GRANITOID CHARACTERISTICS AND RARE EARTH ELEMENTS DISTRIBUTION IN GRANITOID WEATHERED SOIL AND ALLUVIAL CLAY DEPOSITS AT AIR BIAT, WEST BANGKA

• Main Author: Kivlan M.D.P., Mochammad
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/69689
• Institution: Institut Teknologi Bandung

Exploration and research on Rare Earth Elements (REE) deposits on Bangka Island has been carried out on granitoids, granitoid weathered soils, and placer deposits. The focus of REE placer deposits on Bangka Island is still on alluvial sediments in the coarse-fine sand grain size and minimal work on clay grain size. Therefore, this study was conducted to identify the potential of REE enrichment in alluvial clay deposits and compare it with the granitoid weathered soils as its sources on Bangka Island, especially in Air Biat, West Bangka Regency. The number of samples used were two granitoid samples, eight granitoid weathered soil samples, and eight alluvial clay deposits samples. The granitoid and granitoid weathered soil samples were taken from the surface outcrop while the alluvial clay deposits samples were taken from the alluvial tin exploration drilling owned by PT Timah Tbk. Granitoid weathered soil samples were taken per horizon based on the field identification from upper to the lower horizon. The analytical methods used are petrographic and geochemical descriptions, namely XRD (X-Ray Diffraction), SEM (Scanning Electron Microscope), and ICP-MS (Inductively Coupled Plasma – Mass Spectrometry). This method was used to determine the characteristics of granitoids, granitoid weathered soils, and alluvial clay deposits and their relationship to the pattern and control of REE enrichmen on granitoid weathered soils and alluvial clay deposits. Based on the thin section petrography analysis, it is known that all the granitoid samples belong to the S-type alkali feldspar granite. The granitoids are dominated by K-feldspar and quartz followed by biotite, plagioclase, and muscovite. The opaque minerals and sericite are present as secondary minerals with plagioclase and K-felspar are altered into sericite. The accessory minerals are zircon, apatite, and tourmaline. Granitoid weathered soils consists of O, A, B, and C horizons. Quartz minerals dominate in each horizon with decreasing concentrations from A to C horizon. There is a kaolinite as clay mineral in the granitoid weathered soils which indicates that the iv parent rock has undergone intensive chemical weathering. Alluvial clay deposits are dominated by clay minerals such as kaolinite and halloysite followed by quartz and muscovite. Iron oxides and zircon are present in trace amounts. Alluvial clay deposits are divided into three types based on their physical characteristics, namely A-type, B-type, and C-type. A-type is grayish white and deposited in floodplains. B-type is whitish brown and deposited in distal natural levee. C-type is whitish brown with lithic and quartz fragments in the coarse sand-pebble grain size and deposited in proximal natural levee. There is a negative Eu anomaly with LREE concentrations that is higher than HREE concentrations in all samples in the chondrite-normalized spider diagram. The average value of ?REE in alluvial clay deposits is 4.8 times higher than granitoid weathered soils. The REE enrichment in the granitoid weathered soil is controlled by the zircon enrichment as residual minerals in the A horizon. The REE enrichment in the alluvial clay deposits is controlled by the REE ion adsorption on clay minerals. The A-type alluvial clay deposits which is deposited on the floodplain showed a higher REE concentrations than B-type deposited on distal natural levee and C-type deposited on proximal natural levee. The material source and depositional environment affects the process of the REE ion adsorption in alluvial clay deposits. The material deposited in alluvial clay deposits in the study area comes from the erosion of granitoid which contains REE-bearing minerals such as zircon, apatite, and tourmaline. Apatite and tourmaline in the study area have been weathered due to the weathering and erosion processes and then released the elements in the form of ions including REE. The REE ions are then carried away by river along with the material from granitoid erosion and then absorbed by clay minerals in alluvial clay deposits through an ion adsorption process. A calm depositional environment with a pH that is not too acidic as in the floodplain in A-type will accommodate the enrichment of REE. On the other hand, acidic pH conditions and higher river flow energy will make the REE ions easily escape from clay minerals. Groundwater has minimal influence on the REE enrichment in alluvial clay deposits because there is no groundwater basin in the study area so the rainwater that falls will flow directly into the river and not much seep below the surface into groundwater.