Acid Forming Potential of Black Shale in Ban Pong Basin, Northern Thailand

Abstract

The black shale and exposed rocks located at an open-pit mine in the Ban Pong Basin, Chiang Mai province, northern Thailand, have been studied to evaluate acid forming potential using acid-base accounting and net acid generation tests. The mine water chemistry was also investigated to observe the potential of acid mine drainage. The results present that pyrite is found in pale brown marlstone, coaly black shale, and black shale with fossil fragments, whereas calcite is observed in all rock types except for coaly black shale. The acid forming potential outcomes show that only the coaly black shale is classified as an acid forming rock because of high pyrite content, while other rocks are categorized as non-acid forming rocks due to the absence of pyrite and/or elevated calcite content. Moreover, the neutral pH and low metal concentration of mine water indicate no acid mine drainage potential in the study area. The hydrochemical facies of water was classified as Ca-Mg-HCO3 type for upstream and downstream of Ca-Mg-SO4 type for mine water. The enrichment of Ca, Mg, and HCO3 in the surface water is attributed to weathering of carbonate material of the surrounding rocks. In contrast, the dominance of SO4 in mine water is perhaps influenced by pyrite oxidation and gypsum dissolution. To summarize, the coaly black shale is an acid forming rock, but it does not produce acid mine drainage since it is surrounded by calcite-rich rocks acting as the acid neutralizer.