Base metal mineralisation in the Rooiberg group near Rust de Winter : some characteristics of the hydrothermal system

Abstract

The studied hydrothermal system operated in an agglomerate layer which forms part of the "Union Tuff member" that occurs in the uppermost part of the Rooiberg Felsites. In contrast to the Sn and F deposits characteristic of this part of the Rooiberg Group, this hydrothermal system in the Rust de Winter area contains predominately base metals and a small amount of gold. It is believed that this hydrothermal system was possibly the product of a granitic intrusion, but represents a later, cooler mineralisation stage further away from the body. The mineralised areas in the agglomerate layer can be divided into three types, based on differences in the host rock, mineralisation style and mineral associations. The first type occurs in quartz+ chlorite veins in the agglomerate and carries mainly arsenopyrite (with inclusions of native bismuth and gold), pyrite, chalcopyrite, sulphosalts and Cu-sulphides. The second type is disseminated in the agglomerate and consists predominately of sphalerite and chalcopyrite. The third type is found at the contact between the agglomerate and underlying massive rhyolite and consists of quartz+ chlorite + siderite veins with lesser pyrite and chalcopyrite. The sulphide minerals of the quartz+ chlorite veins suggest a history of precipitation in at least three stages. The first stage includes the precipitation of the arsenopyrite, pyrite and chalcopyrite. The sulphide minerals of the second stage (Bi-Pb-Cu-sulphosalt, galena and chalcopyrite) precipitated interstitially with respect to the first generation sulphides. The third stage includes minerals that formed due to supergene alteration by a Cu-rich fluid, for example a variety of Cu-sulphides and the Bi-Cu-sulphosalt. Chamosite (Fe-rich chlorite) and quartz are the dominant silicate minerals associated with the sulphides. Away from the vein system a zone in the agglomerate is encountered where areas dominated by sphalerite alternate with areas dominated by chalcopyrite; no alteration by a Cu-rich fluid was detected here. The chlorite geothermometer indicates temperatures of formation (of the chlorite) in the range 315° - 360°C. Chlorites overprinted by a Fe-rich fluid must have formed at higher temperatures along with some of the primary sulphides (arsenopyrite, pyrite, and chalcopyrite). A second stage of sulphide precipitation followed at slightly lower temperatures resulting in the formation of the Bi-Pb-Cu-sulphosalt and small amounts of galena and a second generation chalcopyrite. These two stages were succeeded by a period of "alteration" by a Cu-rich fluid that represents the last stage of sulphide mineral precipitation.