Diamondiferous gravel barrier beach placer: Morpho-sedimentary dynamics and diamond distribution connectivity, SW Namibia

Abstract

The Atlantic Coast of south-western Namibia is renowned for its Pliocene to Holocene diamondiferous deposits, which contributed more than 63 million carats of diamonds to the global market. A continuum of fossil shoreline deposits associated with barriers to linear and pocket beaches stretches over circa 110 km in length. Of these, the most highly economic are the fossil linear and pocket beach deposits that are preserved over highly competent Proterozoic bedrock. Potholes and gullies, the prominent bedrock-trapsite features, have controlled diamond concentration. By contrast, the barrier shorelines that developed on unconsolidated fluvial and marine sediments at a palaeo-river mouth are less productive. Here, trapsites are absent and diamond accumulation and retention occurred via interstitial trapping in a coarse gravel framework under highly energetic littoral settings. It therefore follows that diamond enrichment is governed by the sedimentological characteristics of the gravel allied to the dynamics of their depositional environments. We examine the relationship of diamond enrichment with the littoral processes at an environmental and subenvironmental level of a latest Pliocene/earliest Pliocene gravel barrier beach complex (BBC). Results show variability in both diamond concentrations and sizes across the barrier that commensurate with a diversity of morpho-sedimentary dynamics.

The study, a first of its kind, provides insights into the diamond enrichment of a BBC. It demonstrates how detailed sedimentological reconstruction of the barrier environments can improve the exploration efficacy of such diamond-bearing littoral placers.