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LOCATION HERMSDORF

LOCATIONGEOLOGYMININGPROCESSINGCHRONICLE

The age of the marble deposit Hermsdorf is still in discussion. Lithostratigraphically, the thick sequence of interlayered marble and phyllite is ascribed to the Herold Formation as part of the Thum Group. This would correspond to a Cambrian age of approximately 510 million years. The deposit is situated in the upper Herold Formation. Marble occurrences in the lower Herold Formation are not known.

The underlying rocks of the deposit are Precambrian gneisses of the Preßnitz Group and Proterzoic metagranitoids (leptynites) as well as Cambrian lithologic equivalents of the Breitenbrunn Formation (muscovite schists). Besides these originally late Cadomian igneous rocks and their subsequent prograde metamorphism, the footwall of the marble unit (quartz-rich phyllites and hornblende-chlorite schists of the lower Herold Formation) represents originally submarine basic volcanic rocks subsequently affected by retrograde greenschist-facies metamorphism.

The deposit comprises six calcitic white marble horizons (k0, k2-k5) and one grey horizon (k1), which consists of magnesium-calcite.

The marble is thought to have a marine sedimentary protolith. The carbonate sedimentation took place under turbulent conditions. One reason for this interpretation is the rough surface relief of horizon k0 (the deepest marble occurrence in the deposit) with high variations in thickness and parallel splitting into several marble layers. These observations suggest a rapid change of depositional conditions. The k1 horizon is characterised by similar but more distinct features. Former about 30-40 m thick reef limestones act as cores of tectonically developed horst structures. In contrast, the k1 horizon has partly been downthrown at synsedimentary graben-like structures.

Such deep sedimentation basins provided the environment for the development of four white marble horizons (k2-k5) with changing facial features and varying thickness. Caused by post-metamorphic brittle tectonic deformation the white marble horizons were divided into fault-bound separated blocks being tilted and thrusted between the horst structures.

The tectonic features of the deposit are of particular importance. The main foliation of the wall rocks forms a girdle fabric with axes striking E-W. Large SW-NE trending fault zones, which are thought to be related with Upper Carboniferous post-kinematic volcanism, divide the phyllites of Hermsdorf into several blocks with displacements of up to 40 m. The central part of these blocks has been downthrown as graben and covered with rhyolites, which preserved the upper Herold Formation (including the marble deposit) from erosion.

The tectonic development of the Hermsdorf deposit comprises five stages:

1. Post-sedimentary brecciation of horizon k1.
2. Pre-metamorphic formation of the graben structures and micro-folds around the horsts of horizon k1.
3. Post-metamorphic collapse of the peripheral zones of the graben structures during the Variscan orogeny (about 330 million years ago) with contemporaneous magnesium-metasomatism in the centre of horizon k3.
4. Formation of large folds in the north-eastern part of the deposit.
5. Reactivation of old faults and fractures combined with intense dissolution processes (karst) and influx of fluids (partial fluoritisation) in horizon k5 caused by influences of the Alpine orogeny in Tertiary.

The white marble from Hermsdorf dominantly consists of calcite with variable but low contents of quartz and phyllosilicates. The metamorphic foliation is traced by oriented crystals of quartz, chlorite and white mica as well as by grain size variations of the calcites resulting in a banded marble structure.

The calcite crystals are characterised by a granoblastic texture, in which single crystals are slightly twinned and lengthened. Such a slight twinning is typical for crystal-plastic deformation under medium temperature conditions of 300°C. The average grain size of the calcites varies between 0.2 and 0.6 mm. The calcites typically have very low iron contents.

Petrophysical parameters of the marble of Hermsdorf were determined to 87.0 N/mm² for compressive strength and 12.0 N/mm² for bending tensile strength. The gross density is 2.69 g/cm³. The remarkable low water absorption of 0.26 wt.-% and high frost resistance of 0.34% mass loss are caused by the intense metamorphic overprint. The marble is free of bacterial and organic components and does not contain any graphitic inclusions.