JUNIOR MINING — Other indicators prove useful in diamond

Although much has been said about the use of “G10” garnets as a indicator of diamond potential, two lesser known geochemical techniques are also effective diamond exploration tools.

Nickel thermometry, a relatively new method developed in Australia, measures the nickel content of pyrope garnets to determine whether the host rock passed through the diamond stability field on its way to the surface. The other technique, ilmenite analysis, will reveal whether diamonds sampled in the field were later preserved in the kimberlitic magma during ascent. Since less than 10% of kimberlites are diamond-bearing, the use of these screening methods can save companies a great deal of time and money that may otherwise be spent testing barren pipes.

“You can blow an awful lot of money just by chasing a target or, once you’ve found it, doing a bulk test,” William Griffin, chief research scientist at the CSIRO division of Exploration Geoscience in Australia, said in Toronto, at a recent short course on diamonds.

“Indicator minerals,” including ilmenites, pyrope garnets and chrome diopsides, are heavy minerals that occur in association with, and in greater abundance than, diamonds in primary deposits. They have been used as an exploration tool since the early 1900s when the Premier mine was discovered at the end of a heavy mineral train in South Africa.

With the invention, in the 1960s, of the electron microprobe, determining the chemical compositions of individual indicator mineral grains became much faster, easier and more reliable.

But John Gurney, at South Africa’s University of Cape Town, who determined that garnets low in calcium and high in chromite (G10s) emanate from diamondiferous pipes, is responsible for the biggest breakthrough in diamond exploration in recent years.

His method is now so refined that it can be used to predict diamond grades with a fair degree of accuracy. For example, well before partners Dia Met Minerals (TSE) and BHP Minerals Canada took a bulk sample from their Point Lake pipe in the Northwest Territories, Gurney predicted a grade greater than 60 carats per 100 tonnes. Subsequent results proved him right. But although the use of G10 garnets to evaluate diamond potential appears to be working in the Slave province of the Northwest Territories, the method has not been effective in all diamond camps.

Nickel thermometry, an alternative technique developed by Griffin, uses nickel content in pyrope garnets to determine if the host kimberlite or lamproite contains diamonds.

By using the nickel content to deduce the temperature and depth at which the garnet was formed, geochemists can determine whether the diatreme sampled the diamond stability field. Diamond-poor pipes contain a high proportion of garnets with low nickel content, suggesting they passed through the graphite field.

Examination of kimberlitic ilmenites takes this process a step further by determining whether the diamonds were then preserved in the kimberlitic magma on its long journey to surface.

The evaluation is based on the knowledge that if oxygen levels in the kimberlitic magma are too high, diamonds will be resorbed. Therefore, ilmenites in equilibrium with diamonds contain very little Fe3+, which indicates that they formed in a highly reduced (low-oxygen) environment. “As kimberlite magma ascends through the uppermost mantle, xenocrystic diamonds in the system are subjected to rapidly changing conditions that may lead to resorption and/or graphitization,” Malcolm McCallum, a geology professor at Colorado State University, said in a recent paper. “The presence of Fe3+-rich kimberlitic ilmenites would suggest an unfavorable environment for the survival of diamond.”