Geochemical behavior of rare elements in Paleogene saline lake sediments of the Qaidam Basin, NE Tibetan Plateau

Abstract

The Paleogene paleo-lake in the Qaidam Basin on NE Tibetan Plateau is one of the largest perennial inland saline lakes in China. Forty-nine samples collected from four depositional environments (shallow lake, lakeshore, sandy mudflat and gypsiferous mudflat) have been tested for major, trace and rare earth elements to study the geochemical behavior of rare elements (including trace and rare earth elements) in sediments of saline lakes. Based on correlation degrees and relationships with major elements, six categories of rare elements in the Qaidam saline lake have been recognized: (1) REE-type (REEs, Nb, Ta, Ge, and Y), (2) nutrient-type (Zn), (3) scavenged-type (Cr, Ga, Sc, and V), (4) conservative-type (Be, Rb, Th, Ni, Cs, and Li), (5) grained-type (Zr and Hf), and (6) independent-type (U, Pb, Co, Cu, Sr, and Ba). Contents of rare elements in saline lakes are comprehensively controlled by detritus inputs, grain size, adsorption by or co-precipitation with clay particles, and biological activities. However, these factors influence the behavior of rare elements of different categories to different degrees. The presence of salts has a diluting effect to all the rare elements except Sr and Ba. The ratio Sr/Ba is proved to be a good indicator of paleosalinity in saline sediments. More depletion of HREEs and relative enrichment of Co, Ni, and Cu have been observed in pure anhydrite samples. The conservative- and independent-type elements are more likely to be enriched in saline lake centers during the process of evaporation.