EXPLORATION 1998 — Trans Hex pins hopes on diamonds

Alexander Bay, South Africa — Although it produces from several operations, Trans Hex Group remains focused on alluvial diamond production at its Orange River property near here.

Situated 50 km upstream from the Atlantic Ocean, the property covers a 10-km-by-75-km strip of the southern bank of the Orange River (which marks the border with Namibia) in the Northern Cape Province’s Namaqualand district.

Trans Hex, which is listed on the Johannesburg Stock Exchange, operates two mines on the property, Baken and Reuning, with aggregate production of between 50,000 and 65,000 carats of gem-quality diamonds per year. Of this amount, more than 75% comes from the Baken mine, making it the company’s flagship operation.

The alluvial diamond deposits in the western coastal region of South Africa owe their existence to the Vaal-Orange drainage system. According to geomorphologic studies, the system has followed two main routes over the last 80 million years. During the first 40 million years, the river flowed hundreds of kilometres south, discharging near the mouth of the present Olifants River. About 30 million years ago, the river was deflected to its present course, which ends at Alexander Bay, about 375 km up the coast from the Olifants River.

Three major episodes of tectonic uplift exposed diamond-bearing sediments and primary kimberlite (formed between 135 and 65 million years ago) in the interior to intense weathering. The first occurred about 10 million years after the river’s birth; the second episode occurred about 15 million years ago; and the third occurred about 2 million years ago. All three events coincided with extremely wet atmospheric conditions, and hence, intense fluvial activity.

Along the modern-day river, diamonds are hosted by two ages of gravels, known as proto- and meso-gravels. The proto-gravels, which were deposited at the time of the second uplifting event, are the only one of the two being mined, as they are higher in grade. This reflects the inland crust being thicker at the time of deposition. Proto-gravels are distinguished from the lower-grading meso-gravels by a deep yellow-brown color and a preponderance of metaquartzite clasts. Where preserved, they occur in irregular and incised channels in basement rocks that represent original flow patterns, or paleochannels.

The Baken mine is situated on one such paleochannel in the shape of a bow 7 km in length and averaging 1 km in width. Mining is currently focused on the northern and southern areas, known as Swartwater and Koeskop, respectively.

Combined reserves in the two areas stand at 7.2 million cubic metres, at a grade varying from 3 to 5 carats of diamonds per 100 cubic metres.

The operation processes 120,000 cubic metres of gravel per month for 11 months of the year, yielding 50,000 carats of diamonds. Minable reserves are sufficient for another six years of production.

Within the paleochannel, ore is generally restricted to the lowermost 5 metres of gravels that directly overlie granitic and metasedimentary rocks of Precambrian age. Above the ore zone there are up to 25 metres of intermediate gravels, which, historically, have averaged 1 carat or less.

Superimposed on the proto-gravels are sediments from the present river, which can reach up to 25 metres in thickness, or terraces of meso-gravels, which were deposited at the time of the third uplifting event.

During a recent visit to the project, The Northern Miner learned that Trans Hex is carrying out a major trenching program in the arc of the paleochannel. It is hoped the program will add another 12 years to the Baken mine’s life.

Two trenches were completed in 1997, and plans call for the excavation of seven more trenches over the next two years. Each trench which will cost about 12 million Rand to complete and will provide between 40,000 and 50,000 cubic metres of basal gravels for processing. Trans Hex hopes to recoup its excavation costs from the sale of the recovered diamonds.

The first trench completed last year, PK 19, contained 1.8 million cubic metres of material, from which the basal unit averaged 3.29 carats of diamonds per 100 cubic metres.

The second trench, PK 20, involved the removal of 1.4 million cubic metres of overburden and 150,000 cubic metres for treatment. Unlike the previous trench, both the intermediate and basal gravels carried economic quantities of diamonds, averaging 3.5 carats. The mineralized section was 21 metres high and 38 metres long, with the basal unit varying in grade from 5 to 8 carats of diamonds. The average stone size was 1.1 carats, only two-tenths of a carat less than the average size of diamonds recovered from the Swartwater and Koeskop areas.

Sampling of the intermediate gravels in trench PK 22 is under way. The sampling technique involves the excavation of successive levels (4 metres per level), from which are taken several widely-spaced, 6,000-cubic-metre samples. If each of the samples is found to carry between 0.65 and 1 carat per 100 cubic metres, then the whole level is directly fed through the plant. If, on the other hand, the samples grade higher than 1 carat per 100 cubic metres, the level is stockpiled for processing at a later date. Trans Hex follows a similar practice in the areas now being mined.

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Recovery plant

Trans Hex currently has four processing plants on site, one of which is used exclusively for exploration purposes. The bulk samples are first screened and washed to separate material having a grain size between 2 and 25 cm.

This material is then passed through a dense-media-separation circuit, where the heavy minerals, including diamonds, are concentrated. The concentrate is then transported in a sealed truck to a central recovery plant, where it is further reduced by X-ray sorting. Finally, the diamonds are hand picked in a secure building and sent to the company’s office in Cape Town, where they are sorted with diamonds mined from other projects.

Trans Hex currently has two 214-tonne excavators, a fleet of 90-tonne haulage trucks and ancillary earth-moving equipment working on the exploration program. Upon completion of the program, the equipment will be returned to the company’s mining operations.

Exploration elsewhere on the property has also borne fruit. At Nxodap, which lies between Baken and Reuning, two large trenches uncovered remnants of proto-gravels underneath meso-gravels. The first trench averaged 3.7 carats per 100 cubic metres; the second ran higher, at 8 carats. Approximately 70,000 cubic metres were taken from each trench, half of which consisted of proto-gravels.

At Reuning South, about 5 km south of the Reuning mine, Trans Hex has found geology similar to that of Nxodap. At Reuning South, the company is excavating two trenches similar in size to those dug at Nxodap.

“There is certainly no shortage of tonnage in the meso-gravels, but it’s low grade,” said Peter Danchin, Trans Hex’s managing director of operations. “So the uncovering of those remnant gravels has changed the whole tone of the exploration.”

Danchin added, however, that a lot more exploration is required before an economic assessment can proceed.

Meanwhile, further upstream on the Namibian side of the Orange River, the company’s foreign arm, Trans Hex International (THI-T), is preparing a trenching program on Block 9, a 2,950-ha concession covering a 3-km-by-10-km strip of the river bank. Trans Hex International owns 50% of the concession and

is operator.

To date, the company has completed 47,000 metres of drilling, which outlined a paleochannel 600 metres wide and 4 km long that is thought to have potential for 30 million tonnes of proto-gravels. The trenching program will test two targets in the paleochannel, Plunge Pool and Trough. Like Baken, the highest concentration of diamonds at both targets is found in the first 3 metres directly overlying bedrock.

Trans Hex expects to recover 30,000 cubic metres of basal gravels from each of the two trenches. The trenches will also provide 52,000 cubic metres of intermediate gravels from Plunge Pool and 96,000 cubic metres from Troug
h.

Plans are in the works for the construction of a 100-tonne-per-hour processing plant on Block 9. The plant, which is expected to be completed by September, will contain a dense-media-separation circuit capable of treating 25 tonnes per hour.

Trans Hex Group owns 65% of Trans Hex International and is itself owned 50% by the Rembrandt Group of South Africa.