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Silberberg |
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Germany |
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S Zn Cu Ag Fe Au
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Super Porphyry Cu and Au
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The small but historic Silberberg pyrite/pyrrhotite deposit with low grade zinc, copper and traces of lead and associated magnetite silver and gold is hosted within a high temperature-low pressure metamorphosed sequence of the Bohemian Massif. The deposit is located on the southeastern outskirts of the town of Bodenmais, ~155 km SE to ESE of Nuremberg in Lower Bavaria, southeastern Germany, ~30 km east of the Czech Republic border (#Location: 49° 3' 32" N, 13° 7' 17"E).
Silberberg is the largest of a series of such deposits in north-eastern Bavaria. Mining at Silberberg, which was first documented in 1436, continued intermittently until 1962. It has subsequently been converted to a tourist mine. During the middle ages it was mined to produce sulphuric acid, also known as 'vitriol', and later also for 'jeweller's rouge', which is fine grained hematite used to polish precious metals (Walther, 1986).
For a description of the tectonic and geological setting, see the appropriate sections of the separate Bohemian Massif record.
Polymetamorphic 'Kieslager' massive sulphide layers near Bodenmais in the Bavarian Forest are intercalated with anatexites/migmatites of the Neoproterozoic Monotonous Group in the Moldanubian Zone of the Variscan Orogen. The anatexites grade into garnet-cordierite-sillimanite gneisses. These migmatites are derived from psammo-pelitic protoliths. Both the host sedimentary rocks and the sulphide layers have been subjected to the same high temperature-low pressure metamorphism during the Variscan Orogeny, with peak metamorphic conditions estimated to be >800 to 850°C and
0.5 to 0.7 GPa (Kalt et al., 1999). The host sequence strikes NW-SE and grades from cordierite-biotite gneiss to the SW over a width of >12 km, before passing into a 2 to 3 km wide biotite gneiss, and then to muscovite schist. Over this width, the degree of metamorphism decrease from south to north, as indicated by semi-conformable metamorphic isograds, as follows, from:
• garnet-cordierite-alkali feldspar-sillimanite to the SW, → cordierite-alkali feldspar-sillimanite to the NE;
• the next isograd cuts obliquely across the biotite gneiss lithofacies into the schist to the NE. Within the biotite gneiss it represents the progression from biotite-sillimanite → biotite-andalusite conditions; whilst within the schist lithofacies it marks the transition from muscovite-sillimanite → muscovite-andalusite (Staude, Raisch and Markl, 2023).
The sulphide layers extend over a strike length of >30 km from NW of Bodenmais to Zwiesel in the SE, and are entirely within the cordierite gneiss lithofacies, to the SW of the garnet-cordierite-alkali feldspar-sillimanite → cordierite-alkali feldspar-sillimanite isograd, on the garnet bearing side. In detail these sulphide bands lie on the contact with a narrow band of garnet-biotite-feldspar gneiss/migmatite conformably enclosed within the broader cordierite-biotite gneiss. Another similar sulphide layer occurs in less strongly metamorphosed biotite gneiss facies, in the neighbourhood of Lam, ~15 km to the north, and close to the biotite-sillimanite → biotite-andalusite isograd detailed above (Staude, Raisch and Markl, 2023; Walther, 1986).
Two main massive sulphide bands occur in the Silberberg district. These average ~0.5 to 2 m (Walther, 1986) in thickness and are found ~30 m apart. A string of higher grade, laterally more restricted sulphide lenses, which may locally be up to 16 m in thickness, have been developed along each of these massive sulphide bands (Linhardt 2015; Weinschenk 1901) with lateral dimensions of from several tens to several hundred metres. These lenses are the individual orebodies mined at Silberberg. Pyrrhotite is the dominant contained sulphide, with lesser pyrite and minor sphalerite, chalcopyrite, magnetite and galena, which is rich in silver, as well as marcasite, magnetite, molybdenite, native Ag and native Bi (Schreyer et al., 1964). The average grade of the massive sulphides was 40% Fe, 25% S, 2.5% Zn, 0.2% Cu and 0.06% Pb, together with up to 200 g/t Ag and 1 to 2 g/t Au (Walther, 1986). Finely disseminated and tens of centimetre thick bands of sulphides are developed laterally between the massive sulphide lenses (Pfeufer 1976). The sulphide lenses and lower grade equivalent disseminated sulphides apparently separate the cordierite-sillimanite-garnet migmatite to the west and the narrow biotite-feldspar migmatite unit to the east (Weinschenk 1901). Locally, an up to 1 m thick lens of pegmatite is found between the sulphides and the biotite-feldspar migmatite/gneiss to the east, whilst a thin quartz-gahnite dominated band is found on both sides of the sulphide lenses, but only at the direct contact between sulphides and either the migmatite or pegmatite (Staude, Raisch and Markl, 2023).
