Development of a Textural Acid Rock Drainage Index for Classifying Acid Formation

Abstract

Textural analyses are largely absent from predictive ARD (acid rock drainage) assessments despite the direct control of texture on acid formation. The objective of this study was to develop a simple method for textural evaluation termed the ARD Index (ARDI) in order to categorise the acid-forming potential of intact samples. The ARDI is intended for use alongside routine predictive geochemical and mineralogical analyses as part of a geochemistry–mineralogy–texture (GMT) approach. The ARD Index (ARDI) assessed iron–sulphide minerals individually by five categories A–E, specifically chosen based on the direct influence on acid formation. Parameters A, B and C (ranked from 0 to 10) examined contents, degree of alteration and morphology of sulphides, while parameters D and E (ranked from –5 to 10) evaluated the neutralising mineral content and the spatial relationship between acid-forming and neutralising minerals. Scores from each category were totaled with values 50–41 considered as extremely acid forming (EAF); 40–31 as acid forming (AF); 30–21 as potentially acid forming (PAF); 20–10 as not acid forming (NAF); 10–0 as NAF/ANC and –1 to –10 as having acid-neutralising capacity (ANC). The ARDI was most efficiently performed on both a meso-scale and micro-scale. Eighty-one samples from an abandoned lode-gold operations and an operational iron-oxide copper gold mine (located in Queensland, Australia) were used to evaluate the ARDI. Samples were mesotexturally grouped (A–Q) and in addition to ARDI evaluations, geochemically and mineralogically analysed through static tests, elemental microanalysis (EA) and quantitative X-ray diffractometry (QXRD). ARDI classification identified lode-Au samples as the most acid forming, with group J (quartz–pyrite) classified as EAF, group H (quartz–arsenopyrite–pyrite) as AF and group G (quartz–galena–sphalerite) as PAF. When used in conjunction with paste-pH and sulphide-sulphur values, classifications were largely in agreement with traditionally used net acid-producing-potential/net acid generating (NAPP/NAG) geochemical methods. This indicates that the ARDI has the potential to be used as a simple textural screening method in predictive ARD studies.