Platreef, Flatreef - Bushveld Complex
Limpopo (Northern) Province, South Africa
PGE PGM Pt Pd Rh Au Cu Ni
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The Flatreef platinum-group element (PGE) deposit and Ivanplats Platreef Operation is located on the Northern Limb of the Bushveld Complex in the Limpopo (Northern) Province of South Africa, some 10 km NNW of the centre of the town of Mokopane, and ~280 km north-east of Johannesburg, and lies on the Turfspruit, Macalacaskop and Rietfontein farms (#Location: 24° 4' 56"S, 28° 57' 34"E).
The host Bushveld Igneous Complex comprises an early bimodal volcanic sequence (the 2059.9±1 Ma Rooiberg Group) that is followed by an intrusive 2055.5±1 Ma layered series of ultramafic and mafic units known as the Rustenburg Layered Suite that is 7 to 8 km thick and ranges in composition from dunite to diorite which contains the main PGE and chromite deposits. These are overlain by the 2054±2 Ma Lebowa Granite and 2061.8±5 Ma Rashoop Granophyre Suites.
For more detail on the regional setting and geological map see the Bushveld Complex overview record.
The Rustenburg Layered Suite has been subdivided as follows, from the base:
• Marginal Zone - composed several hundreds of metres of quenched to fine-grained norite and gabbronorite with variable proportions of accessory clinopyroxene, quartz, biotite and hornblende, indicating magma contamination from the underlying metasediments. This unit is not always present.
• Lower Zone - predominantly consists of interlayered harzburgite, dunite and orthopyroxenite, with rare plagioclase present as a cumulate phase (Cameron 1978; Teigler and Eales 1996). Chromite comprises <1 modal %, irrespective of lithology. It has pronounced lateral variation in thickness and lithology and may be >1 km thick in some trough structures, but it is thinner or absent above swells between those troughs (Grobler, et al., 2019).
• Critical Zone, which is up to 1400 m thick and subdivided into lower and upper zones.
The Lower Critical Zone is 700 to 800 m thick, and is predominantly composed of orthopyroxenite, containing nine major chromitite seams (Lower Group/LG 1 to 7 and Middle Group/MG 1 and 2). The seams have been correlated, across much of the Western and Eastern Bushveld Complex (Cousins and Feringa 1964; Teigler and Eales 1996), although their thicknesses are highly variable. The base of
the Lower Critical Zone has been defined either where there is a significant increase in intercumulus plagioclase (from 2 to 6 %; Cameron 1978), or ~ 200 m lower, at the top of the main olivine-rich interval (Teigler and Eales 1996).
Upper Critical Zone is ~500 m thick, and is defined by the first occurrence of anorthosite which forms a laterally continuous, 1 to 3 m thick layer. Whilst further anorthosite layers are found in the upper portion of the zone, the dominant lithologies are orthopyroxenite (~70 %) and norite (~25 %) (Teigler and Eales 1996). Olivine-bearing harzburgite and troctolite comprise <1% of the zone and are largely restricted to the northwestern Bushveld (Viljoen et al., 1986; Eales et al., 1988; Maier and Eales 1997) where they form part of the Merensky and Pseudo Reefs. These reefs are composed of tens of cm to metre thick coarse-grained to pegmatoidal orthocumulate layers. As well as abundant disseminated chromite, the Upper Critical Zone contains four to five major chromitite seams, including Middle Group/MG 3 and 4 and Upper Group/UG 1 to 3, and tens of minor seams and stringers including those bracketing the Merensky Reef pegmatoid. The so-called cyclic units are particularly typical of the Upper Critical Zone (Cameron 1982; Eales et al., 1986, 1988, 1990). These are generally characterised by basal chromitite, overlain by harzburgite and/or pyroxenite, norite and then anorthosite. The thicknesses of the units range between a few cm to several hundred metres (Eales et al., 1990). PGE mineralisation can be concentrated in the basal ultramafic portions of many of these cyclic units, particularly in the Merensky Reef and UG2 (Upper Group 2) chromitite, the main PGE reefs in the Bushveld Complex, as well as the subeconomic Pseudo and Bastard reefs. The Merensky Reef and UG2 reefs range on average from 0.4 to 1.5 m in thickness with contained PGE (Pt, Pd, Rh, Au) contents typically ranging from 4 to 10 g/t (Cawthorn, 2005)
• Main Zone, which is up to 2800 m this, and is predominantly composed of fairly massive norite and gabbronorite, typically containing 10 to 30% orthopyroxene, 10 to 20 % clinopyroxene and ~50% plagioclase, with occasional anorthosite and pyroxenite bands. Distinct visible layering is rare.
