Fluid mixing in the generation of mesothermal gold mineralisation in the Transvaal Sequence, Transvaal, South Africa
Anderson, Michael R.; Rankin, Andrew H.; Spiro, Baruch
European Journal of Mineralogy Volume 4 Number 5 (1992), p. 933 - 948
published: Oct 14, 1992
manuscript accepted: Jan 22, 1992
manuscript received: Jun 14, 1991
ArtNo. ESP147050405022, Price: 29.00 €
Abstract Fluid inclusion studies of auriferous quartz veins from a wide range of mineralised settings in the Transvaal Sequence of South Africa have revealed the presence of mixed inclusion assemblages. Saline, three-phase, CO2-rich inclusions, with variable phase ratios and a range of densities, coexist with highly saline (NaCl-KCl-CaCl2), aqueous inclusions containing little or no CO2. This is interpreted to represent the heterogeneous trapping of two originally unrelated, partially miscible, end-member fluids in the system (CO2-H2O)-(H2O-NaCl-KCl-CaCl2). Bulk cation analyses of the inclusion contents, essentially the saline end-member fluids, show the dominance of Na, K and Ca, but significant amounts of Mg, Li, Rb, Sr, Ba and B are also present. Regionally, the Na/cation weight ratios of inclusion fluids in samples hosted by the Malmani Dolomite Subgroup are strikingly similar; the majority of Na:K ratios lie between 4 and 8. In contrast, Na/cation ratios from veins hosted by the clastic sediments of the overlying Pretoria Group are very different; here, Na.K ratios of between 25 and 50 reflect the greater evolution of the fluids in the elastics. According to the fluid-rock equilibrium constraints discussed by Giggenbach (1988) the Na.K.Mg ratios of these inclusion fluids are characteristic of waters which are not equilibrated with their hosts. The geochemical trends observed on plots of cation ratios suggest that the saline brines progressively interacted with the more dilute, CO2-rich, end-member fluids. A model is proposed involving the interaction of an homogeneous, gold-bearing CO2-rich fluid of deep-seated (metamorphic/magmatic?) origin, and a saline, basinal (connate/intraformational?) brine. Gold deposition is thought to have been triggered by rapid changes in the physicochemical conditions of the system caused by the partial mixing of the two fluids.