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

Abstract

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.

Factors that influence coal mine drainage chemistry West Coast, South Island, New Zealand

“…Reactions with some minerals, such as carbonates, can have an acid-neutralising effect that mitigates AMD and therefore the chemistry depends on the relative abundances and reactivity of acid-forming minerals and acid-neutralising minerals (Pope et al 2010) under sitespecific geological, hydrological, and environmental conditions. The abundance of acid-forming and neutralising minerals can be quantified by standardised rock analyses (Kleinmann 2000;Smart et al 2002).…”

“…There are standard geochemical methods to predict the acid producing characteristics of rocks (Kleinmann 2000;Smart et al 2002) and rock chemistry results from New Zealand coal measures West Coast coal mine drainage chemistry 119 (Hughes et al 2004;Pope et al 2006;Craw et al 2008) have been published and reviewed (Pope et al 2010).…”

“…Publications including these types of analyses are available (Hughes et al 2004;Pope et al 2006;Craw et al 2008) and there are substantial datasets held by mining companies. Interpretations of acid base accounting data from previous studies are summarised in (Pope et al 2010). …”

A Review of Static Tests and Recent Studies

“…Seven techniques are reviewed in this study, which determine the sample's maximum Acid Production Potential (APP) with its maximum Neutralization Potential (NP). These seven methods are (i) Acid-Base Accounting (ABA), (ii) Modified ABA (MABA), (iii) paste pH, (iv) Net Acid Generation (NAG), (v) Sequential NAG test (SNAG), (vi) Kinetic NAG test (KNAG) and (vii) Acid Buffering Characteristics Curve (ABCC) test (Ehinola and Adene, 2008;Komnitsas et al, 2009;Hesketh et al, 2010b;Huges et al, 2007;Miller et al, 1991;Pope et al, 2010;Shu et al, 2001;;Weber et al, 2004). Each test uses separate methods to evaluate the capacity for acid generation and neutralization.…”

“…• This method does not account for a kinetic rate (Bradham and Caruccio, 1990 Paste pH method: The paste pH method is used to measure a mixture of soil and deionized water that form a slurry or paste together (Pope et al, 2010). The airdried sample is mixed with deionized water at a 1:1 (w/w) ratio and the pH is then measured with a pH meter, calibrated at pH 4.00-7.00.…”