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

Abstract

The rare-earth element (REE) distribution in Precambrian iron-formations (IFs) may be affected by syn-depositional (clastic contamination, precipitation) and post-depositional processes (diagenesis, metamorphism, hydrothermal alteration, weathering). Pyro-and/or epiclastic contamination of the chemically precipitated com-ponent is most important and was found to decrease (Eu/Sm)cN and increase (SnVYb)cN ratios. IF samples yielding high abundances of immobile trace elements (for example Sc> 2 ppm) should therefore be excluded from a sample set. Fractionation of Sm and Eu during the precipitation process is negligible, but semiquantitative REE scavenging from the water column results in elevated (SnVYb)cN ratios in the precipitate. Because of the effects of syn-deposi-tional processes, only the lowermost (Sm/Yb)cN ratios of related IF samples are close to that of contemporaneous seawater. Due to closed-system conditions for the REEs and low water/rock ratios, the REE distribution in IFs is usually not affected by diagenesis and low-to high-grade regional and contact metamorphism. However, hydrother-mal alteration and weathering may have a significant impact. Since their effects on the (Sm/Yb)cN and (Eu/Sm)cN ratios of IFs may be variable, they cannot be predicted in a general approach, but must be considered for each case individually. Thus, such samples should be excluded from a population of IF samples chosen for studies requiring the interpretation of primary REE distributions.