www.gtk.fi Mineralogical characteristics of a metasedimentary hosted Hosko gold deposit from the Archaean Hattu Schist Belt, Eastern Finland Asko Käpyaho 1 , Ferenc Molnár 1 , Grigorios Sakellaris 2 1 Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland 2 Endomines Oy, Pampalo Mine, Pampalontie 11 FI-82967 Hattu, Finland Abstract e late Archaean Hattu schist belt and the associated plutonic rocks in Eastern Finland host several orogenic gold occurrences and deposits (Fig 1). In the Hosko deposit, the ore veins are hosted by a turbiditic metasedimen- tary unit, which is extensively tourmalinised and sericitised along the veins. e subverti- cal quartz-feldspar-tourmaline veins are con- cordant to the layering and schistosity of the rocks. e selvage of the veins is brecciated and contains subangular fragments of the host rock, whereas some parts of the veins show complex ductile deformation features. ese zones also contain boudins of early stage vein- ing. Free gold is present in the quartz-feldspar infillings and as inclusions in arsenopyrite. e gold oſten is associated to Bi-Te minerals. Occasionally, the gold-bearing veins and vein- lets also have minor amount of scheelite. On the basis of the field observations and miner- alogical observations it is concluded that the deposit show similarities with the turbidite- hosted gold deposits described elsewhere. Figure 2 (a) Laminated schist with dark tourmaline band (indicated by an arrow) containing also scheelite and arse- nopyrite with Au and Bi-Te inclusions. (b) Folded and intensively deformed quartz-feldspar vein unit with several generations of veins hosted by a metapelitic schist. (c) e darkest material within the leucocratic vein material mostly consists of fine-grained tourmaline. Figure 3. (a) Quartz-feldspar veins crosscutting extensively tourmalinised host rock. (b) e gold grains (yellow) together with tellurobismuthtite (grey grains) in the cross-cutting late veinlet. Fig 4 (on the right)(a) Quartz dominated vein with a sharp contact to layered and pervasively tourmalinised fine- grained rock unit with gold grains both in the (b) quartz vein and (c) within the tourmalinised rock. Tourmalin- ised gold bearing units reveal internal structures resem- bling sedimentary layering and rhythmic lamination. At microscopic scale, the lamination is caused by pro- portional variation of very fine-grained quartz+feldspar dominating laminas and tourmaline-rich laminas. (d) Quartz-feldspar veins crosscut these laminar features. e veins also contain variable amounts of sulphides; typically anhedral pyrrhotite that is locally associated with arsenopyrite. (e) e pyrrhotite is partially replaced by arsenopyrite and (f) arsenopyrite is zoned. Referenssit: Boyle, R.W. (1986). Geological Association of Canada, Special Paper 32, pages 1-14. Endomines (2013). http://www.endomines.com/hosko.php Eilu et al., (2012). Special Paper 53, 207–342. Huhma, H. et al., (2012). Geological Survey of Finland, Special Paper 54, 74−175. Mustajärvi, J. (1999). Unpublished MSc thesis. Department of Geology, University of Helsinki. 33 p. (in Finnish). Summary ● Hosko Au deposit is hosted by late Archaean schists showing primary sedimentary structures such as layering and lamination. ● Occurrences of gold is related to the quartz-feldspar veins that are hosted by the extensively tourmalinised fine-grained metasedimentary units. Tourmalinisation and sericitisation are the prevailing alteration types. ● e layered and laminar texture of the fine-grained tourmalinised rocks associated with gold-bearing vein units closely resembles those of the unmineralised sedimentary rocks. It considered that such a small scale fea- tures are a result of tourmalinisation of the laminar host rock. is tourmalinisation event is interpreted to be closely related to the hydrothermal event that produced the gold-bearing quartz feldspar veins. ● e relationship between the ductile deformation and generation of the auriferous quartz-feldspar veins is ambiguous: Some of the veins are boudinaged, and therefore predating the intensive ductile deformation. e complex internal structures within the veins sometimes give, however, an impression that ductile deformation may have been taking place simultaneously with the vein generation. ● Based on field observations and microscopic observations the sedimentary host rock underwent a hydrother- mal event that caused tourmaline alteration in the host rock and emplacement of emplacement of banding pa- rallel or crosscutting Au-Bi-Te-As-(W) bearing quartz±feldspar -vein units. ese mineralised rocks were then deformed and metamorphosed. ● Hosko deposit is considered as an Archaean example of “turbidite hosted gold deposits” (cf. Boyle, 1986). AsPy Au(Ag) AsPy Pyrr