Minerals leave 'bread-crumb trail' in the search for diamonds

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The secrets of diamond discovery lie in magnesium-rich rocks.

Diamonds. (Reuters)

A funnel of hard, green-tinged rock spears into the Earth’s crust, rising up out of the mantle 250km below. This rock contains diamonds and the secrets of how to find them. 

Deep inside the mantle, magnesium and carbonate-rich rocks partially melt to form a pool of magma. Fuelled by the release of pressure below and the buoyancy of the rock, the kimberlite violently pushes its way to the surface in less than a day. The kimberlite breaks through the surface and looks like a flesh wound: a spear-tip surrounded by skin flaps of surface rock.

The secret of diamond discovery lies in these dark-green, iron and magnesium rich rocks. As scientists, our interest in these rocks is their unusual mantle-like chemistry, which is seldom found at the surface, and because they are a primary source of mantle material, including diamonds. 

But they are also important for industry: although it is widely acknowledged that kimberlites bring diamonds to the surface by scraping pieces of diamond-bearing mantle rock off the side walls during their ascent, we still do not fully understand how the diamonds are incorporated into the kimberlite and whether a certain spear of kimberlite will contain these valuable gems. 

One of the biggest issues in understanding kimberlite magmas is their deceptive nature: their composition continually changes during their ascent and not all kimberlites are rich in diamonds. Some do not contain any diamonds at all. Scientists believe that all mantle-derived nodules (shards of incorporated wall rock, which are made up of various minerals and diamonds) cannot remain in the magma without being chemically altered or completely dissolved, depending on the amount of time spent in contact with the magma. But we know there are mantle nodules and diamonds in kimberlites, so some obviously do survive.

 Because of the kimberlite-magma’s rapid and violent puncturing into the crust, the magma has less time to interact with the minerals in the nodules, and this increases the minerals’ chance of survival. The details of this ascent are written in the diamonds themselves, through textural and physical characteristics within the individual diamond stones. 

Kimberlites contain mixtures of perfectly shaped and deformed diamonds. This indicates what has historically happened within the magma. Nodules containing perfectly shaped diamonds also contain other distinctive minerals such as apple-green diopside and red garnets. These minerals are flagged as possible diamond indicators and are vital when investigating the diamond resource potential of a new deposit. The indicator minerals, if found, can act as a “bread crumb trail” to the diamonds. However, diamond-free kimberlites also contain minerals of similar composition so the mystery of diamond occurrence remains unresolved.

In an effort to explain the diamond mystery, I simulated a kimberlite ascent path using the equipment in the Centre for Crustal Petrology at Stellenbosch University, to gain a better understanding of how various minerals interact with the kimberlitic magma. Through various high temperate and pressure experiments conducted on a natural kimberlite, sourced from the Kimberley area, the investigation showed how certain indicator minerals dissolve into the magma. This revealed a crucial explanation of how minerals survive in this volatile environment and possibly points to how diamonds themselves are preserved. 

Minerals that contain a high silica (silicon dioxide) levels in their chemical structure are preferentially melted into the kimberlitic magma, while the more calcium-rich minerals are more robust. This means that large mantle nodules — if they contain enough calcium-rich minerals — may protect other minerals, including diamonds, from exposure to the devouring kimberlitic magma. This increases the probability of finding diamonds in the kimberlite funnel at surface. 

Mantle nodules that house less calcium-rich minerals are more likely to be broken up during the ferocious ascent, thus exposing diamonds to the magma at various times. If the nodules spend less time in contact with the magma, you are more likely to find intact diamonds in them, whereas the longer the contact with the magma, the more remote your chances. 

The effects of this research extend beyond the laboratory and scientific interest. If we can unlock the secrets of diamond formation, we can potentially solve the mystery of mineral survivability within the spear-like kimberlite. 

This would enable researchers to identify the diamond potential of current and future kimberlite deposits and, in so doing, increase South Africa’s potential as a global diamond resource competitor.

Sara Burness is a MSc candidate at the University of Stellenbosch.

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