• Upper Zone, which is 1 to 2 km thick, and is composed of cyclic units of magnetitite, gabbronorite, anorthosite and, in the uppermost
portions of the sequence, ferrodiorite (Molyneux 1970; Von Gruenewaldt 1970). As many as 26 magnetite layers are known on the eastern and western limbs (Cawthorn and Molyneux 1986; Tegner et al., 2006), each from a few cm to >10 m thick (e.g., magnetite layer 21).
A craton-wide, ENE to east-west linear structure, the Thabazimbi-Murchison Lineament, separates the Northern Limb of the Bushveld Complex from the remainder of the intrusion. This structure marks a change in the stratigraphy of the Bushveld Complex and the sequence detailed above as well as the intruded succession. On the western and eastern limbs of the complex, south of the Thabazimbi-Murchison Lineament, the Rustenburg Layered Suite is largely intruded into, or on top of, the Magaliesberg Quartzite. In contrast, within the northern limb, the base of the complex progressively transgresses down from the Magaliesberg Quartzite, through the underlying units of the Pretoria and Chuniespoort groups until it directly overlies Archaean basement. Concurrent to these regional transgressive relationships, the igneous sequence undergoes significant lithological change. On the Northern Limb, the Rustenburg Layered Suite is reduced to a compact, >3.2 km thick sequence composed of the Lower, Upper Critical, Main and Upper zones (Hulbert 1983). A succession of intercalated gabbronorite-norite-pyroxenite and anorthosite units, the GNPA, which host the Platreef PGE mineralised interval, occur at the stratigraphic level of the Upper Critical Zone. The Lower Critical Zone and the lower sections of the Upper Critical Zone (including the UG1 chromitite) do not appear to have been developed from south of Mokopane and to the north. Just south of Mokopane, the Lower Zone is separated from the Upper Critical Zone by several tens of metres of microgabbro and metasedimentary rocks thick (Maier et al., 2008). Further north, rocks correlated with the Lower Zone, occur as numerous sill like intrusions within the sedimentary floor rocks.
The Platreef was defined by Van der Merwe (1976) to be the mineralised sequence in the Northern limb of the Bushveld Complex within the that is broadly stratigraphically equivalent to the Upper Critical Zone found to the south of the Thabazimbi-Murchison Lineament. These rocks, which are pyroxenitic and noritic-anorthositic, broadly similar in composition to the Merensky Reef, are stratigraphically located below the Main Zone. However, they are more contaminated and altered than the Upper Critical Zone in the remainder of the Bushveld Complex, and have a gretaer variation in thickness and composition along strike (Grobler, et al., 2019 and references cited therein).
Over much of the northern limb, the Platreef is overlain by as much as 2 km of relatively homogenous gabbronorite, which appears to be the equivalent of the Main Zone to the south of the Thabazimbi-Murchison Lineament (Wagner 1929; van der Merwe 1976; Ashwal et al., 2005). Locally, the lower section of the Main Zone appears to be absent. In the far north, the underlying units also appear to be absent, with basement directly overlain by rocks with the chemical signature of the central to upper Main Zone (Manyeruke 2007; Maier et al., 2008; McDonald et al., 2016). The latter contains a thick, mineralised troctolitic-harzburgitic interval that does not have a stratigraphic analogue anywhere else in the Bushveld Complex (van der Merwe 1978).
North of the Thabazimbi-Murchison Lineament, the Main Zone is overlain by the Upper Zone that hosts >20 massive magnetite layers (van der Merwe 1978; Ashwal et al., 2005), similar to elsewhere in the Bushveld Complex the Bushveld Complex. The roof of the complex is mostly composed of granites. Locally, the Rustenburg Layered Suite is significantly thinned, with the Upper Zone pinching out or being unconformably overlain by the Waterberg red beds (Huthmann et al., 2016; Kinnaird et al., 2017).
