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8–1: Fennoscandian Shield – Proterozoic VMS deposits Pär Weiheda*, Pasi Eilub a
Division of Ore Geology and Applied Geophysics, Luleå University of Technology, SE-971 87 Luleå, Sweden b Geological Survey of Finland, PO Box 96, FIN-02151 Espoo, Finland Received 2 July 2004, accepted 3 April 2005
Fig. 1. Simplified geological map of the Fennoscandiian Shield with major VMS deposits (yellow) and districts (red) indicated. Darker yellow is Archaean, pale yellow Palaeoproterozoic, grey Meso to Neoproterozoic, green Phanerozoic units, and pink Rapakivi intrusions (Mesoproterozoic). Stipple, units under younger cover.
Mining and exploration history: Proterozoic VMS deposits, together with iron ores, have historically been the main ore deposit type mined in the Fennoscandian Shield (Fig. 1). Evidence of continuous mining of the Falun deposit in the Bergslagen area from around 1000 A.D. until the mid 1990s makes this one of the longest operated mines in Europe. Mining of VMS ores is concentrated in a few areas: Bergslagen (and the Uusimaa belt in Finland) and Skellefte district in Sweden, Vihanti–Pyhäsalmi and Outokumpu areas in Finland. Today five mines in the Skellefte district are in operation (Fig. 1), one mine in the Vihanti–Pyhäsalmi area (Pyhäsalmi), and two mines in the Bergslagen area (Zinkgruvan and Garpenberg). Exploration is still strong with the main targets being the Skellefte, Bergslagen and Pyhäsalmi districts. New discoveries have been made in recent years, extending the
*corresponding author:
[email protected] 0169-1368/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.oregeorev.2005.07.007
mines at Garpenberg, Renström, Kristineberg and Pyhäsalmi. In the last few years, new mines have been opened at Maurliden and Storliden. Geological setting: All major Palaeoproterozoic VMS deposits are associated with volcanic arc sequences that accreted to the Karelian (Archaean) craton between 1.93 and 1.83 Ga. The host rocks to the Outokumpu ores have been dated at ca. 1.96 Ga and constitute the first real evidence of accretionary tectonic processes during the early stages of the Svecokarelian Orogeny. The host sequences are interpreted as remnants of ophiolites that were thrust on top of the Archaean basement during the onset of the Svecokarelian Orogeny. The Vihanti–Pyhäsalmi arc is the oldest arc in the region (host rocks ca. 1.93 Ga) and is juxtaposed with exposed Archaean rocks. The host volcanic sequence is composed of a bimodal complex of lowto medium-K rhyolites, transitional between calcalkaline and tholeiitic, and sub-alkaline low- to medium-K tholeiitic basalts and basaltic andesites. Trace element chemistry of the volcanic rocks indicates a mature island arc setting. The Skellefte volcanic arc accreted slightly after the Vihanti–Pyhäsalmi arc, at ca. 1.87 to 1.83 Ga (host sequence is 1.90 to 1.89 Ga) and the VMS ores were formed during extensional volcanism in a juvenile Palaeoproterozoic continental marginal arc setting. The Bergslagen area is more complex. The age of the host rocks is within error the same as for the Skellefte district, but a more continental crustal influence is evident and the VMS ores are related to extensional tectonics within a continental margin. Mineralization features: Due to strong deformation and fluid flow during metamorphism, parts of the deposits have been tectonically mobilized into apparently crosscutting structures. Most ores are steeply dipping elliptical bodies with long axes parallel to the tectonic stretching lineation (Fig. 2). The ores show a variable degree of metal zoning with copper-rich pyritic stringer vein networks beneath massive to semi-massive Zn–Cu–Pb-rich
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parts. Some deposits are enriched in As, Sb, Hg, U, Bi and Te. Gangue minerals are quartz, barite, sericite, biotite, carbonate, and other phases such as talc, tourmaline, gahnite, feldspars (plagioclase and Ba-feldspars), epidote and Fe–Mg silicates. The Skellefte ores are generally Zn-dominated and low in Pb. The most recent discovery, Storliden, contains 4%Cu and 10%Zn and small amounts of Au. The Boliden deposit was high in Au (average 15 g/t) and contained large amounts of arsenic. The Pyhäsalmi deposit contains relatively low grades of base metals and is dominated by Fe-sulphides, but due to the large tonnage (>70 Mt) it has remained economic. The Outokumpu deposits contain Cu, Co, and Au and are associated with mafic rocks, black shales and chemical sediments. The Bergslagen ores are of two main types exemplified by the Garpenberg and Zinkgruvan mines. The former is stratabound and at least partly replacive, associated with calc-silicate rocks and dominated by irregular lenses and pods with massive and disseminated Zn, Pb, Ag, and Cu sulphides, whereas the latter is a stratiform and massive Zn–Pb ore hosted by felsic volcaniclastics, marble, skarn, and siliceous exhalites. Alteration, fluid chemistry and source: All deposits are associated with strong hydrothermal alteration, which can be conformable, semiconformable or discordant. In a few cases, strong hydrothermal alteration also occurs in the hanging wall. Alteration is commonly characterized by chlorite, sericitic, quartz, ± carbonate and andalusite in greenschist facies and by biotite, anthophyllite, cordierite ± quartz, sillimanite, plagioclase, garnet, and orthoamphibole in amphibolite facies rocks.
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In the felsic volcanic-dominated Skellefte district, the ores tend to be associated with a proximal sericitic alteration envelope and a distal chlorite zone grading outwards to propylitic alteration. In a few cases, a Si–Al-rich proximal alteration zone indicates strong cation leaching. In the Pyhäsalmi– Vihanti area, an increase towards ore in the K/Na ratio in felsic rocks and the Mg/Ca ratio in mafic rocks has been detected. A few stable isotope and fluid inclusion studies give inconclusive evidence of source. Oxygen isotope mapping has delineated what has been interpreted as high-temperature (δ18O
