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Chemical calculations based on the molar quantity of neutralizing cations (Na, K, Mg, Ca, Mn) released to solution associated with the acidic dissolution of minerals provide a method to chemically quantify the acid-neutralization capacity (ANC) of carbonates, silicates, simple assemblages of mixed minerals, and waste-rock samples obtained from international mining operations. The acidity neutralized by each cation is equivalent to its valence within the mineral structure. Fe and Al are hydrolyzed during the ANC back-titration and thus are effectively non-acid-neutralizing cations. Sulfur derived from pyrite oxidation is equivalent to the release of two moles of H + , although non-acid-forming S (e.g., gypsum) should be addressed. Calculations based on these principles for the samples tested correlate well with the ANC determined by titration-type tests.

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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.Our 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 nonacid 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.Datasets 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.

Environmental contamination from mines producing acid rock drainage, which is caused by sulphide mineral oxidation, represents one of the most significant environmental problems facing the international mining industry. This work investigates the mineral morphological effects on the rate of pyrite oxidation and the influence of relict morphological features on rapid oxidation and thus acid generation rates. Laboratory-based kinetic tests were performed on potentially-acid forming rock by measuring changes in pyrite mineralogical compositions, metal release and acid generation over time. The rate of pyrite oxidation is strongly dependent on the reactivity of two pyrite morphological forms (euhedral and framboidal). After 210 days 70Á100% of all framboidal pyrite had undergone complete oxidation, which contributed to an initial high acid generation rate (peak concentration of 2927 mg L (1 CaCO 3 after 120 days); subsequent acid generation rates (1730 mgL (1 CaCO 3 after 390 days) were substantially lower. Scanning Electron Microscopy (SEM) micrographs clearly show the persistence of larger euhedral pyrite grains as a contributing factor to this on-going acidity after 390 days. Samples collected from laboratory humidity cells after 390, 480 and 720 days showed evidence of preferential dissolution associated with these large pyritic overgrowth textures. Clearly evident are prior relict framboid networks within larger euhedral pyrite grains suggesting that oxidative dissolution may be related to internal crystallographic defects associated with the overgrowth textures in these samples.

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