Local Geology and Mineralisation
In the Platreef-Flatreef area, geophysics and drilling indicate numerous sills of interpreted lower Zone and Platreef (Critical Zone) rocks. The base of the continuous Rustenburg Layered Suite is apparently located close to the unconformity between the Duitschland Formation with the platform carbonate rocks
of the underlying Chuniespoort Group (Bekker 2001), although some Lower Zone bodies are found near the base of the Transvaal Supergroup (Yudovskaya et al., 2013). To the north, the Rustenburg Layered Suite cuts down through the contact between the Pretoria and Chuniespoort groups, ingesting banded iron formations of the Penge Formation and underlying dolostones of the Malmani Subgroup, of the latter group, towards the base of the Transvaal Supergroup.
The intruded Duitschland Formation has a maximum thickness of between 1000 to 1200 m and is sub-divided into two sequences: i). a basal conglomerate/diamictite overlain by thick carbonaceous and partly pyritic shale and interbedded deep-water carbonate rocks (Bekker 2001); and ii). shallow-marine to shelf-clastic sedimentary rocks with a basal conglomerate, overlain by quartzite which fines upward into shales that form the base of the first of two shale-carbonate cycles. The carbonates are limestone and dolostone, with the latter dominating higher in the formation (Eriksson et al., 2001; Bekker 2001). The Duitschland Formation is overlain by of the conglomerate, breccia and quartzitic sandstone of the locally ~50 m thick Rooihoogte Formation and then by the strongly pyritic black shales (up to 200 m thick), rhythmically interbedded mudstones, siltstones and fine-grained sandstones of the 600 m thick Timeball Formation. All three formations belong to the Pretoria Group and were in contact with the base of the Rustenburg Layered Suite in the Mokopane area.
Surface exposure of the Platreef in the deposit area is limited, although on the northern margin good outcrop of the igneous rocks and the sedimentary
footwall occurs in a creek bed. From drill data Kinnaird et al. (2005) showed that the sedimentary rocks contain sills of Platreef, totalling ~400 m in thickness, consisting of medium-grained melanorite containing numerous centimetre- to metre-sized xenoliths of hornfels and calcsilicate. The contact with the overlying Main Zone is defined by a mottled anorthosite of variable thickness.
Deep drilling commenced in 2007 indicated that the ~40°W dipping Platreef flattened at a depth of ~600 m, and continued at that angle over an interval of from 1 to 2 km before resuming its ~40° western dip. This sub-horizontal section is characterised by thickened mineralised layered units, known as the 'Flatreef' (Kekana 2014). These mineralised units persist westward from the main flat zone, but thin where not flat, and have been traced for some 3 to 4 km west of the surface exposures of Platfreef to a depth of ~1.8 km.
The Bushveld Complex stratigraphy in the deposits area ais as follows, from the base (after Grobler, et al., 2019; and Ivanhoe Mines, 2017):
• Lower Zone - predominantly composed of pyroxenite, dunite and harzburgite that tend to form cyclic units with varying thicknesses and
transitional contacts. Dunite is composed of equigranular unaltered olivine with minor amounts of cumulus chromite. Harzburgite occurs as distinct equigranular and poikilitic varieties. In both, olivine grains tend to be strongly altered to serpentine. Pyroxenites have elongated orthopyroxene grains as well as minor (<2 vol.&%) interstitial plagioclase and clinopyroxene. Noritic units are found locally and are mostly fine- to medium-grained with a distinct fabric of elongated orthopyroxene and plagioclase. Some of these may represent equivalents of the Marginal Zone found elsewhere in the Bushveld Complex. The Lower Zone in the Northern Lobe/Limb occurs as inter-fingering sills and as large, stratified bodies along the basal contact of the Transvaal Supergroup with the
granite basement. Sills of this type up to 700 m thick are intersected in drill core at depth below the Flatreef, emplaced into pelitic, dolomitic, and locally quartzitic and evaporitic rocks belonging to the Duitschland Formation of the Transvaal Supergroup.
• Pyroxenite-Norite Zone (PNZ) - which underlies the main UG2 chromitite seam of the Flatreef and its immediate harzburgitic floor rocks. This zone, which can be up to 570 m thick, is typically composed of poorly differentiated, fine to medium grained pyroxenite/norite containing abundant assimilated floor rocks as hornfels and clinopyroxenites xenoliths and lenses. It also contains intermittent chromitite bands, and possibly represents part of the Lower Critical Zone. These rocks are mostly fine- to medium-grained with a distinct fabric of elongated orthopyroxene and plagioclase. Poorly developed and intermittent chromitite bands, stringers and zones of disseminated chromitite have been encountered within the upper part of this zone, in areas of low sediment contamination. It possibly represents part of the Lower Critical Zone. In the central and eastern portions of the deposit area, the PNZ occurs as thick sill-like bodies within sedimentary floor rocks to the main chromitite seam.
