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Stella Layered Intrusion - Kalplats Project - Crater, Vela, Sirius, Mira, Orion, Serpens North, Serpens South, Crux, Scorpio, Tucana, Pointer, Serpens East |
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North West Province, South Africa |
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PGE PGM Pt Pd Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Stella Layered Intrusion and Kalplats Project platinum group element mineralisation is located ~25 km north of the township of Stella in the Northwest Province of South Africa, ~55 km south of the Botswana border and ~300 km to the west of Johannesburg (#Location: 26° 16' 25"S, 24° 47' 17"E).
Sulphide mineralisation was first detected in the Stella intrusion in 1975-78 by Kafrarian Metal Holdings following up on a Cu-Ni-Zn soil anomaly by drilling into a ferrogabbro. Platinum-group element mineralisation was subsequently discovered by Anglo American Prospecting Services during the 1990s, following a soil sampling and drilling program that delineated several mineralised domains. The project was subsequently acquired by Harmony Gold Mining Company Limited in 2004 who conducted an extensive program of 40 000 m of drilling, metallurgical testing, geotechnical, mining and environmental studies, and the mining of a box cut, resulting in a Feasibility Study that was completed in February 2004. As part of a transaction involving Anglovaal Mining, African Rainbow Minerals and Exploration Investments (ARM) and Harmony, the project was subsequently acquired by ARM. In August of the same year, Platinum Australia Limited (PLA) entered into an agreement with ARM to earn up to 49% of what is now known as the Kalplats Project by completing a Definitive Feasibility Study on the project, with PLA as manager, and each partner contributing 50% of costs. Eight deposits, Crater, Vela, Sirius, Mira, Orion, Serpens North, Serpens South and Crux have been outlined over a 12 km strike length. In addition, several other prospects Scorpio, Tucana, Pointer and Serpens East have had limited testing through thick Kalahari sand cover (Lewins, Hunns and Badenhorst, 2008). In 2014, a Mineral Resource estimate of the platinum, palladium and gold mineralisation was undertaken by Coffey Mining Consultants. In 2015, the rights to the project were purchased by African Thunder from the liquidator of Platinum Australia. In 2021, Canadian based Glacier Lake Resources Inc. undertook to acquire a controlling interest in the Stella Platinum (Pty) Ltd. and Greenstone Platinum (Pty) Ltd, subsidiaries of African Thunder, which controlled the rights to the Kalahari Platinum Project, KalPlats.
According to Maier, Barnes and Smith (2023), the Stella Layered Intrusion is the oldest known example of PGE reef-style mineralisation on Earth.
Regional Setting
The late Mesoarchaean ~3033 Ma Stella Layered Intrusions lie on the western limb of the Kraaipan-Madibe Granite-greenstone Terrane within the Kimberley Block that constitutes the western third of the Archaean Kaapvaal Craton of South Africa. The greenstones of this terrane occur as a number of discontinuous belts and small outliers of deformed and metamorphosed volcano-sedimentary rocks and associated granitoids (Lewins, Hunns and Badenhorst, 2008). The Stella Intrusion lies within the overall NNW-SSE trending Stella Greenstone Belt which forms the western limb of an interpreted 'U-shaped', convex to the south, structure, the eastern limb of which is occupied by the Kraaipan (or Goldridge) Greenstone Belt. This latter belt is parallel to and ~50 km to the east of the Stella Greenstone Belt. Both limbs/greenstone belts have strike lengths of ~100 km in South Africa, persisting for at least another 50 km to the north in Botswana. The third, sub-parallel, north-south trending and isolated, ~100 km long Madibe Greenstone Belt is 20 to 50 km to the east of the Kraaipan Belt, whilst a fourth, the ~60 km long, NNW-SSE trending Amalia Greenstone Belt is aligned with the Stella Belt, after a gap of ~20 km separating it from the Stella-Kraaipan 'closure' (Ramotoroko et al., 2016). The Kraaipan (or Goldridge) Greenstone Belt hosts the Kalgold or Kalahari Goldridge gold deposits.
