Cerium speciation and anomaly formation in mature regolith: implications for ion-adsorption type REE deposit prospecting

Abstract

Cerium (Ce) exhibits distinctive geochemical behaviours in the group of chemically similar rare earth elements (REEs) due to its unique oxidability, making the Ce anomaly (Ce/Ce*) a powerful proxy for tracing variations in redox conditions and weathering processes. Though previous studies have investigated the genesis of Ce anomaly through simulation experiments and theoretical calculations, the detailed mineralogical characterization of field samples is still lacking, limiting the utilization of the Ce anomaly as a geochemical proxy. In this study, the geochemical and mineralogical evolution of Ce during chemical weathering was analysed through an approximately 90-meter-long drill core collected from the Huangshe ion-adsorption type REE (iREE) deposit in South China, which was further linked to the alteration of Fe and Mn (hydr)oxides. Based on the variation of chemical composition and rock texture, the whole weathering profile is divided into four layers displaying distinct Ce anomalies, i.e., topsoil (Ce/Ce* = 2.1–57.7), completely weathered horizon (Ce/Ce* = 0.03–2.88), semi-weathered horizon (Ce/Ce* = 0.90–1.34), and parent rock (Ce/Ce* = 0.90). In the parent rock, Ce-bearing minerals include allanite, titanite, apatite, bastnaesite-(Ce), synchysite-(Ce), monazite, and rhabdophane-(Ce), making up the majority of the accessory minerals. At the initial stage of weathering, the dissolution of these minerals gives rise to Ce mobility. Meanwhile, Fe-bearing minerals are dissolved, and then poorly crystalline goethite forms. The interaction between the goethite and the Ce ions immobilizes the latter to generate a slightly positive Ce anomaly. As the weathering proceed, Ce–Fe–Mn ternary-mixed (hydr)oxides prevail in the lower completely weathered horizon. For Fe (hydr)oxides, they are transformed from goethite into hematite, resulting in the decrease of surface hydroxyl density and specific surface area, and accordingly, the reduction of Ce uptake. As for Mn (hydr)oxides, hausmannite promotes the oxidation of Ce³⁺ to Ce⁴⁺ via direct redox reaction, resulting in the formation of cerianite (CeO₂) in the Fe–Mn–Ce composite and an increase of positive Ce anomaly. In the late weathering period, the well crystallized hematite and the cerianite eventually separate and become ubiquitous in the topsoil, resulting in the obvious Ce positive anomaly. Thus, Fe–Mn (hydr)oxides significantly constrain the migration of Ce through adsorption and oxidative scavenging. Consequently, unlike other REEs that are transported downward to the lower part of the weathering crust, Ce remains isolated in the surface zone, generating a positive Ce anomaly. Thus, the positive Ce anomaly can be utilized as a complementary geochemical exploration indicator to identify iREE deposits hosted in mature regolith in South China and other regions of similar geology and climate globally.