• UG2 Cyclic Unit - which includes hanging wall pyroxenite/chromitite and footwall harzburgite, and occurs above the Pyroxenite-Norite Zone in the western part of the deposit area, comprises a thick, laterally extensive pyroxenite unit containing intervals of massive or semi-massive chromitite. Where the chromitite is massive, as is common towards the base of the pyroxenite unit, it forms a seam that averages ~1 m in thickness. Locally, in the deeper western section of the reef, the immediate hanging wall of this seam contains up to three thin chromitite stringers. The main chromitite seam may have a
mottled texture and contain schlieren and small sub-rounded fragments of chromitite, as well as traces of plagioclase and calcite. In contrast, the ultramafic
silicate rocks of the immediate hanging wall and footwall are essentially devoid of plagioclase. Occasionally, the footwall to the chromitite can to be pegmatoidal pyroxenite/harzburgite. The seam ranges from 0.47 to 2.52 m thick, with a chromitite content from 6.0 to 17.5 wt% Cr. PGE ranges from 2 to 6 ppm, with relatively high Pt/Pd ratios]. The seam has relatively high Rh contents. Sulphur concentrations are as high as 0.85 wt.% and Cu up to 0.17 wt.%.
• Footwall Cyclic Unit (FCU) of Grobler, et al. (2019) or the Norite Cyclic Unit 2 (NC2; Kinnaird et al., 2005; Grobler et al., 2012; Ivanhoe Mines, 2017), which is composed of a repetitive magmatic cyclic layering sequence where laterally removed from the Flatreef. Where intersected in the deep drilling on western and southwestern margins of the deposit it consists of a 45 to 100 m thick sequence of cm to tens of cm thick layers and lenses of alternating pyroxenite-norite-anorthosite sub-units. Many of the contacts between pyroxenite and anorthosite within this unit are occupied by thin (mm-thick) chromitite stringers. The stringers may follow both the bottom and the top contacts of the pyroxenite lenses or layers. Many of the pyroxenite layers contain disseminated sulphides, mostly near their base, but occasionally concentrated in their centre. Where this unit occurs in the central portion of the Flatreef it is lithologically highly variable and consists of varied-textured (commonly pegmatoidal) pyroxenite-norite-gabbronorite containing abundant sedimentary xenoliths which is in contrast to the deep western sector of the deposit area where such variability and xenoliths are less apparent. Magmatic cyclicity in this sector has only been partially preserved in the main Flatreef where it is masked by the inhomogeneity of the xenolith rich intrusion. The resultant modification of magmatic rocks and absence of clear magmatic contacts renders identification of magmatic stratigraphic boundaries complicated and the term 'Footwall Assimilation Zone' (FAZ) is used to collectively identify the unit which may be >200 m in thickness. Magma-sedimentary rock assimilation processes are highly variable in extent and intensity. Zones of assimilation can include most stratigraphic units, such as in the eastern, near-surface extent of the Platreef where the FAZ extends upward into the overlying Flatreef, and possibly as high as the Main Zone. In the western portion of the deposit area footwall assimilation is limited to units below the NC2/FCU.
Variations in the FAZ include i). norite with shale-hornfels xenoliths; ii). para-pyroxenite/para-harzburgite with abundant xenoliths and large, up to >100 m wide and thick, bodies of undigested, but intensely metamorphosed, dolomitic (now calc-silicate) and alumino-silicate (now hornfels) sedimentary rocks of the Duitschland Formation. The latter may form continuous layers at this stratigraphic position, several tens to hundreds of metres in any dimension, and preserve bedding planes that broadly conform to the regional dip, suggesting that they represent in situ relicts of the floor rocks to the Rustenburg Layered Suite. The Bushveld magma interacted with the sedimentary rocks, resulting in pervasive modification of the igneous rocks, including formation of hybrid cumulates. These intervals of strong assimilation and modified rocks can be well mineralised. The main contaminants in the Flatreef are dolomite/limestone, argillite/shale (locally enriched in anhydrite or graphite) and quartzite (notably in the southern sector of the deposit; e.g., Sluzhenikin et al., 2014). The contacts between sedimentary and intrusive rocks varies from sharp to transitional, while sedimentary intercalations persist over centimetres to hundreds of metres. In addition, the amount of metamorphism, metasomatism and/or melting of assimilated sedimentary clasts varies according to the sedimentary rock type and the magmatic unit that consumed the sediment. Argillite xenoliths are typically thermally metamorphosed to hercynite +cordierite ±plagioclase ±hypersthene hornfels and may have been partially or completely melted. These argillite xenoliths tend to be associated with plagioclase-bearing pyroxenite and norite units, with the elevated feldspar and mica contents attributed to the incorporation of partial melts and fluids derived from the clasts.