The bulk of the greenstones belong to the Kraaipan Group, which has been subdivided, from the base, into the Goldridge, Ferndale and Khunwana formations mostly dated at ~3030 to 2930 Ma, with the oldest lithofaceis being iron formations dated at 3410 ±63 Ma (Pb/Pb). The Goldridge Formation at the base of the Kraaipan Group comprises amphibolites after mafic meta-volcanic rocks, and associated iron formations, with minor phyllites, schists and clastic sedimentary rocks. The metavolcanic rocks are generally characterised by a chlorite-actinolite-epidote alteration assemblage, whilst the iron formations comprise alternating laminae of chert and hematite, goethite and magnetite iron oxides. The structurally overlying Ferndale Formation mainly comprises chert that is both ferruginous and jaspillitic, interlayered with rhyolitic felsic volcanic rocks. The Ferndale jaspillites are composed of cryptocrystalline quartz with poorly define magnetite layers, and is absent from the Madibe Greenstone Belt (Keyser and Du Plessis, 1993). The overlying Khunwana Formation is apparently lithologically very similar to the Goldridge Formation, and is composed of amygdaloidal and pillowed mafic meta-volcanic rocks that range from un- to intensely-deformed (Mdze, 2023; Brandl et al., 2006). Where the Ferndale Formation is absent, the Khunwana Formation conformably overlies the Goldridge Formation (Keyser and Du Plessis, 1993).
The greenstone belts are accompanied by granitoids that were episodically emplaced into the Kraaipan-Madibe Granite-greenstone Terrane. Due to the paucity of exposure, the intrusive relationships between the granitoids and the greenstone rocks is uncertain, although the different granitoids all contain xenoliths of amphibolite and banded iron formations of the Kraaipan Group (Anhaeusser and Walraven, 1997). Based on geochemical and petrological characterisitcs, these granitoids have been divided into:
i). an extensive tonalite-trondhjemite granitoid, or TTG gneissic suite with low Na2O/K2O ratios, composed of foliated leucogranites and migmatites, that have been dated at 3008 ±4 Ma at one locality (Mdze, 2023);
ii). K2O-rich, fine to medium-grained pink or grey homogenous syenogranites and granodiorites of the Schweizer-Reneke adamellite and the Kraaipan granodiorite-adamellite that are generally massive, although locally weakly foliated, and have been dated at 2884 ±2 Ma and 2836 ±22 Ma;
iii). K2O-rich, massive, 2782 ±6 Ma (Mapeo et al., 2004), unfoliated Mosita Adamellite.
The Kraaipan-Madibe Granite-greenstone Terrane, which has a prominent north-south trend, appears to represent an Archaean crustal segment that may have been episodically developed and accreted onto the western edge of the proto Kaapvaal Craton (e.g., Anhaeusser and Walraven, 1999). A prominent, generally north-south magnetic feature known as the Carlsberg Magnetic Anomaly (or the Carlsberg Lineament) passes within 20 km to the east of the Madibe, southern Kraaipan and Amalia greenstone belts and subdivides the Kaapvaal craton into eastern and western domains. As such it is taken to reflect the western margin of the proto-Kaapvaal Craton, and separates it from the Kimberley Block to the west.
Geology
The Stella Layered Intrusion was initially outlined in the early 1970s with aeromagnetic data and drilling to establish a strike length of ~13 km and an estimated stratigraphic thickness of ~1.5 km. It is interpreted to represent the upper section of an originally larger and thicker, but tectonically dismembered, layered intrusion (Andrews 2002). It has been divided into the Morester and Kroomdraai sections in the north and south respectively, each of which is ~6 km long. A number of PGE-rich deposits have been delineated by drilling, including Crater and Sirius in the Morester section, and Orion and Crux in the Kromdraai section.
East vergent deformation produced bedding/banding parallel, ductile, locally mylonitic, shearing and thrusting, particularly at the contact between competent and less competent lithologies, e.g., between magnetitite and dolerite dykes. These shear zones can be up to 2 m wide. This deformation progressed to late brittle reverse faulting that produced conjugate fault sets that trend at 10 to 40° and 100 to 115°, and compartmentalise the intrusion, with up to ~50 m of horizontal offset, and unknown vertical displacement. This also led to local duplication of the reef, and consequent thickening to as much as 70 m as at the Crux deposit.
The country rock at the basal contact zone with the intrusion comprises intercalated magnetite-quartz schist, chert, and tectonically emplaced, highly altered slivers of intrusive rocks, all of which are cut by abundant quartz veining. The overlying Stella Intrusion is essentially composed of layers of unfoliated, medium- to coarse-grained, gabbros, leucogabbros, anorthosites and magnetite bearing gabbros and leucogabbros. Contacts between the individual layers are mostly gradational. These layers generally dip sub-vertically or are locally overturned with a westerly dip, with strong local small variations due to faulting and folding.
