A case study of the postdepositional alteration of the Witwatersrand Basal Reef gold placer

An investigation of the mineralogy, quartz texture, fluid inclusions, and gold composition of a crossbedded auriferous specimen of the Archcan Basal reef placer from the Welkom gold field, Witwatersrand, South Africa, has established the degree of postdepositional alteration to which the detrital gold particles were subjected. Three different postdepositional events can be distinguished: (1) regional static recrystallization at a peak metamorphic temperature between 290 degrees and 350 degrees C and a pressure between 2 and 3 kbars, (2) local fluid infiltration along fractures and sedimentary interfaces at a temperature between 240 degrees and 300 degrees C and a pressure similar to the above, and (3) local K (super +) metasomatism during a later fluid infiltration event along the preexisting zones of weakness. Intracrystalline diffusion during these events led to homogenization of the gold on a hand specimen scale with respect to Ag (8.9 wt %) and with respect to Hg (1.0 wt %). Furthermore, during the second event (2), fluid infiltration caused not only homogenization but also partial mobilization of some of the detrital gold with transport over distances of not more than a few millimeters to centimeters. These processes were accompanied by microscopic silicification, calcitization, and possibly Hg metasomatism. The pH value of this infiltrating fluid was around 4. Primary fluid inclusions associated with secondary gold in authigenic quartz consist of CaCl ₂ -rich aqueous solutions whereas secondary fluid inclusions within such authigenic quartz are dilute aqueous solutions, having a very low salinity that is attributed to NaCl and KCl (event 3). It is believed that gold mobilization was coeval with a fracturing and shearing event after deposition of at least parts of the Transvaal Supergroup on top of the Witwatersrand and Ventersdorp Supergroups. The last event (3), the infiltration of K (super +) -enriched fluid after the partial mobilization of the gold, is tentatively correlated with the Bushveld or Vredefort events.