4.4. Schematic E-W section of the Kiannan-niemi area; for location of thesection see Fig. 6.4.2

Figure 6.4.3. Schematic column of the stratigraphic units of the Kiannanniemi area

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According to Luukkonen (1992), the rocks of the Luoma Group display D2 deformation,but the oldest deformation in the other rocks of the SGB is D3. This circumstance cannot beverified in the Kiannanniemi area. The main deformation, D3, manifests itself as a largesynclinorium characterized by open folding with a gently plunging axis striking in a northerlydirection, thus causing the Pahakangas-type tholeiites to occupy a large area (Fig. 6.4.4). Thesubvertical axial plane of F3 trends northwards and is characterized by strong cleavage andshear/fault zones. The axis and axial plane schistosity of the younger, F4, folding strike tonorthwestwards (and the conjugate northeastwards) and is characterized by dextral folds trend-ing northwest to southeast and by conjugate sinistral folds trending northeast to southwest(Luukkonen 1997). The meandering of the key horizons is due to the interference of D3 andD4 structures. The Huutoniemi serpentinite is located in the crest of a F4 fold. The diapiric riseof the granitoids has lifted the lowermost stratigraphic units to form a mantle around the grani-toid domes. Three swarms of Proterozoic mafic dikes intersect the Archaean greenstone beltwith sharp contacts. The Proterozoic metamorphic overprint tends to be weak in the area.

6.4.4 Ni-Cu occurrences of Peura-aho and HietaharjuInkinen (1970) described the exploration and mineral resources of the deposits, and Kokkola(1968), Kojonen (1977, 1981) and Kurki and Papunen (1985) discussed the general geologyand mineralogy, and Papunen (1989) described the PGE geochemistry. This study emphasizesthe relationship of Ni-Cu deposits with volcanic succession, especially with ultramaficmagmatism.

Figure 6.4.5. Geology of the Kiannanniemi village area andlocation of the Hietaharju deposit

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According to Inkinen (1970 a and b), the ore reserves of the two known occurrences are:Hietaharju: 238 000 t at 0.86 wt.% Ni, 0.43 wt.% Cu, 8.5 wt.% S, 18.2 wt.% Fe, 0.05

wt.% Co and Ni/Cu = 2.Peura-aho: 260 000 t at 0.58 wt.% Ni, 0.2 wt.% Cu, 7.4 wt.% S, 14.22 wt.% Fe, 0.04 wt.%

Co and Ni/Cu = 2.9.The subvertical Hietaharju deposit has several subparallel zones of disseminated sulfides

in a serpentinite altered into talc-carbonate rock in the mineralized area (Figs. 6.4.5 and 6.4.6).A zone of tremolite-chlorite rock lies at the eastern margin of the talc-carbonate rock againstthe felsic volcanic rock; similar zones occur in serpentinite and in the talc-carbonate rockproper. These tremolite-chlorite rocks have been interpreted as melt-rich portions of the pri-mary flows where the serpentinites represent cumulates. A mafic volcanic rock occurs to thewest of the ultramafics. The sulfide mineral association is composed of pentlandite, pyrrho-tite and chalcopyrite but, at depth, the intensely carbonated part contains gersdorffite as themain ore mineral, and the PGE contents are relatively high (maximum Pt content 1.9 ppm andPd 2.4 ppm; Papunen 1989).

Figure 6.4.6. Surface level and cross section of the Hietaharju deposit

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The rock sequence at Peura-aho is analogous to that at Hietaharju, but the ultramaficcumulates are not carbonated (Fig. 6.4.7). The deposit is composed of four orebodies that varyin the texture and mineralogy of sulfides. The quality of the disseminated ore in serpentiniteis the best, with Ni tenor ranging from 6.37 to 9.50 wt.% (re-calculated to 100% sulfides). Thetenors of Ni are lower in the massive sulfides in felsic metasediments (1.72-1.99 wt.% calcu-lated to 100% sulfides), but gradually increase in massive sedimentary sulfides close to theserpentinite. In comparison with Hietaharju, the Peura-aho deposit has low contents of PGE:200 ppb Pd and 11-81 ppb Pt.

In the structural classification of Ni sulfides related to ultramafic volcanics, the Kiannan-niemi sulfide deposits resemble the Kambalda type and can be considered sulfide accumula-tions in the basal part of an ultramafic lava flow. The Ni-Cu sulfide accumulations are relatedto the sulphidic sediments and felsic volcanics stratigraphically below the ultramafic cumu-lates. Ultramafic flows along preferred pathways thermally eroded the felsic sulfide-bearingunit and the Ni-Cu sulfides accumulated in the wake of sulfide saturation in the melt. Ther-mal erosion could cause embayments and breaks in the thin felsic sedimentary layer, but, de-spite an intensive search, no such breaks could be indicated. The trace element spectra of theultramafic cumulates are very much like those of the overlying flows, suggesting that no ma-jor wall rock contamination took place even though the Hietaharju deposit displays high con-centrations of certain LIL elements.

