Mineralogical changes and distribution of heavy metals caused by the weathering of hydrothermally altered, pyrite-rich andesite

The weathering of pyrite-rich andesite in the southeastern part of Korea has led to acid rock drainage, creating severe environmental problems in the region. In this study, the mineral compositions and geochemical mobility of heavy metals in the bedrock, in addition to the weathering products, are investigated. Samples of bedrock and weathering products were collected from two sites in the Yangsan area. Site 2 shows considerably more weathering than site 1, due to the longer duration of weathering. The mineral compositions show that the bedrock at site 2 was more heavily altered by hydrothermal activity. The weathering products at site 2 contain jarosite and goethite with lower pH values than site 1, in which the samples are buffered by calcite. This indicates that the mineral compositions and pH values of the weathering products at the two sites are influenced by the primary mineral compositions of the bedrock and the duration of weathering. With the exception of Pb, the concentrations of heavy metals at site 1 are higher than those at site 2, probably due to higher concentrations in the bedrock. Moreover, the labile and acid-soluble fractions, which can be more readily released into the environment, are higher at site 1, mainly due to the shorter duration of weathering. Accordingly, the weathering process at site 1 carries more potential hazards to the environment than that at site 2. However, chemical weathering is locally impeded by calcite buffering at site 1. Although the concentrations of heavy metals at site 2 are lower than those at site 1, they are being continuously released due to the lower pH and longer and more and extensive weathering.