Opaque minerals as keys for distinguishing oxidising and reducing diagenetic conditions in the Lower Triassic Bunter Sandstone, North German Basin

Abstract

The diagenetic evolution in red and white/drab parts of the Bunter Sandstone from outcrops and wells in the North German Basin was unraveled by a detailed petrographical, mineralogical, and chemical study. The diagenetic sequences for the red and white/drab parts differ in the degree of alteration of iron-bearing detrital minerals and in the authigenic opaque minerals. In the white/drab parts, one typically sees reduction of red mica, dissolution of hematite and magnetite, and alteration of ilmenite and titanomagnetite to leucoxene. In contrast, the red parts commonly display oxidation of glauconite; red zonation in ooids; hematisation of magnetite, biotite, and magnetite host in titanomagnetite; and leucoxene replacement of the ilmenite lamellae in titanomagnetite. The red colour of the red bed host is due to red coatings of goethite needles and/or hematite needles. Goethite needles precipitated in the prevailing oxidising conditions, present shortly after deposition, whereas pseudomorphous replacement by hematite occurred during the burial process. The white/drab colour of the reduced areas is the original colour of the sediment. The black core in reduction spots is due to mineralization. Uranium and vanadium minerals (coffinite, montroseite, and V-illite) in reduction spots are located close to carbonate fragments, which probably contained organic matter. Isolated reduction spots may have formed immediately after deposition; inside these were clasts eroded from algal mats formed in the sabkha environment. The copper minerals typically occur in areas with high porosity, reflecting the importance of access to chloride-rich brines, which transported dissolved copper. When these brines reached reducing environments, chalcocite and chalcopyrite were precipitated, possibly by replacement of pyrite. In the red areas, these saline brines may also have promoted the second-stage alteration of ilmenite, in which leucoxene, the first-stage alteration product, was impregnated with iron oxide/hydroxide needles. This study emphasises the importance of opaque minerals, especially Fe–Ti oxides and authigenic opaques, as diagnostic for the diagenetic environment of the Bunter Sandstone, which can thus be divided into primary reduced, primary oxidised, secondary reduced, and secondary oxidised diagenetic environments. Furthermore, the influence of saline brines can be detected using the opaque mineral assemblage.