MATERIALS WORLD December 201032

Diamonds are found in volcanic rocks in theoldest parts of the continental crust. Two typesof diamond-bearing rocks are known –

lamproites that form relatively shallow crater facies orkimberlites that are deep carrot-shaped bodies. Thereis only one lamproite diamond mine in Argyll in WesternAustralia, in contrast to the numerous kimberlite minesin Southern Africa.

Releasing them from rock by crushing may damagestones, however, nature has found a way of releasingthem through weathering. In Southern Africa, thegreatest period of erosion occurred during theCretaceous (154-65Ma) and diminished with timethrough the Tertiary (65-1.8Ma). It is estimated that the uppermost 1,400m of the kimberlite pipes in theOrange/Vaal river catchments were removed and transported into the westward drainage systems.

In kimberlite pipes, grades are known to decreasetowards the root of the intrusion and, as a result, thebest and largest stones are found in alluvial depositseroded from their upper regions.

Exploring for alluvial diamonds, however, requires adifferent skill set to kimberlite exploration. The objectiveis to identify ancient river systems and their residual

sands and gravels. One of the most useful

methods is interpretingaerial photographs toillustrate the subtlechanges that reflect theunderlying topographyand geology. They areparticularly useful whentaken after summer rains,showing standing waterand/or greener vegetation.Infrared spectrum photosare useful here, while geophysical methods are

Main image: Separation plant

(14ft), used for trial mining.

Below: Double 14ft

separation plant with

scrubber and vibrating

grizzly feeder for clay gravels

of limited help, and the final arbiter in any alluvial exploration is the drill bit.

Ground forceParts of the Orange/Vaal river systems have been wellexplored, especially those in the lower reaches whereancient terraces abound. The middle Orange catch-ment poses a different problem. Here, exceptional quality diamonds are found in two gravelhorizons – an upper wind-blown deposit that variesfrom a few centimetres to two metres thick, known asthe Rooikoppie, and a deeper basal primary gravel thatrests on bedrock shales. Overlaying these is a hard calcrete (a natural cement) bed some 8-10m thick.

‘In the past, miners using hand tools were unable topenetrate the calcrete. A few mining operations in theRooikoppie yielded gemstones in the 50-100 caratrange with typical stones in the one-to-two carat rangewith values above US$1,000 per carat’, states MartinPrinsloo of Dreamstone Mining, in London, UK.

His company has a mining license over the VaalRiver gravels that occur in three terraces 20, 60 and105m above the present course. Gravels can be asmuch as 10km from today’s river channel with the105m terrace being the oldest and containing the bestdiamond grade. ‘Ground observations are made difficult by farming and irrigation systems that mask thenatural features’, he says.

Exploration in this terrain for diamonds, found inconcentrations grading 0.3-0.8 carat per hundredtonnes (cpht) of gravel, is not easy. It requiresknowledge of the underlying relief and type of bedrockobtained through drilling, especially in those areaswhere there may be sediment traps. Bedrock rifflesand rock bars cause energy and velocity fluctuations ina river system, resulting in variable hydraulic energy inthe gravel bed.

‘This chaos, in an otherwise perfect river streamcauses diamond enrichment within gravel pockets thatis almost impossible to detect during a drilling

MATERIALS WORLD December 2010 33

On theriver bed

Exploring and extracting alluvial diamonds is challenging. Michael Forrest talksto Martin Prinsloo, of Dreamstone Mining, London, UK, about their methods.

Plant commissioning

in Southern Africa

programme and needs trial mining to confirm,’ explainsPrinsloo.

In the case of the Vaal River, there is also a lot ofhigh-density banded ironstone in the gravels, whichinterferes with the gravity separation of diamonds(heavier specific gravity than the 3.5 of diamonds).Fortunately, magnetic separators can remove 40-50%of banded iron formations, but their magnetic signatureand density can confuse geophysical surveys.

Alluvial diamonds migrate towards the bottom of afast flowing gravel mass If slowed by an obstruction,the bigger stones settle first with the smaller onesaccumulating downstream. This natural sortingprocess shows why big stones are discovered in isolation in relation to the bulk of the parcel.

Eluvial (reworked primary alluvials) deposits tend tobe lower grade as the natural accumulations are dissipated. Percussion is the drill of choice, allowingmeasurement of the thickness and nature of the gravels and overburden. A typical plan is a 50m interval along sections 100m apart.

In selected areas, the density is increased wherebedrock traps may be identified. However, unlike goldor copper disseminated in bedrock, the low diamondconcentrations of less than one carat per hundredtonnes (1 in 500 million) lead to a significant nugget effect where one stone may be the only one in1,000t.

Making the gradeAlthough calculating the tonnage of a gravel deposit isnot difficult, establishing the grade is. Trial mining is theonly answer with bulk samples in the range 50,000-100,000t. Records of historical gravel miningcan be useful.

In the past, exploration in limited areas of the VaalRiver has recorded an average grade of 0.75cpht andaverage value in the US$1400/ct range in Pleistocene,Pliocene and Miocene gravels that range in age fromthe present day to 23Ma.

In the oldest terraces,Dreamstone has indica-ted a resource of3,310,675Mt of primarybasal gravel. Overall, it is estimated that at least another 5.5Mt of diamondiferous gravelsare present.

‘The capital cost of alluvial mining is much lessthan hard rock opera-tions’, states Prinsloo. Thecost of the Vaal Riverdevelopment expansionproject is around US$5mincluding machinery,equipment and runningexpenses for the first 12months.

In addition to gravity separation equipment, the onlymajor requirement is a 85t (or greater) excavator, asthis size is needed to break through the calcrete layerabove the primary gravels.

In operational costs, alluvials are much cheaper witha US$3/t mining cost and a similar processing cost. Amining rate of 145,000t per month, sufficient to supplytwo separating pans, would yield about 1,100ct of quality stones with a value of just overUS$1,500,000.

‘Our agreements with landowners include a 10%royalty that, together with opex, costs a total ofUS$535,000, leaving a 64% profit margin,’ Prinsloosays.

Further information

Martin Prinsloo, Dreamstone Mining, 9 Devonshire

Square, London, EC2M 4HP, UK. Tel: +44 (0)207 548 4092.