Metasomatic Diamond Formation revealed by X-Ray CT Scanning of Diamondiferous Eclogites from Southern Africa

Abstract:

In this study, a private collection of diamondiferous eclogite xenoliths has been made available for non-destructive investigation. All samples have at least one diamond visible. The samples are predominantly sourced from the Excelsior and Newlands mines (South Africa), with additional samples from Roberts Victor mine (South Africa) and Orapa (Botswana).

3D volume models of the samples were created using X-ray tomography. The 3D images reveal abundant secondary veining that is clearly younger than the eclogite. Diamonds are located in fluid pathways and occur in both altered garnet and altered clinopyroxene. Most of the veining is unrelated to the spatial positioning of diamond in the samples. In some instances, early veining has annealed or partially annealed, suggesting a range in timing of at least some of the several metasomatic events that have affected the rock.

Importantly, in the most graphic examples, a clear distinction can be seen between diamond-bearing and non-diamond-bearing veins, even where sulphide is present in abundance in the non-diamond-bearing veins. The amount of diamond detected in the xenoliths varies from a single crystal to well over 50 diamonds forming more than 9% of the rock. This extreme value contrasts with the diamond recovery from currently viable diamond mines of less than 2ppm or 0.0002%.

The morphology of the diamonds includes step-faced flat-faced octahedra, single crystals and aggregates. This is particularly a feature of diamonds in the Excelsior specimens. In the samples from Newlands and Orapa, in contrast, diamond surfaces reflect resorption processes such as rounding and corrosion of the diamonds.

The following conclusions can be drawn from this study:

• Diamonds in this collection, sourced from within the Kalahari craton, appear to have formed by a metasomatic process during which fluids infiltrated pre-existing mantle-derived eclogite;
• Several metasomatic events have occurred during the residence of the eclogite in the mantle;
• Some of these metasomatic events have been diamond-friendly, whilst others have been diamond-neutral or diamond-destructive;
• Diamond can be present at very high concentrations along particular metasomatic fluid pathways in eclogitic mantle rocks;
• The absolute timing of diamond formation is still to be determined.