Grade of Arkansas `Crater of Diamonds’ sought

Diamonds from Arkansas made the big time recently — nothing less than the ultimate in prime-time television. And if a particular Arkansas gem wasn’t the highlight of the event, it was still there and far from inconspicuous: a 4.3-carat, canary-colored stone flashing on Hillary Clinton’s finger. The event was the inauguration of Bill Clinton as the 42nd president of the U.S.

Four carats is not a large stone but other high-quality stones, up to 16 carats, have come from the “Crater of Diamonds” near Little Rock, Ark. The “Crater” is a lamproite, diatreme complex. It was turned over to the state in 1970, designated a state recreational park and for the price of a US$3.75 day pass, anyone can enter the grounds and start digging. Best of all, if a visitor finds a stone, it is “finders, keepers.”

About 170,000 individuals tried their luck in 1992. No mechanical equipment is permitted but for those diehard optimists who work there day in and day out, there is overnight storage space for their manually-driven devices. While the grade of the pipe (carats per 100 tonnes) is not known, gross production of stones is believed to be 200,000 carats or 75,000 stones. The grade of the pipe is one of the factors the state government is in the process of finding out. With the data, the government will then be able to decide whether to lease the property for private development as a commercial diamond mine or to leave it as it is now — a paying recreational resource. To this end, a 4-phase exploration program is being financed by CRA, Kennecott, Arkansas Diamond and Continental Diamonds. Phase I was completed during the summer of 1992 and comprised 26 NQ-size (4.8-cm-diameter) diamond-drill holes totalling 2,600 metres. Angled drill holes confirmed the carrot shape typical of most diamond pipes, and vertical holes determined the structure extends to at least 190 metres below surface. A potential resource of 71 million tonnes is known to exist. Its grade is unknown. The purpose of Phase II is to determine the value of the pipe. The program entails cutting 30 trenches across the 32.4-hectare outcrop of the pipe to recover a targeted 1,000 carats of diamonds. This is the volume of sample needed to reliably determine the size and quality of the diamonds the pipe may be expected to produce in commercial production, and its overall grade. Exemplifying the complexity of valuing diamond pipes, each geological phase of the lamproite (or kimberlite in other cases) must be bulk-sampled separately. Each bulk sample is generally about 2,000 tonnes in size and if the pipe carries six different rock phases, then six times 2,000 tonnes is the size of the bulk sample needed. The amount of ground to be dug at Little Rock is tentatively estimated at up to 5,000 tonnes.

The decision to implement Phase II will be made this month.

Acting as technical adviser to the Arkansas state government, and project manager, is U.S.-based Morgan Mining and Environmental Consultants. Morgan is a sister company to Associated Mining Consultants (AMCL) in Canada and both form part of a global consulting network based in the United Kingdom. AMCL has operated in Calgary since 1976. A new office was opened in Toronto in December, 1992, with Bob Horn as director of minerals. Until recently, Horn was vice-president of exploration for BP Canada (since renamed Talisman Energy).

AMCL was formed to service the growing coal industry of Alberta and British Columbia several years ago but more than 60% of the group’s international business is now in hardrock and the percentage is growing.

According to Horn, the edge AMCL has over the competition is its direct access to a network of multi-lingual, mining, civil and business consultants. Aside from work in North America, AMCL has overseas environmental contracts in the Czech and Slovak republics and recently supplied the professional staff for a mine project in Iran.