Science blog: Estimating the undiscovered Kuusamo-type cobalt–gold resources in Finland
The Geological Survey of Finland (GTK) has been carrying out assessments of undiscovered mineral resources in the Finnish bedrock since 2008. The purpose of the assessments is to produce unbiased information on undiscovered mineral resources for national and regional planning of land use, natural resources management and environmental actions, and for improving assessment tools and their appropriate application in the conditions of Finnish bedrock. During these years, most of the important metals and mineral deposit types in Finland have been covered, and the assessment results have been published in a series of reports published by GTK. As the most recent part of this process, the cobalt and gold resources hosted by undiscovered Kuusamo-type cobalt–gold deposits were estimated down to the depth of one kilometre.
This blog summarises the results of the Kuusamo cobalt–gold assessment. The assessment team consisted of GTK geologists: Kalevi Rasilainen, Pasi Eilu, Irmeli Huovinen, Jukka Konnunaho, Tero Niiranen, Juhani Ojala and Tuomo Törmänen. Detailed results of the assessment have been published in the GTK Bulletin series (Rasilainen et al. 2020).
Although cobalt occurs as a minor commodity in several different types of mineral deposits in Finland, the majority of the known cobalt endowment in the Finnish bedrock is in synorogenic mafic–ultramafic intrusion-related Ni–Cu deposits, komatiite-related Ni–Cu–PGE deposits, Outokumpu-type Cu–Co–Zn deposits, Talvivaara-type Ni–Zn–Cu–Co deposits and Kuusamo-type Co–Au deposits. Undiscovered cobalt resources in the three first-mentioned deposit types in Finland have previously been estimated (Rasilainen et al. 2012, 2014). The undiscovered endowment of cobalt in Talvivaara-type deposits has not been assessed due to the lack of grade-tonnage information required for the construction of a deposit model (Rasilainen et al. 2010). Large mafic–ultramafic layered intrusions in northern Finland might also contain significant cobalt resources, but the lack of cobalt grade information has prevented the estimation of these resources.
The metal association Au–Co±Cu±U±LREE is characteristic for several mineral deposits within the Kuusamo schist belt in eastern Finland (Fig. 1). The main economic interest in the Kuusamo deposits is in gold and cobalt, whereas copper and the REE have been regarded as potential by-products and uranium as a problematic waste product. None of the Kuusamo schist belt deposits has so far proven economic.
Unless otherwise indicated, the following description of the geology and mineral deposits of the Kuusamo belt is based on the work of Pankka (1992), Pankka and Vanhanen (1992) and Vanhanen (2001). The Kuusamo deposits are hosted by a clastic sedimentary-dominated sequence deposited between 2.35 and 2.21 Ga, which also contains basaltic lavas and indications of evaporates. The sequence was intruded by basaltic dykes and sills prior to regional deformation. All deposits have a distinct structural control, and most of them are located at the intersection of a regional northeast-trending anticline with northwest-trending faults. The largest deposit, Juomasuo, is located in a doubly-plunging part of the northeast-trending anticline. The rocks in the Kuusamo schist belt were affected by three regional and at least one localised alteration event. The regional events were characterised by partial to total albitisation, sericitisation, spilitisation, scapolitisation and carbonatisation. The Au–Co±Cu±U±LREE mineralisation is related to the localised stage 4 alteration, which consists of weak to intense sulphidation with K±Fe, Mg alteration. The main ore minerals in the Kuusamo-type deposits are pyrite, pyrrhotite, cobaltite, cobaltian pentlandite and chalcopyrite. Native gold occurs in a free form within gangue and is also associated with bismuth and tellurium minerals that are present as inclusions and in fractures in pyrite, pyrrhotite, cobaltite and uraninite.
The metallic deposits in the Kuusamo schist belt have historically been classified into various types, including orogenic gold with atypical metal association, iron oxide–copper–gold (IOCG), Blackbird type and syngenetic (e.g., Pankka 1992; Pankka and Vanhanen 1992; Vanhanen 2001; D.I. Groves personal communication 2006; Slack et al. 2010; Slack 2013). No exactly similar deposits have yet been discovered elsewhere in Finland; therefore, they are classified as “Kuusamo type”. However, there are similarities with some of the gold deposits in the Peräpohja belt, 200 km to the west of Kuusamo (Ranta et al. 2018). Most of the available evidence suggests that the Kuusamo schist belt deposits are epigenetic. Early, premetamorphic, connate and possibly evaporite-derived brine circulation within the rifted basin caused alteration and rendered parts of the sequence competent. Such brines may survive into an orogeny and also be able to transport metals during orogenic processes (e.g., Yardley & Graham 2002). Premetamorphic alteration hardened the rocks and rendered them competent, and provided brines to transport the metals. Overall, the deposit characteristics are most consistent with the orogenic gold model with an atypical metal tenor. The main reasons for not including the Kuusamo deposits in the sensu lato orogenic gold class include the multiple stages of alteration, the extensive mobility of most metals in the mineralising system, and the metal zoning within the deposits.
