Marine mining at Marshall Fork feature a definite success

Higher-than-expected grades and the successful commissioning of a new marine mining ship have spelled success for Trans Hex Group and Diamond Fields International (DFI-T) in a joint venture off the coast of Namibia.

Operator Trans Hex Group, the South African parent of Trans Hex International (THI-V), and property holder Diamond Fields International have been making an operating profit since beginning mining at the Marshall Fork feature, part of Diamond Fields’ Mining Licence 111, in the near-shore area off Luderitz Bay.

Mining started in May of last year, using Trans Hex’s mining vessel MV Ivan Prinsep while a second ship, MV Namakwa, was being fitted out for the joint venture’s exclusive use. Initial test mining by the Ivan Prinsep — done to assess the usefulness of the ship’s “airlift” mining system — recovered 3,100 carats in the first month, and by the time the Namakwa was ready, in late October, the Prinsep had recovered a further 6,600 carats.

The larger Namakwa has a 50-cm airlift tube for mining and on board is a dense-media separation plant capable of processing 50 tonnes per hour. Separated material goes through an on-board X-ray diamond recovery unit and a fully hands-off sorter.

In addition to the airlift tube, Namakwa uses a water jet system to bring up more sediment. When a second mining pass is made on an area that has been mined using airlift only, the jet system can increase recoveries by about 30%.

Crews do a 28-day rotation aboard the ship.

Results to date indicate that the mined grade has been significantly higher than the grade predicted in earlier resource estimates. Mined grades in January were 70% higher than estimated. Marshall Fork had been estimated to carry a grade of 0.5 carat per sq. metre of seafloor, in an area of 865,000 sq. metres; estimates put the grade of the 2.1-million-sq.-metre Diaz Reef feature at 0.22 carat per sq. metre.

Diamond Fields geologist Randal Cullen suggests this may mean features previously interpreted to have lower grades may be feasible to mine, which could possibly bring as much as 1.7 million sq. metres into the operation’s reserve base.

Added to the operational success of the ships, prices for high-quality rough increased by about 20% in the last quarter of 2001.

At the end of 2001, Diamond Fields had an inventory of 3,800 carats of diamonds, on the books at a cost of US$451,000. The stones were sold in January for US$840,000. Typical offshore operations recover stones of which more than 95% are gem quality.

All told, the joint venture produced 16,470 carats in the six months ended Dec. 31, with 60% of the share going to Diamond Fields and 40% to Trans Hex. In January, the first month of mining on the Western Marshall Fork feature, the Namakwa recovered another 4,000 carats.

Trans Hex is the operator of the joint venture; Diamond Fields handles downstream marketing of the rough. Diamond Fields takes a 60% share in the costs of the operation and in the revenue stream. Once Diamond Fields has recovered all its pre-production costs, to a maximum of US$8.5 million, the joint venture reverts to an equal share.