Geology and geochemistry of the SGC

Geological setting

The SGC belongs to the highly evolved younger granite group of the Erzgebirge batholith (Tischendorf and Förster, 1990; Stemprok, 1993; Förster and Tischendorf, 1994; Seltmann, 1994). Neoproterozoic gneisses of the metamorphic basement and volcano-sedimentary rocks of the Altenberg-Teplice caldera form the host rocks (Fig. 7.2).

Late-collisional extensional tectonics and collapse of the Variscan orogen controlled during the Upper Carboniferous the block and graben tectonics in the area, caldera formation with pre-dominantly ignimbritic rhyolites and porphyritic microgranites, and finally the post-tectonic multiple intrusion of tin granites of the SGC into a subvolcanic level. Uplift, faulting, block tilting and erosion took place mostly in Permo-Silesian and Cenozoic times and formed the recent morphology. NW- and NE-striking faults with vertical dislocations of several hundred metres form the SGC as horst and as a result some deeper pluton parts are uncovered by erosion. The SGC crops out at about 13 km² as a NW-SE elongated body roughly bounded in the north-eastern part by the Weisseritztal fault and in the south-western part by the

Fig. 7.2 Geological sketch map of the Schellerhau granite massif and its geological setting in the Altenberg-Teplice caldera, Eastern Erzgebirge (without Cenozoic).

Pöbelbach fault (Schilka and Baumann, 1996). These marginal fault zones indicate within the structural pattern of the Altenberg-Teplice caldera a NW-striking sinistral strike-slip movement and north-east directed extension controlling in the caldera stage the post-tectonic SGC intrusion. Faults intersecting the SGC are either post-intrusively reactivated or newly formed as indicated by their hydrothermal mineralisation of diffent ages ranging from Upper Carboniferous to Cenozoic.

According to gravimetry data and geochemical studies of drill cores, granites of the SGC type form the mostly hidden Eastern Erzgebirge partial pluton (Tischendorf, 1964). This NNW-striking hidden granite ridge reaches from Dippoldiswalde in the north to Zinnwald in the south with ca. 10 km east-west extension. To the south, in the Czech part of the Erzgebirge, the chemically and texturally similar Preisselberg-Cinovec granite (Stemprok et al., 1994) forms the continuation of this granite ridge.

Phasing

The SGC is characterised by the intrusion sequence of porphyritic (SG1) to weakly-porphyritic (SG2) biotite syeno- to monzogranites, and mostly seriate albite granites (SG3).

The SG1, SG2 and SG3 rocks of the SGC represent individual stages (phases) of a multiple granite intrusion as indicated by field evidence.

The SG1 forms the central and upper part of the SGC and occupies about 2/3 of the SGC at the recent surface. Locally, the SG1 exhibits a marginal facies characterised by the most primitive composition within the SGC that is due to porphyritic texture and muscovite-bearing similar to the “intermediate granites” (IG, “Zwischengranite” type Walfischkopf, Lange et al., 1972; Stemprok, 1986) of the Western Erzgebirge. Pendants of that SG1 variety of the SGC also occur in the multiple intrusions of tin granites at Sadisdorf and Sachsenhöhe.

The textural variability of the marginal facies of the SGC also includes so-called two-phase textures (Cobbing et al., 1992; Seltmann and Stemprok, 1994) where crystals/crystal mush of an earlier SGC intrusion and cooling stage either underwent fluidisation at intrusive contacts or were infiltrated along grain margins but not resorbed by low-viscosity melt batches of a subsequent intrusion stage. The older phenocrysts exhibit sharp contacts to the surrounding groundmass of that second stage.

The SG2 intruded the SG1 along its margins probably due to cauldron subsidence effects in the post-caldera stage of the Altenberg-Teplice crustal unit. The SG2 occupies about 1/3 of the SGC surface. There is field evidence that SG1 enclaves occur in SG2 (locality Paradies-Fundgrube), SG2 dykes cross-cut SG1, and there are sharp intrusive contacts with

stockscheiders and chilled margins of the younger phase against the earlier phase (northern SGC flank at Kipsdorf SG2/SG1, southern SGC flank at Kahleberg SG2/SG1, drill cores SG3/SG1 and SG3/SG2).

The albite granites (SG3) occur as fine- to medium-grained porphyritic and seriate varieties.

They were found in most of the drillings as flat igneous layers of up to several ten metres thickness intercalated with SG1 and SG2 units characterising the SGC as sheeted laccolite body. Small SG3 dykes cross-cutting the earlier granite phases SG1 and SG2 provide evidence for magmatic origin of SG3 and against its interpretation as metasomatic zones.

Exploratory excavations near the former New Galgenteich quarry exposed a small SG3 dyke with sharp igneous contacts to the SG2 (F. Schiemenz, pers. comm.). The occurrence of fluid saturation textures (miarolitic cavities, micrographic quartz/K-feldspar intergrowth) reflects the high fractionation degree of the SG3 melt.

