GHD | Oceana Gold NZ Ltd | 12552081 | Geochemical Assessment Wharekirauponga Underground Mine (WAI985-000-REP-LC-0013) 6 3. Geochemical Assessment 3.1 Acid and Metalliferous Drainage Acid and metalliferous drainage (AMD) is a broad term for the natural process of sulphide oxidation (which occurs when rocks containing sulphide minerals such as pyrite are exposed to air and water) leading to the formation of acid drainage and metalliferous drainage. Although AMD is a naturally occurring phenomenon, this process can be exacerbated by external activities such as mining that reduce particle size and increase oxidising surface areas. Contaminants are often mobilised by water and can then report into the downstream receiving environment. Elevated metal concentrations and depressed pH levels in waterways need to be avoided as they can create toxicity issues for aquatic ecology leading to chronic or acute health issues. AMD can be subdivided into three main categories: 1. Acid Rock Drainage (ARD) – acidic, low pH drainage caused by the oxidation of acid producing sulphide minerals and generally contains toxic heavy metals. 2. Neutral Metalliferous Drainage (NMD) – where the acid generated from sulphide mineral oxidation is neutralised by other minerals such as carbonates resulting in the drainage has circumneutral pH values while containing toxic heavy metals 3. Saline Mine Drainage (SMD) – circum-neutral to alkaline in pH with elevated sulphate concentrations. Adverse environmental effects from AMD due to a depressed pH and/or elevated trace metal concentrations can be avoided by sufficient characterisation, handling practices, and storage and/or treatment strategies. As outlined in this report, characterisation of the spoil material generated from the advancement and excavation of an access tunnel and excavation associated with mining out the Wharekirauponga Orebodies and its AMD potential is a key consideration in the development of appropriate practices and strategies to manage potential adverse outcomes from AMD. This is because the nature and likely geochemical composition of the highly mineralised spoil material from the project (at least in part) suggests that that the spoil may generate AMD. Pyrite (FeS2) is the predominant “acid” forming sulphide mineral present in the area. When pyrite is exposed to air and water, it decomposes into water-soluble components, including ferrous iron (Fe2+) and sulphate (SO4) and generates acid (H+). The reduced water-soluble components are then further oxidised to form ferric iron (Fe3+) and water. The formation of low solubility ferric iron (Fe3+) in water leads to the precipitation of ferric hydroxide type minerals (Fe(OH)3 - an orange precipitate); a process that generates additional acidity (H+). This process can be described using the following chemical reaction: FeS2 + 3.75 O2 + 3.5 H2O → Fe(OH)3 + 2SO4 2- + 4H+ Acidic waters increase the mobility of trace elements that can be elevated as a result of the mineralisation. Acid (H+ ions) generated by sulphide mineral oxidation can be neutralised by carbonate minerals such as limestone (CaCO3) such that the drainage is no longer acidic (low pH), but can still contain elevated metals concentrations: CaCO3 + 2H+ → Ca2+ + H2O + CO2 The actual potential for, and rate of oxidation of pyrite (and other sulphide minerals), and the potential impact to the receiving environment is dependent on many factors. These factors include the concentration of the sulphides in the spoil material, morphology of the sulphides, oxygen concentration and exposure time,
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