On the river bed - extracting alluvial diamonds
Exploring and extracting alluvial diamonds is challenging. Michael Forrest talks to Martin Prinsloo, of Dreamstone Mining, London, UK, about their methods.
Diamonds are found in volcanic rocks in the oldest parts of the continental crust. Two types of diamond-bearing rocks are known – lamproites that form relatively shallow crater facies or kimberlites that are deep carrot-shaped bodies. There is only one lamproite diamond mine in Argyll in Western Australia, in contrast to the numerous kimberlite mines in Southern Africa.
Releasing them from rock by crushing may damage stones, however, nature has found a way of releasing them through weathering. In Southern Africa, the greatest period of erosion occurred during the Cretaceous (154-65Ma) and diminished with time through the Tertiary (65-1.8Ma). It is estimated that the uppermost 1,400m of the kimberlite pipes in the Orange/Vaal river catchments were removed and transported into the westward drainage systems.
In kimberlite pipes, grades are known to decrease towards the root of the intrusion and, as a result, the best and largest stones are found in alluvial deposits eroded from their upper regions.
Exploring for alluvial diamonds, however, requires a different skill set to kimberlite exploration. The objective is to identify ancient river systems and their residual sands and gravels.
One of the most useful methods is interpreting aerial photographs to illustrate the subtle changes that reflect the underlying topography and geology. They are particularly useful when taken after summer rains, showing standing water and/or greener vegetation. Infrared spectrum photos are useful here, while geophysical methods are of limited help, and the final arbiter in any alluvial exploration is the drill bit.
Parts of the Orange/Vaal river systems have been well explored, especially those in the lower reaches where ancient terraces abound. The middle Orange catchment poses a different problem. Here, exceptional quality diamonds are found in two gravel horizons – an upper wind-blown deposit that varies from a few centimetres to two metres thick, known as the Rooikoppie, and a deeper basal primary gravel that rests 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 to penetrate the calcrete. A few mining operations in the Rooikoppie yielded gemstones in the 50-100 carat range with typical stones in the one-to-two carat range with values above US$1,000 per carat’, states Martin Prinsloo of Dreamstone Mining, in London, UK.
His company has a mining license over the Vaal River gravels that occur in three terraces 20, 60 and 105m above the present course. Gravels can be as much as 10km from today’s river channel with the 105m terrace being the oldest and containing the best diamond grade. ‘Ground observations are made difficult by farming and irrigation systems that mask the natural features’, he says.
Exploration in this terrain for diamonds, found in concentrations grading 0.3-0.8 carat per hundred tonnes (cpht) of gravel, is not easy. It requires knowledge of the underlying relief and type of bedrock obtained through drilling, especially in those areas where there may be sediment traps. Bedrock riffles and rock bars cause energy and velocity fluctuations in a river system, resulting in variable hydraulic energy in the gravel bed.
‘This chaos, in an otherwise perfect river stream causes diamond enrichment within gravel pockets that is almost impossible to detect during a drilling programme and needs trial mining to confirm,’ explains Prinsloo.
In the case of the Vaal River, there is also a lot of high-density banded ironstone in the gravels, which interferes 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 signature and density can confuse geophysical surveys.
Alluvial diamonds migrate towards the bottom of a fast flowing gravel mass If slowed by an obstruction, the bigger stones settle first with the smaller ones accumulating downstream. This natural sorting process shows why big stones are discovered in isolation in relation to the bulk of the parcel.
Eluvial (reworked primary alluvials) deposits tend to be lower grade as the natural accumulations are dissipated. Percussion is the drill of choice, allowing measurement 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 where bedrock traps may be identified. However, unlike gold or copper disseminated in bedrock, the low diamond concentrations of less than one carat per hundred tonnes (1 in 500 million) lead to a significant nugget effect where one stone may be the only one in 1,000t.
Making the grade
Although calculating the tonnage of a gravel deposit is not difficult, establishing the grade is. Trial mining is the only answer with bulk samples in the range 50,000-100,000t. Records of historical gravel mining can be useful.
In the past, exploration in limited areas of the Vaal River has recorded an average grade of 0.75cpht and average value in the US$1400/ct range in Pleistocene, Pliocene and Miocene gravels that range in age from the present day to 23Ma.
In the oldest terraces, Dreamstone has indicated a resource of 3,310,675Mt of primary basal gravel. Overall, it is estimated that at least another 5.5Mt of diamondiferous gravels are present.
‘The capital cost of alluvial mining is much less than hard rock operations’, states Prinsloo. The cost of the Vaal River development expansion project is around US$5m including machinery, equipment and running expenses for the first 12 months.
In addition to gravity separation equipment, the only major requirement is a 85t (or greater) excavator, as this size is needed to break through the calcrete layer above the primary gravels.
In operational costs, alluvials are much cheaper with a US$3/t mining cost and a similar processing cost. A mining rate of 145,000t per month, sufficient to supply two separating pans, would yield about 1,100ct of quality stones with a value of just over US$1,500,000.
‘Our agreements with landowners include a 10% royalty that, together with opex, costs a total of US$535,000, leaving a 64% profit margin,’ Prinsloo says.
Martin Prinsloo, Dreamstone Mining, 9 Devonshire Square, London, EC2M 4HP, UK. Tel: +44 (0)207 548 4092.