Figure 6.4.7. Geology of thePeura-aho Ni-Cu occurrence

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The melt, from which the sulfide hosting cumulates crystallized, was less magnesian thanthat of typical komatiites, probably being komatiitic basalt, because the Fo content of cumu-lus olivine calculated from whole-rock analyses is relatively low (Fo74-82) and the Ni/Cu ratiosof sulfides range from 2 to 3 in contrast to the komatiitic value of 20. The Ni/Cu ratios ofkomatiitic basalts in the area range from 1.1 to 40.

6.4.5 Geochemistry of felsic, mafic and ultramafic volcanics andthe dike rocks of the Kiannanniemi area

6.4.5.1 Major elementsThe 338 XRF analyses of the Kiannanniemi samples cover the volcanic rock types from felsicto ultramafic and all types of sediments and mafic dikes found in the area. The analyses werecalculated to volatile-free oxides, and sulfidic Fe was deducted from total Fe using the sulfurcontent of Fe-Ni monosulfide as the basis.

6.4.5.1.1 Komatiite series

The term “komatiite series” was applied to describe both the melt-rich ultramafic lava-derivedrocks (chlorite-amphibole rocks) and the cumulates (serpentinites and talc-carbonate rocks)even though the magnesium content of the magma could have been lower than in properkomatiites.

In the Jensen (1976) cation plot (Fig. 6.4.8), the volcanics of the komatiite series form acontinuous arch extending from the field of peridotitic komatiites to that of Mg tholeiites. Inultramafic cumulates, the content of Al is related to the amount of intercumulus melt and is agood indicator of the cumulus type. Although the array in the plot is continuous, the contentof (FeO*+TiO2) is higher than that in the corresponding sequence of Siivikkovaara andKellojärvi (Fig. 6.4.10). The Mg end of the array follows the “Fe trend” of the Kellojärvi cu-mulates. It is noteworthy that the maximum MgO contents of ultramafic cumulates are < 40

Figure 6.4.8. Ternary plots depicting the chemical compositions of different rock types of the Kiannanniemiarea

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wt.%. The CaO-MgO-Al2O3 ternary plot indicates a normal olivine fractionation trend, withCa/Al ~1 (Fig. 6.4.8).

The x-y diagrams (SiO2, TiO2, Al2O3, MnO, V, Ni, Cr, Al2O3/TiO2 and Mg number vs.MgO) allow us to compare the fractionation trends of different rock types with each other andwith the rock types of Siivikkovaara (Fig. 6.4.9 and 10). The Mg numbers of the cumulatesand ultramafic flow rocks are in general < 90, only two samples of the Vasonniemi cumulatebelt display numbers > 90. Moreover, the Mg numbers of the Kiannanniemi samples are lowerthan those of the Siivikkovaara-Kellojärvi area when the samples with similar MgO contentsare compared; correspondingly, the contents of FeO are higher in the Kiannanniemi rocks.

In the Al2O3/TiO2 vs. MgO diagram (Fig. 6.4.9), most of the ultramafic and basaltic flowsdisplay Al2O3/TiO2 values between 15 and 20, only a few samples from the Vasonniemi ultra-mafic belt having values exceeding 25. The TiO2 vs. MgO diagrams imply that the Kiannan-niemi flows contain more TiO2 than do those of the Siivikkovaara-Kellojärvi area at similarMgO content. The only exceptions are from the Vasonniemi ultramafic cumulate belt. Themaximum forsterite content of fractionated olivine can be approximated from the TiO2 vs MgOand Al2O3 vs. MgO diagrams. The former indicates an olivine forsterite content of 85.5 mol.%and the latter 90.9 mol.%. As the Al contents are scattered, the trend based on TiO2 content ismore reliable.

The Ni vs MgO diagrams of the ultramafic flow and cumulate samples with < 3000 ppmsulfur display 2000 ppm as the maximum content of nickel. The MgO contents of the miner-alized samples are between 27 and 31 wt.%, indicating that the occurrences at Hietaharju and

Figure 6.4.9. X-y diagrams depict chemical compositions of the rock types of the Kiannanniemi area

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Figure 6.4.10. Comparison of chemical compositions of volcanic rocks between Kiannanniemi and Siivikko-vaara areas

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Peura-aho are related to orthocumulates. If the Kiannanniemi non-mineralized samples arecompared with those of Siivikkovaara, the Ni trend of Kiannanniemi is 200-300 ppm lowerthan that of Siivikkovaara, suggesting that the Kiannanniemi flows have either lost some nickelor that the melt originally contained less nickel. The small Hietaharju and Peura-aho sulfideoccurrences cannot explain the nickel depletion of the flows. On the other hand, the lowerAl2O3/TiO2 values, the lower forsterite content of the fractionated olivines, the lower Ni of themagma and the relatively high Cu/(Cu+Ni) values, compared with those of the KellojärviSiivikkovaara area, all imply that the parental magma of fertile cumulates was not komatiiticbut komatiitic basaltic in composition.

