Evaluating waste rock mineralogy and microtexture during kinetic testing for improved acid rock drainage prediction

This study integrates detailed mineralogical and microtextural analyses of waste rock with the results of standard kinetic test procedures to identify the mineralogical changes that influence leachate chemistry over time. The integration of mineralogy and texture provides the opportunity for improved mine waste management strategies and acid rock drainage (ARD) prediction.

Waste rock material from an abandoned gold mine in northern Queensland, Australia, was subjected to column leach kinetic testing over a 30 week period. The column feed comprised of a range of waste rock lithologies (porphyritic rhyolite, massive arsenopyrite, massive pyrite +- galena, and semi-massive polysulphide). In total, 12 individual columns were established to represent six lithologies prepared to two different size fractions (-10 mm and -4 mm). The mineralogy and microtextural characteristics of the column feed material was defined using quantitative X-ray diffractometry (QXRD), scanning electron microscopy and laser ablation (LA ICPMS) at the start of kinetic tests, and at 5 week intervals during the length of the tests. These data were directly correlated with leachate chemistry (i.e., pH, SO₄ and select elements).

Results of this study indicated that sulphide oxidation was strongly influenced by the morphology of sulphide minerals, their trace element contents, the presence of mineral micro-inclusions and galvanic interactions with other sulphide minerals. Waste rock with abundant arsenopyrite was consistently the most acid forming, and oxidised to scorodite (enriched in Zn, Pb and Cu). Pyrite was commonly As-rich as indicated by LA-ICPMS mapping. QXRD results indicated that the abundance of rhomboclase, jarosite, alunite and hydrous ferric oxides increased over time. Galena weathered rapidly to porous anglesite, particularly when in direct physical contact with pyrite. Sphalerite contents decreased consistently over the 30 weeks implying its oxidation, however few reaction products were directly observed. By week 30, the -4 mm fraction material generated lower pH leachate, higher mass release of elements and sulphate for the majority of samples. This indicates that the particle size used in kinetic tests can exert a significant control on leachate chemistry, especially in the absence of abundant neutralising minerals. This contribution demonstrates the value of integrating mineralogy and microtextural analyses during kinetic testwork to improve the interpretation of sulphide oxidation for better prediction of ARD.