A Diamond of an Idea

The price of gold may fluctuate wildly but its more expensive rival – diamond – seems never to lose its lustre. In an attempt to find this precious commodity, explorers strive to develop more accurate and efficient ways of finding the mineral. A new project is being officially launched by the Commonwealth Scientific Research Organisation of Australia (CSIRO) hopes to provide diamond explorers with new investigative technology for their field kits.

The project is a collaboration between CSIRO Exploration & Mining (CEM) and the Minerals and Energy Research Institute of Western Australia (MERIWA) and has been sponsored by both state government geology surveys (Primary Industries and Resources South Australia, Geological Survey of Western Australia and Northern Territory Geological Survey) and industry (North Australia Diamonds, Venus Resources, Flinders Mines and GEM Diamonds).

Diamonds are formed under extremely high pressures and temperatures within the Earth’s mantle. They are most commonly found in pipe-shaped deposits of fragmented volcanic rock called kimberlite or lamproite. Both types of magmas act like conveyor belts, transporting diamonds and other minerals from depths greater than 150km to the Earth’s surface, taking hours or days in transit.

Kimberlite pipes have traditionally been discovered by searching in stream sediments or soil samples for kimberlite indicator minerals or ‘KIMs’ (such as chromite, garnet and Cr-diopside). These are minerals that were brought up from deep within the Earth’s mantle with the diamonds on the kimberlite ‘conveyor belt’.

The northern tropical areas of Australia are famous for the presence of diamondiferous lamproite (such as the Argyle Mine in the Kimberley in Western Australia) and kimberlite pipes (such as the Merlin Mine in the Northern Territory).

However, as the new project’s leader, Dr Noreen Evans explains, KIMs are prone to chemical dissolution in tropical weathering conditions which means significant finds may have been missed using this traditional method.

An alternative approach

"Explorers need to find new exploration methods based on analysing indicator minerals which are resistant to chemical dissolution – such as zircon," says Dr Evans.

"MERIWA 405, as the project is known, will examine the feasibility of alternative approaches to diamond exploration involving the chemical analysis of zircon and other resistant minerals. This method will be of most value in places where traditional KIMs are often not preserved and where the kimberlite has been buried by geological processes."

The CSIRO research team (Dr Brent McInnes, Brad McDonald and Dr Noreen Evans) has already successfully tested a method called (U-Th)/He thermochronology to detect ‘hot zircon’ originating from deep crustal regions.

This zircon is termed ‘hot’ because its (U-Th)/He signature indicates that it spent most of its life deeper than 6km before being transported to the surface by the kimberlite. ‘Cold zircon’ has a totally different chemical signature, having spent most of its life in the cooler conditions near the Earth’s surface.

"We developed a zircon ‘double-dating’ technique involving (U-Th)/He and U/Pb dating of single zircon grains, and applied it to samples obtained from the Merlin Kimberlite Field in the Northern Territory in a proof-of-concept study," says Dr Evans.

"The study found that more than 95% of zircons analysed from the kimberlite pipe were hot zircons, which originated in the deep crustal region and had been carried to the surface during kimberlite emplacement roughly 380 million years ago." However, in contrast, the team found that detrital heavy mineral separates from the North Kimberley craton contained only old, cold zircons with ages ranging from 1,400 - 1,600 million years.

From these results the team deduced that as a kimberlite pipe erodes it sheds hot zircon and traditional KIMs into the regional drainage system surrounding the kimberlite. The ratio of hot/cold zircons in stream sediment samples in the vicinity of kimberlite could be used as an exploration targeting tool.

Looking ‘hot’

‘Like diamonds, zircons "are forever" which is why they are so useful in applications like this one,’ says Dr Evans. ‘In tropical countries such as Australia or Brazil, the KIMs are likely to be weathered away. In such cases, it’s the hot zircon the explorers are looking for. If you collect zircon samples in a regional survey and double-date them, you should see more of the young hot zircon closer to the kimberlite field.

‘The contrast in age between the zircon eroded from old continental rock (cold zircon) and that eroded from a kimberlite pipe. A number of diamond explorers are supporting MERIWA 405, including North Australia Diamonds, Flinders Diamonds, GEM Diamonds and Venus Resources.

North Australia Diamonds chief mineralist Janusz Jakimowicz says it is still early days to say how successful this method for diamond exploration will be. ‘However, hot zircon may become a very important kimberlitic indicator - or the only indicator in the heavily weathered terrain."

Exploration manager (Diamonds) for Flinders Mines, John Ceplecha, agrees.
"The technique would eliminate much of the guesswork associated with interpreting indicator results detected beneath sedimentary cover."

Jakimowicz believes that in this type of environment only diamond has a chance to survive and other typical kimberlitic indicators are usually wiped out.
"Diamonds are very rare and require the collection of large sample volumes," he says.

"Zircon are also strong survivors and abundant in fine fractions (<300 microns). In most cases a few kilograms of stream sediment is enough to find zircon.
Of course the common zircon is useless for us but hot zircons (those with the kimberlite fingerprints) would be ideal kimberlitic indicators.

"A new kimberlitic indicator such as this would have an enormous impact on the speed and cost of sampling. The standard sample taken for kimberlitic indicators is at least 20-50kg which limits the sampling programme. It is equivalent to 10-25 hot zircon samples, which I estimate shouldn’t be bigger than 2kg. Smaller samples mean quicker results at a significantly smaller exploration budget."

MERIWA 405 aims to test this hypothesis further via local and distal sampling of stream sediments and paleochannels around several Australian diamond occurrences such as Ellendale, Argyle, Aries, Seppelt, Merlin, Elliston and Timber Creek.

The project team has prepared a technology transfer and implementation strategy for the practical application of the zircon doubledating method, with customers being the:

• The diamond industry looking to reduce the risk of exploration through the detection of hot zircons in standard KIM-type stream sediment samples, and the ability to trace these grains back to a chronological time horizon in the regional stratigraphy.
• State/Federal geoscience surveys keen to provide new pre-competitive, stream sediment geochemistry data sets to attract diamond explorers into prospective regions.
• Mineral services sector looking for new analytical methods and instruments to add to their existing geochemical and mineralogical analysis capability.

Future outcomes

Three key outcomes of MERIWA 405 will be:

• A new greenfield exploration tool that readily discriminates hot kimberlitic zircons from cold crustal zircon in stream sediments or soil samples
• A U-Th-Pb-He database/training set for zircon from key Australian kimberlite Occurrences.
• The determination of uplift/erosion rates of kimberlite deposits to determine the potential for secondary alluvial diamond accumulations

This piece was provided by Australia’s Commonwealth Scientific Research Organisation (CSIRO) and first appeared in its earthmatters magazine.