Mineralogical, geochemical and economic potential of rare earth element deposit in andesitic rock and their weathered crusts at Riih Serian area, Sarawak Malaysia

Abstract

Systematic research involving sampling and analysis of critical minerals which is rare earth elements (REE) has been conducted in the Riih Serian area, Western Region of Sarawak, Malaysia. The scope of the research focuses in the mineralogical and geochemical characterization that relatively influence the enrichment process of REE in the weathered crust of volcanic rocks in the study area. In general, the geology of this region is mostly composed of Palaeozoic and Mesozoic rock sequences, which are overlain by Cenozoic to Recent sedimentary rocks. The Sadong Formation and the Serian Volcanic Formation, two major rock lithologies of the Late Triassic provide the basis for the occurrence of REE. The research method includes outcrop mapping and in-situ sampling along four volcanic weathering profiles at intervals of 1-meter from the surface to the bedrock, as well as sample analyses in the laboratory such as petrography, X-ray diffraction (XRD), X-ray luminescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS) and scanning electron microscopy-mineral liberation analysis (SEM-MLA). The horizon of the weathering profile can be divided into horizons A, B, C and D based on the observations and soil characteristics of the weathering profile. Based on the XRF analysis results, all soil samples were classified as laterite, whereas the two fresh rocks collected with loss on ignition (LOI) values <3%, are classified as trachy-andesite and are of the tholeiitic group. The ICP-MS data show that the trend of REE enrichment from the bedrock to the B horizon (accumulation) can also be identified in some of the weathering profiles studied, with the maximum total rare earth elements (TREE) reaching 1715.98 ppm. Chondrite normalization analysis of laterite soil samples reveals a positive relationship between negative Ce/Ce* anomaly and REE-rich horizons. The Eu/Eu* anomaly in the entire sample exhibits a relatively weak negative trend, which is strongly connected to the trend of decreasing Na and Ca oxide content with increasing Si oxide values. The XRD results of soil samples from enrichment horizons with high TREE content also shows the existence of secondary minerals, namely kaolinite as the dominant mineral, followed by other minerals such as goethite and gibbsite, which cause the accumulation of REE in the particular horizon. This observation is further supported by SEM-MLA analysis, which demonstrates that kaolinite particles contribute the majority minerals composition in REE-rich laterite soil samples. This implies that REE adsorption process on adsorbing minerals, specifically kaolinite is dominant in the soil samples, as evidenced by the extraction rate through single-step extraction analysis, which shows that the rare earth elements and yttrium (REY) ion exchangeable percentage is greater than 75%. The negative Ce/Ce* anomaly (<0.07) value on the REE-rich soil sample, as well as the extraction percentage of thorium (non-detect) and uranium (<0.68%), indicate that the research area, which consists of volcanic rock, has the potential for the presence of REE ion-adsorption clay deposit. In conclusion, the findings of this study provide insight into the potential occurrence of REE ion adsorption clay deposit from volcanic rock (Serian Volcanic Formation) covering an estimated area of 510 km2 in the Western Region of Sarawak.