Behavior of major and trace elements during weathering of sericite–quartz schist

Two regolith profiles developed on the sericite–quartz schist in subtropical humid environment were selected to investigate behaviors of major and trace elements during weathering in Mengman gold deposit of Yunnan province, China. One profile located in the mining district sheared by a fault and the other was outside the mining area which represented the normal weathering profile on the schist. Regolith samples were collected in both profiles sequentially. Thirteen major oxides and 23 trace elements (including REE) were analyzed and their behaviors were compared in these two profiles. Based on the idea that immobile element is just a relative notion, we presented a method of immobile plateau to determine immobile elements during each stage in a progressive geochemical process and used mass ratio (MR) to calculate the percentage of gain or loss (Xgp) of each element during the whole process.In both profiles, only TiO2 was immobile during the whole weathering. The regolith profile formed on the mineralized schist recorded the weathering process more sensitively than the regolith profile on the normal schist. REE was mobile and fractionated during the schist weathering. LREE was loss in mass during the soil development stage which resulted from the chemical leaching, but was gain in mass during the pedogenesis stage because of the preferential absorption of soil to LREE. The LREE depletion near the fault during weathering was the collective effects of chemical leaching and physical accumulation. HFSE were all mobile in the mineralized regolith profile especially near the fault. But Nb–Ta and Zr–Hf were covariant in both profiles during the schist weathering.

► A method of immobile plateau was presented to determine immobile elements during each stage in a progressive geochemical process.
► REE was mobile and fractionated during the schist weathering.