Geochemical and isotopic studies of bauxitization in the Tatun volcanic area, northern Taiwan

Abstract

The Tatun volcanic area in the northern tip of Taiwan is composed of Pleistocene andesite and andesitic tuff. Its high relief causes extraordinarily heavy rainfall (> 3000 mm yr −1), especially during the winter monsoon season. Bauxite deposits were formed on the windward northern slopes. Soil samples from two profiles in the bauxite deposits were studied by chemical analysis, X-ray diffraction, differential thermal analysis and hydrogen and oxygen isotopic analyses. The soil profiles can be divided into two zones by chemical characteristics. In the upper zone (zone A), the chemical composition of the soil samples undergoes little change except depletion of Na and Ca due to leaching. The major minerals are quartz, chlorite and illite. In the lower zone (zone B), all major elements suffer rapid dissolution except Al 2O 3. Gibbsite grows drastically at the expense of clays, while quartz diminishes sharply. Fe 2O 3 and TiO 2 are enriched at the boundary between the two zones due to coprecipitation of Fe and Ti as hematite and goethite, probably as a result of increasing oxygen fugacity. Mobilization of Fe in zone B is attributed to its low Eh. Thus zone A is inferred as a permeable layer and zone B as an aquifer. The weathering rate in this area is estimated to be 12 cm ka −1. The most thoroughly leached soil samples contain up to 80% gibbsite. The high gibbsite contents and the analyses of coexisting waters allow hydrogen and oxygen isotopic fractionation factors between gibbsite and water to be calculated as 0.992 ± 0.002 and 1.016 ± 0.001, respectively.