A Geoenvironmental Model for Orogenic Gold Deposits to Predict Potential Environmental Effects

Orogenic deposits are an important source of gold around the world, with associated environmental impacts. New Zealand has a broad spectrum of these orogenic deposits, providing an ideal setting to develop a general conceptual model that can be used to predict potential environmental issues associated with their exploration and mining. This model provides a practical and quantitative framework for permitting and managing mine operations, with a focus on downstream water quality. The model has been quantified using data collected from natural mineralised occurrences, two active mines, and numerous historic mine sites. Mine waters in and around orogenic deposits almost invariably have a pH of 7-8. Minor localised acidification occurs in excavations and waste rock, but it is readily neutralised by the abundant calcite in the host rock. The ore can have strongly elevated levels of As and Sb; the proportions of these metalloids are controlled by geological factors, especially the crustal level of emplacement and the structure of the mineralised rocks. Agitation of sulfide mineral slurries during processing and pressure oxidation in the processing system can lead to dissolved metalloid concentrations of tens to hundreds of mg/L in mine tailings waters. The gold also commonly contains Hg, up to 40 wt%, and discharge of this Hg to the atmosphere during processing is possible, but Hg is not significantly mobilised from other mine rocks. High metalloid concentrations are the most significant environmental issue, but are decreased by adsorption to iron oxyhydroxide as the water percolates through mine rocks and tailings. Even so, additional treatment may be needed to lower metalloid concentrations for discharge to rivers and lakes.