Mining industry consultancy The MSA Group says the significant increase in cost-estimate requests it has received for diamond exploration testing this year is continuing.

“Even if these requests are for budgeting purposes, many clients still have to secure funding to start with a proposed project. However, interest in diamond exploration is definitely picking up,” notes MSA laboratory manager Hilde Cronwright.

Speaking to Mining Weekly in July, she said diamond exploration enquiries received by the company from junior explorers in Botswana, Lesotho and locally in the Free State, had increased considerably since May this year.

“MSA is currently busy with projects at different stages of exploration. In Sierra Leone, our client is doing early-stage exploration of kimberlite targets using systematic stream sediment sampling and recovering kimberlite indicator minerals (KIMs).

“Other clients from Lesotho have located some kimberlite dykes and need to test their diamond potential – based on the mineral chemistry – whereafter we will perform preliminary grade analysis using microdiamond testing at our new South African caustic fusion facility, the only one in Africa, constructed by [mining equipment supplier] Min-Met Equipment in Brakpan, Johannesburg. Another MSA client in Botswana is about to start bulk sampling of kimberlite on which MSA will perform microdiamond testing.”

Cronwright explains that, geologically, not all kimberlites are diamond bearing, and even if they are diamondiferous, it might not be economical to mine or they could contain only low-value stones.

Securing funding for early-stage exploration projects is challenging, as it is probably regarded as the riskiest commodity in which to invest, she adds. Therefore, MSA advises its clients on how to cost-effectively approach their project to make an informed decision on a project’s viability before embarking on more expensive bulk sampling.

Analytical Process
The exploration for kimberlite is typically done in several stages. Stage one involves regional targeting that includes mapping the Archean-age basement, desktop studies, as well as investigating historical diggings.

Stage two includes kimberlite detection through regional till or stream sediment sampling, and/or high-resolution aeromagnetic surveys to find pipe-like bodies.

Stage three comprises heavy mineral analysis to recover KIMs for mineral chemistry analysis to determine any diamond potential.

Stage four involves deposit delineation drilling to determine volume and lithology.

The fifth and final stage comprises the evaluation of the diamond grade and a mining feasibility study.

Different analysis methods are employed, depending on the stage of exploration.

“When a kimberlite still needs to be located, regional sampling programmes are designed to sample material from first order stream sediments, or glacial still samples that are taken over large areas,” Cronwright explains.

“The preparation of samples entails screening, concentration using heavy liquids like tetra-bromo-ethane with a density of 2.96 cm³ to concentrate all the heavy minerals, including KIMs. These concentrates are then sorted by experienced mineral sorters who can visually distinguish between kimberlitic minerals or similar minerals formed by other geological processes.”

Once the kimberlite target is located, several 5 kg to 20 kg samples can be taken to confirm their kimberlitic nature through the petrographic description of mineralogy in a thin section, step-wise crushing (if required, for rock samples or drill-core) to disaggregate minerals, heavy mineral concentration and KIM sorting. The recovered KIMs, comprising garnets, chrome-spinels, ilmenites and chrome-diopsides, are placed on mineral cards and analysed by electron microprobe.

Cronwright speaks highly of a published classification scheme authored by Herman Grütter and associates for use by diamond explorers, which explains the use of several mantle mineral chemistry discrimination diagrams used to predict whether the kimberlite has sampled crustal lithospheric mantle deep enough (and cold enough) to have formed and preserved diamonds.

The different garnet types discernible (discrimination between G9 and G10 garnets) indicate varying levels of favourability as to whether mantle conditions indicate diamond formation.

“After the kimberlite is located and the KIM chemistry confirm diamond potential, traditionally, the diamond grade will be determined by processing several tonnes of kimberlite material through a bulk sample diamond recovery plant by means of dense-media separation, X-ray diamond and/or grease-table recovery,” explains Cronwright.

“However, microdiamond analysis of a 200 kg sample can be done as a first-stage cost-effective evaluation of diamond grade to help prioritise targets for bulk sampling or [whether they should be discarded]. Should sufficient diamonds be recovered, the size frequency distribution from microdiamond data can be modelled to predict macro diamond grade and large stone frequency distribution information.”

MSA’s client base includes some of the world’s largest multinational mining companies, as well as private equity and sovereign wealth funds, governments and mining juniors listed globally.

On average, MSA undertakes 150 to 200 projects a year and routinely reports on all the mining stock exchanges.

MSA is ISO 9001-certified for quality and OHSAS 18001-certified for health and safety.  The Diamond Laboratory ISO/IEC 17025 accreditation is under temporary voluntary suspension owing to a recent MSA office move, and will be reinstated at the next South African National Accreditation System assessment scheduled for February 2017.