Development of a textural acid rock drainage index for classifying acid formation

Textural analyses are largely absent from predictive ARD (acid rock drainage) assessments despite the direct control of texture on acid formation. Therefore, the ARD Index (ARDI) was developed as a simple textural evaluation to be used as a stage-one test as part of an integrated geochemistry-mineralogy-texture (GMT) approach to predicting ARD. Parameters A, B and C (ranked from 0-10) examined contents, degree of alteration and morphology of sulphides, whilst parameters D and E (ranked from -5-10) 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 tested using samples from an abandoned lode-gold mine and an operational iron-oxide copper gold mine, both located in Queensland, Australia. Samples were mesotexturally grouped (A-Q) and analysed as per the GMT approach. The ARDI identified group J (quartz-pyrite), H (quartz-arsenopyrite-pyrite) and G (quartz-galena-sphalerite) from the lode-Au operations as the most acid forming.