Diagenetic history of the Korallenoolith (Malm) of northwestern Germany: Implications from in-situ trace element and isotopic studies

We have collected rock samples from the abandoned Konrad iron ore mine near Salzgitter/Germany, to elucidate the diagenetic history of the Korallenoolith formation in northwestern Germany. Petrographic and cathodoluminescence investigations showed that the rocks contain a wide range of particles, from primary biogenic material to various generations of interparticle cements and cements in fissures and fractures. The diagenetic environment of cement precipitation was reconstructed using in-situ trace element microanalyses with PIXE, using the Bochum proton microprobe. Furthermore, the different components were analysed for their radiogenic (87Sr/86Sr) isotope signatures. 'Rim cements' around echinoid fragments display several stages of cement precipitation. Early cement generations show high Sr concentrations and low 87Sr/86Sr values. In contrast, later generations have relatively low Sr values, around 300 ppm, and iron concentrations of up to 3%. Fissure cements represent an even later stage of diagenetic history. They consist of calcite, anhydrite and celestite and are characterised by significantly higher 87Sr/86Sr values. The results of combined trace element and isotopic measurements indicate that the biogenic material was deposited in a shallow marine environment with normal salinity and a relatively high primary availability of iron. The early generations of diagenetic cements have similar, marine, signatures and were therefore precipitated in a closed system, where the trace element and isotopic properties were inherited from the dissolved precursor phases. In contrast, the later generations of rim cements and particularly the fissure cements show completely different and variable chemical signatures. They were precipitated in an open system from formation waters.