Cr contents range from 16 to 6085 ppm. A group of ultramafic rocks with MgO between27 and 34 wt.% contain > 4200 ppm Cr. The samples richest in chromium (6085-5336 ppm)derive from the ultramafic cumulates of Peura-aho and Hietaharju, indicating that crystalliza-tion of cumulates started in the liquidus field of chromite. This is typical of cumulates derivedfrom a melt that contains less MgO than typical komatiites. On the other hand, some of theMgO-rich cumulates of the Vasonniemi komatiite belt display low Cr values due to crystalli-zation of olivine adcumulates before the liquidus of chromite.

Two types of mafic (MgO 4-10 wt.%) volcanics in the Kiannanniemi area can be distin-guished by their Cr contents: the “Pahakangas type” of tholeiitic basalts, which have Cr con-tents between 85 and 402 ppm, and the “medium-Cr basalts”, which occur in the sequencebetween the ultramafic flow rocks, and display 352-828 ppm Cr. Eighty-three samples of the97 mafic volcanics analysed were tholeiitic basalts and 14 “medium Cr basalts”, indicating therelative abundance of basalt types.

Due to their high Fe and Ti contents, the Proterozoic tholeiitic dike rocks plot in the fieldof Fe tholeiites in the Jensen cation diagram, whereas the boninitic dikes plot in the trend ofkomatiitic rocks. In the SiO2 vs. MgO diagram, the most magnesian boninites have higher SiO2contents than the komatiites. The presence of orthopyroxene in boninites is due to the highSiO2 content.

In the Mg number vs. MgO diagram, the Mg numbers of the boninitic samples are higherthan are those of the ultramafic flows with similar MgO contents, suggesting that the ultrama-fic flows were actually komatiitic basalts with variable amount of cumulus olivine.

The Fe-tholeiitic dikes display high, and the boninitic dikes low TiO2 values in the TiO2vs. MgO diagram. Moreover, the Al2O3 values of boninites are lower than the correspondingvalues of tholeiites. The Al2O3/TiO2 values of Fe tholeiites range from 3 to 12 whereas theboninitic dikes have high “komatiitic” ratios.

6.4.5.2 REE analysesTo establish the contamination, if any, of the flow rocks, 20 samples were analysed for

REE from the ultramafic sequences of Hietaharju, Peura-aho and Vasonniemi, the tholeiiticbasalts underlying the ultramafic sequence (samples 29-TOH-96 and 83-TOH-96), a gabbroiccumulate related to the fractionated komatiitic flows (63-TOH-96) and the Proterozoicboninitic dikes and intrusions (11-3-TOH-96, 12-1-TOH-96, 12-3-TOH-96 and 79-1-JSK-94).

The chondrite-normalized REE distribution patterns of tholeiitic basalts (Fig. 6.4.11) areflat and similar to those of the Siivikkovaara and Arola areas, but one of the samples (83-TOH)is slightly depleted in LREE.

Four analyses of the Hietaharju sequence display an LREE-depleted pattern, the mostdepleted being the samples close to the Ni-Cu sulfides. The Al2O3/TiO2 values are 13 and the

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Mg numbers < 80, suggesting that the magma was close to komatiitic basalt in composition.Two other samples (drill core SMS-20 at a depth of 82-141 m) are HREE depleted (Al2O3/TiO2= 15), probably due to talc-carbonate alteration of the rocks. These samples are the most mag-nesian of the sequence, with Mg numbers > 80. Despite these samples being barren, sulfidemineralization was met with in other holes drilled through this zone.

It is interesting to note that the ore samples of Hietaharju display high LREE, Ba, Cl andPb but low Y, Zn and Zr values. Also, the As and PGE values are high in the deeper portion

Figure 6.4.11. Chondrite normalized REE distribution patterns of the Kiannanniemi rocks calculated with theNewPet programme (Clarke 1987-1994). Chondritic concentrations for La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yband Lu according to Nakamura (1974) and for Pr, Tb, Ho and Tm according to Sun (1980).

