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Sakatti |
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Finland |
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Cu Ni Pt Pd PGE PGM Co Au
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Super Porphyry Cu and Au
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The Sakatti copper-cobalt-nickel-platinum group element (PGE) sulfide deposit deposit is located 15 km north of Sodankylä, in Lapland, Finland, ~150 km north of the Arctic Circle and ~785 km north of Helsinki.
The deposit was discovered in 2009 by Anglo American plc, after detecting the first signs of mineralisation in 2006. By 2016, the mineral resources had been delineated and resources were classified and reported.
Regional Setting
Sakatti, which is located ~20 km SSW of the similar Kevitsa Ni-Cu-PGE deposit, falls within the Central Lapland Greenstone Belt in northern Finland. The greenstone belt forms an almost uninterrupted zone from belwo the Caledonian Thust in northern Norway, through central Finnish Lapland, where it bifurcates into limbs that extend to both the west coast of Finland and crosses its eastern border into Russia. Overall the belt has a NNW-SSE length of ~550 km and width of up to 150 km. It is composed of volcano-sedimentary sequences, that extends from deposition of the 2.44 Ga Salla Group shallow sedimentary clastic rocks and bimodal felsic-intermediate metavolcanic sequence with komatiite to rhyolite composition, deposited during a period of intracratonic rifting. This rifting progressed to a period of deepening with sulphidic shales before being overlain by the 2.0 Ga Kittilä Group, reflecting opening of an oceanic basin. The sequence ended with the ~1.88 Ga molasse-type metasedimentary rocks of the Lainio and Kumpu Groups (Hanski and Huhma 2005).
A number of intrusive mafic magmatic events have been recognised within the greenstone belt. The oldest of these is represented by the 2.44 Ga layered mafic intrusions of the Koitelainen and Akanvaara complexes 20 km NE and 70 km SE of Sakatti respectively. These intrusions host a number of metre-scale chromitite ±PGE horizons as well as V-enriched magnetite gabbros. At 2.20 Ga a suite of differentiated mafic sills were intruded into the southern part of the greenstone belt. This was followed at ~2.15 Ga by gabbroic sills and dykes exposed in the central and south-eastern part of the schist belt. The final major mafic intrusive event is the 2.05 to 2.0 Ga Kevitsa-type layered intrusion and sills, as well as minor gabbroic intrusions and dykes (Rastas et al., 2001; Rasanen and Huhma 2001; Hanski and Huhma 2005). The known magmatic Ni-Cu-PGE deposits, including Sakatti, are hosted within the 2.05 Ga 'Kevitsa intrusions', intruded during the late rift phase, prior to the opening of the oceanic basin. Mineralisation is also associated with some of the small ultramafic intrusions of unknown age in the Sakatti area. Sakatti lies towards the centre of the greenstone belt.
Geology and Mineralisation
Three separate mineralised ultramafic bodies have been defined in the area, namely the Main, NE and SW bodies. Regionally, these intrusions are surrounded by quartzites with minor mafic to felsic volcanic rocks of the cratonic to marginal marine 22.2 Ga Sodankylii Group and pelitic metasediments of the Savukoski Group, specifically the ~2.06 Ga Matarakoski Formation. The Savukoski Group corresponds to a deepening of the basin between 2.2 and 2.05 Ga, and is characterised by sulphide bearing carbonaceous shales intercalated with mafic to ultramafic volcanic rocks that are picritic and komatiitic, thought to have been generated deep in the mantle. Several Ni-Cu-PGE deposits, including Sakatti, are associated with layered intrusion associated with this mafic to ultramafic magmatic event within the rift basin between 2.2 and 2.05 Ga (Fröhlich et al., 2021 and references cited therein).
The main intrusive body at Sakatti subcrops over an ~250 x 500 m area and forms an irregular, 200 to 400 m diameter, NW-plunging, cylindrical intrusion that persists to a known depth of ~1200 m (Brownscombe et al., 2015). The intrusion occurs in the lower part of the is mostly composed of serpentinised peridotite, which was an olivine cumulate with interstitial clino- and orthopyroxene, with subordinate dunite, an olivine adcumulate, at depth. The immediate wall rocks to the main intrusion is a fine grained, aphanitic lithology, composed of small olivine and plagioclase phenocrysts set in a fine grained plagioclase-pyroxene-olivine groundmass. Near the contact with the intrusion, it contains mm- to cm-scale 'fingers' or veins of intrusive peridotite. This wall rock cannot be readily related to the regional country rock, but on the basis of the mineralogy and textures, it is thought to be of volcanic in origin, compositionally similar to a komatiite-picrite. In the south western part of the deposit, the wall rocks comprise a heterogeneous package that includes mafic volcanic rocks and scapolite-biotite schist, intruded by gabbroic sills. Upwards in the stratigraphy, the aphanitic unit and the mafic suite are followed by a 100 to 300 m thick hematite-carbonate-albite-talc altered polymictic breccia unit. The uppermost unit of the wall rock sequence is a 600 m thick phyllitic unit (Brownscombe et al., 2015).
