ORE MINERALOGY OF THE WORLD-CLASS GAMSBERG ZINC SULFIDE DEPOSIT, SOUTH AFRICA

The Gamsberg deposit is the largest deposit of the world-class Aggeneys-Gamsberg district, located in the Northern Cape Province of South Africa. Since its discovery in 1974, the Gamsberg deposit has and continues to be the country’s largest (265 Mt at 6.1% Zn and 0.3% Pb) and most important unexploited base metal resources. The deposit consists of disseminated to massive bodies of Fe-Zn sulfides and oxides hosted by calc-silicate-rich pelitic schists, which collectively experienced intense deformation and metamorphism during the Namaquan Orogeny (1000-1200 Ma).

A detailed mineralogical and mineral chemical assessment of the Gamsberg deposit has resulted in the definition of four sphalerite populations (i.e. Fe-rich, Mn-poor, Mn-rich, and Zn-rich). Petrographic analyses indicate that the Fe-rich population, which consists of very fine-grained (10’s micron) crystals disseminated throughout the host rock, represent the composition of the original sulfide mass. The Mn-poor and -rich (i.e. Mn-enriched) populations comprise massive, coarsely crystalline, overgrowths and annealed crystals with minute (micron-sized) inclusions of albandite. Combined, these attributes suggest that the Mn-enriched populations represent the composition of the sulfide mass after metasomatic alteration during peak metamorphism. Morphologically, the Zn-rich population is characterized by anhedral to subhedral, fine- to coarse-grained, porous to void-filling crystals commonly intergrown with microscopic phyllosilicates, suggesting growth under retrograde metamorphic conditions.

The apparent mixing line observed between the Mn- and Zn-rich populations, indicates the introduction of a secondary fluid that interacted with the pre-existing Mn-rich population, resulting in the precipitation of the secondary Zn-rich population, which displays a high degree of scatter on a bivariant plot of Zn versus Mn, a feature common to hydrothermally altered minerals.