F Kiannanniemi “boninitic” dikesE Kiannanniemi komatiitic flows 1996

D Kiannanniemi komatiitic cumulates 1996C Peura-aho, drill core SMS-1

B Hietaharju, drill core SMS-20A Tholeiitic basalts

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of the mineralized zone, probably indicating the effect of alteration by carbonate fluids.Two of the Peura-aho drill core (SMS-1) samples analysed for REE derive from below

the mineralized zone (at a depth of 100 m) and three from above it. Two upper samples (drillcore depth 76.25 m and 92.75 m) are typical komatiitic basalts with Al2O3/TiO2 < 13, Mgnumber 69-75 and the REE pattern flat or slightly depleted in LREE. The other samples arefrom the ultramafic cumulate hosting the mineralization and display a pattern slightly depletedin LREE, Al2O3/TiO2 values 15-16 and Mg numbers > 80.

The data suggest that the mineralized sequence is composed of komatiitic basalts withvariable amounts of olivine ± clinopyroxene xenocrysts and olivine ± clinopyroxene cumu-lates of the same magma type as that hosting the mineralizations. Typical komatiitic compo-sitions have not been met with in the drill sections. Note that the chromium contents of cumu-lates are relatively high, indicating early chromite crystallization. The cumulates can be clas-sified on the basis of their Al and Ca contents as chromite-olivine mesocumulates, with the oli-vine forsterite content well below 90 mol.%.

The komatiitic rocks of the Vasonniemi belt display a flat or LREE-depleted REE patternand the concentrations are only 3-4 times higher than those of chondrite.

The boninitic dikes display a typical LREE-enriched REE pattern with no Eu anomaly.The Kiannanniemi boninitic dikes have somewhat lower concentrations than the Penikat lay-ered mafic complex and the Loljunmaa feeder dike, and the fractionation pattern is slightly lesssteep than in the marginal facies of the Penikat layered complex. Otherwise the general pat-tern is very similar, implying that the Kiannanniemi boninitic dikes are of the same magma typeas the 2.44-Ga layered mafic complexes.

6.4.5.3 PGE analysesPapunen (1989) noted high PGE values in assays of the arsenide-bearing samples from

the Hietaharju deposit. In this study the highest assays of samples from close to both depos-its were 225 ppb Pd and 165 ppb Pt and the concentrations tended to be well below 100 ppb.The arsenides of Hietaharju have thus effectively collected PGE but the other ore types are lowin PGE.

6.4.6 Volcanic interpretations and applications to explorationThe Vasonniemi ultramafic belt parallels a prominent EM anomaly caused by sulphidic

sediments and felsic volcanics. The contact of the felsic volcanics with underlying (?) tholeiiticmetabasalts to the east of felsic rocks had previously been intersected by 11 drill holes. Thicklayers of sulfidic sediments were observed but no base metals were associated with the ironsulfides. The western contact with the overlying ultramafic rock was not investigated. Geo-chemistry suggests that the ultramafics are of the komatiitic magma type, and that the serpenti-nites represent former olivine mesocumulates derived from preferred lava pathways (“chan-nel facies”). The environment is thus fertile for sulfide Ni deposits, but the sparse outcropsprevent accurate geological and stratigraphic interpretations from being made. Only a fewsamples have been analysed from the outcrops, but none of them indicate contamination orsulfide saturation of the melt. Geophysical maps nevertheless indicate that the belt still hasuntouched dimensions.

The ultramafics of the Hietaharju-Peura-aho belt are chlorite-amphibole rocks derived

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from komatiitic basalts and related cumulates, which were altered to serpentinites and talc-carbonate rocks. The cumulate bodies, which are lens-shaped in surface section and composedof olivine and chromite, crystallized from komatiitic basalt. The flows formed channels, andolivine-chromite mesocumulates crystallized at the base of them in much the same way as truekomatiites. The cumulates host Ni-Cu mineralizations formed by a reaction in which sulphurderived from the wall rock, and nickel and copper from a magmatic source. The geochemis-try of the host rocks does not directly indicate contamination, but some high contents of LILelements in the Hietaharju orebodies may derive from the same sedimentary source as sulphur.

The Huutoniemi ultramafic cumulate on the eastern side of Lake Kiantajärvi may be anextension of the Hietaharju-Peura-aho belt. It is surrounded by graphitic and sulfidic meta-sediments and felsic volcanics that could have supplied sulfur for the ultramafic rocks. The areawas recently drilled by GSF and a study on the petrology, geochemistry and ore potential ofthe ultramafics is under way at the Department of Geology, University of Turku (Jani Rautio).

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Inkinen, O. (1970b) Suomussalmen Peura-ahon malmiaiheen jatkotutkimukset kesällä 1970 [The continuedexploration at the Peura-aho prospect in Suomussalmi, summer 1970]. Unpub. report, Outokumpu Oy,Exploration Department.

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