3.   GEOLOGY

3.2.1  

Sishen Mine

The iron ore deposit at Sishen Mine is one of a number of genetically related high-grade haematite deposits situated in the Sishen – Postmasburg area. Superior-type banded iron formations (“BIF”) of the Transvaal Supergroup crop out along the western margin of the Kaapvaal craton in the Northern Cape Province. These iron formations can be traced as a prominent range of hills in a broken arc for some 400km from Pomfret in the north, to Prieska in the south. The bulk of the hematite mineralisation is found in the vicinity of Postmasburg and Sishen Mine. Within this sub-region, iron ore and associated lithologies of the Transvaal and Olifantshoek Supergroups crop out intermittently along an arcuate belt for 60km.

The outcrops define a regional anticlinal structure known as the Maremane anticline. Sishen Mine is located at the northern end of the anticline. At this locality, the bulk of the hematite ores is buried beneath younger cover lithologies.

The Transvaal Supergroup lithologies were deposited in an extensive epeiric sea on the central part of the Kaapvaal Craton. The strata developed within two related basins of which the westernmost (the Griqualand West basin) is preserved in the Northern Cape Province. The Transvaal Supergroup, as preserved within the Griqualand West basin, comprises an extensive, basal carbonate platform sequence (the Campbell Rand Subgroup) conformably overlain by iron-formations of the Asbestos Hills Subgroup. South of Postmasburg, the BIF of the Asbestos Hills Subgroup are in turn overlain by a mixed chemical and clastic unit termed the Koegas Subgroup. The Koegas Subgroup is conformably overlain by diamictite of the Makganyene Formation upon which lavas belonging to the Ongeluk Formation have been subaqueously extruded. North of the Sishen Mine area, the Ongeluk lava is in turn conformably overlain by BIF with interbedded manganese (Hotazel Formation) and carbonates of the Mooidraai Formation. The latter formations constitute the Vöelwater Group. At the Sishen Mine deposit, the upper parts of the Asbestos Hills Subgroup have been ferruginized to ore grade. These stratiform, laminated and massive ores constitute the bulk of the resource. They are unconformably overlain by a thick package of sedimentary rocks (conglomerates, shales, flagstone and quartzite) termed the Gamagara Subgroup. At Sishen Mine, diamictite of the Makganyene Formation and lavas of the Ongeluk Formation have been thrust over the sedimentary rocks of the Gamagara Subgroup. The diamictite and lava have been eroded by later events. Tillite of the Dywka Group and pebble beds, clay and calcrete of the Kalahari Group, have been deposited on these erosional unconformities.

Sishen Mine is situated on the northern extremity of the Maremane anticline. At this location the lithologies strike north-south and plunge from the centre of the anticline in a northerly direction. The bulk of the resource comprises high-grade, laminated and massive ores belonging to the Asbestos Hills Subgroup. These ores are truncated by an erosion surface upon which lower-grade conglomeratic ores and sedimentary rocks of the Gamagara Subgroup have been deposited. The orebodies are intensely folded and faulted. Dips vary according to local structures but at Sishen Mine a regional dip of 11° in a northwesterly direction prevails. Continuous, alternating basin and dome structures occur at Sishen Mine mine. These interference folds are further modified by normal faulting and low-angle thrusts. Ore bodies are best preserved in basinal and pseudo-graben type structures. The anticlinal structures normally comprise barren footwall lithologies. Highly deformed, isolated orebodies occur close to the Maremane anticline. The orebodies tend to be less deformed and more continuous, the further they are situated from the anticline. Hangingwall lithologies also thicken down plunge.

The carbonates of the Campbell Rand Subgroup are separated from the overlying BIF of the Asbestos Hills Subgroup, by a siliceous, residual breccia. This breccia is known locally as the Wolhaarkop Breccia and is developed on an irregular, karst surface. The BIF’s of the Asbestos Hills Subgroup are characteristically fractured and brecciated, especially near the contact with the Wolhaarkop breccia. Both upper and lower contacts are erosion surfaces and together with the lack of easily identifiable marker horizons, make correlation of individual beds virtually impossible. A highly altered, slickensided, intrusive sill is commonly found separating the BIF from the overlying laminated ore. At Sishen Mine it is generally less than 2m thick. The sill is invariably folded into the basinal geometry and only rarely crosscuts (intrudes) the ore bodies. The laminated and massive ores are commonly folded and faulted into basinal and pseudo-graben structures. Deep palaeosinkholes, filled with brecciated ore and Gamagara sedimentary rocks are found on the southern parts of the Sishen Mine properties. The sinkholes are restricted to antiformal structures close to the Maremane Dome on the southern portions of the mine. They are an important mechanism for preserving collapse breccia ore.

