Distribution of lead- and zinc-bearing minerals in the gossan of the Gamsberg Zn deposit, South Africa

The giant Gamsberg massive sulphide deposit is currently being developed by Vedanta Resources. During mine development, the massive gossan zone is stripped in order to expose the sulphide orebody. During a field visit In September 2016, a suite of samples was collected from the gossan in the developing open pit. Three lithologically distinct zones were recognized. The topmost zone is a goethitic hard cap that is also well exposed at the present-day land surface (Rozendaal, 1986). The goethitic hard cap grades down into an iron oxide-poor zone that comprises of very friable siliceous material. The latter is cross-cut by fractures filled by hematite. The third zone of the gossan is marked by an abundance of fine-grained earthy hematite intergrown with semi-friable silica. Vugs are commonly lined by chalcedony. A detailed study revealed the presence of a complex suite of secondary lead and zinc minerals. Zinc is most abundant in the goethite cap – apparently related to the occurrence iron-rich smectite group minerals. Minor amounts of Pb-rich minerals, such as anglesite and members of the corkite-beaudantite were also identified. Most abundant, however, is lead below the goethitic cap where anglesite and members of the mimetite- pyromorphite and corkite-beaudantite series are very common. The lowermost gossan zone consists of mainly fine grained silica and hematite. Lead- and zinc-bearing minerals are conspicuously absent. The results obtained are consistent with observations by Rozendaal (1986) – and the apparent enrichment of lead-bearing secondary minerals in the gossan - relative to scarce nature of galena in the sulphide orebody. This apparent enrichment is attributed to the greater mobility of zinc – relative to lead - in the supergene environment.