A comparison of fluids causing post-depositional hydrothermal alteration in Archaean basement granitoids and the Witwatersrand Basin

Some Archaean granitoids occurring along the north-western and western edge of what is classically known as the Witwatersrand Basin have been hydrothe rmally altered in two different styles. Both styles - pervasive and vein-co ntrolled alteration - are characterised by sulphide mineralisation as well as by thorium-, uranium-, gold- and REE-rich nodules of carbonaceous materi al (fly-speck carbon). All of the granitoids displaying these characteristi c alteration styles are associated with secondary moderate- to high-salinit y, Ca-rich fluid inclusions with low homogenisation temperatures in magmati c quartz and quartz veins. C-isotopes of the fly-speck carbon and the fluid composition of the associated fluid inclusions are typical of modified bas in brines. Similar types of Ca-rich fluid inclusions were found in authigen ic quartz and hydrothermal quartz veins from gold-bearing, hydrothermally a ltered sediments of the Witwatersrand Basin, although the salinities of the fluids in the basement granitiods are somewhat higher. The moderate- to hi gh salinity of this inclusion fluid and its present composition is consider ed to be the result of modifications by fluid-rock interaction during trans port and subsequent metamorphism within the Witwatersrand Basin. Available age data on the hydrothermally derived minerals in these granitoids range b etween 2.7 and 2.0 Ga indicating several fluid pulses, with fluids repeated ly expelled during the dewatering of the Witwatersrand Basin. These data ar e consistent with the conclusion that major parts of the alteration are the result of sediment dewatering which affected both the,granitoids and the g old-bearing strata within the Witwatersrand Basin.