File:Fossiliferous sulfidic shale (Kupferschiefer Formation, Upper Permian; near Eisleben, Germany) 1 (45787351935).jpg

Fossil plant in sulfidic shale from the Permian of Germany. (bedding plane view; ~13.7 centimeters across at its widest)

This is a sample from the famous Kupferschiefer of Germany. The name means "copper shale". It is a thin (<1 meter thick), black shale horizon in the Permian of many parts of northern Europe - for example, Germany, Poland, and parts of Britain. The horizon is estimated to be present at the surface or in the subsurface over an area of at least 20,000 square kilometers.

The dark color of the rock is due to a high organic carbon content. The material bubbles slightly in acid - it is calcareous. Kupferschiefer samples are heavy for their size - they contain a relative abundance of finely disseminated sulfide minerals. Sometimes, veins and veinlets of sulfides are present. Reported minerals include chalcocite (Cu2S - copper sulfide), chalcopyrite (CuFeS2 - copper iron sulfide), bornite (Cu5FeS4 - copper iron sulfide), pyrite (FeS2 - iron sulfide), galena (PbS - lead sulfide), sphalerite (ZnS - zinc sulfide), tetrahedrite (Cu12Sb4S13 - copper antimony sulfide), and others. Minor amounts of precious metals, such as gold and platinum-group elements, are also known.

The copper content has made the Kupferschiefer a mining target in Germany since Medieval times. Surficial smelting of Kupferschiefer outcrops was done as far back as the Bronze Age. Mining is also done in parts of Poland.

The origin of the Kupferschiefer's mineralization has been explained by several hypotheses in the literature. Traditionally, this stratabound copper sulfide deposit was interpreted as having formed by metal sulfide precipitation on an ancient Permian seafloor in stagnant water with reducing conditions.

Subsequent investigations have demonstrated that metal-rich fluids have gone through the Kupferschiefer, plus some overlying and underlying rocks, and precipitated various sulfide minerals. Two pulses of sulfide mineralization have been identified: at around 149 Ma (Late Jurassic) and 53 Ma (Eocene). Suggested causative events for the mineralization are the breakup of Pangaea during the Mesozoic and the closure of the Tethys Sea during the early Tertiary (see Borg et al., 2012).

The fossil plant in this rock has been preserved by carbonization, which refers to dark, flattened, carbon-rich films. Many fossil plants in the rock record have been carbonized, but animals can also be preserved this way (for example, graptolites).

Classification: Plantae, Gymnospermae, Coniferophyta

Stratigraphy: Kupferschiefer Formation, lower Zechstein Series, Upper Permian

Locality: undisclosed/unrecorded site at or near the town of Eisleben, Saxony-Anhalt State, eastern Germany

Mostly synthesized from: Guilbert & Park (1986) - The Geology of Ore Deposits. 985 pp.

Borg et al. (2012) - An overview of the European Kupferschiefer Deposits. Society of Economic Geologists Special Publication 16: 455-486.

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