3 Mine drainage and downstream water chemistry
Prediction of mine drainage chemistry is economically and environmentally significant for all parts of the mining cycle. Mine drainage chemistry influences mine planning, mine operations, and mine closure and post-closure activities and costs. During mineral exploration a cursory prediction of mine drainage chemistry can be used to prioritise exploration targets. During mine planning and operations, detailed predictions of mine drainage evolution will assist with water management and treatment. Prediction of mine drainage chemistry determines treatment requirements during mine closure and post-closure. The following section covers the types of information that should be collected to predict mine drainage chemistry.
Acid generation from rocks disturbed by mining and the subsequent formation of AMD has caused significant ecological impacts on streams on the West Coast. Therefore prediction of acidity from rocks disturbed by mining is a key piece of information required to determine water quality downstream of proposed mines. As such, rocks that are disturbed by mining are separated into two groups: potentially acid-forming rocks and non-acid-forming rocks. The acid-forming potential of rocks disturbed by mining is highly variable and can be predicted qualitatively and quantitatively. Qualitative prediction of mine drainage chemistry can be completed by evaluation of background information. Quantitative predictions are made by a number of standardised laboratory procedures conducted on fresh rock samples. Other factors that can lead to aquatic ecological impact in mine drainage include elevated trace element concentrations and increases in suspended sediment.
3.1.1 Commodity and region
Historical information can enable a qualitative desktop assessment of likely mine drainage chemistry. The key information to be determined is the geological formation(s) that will be disturbed by a proposed mine, which can be determined from knowledge of the commodity being mined and the location. Assessments based on historical information are indicative and analyses of the various rock types that will be disturbed at a proposed mine site are required to provide quantitative assessment.
A geological formation is a group of rocks that are recognisable over a large area (typically >10s of km2). These groups of rocks have similar characteristics such as age, composition, geological history and depositional process. Rocks of a single geological formation have a limited range of mineral compositions, and mineral composition of rocks disturbed by mining determines the characteristics of mine drainage. Therefore identification of the different geological formations that will be disturbed by a proposed mine can provide qualitative information on likely mine drainage chemistry (Pope et al. 2006, in press; Craw et al. 2008).
The way in which mining disturbs the rocks will influence the resultant drainage. Mining disturbances may range from groundwater chemistry changes in the rocks surrounding mines to removal, crushing, pulverising and chemical alteration of rocks during ore processing. Of particular interest for the prediction of mine drainage chemistry is the acid-forming or neutralising potential of rocks and the trace element concentration and mobility. Rocks that are unlikely to form acid during mining are called non-acid forming (NAF); rocks that are likely to form acid during mining are called potentially acid forming (PAF). PAF rocks can acidify drainages naturally in the absence of mining and the term acid rock drainage (ARD) is used to differentiate natural acidic discharges from mining discharges, which are termed acid mine drainage (AMD). Both NAF and PAF rocks can release trace elements at elevated concentrations.
New Zealand is the 1:250 000 Q Map series (GNS Website - Relatively few geological formations host gold or coal deposits in the West Coast and Southland regions and these differ for each region. Typically gold deposits occur within different geological formations to coal deposits. Figure 7 shows the geological formations that are likely to be disturbed by coal and gold mining on the West Coast and in Southland, along with comments on likely mine drainage characteristics. Detailed information relating to the location and extent of geological formations can be obtained by analysis of geological maps. The most up-to-date compilation of geological maps for http://www.gns.cri.nz/research/qmap/aboutqmap.html). More detailed geological maps for specific areas may be available from university theses, published reports and scientific papers, or from mineral exploration reports (Crown Minerals Website - http://www.crownminerals.govt.nz/cms). Generalised maps and descriptions for each of the geological formations in Figure 7 drawn from these sources are shown in Appendix C.1 (West Coast) and C.2 (Southland).
Where proposed new mine operations overlap or are adjacent to historical mines, information on drainage from historical mines gives an indication of mine drainage chemistry for the proposed operations. However, to ensure these interpretations are valid, care must be taken to ensure both the proposed and historical mines are in the same geological formation. Different geological formations with different mine drainage characteristics can occur adjacently and thus will have different mine drainage chemistry.
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