New Mexico Geological Society Annual Spring Meeting — Abstracts

Approximately 45000 metric tons of gold have been won by mankind from one extraordinary basin, the Witwatersrand Basin of South Africa. Gold is hosted almost exclusively in pyrite-bearing quartz pebble conglomerates which have been metamorphosed to lower greenschist facies. The source of this gold and how it was introduced remains enigmatic and heavily debated. There is evidence to support a sedimentary control and other evidence to support hydrothermal control. The problem with the hydrothermal hypothesis is that no mechanism has been proposed to mobilize such vast quantities of gold. Geologists have speculated that gold solubility was in the order of 0.1 ppm, however, preliminary work indicates that gold was more mobile.

Material collected from the Basal Reef, Welkom Goldfield, was used for this exercise. In order to test the hypothesis that gold was more soluble than 0.1 ppm, one needs to determine the temperature of the fluids, the oxygen and hydrogen-sulfide fugascity material, and the salinity of the fluids. Samples were crushed in vacuo and the volatile species analyzed with a quadrupole mass spectrometer, thereby determining the chemistry of fluid inclusions. A minimum temperature of 345 degrees Celcius has been constrained based on the mineral carpholite in bedding parallel fractures, temperature being an important control on gold solubility. The bedding parallel fractures are also host to fluid inclusions with estimated 2.3 molar NaCI, based on the freezing depression of ice. Based on the gas analysis, the oxygen and hydrogen sulfide fugascities have been calculated giving log values for oxygen fugascity of -27 to -30. Salinity has a profound effect on the determination of gold solubility using Henry's Law of constants, thereby increasing the solubility of gold thio-complexes by nearly three orders of magnitude. The data shows that metamorphic fluids trapped by the secondary fluid inclusions had the potential to dissolve 0.1 to 10 ppm of gold.

Perhaps gold solubility during metamorphism may be the key to understanding why the Witwatersrand Basin has exceeded all other gold deposits in terms of reserves in that there existed an efficient transport mechanism and an effective trap.