Bulk Sampling

Diamond grades are usually expressed in carats per tonne (cpt) or more often, carats per hundred tonnes (cpht). World-wide, kimberlite grades in producing mines vary from approximately 25 cpht to over 400 cpht – it is the average dollar per carat value which is a determining step in the viability of a diamondiferous kimberlite to be a mine.  However, one metric carat is equivalent to 0.2 grammes in weight.  To obtain a sample to determine either the average value of a statistically valid number of diamonds or to be reasonably certain that the grade is representative of the kimberlite type, samples of usually several thousand tonnes are collected.

Operation of the bulk sampling plant falls under the control of Dr Alex Dak, Savannah's Laboratory and Special Projects manager.  Two additional plant personnel, of whom one has previous DMS experience have joined the team and were present during the commissioning of the plant in January.  The plant area is currently being upgraded into a secure zone with restricted entry before the first of the planned kimberlite samples are treated. Procedures are in place (and under continual review) to safeguard both the integrity of each sample and the handling and sorting of the DMS concentrate.

In Savannah’s exploration strategy, the kimberlites which have been located by the drilling of aeromagnetic targets are prioritised for further work based upon the mineral chemistry of their contained indicator minerals.  Modern diamond exploration often then uses the microdiamond content (all dimensions less than 0.5mm) of those prioritised kimberlites to determine which should be bulk sampled for macrodiamonds (greater than 0.5mm in all dimensions).  The extraction of microdiamonds requires caustic dissolution of the kimberlite sample and there are currently very few laboratories world-wide which undertake this hazardous work; the results backlogs vary from several months to two years.

Savannah’s technical team decided that the company cannot afford the delays occasioned by microdiamond sampling and proposed that those kimberlites which, after detailed indicator mineral analysis show a propensity be diamondiferous, be mini-bulk sampled to obtain macrodiamonds.  Samples will range from a few tens of tonnes to approximately one hundred tonnes, dependent upon the size of the kimberlite and the ease of collection of the sample.

A state-of-the-art one tonne per hour DMS unit was purchased from Bateman’s in South Africa and a crushing and screening front-end unit was fabricated by one of the Clarity Group companies, also in South Africa.  To effect diamond recovery, a Flowsort X-Ray sorter was purchased direct from the manufacturers in Johannesburg.   The DMS and its front-end were transported to Tanzania in November and December 2007 and both units were commissioned in early January 2008 at a site in Tabora.  The Flowsort unit is currently en-route to Tanzania

The first production sample to be treated in February 2008 will be 100 tonnes from the diamondiferous kimberlite Loya 1, to be followed by 100 tonnes from a known microdiamond producing kimberlite, Eagle 1. Thereafter, samples will be collected from the drilled kimberlites, prioritised for further work ,and a pipeline of bulk sampling targets developed.  At a DMS capacity of 1 tph, it is estimated that the first samples will take 3-4 weeks to process fully but that treatment time, with experience and practice, will decrease to a maximum of two weeks per 100 tonne sample.

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