Finding a Prospective Kimberlite

PRELIMINARY STAGE:

Area selection: Diamond-bearing kimberlites tend to be located on Archean cratons, unaffected by any Major tectonic event or granitisation for at least 2.5 billion years. Extensional fracturing, extensions of oceanic transform faults, hotspot tracks, cover of the craton by horizontal sedimentary layers can all be favourable, as is limited previous exploration activity for diamonds.

STAGE 1: 6 TO 12 MONTHS

Regional survey, using airborne geophysics or regional indicator mineral sampling. Purpose is to find, in large cratonic areas covering several 10,000 square km, magnetic anomalies or indicator mineral trails that may lead to kimberlites. Initially a kimberlite province might be detected of say 30 by 30 km in size.

STAGE 2: 1 TO 2 YEARS (DEPENDING ON NUMBER OF TARGETS FOUND)

Detailed ground follow-up of magnetic anomalies and indicator mineral trails by ground magnetics, closely spaced indicator mineral sampling, drilling and trenching, to locate the individual kimberlite pipes in the kimberlite province.

Kimberlitic Indicator Minerals are minerals formed together with diamonds at depths of 150 to 200 km include pyrope garnets, ilmenites, chrome-diopsides and chromites. These minerals are more abundant than diamonds and are therefore more easily detected in samples taken in the rivers downstream of the pipes. For this reason they are called kimberlite indicator minerals and are an important exploration tool.

Evaluation of a Diamond Prospect

STAGE 3: 3-6 MONTHS

TESTING THE PIPE FOR DIAMONDS – MICRODIAMONDS

After the discovery of a kimberlite pipe the most common initial test to determine if a pipe is diamondiferous is to assay a rock sample for the presence of microdiamonds.

A microdiamond is a diamond less than 0.4 mm in size (less than 0.001 carats in weight) and is only of geological importance, as these microdiamonds are not recovered by commercial diamond mining operations. Microdiamonds are recovered by dissolving about 50 – 100 kg of rock in a hot caustic solution by a commercial laboratory. Most, but not all, economic diamond mines have significant numbers of microdiamonds. Some economic pipes and dykes (i.e. pipes counting sufficient quantities of macrodiamonds) have no, or very minor, amounts of microdiamonds.

Although microdiamond analysis gives an initial indication of the economic potential of a diamond pipe, it is the macrodiamond size distribution and the diamond value, that ultimately determines its economic value.

It is widely thought that the microdiamonds form part of a continuous size distribution from large to small diamonds, but research has not conclusively proven that they are part of the same size distribution, and not merely a mix of independent microdiamond and macrodiamond sizes.

It is becoming increasingly common to quote the ratio of microdiamonds per 100 kg of rock and the ratio of microdiamonds to macrodiamonds. In Canada, a rule-of thumb is to recover at least one microdiamond larger than 0.1 mm per kilogram of rock and one macrodiamond (>0.5 mm) per 10 kg of rock to indicate economic potential.

STAGE 4: 6-12 MONTHS

If microdiamond results are promising, bulk testing of the kimberlites is undertaken to assess the grade of the commercial-sized diamonds and their average value.

MINI-BULK SAMPLING by large diameter drilling – recovery of macro-diamonds in plant for grade estimation followed by BULK SAMPLING – treatment in plant for diamond recovery and valuation (at least a 2,000 carat parcel is needed for reliable valuation as US$/carat)

BULK SAMPLING

The only way to get a definitive answer is to collect a “bulk sample” of 10 to 100 tonnes, for example, and process the sample to recover macrodiamonds (>0.5 mm in size). Usually sample processing is done via a dense medium separation plant (DMS or HMS), jig or diamond pan.

GRADE ESTIMATION

The estimation of diamond grade is the process of determining the average weight of diamonds per unit mass or volume of source rock. The diamond weight is a function of the number of the number of diamonds and the size of these diamonds. The average or mean stone size of recovered diamonds is obtained by dividing the total carats by the number of diamonds.

Grade can be expressed as;

Grade = diamond weight (carats) / sample weight (tonnes) or volume (m3),
and diamond weight = number of diamonds x mean stone size.

A sample size that returns about thirty diamonds (or “stones”) will give a “statistically significant” result but this may be impractical at the early stages of an evaluation program.

In a preliminary sampling program, where there is no past data on the grade of the deposit, a useful starting point to use the world average grade for commercial diamond mining operations as follows;

- average grade of 50 carats per hundred tonnes (cpht),
- average diamond size of 0.1 carats per stone (ct/st) and
- average diamond value of 50 US$/ct.

With this ‘guesstimate’ of the diamond grade and size, the number of stones per unit volume (or mass), the “stone density” in stones per cubic metre (st/m3) or stones per tonne (st/t), can be calculated. Using the relationship above and the world’s average data, then the following can be derived;

Stone Density = (Grade / Av Stone Size) per unit volume or mass,
therefore if the Grade = 50 cpht = 0.5 carats per tonne (ct/t)
and the Mean Stone Size = 0.1 ct/st,
then the Stone Density = 0.5/0.1 = 5 stones per tonne.

The results indicate that, on average, in the above example, 5 diamonds should be recovered from each tonne of processed source rock. To recover 30 diamonds 6 tonnes of source rock would have to be processed.

Typically, a deposit with a high stone density will be easier to evaluate than a deposit with a low stone density because the sample size required to recover a significant number of stones will be smaller.

The number of diamonds recovered from a sample will also depend upon the smallest size of particle recovered. In pipe evaluation, the macrodiamond is usually considered to be of a commercial size, i.e. greater than about 0.5 mm, and below this size the diamonds are considered microdiamonds and are not usually recovered during commercial mining operations. However, many exploration companies use different definitions of what is a macrodiamond or microdiamond and care should be exercised in using this term; unless it is clearly defined. Australian and North American exploration companies usually use the term microdiamonds for diamonds less than 0.4 or 0.5 mm in size and usually recovered during early stages of an exploration program.

STAGE 5: 3 MONTHS

If average value content seems better than US$ 40/tonne and near-surface tonnages exceed several million tonnes, a pre-feasibility study may commence, with more drilling, delineation drilling, geotechnical drilling and more bulk sampling to firm up tonnages, grade and value (resources) estimates.

STAGE 6: 6 MONTHS

Full feasibility study and permitting (environmental, legal, etc), with detailed mine planning.

STAGE 7: UP TO 2 YEARS

Mine and plant construction, infrastructure works.

STAGE 8

4 to 6 years from the initiation of exploration, production should commence.