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Kongsberg - Kongens Gruve |
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Norway |
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Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Kongsberg or Kongens Gruve silver deposits are located ~70 km WSW of Oslo in southern Norway (#Location: 59° 39' 1"N, 9° 35' 34"E).
The silver mines of the Kongsberg district constitute the largest mining field in Norway, with over 130 mines which are recorded to have produced ~1300 t of Ag between 1623 and 1957, although an additional 700 t is estimated to have been lost in beneficiation and by theft. The deepest of these mines was Kongens Gruve which was worked to 1076 m below surface. Kongsberg was the largest pre-industrial working place in Norway, with over 4000 workers at its peak in the 1770s, supplying over 10% of the gross national product of the Danish-Norwegian union.
Kongsberg is one of a number of deposits associated with Permian magmatic activity within the NNE-SSW trending, 40 to 70 km wide and 250 km long Oslo Graben, which was filled by Cambro-Silurian sedimentary rocks and volcanic and intrusive rocks of Permian age. The lower sections of the Cambro-Silurian succession are mostly composed of shales, whereas limestones and sandstones predominate higher up. The Permian volcanic rocks are predominantly basalts and porphyries with some felsic pyroclastic rocks. Plutonic rocks intruding these sequences include some gabbros, but are mainly syenodiorites, syenites, alkali granites and nepheline syenites. These rock types are predominantly confined to the graben itself, although a number of minor intrusions of similar age are found in the bordering basement Precambrian rocks to the east and west. Mineralisation associated with this complex include (Bugge, 1978):
• Deposits within the igneous rocks (Fe-Ti in ultramafic jacupirangite dykes; Mo in granitic pegmatites),
• Metasomatic and fissure vein deposits at the igneous contacts (Fe and Pb-Zn skarns in mainly Palaeozoic carbonate wall rocks) and
• Vein deposits in Precambrian basement rocks (native silver, e.g. Kongsberg silver mines; fluorite vein deposits, e.g. Lassedalen and Gjerpen; silver-bearing lead-zinc-copper deposits, e.g., Tråk).
Kongsberg is hosted by basement Palaeoproterozoic Svecokarelian granitoid and ortho- to paragneisses of the Kongsberg-Bamble Formation several kilometres west of the western margin of the Palaeozoic Oslo Graben. This formation comprises, from oldest to youngest, banded gneisses and dioritic gneisses of the Grey Kongsberg Gneiss, dykes of the Vinor Amphibolites (after a noritic protolith), and the Kongsberg Granite. This suite of gneisses is separated from the mafic meta-lavas and interbanded sedimentary rocks of the Mesoproterozoic Telemark Formation to the NW by a major mylonite zone. In the immediate Kongsberg district, the Kongsberg-Bamble Formation is composed of narrow alternating light and dark bands of metamorphic origin (Meurant, 1984).
The Kongsberg deposit is characterised by native silver-calcite-nickel-cobalt-arsenide veins. The grades of these veins generally ranged from 100 to 350 g/t Ag, with some oreshoots contained twice this amount, whilst lumps of native silver weighing up to 500 kg were sometimes found. Most of the silver deposits occur as veins cutting north-south striking 'fahlbands' within the host gneisses. Fahlband is a term applied to elongated schist zones impregnated with sulphides that often extend for hundreds to thousands of metres, parallel to sub-parallel to the strike of the enclosing gneisses and migmatites, i.e., a mineralised retrograde shear zone. In the Kongsberg district, fahlbands were impregnated with sulphides following solidification of the host protoliths. They are not restricted to any particular lithology within the district, occurring in biotite schist, chlorite schist, biotite-garnet schist, chlorite-garnet schist, quartz-feldspar-biotite gneiss and quartzite. Individual fahlbands are made up of hundreds of small sulphide-rich lenses with long dimensions parallel to the dominant strike of the host metamorphic rock, seldom with any dimension of >2 m. Sulphides generally comprise ~8% of the fahlband in the core of any lens, dropping to near zero at its the margin. Single fahlbands may be as much as 15 km long and rarely up to 300 m thick. Internally sulphides are disseminated as an integral part of the host rock, with mica trains flowing around sulphide aggregates in the same way as around garnet porphyrobblasts. In more siliceous hosts, aggregates of quartz grains, with coarser than usual sulphides parallel the host foliation and fine quartz-sulphide veins may locally cut the host. Otherwise, sulphides do not cut across the metamorphic foliation (Meurant, 1984).
Silver was not a primary constituent of the fahlbands, but it is thought that the sulphide-rich fahlbands extracted and precipitated native silver from circulating silver-rich solutions (Bugge, 1978).
The following styles of silver-bearing veins have been differentiated on the basis of their physical occurrence (Bugge, 1978):
• Hovedganger or main veins - first-generation veins that have been permeated and upgraded by solutions of a second generation.
• Justits type veins, which occur as systems of short parallel calcite veins within the fahlbands, and
• Gottes Hülfe type veins, which are calcite veins branching from the main veins.
The principal mineral associations within these veins are (Bugge, 1978):
• quartz, carbonaceous material (bitumen containing >90% C with minor O2, H2, N and S), fluorite and pyrite;
• calcite, barite, fluorite, argentite and native silver with cobalt and nickel arsenides; and
• calcite and zeolites.
More than 100 minerals have been reported from the deposit, including native elements (Ag, Au, As, Cu, S, C), sulphides, sulphosalts, selenides, arsenides, carbonates, sulphates, arsenates and halides, in addition to a range of silicate minerals (Neumann, 1944; Johnsen, 1986, 1987; Bancroft et al., 2001; Kullerud et al., 2015).
