Authigenic heavy minerals a clue to unravel supergene and hypogene alteration of marine and continental sediments of Triassic to Cretaceous age (SE Germany)

Siliciclastic continental–marine sediments of Triassic through Jurassic age, resting unconformably on the crystalline rocks of the Central European Variscides, and marine-(deltaic) sediments of Cretaceous age deposited in a narrow embankment are well exposed in the Wackersdorf area, SE Germany. These sediments were investigated for their variegated spectrum of authigenic heavy minerals (HM) and heavy mineral (HM) aggregates, the majority of which belongs to the (semi)opaque group of minerals, using petrographic and ore microscopy, supplemented by SEM-EDX and by EMPA. These HMs originated from supergene (weathering + diagenesis) and hypogene alteration (hydrothermal processes related to faults and unconformities). The physical and chemical conditions during which these authigenic minerals developed are discussed by means of Eh–pH diagrams under different temperature conditions (< 200 °C). Minerals indicative of supergene alteration in these sediments are apatite, Fe–Mn “limonite”, barite, ankerite, Fe–(Ni) sulfides and Pb–Cu–Zn sulfides. The ilmenite–pseudorutile–anatase series may be used as a measure for the intensity of supergene alteration under rising oxygen fugacity. Pseudorutile and anatase also formed during hypogene alteration, and developed different crystal morphologies compared with their supergene counterparts found in the above series. The authigenic HM fluorite, barite, niccolite, acanthite, silver, bravoite, Ni marcasite, pyrite, sphalerite, galena, chalcopyrite, chalcocite and rhabdophane-(Ce) were produced by hypogene/ hydrothermal processes. The hypogene mineralization in the Mesozoic foreland sediments belongs to two types: (1) epi-mesothermal REE-F mineralization controlled by the structural reactivation along a deep-seated lineamentary fault zone, and (2) epithermal Ni–Cu–Zn–As–S mineralization related to the geohydraulic plane of the late Variscan unconformity/Permo–Triassic peneplain.