Kyaukpahto gold mine located in the Kawlin Township, Sagaing Region of Myanmar is the first open pit and the largest gold mine of Myanmar. As the gold is exploited by open-pit mining, a large amount of waste rocks are disposed at waste dumps near the mine site. The metal sulfides such as pyrite, arsenopyrite, and chalcopyrite in the waste rocks are exposed to the surface, favoring the oxidation of these metal sulfides and generating acidic water. There is a possibility of AMD generation in the open pit and at the low grade ore dump. This work focuses on assessment of the potential acid-forming waste rocks and characterization of these waste rocks in Kyaukpahto gold mine. Waste rocks and water samples were taken from the open pit, low grade ore dump, and waste dumps and subjected to various tests and analyses such as NAG test, Paste pH test, Paste EC test, ANC test, XRF, XRD, and ICP-MS analyses. Based on the results of the chemical tests and analyses, waste rock samples collected from open pit and low grade ore dump are identified as PAF rocks which particularly concerned with the potential generation of acidic mine water. Two-step batch leaching test indicated that low pH value (pH 4) and elution of metal ions such as As, Al, Fe, Cu, Zn, and SO42− were observed with the high concentration from the samples. Elution of As is higher than other metal ions and this elution process will take place over a longer period than other metal ions. Thus, it is very critical to take appropriate measures against generation of AMD, such as controls on sulfides oxidation, and reduction of metals elution.
Acid mine drainage (AMD) which is also known as acid rock drainage (ARD), causes environmental problems that affect many countries with historic or current mining activities. AMD is resulted from the exposure of sulfide ores and minerals to water and oxygen. When the ore is exposed to generate AMD, sulfate and heavy metals such as iron, copper, lead, nickel, manganese, cadmium, aluminium and zinc are also released to contaminate into that water (Moodley et al., 2017). Acidic, metals-rich waters may also form in spoil heaps, waste rocks and mine tailings, essentially by the same biologically reactions as in mine adits, shafts, pit-walls and pit-floors. Due to the more disaggregated (and more concentrated, in the case of tailings) nature of the acid-generating minerals in these waste materials, AMD that flows from them may be more aggressive than that discharges from the mine itself. Another important consideration is that it poses the potential long-term pollution problem as the production of AMD may continue for several years after mines was closed and tailing dams was decommissioned. (Johnson & Hallberg, 2005). The most common acid-generating sulfide minerals are enlisted as pyrite/marcasite (FeS2), pyrrhotite (FeS), chalcopyrite (CuFeS2) and arsenopyrite (FeAsS). In general, pyrite, the most common sulfide mineral of waste rocks and typical of many oxidation processes during weathering, is oxidized in accordance with the following reactions (Sengupta, 1993):