The Gamsberg zinc deposit, located in the Northern Cape Province, represents South Africa’s greatest and most important unexploited base metal resource. Yet, in spite of its size, the low zinc grade combined with the high manganese content of the sphalerite continues to prohibit development of the deposit, as the high manganese content makes the zinc concentrates unsuitable for processing by conventional smelting methods, i.e. roasting, leaching/solvent extraction and/or electrowinning. Although numerous published investigations of the Gamsberg deposit have been undertaken, few have focused on the correlation between petrography and mineral chemistry of sphalerite and their implications on mineral processing. In accordance, the aims of this investigation are to highlight the importance of conducting a detailed geometallurgical investigation, as well as provide some new constraints on the metallurgical behavior of sphalerite and zinc concentrates from the Gamsberg deposit. Detailed analysis of sphalerite by petrographic and various microanalytical techniques reveal a marked absence of correlation between crystal morphology and mineral chemistry, necessitating a chemically based classification scheme. As such, six sphalerite populations where identified. They are: zinc-, cobalt-, iron- and manganese-rich, as well as manganese-enriched and manganese-poor. The zinc- and cobalt-rich populations display high zinc contents and low concentrations of iron and manganese, while the iron-rich population displays higher iron contents. Dominating the sphalerites of the mineralized pelitic schist is the manganese-enriched population. Of volumetrically lesser importance are the manganese-poor and -rich populations. The manganese-rich population contains high levels of iron, manganese, cobalt and cadmium relative to the zinc-rich population. In contrast, the manganese-poor population displays similar concentrations of cobalt and cadmium to the zinc-rich population, with iron and manganese intermediate between the zinc- and iron-rich populations. Comparisons between the sphalerite populations and milling size fractions reveal a consistent correlation between sphalerite mineral chemistry and size fractions with the coarser-grained concentrates being economically less favorable compared to the finer-grained concentrates, which display higher zinc contents. Combined with other lines of evidence, this suggests that the sphalerites of mineralized pelitic schist exhibit extensive solid solution that will have an effect on process optimization. For instance, it suggests that milling to a finer grain size will not remove or reduce the iron and manganese content. Another concern to processing will be the variable mineral chemistry of sphalerite, which might cause some sphalerites to display a delayed response during flotation or misreport to the lead concentrate. In spite of this, using solvent extraction, the elevated manganese content of the concentrates could be controlled, while simultaneously recovering zinc and manganese for sale. Hence, the recovery of manganese metal as a by-product could potentially enhance the economics of the deposit. Alternatively, other forms of mineral processing and metal extraction must be evaluated to meet the target grades and recoveries.
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Jun 9, 2012
B Davoodi + 1
Open Access