The first reported samples of phosphate rich rock were dredged from the Agulhas Bank on the continental shelf of southwest Africa as early as 1891. In 1961, with the contemporaneous discovery of marine diamonds on the shelf and the recognition of the wealth of fisheries; detailed studies of bedrock and the oceanographic regime were undertaken by the Marine Diamond Company and the Russian Research Institutes of Oceanology, Marine Fisheries and Oceanography. During a series of investigative programmes, which commenced in 1976, under the direction of SANCOR (South African National Committee for Oceanographic Research), personnel from the Joint University of Cape Town/Geological Survey of South Africa Marine Geoscience Unit undertook to sample and map the coastal margin and shelf of SW Southern Africa. Over 3,000 seabed samples were taken from the continental shelf over an area extending northwards from the Agulhas Bank to the Namibian/Angolan border at the Kunene River. This research showed extensive deposits of phosphate rock (P2O5) and potassium-rich glauconite (K2O).
In 1999, in response to the recent developments in marine technology and a growing requirement for alternative fertilizer sources, Oceanic Minerals Namibia explored the commercial viability of mining glauconite sand, offshore Namibia. Oceanic Minerals held six exploration licenses covering an area of 6,600km squared located over an anomalous middle-shelf high, the Luderitz Bank, within water depths of 180-500m.
Reconnaissance geophysical survey data were acquired in September 1999. The survey was designed to identify the geological character of within the shallower eastern regions of the bank. The geophysical spread included single-line echo soundings, side scan sonar, 34.5kHz pinger and a 1" bolt airgun. This survey was followed in July 2000 by a bulk sampling campaign utilising a large volume grab. 82 bulk sampling stations were occupied and representative samples acquired.
The samples were processed for physical property analysis and X-ray diffraction (XRD) and X-ray fluorescence (XRF) measurements made on representative glauconite sands. This investigation was followed by series of petrographic studies, concentrating on the structure of individual glauconite grains using SEM and EDEX analysis.
The glauconites on the Luderitz bank showed several different grain types and colours, including composite grains.
In 2001, trials were carried out utilising washed Luderitz Bank glauconite as a potential organic slow-release K20 fertilizer at the University of Stellenbosch.
Due to interests elsewhere, privately-owned Oceanic Minerals gave up the permits in 2003.
Ocean Mineral Projects personnel were involved in the following:
- Geophysical Surveys
- Bulk Sample Site locations
- Sediment Analysis
- Testing glauconite on plants
- Diamondiferous potential of the Luderitz Grants
- Ilmenite potential of the Luderitz Grants
Why does glauconite occur on the Namibian Shelf?
Glaconitization commences immediately below the sea floor, often within the semi-confined microenvironments within the sediments. Mineralogically, glauconite may describe either greenish, round lobate pellets of sand size and the mineral glauconite, which is a secondary mica and the ferric equivalent of illite. The crystal structure can be well-ordered or disordered. Mixed layer glauconite/montmorillonites and composite glauconite/phosphorite grains also occur.
There is a general agreement that glauconites form by alteration of micaceous material. Four environmental prerequisites for phosphate and glauconite deposition are globally recognised:
- There must be free K2O and PO4 available in the system, either in the water column or liberated from decay of organic matter. Upwelling cells off Namibia, related to the Benguela Current system maintain high levels of free K2O and P2O4
- Ph must be high and free Mg must be low
- A surface must be available to grow the minerals, biotite, foecal pellets, foraminifera or radiolarian tests and other detrital silicates
- The area should be relatively free of continental run-off, which dilutes the process
Glaconitization commences immediately below the sea floor, often within the semi-confined microenvironments within the sediment. As glauconite matures, surface cracks develop (sutures), the K2O content increases and the lattice becomes more ordered. As the grains age, the sutures can become infilled with secondary glauconite or other minerals. As these infills mature, a natural beneficiation can occur, increasing the maturity and K2O content.
To read more about P2O5 on the Namibian Shelf go to our Sandpiper Project.