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Mineralogical and petrological investigation on Kupferschiefer type mineralization in the Wolfsberg mine, Richelsdorf mining dis

Nico Kropp

The spatial distribution of the Kupferschiefer mineralisation is linked to the position of the Central European Crystalline Rise, which underlies parts of the Richelsdorf Mountains and is largely considered as source of the metals. Base metal-rich fluids migrate along displacements or faults, depending on structural history.

Scanning electron microscope and cathodoluminescence were carried out on dump material from the Wolfsberg Mine, Richelsdorf Hills (Fig. 1), Hesse.

Fig. 1: Cross section of the Wolfsberg Mine; the red colour stands for Rotliegend-sedimens, blue for Zechstein-sediments with the 'Kupferschiefer'-strata at the base, Buntsandstein-layers are yellow.

Fig. 1: Cross section of the Wolfsberg Mine; the red colour stands for Rotliegend-sedimens, blue for Zechstein-sediments with the 'Kupferschiefer'-strata at the base, Buntsandstein-layers are yellow.

Fig. 1: Cross section of the Wolfsberg Mine; the red colour stands for Rotliegend-sedimens, blue for Zechstein-sediments with the 'Kupferschiefer'-strata at the base, Buntsandstein-layers are yellow.

The studies were focussed on investigations of cements of the black shale footwall sandstone that are enriched in base metal sulphide ore representing the Kupferschiefer-mineralisation.

Based on the ratio between quartz, feldspar and lithic clasts, the sandstone can be lithologically classified as ore-bearing, conglomeratic  arkose dominated by basement clasts. The crystalline basement bordering the Richelsdorfer Mountains can be regarded as source of the clastic components.

The cement between the lithic clasts can be composed differently. They can be of sulphidic, sulphidic-carbonatic and carbonatic composition. Microscopically, some of the carbonates show red coatings, which indicate iron oxihydroxides. This observation is confirmed by cathodoluminecence (CL) studies that show a lighter edge of the carbonates, too (Fig. 2). Besides iron oxihydroxides, the carbonates contain manganese, which represents an activator element of luminescence. Lattice defects in quartz show blue luminescence colours, feldspars with low content of manganese show green luminescence colours.

Fig. 2: CL-image showing orange carbonates, green feldspar and blue quartz.

Fig. 2: CL-image showing orange carbonates, green feldspar and blue quartz.

Fig. 2: CL-image showing orange carbonates, green feldspar and blue quartz.

The Kupferschiefer-type ore mineralisation derives from different phases, whereas obviously the youngest stage shows hydrothermal character by replacements textures. The oldest phase mostly consists of sulphides like bornite, chalcopyrite, chalcocite, pyrite and marcasite as well as tennantite. The youngest phase shows high contents of cobalt and nickel sulphides  and arsenides associated with pyrite. Ore texture is displayed by interstitial fillings or replacements of fossils (Fig. 3 & 4) and can be associated with hydrothermal vein mineralisation.

Fig. 3: SEM-image of a mineralized foraminifer which is most pyritic with shelled intergrowth of arsenic pyrite that shows a high content of cobalt and nickel.

Fig. 3: SEM-image of a mineralized foraminifer which is most pyritic with shelled intergrowth of arsenic pyrite that shows a high content of cobalt and nickel.

Fig. 3: SEM-image of a mineralized foraminifer which is most pyritic with shelled intergrowth of arsenic pyrite that shows a high content of cobalt and nickel.

Besides the occurrence of cobalt- and nickel sulphides, the alteration  of feldspar and alkali feldspar to sericite or kaolinite indicates a  hydrothermal formation of the last ore stage.

Fig. 4: Reflected-light-microscopy showing brownish bornite and grey chalcocite which are replaced by shelly arsenic pyrite.

Fig. 4: Reflected-light-microscopy showing brownish bornite and grey chalcocite which are replaced by shelly arsenic pyrite.

Fig. 4: Reflected-light-microscopy showing brownish bornite and grey chalcocite which are replaced by shelly arsenic pyrite.

Postdate mineralisation phase is characterized by sulphides intergrown with carbonates and feldspars or alkali feldspars. Textural features are complete to partial replacement of feldspar components. Solution embayments have been observed on quartz clasts, but stand apparently in relation to the late hydrothermal phase of the ore mineralisation that reached, temperatures over 200°C required to solve quartz. Additionally, arsenic pyrite associated with the late stage sulphide assemblage is formed between 150 and 200°C and support the achievement of mesothermal conditions. Arsenic-cobalt-nickel-sulphides also associated with the late mineralisation stage exhibit zonation textures caused by changing composition of the ore forming fluids. Zonation patterns show decreasing content of arsenic from the centre to the outside. Similar distribution patterns have been noticed for nickel and cobalt.

As a secondary product covellite occurs associated with bornite, chalcopyrite lamellae as secondary filling also occurs with bornite. Surfaces of the hand specimen are slightly covered with green coatings of secondary copper minerals formed by weathering. Some hand specimen show reddish cement around some clasts that could be interpreted as oxidized areas ('Rote Fäule'), but the relation between red cement, observed in this study, and Rote Fäule is very speculative.

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