Evaluating kimberlites adds dramatically to costs

Having recently intersected a kimberlite on their Lac de Gras area property, Aber Resources (TSE) and partners SouthernEra Resources (TSE) and Commonwealth Gold (VSE) must now await processing results to see if the pipe has the potential to be economic. This could be an expensive undertaking, since costs for processing kimberlitic material start around $1,000 for a 20-30-kg sample.

There are about 5,000 known kimberlite bodies in the world. Of these about 1,000 are diamondiferous and only about 50 are considered to be economic. To estimate the economic potential of a kimberlite body, bulk samples are collected and evaluated for indicator minerals, microdiamonds and macrodiamonds. Christopher Jennings, president of SouthernEra, says it doesn’t really matter if the sample is fresh or weathered, but it is most important to collect a large volume of material. The only problem with weathered material, Jennings says, is that it contains clay material which may cause processing problems.

In order to gather large volumes of material, composite bulk samples are often collected from one or more pipes. Samples are usually collected by diamond or reverse circulation drilling. However, Jennings cautions that care must be taken when using diamond drill bits since they can contaminate the samples.

Evaluation for indicator minerals and microdiamonds usually involves petrographic examination and attrition milling. The kimberlite material is gently crushed and material of 0.1-2 mm is classified using magnetic and density separation techniques. The various specific gravity fractions collected are then visually inspected for indicator minerals and/or microdiamonds.

Costs for this type of processing average $1,000-1,300 for a 30-50 kg sample. Microdiamonds can also be separated and collected using a process that utilizes their property of resistance to chemical attack. Kimberlite material is first heated in caustic soda which destroys most of the minerals present. A fused residue greater then 0.5 mm is then sieved, neutralized and classified by gravity and magnetic separations into magnetic fractions and a non-magnetic microdiamond-bearing concentrate. The fractions are again visually inspected for microdiamonds.

The caustic fusion method will also recover some relict indicator minerals such as graphite and garnet. However the main focus of this processing technique is the recovery of microdiamonds.

To process 25 kg of material using this method costs $1,000-1,200. Once it has been determined that the kimberlite contains the correct indicator mineralogy, and a certain number of microdiamonds, larger samples are then evaluated for macrodiamonds. Usually 50-200-tonne samples are collected and analyzed using a combination of gravity separation and chemical dissolution techniques. Costs for recovering macrodiamonds depend highly on the amount of material processed and the number of diamonds recovered. Kimberlite bulk samples are currently being processed at the Saskatchewan Research Council Laboratory in Saskatoon and at Lakefield Research, a division of Falconbridge, near Peterborough, Ont.