Summary:
The Namakwa Sands HM deposit occupies an ellipsoidal area of 15 kilometers northeasterly by 4 km wide and is interpreted to be an ancient dune complex shaped by prevailing winds at the time of its formation. Repetitive cycles of erosion from crystalline source rock, fluvial transport and prolonged reworking by water and wind formed the deposit. The Namakwa Sands heavy mineral assemblage is heterogeneous, creating challenges to efficient recovery of valuable heavy minerals.
The NE-SW dimension is interpreted to reflect prevailing winds at the time of the deposit’s formation. A narrow sub-economic corridor divides the reserves into two proximal ore bodies, Graauwduinen West and Graauwduinen East, which are more commonly called the “West” and “East” deposits. Nearly two-thirds of historic ore production has been extracted from the West mine pit, to a maximum depth of about 45 meters. In the medium term, 60-65% of extracted ore will be mined from the West pit, but the long-term LOM Plan calls for a nearly even split between West and East mines.
The very large Namakwa HM deposit is broadly the result of prolonged, repetitive weathering-erosion-deposition cycles that were initiated with the breakup of the Gondwana Supercontinent approximately 100 million years ago. The separation of the African and South American proto-continents triggered weathering and erosion of massive volumes of sediment from high-grade metamorphic crystalline “basement” rocks of the one billion-year-old Namaqua-Natal orogenic belt. The sandy sedimentary sequence that hosts our Namakwa HM deposit is interpreted as being derived mostly from the Namaqua-Natal metamorphic belt which was welded onto the western and southern margins of the 2.6+ billion-year-old Kaapvaal Craton. High grade metamorphism facilitates the partitioning of titanium into ilmenite and rutile crystals making them available for erosion, transport and deposition to form economic deposits. Heavy mineral concentrations in beach placers, marine terraces and in coastal dunes were reworked by water and wind into what is now our Namakwa heavy mineral deposit, the end-product of 100 million years of geologic evolution.
The Namakwa Sands deposit consists of two adjacent orebodies, referred to as the Graauwduinen West orebody and the Graauwduinen East orebody, which are named after the farm Graauw Duinen, the discovery site. A SE-NW trending depression called Langlaagte Corridor defines the border between the two orebodies.
The Graauwduinen West orebody
The Graauwduinen West orebody comprises a barren paleodune complex that is overlain by a series of elevated strandline deposits, which in turn have been largely reworked into a dune sequence superimposed with duricrust. Free-flowing cover sands terminate this stratigraphy.
The eastward-thickening, shallow-marine succession of Strandline East represents the first major stage of local marine sedimentation. This raised, fossilized strandline deposit lies approximately 2 km inland from the current coastline and displays typical log spiral morphology.
In a northerly direction Strandline East is about 5.5 km long, up to 1 km wide, and 5 m thick on average. Eastward it pinches out around 50 m amsl. Northward the Other Sand underlie Strandline East, but to the south downward to 20 m amsl, it covers bedrock directly.
The basal unit of the Orange Feldspathic Sands Waste (OFSW) comprises a 1m to 2m-thick, fairly developed duricrust horizon. Thin, localized mud pans and sandy colluvial lithologies are often interfingered. The following lithology consists of an unconsolidated, distinctly dark-yellow, moderately sorted, medium-grained sand. The aeolian fossil contents peak in this unit, resulting in an anomalous phosphorous signature particular to the 75- to 90-m amsl level and surrounds.
Strandline West characterizes the next major marine transgression to a maximum elevation of 30 m amsl. This strandline deposit exhibits similar features to Strandline East but is about half the size.
The third mineralized dune succession called Orange Feldspathic Sands Mineralized (OFSM) hosts the bulk of the ore of the Namakwa Sands deposit. Its four lithologies form a relatively massive, seaward-thickening wedge, which can be up to 30 m thick. The basal portion constitutes a yellow, well developed duricrust horizon, referred to as the Hards, which cemented an assortment of terrestrial fossil types. In the western part of the Graauwduinen West orebody the Hards overlie reworked Strandline West, but toward the east it rests on the top of the Orange Feldspathic Sands Waste. Compared to the Subhards, the Hards are also predominantly calcareous but have a higher clay content.
The fourth mineralized dune succession, which is distinctly rubified, includes a complex duricrust horizon called Dorbank, which is overlain by free-flowing Red Aeolian Sands. The characteristic red coloration of both these units relates to prolonged oxidation of ferruginous minerals in a hot and arid climate that has marked the area since the Quaternary. The Dorbank occupies the top part of the Orange Feldspathic Sands Mineralized and is mapped across the entire Graauwduinen West orebody. The vertical thickness cementation is inconsistent, ranging up to 15 m, and laterally it can be extremely discontinuous. On a larger scale, the Dorbank of manifests thickest in topographic depressions, thinning to the northeast and southwest flanks, approximating basin-like morphology.
Intensely reddened, fine-grained, moderately sorted, up to 5 m thick, free-flowing sands of the Red Aeolian Sands (RAS) cover the Dorbank unconformably. These aeolian sands are generally structureless with abundant fauna and flora taxa relics.
The Graauwduinen East orebody
The Graauwduinen East orebody represents surficial aeolian sands, overlying a clayeous dune sequence, cast on top of barren sands. Unlike the generally flat and scoured bedrock profile encountered in the Graauwduinen West orebody, the bedrock in the Graauwduinen East orebody displays extreme undulation and outcrops frequently, noticeably to the southeast.
The bulk of the ore in the Graauwduinen East orebody is represented by the above-lying Orange Feldspathic Sands Mineralized, but the two constituting lithologies are much thinner than found in the Graauwduinen West orebody. The average thickness is about 5 m, but in the eastern part of the Graauwduinen East orebody the Orange Feldspathic Sands Mineralized can be up to 20 m thick.
In the Graauwduinen East orebody, the base of the Orange Feldspathic Sands Mineralized is cast as a laterally continuous, single layer duricrust horizon, called Hardpan that can reach up to 5 m in thickness. Its streaky, orange-white, rust-like appearance is very different to the Subhards or Hards found in the Graauwduinen West orebody. Instead, it resembles the type of duricrust that underlies much of the Namaqualand coastal plain. The physical competency of the Hardpan is also considerably weaker than the duricrust mapped in the Graauwduinen West orebody and it appears compacted rather than lithified.