Mineralogical and textural controls on acid rock drainage formation: a case study from Northern Queensland

A clear understanding of mineralogical and textural characteristics of a mineral deposit are required in order to improve predictions of acid rock drainage (ARD) formation from solid mine waste. A plethora of geochemical tests and protocols have been designed to predict ARD (e.g., static and kinetic tests) and are routinely utilised by industry. However, the estimated costs for total worldwide liability associated with current and future ARD remediation are approximately US $100 billion, indicating that these tests are ineffective (Tremblay and Hogan, 2001 in Hudson Edwards, 2011). To reduce liability costs, predictive tests based on mineralogical and textural ARD characteristics must be developed and implemented at early-stages of the mine life cycle.

This study focuses on mineralogical, textural and geochemical characterisation of solid waste (n=58) from a historic mine located in Northern Queensland. Site geology consists of hydrothermally altered Mesoproterozoic rhyolites. Gold and base-metal mineralisation is hosted within massive quartz veins and is commonly associated with pyrite, galena, arsenopyrite and sphalerite. The runoff and basal seepage from a major waste rock pile (3.9 Mt) enters a pit lake (pH 3.09) and a catch dam (pH 3.89), discharge from which has impacted adjacent creeks.

Waste rock was domained by hand-specimen texture, with sulphide-bearing groups subjected to detailed microanalytical characterisation and kinetic testing. Results showed that groups demonstrating a massive-sulphide texture were the most acid forming. Rhyolite samples containing minor disseminated pyrite in a muscovite altered groundmass were potentially acid forming, but where contained in quartz veinlets, were non-acid forming. This study demonstrates that through integrated mineralogical, textural and geochemical evaluations, ARD sources can be better identified and waste rock management improved