Numerous methods have been developed and applied to the estimation of undiscovered mineral resources during the past decades, but the task still remains challenging and there is no universally accepted, definitive procedure (e.g. Lisitsin et al. 2007 and references therein). Published methods for quantitative estimation of mineral resources include the three-part assessment (Singer & Menzie 2010), the Zipf’s law approach (Rowlands & Sampey 1977, Merriam et al. 2004, Mamuse & Guj 2011), regression-based techniques (Mamuse et al. 2010), one-level assessment (McCammon & Kork 1992; McCammon et al. 1994) and various combinations of these (e.g. Chudasama et al. 2018). The three-part approach is the most widely used method for quantitative assessment of undiscovered mineral resources, and it has also been used in the GTK assessments.
The three-part method consists of the following components: (1) evaluation and selection or construction of descriptive models and grade-tonnage models for the deposit types under consideration; (2) delineation of areas according to the types of deposits permitted by the geology (permissive tracts); and (3) estimation of the number of undiscovered deposits of each deposit type within the permissive tracts. The estimated number of deposits is combined with the grade and tonnage distributions from the deposit models to assess the total undiscovered metal endowment. In addition to the three components, the process flow of a typical assessment project applying the three-part method contains one or more data gathering phases (Fig. 2).
The Kuusamo cobalt–gold assessment process began with the selection of the team of experts. The roles and responsibilities of the experts were agreed in the start-up workshop. Preliminary permissive tracts were defined by the assessment team members who had personal working experience in the areas under study. The tracts were finalised after a review by other team members. The main criteria for delineating the Kuusamo-type tracts included: 1) a tectonic setting of an intracratonic rifted basin, 2) local indications of pre-existing evaporites, 3) extensive and intense sodic ± CO2 (albite ± carbonate) and Na-Cl (scapolite) alteration predating the orogeny and mineralisation, and 4) localised multi-stage, syn to late orogenic, structurally-controlled alteration proximal to Au–Co±Cu mineralisation. A grade-tonnage model was constructed using data from the well-known Kuusamo-type Co–Au deposits within the area. The assessment team members estimated the number of undiscovered Kuusamo-type deposits within each permissive tract in two assessment workshops. After the workshops, Monte Carlo simulations were run to estimate the probability distributions of the undiscovered cobalt and gold endowments in the permissive tracts.
Kuusamo-type deposits are not known with certainty outside of Finland, and the number of deposits with a published resource estimate is small. Because of this, the grade-tonnage model is only based on 10 deposits, all within the Kuusamo area. Statistical tests indicate that the distributions of ore tonnage, gold grade and cobalt grade for the 10 deposits do not significantly differ from log normality (Table 1). There are weak correlations between logarithmic ore tonnage and metal grade values, but Bonferroni-adjusted probabilities for the correlation coefficients indicate that the correlations are not significant. Because of the small number of deposits used to construct the grade-tonnage model, and the uncertainties concerning the completeness of the reported resources, it is probable that the model gives a downward-biased representation of the true resources of the Kuusamo-type cobalt–gold deposits.
|Table 1. Summary statistics for the Kuusamo Co–Au grade-tonnage model.|
|Tonnage (Mt)||Co (%)||Au (g/t)|
|Number of deposits||10||10||10|
|Mt: Million metric tons. Tonnages are rounded to full thousands and grades to two significant digits.
* The Shapiro-Wilk normality test p-value was calculated for logarithmic tonnage and grade values.
In total, eight permissive tracts were delineated for Kuusamo-type Co–Au deposits (Fig. 3). These tracts contain all the known Kuusamo-type deposits and occurrences in Finland. In total, the tracts cover an area of 21,082 km2, which is approximately six per cent of the land area of Finland. The size of the permissive tracts varies from 780 km2 to 5611 km2 and the median area is 2254 km2. Most of the tracts are located in northern Finland.
The expected (mean) number of undiscovered deposits for a permissive tract, rounded to whole numbers, varies between two and 14, and the sum of the mean estimates across all tracts is 58 deposits. Over 60% of the undiscovered deposits are estimated to be located within three permissive tracts: Kuusamo (24%), Peräpohja (21%) and Pelkosenniemi (16%) (Fig. 2).
The median estimate of the total in situ cobalt and gold content in undiscovered Kuusamo-type Co–Au deposits in Finland is at least 100,000 t of cobalt and 85 t of gold (Table 2). Approximately 50% of this endowment is estimated to be located in undiscovered deposits in the Kuusamo and Peräpohja permissive tracts (Fig. 2). The assessment results indicate that at least 80% of the remaining Kuusamo-type cobalt and gold endowment within the uppermost one kilometre of the Finnish bedrock is in poorly explored or entirely unknown deposits.
|Table 2. Summary of the identified and estimated undiscovered resources in Kuusamo-type Co–Au deposits in Finland.|
|Identified||At least the indicated amount at the probability of||Mean||Probability of|
|0.95||0.90||0.50||0.10||0.05||Mean or greater||None|
|Ore: Mineralised rock containing the metals. Well-known resources as of 29th May 2017. Data sources are listed in Appendix 2. All resources are rounded to two significant digits.|
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Text: Kalevi Rasilainen
Kalevi Rasilainen completed his PhD thesis on orogenic gold mineralisation in the Ilomantsi region in 1996. He has worked at GTK since 1985, mostly carrying out research on mineral resources-related subjects. For the last 13 years, he has coordinated a GTK project assessing the undiscovered mineral resources in Finland. He is the scientific coordinator of the ongoing EIT RawMaterials co-funded project MAP, which is concentrated on developing the methodology and software for the assessment of undiscovered mineral resources.