BiTe Qz+ plg+ musc+ pyrrh+ vein 5 mm Laminated Tourm+ musc+ Qz+ ilmenite rock 70 µm Te-Bi Au Au (a) (c.) Plg Qz+ plg+ musc+ pyrrh+ vein (b) (e) (f) (a) (d) Au(Ag) BiTe Fig 5 (a) Some parts of the veins are dominated by euhedral or subhedral medium- to coarse -grained arsenopyrite (shiny white in the figure). (b) e arsenopyrite contains gold inclusions and tellurobismuthite next to small vugs. Au Bi-Te AsPy (b) (a) (c) (b) (a) Figure 1. Bedrock map aſter Korsmann et al., (1997) showing the known gold showings or deposits classi- fied as orogenig in origin (Eilu et al., 2012) and form- ing Karelian gold line. e northernmost of the cur- rently known Au deposits is the metasedimentary rock hosted Hosko deposit that is currently known to be at least ca. 600 m long, 2-15 m wide and it is at least 100 m deep (Endomines, 2013). For detailed map see Mustajärvi (1999). Host rocks e host rock for the Hosko gold deposit is a fine- grained and equigranular metapelitic schist mainly composed of plagioclase, quartz, muscovite, biotite and ilmenite as main minerals (Fig 2). Primary sedi- mentary features such as layering and lamination are well preserved in places. e schist is hosting auriferous quartz-feldspar veins. Tourmalinisation and sericitisation are the most common alterations types, especially surrounding the complex vein sys- tems (Fig 3). Occurence of gold e gold is typically observed in the veins or in the tourmalised wall rock next to veins (Figs 3 and 4). e veins contain variable amounts of sulphides in- cluding arsenopyrite, pyrrhotite, chalcopyrite and minor pyrite. Gold together with Te-Bi minerals can be seen both as free gold with silicates (commonly with quartz or tourmaline) and as inclusions within arsenopyrite (Fig 5). Scheelite is locally present in the veins or in the tourmalinised parts of the host rock. 50 μm AsPy Qz Qz Acknowledgements: Harri Kutvonen is thanked for designing the layout of the poster and Jari Väätäinen is thanked for the Fig 5a. 1 cm
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
www.gtk.fi
Mineralogical characteristics of a metasedimentary hosted Hosko gold deposit from the Archaean Hattu Schist Belt, Eastern FinlandAsko Käpyaho1, Ferenc Molnár1, Grigorios Sakellaris2
1 Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland2 Endomines Oy, Pampalo Mine, Pampalontie 11 FI-82967 Hattu, Finland
Abstract
The late Archaean Hattu schist belt and the associated plutonic rocks in Eastern Finland host several orogenic gold occurrences and deposits (Fig 1). In the Hosko deposit, the ore veins are hosted by a turbiditic metasedimen-tary unit, which is extensively tourmalinised and sericitised along the veins. The subverti-cal quartz-feldspar-tourmaline veins are con-cordant to the layering and schistosity of the rocks. The selvage of the veins is brecciated and contains subangular fragments of the host rock, whereas some parts of the veins show complex ductile deformation features. These zones also contain boudins of early stage vein-ing. Free gold is present in the quartz-feldspar infillings and as inclusions in arsenopyrite. The gold often is associated to Bi-Te minerals. Occasionally, the gold-bearing veins and vein-lets also have minor amount of scheelite. On the basis of the field observations and miner-alogical observations it is concluded that the deposit show similarities with the turbidite-hosted gold deposits described elsewhere.
Figure 2 (a) Laminated schist with dark tourmaline band (indicated by an arrow) containing also scheelite and arse-nopyrite with Au and Bi-Te inclusions. (b) Folded and intensively deformed quartz-feldspar vein unit with several generations of veins hosted by a metapelitic schist. (c) The darkest material within the leucocratic vein material mostly consists of fine-grained tourmaline.
Figure 3. (a) Quartz-feldspar veins crosscutting extensively tourmalinised host rock. (b) The gold grains (yellow) together with tellurobismuthtite (grey grains) in the cross-cutting late veinlet.
Fig 4 (on the right)(a) Quartz dominated vein with a sharp contact to layered and pervasively tourmalinised fine-grained rock unit with gold grains both in the (b) quartz vein and (c) within the tourmalinised rock. Tourmalin-ised gold bearing units reveal internal structures resem-bling sedimentary layering and rhythmic lamination. At microscopic scale, the lamination is caused by pro-portional variation of very fine-grained quartz+feldspar dominating laminas and tourmaline-rich laminas. (d) Quartz-feldspar veins crosscut these laminar features. The veins also contain variable amounts of sulphides; typically anhedral pyrrhotite that is locally associated with arsenopyrite. (e) The pyrrhotite is partially replaced by arsenopyrite and (f) arsenopyrite is zoned.
