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Modified: 14.08.2009

Valkeasuo - Gold Database

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Name Valkeasuo DATA UPDATED 12.9.2008
Alternative names Hosko
Deposit summary VALKEASUO (HOSKO) has an in situ resource estimate of 2750 kg gold (a JORC-compliant resource). It is an Archaean orogenic gold deposit comprising a set of pipe-like subvertical lenses in a mineralised domain at least 1.5 km long. The deposit is hosted by intermediate volcanogenic metasedimentary rock and located close to the N-S trending Rosvohotu shear zone. Native free gold associated with quartz, tourmaline and K feldspar.
LOCATION
Geological domain Archaean Belt Ilomantsi
Site photo   Regional map kareliamap1_th
Map sheet 433305
Northing (kkj) 6998000 Easting (kkj) 4559200
Latitude 63.08034N Longitude 31.16835E
Municipality Ilomantsi
Nearest town, roads 40 km NE from Ilomantsi, 100 km NE from Joensuu. A sealed road 300 m from the area.
MINING
Exploration licence no 4853/1, 5511/1, 5840/4, 6926/1 Mining concession no  
Present holder Endomines Oy (1991–)
Previous holders Geological Survey of Finland (GTK) (–1991)
Mine photo 1 hosko_03092001_testpit_th Mine photo 2  
Mine photo 3   Mine photo 4  
Status of development Prospect
When mined  
Resources Measured 0.014 Mt @ 4.8 ppm, indicated 0.44 Mt @ 3.3 ppm, and 0.721 Mt @ 1.7 ppm Au, cut off grade at 0.5 ppm Au [18,21]. Indicated 0.1 Mt @ 20 ppm Au, inferred 0.1 Mt @ 10 ppm Au (2 ppm cut off) [11]. 1 Mt @ 4 ppm Au [17].
Deposit size (Mt) 1,175 Reference (size) [18,21]
Total in-situ gold (kg) 2745 Reference (in-situ Au) [18,21]
Total gold production (kg)   Reference (gold prod)  
Production of other metals  
Extent of mineralisation More than >1.5 km, possibly even 5–7 km long, a few tens of metres wide, subvertical mineralised zone which is open at the depth of 50–100 m [6,8,10,11].
Lodes A set of pipe-like, subvertical lenses, steeply plunging to S or SE, at about 85°, each 5–25 m wide [10,13].
Best sections 1.95 m @ 42.2 ppm, 1.8 m @19.7 ppm, 1.3 m @ 4.7 ppm, 0.9 m @ 6.1 ppm Au [23]. 24.3 m @ 26.6 ppm, 2.7 m @ 11.0 ppm Au [22]. 14.2 m @ 7.5 ppm, 17 m @ 2.3 ppm Au [17]. 12 m @ 11 ppm Au, 1 m @ 1777 ppm Au [6]. 9 m @ 2.2 ppm, several 1–3 m zones @ 3–6 ppm Au [1,15].
EXPLORATION
Discovery year 1992
Discovery By GTK, guided by regional and localised till-geochemical survey [12].
Exploration history GTK (1987–1996) [1,2,4,12,14,15]: No natural outcrops at all. Detailed geochemical till sampling: sampling grid 250x250 m over the greenstone belt covering 400 km2. Follow-up as till-bedrock interface geochemistry, samples collected across the Au anomaly along traverses 100 m apart with sampling distance 10-30 m. Low-altitude air- and ground-magnetic, slingram and IP survey. Bedrock mapping based on outcrops, geophysics, trenching and diamond drilling. Special studies on Quaternary geology, ore mineralogy and geochemistry, and petrogenesis. SIP investigations.
Endomines Oy (1996–) [3,6,8,11,13,20,21,22]: Re-logging of older core, RC and diamond drilling, ground-geophysical surveys, extensive trenching, preliminary benefication tests, detailed mineralogical and structural studies.
Section figure 1 hosko_section_th Plan figure 1  
Section figure 2 hosko_3d_drilling_th Plan figure 2  
Section figure 3   Plan figure 3  
Trench fig 1   Trench fig 4  
Trench fig 2   Trench fig 5  
Trench fig 3   Trench fig 6  
Explor site photo 1   Explor site photo 2  
Geophysical response [6]: An IP anomaly indicates the presence of the Au-mineralised zone for, at least 1.5 km. However, the IP anomaly continues for 6 km, most of which is, still, unchecked for mineralisation [10].
[2]: No response on magnetic or slingram methods. Magnetic and electric methods do show the structural features of the area, including those which control gold mineralisation.
Drilling GTK: (1992–1996) [1,6,14,15]: 21 diamond-drill holes, total 2518 m.
