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Last Updated: Tuesday, 25 November, 2003, 12:01 GMT
Quest for space impact riches
Sites where asteroids struck the Earth millions of years ago may be the key to discoveries of new mineral and metal deposits in the future.

Comet impact, BBC
Some geologists believe that sudden, catastrophic impacts could have created some of the world's biggest deposits - in an instant.

Mineral exploitation currently occurring at impact areas includes the world's most profitable gold mine, in South Africa, and a massive nickel and platinum deposit in Canada.

"On average I would say that one quarter of the known impact structures on the Earth have some sort of deposit associated with them," Canada's Natural Resources Department chief scientist Richard Grieve told BBC World Service's Science In Action programme.

"Of that quarter, maybe about half have been actually exploited, either in the past or currently so."

Increasingly, some geologists are questioning the theory that the Earth's rock record changes slowly over time.

Many are now looking for evidence of where rocks have been "shocked" - which would indicate the impact of an asteroid or comet.

Impact lines

"The pressures required to make the textures that we're going to look at can only be made by impact of something like a meteoroid or an asteroid or a comet," said Dr Adrian Jones, of University College London.

The keys to finding such sites are grains of quartz, which, under the microscope, have tell-tale parallel lines that reveal if they were part of an "impact structure", the area where an extraterrestrial body struck the Earth.

Dr Jones added that one recently discovered major nickel deposit in Russia - coupled with two other, previous finds - suggested that some metals might come from the impactors themselves.

"It makes it rather interesting that two or three large impact structures are now associated with the same association of nickel-rich metals," he stated.

"The idea from our modelling and our smaller experiments [is] that the impact crater itself may still retain a mixture of materials, both from the melted crust and from the residue of the meteorite impact that has been redistributed around the crater.

"That would contain a lot of nickel-rich metals and platinum-group elements."

Gas creep

Alternatively, it may be that the impact causes such massive, immediate change that minerals become present in ways they otherwise would not have done.

The disruption caused to the Earth's underlying crust can create the ideal conditions for the deposition of minerals and hydrocarbons, geochemist Ian MacDonald of Cardiff University told Science In Action.

"It's the excavation of the crater - the way that the rocks have been broken up and smashed - that has allowed oil or gas to creep into that structure and accumulate there, for us to drill into and then tap off," he said.

"Or it's been the way that the rocks have melted at the moment of impact that has allowed important metals like nickel or copper or platinum to concentrate or segregate at the bottom of the crater."

But the impact theory is not popular theory with everyone.

"Geologists have always viewed the rock record as something that changes very, very slowly," Dr MacDonald said.

"These catastrophic events, for many of them, were difficult to accept, because they seem to be so at variance with the slow change of geological time that generally happens.

"Ninety-nine percent of the time things happen very slowly."

Craters, (BBC)
Vredefort [1], at 300 km in diameter, is the Earth's biggest impact structure. The hole was gouged out two billion years ago
Chicxulub [3] was made in an event linked to the extinction of the dinosaurs. The impact was dated to 65 million years ago
Beaverhead [10], 60 km wide, was created 600 million years ago.

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