R. Klemd, A comparison of fluids causing post-depositional hydrothermal alteration in Archaean basement granitoids and the Witwatersrand Basin, MINER PETR, 66(1-3), 1999, pp. 111-122
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.