Estimation of Calcium and Magnesium Carbonate Neutralization Potential for Refined Acid-Base Accounting Using Electron Microprobe and X-Ray Diffraction

Acid-base accounting (ABA) is intended to estimate the quantities of minerals potentially capable of generating and neutralizing acid using chemical procedures. The neutralization potential (NP) component of ABA is more complex than acid potential (AP) because the minerals quantified in the analytical result may or may not contribute significantly to acid neutralization under field conditions. Calcium and magnesium carbonates (e.g. calcite, dolomite) are highly effective acid neutralizing minerals. However, alumino-silicates are digested to varying degrees during the NP determination but have weak actual acid neutralizing effect at near neutral pH due to the release of aluminum and to kinetically-slow reaction rates. NP determinations can therefore over-estimate effective NP and lead to non-conservative acid-base accounts. This paper presents a method for calibrating routine NP and/or inorganic carbon determinations to calcium and magnesium carbonate content using electron microprobe to estimate the actual compositions of carbonate minerals, and quantitative x-ray diffraction (XRD) to estimate the quantities of the minerals. A small lithologically representative sub-set of samples is used to develop correction factors which are then applied to routine ABA data. Application of the method is illustrated for the Galore Creek Project, a copper-gold porphyry-type deposit in northwestern British Columbia which has the carbonate minerals calcite (CaCO3), dolomite (CaMg(CO3)2), ankerite (CaFe(CO3)2), siderite (FeCO3) and rhodochrosite (MnCO3). Additional