Facies-related baryte mineralization bearing Cu–Zn sulfides in Miocene estuarine deposits of the upper Rhein Graben (Wetterau, Central Germany)

Baryte with or without base metal sulfides is quite common in sediments deposited in open marine environments or in continental sedimentary basins. Its precipitation is caused by hydrothermal processes, related to diagenesis, and frequently mediated by biogenic processes. The current study is focused on siliciclastic sandstones of Miocene (Aquitanian) age in an estuarine environment in the Wetterau region of the Rhein Graben, central Germany. In the estuarine environment only the central basin and the landward delta are host to a diagenetic and subsequent hydrothermal mineralization.Diagenesis took place under near-ambient (T ≈ 25 °C) conditions and resulted in strong pyritization (− 0.75 < Eh < + 0.25, pH > 5) in the central basin. Diagenesis is more landward represented by a pervasive silicification (pH < 12) in deltaic sandstones.Epigenetic mineralization (100°–130 °C) with pyrite in the central basins was succeeded by Cu–Zn–(Sb) minerals (0.75 < Eh < 0/5 < pH < 11), silicification and kaolinization (2 < pH < 9.5) and eventually by the formation of gibbsite (3 < pH < 8). At the transition from the delta to the estuarine funnel, baryte is of very widespread occurrence. Its variegated texture and crystal morphology allow for a precise determination of the hydraulic system as marine phreatic, freshwater phreatic, and freshwater vadose. The narrow size of the rift graben and its sealing against the open sea fostered concentration of Ba and enhanced the redox processes. Hypogene brines along with Miocene volcanic activity provided the metals, and marine ingressions in this transitional environment supplied the sulfur. Sulfides were concentrated in the finer-grained rocks because of their enrichment in organic material, while sulfates accumulated in the more permeable coarser sandstones.