The genetical position of alkali feldspars, especially the occurrence of sugar-grained albites, remains unsolved and is controversially discussed for similar rocks (Beus et al., 1962;

Schwartz, 1992; Stemprok, 1993). Regardless of many subsolidus features caused by deuteric alteration and post-magmatic fluid-rock reactions, we classify the SG3 rocks as to be of predominantly magmatic origin (Just et al., 1987; Seltmann et al., 1992) as they are characterised by snowball quartz that is bearing melt inclusions (R. Thomas, pers. comm.).

These melt inclusions show features of trapped silicate melt similar to those studied in topaz-bearing granites from Karelia (Poutiainen and Scherbakova, 1998) and do not link to the interpretation of crystallised silica colloid trapped as a hydro gel during greisenisation as described from a Cornish topaz granite (Williamson et al., 1997). The snowball quartz contains also inclusions of late-magmatic matrix albite. Many SGC samples underwent supplementary metasomatic albitisation (Haapala, 1997) and therefore only relics of the primary texture and structural relationships allow to identify the original granite type.

Within the Eastern Erzgebirge, marginally to the SGC, a series of stock-like granites (Altenberg, Sadisdorf, Sachsenhöhe, Zinnwald, Preisselberg), each with 1-5 km² outcrop size at the recent surface, form as multiple intrusions cupola-shaped elevations of the hidden pluton. These stocks we interprete as channelised products of evolved melt batches and the accompanying ore-forming greisen fluids as originated from deeper parental magmas. They are, similar to the SG1 to SG3 sequence, composed of rock types with fine-grained porphyritic via medium-grained equigranular syeno- to monzogranites to seriate albite granites. The granite elevations represent the country rocks for endocontact tin mineralization of the greisen type. Numerous drillings made in the area between Cinovec (Zinnwald) and

Krupka on the Czech side where granites were continuously followed in the drill cores appear equivalent to those found in the SGC. Our studies confirm the evolution series of protolithionite, zinnwaldite and lepidolite granites such as decribed as upward sequence from the deep drilling at Cinovec (Stemprok and Sulcek, 1969; Rub et al., 1997).

Due to the recent erosion level of the SGC of estimated ca. 500-1000 m (Schust, 1980, Spengler, 1949), any mineralised cupolas or elevations over the Schellerhau granite body were eroded. Greisen occurrences within the SG1 (Fig. 7.1) represent either the root zones of eroded tin mineralization or are exogreisens of hidden intrusions of SG2 and SG3 intruding at depth the SG1.

Geochemistry

The SG1, SG2 and SG3 rocks chemically represent the suite of P-poor, Li-F-enriched series of leucogranites that exhibit some distinct A-type tendency (Förster et al., 1995; Breiter et al., 1999). The latter are weakly peraluminous (A/CNK ≤ 1.2), enriched in HREE, Y, Th, Hf, Zr, Sc, Nb, Ta, and U and display elevated abundances of Rb, Li, F, and Sn (Förster et al., 1996) increasing from SG1 to SG3. There is only a moderate chemical contrast between the SG1 and SG2 rocks. The SG3, however, exhibits in distinction to the SG1 and SG2 the chemical and petrographic patterns of alkali feldspar leucogranites (Table 7.1). The SG3 rocks are more highly evolved as also shown by the decreased Zr/Hf and Y/Ho values. Its modal composition (Table 7.2) is due to feldspathisation different to that of the SG1 and SG2 rocks. Topaz occurs mostly poikiloblastic and was classified in Table 7.2 as of secondary nature but in few cases primary magmatic topaz exists in the SGC rocks (R. Thomas, pers. comm.).

The SG1, SG2 and SG3 rocks are interpreted as products of in-situ fractionation of a magma derived from a common deep-crustal parental magma. The chemical and textural specifics of the SG3 rocks were produced when the rock underwent deuteric alteration, and only in few cases (drill cores) the primary textural and chemical features were preserved.

Table 7.1 Chemical analyses of major and trace elements from representative SGC samples.

TiO2 0.057 0.094 0.068 0.022

Al₂O₃ 12.60 13.84 14.64 16.97

Table 7.2 Modal mineral composition of representative samples of the SGC.

Samples Sh-22 (SG1) # 16 (SG2) Sh-32 (SG2) Sh-18 (SG3)

Characteristics Very fine-grained, weakly PRIMARY MINERALS: PHENOCRYSTS and GROUNDMASS, vol.%

Quartz 31.7 30.7 23.6 19.7

to Zinnwaldite) 3.4 3.0 9.0 4.9

Accessories (Titanite, Rutile, Monazite, Zircon, Thorite, Apatite, ...)

0.1 0.8 0.1 0.1

Opaque (Ore) Minerals:

Ilmenite, Sulphides 0.1 0.1 0.1 0.1

SECONDARY MINERALS, vol.%

Albite

(Afs, An <5 vol.%) 3.3 6.3 3.0 38.1

Mica from veinlets - 0.6 -

-Sericite

(+ Hydrosericite) 4.4 3.6 2.5 6.3

Muscovite 0.1 0.5 -

-Kaolinite 0.1 1.6 1.5

-Fluorite 0.3 0.7 0.8 0.6

Topaz 0.2 0.2 2.1

-Carbonate - 0.4 -

-SUM, vol.% 100.0 100.1 99.9 100.0