The PGM mineralisation of the Stella Layered Intrusion is hosted in the magnetite-rich gabbros and leucogabbros lithofacies that are ~150 to 230 m stratigraphically above the contact with the Kraaipan Greenstone country rocks described above. The magnetite content of these layers ranges from weakly 1 to 2% disseminated, to strong segregations forming layers of 50 to 90% magnetite to produce more than 10 magnetitite layers that vary from 0.4 to 4 m in thickness (Lewins, Hunns and Badenhorst, 2008; Maier, Barnes and Smith, 2023).
However, while the mineralistion is well above the stratigraphic base of the intrusion, in some of the deposits, the footwall gabbros have virtually been completely sheared and thrust out, such that the proximal footwall to the mineralised package comprises Kraaipan Greenstone and/or granites (Lewins, Hunns and Badenhorst, 2008).
Maier, Barnes and Smith (2023) and Lewins, Hunns and Badenhorst, 2008 studied drill holes through the intrusion. The following is an amalgamation of their observations of the intrusive succession, from the basal contact with the Kraaipan Greenstone wallrocks. The entire intrusive sequence has been overturned and a dips of 80 to 85°WSW.
• The stratigraphically lowest, basal 20 to 80 m of the section, is largely composed of magnetite-poor leucogabbro, described by Lewins, Hunns and Badenhorst (2008) as the Deep Footwall, comprising a homogenous, extensively chloritised, mafic dominated, fine to medium grained, unfoliated gabbro, virtually devoid of magnetite. This is locally followed by,
• a Pegmatoid Phase that is 20 to 30 m or more in thickness and comprises a late cross-cutting leucogabbro composed of coarse grained 'pegmatoidal', feldspar-rich, and locally chloritised and saussuritised gabbro. It contains occasional coarse, inter-cumulate, magnetite segregations. It shows no evidence of layering and is bound by intrusive contacts (Lewins, Hunns and Badenhorst, 2008).
• This is followed by up to 70 m or more of the Footwall sequence comprising a predominantly mafic rich succession of medium grained magnetite gabbro but with thick 10 to 20 m bands of leucogabbro and occasional <2 m thick magnetitite zones and anorthositic layers. This interval is also cut by zones of amphibole schist after shearing.
• The sequence is punctuated by the 2 to 5 m thick Alpha Reef (Lewins, Hunns and Badenhorst, 2008) or 'Pre reef kick'/ PRK (Maier, Barnes and Smith, 2023) which contains ~1.5 g/t PGM in a magnetitite/magnetite gabbro.
• This is followed by the pre-LG Footwall which is ~10 to 45 m of feldspar-rich, and mafic-rich, partially chloritised, medium grained, grey/green magnetite gabbro to leucogabbro containing 1 to 15 modal % magnetite.
• The 40 to 45 m, locally up to 80 m, thick main mineralised interval, which is described in the Mineralisation section below. Most of the rocks hosting this mineralised interval, and its immediate hanging wall and footwall, are leucogabbro, which contains >15% magnetite, and numerous massive (i.e., >50 wt.% oxide) to semi-massive (20 to 50 wt.% oxide) magnetite layers. These rocks are predominantly medium- to coarse-grained, or locally pegmatitic, composed of euhedral and subhedral plagioclase replaced by amphibole and chlorite, with interstitial magnetite.
• Hangingwall rocks, comprising ~30 m of magnetite-rich, cumulate textured gabbro, magnetite anorthosite and magnetitite with relatively low levels of <0.3 ppm Pt and traces of Au which persist for ~10 m, whereas elevated Cu continues for ~30 m, to a horizon known as the 'Pseudoreef', a prominent cumulate-textured magnetitite layer several metres thick with abundant coarse chalcopyrite, but no PGE or Au.
• High Hangingwall, at least 20 m thick, made up of predominantly magnetite-rich gabbro with narrow <5 m thick magnetite layers. The gabbro has a cumulate texture with <1 cm euhedral feldspar crystals and 1 to 20% interecumulate magnetite.
• Lewins, Hunns and Badenhorst (2008) suggest his sequence is stratigraphically overlain by as much as 1000 m of magnetite-rich gabbros that have since been eroded and or structurally displaced.
