Distribution and geochemical characteristics of barite and barium-rich rocks associated with the Broken Hill-type Gamsberg Zn-Pb deposit, Namaqua Province, South Africa
Bedded barite forms a distinct component of four Broken Hill-type deposits in the amphibolite-facies metamorphosed Bushmanland sequence of South Africa. In the Gamsberg Zn-Pb deposit, barite has been effectively fractionated from the base metal sulphide facies and concentrated as a separate deposit towards the eastern part of the Gamsberg inselberg. The barite occurs as massive to laminated layers, associated with other oxidised lithologies, such as hematite-quartz rocks and manganiferous iron formations. Geochemical and isotopic evidence favours a non-marine, hydrothermal origin of the Ba and supports a close genetic relationship with the base metal sulphide ores. Fractionation of these hydrothermal components occurred due to redox conditions within the depositional basin, which restricted base metal precipitation to an anoxic basin facies and barite to an oxidised shelf facies. The spatial distribution of barite and other oxidised lithologies indicate that redox transitions occurred immediately before and after deposition of the base metal sulphide ores. Such rapid facies changes are typical for sediment-hosted Zn-Pb deposits, reflecting tectonically induced sub-basin formation and reactivation of feeder conduits, which represent important prerequisites for the formation of these deposits. In addition, the presence of barite in the Bushmanland deposits indicates that the ore-forming fluids were reduced, since Ba cannot be transported in the presence of sulphate. Reduced fluids are sensitive to temperature changes and a decrease in brine temperature most probably resulted in the termination of base metal sulphide deposition. Post-dating deposition of the sulphide ores, Fe, Mn and Ba were still supplied to the system and precipitated as manganiferous iron formations and bedded barite lenses that overlie the sulphide horizon. Anomalous concentrations of Ba within the wall rocks of the Aggeneys-Gamsberg deposits indicate that hydrothermal discharge commenced before and continued after peak base metal sulphide deposition. The presence of this Ba halo has implications for exploration of similar deposits in the Namaqua Province. © 2005 Geological Society of South Africa.