Dolostone and limestone xenoliths are metamorphosed and melted, and retrograde altered to calc-silicate and marble, with associated metasomatic and mineral replacement processes leading to the formation of localised skarn assemblages. These skarns may be developed along sedimentary bedding planes within dolomitic limestone and/or limestone and along contacts between carbonaceous xenoliths and the igneous rocks. Magnesian skarns are only found within the contact aureoles of dolomite/limestone xenoliths, and include a potassic suite of metamorphic olivine (monticellite) and clinopyroxene (fassaite) with retrograde alteration of periclase to brucite and other alteration products such as talc and serpentine (Sluzhenikin et al., 2014). Magmatic units were modified due to the introduction of magnesian components into the magma at high temperatures during assimilation, giving rise to the formation of para-pyroxenite (clinopyroxene dominant) and para-harzburgite (serpentinised olivine dominant) rock types (Gain and Mostert 1982; White 1994; Buchanan et al., 1981).
• Flatreef of Grobler, et al. (2019), also referred to as the Turfspruit Cyclic Unit (TCU; Kinnaird et al., 2005; Grobler et al., 2012; Ivanhoe Mines, 2017). This subdivision is the down-dip extension of the Platreef, where it forms a laterally continuous, sub-horizontal sequence of PGE mineralised magmatic cyclical units composed of interlayered chromitite-pyroxenite-norite-anorthosite. These units may be several tens to locally >100 m thick, and are found below the base of the Main Zone of the Northern Lobe of the Bushveld Complex. The Turfspruit Cyclic Unit has been variously divided, from the base, by Grobler, et al. (2019) and Ivanhoe Mines (2017), into:
- Main Reef which has a lower, variably developed, mineralised pegmatoidal harzburgite and/or pegmatoidal olivine-bearing pyroxenite (T2 Lower), locally containing a chromitite stringer at its bottom contact. These are overlain by T2 Upper, a mineralised pegmatoidal orthopyroxenite commonly with a thin, ~5 mm chromitite stringer marking its upper contact. Where both T2 Lower and Upper are present, poikilitic harzburgite typically forms the lower portion of the pegmatoidal layer and orthopyroxenite the upper portion. T2 Upper forms a contiguous mineralised layer with the underlying T2 Lower, where present. The pegmatoidal interval has an average thickness across the deposit of ~15 m, but can locally reach >60 m in some areas (Peters et al. 2016).
As detailed previously, the composition of these rocks is affected by the composition of both the magma and assimilated country rocks. A coarse-grained, plagioclase-rich rock is formed where the T2 magma assimilates shale, and an olivine-rich coarse-grained rock where the same magma interacts with dolostone or calc-silicate.
The mineralised T2 Upper pyroxenite can reach a grade of >10 ppm PGE. Higher PGE and Ni-Cu grades (>4 g/t PGE, >0.4% Ni, >0.2% Cu) are commonly associated with the T2 pegmatoid and chromitite. The Pt/Pd ratios also tend to be higher (>1.0) in association with chromitite and pegmatoid. The T2 mineralised zone is defined, based on a 1 g/t 3PE+Au cut-off that exhibits an average thickness of ~25 m. [NOTE 3PE = platinum + palladium + rhodium]
- Middling Unit of Grobler, et al. (2019) appears to correspond to the T1 Unit of Ivanhoe Mines (2017), which overlies the T2 units. The base of this unit is typically marked by a 2.5 to 10 m thick sulphide mineralised orthopyroxenite with a sporadically developed upper-, and a more persistently developed basal, chromitite stringer. Above this interval, the Middling Unit comprises an un-mineralised, non-pegmatoidal medium to coarse-grained, variably feldspathic pyroxenite overlain by a generally non-pegmatoidal mineralised zone near its top, the T1 Mineralised Zone. The T1 pyroxenite is generally the thickest unit within the Turfspruit Cyclic Unit, averaging ~31 m. The T1 Mineralised Zone of Ivanhoe Mines (2017) appears to represent the mineralised interval of the Upper Reef of Yudovskaya et al. (2017) and Grobler, et al. (2019). The latter authors describe their Upper Reef as comprising a variably developed 5 to 10 m thick pyroxenite that is commonly mineralised within its basal 2 to 5 m, with a thin 0 to 2 mm thick, poorly developed, basal chromitite stringer. Ivanhoe Mines (2017) describe their T1 Mineralised Zone as comprising disseminated, medium to coarse-grained sulphides hosted within a typically equigranular feldspathic pyroxenite with local chromitite stringers. The contacts of the T1 Mineralised Zone are gradational with adjacent weakly to un-mineralised T1 pyroxenite. Mineralisation is better developed where the T1 feldspathic pyroxenite is thickened. The average thickness of this mineralised zone is 4.5 m using a 2 g/t 3PE+Au cut-off.
