Kuusamo Schist Belt - Juomasuo, Hangaslampi, Kouvervaara, Meurastuksenaho, Pohjasvaara


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The Juomasuo gold-cobalt deposit is located within the Kuusamo Schist Belt, ~700 km NE of Helsinki and ~45 km north of the town of Kuusamoin, in northeastern Finland.

Modern investigations in the Kuusamo Schist Belt began in the late 1950s when the Finnish companies Suomen Malmi and Outokumpu Oy began systematic mineral exploration. Significant mineralisation was discovered in several prospects, encouraging continued Au, Fe and U exploration by several companies, including Kemi Oy and Rautaruukki Oy. In the late 1970s, the Geological Survey of Finland (GTK) undertook an exploration and research project in the area that continued through the 1980s and 1990s. The Juomasuo deposit was discovered in 1985 with the application of airborne magnetic and electromagnetic data. Mining rights over the deposit were first granted to Outokumpu Oy at the beginning of the 1990s, followed by test mining and a prefeasibility study. The deposit has changed ownership several times since then, although to at least 2021, there has been no mining at Juomasuo or any of the deposits within the Kuusamo Belt (Vasipoulos, et al., 2021).

Regional setting

  The deposit is situated within the Palaeoproterozoic Kuusamo Schist Belt and is part of the Kuusamo-Kuolajärvi orogenic Au metallogenic district (Eilu et al., 2012), which contains several epigenetic Au-Co (±Cu) deposits and occurrences, of which Juomasuo is the largest (Pankka 1992; Vanhanen 2001). The Kuusamo Belt is, in turn, part of the larger Karasjok-Central Lapland-Kuusamo-Lake Onega Greenstone Belt that extends from northern Norway to Lake Onega in Russia (Pankka 1992). This greenstone belt is composed of a 2.4 to 1.9 Ga Karelian supracrustal succession (Silvennoinen 1972, 1992; Huhma et al., 2018), whilst the Kuusamo Schist Belt was at least partially formed within an intracratonic rift, the result of the Palaeoproterozoic break-up of the Archaean Karelian Craton (Hanski and Huhma 2005). The stratigraphy of the Kuusamo Schist Belt is made up of several volcanic and sedimentary units, including rocks formed during three stages of mafic volcanism with associated mafic sills and dykes (Pankka 1992). It has been dividing into 11 formations, which belong to the Salla, Kuusamo, Sodankylä and Savukoski groups, the same units that are recognised in the Central Lapland Greenstone Belt, and a further formation belonging to the Kumpu Group as the uppermost unit (Silvennoinen, 1972; 1991; Huhma et al., 2018; Köykkä et al., 2019; Lahtinen and Köykkä 2020). The total thickness of the volcano-sedimentary sequence in the Kuusamo Schist Belt varies from 5 to 7 km (Lahtinen and Köykkä 2020). The sequence comprises, from the base, which unconformably overlies Archaean basement:
Posio Suite - a continental clastic and shallow marine succession of quartzite and paragneiss (Köykkä et al., 2019);
Sirniö Formation of the Salla Group - which contains felsic metavolcanic rocks;
Kuusamo Group which is made up of:
  - an un-named polymictic conglomerate unit that is generally <50 m thick; overlain by the
  - Vehnäsvaara, Kuusijärvi, and Kuntijärvi formations collectively known as Greenstone Formation I, which comprises mafic and intermediate-composition massive lava flows with some tuffaceous intercalations that were deposited in a subaerial environment;
Sodankylä Group which includes:
  - Hukkavaara Formation, formerly known as the Sericite Quartzite, Sericite Schist, and Quartzite Schist formations. These formations comprise basal tuffite schists and overlying sericite quartzite with interlayers of stromatolitic dolostone (Pekkala, 1985) and silicic schist, which is considerd to likely have a felsic volcanic origin (Silvennoinen 1972, 1992);
  - Petäjävaara Formation, formerly known as Greenstone Formation II, comprising a <50 m thick metabasaltic lava unit with pillow structures;
  - Vaimojärvi Formation, formerly the Siltstone Formation, which is composed of arkose, orthoquartzite, phyllite and dolomite units;
  - Ruukinvaara Formation, formerly known as theGreenstone Formation III, made up of subaerial plateau basalt flows;
  - Rukatunturi Formation, the uppermost unit of the Sodankylä Group in the Kuusamo Schist Belt, comprising a 600 to 800 m thick unit of sericitic quartzite with dolostone beds that grade upwards into arkose and finally glassy orthoquartzite;
Savukoski Group, dolostone, black shale and basaltic tuff;
Kumpu Group, sparsely outcropping conglomerate-sandstone associations, forming the uppermost unit in the Kuusamo Schist Belt (Köykkä et al., 2019).

