Factors controlling the trace-element distribution in fine-grained rocks: the Albian kaolinite-rich deposits of the Oliete Basin (NE Spain)

Abstract

This paper combines mineralogical data (X-ray diffraction, petrographic and scanning electron microscopy techniques) and geochemical analyses in order to investigate the composition of a set of samples (claystones, siltstones and sandstones) which belong to the Albian deposits of the Oliete Basin in Spain. The aims of the study are to characterize the distribution of major and trace elements in kaolinite-rich rocks; to infer the role played by accessory phases and clay minerals on the trace-element distribution; and to assess the source-area composition of these materials. Mineralogical analyses show that these rocks are mainly composed of different proportions of quartz and clay minerals (kaolinite and illite) with minor feldspar. Micron accessory minerals such as monazite, zircon, pyrite, marcasite, Fe oxides and Ti oxides have also been detected. The sandstone framework implies that the source area of these sediments was probably a granitoid source which underwent low rank metamorphism. The major element distribution reflects the mineralogy of the rocks. Plots in the A-CN-K diagram, as well as their CIA values, indicate that these sediments were generated from an intensely weathered source area. This produced the development of relatively cation-poor clays such as kaolinite, and claystones with large rare-earth elements (REE) contents. The fact that transition element contents in claystones are greater than in siltstones and sandstones suggests that they are preferentially linked to clay minerals, probably by surficial sorption. In comparison with other lithologies, the high abundances of high field strength elements in siltstones imply that accessory minerals, such as zircon and Nb-bearing phases, are mainly concentrated in siltstones. It should be noted that siltstones are also the lithology which displays the highest REE abundances. The statistical analysis of the geochemical data suggests that REE are mainly controlled by Ti- and Nb-bearing phases, such as titanite and Nb- and Ti-oxides. Although clay minerals are not the main phases in the REE control, they probably also have some influence, as the analyses show. In addition, other accessory phases such as monazite, secondary phosphates, and zircon also contribute to the REE contents, although much less than amorphous Fe oxides in some samples. According to the REE patterns and elemental ratios such as La/Sc, Th/Sc, Th/Co, etc., the source area of the studied outcrops was probably felsic and with intermediate composition between granite and granodiorite.