Correlation of acid base accounting characteristics with the Geology of commonly mined coal measures, West Coast and Southland, New Zealand

摘要

Assessment of the potential for waste rock at coal mine sites to produce acid mine drainage is an important part of mine planning and operations and is commonly assessed using acid base accounting analyses. The underlying factor that controls mine drainage chemistry is the mineralogical composition of the coal measures sequences which in turn is controlled by several geological factors including provenance, depositional environment, diagenetic processes and tectonic setting. Therefore, coal mine drainage chemistry is directly linked to the geology of coal measures sequences.rnOur research uses acid base accounting data to identify relationships between coal measures geology and acid production potential for Brunner, Paparoa and Morley Coal Measures as well as Gore Lignite Measures. Our data is from several sources and reflects areas in these sets of coal measures where mining or exploration is underway. In general Brunner Coal Measures are strongly acid producing especially fine grained rocks and this relates to a coastal depositional environment, common diagenetic pyrite, overlying transgressive marine rocks and compositional maturity. Morley and Paparoa Coal Measures are mostly non-acid forming relating to fluvial - lucustrine depositional environments, overlying coal measures, diagenetic carbonates, and possibly carbonates from source rocks. Gore Lignite Measures are mostly non-acid forming, however, some acid forming rocks are present. Gore Lignite Measures are delta plane deposits with occasional marine influence.rnDatasets from Brunner, Paparoa and Morley Coal Measures as well as Gore Lignite Measures also demonstrate some of the limitations of acid base accounting analyses. For example, oxidation steps in acid base accounting analyses designed to dissolve pyrite also react with organic material and this causes a false positive analysis. Other assumptions common in acid base accounting such as use of total S content to calculate pyrite related acidity are inappropriate for some samples because of other forms of S in these samples. Despite the interferences, standard acid base accounting tests and procedures are very useful for first pass predictions of mine drainage chemistry and datasets from commonly mined New Zealand coal measures can be successfully related to geological processes.