Contrasting genesis of lateritic bauxite on granodioritic and andesitic rocks of Mempawah Area, West Kalimantan
Abstract
The lateritic bauxite deposits of Mempawah Area, West Kalimantan were formed by chemical weathering of Cretaceous granodioritc and andesitic rocks. They occur locally on low hills surrounded by swampy areas. Detailed surface geological mapping, test pits, mineralogical and geochemical analyses were performed to determine characteristics and genesis of bauxite from different parent rocks. From the bottom upward, the deposits are generally composed of fresh parent rocks, alterite, bauxite, and lateritic soil with a few sparse conakryte at the top of the bauxite zone. Bauxite on granodioritic rocks is brownish-red, massive, boulder- to gravel-sized concretion in clay matrix, consists of kaolinite, quartz, gibbsite, goethite with minor magnetit and hematite meanwhile, andesitic rocks form reddish-brown bauxite with more goethite. SiO2 as mobile compound decreases significantly in neutral pH leaching process, meanwhile Al2O3 and Fe2O3 precipitate as residual materials and form bauxite. Enrichment anomaly of bauxite on andesitic rocks is caused by physicochemical changes from hydrothermal alteration. Bauxite was formed by indirect bauxitization through leaching of primary minerals in tropical-humid climate.
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