Indicator minerals aid in search for diamonds

(Second of a 3-part series)

Using surface geochemical sampling, Dia Met Minerals (VSE) followed a 700-km-long trail of indicator minerals for five years in the Northwest Territories until it made the kimberlite discovery that sparked one of the largest claim-staking sprees in Canadian history.

Dia Met spent about $800,000 on diamond exploration in the Northwest Territories, most of it on surface sampling. The technique has proved so useful and cost-effective that it has become standard practice in Canada where weathered kimberlitic material has been smeared across the land by fluvial and glacial processes.

Dia Met followed kimberlite indicator minerals pyrope garnet, ilmenite, and chrome diopside from the Mackenzie River eastward to Lac de Gras, N.W.T., collecting thousands of overburden samples as the program progressed. Since the 1989 discovery, some 15-20 million acres have been staked. Increasing concentrations of indicator minerals are theoretically followed up-ice to locate the bedrock source of the minerals. Chrome diopside is generally destroyed within 50 km of the source, pyrope within 150-200 km. Essentially, three types of surficial sample material are collected: glacial overburden or till, eskers and stream sediments. Since streams and eskers drain and sample large areas, these are the types of sediments most commonly collected.

Although sampling techniques vary widely from company to company, sample collection programs are generally very similar.

SouthernEra Resources (TSE) President Christopher Jennings says that in Canada one must understand the glacial history of an area before sampling can begin. It is important, Jennings says, to know how many till sheets are present and the direction of transport.

Most companies first carry out an orientation survey to determine the best type of material to be sampled and the sampling density. According to Bryan Schreiner, manager of the resources division at the Saskatchewan Research Council, sampling density is generally based on two factors, project budget and property size.

After the orientation survey, reconnaissance sampling is carried out. During this phase of sampling, overburden samples are typically collected across a property at 7-10-km intervals. Christopher Gleeson of Gleeson and Associates, an exploration consulting firm, stresses that is important to be systematic and note the specific type of overburden sampled.

If indicator mineral anomalies are found, a follow-up program of detailed sampling is undertaken. As with the Dia Met case, it can take years of diligent sampling to locate a kimberlite body. Most companies also utilize airborne and ground geophysical surveys to aid in their search. Samples collected typically weigh 10-30 kg and are often preconcentrated in the field so that 1-3 mm and finer-sized material is retained. On average a 2-man crew can collect about eight samples per day. Field collection costs in the Northwest Territories (including transportation costs) are about $500-650 per sample, and in places like Saskatchewan and Ontario, where access is better, costs drop to about $200 per sample.