As junior mining companies struggle to keep the great Canadian diamond hunt alive in the Northwest Territories, African diamond producer Redaurum (TSE) is preparing to triple production at its River Ranch mine.
During a recent site visit, The Northern Miner learned that the company has embarked on the last phase of a 3-part expansion program that will see production increase from half a million to 1.5 million tonnes of kimberlite ore per year by the end of 1995. With the expansion, diamond output over the next two years is expected to reach 500,000 carats per year. The River Ranch kimberlite was discovered in 1974, just inside the border with South Africa, about 12 km west of Beit Bridge. The discovery came two years after Kimberlitic Searches, a wholly owned subsidiary of De Beers, found an alluvial diamond in a stream draining the deposit. De Beers subsequently relinquished control of the deposit following a dispute with the Zimbabwean government.
In June, 1991, Cornerstone Investments and its 50% joint-venture partner, Auridiam Consolidated NL of Australia, acquired the property through open tender. During July, 1993, Redaurum completed the acquisition of Cornerstone, as well as that company’s sole asset, a half interest in the private operating company Auridiam Zimbabwe, which owns and operates River Ranch under a 25-year mining lease. The lease can be extended for a further 25 years when the current licence expires.
Construction of Phase I, a trial rotary pan plant, began in December, 1991, and initial pilot plant production followed in March, 1992. Late in 1993, a Heavy Media Separation (HMS) module and two X-Ray Sortex diamond recovery units were added to the operation. Last year, total diamond production reached 151,396 carats.
The River Ranch kimberlite is shaped like a dumbbell, covering a total surface area of 5.2 hectares. The pipe measures 500 metres long in an east-west direction, and its width ranges from 60 to 150 metres. Although significantly different from the theoretical carrot shape of most diatremes, the River Ranch is similar in shape to the Dokolwayo kimberlite in Swaziland and the K1 Kimberlite in Venetia, South Africa.
The kimberlite orebody is steeply dipping and consists of three zones: the upper crater; the middle diatreme; and the lower root. It is a Group I Kimberlite and consists of six intrusive phases and two reworked crater facies phases which occur in the diatreme as “floating reef.” All of these phases are diamondiferous but of varying grade.
Initially, it was envisioned that the deposit would be mined to a vertical depth of 150 metres via an open pit. Drill-indicated reserves to this level are 17.5 million tonnes, containing an estimated 5 million carats. Under the current mine plan, the deposit will be exploited to a depth of 200 metres, which should result in a 30% increase in reserves.
With an overall stripping ratio of 2.1-to-1, the pit is expected to have a life of 15 years, after which mining will be carried out from underground. Mining is performed by a contractor, with loading and haulage of ore and waste carried out on two 10-hour shifts. To date, little blasting has been required since the deposit is oxidized to a vertical depth of 30 to 40 metres. Ore removed from the pit is passed through a scrubber, where it is reduced in size. Material finer than 1 mm is sent to the tailings, while material greater than 35 mm is sent to a coarse stockpile for later processing. Anything between these two size ranges is then processed in either the rotary pan plant or the HMS circuit. Resulting concentrates are sent through the X-Ray Sortex plant, with the rejects being passed over grease tables. At present, mining and processing costs amount to less than US$5 per tonne. The Phase III plant will include in-pit primary crushing, re-crushing, secondary scrubbing and screening circuits. The introduction of these units and conveyor stock feed will significantly increase the volume of ore processed. Two additional HMS units will also be introduced to avoid any bottlenecks at the concentration stage. The existing Sortex units will have the capacity to handle the increased throughput of concentrates from the new HMS units.
Once the third phase is completed, capital costs incurred by both partners at River Ranch will total US$13.6 million. Most of Redaurum’s financial commitments have been met through cash flow from diamond sales. With the Phase III expansion, diamond output over the next two years is expected to reach 500,000 carats per year as weathered kimberlite ore is mined. Thereafter, annual recoveries are forecast to stabilize at a level above 400,000 carats per annum as the kimberlite hardens at depth. All production from River Ranch is subject to a 0.8% royalty, which is payable to the government of Zimbabwe.
According to Redaurum President Anthony Hamilton, 65% of the diamonds recovered are gem quality, with 10% being greater than 1 carat in size. Redaurum’s share of production is sold by tender in Antwerp, Belgium. “We take the goods to market as they are produced,” Hamilton told The Northern Miner.
In order to get better prices for its stones, Redaurum does not disclose the actual prices received per parcel. However, Hamilton did estimate that in 1995, the company will generate revenue in excess of US$10 million from its sale of production, estimated at 210,000 carats.
With the expansion program well in hand, the company plans to carry out additional exploration on the property. Redaurum Chairman Baxter Brown, a well-known diamond explorationist, explained that over the next two months three other potential kimberlite targets will be investigated. He said these targets were identified through spectral analysis, and they all exhibit coincident indicator mineral anomalies.
Brown is also supervising the company’s exploration efforts on other properties in Zimbabwe, as well as in Botswana, Namibia and South Africa. Elsewhere on the production front, Redaurum signed a deal to acquire the operating Quaggas Kop alluvial mine, 275 km north of Cape Town, South Africa. Last year, the mine produced 4,800 carats of gem-quality diamonds. At the Kelsey Lake project in Colorado, Redaurum is constructing a 250,000-tonne-per-annum processing plant to facilitate pilot production in July at an annual rate of 25,000 carats. In addition, a prefeasibility study will be carried out, which, if positive, could lead to full-scale mining at a rate of 100,000 carats per year by 1997.
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