  This succession underwent deformation and regional metamorphism during the 1.93 to 1.76 Ga Svecofennian orogeny (Silvennoinen 1972, 1992; Lahtinen and Köykkä 2020). Four deformation stages are recognised (Lahtinen and Köykkä 2020), commencing with a D1 east-west compressional phase, followed by north-south D2 shortening producing F2 folds. These were succeeded by overprinting stages of folding during D3 and D4. Metamorphic grades vary from greenschist facies in the south and central parts of the Kuusamo Schist Belt to medium-pressure amphibolite facies in the western parts, near the contact with the Central Lapland Granitoid Complex (Hölttä and Heilimo 2017). Higher metamorphic grades are also found to the north and east within the Kuusamo Schist Belt. This metamorphic gradation from greenschist facies in the south to amphibolite facies in the north can be explained by the presence of a long, major, 70 to 100 km wavelength D4 anticlinorium, with metamorphic grades increasing towards the hinge zone (Lahtinen and Köykkä, 2020). Wide-angle reflection and refraction surveys (Tiira et al., 2014) showed an ~8 km upward bulging of lower and middle to upper crust from the southern Kuusamo Schist Belt to the Archaean Suomujärvi complex north of the belt as part of this structure. In the central part of the Kuusamo Schist Belt, the 25 x 5 km, NE-trending F2 Käylä-Konttiaho anticline is the major structure associated with the bulk of the significant Au-Co occurrences (Vanhanen 1990).

  The epigenetic hydrothermal Co-Au deposits of the Kuusamo Schist Belt all share similar alteration assemblages and ore mineralogy, although there is some variation in the metals enriched in each deposit (Vanhanen 2001). The common sequence of alteration in these deposits of the Kuusamo metallogenic district, including Juomasuo, developed through several stages (Pankka 1992; Pankka and Vanhanen 1992; Vanhanen 2001). The earliest involved extensive albitisation, predating both regional deformation and metamorphism. This was succeeded by syn- to late-metamorphic alteration, which began with Mg-Fe metasomatism, followed by K-metasomatism and local sulphidation during ductile deformation. The final alteration stage includes the addition of carbonate and silica in a brittle deformational regime. Pankka (1992) subdivided Au-Co mineralisation in the Kuusamo Belt into a replacement type, best represented by Juomasuo, and a breccia style, of which the Konttiaho deposit is a representative end member. The bulk of the known Au-Co deposits in the Kuusamo Schist Belt occur in units near the upper and lower boundaries of the Petäjävaara Formation (Vanhanen 1990, 2001). Re-Os and U-Pb dating indicates Au-Co mineralisation in this belt, including the Juomasuo deposit, was formed by multiple hydrothermal processes during the late stages of the 1.81 to 1.76 Ga Svecofennian orogeny, postdating regional peak metamorphism (Pohjolainen et al., 2017; Molnár et al., 2020).

  Apart from Juomasuo, the most notable of these and their estimated resources, include Hangaslampi, ~1 km south of Juomasuo, with 0.369 Mt @ 5.1 g/t Au, 0.07 wt.% Co, 0.1 wt.% Cu; Kouvervaara with 1.58 Mt @ 5.1 g/t Au, 0.07 wt.% Co, 0.1 wt.% Cu; Meurastuksenaho with 0.892 Mt @ 2.3 g/t Au, 0.2 wt.% Co, 0.1 wt.% Cu; and Pohjasvaara with 0.133 Mt @ 3.8 g/t Au, 0.09 wt.% Co (Vanhanen 2001).

  The current total Au-Co mineral resource estimate for the Juomasuo deposit is 2.37 Mt @ 4.6 g/t Au, 0.13 wt.% Co, plus an additional Co only resource of 5.04 Mt @ 0.12 w.t% Co accompanied by varying low-grade Au concentrations (Dragon Mining ASX announcement, 2014). Apart from Au and Co, the most commonly associated trace metals in the deposit are Cu, Mo and REE, together with local enrichment of U (Vanhanen 2001).

