Fluid evolution and mineralising potential in the outer margin of the southern Gariep Belt

Abstract

Abstract provided by author:

Fluid inclusion analyses of quartz veins of various generations from the outer margin of the southern Gariep Belt reveal the presence of two chemically distinct fluid populations: the pre-orogenic, rift-related Rosh Pinah ore-forming fluid and the Orogenic fluids. The ore-forming, fluid is more saline and chemically more complex than the Orogenic fluids, probably representing hot, chemically evolved hypersaline brines induced by rift-related magmatism. Four generations (D1/D2, D3, D4 and D5) of Orogenic veins are recognised on the basis of their structural relationships. The Orogenic fluids are chemically similar, dominantly aqueous and generally of moderate to low salinities (10 wt percent NaCleq), with the few higher salinity estimates (10-16 wt percent NaCleq) being attributed to the interaction of some of the Orogenic fluids with evaporitic horizons. A trend of decreasing temperature with a decrease in age (D4D1/D2D3D5) is developed within the Orogenic fluids, with the exception of those fluids related to the emplacement of the Kuboos Pluton, which yielded the highest temperatures. The D1/D2 fluids from the northern and Vanrhynsdorp areas were hotter than the fluids from the central area, whereas the D3 and D5 fluids exhibit no apparent temperature variation as a function of area. D4 fluids show a decrease in temperature further away from the pluton. Estimates of the peak metamorphic pressure in the outer margin of the belt have been further constrained, using microthermometry results in conjunction with mineral assemblages, to 2. 9-3. 6 kbar, 2. 9-3. 1 kbar and 1. 8-3. 1 kbar for the northern, central and Vanrhynsdorp areas, respectively. Field observations and fluid inclusion analyses suggest channeled fluid flow on a regional rather than on an intra-formational scale, even though a rock-buffered system is suggested by oxygen isotope results. A different, largely rock-dominated fluid system was developed within the relatively impermeable carbonate units compared with the fluid system developed within the rest of the outer margin of the belt. The D1/D2 fluids probably represent circulating formation and metamorphic waters that were advectively forced out of the orogen and into the foreland, whereas channeled meteoric fluid flow most likely dominated during the transtensive phase of the orogeny. The D4 fluids probably contain a magmatic component, in addition to the meteoric component. Comparisons between the ore-forming fluid and the Orogenic fluids indicate that the latter have neither the required salinities, nor the required chemistry necessary in order to transport sufficient metals to produce a massive sulphide deposit the size of Rosh Pinah. The chemistry of the Orogenic fluids are, however, similar to those associated with gold remobilisation and as such the potential for gold mineralisation related to these Orogenic fluids should not be ruled out. Base metal sulphide exploration within the outer margin of the belt should be confined to rocks of the Hilda Subgroup. The recognition of highly saline, chemically complex non-orogenic quartz vein-hosted fluid inclusions is considered critical from the point of view of base metal sulphide exploration