Browsing by Author "Frimmel, H"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Restricted
    Gold mobilizing fluids in the Witwatersrand Basin: composition and possible sources.
    (Springer, 1999) Frimmel, H; Hallbauer, D
    Crush-leach data were obtained, using High Performance Gradient Ion-Chromatography and Capillary Electrophoresis, on individual generations of aqueous fluid inclusions in hydrothermal quartz from three different auriferous conglomerate horizons (reefs) in the late Archaean Witwatersrand Basin, South Africa. These data, supplemented by oxygen isotope analyses of hydrothermal quartz and in combination with microthermometric analyses, help to constrain the chemical composition, pH, temperature of formation and the possible source of the mineralizing fluid which, in places, was capable of mobilizing some of the primarily detrital gold in the fluvial Witwatersrand sediments. The dominant cations in the aqueous fluid inclusions are Na + and Ca 2+, with C1- or HCOj being the dominant anion, whereas K ÷, Mg 2+, and SOlare subordinate. Most fluid inclusions have elevated NH~- concentrations which are directly correlated with those of NO 3. In a number of samples small amounts of organic acids (formate, propionate, and acetate) were also detected. A largely meteoric source is inferred for the gold-mobilizing fluids in the Witwatersrand reefs because of a lack of Br- in the fluid, a composition distinctly different from that of seawater, the presence of organic acids, and ~18Ofluid values around 0%0. The fluids are ascribed to hydrothermal infiltration triggered by the 2020 Ma Vredefort impact which also created a secondary permeability in the form of a dense network of micro-fractures preferentially in the conglomerate beds of the already metamorphosed Witwatersrand rock sequence. This fluid differs from the regional metamorphic fluid in the basin by having a considerably higher pH (5.7-7.2). The difference in pH might explain why the older, fairly acidic metamorphic fluid was apparently less capable of mobilizing the gold as gold solubility reaches its peak at the pH calculated for the fluid ascribed to the impact.
  • Thumbnail Image
    Item
    Open Access
    New Pb-Pb single zircon age constraints on the timing of neoproterozoic glaciation and continental break-up in Namibia.
    (University of Chicago Press, 1996) Frimmel, H; Klötzli, U; Siegfried, P
    Dating of single zircons from low-grade metamorphosed rhyolites in the Rosh Pinah Formation of the Gariep Belt in southwestern Namibia, using the Pb evaporation technique, yielded a primary crystallization age of 741 ± 6 Ma. Both the stratigraphic position and the geochemistry of the volcanic rocks indicate an early continental rift environ- ment. The new data not only provide an age for the massive Zn-Pb-Cu sulfide mineralization associated with these volcanic rocks, but they also set a maximum age limit for Neoproterozoic continental break-up in southern Namibia. This age is statistically indistinguishable from a recently reported age of 748 + 3 Ma for stratigraphically equivalent volcanic rocks in the northern rift of the Damara Belt, suggesting that the onset of the formation of the N-S-trending Adamastor ocean and of the NE-trending Khomas ocean in central and northern Namibia occurred at the same time. The volcanic unit directly overlies, in both rift grabens, a diamictite horizon with glaciogenic features. Our new results further constrain the age of this glacial epoch, which may be correlated with the Sturtian glaciation, to around 750 Ma