Sedimentary rocks of the Gamagara Subgroup, overlie the laminated, massive and breccia ores. Conglomerates of ore grade with well-rounded clasts and fine-grained, well-sorted, gritty ores are common at Sishen Mine. Partly ferruginized shales, interbedded with ore conglomerates and thick flagstones are also a feature of the Gamagara Subgroup. Along the western margin of Sishen Mine, Makganyene diamictite and lavas of the Ongeluk Formation have been thrust over the sediments of the Gamagara Subgroup. A few thin, diabase dykes with north south and northeast orientations, have intruded the stratigraphic sequence. They form impervious barriers and compartmentalise the groundwater. A buried glacial valley, filled with Dwyka tillite and mudstones has been identified with reconnaissance drilling. The valley is located between the mine and Kathu. It has a north-south orientation that changes to northwest between Dibeng and the mine. The valley does not fall within the planned open pit. The Kalahari Group comprises boulder beds, clays, calcrete, dolocrete and windblown sands. The Kalahari Group is developed to a maximum thickness of 60m. The clay beds at Sishen Mine can attain a thickness of up to 30m on the northern parts of the deposit. The Kalahari beds of calcrete, limestone and clay and quaternary sand and detritus, blanket more than 90% of the Sishen Mine mining area. Only scattered outcrops of iron ore and banded iron ore formation, on the south-eastern parts of the Kumba properties, and quartzite further west, crop out on the surface.

The ores at Sishen Mine are composed of hematite and specular hematite with minor to trace amounts of limonite. Four distinct ore types can be classified (Table 3.1). Each has unique chemical, physical and metallurgical properties. The genesis of each ore type has been influenced by regional tectonism and the preservation of each orebody is primarily determined by local geological structures. The Laminated and Massive Ore are the most important sources of high grade, lump ore in the region. The upper portions of the Asbestos Hills Subgroup comprise fairly undisturbed, thinly laminated, hematite ore which grades upward into thickly bedded, contorted and even massive ores. Breccia ores comprise a chaotic arrangement of very angular and poorly sorted fragments of laminated and massive ore types, cemented by specular hematite. The breccias fill palaeosinkholes developed in the carbonates of the Campbellrand Subgroup. Angular fragments of BIF and some argillitic material are also found in the collapse breccia. Specularite is very common in the porous breccia matrix.

Conglomeratic ore belonging to the Gamagara Subgroup is preserved along the north, western and southern flanks of the Maremane anticline. At Sishen Mine, the conglomeratic beds occur within basinal and synclinal structures and also on the western side of major fault planes. The conglomeratic ores are invariably situated adjacent to, or are in close proximity to, laminated and massive ore bodies.

3.2.2  

Sishen South Project Phase I and Phase II

Iron ore at the Sishen South Project is preserved in chemical and clastic sediments of the Proterozoic Transvaal Supergroup. These sediments define the western margin of the Kaapvaal Craton in the Northern Cape Province. The stratigraphy was deformed by thrusting from the west and has undergone extensive karstification. The thrusting produced a series of open, north-south plunging anticlines, synclines and grabens. Karstification was responsible for the development of deep sinkholes in the dolomite, within which the iron ore at Sishen South Project has been preserved from erosion. These structures are therefore extremely important targets in the prospecting process.

Almost 80% of the Sishen South Project area is covered by sand, dolocrete and calcrete of the Kalahari Group. Outcrops of the Campbell Rand Subgroup and Kuruman Formation are found on the western portion of the prospecting area and banded iron formation of the Asbestos Hills Subgroup occur along the extreme eastern margin. Geophysical surveys and borehole drilling have established the presence of a number of covered deposits in the central and eastern parts of the prospecting area.

