Their theory challenges long-held ideas about when the Earth's atmosphere became enriched with oxygen, and pushes the likely date for formation of an atmosphere resembling today's far back into the early history of the planet.

It may also revolutionise the worldwide search for gold and other minerals, and raises new questions about when and how life could have arisen.

Evidence for the presence of oxygen in the primitive atmosphere has been put forward by the Chief of CSIRO Exploration and Mining Professor Neil Phillips, Australian-based South African geologist Mr Jonathan Law and US gold mining consultant Dr Russell Myers in a publication by the Society for Economic Geology.

"These findings may have enormous economic implications in that we may simply have been looking in the wrong places for massive gold deposits like South Africa's Witwatersrand," says Professor Phillips.

"Or we may actually have found them - and not recognised them for what they are, because we did not understand the processes involved in their formation."

The scientists base their case on the presence of iron-rich nodules in the deep strata of the Witwatersrand - nodules they believe are pisoliths, small balls containing ferric iron produced by exposure to an oxygen-rich air.

Pisoliths still form nowadays and provide important clues in the search for minerals, including gold. Those found in the Rand come from levels 3-4 kilometres down, which are securely dated at 2.7 to 2.8 billion years old.

The researchers' theory has been lent additional weight by evidence from the Western Australian Pilbara region for the presence of sulphates in rocks up to 3.5 billion years old. These, too, could not have formed without an oxygen-rich atmosphere.

Pisoliths have been a vital tool in the discovery of $5 billion worth of new gold deposits in WA in recent years, using techniques developed by CSIRO's Dr Ray Smith, Dr Charles Butt and Dr Ravi Anand.

The small iron-rich balls form from iron in groundwater and 'scavenge' traces of other minerals in the local environment. They provide clues, like fingerprints, which point to deposits lying hidden beneath metres of inscrutable surface rubble.

By analysing pisoliths over a wide area for gold content, geologists can construct a pattern of steadily enriching traces, with the hidden deposit lying like a bullseye at the heart of it, usually a bit uphill.

Some geologists believe living organisms may play a part in the formation of pisoliths, raising tantalising questions about the nature and role of life in shaping the Earth's early surface and mineralisation.

The presence of pisoliths in the deep strata of the Rand suggests that the conditions for mineral formation 3 billion years or so ago were different to what many geologists have believed for the past half-century, the team say. These ideas have already been integrated into a new exploration model for the formation of the Rand deposits by the same researchers.

The Rand is unique on Earth - a vast body of rock very rich in gold. The mightiest gold deposit ever found. Nothing like it has been discovered elsewhere.

Professor Phillips says that this may be because we didn't know what to look for, because we made wrong assumptions about the conditions in which it formed.

In other words, fresh Rands may still await discovery. Some geologists speculate one of them, at least, lies in central Western Australia.

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