Mineralogical characterisation of an antimony tailings impoundment located in the Murchison Greenstone Belt, South Africa: Implications for metal recovery and environmental impact

The growing demand for critical and strategic materials, coupled with environmental concerns, has renewed interest in mine tailings. However, limited geochemical and mineralogical data on these repositories constrain their effective reprocessing and the evaluation of environmental risk. The present study serves to present an integrated geochemical and process mineralogical characterisation of a tailings impoundment derived from historical antimony mining in the Murchison Greenstone Belt, with emphasis on antimony recovery and environmental assessment. Five vertical auger holes were drilled according to a site-specific grid spacing of 180 m, in order to obtain material for study. Antimony (Sb) concentrations range from 3 740 to 12 125 ppm, while gold (Au) contents range from 90 to 2 200 ppb. Major oxides are dominated by SiO2 (41.7 to 51.9 wt%), MgO (9.6 to 19.0 wt%) and Fe2O3 (6.0 to 11.1 wt%). An FEI 600F Mineral Liberation Analyser was employed to determine the modal and relative mineralogy, mineral associations, Sb deportment, liberation characteristics, and grain-size distributions. Sb occurs predominantly in sulphide phases associated with silicate and carbonate minerals, and these sulphide phases are preferentially concentrated at the bottom of the tailings impoundment. In contrast, Sb oxides are more concentrated near the surface, probably due to oxidative alteration. Integrated geochemical and mineralogical results demonstrate an economically significant resource of Sb, with an estimated 94 457 tonnes of contained Sb, which is predominantly hosted in recoverable sulphide phases. The tailings are characterised by a range of coarse to fine particle sizes with varying degrees of locking among the Sb minerals present. Based on the current data, it is considered that further milling may be required to achieve sufficient liberation of Sb phases and to improve metal recovery efficiency. Flotation and bulk alkaline leaching present promising and environmentally benign routes for recovering Sb from fine-grained, low-grade ores. Geochemical indices such as geo-accumulation index, as well as enrichment factor, were calculated and applied to evaluate levels of Sb enrichment and associated potential environmental risk represented by the samples collected. Elevated geo-accumulation index and enrichment factor values confirm strong Sb and arsenic (As) enrichment. Acid-base accounting results indicate that the tailings are non-acid-generating due to the abundance of carbonate minerals, which buffer acidity and restrict metal mobility. The limited oxidation potential and stable neutral to slightly alkaline characteristics of the Murchison Greenstone Belt Sb tailings are indicative of low environmental risk, supporting the potential for comparatively safe and efficient resource recovery. However, inhalation and ingestion of Sb- and As-laden dust emanating from the tailings, by residents and livestock in surrounding communities, remain an area of concern. This aspect should also be carefully considered prior to commencing tailings retreatment.