Mineral textures indicate the native silver formed during at least two separate stages and an intermediate phase, as reflected by the composite vein types and mineral association observations above and more detailed mineralogic studies (e.g., Kullerud et al., 2015; Kotkova et al., 2018).
The earliest native silver typically occurs as euhedral to subhedral crystals up to 1 mm long. These commonly enclose rounded inclusions of acanthite, chalcopyrite and polybasite (Kullerud et al., 2015).
The two native silver generations were temporally separated by the formation of Ni-Co-Fe sulphoarsenides (including rammelsbergite and nickelskutterudite) and the monoarsenide niccolite which rim the early stage silver crystals. Vughs are also frequent during this interim phase, containing many spectacular and large crystals.
The second generation of native silver occurs as fracture fillings, often enclosing the earlier Ag generation with its rim of sulphoarsenides. Native silver shows significant variations in Hg, Au and Sb contents, locally containing up to 20 wt.% Hg and up to 2.7 w.t% Sb in silver (Kullerud et al., 2015).
The presence of acanthite, chalcopyrite and polybasite as inclusions in the earliest generation of silver suggests Cu, Ag, Sb and S were introduced during an early pre-native Ag stage. This was followed by a stage of sulphide breakdown and formation of the first generation of native silver. Subsequently, Co, Ni and As bearing fluids were introduced, resulting in the growth of sulphoarsenide along the rims of the early euhedral crystals of native silver. This stage was again followed by new growth of native silver through the influx of additional Ag-bearing fluids and silver remobilisation (Kullerud et al., 2015; Kotkova et al., 2018).
This first generation of silver is associated with quartz-breccia veins which generally strike ~north-south and dip very steeply, generally to the ENE. They range from near zero to 10 m in thickness, averaging ~1 m. They may extend for as much as 6 km along strike, although usually much less. Generally they include fragments of wall rock, which in places are so abundant the vein may be designated as a mineralised breccia zone. Quartz is abundant within these first stage veins and is compact and grey in colour. Sulphides, in the order or prevalence are pyrite, pyrrhotite, sphalerite, chalcopyrite and galena. In addition to the quartz breccia veins, there are some calcite veins with the same strike as the quartz-breccia, but are wider than and have a different mineralogical association to the later calcite with Ni-Co arsenides and silver veins. These early calcite veins also contain 30 to 40% fluorite, plus sphalerite and galena and have been followed for several hundred metres without any silver (Meurant, 1984).
The quartz and calcite veins of the first stage are estimated to have formed at 500 to 400°C, with the progressive precipitation of quartz, calcite and pyrite, in that order, although pyrite continued to be precipitated to much lower temperatures. Fluorite and axinite were also deposited in this temperature range, followed by two additional fluorite stages at ~300 and ~200°C. Barite commenced precipitation at just below 400°C, followed by galena, sphalerite and chalcopyrite, and the intermediate phase of sulphosalts (Neumann, 1944). These minerals were deposited at a pressure corresponding to a depth of 2 to 3 km (Bugge, 1978). Both sphalerite and chalcopyrite form two populations, one older and the other younger than native silver. The native silver of both generations was deposited after an appreciable gap at ~250°C, followed by pyrrhotite, niccolite and the diarsenides, and finally zeolites. Minor argentite formed after the second stage during two low temperature windows (Neumann, 1944).
The second generation native silver bearing veins generally strike east-west, normal to the main north-south striking metamorphic foliation of the host gneisses and trend of the fahlbands, and occupy what was a network of pre-existing open fissures, forming clusters of small parallel veins. Mineralisation is essentially confined to the fahlbands, with calcite veins only being mineralised for a few metres into the surrounding gneisses away from the fahlbands. Individual veins are quite narrow, varying from a few millimetres up to 0.5 m in thickness averaging 5 to 10 cm, although veins up to several metres thick have been also reported (Meurant, 1984; Kotkova et al., 2018). These silver-bearing veins dip at between 80 and 90°S, although some dip steeply north, while others are as shallow as 45°S in the northern part of the district. The native silver occurs as threads, moss-like masses or plates. Crystals of silver are rare. Argentite is present but only accounts for 3 to 4% of the silver content. Pyrargyrite, stephanite and other sulphosalts are present in minor amounts. In addition to calcite, the gange includes quartz, Ba-bearing feldspar, barite, axinite, albite, chlorite, zeolite and bitumen. Some of the fissures, instead, parallel the fahlbands and were mineralised with calcite with some zeolite and adularia, but seldom contain silver (Meurant, 1984).
Silver mineralisation is closely associated with fahlbands, with silver only being present where veins cross a fahlband, although not all veins crossing a fahlband are mineralised. Conversely calcite veins that do not cross a fahlband are not silver bearing. At Kongens Gruve, as elsewhere in the district, clusters of rich veins within a fahlband was separated from the next cluster by as much as 100 m of barren ground. There are two main fahlbands that control the bulk of the native silver mineralisation in the district, the Underberget and Overberget bands, as well as 13 others that are of less significance.
The most recent source geological information used to prepare this decription was dated: 2018.
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.
Kongsberg
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Kotkova, J., Kullerud, K., Srein, V., Drabek, M. and Skoda, R., 2018 - The Kongsberg silver deposits, Norway: Ag-Hg-Sb mineralization and constraints for the formation of the deposits: in Mineralium Deposita v.53, pp. 531-545.
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