In the relatively deep western and south-central portions of the deposit, the Upper Reef is underlain by a 14 to 100 m thick interval of interlayered pyroxenite-norite-anorthosite that locally contains up to several 0.1 ppm PGE in pyroxenite. However, in the relatively shallow central and eastern portions of the deposit, this layered suite is rarely present, and the Upper Reef typically directly overlies unmineralised medium grained feldspathic orthopyroxenite of the T1 Unit. The Upper Reef/T1 Mineralised zone is at the same stratigraphic position as the Bastard Cyclic Unit described from other parts of the Bushveld Complex (Davey, 1992; Viljoen et al., 1986; Viring and Cowell, 1999).
- Hanging Wall Unit 1 of Grobler, et al. (2019), which varies from 0 to 20 m in thickness, and is composed of interlayered mela- and leuco-gabbronorite.
- Hanging Wall Unit 2 of Grobler, et al. (2019), which is the uppermost unit of the Flatreef. It is a strongly continuous laterally, and is a 0.3 to 30 m (averaging 4.2 m) thick mottled anorthosite that contains autoliths of ultramafic rocks. In some locaions, mottles near the base of the layer, and sometimes along its top contact, are highly elongated suggesting flow, whereas the pyroxenite/norite above and below is undeformed.
• Norite Cyclic Unit 1 of Ivanhoe Mines (2017), which separates the Flatreef from the overlying Main Zone. It consists of a sequence of multiple anorthosite to norite to pyroxenite units with sub-horizontal to horizontal layering. It is laterally extensive, but exhibits significant variations in thickness and lateral facies variation from norite cyclic units to feldspathic pyroxenitic units. The upper margin of this unit is occupied by a laterally extensive mottled anorthosite that ranges from 0 to several tens of metres thick, and separates it from the gabbro-norite of the overlying Main Zone. This anorthosite is at the same stratigraphic position as the Giant Mottled Anorthosite of the Eastern and Western lobes of Bushveld Complex, and represents the Hanging Wall Unit 2 of Grobler, et al. (2019). The latter authors describe the transition zone between the Main Zone and the Flatreef (i.e. Hanging Wall units 1 and 2) in some locations being occupied by an ~60 m thick suite of interlayered pyroxenite, norite, mottled anorthosite and gabbro, with sporadic low-grade sulphide mineralisation occurring in both anorthosite and pyroxenite. The mineralised pyroxenite layers/lenses are commonly altered near their lower and upper contacts. This suite is assumed to be the Norite Cyclic Unit 1.
• Main Zone, which has an apparent thickness of up to 1450 m, with several marker layers traceable along the entire strike length and dip-extent of the deposit area. It is predominantly composed of gabbronorite that is massive on a scale of tens of metres. Three marker units have been identified, i). the Tennis Ball Marker, ~280 m above the base of the Main Zone, named after a band of up to 5 cm wide melanorite-pyroxenite spheres within the gabbronorite. ii). the Basal Melagabbro-norite, a 30 m thick melanocratic gabbronorite layer situated 30 m below the Tennis Ball Marker. It generally has a transitional upper contact, grading downwards from mesocratic gabbronorite to melanocratic gabbronorite. A central gabbronorite is occasionally developed, while the lower contact is either sharp, or gradational into underlying finely layered leuco-gabbronorite. iii). the Ping Pong Marker, a 'fairly continuous' layer which averages ~5 m in thickness, and is located between the other two markers. It comprises a horizon of erratically dispersed centimetre-scale anorthositic spheres, globules or blebs, within a gabbronorite horizon. It can be readily traced within the eastern and southeastern parts of the deposit area, although it is also present, with marginally less continuity within the central and down-dip western parts. Notably the marker units, which dip gently at 25 to 35°W, are not parallel to the Flatreef, which dips subhorizontal to 35 to 45°W (Grobler, et al., 2019).
