mineralization

Mineralization stages of the unique shear zone-hosted "Felsit-type"
Sn-polymetallic mineralization in the eastern Erzgebirge, Germany
Tom Járóka and Thomas Seifert
INTRODUCTION
MINERALOGY
Großschirma
The Erzgebirge with his former world class tin deposits Altenberg/Zinnwald and
Ehrenfriedersdorf is regarded as one of the most important tin districts in the past. In the
last three years geological investigations of two tin occurrences, situated in the Geyer
and Gottesberg area, show a possible future for the tin mining industry in Germany.
Microscopic and geochemical studies of the samples show that the Sn-polymetallic mineralization of the
“Felsit-type” can be distinguished into three different mineralization stages which telescope each other in
many cases (Fig. 2).
FZ
First mineralization stage
Another tin occurrence in the Erzgebirge is the “Felsitzone”, an arch-shaped polymetallic
cassiterite-bearing mineralization zone with a lateral extension of about 18 km, situated
in the NW and N part of the Freiberg mining district within the Großschirma area (Fig. 1).
The first one is dominated by a Fe-rich chlorite and quartz. The shear zones are characterized by a finegrained chlorite-quartz matrix (Fig. 5, Fig. 6). Cassiterite probably appears in two generations with different
grain shapes: acicular (< 1 – 100 µm) and isometric cassiterite (< 10 – 650 µm) (Fig. 7, Fig. 8). Some larger
cassiterite aggregates feature a spongelike intergrowth of acicular and isometric crystals (Fig. 9). Small
amounts of fluorite, rutile, apatite and scheelite are also associated with the first stage (Fig. 10).
It was found during mining activity in the “Kurprinz Friedrich August” mine field in the
area of Großschirma at the beginning of the 19th century. In 1961 extensive studies of
the “Felsitzone” in the area of the “Ludwig Spat” and “Drei Prinzen Spat” revealed also a
minor cassiterite content.
Second mineralization stage
REGIONAL GEOLOGY
The mineralization is hosted by metamorphic rocks of the Preßnitzer Group unit that form
the most northeastern part of the Erzgebirge metamorphic core complex. In the
Großschirma area this unit is predominantly composed of two-mica gneisses and mica
schists, whereas intercalations of muscovite gneiss (“red gneiss”), amphibolites and
metacarbonates occur less commonly. These metamorphic rocks were deformed by
several NE – SW striking fault zones.
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Fig. 1 The arch-shaped “Felsitzone“ (FZ) is located about 10 km NW of Freiberg. The
“Felsit-type” mineralization appears especially in the Großschirma (G) area.
The second mineralization stage is dominated by pyrite which is the most abundant ore mineral of this
paragenesis and occurs in at least three generations. The first pyrite generation is a fine-grained primary
generation. The second generation is caused by a pyritization of pyrrhotite, whereby pyrite, marcasite and
also some magnetite was formed (Fig. 11, Fig. 12). The third generation results from the recrystallization of
the second one. Chalcopyrite, galena, sphalerite, arsenopyrite and bismuthinite appear only subordinately
(Fig. 12, Fig. 13). The sulfides of the second mineralization stage corroded and displaced the fine-grained
chlorite-quartz matrix of the first stage (Fig. 8).
Third mineralization stage
The third stage is dominated by carbonates that are often associated with some fine-grained hematite and
limonite.
Fig. 2 Mineralization stages of the “Felsit-type” Sn-polymetallic mineralization. The ore and gangue minerals of
the first stage (especially chlorite) are usually corroded
and displaced by sulfides of the second stage. The
carbonates of the third stage corroded the minerals of
the first two stages. (1) Probably two cassiterite
generations: acicular and isometric cassiterite, (2) Hematite and limonite.
The genesis of the “Felsitzone” mineralization is still in debate. Baumann & Weinhold
(1963) favored a pre-Variscan age of the mineralization and postulated a syngenetic
model with a submarine exhalative volcanism, while Lorenz & Schirn (1987) and
Kormilicyn (1987) preferred an epigenetic model.
MINERALIZATION
The hydrothermal Sn-bearing fluids migrated within shear zones that developed primarily
at the contact between different lithotypes of the Preßnitzer Group unit (Fig. 3). The
shear zones are characterized by strong pervasive metasomatic alterations which were
triggered by small chlorite-cassiterite-quartz-sulfide-veins (Fig. 3, Fig. 4). The rockforming minerals are corroded and displaced by the ore and gangue minerals within the
shear zones (Fig. 5).