Endomines Oy (1996–) [6]: 5 diamond-drill holes, total 306 m, 24 RC holes, total 1912 m, and 53 percussion holes, total 756 m. The area for the resource estimate is drilled in 50x50 m grid. RC drilling in profiles 10 apart [13]; 2004: Nine diamond-drill holes [17]; 2007: 30 diamond-drill holes, total 3295 m [22].
Elements analysed [5]: Main components, Cl, Sn and Zr by XRF; Ag, As, Au, Bi, Pd, Sb, Se and Te by GFAAS; Hg by wet-chemical method; B by DCP; Ba, Cd, Co, Cr, Ga, La, Li, Mo, Nb, Ni, Pb, Rb, Sb, Sc, Sr, Th, Tl, U, V, W, Y and Zn by ICP; C and S by Leco.
Primary dispersion [2]: Au and Te show good correlation; Ag and Bi show moderate correlation with Au. No consistent chemical zoning found yet. The local background threshold for Au anomaly is 10 ppb [10].
Secondary dispersion [2]: Regional Au, As and B till anomaly, local Au, Te and Bi anomaly. Au content within the till anomaly is from tens of ppb to >1 ppm. Best combination for defining exploration targets: Au + Te + Bi- better than Au alone.
Primary anomaly fig 1 ilomantsi_magnmap_th Secondary anomaly fig 1 hattubelt_regional_au_till_th
Primary anomaly fig 2   Secondary anomaly fig 2  
Primary anomaly fig 3   Secondary anomaly fig 3  
Primary anomaly fig 4   Secondary anomaly fig 4  
Primary anomaly fig 5   Secondary anomaly fig 5  
Economic evaluations Resource estimates for Endomines by GTK in 1999 and 2001 [10,11].
Exploration geologist in charge Endomines Oy: Jaakko Liikanen; GTK: Martti Damsten, Timo Heino
ORE
Siting of gold 97.4% free milling gold [6]. Gold is associated with quartz, tourmaline and K feldspar [11].
Fineness  
Major opaques Pyrrhotite, arsenopyrite [5,6,13].
Minor opaques Pyrite, marcasite, sphalerite, chalcopyrite, magnetite, native gold [5,6,13].
Gangue Quartz, feldspar, scheelite, tourmaline, muscovite, apatite [5,6,13].
Ore miner. photo 1   Ore miner. photo 5  
Ore miner. photo 2   Ore miner. photo 6  
Ore miner. photo 3   Ore outcrop photo 1 hosko_03092001_outcrop1_th
Ore miner. photo 4   Ore outcrop photo 2 hosko_03092001_outcrop2_th
Ore composition Diamond-drill core [5]: 10.20 ppm Au, 1.60 ppm Ag, 1740 ppm As, 1400 ppm B, 514 ppm Ba, 1.13 ppm Bi, 13.9 ppm Co, 53.6 ppm Cu, 16 ppb Hg, 14.1 ppm Li, 1.5 ppm Mo, 28.9 ppm Ni, 3.2 ppm Pb, 80 ppm Rb, 8400 ppm S, 3.73 ppm Sb, 0.41 ppm Se, 491 ppm Sr, 3.00 ppm Te, 5.4 ppm Th, 1.00 ppm Tl, 1.9 ppm U, 96 ppm V, 230 ppm W, 16 ppm Y, 167 ppm Zn, 161 ppm Zr; 67.9% SiO2, 0.47% TiO2, 16.29% Al2O3, 4.63% Fe2O3, 0.33% MgO, 2.40% CaO, 3.78% Na2O, 1.80% K2O, 0.15% P2O5.
Enriched elements Au, Ag, As, B, Ba, Bi, S, Sb, Te, W [5].
Ore fluid H2O-CO2-CH4 fluid, 10-25 vol-% CO2, Th = 205–396°C [16].
Stable isotopes  
Pb isotope data  
GEOLOGY
Geological setting The mineralisation is in the northern part of the 2754–2 726 Ma Hattu Schist Belt [2,19], in a volcanogenic(?) metasedimentary sequence [6], in a limb of a regional synform [13].
Major host rocks Mica schist: intermediate metatuffite [1] or metagreywacke [6,10].
Minor host rocks Metagreywacke [6], meta-arkose [13].
Intrusives The nearest plutons clearly predate mineralisation. The age of the tonalitic plutons bounding the schist belt is ca. 2750 Ma [2,19]. Granitic pegmatites intrude the hosting sequence and postdate the gold mineralisation [13].