Referenssit:Boyle, R.W. (1986). Geological Association of Canada, Special Paper 32, pages 1-14.Endomines (2013). http://www.endomines.com/hosko.phpEilu et al., (2012). Special Paper 53, 207–342.Huhma, H. et al., (2012). Geological Survey of Finland, Special Paper 54, 74−175.Mustajärvi, J. (1999). Unpublished MSc thesis. Department of Geology, University of Helsinki. 33 p. (in Finnish).
Summary
● Hosko Au deposit is hosted by late Archaean schists showing primary sedimentary structures such as layering and lamination.● Occurrences of gold is related to the quartz-feldspar veins that are hosted by the extensively tourmalinised fine-grained metasedimentary units. Tourmalinisation and sericitisation are the prevailing alteration types.● The layered and laminar texture of the fine-grained tourmalinised rocks associated with gold-bearing vein units closely resembles those of the unmineralised sedimentary rocks. It considered that such a small scale fea-tures are a result of tourmalinisation of the laminar host rock. This tourmalinisation event is interpreted to be closely related to the hydrothermal event that produced the gold-bearing quartz feldspar veins. ● The relationship between the ductile deformation and generation of the auriferous quartz-feldspar veins is ambiguous: Some of the veins are boudinaged, and therefore predating the intensive ductile deformation. The complex internal structures within the veins sometimes give, however, an impression that ductile deformation may have been taking place simultaneously with the vein generation.● Based on field observations and microscopic observations the sedimentary host rock underwent a hydrother-mal event that caused tourmaline alteration in the host rock and emplacement of emplacement of banding pa-rallel or crosscutting Au-Bi-Te-As-(W) bearing quartz±feldspar -vein units. These mineralised rocks were then deformed and metamorphosed. ● Hosko deposit is considered as an Archaean example of “turbidite hosted gold deposits” (cf. Boyle, 1986).
AsPy
Au(Ag)
AsPy
Pyrr
BiTeQz+plg+musc+pyrrh+vein
5 mm
LaminatedTourm+musc+Qz+ilmeniterock
70 µm
Te-Bi
Au
Au
(a)
(c.)
Plg
Qz+plg+musc+pyrrh+vein
(b)
(e)
(f)
(a)
(d) Au(Ag)
BiTe
(g)
BiTeAsPy
Pyr
rh
Fig 5 (a) Some parts of the veins are dominated by euhedral or subhedral medium- to coarse -grained arsenopyrite (shiny white in the figure). (b) The arsenopyrite contains gold inclusions and tellurobismuthite next to small vugs.
Au
Bi-Te
AsPy
(b)
(a)
(c)(b)
(a)
Figure 1. Bedrock map after Korsmann et al., (1997) showing the known gold showings or deposits classi-fied as orogenig in origin (Eilu et al., 2012) and form-ing Karelian gold line. The northernmost of the cur-rently known Au deposits is the metasedimentary rock hosted Hosko deposit that is currently known to be at least ca. 600 m long, 2-15 m wide and it is at least 100 m deep (Endomines, 2013). For detailed map see Mustajärvi (1999).
Host rocks
The host rock for the Hosko gold deposit is a fine-grained and equigranular metapelitic schist mainly composed of plagioclase, quartz, muscovite, biotite and ilmenite as main minerals (Fig 2). Primary sedi-mentary features such as layering and lamination are well preserved in places. The schist is hosting auriferous quartz-feldspar veins. Tourmalinisation and sericitisation are the most common alterations types, especially surrounding the complex vein sys-tems (Fig 3).
Occurence of gold
The gold is typically observed in the veins or in the tourmalised wall rock next to veins (Figs 3 and 4). The veins contain variable amounts of sulphides in-cluding arsenopyrite, pyrrhotite, chalcopyrite and minor pyrite. Gold together with Te-Bi minerals can be seen both as free gold with silicates (commonly with quartz or tourmaline) and as inclusions within arsenopyrite (Fig 5). Scheelite is locally present in the veins or in the tourmalinised parts of the host rock.
50 μm
AsPyQz
Qz
Acknowledgements:Harri Kutvonen is thanked for designing the layout of the poster and Jari Väätäinen is thanked for the Fig 5a.
1 cm
Related Documents
