How gold formed in the Vaal Reef deposit

Arstechnica : The Vaal Reef deposit, which formed 3 billion years ago in South Africa’s Witwatersrand basin, is one of the best gold deposits in the world. How the gold got there is uncertain, and the geological evidence is contradictory. In Nature Geoscience, ETH Zürich geologist Christoph Heinrich suggests a scenario that seems to fit all the observations. His hypothesis centers on the voluminous eruptions of acidic volcanic gases such as sulfur dioxide and the low oxygen environment that prevailed on Earth at the time. When the gases dissolved in rainwater and then entered rivers, the SO₂ formed a low oxygenated sulfuric acid and dissolved the gold out of the volcanic rock. Downstream, the gold-laden water encountered mats of living microbes, dead organic matter, or methane, which chemically reacted with the compound and precipitated out the gold.