Regional geol map 1 hattu_belt_map1_thumb Outcrop photo 1  
Regional geol map 2   Outcrop photo 2  
Local geology map 1   Outcrop photo 3  
Local geology map 2  
METAMORPHISM  
Metamorphic history [2]: Progressive regional metamorphism on ca. 2750–2700 Ma, apparently peaked soon after gold mineralisation, at a temperature of about 550±50°C. Thermal peak was synchronous or outlasted deformation.
A relatively strong, but unevenly distributed Palaeoproterozoic overprint [9].
Metamorphic grade Greenschist-amphibolite facies transition [2]. 500–600°C, 4–5 kbar [13].
Metamorphic mineral assemblage Mica schists: quartz-muscovite-K felsdspar-plagioclase ± tourmaline, sericite, sillimanite, chlorite, garnet, pyrrhotite [13].
Meta-arkose: quartz-plagioclase ± K feldspar, muscovite, tourmaline, biotite, garnet [13].
Metamorph photo 1   Metamorph photo 2  
STRUCTURE  
Structural style Brittle-ductile [6].
Closest major shear Rosvohotu Shear Zone system 200 m west from the deposit [2].
Controlling structure N-S trending Rosvohotu shear zone [2]. The host rocks follow a sinistral fold pattern and the largest pipe (lode) is exactly around a fold crest [11].
Deformation history Syn-mineralisation brittle-ductile deformation, followed by another stage of deformation characterised by sinistral folding [6], in a transpressional regime [19].
Ore fabric Massive? [6].
Veins Auriferous, 1 cm to 1 m wide, syn-peak deformation quartz-feldspar-tourmaline veins [13].
Structure photo 1 hosko_au_tourm_qzveins_sericitisation1_260905txt_th Vein photo 1  
Structure photo 2   Vein photo 2 hosko_au_tourm_qzveins_sericitisation2_260905txt_th
Structure photo 3   Vein photo 3 hosko_au_tourm_qzveins_sericitisation_260905txt_th
ALTERATION  
General alteration Sericitisation is most prominent; alteration predates the final stages of deformation within the shear zone [2,3,14].
Tourmaline selvages at the auriferous veins [13,14].
Proximal alteration  
Intermediate alteration  
Distal alteration  
Zonation figure   Prox alteration photo 1  
Alteration photo 1   Prox alteration photo 2  
Alteration photo 2   Intermed alteration photo  
Alteration photo 3   Distal alteration photo 1  
Post-mineralisation modifications An Archaean post-mineralisation metamorphic overprint at about 500±50°C with deformation, sinistral folding, and local remobilsation of gold [2,6]. During this stage, gold concentrated into axial planes of folds dipping 80° to the south [6]. On ca. 1800 Ma, a Proterozoic regional metamorphic overprint which is shown by K-Ar and Rb-Sr ages of micas [2].
TIMING [2]: Either pre-peak metamorphic and formed under greenschist-facies conditions, or syn-peak metamorphic. Minimum age 2708–2693 Ma (U-Pb of titanite and monazite indicating peakmetamorphism).
GENETIC MODEL [2]: Formed in a structurally favourable, the most competent lithological units in the area. Precipitation of gold by desulphidation of fluid and, possibly, by decomposition of Au-bisulphide, -thiosulphide and -telluride complexes of fluid due to cooling and/or changes in pH and fO2. Probably, gold precipitated just below 500°C with sulphides due to reaction between the mineralising fluid and wall-rock (chiefly by sulphidation). The formation of the present low-temperature Te and Bi minerals probably took place as subsolidus reactions with cooling temperature.
Mineralisation in the locally most competent, argillic, units of the sequence during deformation [6]. Fluid inclusions suggest a single-event mineralisation at about 2–3 kbar, 300–3 50°C [16].
Genetic type Orogenic References [2,6]
Alternative genetic type 1   References  
Alternative genetic type 2   References  

References

1. Nurmi, P. A. 1993. Archaean Au in Finland. Engineering and Mining Journal, Nov., 32–34.
2. Nurmi, P. A. & Sorjonen-Ward, P. (eds) 1993. Geological Development, Gold Mineralization and Exploration Methods in the Late Archaean Hattu Schist Belt, Ilomantsi, Eastern Finland. Geol. Surv. Finland, Special Paper 17. 386 p.
3. Lindborg, T. 1997. Personal communication 2/10/1997.
4. Vanhala, H. 1997. Laboratory and field studies of environmental and exploration applications of the spectral induced-polarization (SIP) method. Geol. Surv. Finland. 104 p.
