The origin of sideritegoethitephosphate mineralization in the karst-related faultbound iron ore deposit Auerbach, Germany, a clue to the timing of hypogene and supergene Fe-Al phosphates in NE Bavaria

Abstract

The Auerbach siderite–goethite ironstone deposit evolved during the Cenomanian in a fl uvial-lacustrine depositional environment on the Jurassic karstifi ed carbonate platform. Long-lasting normal faulting controlled the subsidence of the Fe sinkholes, protected the ironstone deposit from erosion and was responsible for the precipitation of Fe phosphates (vivianite, rockbridgeite, phosphosiderite, ferroan variscite, cacoxenite, oxi-beraunite), Al phosphates (wavellite, crandallite s.s.s.) and churchite-(Y) as it was active into post-Coniacian times as pathway for low-temperature hydrothermal solutions. Based upon mineralogical studies involving SEM-EDX, XRD, IR spectrometry, DTA and chemical studies by means of XRF, fi ve mineral assemblages were established: 1. sedimentary-diagenetic, 2. early hydrothermal, 3. late hydrothermal/epithermal-reducing to oxidizing, 4. late hydrothermal/epithermal-oxidizing, 5. supergene/weathering. Mineral assemblage 1 evolved during the Cenomanian, mineral assemblages 2 through 4 from the Coniacian through the Neogene and mineral assemblage 5 during the Pliocene. The Auerbach Fe-P mineralization within well-dated Cretaceous platform sediments is taken reference to chronological constrain similar Fe-P mineralizations in pegmatites and vein-type deposits in the basement. Minerostratigraphic comparisons between Fe-P mineralization in well-dated sedimentary host rocks and equivalent Fe-P mineralization of unknown age in the basement are performed using the crystal morphology of marker minerals such as apatite, vivianite and strengite. Fe-P mineralizations in the basement tend to evolve more complex crystal morphologies than equivalent minerals in carbonate rocks, with a simple parent ore and undergoing only monophase minero-tectonic processes. “Telescoping” of phosphate minerals is typical of these mineral assemblages, characterized by a small-scale fl uctuation of the redox conditions in the phreatic-vadose hydrological system at temperatures < 100 °C. This epithermal phosphate mineralization is genetically bound to NW to NNW trending faults and related to the Alpine unconformity as a horizontal reference plane.