Mineralogical Assessment of the Metamorphosed Broken Hill Sulfide Deposit, South Africa: Implications for Processing Complex Orebodies

Abstract

The Broken Hill Pb–Zn–Cu–Ag ± (Au) deposit is located in northwestern South Africa, ~750 km NNE of Cape Town in the Northern Cape Province of South Africa. In conjunction with the other deposits of the world-class Aggeneys-Gamsberg district (i.e., Broken Hill-Broken Hill Deeps, Swartberg/Black Mountain, Big Syncline, and Gamsberg deposits), the Broken Hill and associated deposits are of particular importance as they represent the only active lead and zinc mining operation in South Africa. Yet, in spite of their economic importance, the mine reports diminished recovery rates compared to similar mines, regularly incurs penalties for high concentrations of deleterious elements and no credit for the small quantities of Au recovered. Therefore, the aims of this investigation are to provide a detailed mineralogical assessment of the sulfide minerals to provide new insight that could aid in improving the ore recovery and potential environmental concerns. A detailed investigation of the Broken Hill deposit highlights the complexity of the sulphide assemblage, which comprises various types of base metal and iron-sulfides/sulfosalts, texturally and chemically modified during subsequent phases of deformation and metamorphism. The high degree of complexity and heterogeneity within the deposit may explain the poor recovery rates. Therefore, mineralogical investigations should be conducted on a regular basis, ensuring efficient and optimal recovery. Detailed mineralogical investigations of the sulfide minerals reveal a high level of complexity with multiple varieties of base metal and iron sulfide, and sulfosalt minerals deposited during several paragenetic stages, which have been confirmed by whole rock and mineral chemical analyses. In spite of these obvious complexities, and analogous to the paragenetic sequence of previous investigations, four paragenetic stages of sulfide deposition have been identified at the Broken Hill deposits. They are: (T 1) Primary (pre-metamorphism); (T 2) early metasomatic (retrograde-metamorphism); (T 3) late metasomatic (retro-/pro-grade-metamorphism); and (T 4) late open-space-fill (retrograde metamorphism). Primary (T 1) sulfides occur within primary magnetite quartzite where they comprise massive to annealed groundmass bodies. Early metasomatic (T 2) sulfides are restricted to remobilized and reconcentrated massive magnetite to magnetite quartzite and embody the bulk of the minable sulfide bodies. In contrast, late metasomatic (T 3) sulfides are characterized by cataclastic sulfide matrix breccias, while the final stage (T 3) occurs as a late open-space-fill in paragenetically younger sulfides, oxides, and rarely silicates. In conclusion, this investigation provides insight into the mineralogical information that can be used to improve ore recovery and mitigate potential environmental concerns. The high degree of complexity and heterogeneity within the deposit may explain the poor recovery rates. Therefore, further mineralogical investigations should be conducted on a regular basis, ensuring efficient and optimal recovery.