Norsk mineralsymposium
2016

Redigert av
Alf Olav Larsen og Torfinn KjærnetStathelle 2016

Ag-Sb bearing minerals from the Kongsberg SilverDeposit, Buskerud, NorwayKåre Kullerud1, Jana Kotková2,3 & Radek Škoda3
1

Norwegian Mining Museum, P.O. Box 18, N-3602 Kongsberg (kare.kullerud@bvm.no)
Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic (jana.kotkova@geology.cz)
3
Department of Geological Sciences, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic(rskoda@sci.muni.cz)
2Introduction
The historical silver mines at Kongsberg, Norway, were operative more or less continuously from1623 to 1958. The mines are world-famous for spectacular samples of wire silver exhibited innumerous mineralogical museums around the world. Minerals reported from the deposit include
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). Knowledge of the compositional variations of native silver andother minerals from the deposit is limited as little data acquired by modern analytical techniques e.g.
electron microprobe has been published so far. The present paper gives a first modern documentationof the compositional variations of Ag-Sb bearing minerals from the silver deposit.

Geological setting
The silver mines occur in an about 30 km long and 10 km wide area (Fig. 1) within the Kongsberg
Terrane belonging to the Sveconorwegian orogenic belt in S. Norway (Bingen et al. 2008). The hostrocks of the mines are dominated by Proterozoic (≈ 1.6 Ga) quartz-plagioclase-biotite gneisses, micaand chlorite schists, amphibolites and granitic gneisses (Bugge 1917, Jacobsen and Heier 1978,Starmer 1985). Presence of fahlbands (Fig. 1), i.e. north-south striking subvertical zones enriched insulphides, predominantly pyrite and pyrrhotite (e.g. Bugge 1917, Gammon 1966), is typical for thearea. The two largest fahlbands, which occur in the central mine area, are called Overberget andUnderberget. The fahlbands are cross-cut by nearly vertical E-W trending mafic dikes and quartz andcalcite veins. K-Ar whole rock data from the mafic dikes suggest that they are Permian (270 - 276Ma) and related to the Oslo Rift. Field relationships indicate that the emplacement of the mafic dikes
occurred close in time with the formation of the quartz and calcite veins (Ihlen et al. 1984). In theearly days workers in the mine became aware that the native silver mineralizations occur exclusively
at the intersections between the calcite veins (subvertical, running E-W) and the sulphide-enrichedfahlbands (e.g. Bancroft et al. 2001).
Field observations suggest that the quartz veins, which carry sulphides of iron, copper, zinc and lead,are older than the calcite veins (Neumann 1944). Neumann (1944) reported two types of calcite veins,
one silver-bearing and one silver-free. The latter veins are thicker and more extensive than the silverbearing ones. In addition to calcite, the silver-free veins carry fluorite, sphalerite and galena.
Neumann (1944) referred to the silver-bearing veins as “calcite – nickel-cobalt-arsenide – native
silver veins”, and described them as very narrow, varying in width from a few mm, rarely up to 0.5 m,53

with an average thickness of 5 to 10 cm. He mentioned, however, that the silver-bearing calcite veinsin places can be as big as an ordinary sitting-room.

Fig. 1. Geological map of the Kongsberg Silver Deposit.
Modified after Gammon (1966) and Dons and Jorde (1978).

Antimony bearing minerals
Previous work
Neumann (1944) reported the antimony bearing minerals dyscrasite, tetrahedrite, stephanite,polybasite and pyrargyrite from the Kongsberg Silver Deposit (see Table 1 for mineral formulas). He
inferred that polybasite and tetrahedrite formed at an early stage prior to native silver, while
pyrargyrite and stephanite formed during replacement of native silver. However, Neumann (1944) did
not make any conclusion concerning the age relationship between native silver and dyscrasite.
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Table 1. Antimony-bearing minerals from the Kongsberg Silver Deposit
MineralAntimonial silver
Dyscrasite
Allargentum
Pyrargyrite
Miargyrite
Stephanite
Polybasite
Tetrahedrite

Formula(Ag,Sb), Sb < 5%
Ag3Sb
Ag6Sb
Ag3SbS3AgSbS2
Ag5SbS4[(Ag,Cu)6(Sb,As)2S7][Ag9CuS4](Cu,Fe)12Sb4S13