A number of felsic rock types are logged within the drill core. These include oligoclase-quartz rocks that contain xenoliths of the host leucogabbro and have strongly sheared contacts with the host rocks, implying pre- to syn-tectonic emplacement. Abundant fine-grained quartz-albite veins that are centimetres to tens of metres thick, cut the intrusion and country-rock, and are oriented sub-parallel to the igneous layering. Many have chilled margins and are unaltered and undeformed, suggesting emplacement subsequent to the deformation of the leucogabbro. Less common coarse to medium-grained granite veins are up to a few metres in thickness, and are composed of plagioclase, quartz, alkali feldspar, locally biotite and myrmekite, and contain blocks of sheared gabbro indicating they are post-tectonic in age.
Two distinct sets of mafic dykes are recognised, namely: i). Fe-rich, tholeiitic dolerite, predominantly oriented sub-parallel to the igneous layering and altered to an amphibole schist composed of actinolite, chlorite, quartz, plagioclase and epidote. These dykes are less altered and deformed than the Stella cumulates, although they have mostly sheared contacts with the cumulates and thus were likely intruded during the late stages of tectonism. Geochemical simulation yielded a crystallisation sequence and mineral compositions matching the Stella cumulates, interpreted to suggest these dykes could represent the parent magmas to the Stella intrusion. ii). A second suite of mafic dykes with chilled margins and brecciated contact zones, that are composed of magnetite gabbro, trend ENE, and evident on aeromagnetic data.
Some drill holes intersected up to 200 m of diamictite overlying the intrusion, possibly belonging to the Kameeldoorn Formation of the Neoarchaean Ventersdorp Supergroup. These are dated at 2.754 to 2.709 Ga (Gumsley et al., 2020). Their contacts with the Stella intrusion dip steeply, taken to suggest deposition in fault troughs or glacial channels.
Overburden is typically ~4 m thick, ranging from 1 to 11 m thick, the latter being in paleochannels. The depth of weathering varies from a few to 45 m, averaging 25 m.
The rocks of the Stella Layered Intrusion are seldom fresh. The entire lithostratigraphic sequence has been affected by a number of episodes of intense alteration which can be attributed to either igneous alteration or greenstone belt deformation and alteration. The only alteration definitely linked to a
magmatic source is the high-temperature clinozoisite/epidote, interpreted to have taken place during the cooling phase of the magma, and a late, sparse white mica that mainly affected the plagioclase (Lewins, Hunns and Badenhorst, 2008).
Mineralisation
The mineralised interval within the Stella intrusion is up to 100 m thick and can be sub-divided into a number of laterally continuous sub-reefs, predominantly based on PGE, Cu and Au contents as well as the Pt/Pd ratio (after Maier, Barnes and Smith, 2023). The following is from a drill hole studied by those authors. It was inclined at ~45°E, cutting foliation at an angle of 45°, suggesting thicknesses measured and quoted below need be reduced by 0.71 to true width. It entered the intrusion from the base.
• The stratigraphically lowermost PGE-enriched horizon, known as the 'Pre reef kick' or PRK (or Alpha Reef), occurs ~15 to 45 m stratigraphically below the main mineralised interval and contains 0.5 to 3 ppm PGE. The pre-LG Footwall separating this reef from the main mineralised interval contains <0.5 ppm PGE.
• The main mineralised interval averages 40 to 45 m, but is locally up to 80 m, in thickness, and comprises a
- basal low-grade reef or LG, hosted by magnetite leucogabbro and minor magnetitite. This reef has slightly higher Pt than Pd values, with an overall grade of ~1 g/t Pt+Pd+Au over widths of 10 to 20 m;
- The succeeding 10 to 15 m thick Mid Reef is enriched in Pd by a factor of 2 to 4 x Pt, and usually contains two higher-grade reefs, termed MR1 or Mid Reef 1 and MR2 or Mid Reef 2 respectively, characterised by elevated magnetite content and grades reaching 4 ppm, but averaging 2.5 ppm Pt+Pd+Au over widths of 1 to 3 m;
- Lower grade 5 to 10 m thick interval of predominantly non-magnetic anorthosite with <0.5 g/t Pt+Pd+Au;
- The Main Reef, which has a width of up to 20 m, and an average grade of 1.9 g/t Pt+Pd+Au over 14 m, which includes two high-grade zones, the 2.3 m thick Lower Main Reef or LM at the base, averaging 3.5 g/t Pt+Pd+Au), and the 5 to 8 m thick, magnetite-rich, Upper Main Reef or UM at the top averaging 4.4 g/t Pt+Pd+Au. Gold and Cu increase sharply over the top few metres of UM and extend into the hanging wall rocks.
