GEOLOGY, CHARACTERISTIC, AND GENESIS OF LATERITIC BAUXITE OF MEMPAWAH, WEST KALIMANTAN

• Main Author: Mildan, Deni
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/47326
• Institution: Institut Teknologi Bandung

Bauxite is a product of weathering of alumina-rich rocks, low alkaline, alkaline earth, and silica. Besides, bauxite also can be formed from weathering of metamorphic rocks, sedimentary rocks, carbonate rocks, and as clastics. Mineralogy and characteristic of bauxite vary depending on some main factors, especially texture and mineralogy of parent rock. Characteristic and genesis of lateritic bauxite will affect ore processing. Special handling of bauxite ore with certain characteristics and genesis will help optimizing ore processing to obtain an optimum result. However, research on the characteristics and genesis of lateritic bauxite is lack. Therefore, further research regarding the characteristics and genesis of lateritic bauxite deposit is necessary. The research was conducted in Mempawah, West Kalimantan as one of the lateritic bauxite prospect areas in Indonesia. Methods used in this research were geological surface mapping and test pit. Geological surface mapping was used to determine the geological condition of the research area and identify the parent rock of lateritic bauxite. Test pit observation was used to determine the vertical distribution of lateritic bauxite and the relationship with parent rock. Sixteen thin sections and 12 polished sections were analyzed to identify mineralogy and alteration of minerals in the fresh parent rock, alterite, and bauxite as a result of weathering process. XRD analysis was performed to 2 bauxite samples to identify the mineralogy of bauxite. XRF analysis was performed on 2 fresh rocks, 2 alterites, and 28 bauxites so that the pattern of enrichment of SiO2, Al2O3, and Fe2O3 can be determined. Bauxite samples were taken systematically from test pit every 2 meters. The research area consists of 3 lithological unit, they are andesite, granodiorite, and alluvial deposit. Hydrothermal alteration in andesite is propylitic type. Geological structure is hardly identified because of intensive weathering process. Bauxite in andesite is reddish-brown, massive, boulder- to gravel-sized concretion in clay matrix, consists of goethite, kaolinite, gibbsite, magnetite, and hematite, SiO2 3,493 - 45,655%, Al2O3 47,052 - 93,137%, and Fe2O3 3,277 - 7,293%, with moderate - stromg lateritization. Bauxite in granodiorite is brownish red, massive, boulder- to gravel-sized concretion in clay matrix, consists of kaolinite, quartz, gibbsite, goethite, magnetite, and hematite, SiO2 34,814 - 57,775%, Al2O3 41,503 - 62,906 %, and Fe2O3 0,692 - 2,302%, with weak - moderate lateritization. Bauxite in the research area can be classified as orthobauxite. Bauxite was formed by indirect bauxitization through leaching of primary minerals to become kaolinite then gibbsite. SiO2 as mobile compound decreases significantly in neutral pH leaching process, meanwhile Al2O3 and Fe2O3 total precipitate as residual materials and form bauxite. Enrichment anomaly of bauxite in andesite is caused by physicochemical changes from hydrothermal alteration. Furthermore, contact between groundwater, oxygen, and sulphide minerals during weathering process leads to pH and Eh condition so that Fe, Al, and other heavy metals become mobile. Laterite generally consists of alterite zone, bauxite zone, and latosol zone, along with conakryte at the top of the bauxite zone was formed in tropical-humid climate. Hematite as advanced weathering product has less OH than goethite characterized climate change to become drier.