Neumann (1944)
xxxxxThis studyxxxxxxxx

Petrographical observations
For the present study, polished ore samples and thinsections were prepared from 14 samples from thenorthern and central parts of the silver deposit. Antimony bearing minerals were identified in 7 of thesamples. Polybasite was observed in six samples collected from the South Vinoren area and the areaaround the mines Gottes Hülfe in der Not and Mildigkeit Gottes (Fig. 1). Minerals belonging to thetetrahedrite isotypic series were present in three samples from the same areas. Antimonial silver,
dyscrasite, allargentum, pyrargyrite and miargyrite occur in one of the samples from the Norske Løve
Mine, South Vinoren, while stephanite occurs in one sample from Mildigkeit Gottes Mine.
The ore minerals show complex textural relationships (Figs. 2 and 3), making the determination of
their age relationships difficult. The presence of acanthite, polybasite and chalcopyrite as inclusions in
native silver (Fig. 2a) suggest that these minerals formed at an early stage, prior to native silver.
However, the occurrence of polybasite in the matrix and occasionally replacing native silver suggests
several stages of polybasite formation. The earliest generation of native silver often occurs as sub- toeuhedral grains, commonly with a thin rim of a [(Ni,Co,Fe)AsS] polymorph, possibly alloclasite (Fig.
2b). The second generation of native silver envelopes the earliest silver generation with its rims ofalloclasite (Fig. 2b). Tetrahedrite shows several types of textural occurrence (Fig. 2b and c). One
possible interpretation of the textures in Fig. 2b is that tetrahedrite replaced parts of the first
generation of native silver, after formation of the thin rims of alloclasite, but prior to the formation of
the second generation of native silver. More work is needed before the age relationships between
native silver and the two phases belonging to the tetrahedrite isotypic series shown in Fig. 2c can beestablished.
Textural relationships between native silver, allargentum, dyscrasite and pyrargyrite (Fig. 3) suggestthat allargentum formed as the latest phase during reactions between pyrargyrite and native silver, anddyscrasite and native silver. However, the age relationships between native silver, dyscrasite andpyrargyrite are uncertain. Stephanite replacing native silver was observed in one sample.

Mineral compositions
Chemical analyses of the minerals were performed using the CAMECA SX100 electron microprobe
at Masaryk University in Brno, Czech Republic. Compositional variations of dyscrasite, allargentum
and antimonal silver are shown in Fig. 4, and compositional variations of phases belonging to thetetrahedrite isotypic series in Fig. 5. Dyscrasite contains 73.9-76.3 wt% Ag, corresponding to 76.278.4 atomic % Ag per formula unit (p.f.u.), which is higher than for the ideal formula (Ag3Sb – 7555

atomic % Ag). Allargentum contains 85-86 atomic % Ag p.f.u., which is close to the ideal formula(Ag6Sb – 85.7 atomic % Ag).

Fig. 2a-c. Back-scattered electron images showing theoccurrences of polybasite (Plb), acanthite (Acn), native silver(Ag), chalcopyrite (Ccp), pyrite (Py), pyrargyrite (Pgy),
miargyrite (Mgy), dyscrasite (Dsc), tetrahedrite (Ttr), andalloclasite or one of its polymorphs (Alc). Note the presence of
two compositionally different types of tetrahedrite (Ttr-1 andTtr-2, indicated by two different shades of grey) in c.

Fig. 3. Back-scattered electron image (a) andreflected light photomicrograph (b) of a sample
from the Norske Løve Mine, South Vinoren,
showing textural relationships between antimonialsilver (Ag), pyrargyrite (Pgy), dyscrasite (Dsc),
allargentum (AlAg) and calcite (Cal). The
brightness of the mineral phases in a is controlled
by the atomic weight of elements constituting theminerals. Colors in b: pyrargyrite = bluish grey,
native silver = white, dyscrasite and allargentum =light grey, calcite = black.

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Minerals from the deposit belonging to the tetrahedrite isotypic series contain about 2 atoms Fe, 4atoms Sb and 11.75-13.15 atoms S p.f.u. The high-sulfur grains contains about 3.5 atoms Ag and 6.5
atoms Cu per formula unit, while the low-sulfur grains contain about 6 atoms Ag and 4 atoms Cu per
formula unit. Both the S-rich and the S-poor types can be classified as freibergite[(Ag,Cu)10(Fe,Zn)2Sb4S12-13]. Native silver contains up to 2.8 atomic % Sb and can be termed
antimonial silver.

Fig. 4. Binary Sb-Ag diagram showing the compositional variations of dyscrasite, allargentum andnative silver (including antimonial silver) from the studied samples.

Fig. 5. Cu (blue diamonds) and Ag (red squares) vs. S of minerals belonging to the tetrahedrite
isotypic series, showing variations between the approximate compositions Ag3.5Cu6.5Fe2Sb4S13.15 andAg6Cu4Fe2Sb4S11.75.

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Discussion and conclusions
Antimony bearing minerals have been identified in 7 of 14 investigated samples from the Kongsberg
Silver Deposit, suggesting that antimony is a common trace element at the deposit.
The antimony bearing minerals include native silver (antimonial silver), allargentum, dyscrasite,miargyrite, pyrargyrite, stephanite, polybasite and a mineral belonging to the tetrahedrite isotypicseries, which classifies as freibergite.
It is inferred that polybasite formed at an early stage together with acanthite and chalcopyrite, prior to
precipitation of native silver. Furthermore, polybasite formed at a late stage, as well as allargentum
and stephanite, replacing native silver. Native silver formed during at least two stages, interrupted bya stage of alloclasite formation. The age relationships between native silver, dyscrasite, pyrargyrite,miargyrite and freibergite cannot be determined based on the present data.
The ore mineralogy of the deposit suggests significant fluctuations in the contents of sulfur andarsenic of the ore-forming fluid through time. The observed distribution of antimony-bearing minerals
in the studied samples might suggest variations in the Sb-content of the source of the ore-forming
fluid as well.

References
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Geochronology of high-grade metamorphism in the Sveconorwegian belt, S. Norway: U-Pb,Th-Pb and Re-Os data. Norwegian Journal of Geology 88, 13-42.
Bugge, C. (1917): Kongsbergfeltets geologi. Norges Geologiske Undersøkelse 82, 272 s.
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Johnsen, O. (1987): Silber aus Norwegen. Zur Bergbaugeschte und über die Mineralienschätze. Emser
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