Staude, Raisch and Markl (2023), describe a section across one of these ore lenses, commencing with the cordierite-sillimanite-garnet migmatite to the west → a sulphide matrix breccia → ~1.5 m of massive sulphide → 1.5 to 2 m of net-textured semi-massive sulphide diluted by numerous quartz grains → pegmatite, which is ~1 m thick, brecciated and cemented by semi-massive sulphide → biotite-feldspar migmatite to the east.
In this transect, the lithofacies are described as follows (Staude, Raisch and Markl, 2023):
• The cordierite-sillimanite-garnet migmatite has a foliation at outcrop scale, which is not visible in thin section. The migmatite mesosome is largely composed of cordierite overgrowing quartz, K feldspar and sillimanite, with local large garnet poikiloblasts and minor euhedral biotite, accompanied by accessory ilmenite, hematite and hercynite, as well as occasional globular aggregates of pyrrhotite, galena and pyrite and minor sphalerite. The leucosome is dominated by quartz and K feldspar with minor cordierite and biotite.
• The biotite-feldspar migmatite has a distinct foliation, both macroscopically and in thin section. It's melanosome contains a large amount of biotite, large garnet grains, quartz, K feldspar, sillimanite and accessories ilmenite, hematite and hercynite. The leucosome, in contrast, comprises quartz, K feldspar, muscovite and minor biotite.
• The quartz-gahnite rock is virtually entirely composed of equant euhedral crystals of gahnite containing magnetite exsolutions, set within a quartz matrix. Accessories are monazite-(Ce), ilmenite, hematite, cassiterite and undetermined Nb-phases.
• The pegmatite is composed of quartz, microcline, orthoclase, muscovite and biotite, with symplectic intergrowths with quartz at the contact with K&npsp;feldspar. It contains patches of skeletal textured quartz-gahnite close to the contact with the quartz-gahnite layer. It also contains rounded inclusions of galena, pyrite and chalcopyrite, with minor pyrrhotite and sphalerite, and accessory cordierite, sillimanite, and cassiterite, likely representing trapped sulphide melt in crystallising anatectic silicates (e.g., Frost et al., 2002). Rounded pegmatitic inclusions in sulphides adjacent to the sulphide-pegmatite contact contain symplectic intergrowths of K feldspar and quartz.
• The massive sulphides are predominantly composed of coarse-grained, up to 3 cm, pyrrhotite with 120° contact angles, intergrown with medium-grained, up to 1 cm, sphalerite and coarse-grained, up to 5 cm, euhedral pyrite. Pyrite often exhibits symplectic intergrowths with sphalerite. Chalcopyrite commonly occurs as centimetre sized aggregates containing abundant inclusions of Ag-Hg alloys. Micron-sized galena commonly occurs a symplectic intergrowths with pyrrhotite, with abundant platy exsolution lamellae of native Bi.
• Semi-massive sulphides form the bulk of the ore, and comprise the same mineral assemblage and grain size distribution as those of the massive sulphides. Two different styles are recognised: i). the most common, which contains numerous anhedral quartz grains and lesser subhedral cordierite and K feldspar crystals. Sphalerite in this style preferentially surrounds quartz grains or round symplectic intergrowths of quartz and cordierite which are only seen as schlieren-like textures. Chalcopyrite then preferentially surrounds sphalerite. ii). the second style is rare, and occurs within the massive sulphides, as lenses composed of an intergrown network of euhedral actinolite, or rarely biotite, with interstitial pyrrhotite. The massive sulphides surrounding this style os semi-massive sulphides texture tend to have a larger grain size and is free of silicates.
