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32 www.resourceworld.com O C T O B E R / N O V E M B E R 2 0 1 8 2,000 times the Fort Knox gold reserves in the US, researchers said. "This was a time when the world's greatest gold ore deposits were formed in South Africa in the Witwatersrand Basin," said Professor Ross Large, from CODES. "Over the next 400 million years, gold levels remained high in the oceans and many other important deposits formed, including the Golden Mile in Western Australia," said Large. The Golden Mile is the ore deposit being mined in the Superpit in the photographs (see photo page 30). Large said the concentration of gold in the oceans indirectly relates to gold ore generation in the shallow crust. "This means peak times of gold in the oceans correspond to the best times in Earth history for gold ore formation. The very ancient oceans were therefore enriched in gold but highly toxic," Large said. There is a theory that gold in the Witwatersrand Basin came from rivers bearing acid rain-leached gold, from the belching of ancient volcanoes, which was trapped and concentrated in sediments by cyanobacteria. See Nature Geoscience, February 2, 2105 by Christopher A. Heinrich Witwatersrand Gold Deposits. The deposits are formed by volcanic rain, anoxic rivers and Archaean life. This is a theory and there is some debate about exactly when cyanobacteria first evolved and to what extent it played in the formation of the Witwatersrand gold deposit, the earth's largest deposit of gold, but what is clear is that in ancient times, especially around three billion years ago, large quantities of gold were deposited in ore deposits around the world. It is also a theory that cyanobacteria evolved at this time. Coincidence? I think not. Cyanobacteria flourished in the ancient oceans, pulling gold out of the seawater into sediments, while the oxygen concentration of the atmosphere steadily climbed. These bacteria do not like oxygen so their impor - tance declined as the oxygen content of the atmosphere went up. This theory strongly suggests that the period in earth's history from three billion years ago to 2.6 billion years ago was an especially productive time for gold deposit formation. Following this period of gold-rich oceans, the time series curve shows that in middle Earth history, during the Proterozoic period, gold was at an all-time low in the oceans. This helps to solve a great conundrum in Earth Sciences of why so few gold deposits are present during this billion- year period, researchers said. Research is now pointing towards a much larger role of microscopic life in the process of creating vast deposits of gold at one time in our earth's history, and then continuing on to the present day actively depositing and transporting gold in sur - face environments. n GOLD