Gold mobilizing fluids in the Witwatersrand Basin: composition and possible sources.
Journal Article
1999
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Mineralogy and Petrology
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Springer
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University of Cape Town
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Abstract
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 รท, 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.
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Reference:
Frimmel, H. E., & Hallbauer, D. K. (1999). Gold mobilizing fluids in the Witwatersrand Basin: composition and possible sources. Mineralogy and Petrology, 66(1-3), 55-81.