Effects of syn- and post-depositional processes on the rare-earth element distribution in Precambrian iron-formations

The REE distribution in these rocks may be affected by syn- depositional (clastic contamination, precipitation) and post- depositional (diagenesis, metamorphism, hydrothermal alteration, weathering) processes. Pyro- and/or epiclastic contamination of the chemically precipitated component was found to be important and to decrease (Eu/Sm) (sub CN) and increase (Sm/Yb) (sub CN) ratios. Samples with high abundances of immobile trace elements (e.g. Sc > 2 ppm) should be excluded from a sample set. Due to closed-system conditions for the REE and low water/rock ratios, the REE distribution in iron formations is usually not affected by diagenesis or contact metamorphism but hydrothermal alteration and weathering may have a significant impact. Such samples should be excluded from a population of iron-formation samples chosen for studies requiring the interpretation of primary REE distributions.