The only outcrops of iron ore at the Sishen South Project strike northeast-southwest across the western portion of Welgevonden 486 and Kapstevel 451. These outcrops form part of the eastern limb of an anticlinal structure. The iron ore comprises laminated and conglomeratic types and crops out as thin, lenticular bodies that dip between 40° and 50° to the east. At depth, shale, quartzite, conglomerate and lava overlie this ore. Four distinct iron ore types have been described at Sishen South Project, with most of the ores being similar (slightly different chemistry) to those found at Sishen Mine. Sishen South Project comprises clastic-textured (28.8% of total), laminated (52.9% of total), collapse breccia (9.8% of total) and conglomeratic (8.6% of total) ores. The synclines generally preserve laminated, clastic-textured and conglomeratic ores, whereas conglomeratic ore is often preserved within the grabens. Both conglomeratic and collapse breccia ore types are found in the sinkholes. Typical hanging-wall lithologies are conglomerate, shale, quartzite, tillite, clay and calcrete, although each geological structure contains a unique combination of ore types and waste lithologies. The ore minerals at the Sishen South Project are hematite and minor specularite. Mineralogically the iron ore comprises clasts that may be both massive and featureless, or porous and distinctly laminated. The matrix comprises mostly intergrown clay minerals, together with numerous small clasts and flakes of hematite entrained in the silicate gangue material. Clasts of massive hematite are characteristically non-porous or with only low levels of porosity, and as such, there is minimal clayey gangue material incorporated into the clasts. Clasts of finely laminated ore typically contain above average levels of initial porosity, with gangue minerals often infilling or partially infilling pore space. Small amounts of specularite are often associated with this type of clast. Clay-rich clast commonly appear to have been affected by secondary ferruginisation.

3.2.3  

Thabazimbi Mine

Iron ore is defined as comprising iron-rich rock characterised by an iron content exceeding 60% by weight and containing less than 15% by weight of SiO₂. Low grade iron ore is defined as having an iron content between 55%Fe and 66%Fe.

The iron ore deposits at Thabazimbi Mine are hosted within the lower parts of the Penge Formation, which is the uppermost formation of the Chuniespoort Group within the Transvaal Supergroup. The base of the Penge Formation consists of chert-rich shale unit that averages 10m in thickness. This shale member is overlain by 300m to 400m of ferruginous sediments, consisting of thick autochthonous banded iron formation interlaminated with thin units of orthochemical iron-formation. The iron-ore bodies are typically located within the basal 50m to 80m of the Penge Formation within iron-oxide dominated rhythmites. Iron ore bodies are laterally discontinuous bodies that pinch out along strike and down-dip. The ore thickness ranges from 2m to in excess of 100m; the average mineralised thickness is approximately 20m. Rocks of the Transvaal Supergroup within the Thabazimbi Mine area strike east-northeast and dip to the south at angles between 45º and 60º. The Penge Formation crops out in three easterly trending belts reflective of thrust-related duplication of the Transvaal Supergroup associated with Waterberg-age tectonism. The three belts are referred to as the Northern Range, the Central Range and the Southern Range; weathering and erosion has resulted in these three belts having prominent topographic expression.

The Thabazimbi Mine iron ore deposits are considered to be the result of initial iron deposition during primary chemical sedimentation at the top of the dolomite-dominated Chuniespoort Group, giving rise to a thick zone of banded-iron-formation. Subsequent metamorphism and supergene processes have occurred and have resulted in local chemical modification of the rocks resulting in the formation of high-grade hematite rock within localised ore-bodies. Joint systems developed with the ores are also identified within the adjacent host lithologies; in addition, there is a strong spatial association between the presence of ores and faulting and brecciation of the lower Penge Formation. Ores grade laterally into banded-iron-formation with an increase in SiO₂ and Al₂O₂ content. Geochemical considerations suggest that the ores are a result of ferruginisation, in which chert, talc and carbonates within the primary banded iron-formation lithologies were selectively replaced by hematite or martite/goethite. Minimal instances of silicification have been identified within the Thabazimbi Mine ores suggesting that SiO₂ dissolved within the ore zones was not reprecipitated, but was removed by mobile solutions. Formation of the high-grade ores resulted from the selective removal of goethite, resulting in porous hematite rich ores.

