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| Titel |
Silver and Co-Ni sulphoarsenides from the Kongsberg silver deposit, Norway |
| VerfasserIn |
Kåre Kullerud, Jana Kotkova, Radek Škoda |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250107826
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| Publikation (Nr.) |
 EGU/EGU2015-7542.pdf |
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| Zusammenfassung |
| The historical silver mines at Kongsberg, Norway, are world-famous for spectacular samples
of thread silver exhibited at numerous mineralogical museums. 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). However, documentation of the minerals
using state-of-the-art methods such as electron microprobe, with implications for
the genesis of the deposit, is largely missing. In our contribution, we present new
data on the compositional variations of silver and sulphoarsenides from the silver
deposit.
Most of the ore minerals studied here occur in calcite veins in the hosting gneiss. For
some samples, however, fluorite is the dominating vein mineral associated with the ore
minerals. Mineral textures show that native silver formed during at least two separate stages.
The earliest generation of native silver is typically represented by euhedral to subhedral
crystals, up to 1 mm long. These crystals frequently contain rounded inclusions of
acanthite, chalcopyrite and polybasite, and they are commonly rimmed by a zone
consisting of Co-Ni-Fe bearing sulphoarsenides. The later 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. Concentrations of up
to 20 wt% Hg and up to 2.7 wt% Sb in silver have been measured during the present study.
Dyscrasite and allargentum have also been observed in association with native silver.
Neumann (1944) reported auriferous silver with up to 50 % Au. The sulphoarsenides
rimming the crystals of the first silver generation vary in composition along the
cobaltite-gersdorffite series with approximate limiting compositions Ni0.8Fe0.2AsS and
Co0.51Ni0.33Fe0.16AsS.
Existing data indicate that the Kongsberg silver deposit formed in veins in the host
gneisses during several stages of influx of external fluids carrying ore-bearing components.
The presence of acanthite, chalcopyrite and polybasite as inclusions in the earliest generation
of silver suggests that Cu, Ag, Sb and S were introduced during an early stage. This was
followed by a stage of sulphide breakdown and formation 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 silver. This stage was again followed by new growth of
native silver. The presence of ore minerals in both calcite and fluorite filled veins suggests
that both CO32- and F--bearing fluids were important transporting agents for the ore
forming elements.
References cited:
Bancroft P, Nordrum FS, Lyckberg P (2001): Kongsberg revisited. Mineralogical Record
32, 181-205.
Johnsen O (1986): Kongsberg. Famous mineral localities: the Kongsberg Silver Mines,
Norway. Mineralogical Record 17, 19-36.
Johnsen O (1987): Silber aus Norwegen. Zur Bergbaugeschte und über die
Mineralienschätze. Emser Hefte 8, 1-48.
Neumann H (1944): Silver deposits at Kongsberg. Norges Geologiske Undersøkelse 162,
133 p. |
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