Processing of diamond samples can be costly

At the height of the field season in the Lac de Gras area, it was rumored that Monopros, the Canadian subsidiary of De Beers Consolidated Mines, was flying out a Twin Otter load of overburden samples on a daily basis. With a payload of around 1,450 kg, and processing costs for overburden samples in the range of $250-375 for a 30-kg sample, analytical costs for each plane load would be in the order of $12,000-18,000.

By comparison, costs for heavy mineral processing of till samples for gold are in the range of $80-100 for a 30-kg sample, or about a quarter of the cost of processing indicator mineral samples.

Major overburden and stream sampling programs are either in progress or have recently been completed on several diamond properties across North America. Surface material is collected with the hope that it contains kimberlite indicator minerals, the “fingerprints” used to locate bedrock kimberlite occurrences.

The sampling programs have generated large volumes of material that require specialized processing, but there are few facilities that can do the work. Commercial laboratories designed to process overburden samples for kimberlite indicator minerals are present in British Columbia, Saskatchewan and Ontario. Most laboratories are processing 500-1,000 kg of material per day. Al Holsten, manager of geoscience services at the Saskatchewan Research Council Laboratory in Saskatoon, estimates his lab will be processing material well into the new year.

Although advances in technology have helped to refine processing techniques developed in the 1960s, sample processing is still a costly and time-consuming process. Russ Calow, manager of analytical services at Lakefield Research, a division of Falconbridge, near Peterborough, Ont., says that in reconnaissance sampling programs where kimberlite indicator minerals are sparse and the sample areas covered are very large it is important to ensure the highest possible recovery of indicator minerals during sample processing. This means that the efficiency rates of all the processing stages must be closely monitored and tested. High standards of quality control thus add to the cost of processing.

Sample Processing involves several stages:

— 10-30-kg field samples are weighed and wet-sieved into various size fractions. All fractions are saved, and the 0.2-2-mm-sized material is retained for further processing.

— Optional processing of the 0.2-2-mm fraction on a shaker table (similar in some ways to gold panning).

— Strongly magnetic minerals are then separated from the 0.2-2-mm-sized fraction.

— The material is then processed to collect a heavy mineral concentrate (HMC) with specific gravity greater than 3.0. Processing can be done with heavy liquids or a Magstream 200. The Magstream 200 produces a heavy media-like separation, using a non-toxic ferrofluid in a magnet-ringed centrifuge.

— The HMC with a specific gravity greater than 3.0 is then further separated into various fractions using a Frantz Isodynamic separator.

— The fractions are then examined with a binocular microscope to identify indicator minerals such as pyrope garnet, chrome diopside, ilmenite, and chromite. This is probably the most time-consuming stage of the sample evaluation.

— Optional chemical analysis of indicator minerals with an electron microprobe.