Geology and Mineralisation

  The Juomasuo deposit is concealed below 5 to 8 m of glacial till and occurs in the northern section of the 25 km long, NE-trending, F2 Käylä-Konttiaho anticline, accompanied by several other Au-Co occurrences, as detailed above. Mineralisation is structurally controlled by a NW-trending shear zone cross-cutting the Käylä-Konttiaho anticline (Vanhanen 2001; Eilu et al., 2012).
  The deposit comprises one main mineralised lode and several smaller sulphidised zones in the same vicinity, localised within an area of approximately 0.5 km2 (Vanhanen 2001). Native gold is chiefly associated with bismuth and tellurium minerals, occurring as inclusions in pyrite, cobaltite and uraninite, between silicates, and in tiny gold-bismuth-tellurium rich veinlets oriented parallel with foliation, and enveloped by silicates. The main lode is composed of foliation controlled sulphide disseminations and relatively narrow (<1 m) massive or semi-massive seams or veins that parallel the dominant foliation. It has an average dip of 50°SW, with known surface dimensions of ~50 x 100 m (Vanhanen 2001), and has been drilled to as deep as 300 m depth below surface where it is still continuous. The associated smaller sulphidised zones appear to pinch out with depth although these have not been drilled to similar depths (Pankka 1992; Pankka and Vanhanen 1992; Vanhanen 2001).

  Two main types of mineralisation have been recognised at Juomasuo, Au-Co and Co-only ore that are pyrrhotite-rich (with minor pyrite and chalcopyrite), and pyrite-dominated, respectively (Witt et al., 2020). Both are foliated and characterised by ductile deformation (Witt et al., 2020).

  Rocks outside of the Juomasuo mineralised zones are mostly albitites, almost regardless of protolith, a product of pre-mineralisation and pre-deformational hydrothermal alteration. In some instances, even rocks within the ore zones can be described as albitites. These intensely albitised rocks have high Na
2O contents, which can be up to 10 wt.%. These albitites are predominantly composed of fine-grained pink albite with varying amounts of fine-grained quartz, and typically lack observable primary features. Protoliths of the albitites may sometimes be differentiated by locally preserved lithogeochemical parameters such as original sedimentary laminations and grading in albitites of metasedimentary origin. Proximal to the mineralised zones, the albite abundance is significantly reduced due to superimposed alteration. In these areas, quartz, chlorite, biotite and sericite predominate, along with variable amounts of preserved albite, amphibole, talc and accessory minerals. Among this assemblage, chlorite, biotite and sericite dominate the ore zones, and are spatially associated with the enrichment of Au and Co. Within the mineralised zones, the host rocks are chlorite ±biotite or sericite schists irrespective of the protolith. Chlorite forms tabular sheets, and in some parts of the deposit is the dominant mineral. Chlorite and biotite are typically closely associated, with the latter being deposited synchronously and/or slightly later. They occur as zones of intercalated biotite and chlorite where the latter is most intense. Petrographic evidence suggests sericitisation post-dates chlorite-biotite alteration.
  However, Pankka (1989), suggests the host rocks of the Juomasuo deposit are principally metasedimentary in origin and these have been the most intensely albitised, while intercalated meta-igneous rocks of ultramafic, mafic, intermediate and felsic composition are less albite altered.
  Ultramafic rocks occur as a fine-grained, dark unit composed of a chlorite-biotite-talc assemblage with large euhedral-subhedral carbonate porphyroclasts up to 6 mm in diameter and some albite pseudomorphs after plagioclase. Biotite seams define the dominant foliation. This rock unit corresponds to the ultramafic sill of Vanhanen (2001). The fact that albitisation is not as intense has been taken to suggest emplacement following the bulk of regional albitisation. The mafic metavolcanic rocks are by far the most abundant meta-igneous rocks at Juomasuo. They are fine-grained and equigranular with chlorite and biotite being dominant minerals. In places, they are also sericitised and can be differentiated on their chemical composition. Some of these rocks have been folded and brecciated; others are cut by abundant quartz veins. Felsic metavolcanic rocks are light grey, fine to medium grained, and contain abundant sericite, albite and quartz, accompanied by subordinate biotite and chlorite. Crosscutting quartz veins are common. The intermediate metavolcanic rocks have similar textural characteristics to the felsic metavolcanic rocks, but can be distinguished on geochemical criteria.

The Co-only ore was deposited during the stage 1 chlorite alteration, with some Au-Co ore possibly also deposited then. The later stage 2 mineralisation is linked to sericitisation and the Au–Co ore with higher Au and lower Co contents, compared to the previous mineralising stage, and possibly overprinted parts of the earlier, mostly Co-only mineralisation.

This summary is largely drawn from Vasilopoulos et al., 2021.

The most recent source geological information used to prepare this summary 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.

  References & Additional Information
   Selected References:
Vasilopoulos, M., Molnar, F., OBrien, H., Lahaye, Y., Lefebvre, M., Richard, A., Andre-Mayer, A.-S., Ranta, J.-P. and Talikka, M.,  2021 - Geochemical signatures of mineralizing events in the Juomasuo AuCo deposit, Kuusamo belt, northeastern Finland: in    Mineralium Deposita   v.56, pp. 1195-1222.

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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