The sulphide mineralisation comprises:
• Disseminated sulphides, which extend from the subcropping southeastern section of the intrusion, through its central part, to its deeper reaches, although the intrusion is not mineralised throughout. These sulphides are predominantly composed of chalcopyrite with minor pyrrhotite, pentlandite, and pyrite. They tend to be more Cu-rich in the shallower parts and Cu-poor in the deeper sections of the intrusion (Brownscombe et al. 2015). Typical grades within thick intersections are of the order of 0.51% Cu, 0.23% Ni, 0.44 ppm Pt, 0.22 ppm Pd, and 0.13 ppm Au (e.g., over 62.70 m in DDH 08MOS8007; Coppard et al., 2013).
• Massive to semi-massive sulphide veins, which predominantly occur in the shallow and central parts of the intrusion, accompanied by disseminated sulphides. These veins are Cu-rich, Au-enriched, as compared to the disseminated and massive sulphides, and are typically 5 to 20 cm thick (Brownscombe et al., 2015; Coppard et al., 2013).
• Massive sulphides, found in the central section of the intrusion, occurring as stacked lenses that persist for up to 150 m into the aphanitic wallrocks. These lenses are up to 25 m thick within the centre of the intrusion, tapering to ~0.5 m towards the SE and NW. An example intersection in drill hole DOH llMOS8049 was 26.5 m @ 3.69% Cu, 4.16% Ni, 0.18% Co, 1.10 g/t Pt, 1.27 g/t Pd, and 0.24 g/t Au (Coppard et al., 2013).
In general, disseminated and vein sulphides have low Ni/Cu and high Pt/Pd ratios of <1 and 2, respectively. The massive sulphides have highly variable Ni/Cu ratios, as high as Ni/Cu of >10 in the deeper parts of the intrusion, grading upwards to become Cu-rich in the upper levels with Ni/Cu of <0.1. Platinum dominates over palladium in the disseminated and vein-style mineralisation with Pt/Pd ratios of ~2. In the massive sulphides, Cu-rich zones are Pt-rich, whereas Ni-rich blocks are more Pd-rich (Brownscombe et al., 2015).
Fröhlich et al. (2021) undertook a detailed petrographic study of exploration drill core intersecting vein-style mineralisation with a classic magmatic sulphide assemblage of chalcopyrite ±pyrrhotite, pentlandite and pyrite. The bulk (>1000) platinum-group bearing mineral grains studied belonged almost exclusively to the moncheite (PtTe2) - merenskyite (PdTe2) - melonite (NiTe2) solid solution series, with almost two thirds of the platinum-group element-bearing minerals being melonite. Some of the platinum-group minerals contain inclusions of Ag-rich gold (AgAu2) and muthmannite (AuAgTe2). Most of the platinum-group minerals occur as inclusions in chalcopyrite, although a few grains are located at base-metal sulphide grain boundaries and in fractures in base-metal sulphides.
Mineral Resources
Published Mineral Resources (Anglo American plc Ore Reserves and Mineral Resources Report 2020) were:
Measured + Indicated Resources - 3.5 Mt @ 3.45% Cu, 2.47% Ni, 2.49 g/t 3E PGE (Pt+Pd+Au);
Inferred Resources - 40.9 Mt @ 1.77% Cu; 0.83% Ni, 1.37 g/t 3E PGE (Pt+Pd+Au).
These Mineral Resource estimates are based on a predominantly underground Cut & Fill mining and fall within a volume defined using a USD45/t Net
Smelter Return value. This equated to an ~1% Cu equiv. cut-off. Sakatti co-product estimated average grades include:
Indicated Resources - 0.11% Co; 0.98 g/t Pt; 1.18 g/t Pd and 0.33 g/t Au; which equates to 11.41% Cu equiv. average grade.
Inferred Resources - 0.04% Co; 0.61 g/t Pt; 0.43 g/t Pd and 0.33 g/t Au; which equates to 4.68% Cu equiv. average grade.
The information in this summary is drawn from Törmänen and Karinen (2016) and Fröhlich et al. (2021) as cited below.
The most recent source geological information used to prepare this decription was dated: 2021.
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.
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Selected References:
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Frohlich, F., Siikaluoma, J., Osbahr, I. and Gutzmer, J., 2021 - Genesis of sulfide vein mineralization at the Sakatti Ni-Cu-PGE deposit, Finland,: in The Canadian Mineralogist v.59, pp. 1485-1510.
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Tormanen, T. and Karinen, T., 2016 - The Kevitsa and Sakatti Ni-Cu-PGE deposits: in Boyd, R., Bjerkgard, T., Nordahl, B. and Schiellerup, H., 2016 Mineral Resources in the Arctic Geological Survey of Norway Special publication pp. 351-353.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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