| Summary: | 碩士 === 國立成功大學 === 地球科學系碩博士班 === 100 === Chemical weathering of sulfide-bearing ore rocks releases abundant heavy metals and hydrogen ions, and can make mine drainage highly acidified and enriched in heavy metals. Such an acid mine drainage (AMD) is often a threat to the environment. Golden Falls and its downstream Lian-Dong Creek and Lian-Don Bay are a typical example of AMD contamination in the abandoned Chinkuashih Au-Cu deposits, northern Taiwan. The riverbed is covered with abundant iron oxyhydroxides having significant arsenic, sulfur, and heavy-metal contents. This study was deeply inspired by the example. The aims were to investigate the drainage characteristics and precipitate mineralogy of the Changjen first mine tunnel in Chinkuashih and help understanding possible sources or transitional precipitates of contaminants for AMD. Stalactic and encrusting precipitates of a number of minerals and associated waters were collected from the tunnel and analyzed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photometry, and inductively coupled plasma emission spectrometry (ICP-OES).
The XRD, SEM, and TEM results and compositions of dissolved solids indicated that crimson stalactites consisted of 6-line ferrihydrite and goethite, reddish orange stalactites were composed of goethite and schwertmannite with up to 9.5 wt.% As2O5, and black-skinned stalactites comprised birnessite with interlayer cations dominated by Ba and K, and a yellow body of 2-line ferrihydrite. The tunnel walls at various sites were encrusted by goethite, tyrolite, felsobanyaite, and jarosite precipitates. Most of the precipitates were very poorly crystalline and had non-stoichiometic compositions containing P, As, Mn, Zn, Cu, Pb, Sb, Sn, Cr, Ni, Ba, or Al. Some of the toxic elements were the main constituents of the precipitates, e.g., up to ca. 9 and 15 wt.% of As2O5 in the felsobanyaite and tyrolite, respectively. The waters associated with the precipitates had pH of 3.81~6.95. Geochemical modeling with PHREEQC suggested that the saturation index with respect to felsobanyaite was positive in the corresponding water whereas the other waters were all undersaturated with respect to their coexisting minerals, possibly due to variations in water chemistry with time and/or limited thermodynamic database that are currently insufficient for consideration of the variable microbial effects and mineral stoichiometry.
The number of types and the compositional ranges of the identified precipitates were apparently greater than those found in common AMD sites but rather distinct, implying that the mine drainages were strongly influenced by local changes in lithology. The poorly crystalline precipitates can be regarded as a metastable host of arsenic and heavy metals. They can be dissolved upon subtle changes of chemical environments, releasing potentially toxic contaminants into the surface AMD and streamwaters.
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