3.4.1 |
KwaZulu-Natal Deposits
The heavy mineral deposits within the KwaZulu-Natal dune corridor have been interpreted to represent remnant beach deposits of a large coastal dune system that developed on the Mozambique coastal plain during the late Pliocene Era, approximately three million years ago.
Hillendale and Fairbreeze Project deposits are situated within the same geological environment, consisting of the Late Tertiary-Pleistocene dune corridor developed along the Natal coastline. The original South African coastline was generated by rifting processes involved in the fragmentation of Gondwanaland during Jurassic and Cretaceous times. By the end of the Cretaceous, the present configuration of southern African coastline was established and since that time local modifications have taken place, resulting from tectonic uplift, eustatic changes and flexuring of the sub-continent.
Tectonic events and coupled eustatic changes have resulted in the Mozambique coastal plain being subject to several marine transgression-regression cycles during Tertiary and Pleistocene times. A major Eocene transgression resulted in deposition of Eocene marine sands on the Mozambique coastal plain. Relict shoreline features occur around the southern African coastline, often at high levels (+100m) above the current sea level. Beach gravels and dunes up to 100m above mean sea level are common on the eastern seaboard of South Africa and there appear to be at least two major beach elevations at 30m above sea level and 60m above sea level. Pleistocene dune ridges are prominent along the Natal shoreline and Eastern Cape. The highest dunes are dominated by homogenous red sands of the Berea Formation that preserve many aeolian features. The red colouration of the sand is believed to be associated with weathering of heavy minerals under tropical conditions. Detailed histories of these deposits are very complex and evidence for submergence of some of the Pleistocene dunes is unequivocal. Relict submerged dunes are also present off the coast of Natal, recording periods when the sea-level was 70m lower than the present sea-level. The heavy minerals that are present within these sand units were derived from the weathering of rocks from the Natal Metamorphic Belt, the Natal Group and the Karoo Supergroup. These minerals underwent fluvial transport to the sea during protracted erosion and these minerals have subsequently been concentrated within the beach environments by near shore wave action.
The Hillendale deposit is considered to represent a paleo-spit formed at the mouth of the paleo-Mhlatuze River. It consists of dominantly Berea-type red sands but also contains discontinuous lenses of medium to coarse yellow to dark orange sands that are low in silt and clay and THM. The deposit is approximately 3.8km long, 600m wide and generally between 18m to 21m thick. THM content is between 1% and 25% with Valuable Heavy Minerals (“VHM”) making up 10% to 70% of the THM suite. Grades are locally variable but as a unit Hillendale can be considered relatively homogeneous. Silt content of the sands is largely due to weathering and ranges between 15% and 25%.
The Fairbreeze Project group of deposits is considered to represent paleo-strandlines and beaches that developed adjacent to headlands. The deposits are distributed over a total strike length of approximately 10km and a width of 700m.
Fairbreeze Project deposit is divided into five separate blocks, Fairbreeze Project A, B, C Extension and D Blocks. Like Hillendale, the Fairbreeze Project deposits are hosted within Berea Formation sands, although at Fairbreeze Project, these sands are distributed above a variable bed-rock surface. In the extreme north, Vryheid Formation rocks (sandstones and shales) crop out to the southeast of the Fairbreeze Project C deposit. Between the A and B blocks Natal Group lithologies (sandstones and grits) are exposed. In this area the Berea Formation sands are generally absent and the overburden that is developed above the bedrock consists of fine-grained, silt-poor wind blown sands.
The Fairbreeze Project A and B deposits are considered to represent two sections of a set of strandline deposits. Strandlines represent tabular zones of concentrated heavy mineral accumulations that are preserved by gradual marine regression that leaves the strandline above the level of marine erosion.
Fairbreeze Project C deposit has been interpreted to be a beach deposit formed on the low energy side of an ancient headland that projected into the sea. The heavy minerals accumulated against the headland, whilst the lighter minerals were continually remobilised by wind transportation, resulting in concentration of the heavy minerals. Fairbreeze Project D deposit has been interpreted to represent a set of strandlines deposited to the east of Fairbreeze Project A, B and C deposits, during progressive sea-level regression.
The Fairbreeze Project A deposit is 18m to 63m thick with an average thickness of approximately 38m. Rapid variation in thickness is apparent with vertical differences of 15m over lateral distances of 50m. Unlike Hillendale the deposit is heterogeneous with three heavy mineral horizons dipping to the east, centred on strandlines. THM grades in the strandlines are typically between 5% and 15% with 50% to 70% of VHM.
