Summary:
The Broken Hill deposits form part of the Bushmanland Group which is a volcano- sedimentary sequence that hosts ore deposits (Reid et al., 1997). This group has metapelitic rocks that are CaNa-Mg depleted. The group’s ore body is divided into two types: a pyrrhotite-galena-sphaleritechalcopyrite ore hosted by quartz-garnet-rich gangue (Black Mountain and Broken Hill) and a pyrite-pyrrhotite-sphalerite ore hosted by quartz- biotite-sillimanite gangue (Big Syncline and Gamsberg). The principal ore minerals are sphalerite, galena and chalcopyrite with garnet and apatite being minor constituents. Barite horizons occur as lateral equivalents of the sulphide ores at Black Mountain and Gamsberg and calcitic marble bands are associated with the Big Syncline and Gamsberg (Moore et al., 1990).
Despite the discovery of Gamsberg zinc deposit in 1972, the ore body continues to be South Africa’s greatest unexploited base metal resource. Regardless of its size, the low zinc and high manganese content of the sphalerite combined with the low zinc price prohibits the development of the deposit. The high manganese content of the sphalerite makes it unsuitable for processing by conventional methods i.e. roasting, leaching/ solvent extraction and/or electrowinning (McClung & Viljoen, 2011). The ore body has two distinct ore types, namely pelitic and garnetmagnetite ore. The pelitic ore is characterized by a high Fe-suphide content whilst the garnet-magnetite ore have higher levels of garnet, FeMn-silicates and magnetite (Schouwstra et al., 2010).
The economic value of the deposit is dependent on the mineralogical characteristics and associations present in the ore assemblage, i.e. the loss of sphalerite to tailings in flotation circuits due to inadequate particle separation reduces the economic viability of the process option. Given the low grade of the ore and the sensitivity to manganese level in concentrates, the pelitic ore is the more attractive option to process hence this ore type is discussed in detail below.
Mineralization for this ore reserves occurs within the Gams formation and is hosted by a thin, less than 1 km thick deformed and metamorphosed, continental margin volcano-sedimentary rocks (McClung & Viljoen, 2011). According to Porter GeoConsultancy (2001), the Gams ore comprises three units, namely:
- A lower member composed of a diverse suite of quartz-felspar-garnet-clinopyroxene rock, garnet- clinopyroxene-feldspar marble and garnet-clinopyroxene-quartz-magnetite rocks.
- A middle sulphide zone with quartz-garnet-amphibole rocks and graphitic quartzsillimanite- muscovite-feldspar containing major amounts of pyrite, pyrrhotite, sphalerite and galena.
- An upper unit of diverse garnet, pyroxenoid, clinopyroxene, orthopyroxene, amphibole, quartz, apatite, carbonate, magnetite, hematite and barite bearing rocks.
The Gams ore is underlain by white quartzite with minor schists and conglomerate which succeeds the Hoogoor Suite leuco-gneisses. The deposit is located at the contact between the underlying quartzite and overlying Koeris formation (Porter GeoConsultancy, 2001). It is confined to a single, large sheath-fold that forms a steep-sided inselberg measuring 7 km by 5 km (McClung & Viljoen, 2011). Zinc is only found in sulphides in the Gams ore body; hence, the middle zone is discussed in detail.
The sulphide zone is irregularly present within the shear fold structure and is weakly mineralized containing 1 to 4 % Zn. The intervals of weaker mineralization include a number of higher grade cores with 7 % Zn embracing smaller cores containing of above 10 % Zn. The sphalerite ore rich in pyrrhotite and pyrite is located near the centre of the Gams ore Formation, flanked by iron sulphides, predominantly pyrite below and pyrrhotite above. These iron sulphides are in turn sandwiched by magnetite and hematite rich zones towards the outer margins of Gams Formation (Porter GeoConsultancy, 2001). There are six sphalerite populations in the Gams ore: 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 (McClung & Viljoen, 2011).