The Earth may have had an oxygen-rich atmosphere as long ago as three billion
years and possibly even earlier, three leading geologists have claimed.
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 was 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 .
Professor Neil Phillips, CSIRO, 03 9662
7454,mobile 0417 300 400
neil.phillips@csiro.au
Dr Ravi Anand, CSIRO 08 6436
8672
Keith Bashford, CSIRO 0438 173
654
