Timing and genesis of base- metal mineralisation in black shales of the Upper Permian Ravnefjeld Formation, Wegener Halvo, East Greenland

• Published in: Mineralium Deposita
• Main Authors: Pedersen, M., Nielsen, J.K., Boyce, A.J., Fallick, A.E.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Springer 2003
• Subjects: QE Geology; Greenland; Ravnefjeld; East Greenland
• Online Access: https://eprints.gla.ac.uk/270/; https://doi.org/10.1007/s00126-002-0283-6

Bituminous mud shales of the Upper Permian Ravnefjeld Formation (Zechstein I equivalent) are mineralised with zinc, lead and copper within a ca. 50 km 2 area on Wegener Halvo in central East Greenland. The occurrence of base-metal sulphides in shale nodules cemented prior to compaction indicates an early commencement of base-metal mineralisation. In other cases, post-compactional sulphide textures are observed. Homogeneous lead isotope signatures of galena and sphalerite from the shales (Pb-206/Pb-204: 18.440-18.466; Pb-207/Pb-204: 16.554-16.586; Pb-208/Pb-204: 38.240-38.326) suggest that all base metals were introduced during a single hydrothermal event. Therefore, post-compactional textures are believed to result from recrystallisation of early diagenetic sulphides during deep burial in the Upper Cretaceous to Tertiary. Lead isotope signatures of galena hosted in Upper Permian carbonate build-ups are relatively heterogeneous compared to those of the shale-hosted sulphides. The observed relations indicate a shared lead source for the two types of mineralisation, but different degrees of homogenisation during mineralisation. This suggests that lead was introduced to the carbonate rocks and black shales during two separate events. delta(34)S of base-metal sulphides in the Ravnefjeld Formation lie between -12 and -4parts per thousand, whereas synsedimentary and early diagenetic pyrite in unmineralised shales in general have 6 4 S between -47 and -16.5parts per thousand. Early diagenetic pyrite in the Wegener Halvo area in general has delta(34)S 15 to 20parts per thousand higher than the same pyrite morphotype in Triaselv in the western part of the basin. This relatively high delta(34)S can be explained by extensive microbial sulphate reduction within persistent euxinic (super-anoxic) bottom waters under which supply of isotopically light seawater sulphate (and disproportionation of intermediate sulphur compounds) was restricted. The sulphur in the base-metal sulphides is believed to represent sulphide-dominated ...