DISTRIBUTION OF REE AND TRACE-ELEMENTS IN SIZE AND MINERAL FRACTIONS OF HIGH-PURITY QUARTZ SANDS

Eight sample sets - raw quartz sand, quartz fraction, < 2-mum clay fra ction, 2-20-mum fraction and heavy minerals - of the Cretaceous quartz sand deposits of Haltern and Weferlingen (central Germany) and the Te rtiary quartz sand of Frechen (western Germany) were studied to charac terize the distribution of REE and trace elements and to get genetic i nformation about provenance and sedimentologic history of the sediment s. The quartz sand samples from different localities have rather simil ar trace-element distribution and REE patterns with enriched LREE and negative Eu anomalies. However, these relations can be modified by loc al changes in syn- or postsedimentary geochemical conditions. Adsorpti on of REE and trace elements on humic substances under reducing condit ions and postsedimentary weathering in the quartz sand of Weferlingen resulted in significantly modified element abundances. The concentrati on of trace elements is most important in the < 2-mum fraction and in the heavy-mineral fraction where they may be enriched by a factor of u p to 100. The chondrite-normalized REE distribution of the < 2-mum fra ction is quite similar to those in the North American Shale (NAS) with average Eu/Sm ratios of 0.21 and (La/Yb) cn. of 6. 1. Low trace-eleme nt abundances in the < 2- and 2-20-mum fractions were attributed to qu artz dilution and quantified. Among the common minerals in the heavy-m ineral fractions, especially zircon and the Fe-Ti-oxides (rutile, ilme nite, magnetite) influence the trace-element distribution. Investigati ons concerning the provenance of the quartz sands illustrate that the host sediments have a rather homogeneous geochemical signature due to their sedimentological history. Taking into account Th/Ta variations t he quartz sands of Weferlingen and Haltern can be distinguished from t he Frechen quartz sand. Additionally, discriminations based on element ratios within the quartz fraction (Th/Sc, Ba/Sc, Ba/Co, Cs/Sc or Li/S c) seem to be promising.