Tertiary-age diamondiferous fluid deposits of the lower Orange River valley, Southwest Africa

Abstract

The Orange River, the principal conduit for transportation of diamonds from the southern African interior to the Atlantic coast, has within its lower valley two recognized suites of gravel terraces in which part of the passing diamond population has been trapped. Both suites lie on eroded bedrock and both are downstream thickening-and-fining coarse clastic wedges. The older and higher terrace suite, comprising the Arries Drift Gravel Formation, is early mid-Miocene in age (19-17 Ma) and is referred to locally as the Proto-Orange terrace. The lower and younger terrace suite, locally known as the Meso-Orange terrace, has not yet been dated but is considered to be Plio-Pleistocene in age. Middle to Late Proterozoic bedrock, underlying both suites of terrace deposits, was deeply eroded during the incision of the Orange River which began in Late Cretaceous times. The Proto-Orange terraces, richer in diamonds than the younger Mesozoic deposits, form low-grade but large-average, diamond-size, gem quality placers which are mainly discussed here. Diamonds are best concentrated in cobble-boulder basal gravels on or close to the rough bedrock contact in three main trapsite settings: scour pools hosting oversize (obstacle) clasts return grades of 10 to 40 carats per hundred ton (cpht); push bars, where the grades range from 10 to 55 cpht; and bedrock highs with their associated oversize clasts, yielding grades of 6 to 12 cpht, do not concentrate diamonds to the same extent as the two former trapsites, but occur over a wider area and thus potentially enrich a greater volume of gravel. The average diamond (stone) size in these three trapsites is also upgraded, ranging from 1.5 to 2.8 carats per stone (ct/stn). The key factors in concentrating diamonds at the bedrock-gravel interface are turbulence scale and intensity created by the rough boundary conditions and the presence of fixed bedrock sites of turbulence, which are stable enough to initiate and retain around them the slow and stable growth of gravel that hosts the concentrated diamonds. The upper, more mobile gravels have far lower concentrations of diamond than the basal gravels, with grades dropping to an average of 0.3 cpht and stone size decreasing to 1 to 1.3 ct/stn. Sites of higher concentration in the upper gravels of the Proto-terraces are bar platforms/riffles (1 to 3 cpht and average stone sizes of 1.1 to 1.3 ct/stn) and bar heads (0.7 to 1.5 cpht and average stone sizes of 1 to 1.3 ct/stn) while bar tails are practically barren. Although the vertical decline in diamond concentration may be attributed to a decrease in diamond supply during the aggradation of the Proto-Orange gravels, the younger, lower grade Mesozoic gravels also display a similar vertical profile. Hence, diamond concentration in the lower Orange River deposits is influenced positively by the occurrence and nature of the localized fixed bedrock trapsites. From an exploration point of view, these fixed trapsites are more difficult to locate, often requiring extensive drilling, whereas the areally extensive but low-grade upper gravels are relatively easy to delineate. During exploitation, the localized but higher grade fixed trapsites provide a reliable source of above average-sized diamonds in an otherwise low-grade.