• Hanging wall rocks, comprising magnetite gabbro, magnetite anorthosite and magnetitite with relatively low levels of <0.3 ppm Pt and traces of Au which persist for ~10 m, whereas elevated Cu continues for ~30 m, to a horizon known as the 'Pseudoreef' composed of magnetitite with chalcopyrite, but no PGE or Au.
Weathering has leached Pd from the hosts, with the estimated loss of 25% of the hypogene Pd within 10 m of surface, and 15% within 30 m (Maier, Barnes and Smith, 2023).
The main platinum group minerals (PGM) include kotulskite, moncheite, sperrylite, arsenopalladinite, ferroplatinum, rare gold and electrum, although PGE-sulphides are absent. Most PGM are hosted within silicates, whilst sulphides and magnetite contain few PGM. In some cases, the PGM, particularly sperrylite, may replace silicates and form encrustations or 'atoll structures' surrounding Cu sulphides, up to 35 µm in diameter. Some variation has been noted in the composition of PGM between reefs. The LG contains mainly merenskyite and sperrylite, the Mid Reef has mainly merenskyite and stibiopalladinite, whereas the Main Reef has mainly stibiopalladinite and sperrylite. The weathered Main Reef has abundant PGE alloys and gold, similar to weathered reefs elsewhere (Maier, Barnes and Smith, 2023).
The oxide layers are predominantly composed of ~85% titanomagnetite, with much of the remainder being ilmenite. Some rutile, leucoxene and hematite are found close to faults. Grades in the magnetic fractions of the Main Reef are 1.14% V2O5, 4.83% TiO2, 81.1% Fe2O3 (in the Crater deposit) and 0.93% V2O5, 5.59% TiO2, and 80.7% Fe2O3 (Orion deposit; Andrews 2002; Maier, Barnes and Smith, 2023).
Mid Reef oxide grades (at Orion) are 1.06% V2O5, 4.07% TiO2, 74.2% Fe2O3, whereas LR grades at Orion are 0.41% V2O5, 2.36% TiO2, 51.3% Fe2O3. Andrews (2002) reported the results of microprobe studies across the drilled interval of the intrusion indicating that magnetite contains an average of 1.3 wt.% V2O5, 0.43 wt.% TiO2, and 0.1 wt.% Cr2O3. Vanadium grade drops to 0.9 to 0.95% V2O5 near carbonate alteration but is otherwise relatively constant (Maier, Barnes and Smith, 2023).
Maier, Barnes and Smith (2023) note that the PGE mineralisation of the Stella Intrusion is unusual in that it is concentrated in the upper parts of the intrusion, where it comprises interlayered magnetitite and anorthosite-leucogabbro. Much of the mineralised interval is relatively sulphide poor with <500 ppm S, but contains abundant PGM, mainly as antimonides and arsenides, typically hosted by silicates rather than sulphides or oxides. The sulphides present occur as approximately equal proportions of chalcopyrite and pyrite (Maier, Barnes and Smith, 2023).
Resources
The following Mineral Resource is quoted by Maier, Barnes and Smith (2023) after Coffey Mining Consultants (2014):
Measured + Indicated Resources - 69.91 Mt @ 1.48 g/t Pt+Pd+Au, (3E);
Inferred Mineral Resources - 56.68 Mt @ 1.62 g/t Pt+Pd+Au, (3E).
The most recent source geological information used to prepare this decription was dated: 2023.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Stella
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Selected References:
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Lewins, J.D., Hunns, S. and Badenhorst, J., 2018 - The Kalahari Platinum project: in Third International Platinum Conference, Platinum in Transformation, The Southern African Institute of Mining and Metallurgy, pp. 355-366.
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Maier, W.D., Barnes, S.-J. and Smith, W.D., 2023 - Petrogenesis of the Mesoarchaean Stella layered intrusion, South Africa: implications for the origin of PGE reefs in the upper portion of layered intrusions: in Mineralium Deposita v.58 pp. 1477-1497.
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Poujol, M., Anhaeusser, C.R. and Armstrong, R.A., 2002 - Episodic granitoid emplacement in the Archaean Amalia-Kraaipan terrane, South Africa: confirmation from single zircon U-Pb geochronology: in J. of African Earth Sciences v.35, pp. 147-161.
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Ramotoroko, C.D., 2014 - Gravity and magnetic mapping of the extension of the Archaean Madibe- Kraaipan granite-greenstone terrain, Kaapvaal Craton in southeast Botswana: in A dissertation Submitted in Partial Fulfilment of the Requirements for Master of Science Degree in Physics, Physics Department, University of Botswana, Gaborone, Botswana, 101p.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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