Distribution of REE and trace elements in size and mineral fractions of high-purity quartz sands

Abstract

Eight sample sets — raw quartz sand, quartz fraction, < 2- μm clay fraction, 2–20-μm fraction and heavy minerals — of the Cretaceous quartz sand deposits of Haltern and Weferlingen (central Germany) and the Tertiary quartz sand of Frechen (western Germany) were studied to characterize the distribution of REE and trace elements and to get genetic information about provenance and sedimentologic history of the sediments. The quartz sand samples from different localities have rather similar trace-element distribution and REE patterns with enriched LREE and negative Eu anomalies. However, these relations can be modified by local changes in syn- or postsedimentary geochemical conditions. Adsorption of REE and trace elements on humic substances under reducing conditions and postsedimentary weathering in the quartz sand of Weferlingen resulted in significantly modified element abundances. The concentration of trace elements is most important in the < 2- μm fraction and in the heavy-mineral fraction where they may be enriched by a factor of up to 100. The chondrite-normalized REE distribution of the < 2- μm fraction 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-element abundances in the < 2- and 2–20-μm fractions were attributed to quartz dilution and quantified. Among the common minerals in the heavy-mineral fractions, especially zircon and the Fe/1bTi-oxides (rutile, ilmenite, magnetite) influence the trace-element distribution. Investigations 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 the quartz sands of Weferlingen and Haltern can be distinguished from the Frechen quartz sand. Additionally, discriminations based on element ratios within the quartz fraction (Th/Sc, Ba/Sc, Ba/Co, Cs/Sc or Li/Sc) seem to be promising.