Karstification within the underlying Frisco Formation results in slumping of the Penge Formation into these features. These structures are commonly associated with an increase in iron content with depth, which is suggestive of a second stage of ferruginisation of already Fe-enriched lithologies. Gravitational collapse of iron-formation and iron-ore units into solution-cavities within the underlying Frisco Formation dolomites resulted in brecciation and fragmentation of the iron ore units and increased permeability that was subjected to a further stage of epigenetic ferruginisation by cooler groundwater circulating within the zones of enhance permeability. Further epigenetic ferruginisation is evident around post-Karoo faults that transect the iron ore bodies.

Post-Karoo dolerite dykes transect the deformed Penge Formation lithologies and also displace ore bodies, as well as bounding ore zones in some of the deposits. In addition, the Pretoria Group unconformably overlies the Penge Formation, yet basal conglomerates of the Pretoria group contain no clasts of high-grade hematite implying that these rocks had not formed at the time of deposition of the Pretoria Group (±2.2 Ga). There have been two major metamorphic events over the Transvaal Supergroup lithologies at Thabazimbi Mine: the first event was the Bushveld contact metamorphism (±2.05 Ga), followed by dynamic metamorphism resulting from Waterberg tectonism.

The main ore deposits are hosted within the Northern Range and are referred to as the Kwaggashoek-East (Kwaggashoek 345KQ), East-Mine (Wachteenbietjiesdraai 350KQ and Kwaggashoek 345KQ), Donkerpoort (straddling Donkerpoort344KQ and Wachteenbietjiesdraai 350KQ) and Donkerpoort-West deposits (Donkerpoort 344KQ). These deposits share similar lithological, mineralogical and geochemical characteristics and cover a total strike length of approximately 11km. Iron ore mineralisation within the Southern Range is more sporadically distributed, when compared to the Northern Range; mineralisation is present over a strike length of approximately 5km. Mineralisation is most specifically present on Wachteenbietjiesdraai 350KQ, Buffelshoek 351KQ and Groenfontein 352KQ. Mineralisation within the Central Range has only been located on Kwaggashoek 345KQ.

Within the Northern Range, a diabase sill occurs within the iron formations of the Penge Formation approximately 90m above the basal contact of this formation with the underlying Chuniespoort Group Dolomite. The iron ore bodies are located within the basal portions of the Penge Formation.

The iron ore bodies of the Northern Range, as defined by a Fe cut-off grade of 55%, are characterised by irregular, tabular morphologies. The iron ore is commonly present, with the footwall contact of the orebody coincident with the basal shale unit of the Penge Formation. The iron ore frequently displays brecciated textures, with fragments of hematite contained in a secondary hematite matrix. Lenses of iron formation may be identified within the iron ore bodies, and locally shale may also be included within the iron ore. Thicknesses of orebodies in the Northern Range are typically in the range 15m – 30m. The iron ores near surface consist of hard, compact massive hematite rock. Down-dip, the rock textures change gradually towards more friable forms which grade into talc-hematite and calcite-hematite rocks.

Iron ore bodies within the Southern Range occur distributed within the banded iron formation and are generally significantly smaller and less laterally continuous than those of the Northern Range. The Southern Range deposits include Buffelshoek East and Buffelshoek West, Bobbejaanwater and the Meyer and Kumba Mines. The western block of Buffelshoek West comprises thick iron-ore mineralization (30m), which terminates against a diabase dyke. Structural complexities within this deposit are recognised, but have not been fully resolved. Iron ore mineralization at the Bobbejaanwater deposit consists of irregular lenses of hematite rock in shales and banded iron formations. The mineralisation consists of fine-grained hematite that yields a high proportion of fines when mined; only a small proportion of the orebody can be beneficiated and yields a minor component of lump ore.

The Thabazimbi Mine iron ore deposits are dominated by hematite as the major iron oxide mineral present. The high grade ores consist predominantly of hematite with variable textures, accompanied by minor silicate minerals. With depth, the hematite rock grades into talc-hematite rock and calcite-hematite rock. Goethite and limonite are also present within the ore and low-grade ores at Thabazimbi Mine.