• A sulphide matrix breccia occurs at Silberberg, apparently somewhat similar to that seen in magmatic sulphide deposits. It occurs at the base of the massive sulphides, and comprises clasts of country-rock engulfed within a sulphide matrix. Clasts are surrounded by multiple mineral grains of the disintegrated silicate rock within the sulphide matrix, or, when the country-rock silicates are interpreted to have been molten, they form an emulsion layer around the silicate clast.
This mineralisation has many of the characteristics of Broken Hill Type deposits that include, Broken Hill (Australia), Rampura Agucha (India), Gamsberg/Aggeneys (South Africa). Staude, Raisch and Markl (2023) conclude from their observations, including those outlined above, and others detailed in their paper (cited below), that Silberberg represents a pre-peak metamorphism, stratabound, sediment hosted sulphide deposit that has undergone high temperature-low pressure metamorphism during the Variscan Orogeny. They cite textures at Silberberg that are similar to those of magmatic sulphide deposits, including sulphide-matrix breccia, emulsion textures, pegmatitic leucosomes, and massive sulphides overlain by net-textured intergrowths of refractory quartz, the latter of which is interpreted to be a relic of silicate anatexis. The authors ascribe these textural observations and outcrop-scale zonation of sulphide-silicate textures to imply the sulphide deposit was largely molten during peak-metamorphic conditions which were at least 800 to 850°C and 0.5 to 0.7 GPa. They suggest that both the silicate country rocks and sulphides underwent anatexis to form two immiscible, partial to complete silicate and sulphide melts that respectively cooled to produced migmatites and sulphide bodies. Molten sulphides would be amenable to locally migrating to structurally favourable dilational sites to cool as massive to semi-massive lenses. This is consistent with the sulphide growth whereby sphalerite is preferentially overgrown by chalcopyrite, and the zonation of sphalerite/pyrrhotite to chalcopyrite to galena to Ag-Au-Bi minerals in fractures, and the overall chemical zonation of pyrrhotite-rich, sphalerite-rich, and galena-rich portions of the orebody (Weinschenk 1901). The formation of two immiscible melts is supported by the differences in chemical composition of the same minerals in the sulphide bodies and adjacent migmatitic host rocks, e.g., garnet in sulphides is Mn-rich spessartine, while Fe-rich almandine predominates in migmatites; and sulphide-enclosed cordierite is similarly more enriched in Mg in contrast to migmatitic cordierite. It may therefore be concluded that formation of the original mineralisation predated, and was unrelated to anatectic melting, but that the latter may have been instrumental in subsequent remobilisation, recrystallisation and likely upgrading in structrally favourable dilational locations.
According to Walther (1986), some 220 000 tonnes of ore was extracted between 1869 and the mine's closure in 1962. Geological reserves were quoted as just on 1 Mt of ore. Grades are of the order of 40% Fe, 25% S, 2.5% Zn, 0.2% Cu, 0.06% Pb, with up to 200 g/t Ag and 1 to 2 g/t Au.
The most recent source geological information used to prepare this decription was dated: 2023.
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.
Silberberg
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Selected References:
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Staude, S., Raisch, D. and Markl, G., 2023 - Sulfide anatexis during high‑grade metamorphism: a case study from the Bodenmais SEDEX deposit, Germany: in Mineralium Deposita v.58, pp. 987-1003. doi.org/10.1007/s00126-023-01166-y.
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Walther, H.W., 1986 - Federal Republic of Germany, Mineral Deposits in the Variscan Orogenic Belt, Moldanubian and Saxothuringian Zones,: in Dunning, F.W. and Evans, A.M., (Eds.), 1986 Mineral Deposits of Europe, The Institution of Mining and Metallurgy and The Mineralogical Society, London, Vol. 3, Central Europe, p. 183.
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