5. Bornhorst, T. & Nurmi, P. 1999. Personal communication 20/1/1999.
6. Liikanen, J. 1999. Ilomantsin Hattuvaaran liuskejakso: Hoskon kultamalmi kooste. Written communication. 3 p.
7. Klemetti, M. & Laukkanen, J. 1998. Hoskon esiintymän rikastettavuuselvitys. VTT, Report KET4030/98.
8. Endomines Oy. 1999. www.endomines.fi on 09/12/99.
9. Korsman, K. (ed.) & Glebovitsky, V. (ed.) 1999. Raahe-Ladoga Zone structure-lithology, metamorphism and metallogeny: a Finnish-Russian cooperation project 1996–1999. Map 2: Metamorphism of the Raahe-Ladoga Zone 1:1000000. Geol. Surv. Finland.
10. Parkkinen, J. 1999. Ilomantsin Hoskon ja Raahen Laivakangas: mineraalivarantoarvioiden menetelmätarkistus Endomines Oy:lle. English summary: The Hosko gold prospect at Ilomantsi and the Laivakangas gold occurrence at Raahe: technical revision of evaluations for Endomines Oy. Geol. Surv. Finland, Report M10/1999/4. 19 p. (38.4 MB)
11. Parkkinen, J. 2001. Quality control of mineral resource estimates: Laivakangas, Hosko, Kuivisto E, Other, Osikonmäki, Pirilä, Housuvaara, Pahkalampi. Geol. Surv. Finland, Report. 27 p.
12. Hartikainen, A. & Niskanen, M. 2001. Maaperägeokemialliset kultatutkimukset Hatun liuskejaksolla Ilomantsissa vv. 1983–1995. Geol. Surv. Finland, Report S/41/4244/1/2001. 22 p.
13. Mustajärvi, J. 2001. Ilomantsin Hoskon kultamineralisaation ja sen rakennegeologinen kontrolli. Unpublished MSc thesis. Department of Geology, University of Helsinki. 33 p. (in Finnish)
14. Heino, T., Hartikainen, A., Koistinen, E. & Niskanen, M. 1995. Tutkimustyöselostus Ilomantsin kunnassa valtausalueilla Sivakko 1 (5188/1), Korpilampi 1–2 (5402/1–2), Korpi 1–4 (5356/1–4), Kuivisto 1 (5210/1), Kuivisto 2–3 (5356/5–6), Pihlajavaara 1–2 (5511/2–3) ja Valkeasuo 1–2 (4853/1, 5511/1) suoritetuista kultamalmitutkimuksista vuosina 1992–1995 sekä neljältä vireillä olevalta valtaukselta (Sivakko 2–3 ja Valkeasuo 3–4). English summary: Report on exploration in Ilomantsi in 1992–1995, within claims Sivakko 1 (Mine Register no. 5188/1), Korpilampi 1–2 (5402/1–2 ), Korpi 1–4 (5356/1–4), Kuivisto 1 (5210/1), Kuivisto 2–3 (5356/5–6), Pihlajavaara 1–2 (5511/2–3) and Valkeasuo 1–2 (4853/1, 5511/1). Geol. Surv. Finland, Report M06/4333/-95/1/10. 17 p.
15. Luukkonen, E., Halkoaho, T., Hartikainen, A., Heino, T., Niskanen, M., Pietikäinen, K. & Tenhola, M. 2002. Itä-Suomen arkeeiset alueet –hankkeen (12201 ja 210 5000) toiminta vuosina 1992–2001 Suomussalmen, Hyrynsalmen, Kuhmon, Nurmeksen, Rautavaaran, Valtimon, Lieksan, Ilomantsin, Kiihtelysvaaran, Enon, Kontiolahden, Tohmajärven ja Tuupovaaran alueella. Geol. Surv. Finland, Report M19/4513/2002/1. 265 p. (in Finnish, 130 MB)
16. Poutiainen, M. and Partamies, S. 2003. Fluid inclusion characteristics of auriferous quartz veins in Archean and Paleoproterozoic greenstone belts of eastern and southern Finland. Econ. Geol. 98, 1355–1369.
17. Endomines Oy 2005. www.endomines.fi in 12/12/2005.
18. Micon International ltd 2007. Hosko mineral resources at 4th April 2007 classified following the guidelines of the JORC code (2004). 1 p.
19. Sorjonen-Ward, P. & Luukkonen, E.J. 2005. Archean rocks. In: Precambrian Geology of Finland – Key to the Evolution of The Fennoscandian Shield. Elsevier Science B.V., Amsterdam, 19-99.
20. Endomines 2007. Pressmeddelande den 20 juni 2007. (in Swedish)
21. Endomines 2007. Pressmeddelande den 24 oktober 2007. (in Swedish)
22. Endomines 2007. Pressmeddelande den 20 november 2007. (in Swedish)
23. Endomines 2008. Pressmeddelande den 3 mars 2008. (in Swedish)
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