Fairbreeze Project B constitutes an extension to Fairbreeze Project A separated by a bedrock outcrop. It is similar to Fairbreeze Project A but is characterised by lower average THM grades; silt and clay content of Fairbreeze Project B deposit exceeds that of the A deposit.
Fairbreeze Project C and C Extension are more homogeneous in nature than Fairbreeze Project A or Fairbreeze Project B and average 10% to 15% THM with a VHM fraction of 60% to 80%. Grades typically increase with depth and this deposit also has a higher zircon and rutile content than the A and B deposits.
Fairbreeze Project D is similar to Fairbreeze Project A and Fairbreeze Project B but is lower in grade and varies in thickness from 18m in the South to 34m in the North. Two strandlines are present and grades average between 5% and 8% THM in the strandlines with a VHM component accounting for approximately 50% of the THM. The Fairbreeze Project deposits contain approximately 30% silt.
Block P is located north of Hillendale and there is one significant difference between this body and both Hillendale and Fairbreeze Project; the basal unit of the deposit consists of a pebble bed that underlies a mineralised red sand unit. Granite-gneisses are present immediately below the pebble bed. With respect to the red sand orebody, Block P deposit is broadly similar to the Fairbreeze Project deposit style in that it is a strandline in nature. The Block P deposit has THM grades of around 5%. Silt and clay content is in the order of 25% for Block P.
|
3.4.3 |
Limpopo Province – Gravelotte
The Limpopo Province assets of Kumba are located approximately 8km north of the town of Gravelotte, in southwestern South Africa and comprise both elluvial and hard rock ilmenite deposits. The principal resource consists of elluvial sand deposits containing ilmenite linerated from the weathering of Archaean magnetite rich rocks of the Rooiwater Complex, developed along the northern flank of the easterly-trending Murchison Greenstone Belt.
The Rooiwater Igneous Complex consists of the Novengilla Gabbro, the Quaqqa Quartz-Amphibolite and the Free State Hornblende Granite. The Novengilla Gabbro hosts the magnetite rich bands that are the source of the ilmenite within the Gravelotte deposits. During metamorphism of the Rooiwater Igneous Complex the ilmenite, which is typically present as exsolution lamellae within magnetite, has recrystallised as discrete crystals of ilmenite. The weathering of this material has largely disaggregated the magnetite and ilmenite making it possible to segregate the ilmenite from magnetite. The residual sand and soil derived from weathering is accumulated above a pebble layer that overlies the bedrock.
Mineral Resources estimates have been developed for the sand unit, as well as the pebble layer and the underlying hard rock bodies. The sand layer is on average 0.9m thick and the pebble layer is on average 0.3m thick; the sand resource is present within an area with a strike length of approximately 24km and a width of 2.5km, over the farms Begin 765LT, FreeState 763 LT, Mon Desir 782LT, Gravelotte 783LT, Quagga 759LT, Malati 764LT, Rubbervale 784LT and Solomons Mine 76LT.
|
3.4.4 |
Tiwest JV
The Perth Basin of Western Australia has historically provided a significant proportion of the world’s ilmenite, rutile and zircon. These detrital heavy minerals originate from igneous and metamorphic rocks in the Achaean shield in the interior of Western Australia, but have been concentrated through multiple phases of weathering, erosion and deposition. Economic accumulations of heavy minerals are mostly found in high energy Cainozoic shoreline deposits, although significant accumulations also occur in older Cretaceous fluvial sediments.
The Perth Basin is a narrow, north-south trending longitudinal trough bounded to the east by the Darling Fault and to the west by the continental shelf (Cockbain, 1990). The basin is approximately 1,000km long, and up to 15km deep. The basin has undergone two major stages of evolution, as represented by the sedimentary fill:
- An early phase extending from the Silurian to the early Cretaceous consisting of thick sequences of continental siliciclastic sediments and minor shallow marine sediments. During this period the basin was probably bounded by continental crust to the east and the west.
- From the early Cretaceous to the present, the basin sediments are typical of a marginal sag basin with generally thin sequences of shallow marine sediments. Eustatic sea level changes over this period, and possibly some tectonic movement, have caused near-shore sediments to be deposited over much of the east-west extent of the basin, especially over the coastal plain regions.
