MINERALOGICAL AND GEOCHEMICAL CONTROLS OF ARSENIC IN A GOLD MINE TAILINGS DAM. A CASE STUDY AT ANGLO GOLD ASHANTI, OBUASI, GHANA

The Anglogold Ashanti gold mine at Obuasi, Ghana, processes gold ores that contain approximately 1% arsenic, using the bacteria oxidation (BIOX) pretreatment method. Produced tailings are stored in the Sansu tailings dam. Weathering and oxidation of these materials mobilize As and other toxic substances into the impoundment. Some mineralogical and geochemical controls of arsenic distribution in the tailings dam have been studied. Sediment mineralogy was determined by X-ray (XRF and XRD) techniques, electron microprobe analyses and petrographic methods. Tailings pond, interstitial (pore), and seepage waters were analyzed. Pyrite and arsenopyrite are principal ore minerals remaining in the dam and their crystals show rims of oxidation (hematite/goethite), which are more prominent at depths of high arsenic concentrations. Arsenic III is the dominant form in the dam. Total As concentrations in the dam vary with depth and are influenced by the pH, Eh, and DO. Illite, kaolinite, and carbonates, present in the dam, act as pH buffers, and consume the generated acid during pyrite oxidation and raise the pH to near neutral; a condition that mobilizes arsenic that may be transported into the seepage ponds.