GEOCHEMISTRY
Fig. 3 Mineralized shear zone that developed at the contact between muscovite gneiss and
actinolithe fels. The shear zone is characterized by a pervasive metasomatic alteration which
is triggered by small chlorite-cassiterite-quartz-sulfide-veins (Chl-Cst-Qtz).
Fig. 6 Fine-grained cassiterite (Cst) in
a matrix that consists predominantly of
chlorite (Chl) and quartz (Qtz).
Fig. 8 Isometric cassiterite (Cst) in
intergrowth with chlorite (Chl), quartz
(Qtz) and younger pyrite (Py) and
carbonate (Cb).
Fig. 10 Fine-grained scheelite (Sc) is
corroded by carbonates (Cb).
Fig. 12 Pyrite-magnetite aggregates
(Py + Mag) in intergrowth with pyritemarcasite aggregates (Py + Mrc) next
to chalcopyrite (Ccp).
The mineralized shear zones are characterized by grades up to 0,28 wt % Sn, 0,15 wt %
Cu, 300 ppm Pb, 140 ppm Zn, 1,1 wt % F, 250 ppm Li, 820 ppm Rb, 90 ppm Cs and
130 ppm W. The bulk geochemistry of selected drill core samples revealed a positive
correlation between Sn, F, Li, Rb, Cs and W. A significant correlation between Sn, Cu, Pb
and Zn could not be observed.
CONCLUSIONS
The shear zone-hosted “Felsit-type” Sn-polymetallic mineralization of the Großschirma
area are untypical for the Freiberg mining district which is normally characterized by lateand post-Variscan hydrothermal Ag-polymetallic veins. Thus far a paragenesis with
cassiterite, chlorite and quartz, together with younger sulfides, was not known for the
Freiberg mining district. Beside Sn an enrichment of F, Li, Rb, Cs and W suggest a
genetic link between the “Felsitzone” mineralization and the late-Variscan Sn-WAssociation of the Erzgebirge which is actually not common in the Freiberg mining
district.
Fig. 4 Chlorite-cassiterite-quartzsulfide-veins (Chl-Cst-Qtz) disrupt
a strong metasomatic altered amphibolite.
REFERENCES
Baumann, L. und Weinhold, G. (1963). Zum Neuaufschluss des sogenannten „Felsithorizontes“ von Halsbrücke. - Zeitschrift für angewandte Geologie, 9: 7, 338-345, Berlin.
Járóka, T. (2011). Mineralogische und geochemische Charakterisierung von Kassiteriten-führenden Mineralisationen in metamorphen Gesteinen der „Felsitzone“ im Raum Großschirma, Lagerstättendistrikt Freiberg. Unpub. M.Sc. Thesis, Freiberg.
Kormilicyn, V. S. (1987). Über die Genese und die metallogenetische Bedeutung der Sulfid-Kassiterit-Vererzung des „Felsithorizontes“ im Erzgebirge (DDR). - Zeitschrift für geologische Wissenschaften, 15: 5, 599-618, Berlin.
Lorenz, W. und Schirn, R. (1987). Mylonite, Diaphthorite und epigenetische Zinnmineralisation in der Felsitzone nordwestlich von Freiberg, Erzgebirge. - Zeitschrift für geologische Wissenschaften, 15: 5, 565-597, Berlin.
Tichomirowa, M. (2001): Die Gneise des Erzgebirges - hochmetamorphe Äquivalente von neoproterozoisch-frühpaläozoischen Grauwacken und Granitoiden der Cadomiden. Unpub. Habilitation Thesis, Freiberg.
Fig. 5 A fine-grained matrix, consisting of chlorite
(Chl) and quartz (Qtz), displaced the rock-forming
minerals within the shear zone. This matrix is
displaced by younger pyrite (Py).
Fig. 7 Fine-grained acicular cassiterite
(Cst) in intergrowth with chlorite (Chl)
and quartz (Qtz).
Fig. 9 Fine-grained acicular cassiterite
(Cst I) in a spongelike intergrowth with
isometric cassiterite (Cst II).
Fig. 11 Intergrowth of pyrite (Py) and
marcasite (Mrc) which results from the
pyritization of pyrrhotite.
Fig. 13 Fine-grained sphalerite (Sp) is
marked by chalcopyrite-disease (Ccp).
Tom Járóka ([email protected]), Thomas Seifert ([email protected])
TU Bergakademie Freiberg I Department of Mineralogy I Division of Economic Geology and Petrology
Brennhausgasse 14 I D-09599 Freiberg/Germany I http://tu-freiberg.de

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