Many of the high grade heavy mineral deposits in the Perth Basin lie within shoreline deposits sitting unconformably against Mesozoic sediments or weathered Precambrian basement. The extensive deposits against the Darling Scarp and the Whicher Scarp in the South Perth Basin (Yoganup, Boyanup, Waroona), and the Gingin Scarp in the North Perth Basin (Gingin, Cooljarloo, Eneabba) belong to this group. Despite the near-shore origins of these sediments, a lack of fossils has made them difficult to date. They are correlated together as the Yoganup Formation on the basis of similar lithology and geomorphological position (shoreline deposits between 26 – 115m ASL). This formation has been correlated with the fossiliferous Ascot Beds, of likely Pliocene age (Cockbain, 1990).
A younger series of shorelines is found to the west of these deposits, within Quaternary sediments. These shorelines have been economically significant in the South Perth Basin near Capel, but have not been as significant in the north. The younger strandlines are known to exist in the North Perth Basin (Dongara, Jurien and possibly Eneabba West), but a large limestone ridge covers most of the prospective areas. The younger strandlines typically contain higher garnet concentrations and less altered ilmenite than the older strands.
The Capel and Eneabba districts have been the most significant mineral sand producing areas in Western Australia. The various deposits within these districts are situated in what were northward facing embayments (sometimes called J-shaped bays). Heavy minerals were probably accumulated with a combination of longshore drift and wave action. Mineralisation is also partly controlled by the location of paleodrainages and sediment sources.
The mineralisation at Cooljarloo comprises the valuable detrital heavy minerals ilmenite, rutile, leucoxene, and zircon with subordinate monazite and trash aluminosilicates kyanite, staurolite, andalusite and tourmaline. These minerals were concentrated in near-shore sediments deposited during transgressive, interglacial peaks, probably in the Late Pliocene or Early Pleistocene. The deposits form a swathe of about 15 north-westerly trending sub-parallel strands over a three-kilometre wide belt. These have been described by Baxer (1977) as the Munbinea shorelines and can be followed from Cataby to Badgingarra over a 40-kilometre section of the Gingin Scarp.
The shallow marine and near-shore sediments that host the mineralisation are between 20m and 50m thick and unconformably cover the fluvial sands and silts of the Upper Jurassic Yarragadee Formation. At Cooljarloo, some of the high grade mineralisation abuts the Gingin Scarp, which marks the eastern extent of the Cainozoic marine transgressions. However, a significant proportion of the mineralisation lies in a fan of strandline deposits that lie west of the scarp. The mineralised sands near Cooljarloo were deposited in at least three sequences of marine transgression and regression.
First Transgression and Regression (“33000 Layer”): The locally named 33,000 layer unconformably overlies the Yarragadee Formation as a sheet over much of the north-eastern area at Cooljarloo. It is a variable layer of silty, weakly carbonaceous, poorly sorted sand that is often grey-green in colour, or yellow-brown (possibly an oxidised version of the grey-green material). The layer is typically 5m to 7m thick and contains moderate levels of heavy minerals.
The heavy mineral in the 33,000 layer has been well-studied and is considerably finer than the overlying strandline mineralisation (D50 of 0.12mm compared with 0.18 for the overlying HM. It has a significantly
higher level of monazite (1.3% versus 0.4%) and the ilmenite is less altered (TiO2 of 56% versus 62% in the
overlying strandlines). All of these properties make the 33,000 layer less attractive for mining than other areas.
Second Transgression and Regression (Mid-level Deposits): The mid-level deposits were emplaced in three episodes between R.L. 44m and 64m. Drill core shows thin lamellar, mineral sands, small cross-set beds and massive disseminated mineral sands. The low energy conditions and shallow marine environment are evident. These conditions were periodically interrupted by storm events that reworked up to five metres of sand and destroyed any previous structure. The sands are grey to white, medium-grained, well-rounded and well-sorted mature sediments. They differ from units in other transgressions by the absence or low content of feldspar. The sands are generally very clean with less than 5% clays. These are the sequence currently mined at Cooljarloo, together with the overlaying third transgression minerals.
Third Transgression and Regression (Near-surface Deposits): The third and final transgression must have been quite rapid, as much of the previous regressive sands were undisturbed. The sands in this regression are coarser and more angular than the second transgression and contain more feldspar. These facts all support the theory that the sands are less mature than the older lithologies. The valuable minerals are generally coarser and contain less rutile and zircon than the older, more mature mineral suits. There are also more alumino-silicates in the heavy mineral assemblage. The near-surface strands were deposited in seven or eight events over a three-kilometre wide coastal strip.
|
back to top