Some intervals within the Main Zone have a pronounced layering of gabbronorite, melagabbro-norite and mottled anorthosite, representing well-developed cyclic units similar to those in the Upper Critical Zone elsewhere in the Bushveld Complex. Anorthosite layers, however are relatively rare and usually range from a few centimetres to a few metres in thickness and typically contain mottles (oikocrysts) of anhedral pyroxene that may become progressively elongated towards the contacts of the layers. Autoliths or xenoliths of ultramafic rocks and of anorthosite are observed in several drill holes, whereas calcsilicate and hornfels xenoliths are rare (Grobler, et al., 2019).
• Bikkuri Reef - Up-dip from the main Flatreef section, towards the northeastern sector of the deposit area, part of the Turfspruit Cyclic Unit has been intersected stratigraphically 'out of position' at depths ~400 m, above the main Flatreef which is at ~700 m below surface, separated by Main Zone rocks. This 'Bikkuri Reef' is represented by thin T1 and T2 reefs (denoted B1 and B2 reefs) directly in contact with highly contaminated calc-silicate footwall rock. The Bikkuri Reef is basically devoid of olivine-bearing harzburgitic lithologies. The B2 pegmatoidal pyroxenite is not well developed, and the associated mineralisation is generally disrupted and of lower-grade. However, Turfspruit Cyclic Unit rocks containing chromitite stringers are recognisable, and are regarded as part of the T2 Reef. The Bikkuri Reef is interpreted to be the result of semi-consolidating magma that slumped back into the crystallising magma chamber (Grobler et al., 2013) and was displaced downward to the west on a shallow extensional listric fault to overlie down dip T2 Unit equivalents.
• Granite Dykes form swarms with two different orientations are recognised within the deposit area, although they are interpreted to both be part of an anastomosing swarm of syn-Bushveld intrusions contiguous with tension fractures and dilational zones in response to regional transpression. These dykes are commonly orientated sub-parallel to the ductile shear zones, and range from several centimetres to tens of metres in thickness. They are: i). Low Angle Granite Veins that strike 335° and dip at 32°NE; and ii). Steep Granite Veins which strike 329° and dip at 68°NE. The bulk of these dykes occur within Main Zone rocks, with a relatively minor number of intersections within the mineralised reef horizons. The granites are concentrated to the NE, concordant with the gently dipping 'Flatreef', whilst the frequency of intersections decreases markedly to the west and NW (Ivanhoe Mines, 2017).
Five separate PGE mineralised zones have been identified in the Flatreef deposit (Ivanhoe Mines, 2017):
• Within the Upper (Bastard) Reef, occurring as fine to medium-grained magmatic sulphides hosted in feldspathic pyroxenite.
• The Main Reef in the T2 and T1 units have the highest PGE grade, and are the best developed and most continuous of the five zones. They have less contamination from meta-sedimentary xenoliths compared to reefs lower in the igneous stratigraphy. Those in the T2 Unit comprise very coarse-grained magmatic sulphides hosted in pegmatoidal orthopyroxenite and pegmatoidal poikilitic harzburgite. The top of the mineralised zone is commonly marked by a chromite stringer. The T1 Unit mineralisation consists of medium to coarse-grained magmatic sulphide grains hosted in feldspathic pyroxenite.
• Mineralisation within the Footwall Assimilation Zone occurs as medium- to coarse-grained magmatic sulphides hosted in pyroxenite, feldspathic harzburgite/clinopyroxenite, parapyroxenite and paraharzburgite and have a high percentage of associated base metals (Ni and Cu). This mineralisation is less continuous than that of the Main Reef. A clinopyroxenite domain can form a continuous zone of mineralisation below the base of the T2 Mineralised Zone in the north-western portion of the deposit. It is a distinct lithological zone that hosts continuous low-grade Ni mineralisation with local 3PE+Au mineralisation. No meta-sedimentary xenoliths have been identified within this domain, suggesting xenoliths have been completely assimilated.
• The UG2 Reef is also less continuous and composed of fine-grained sulphides hosted in chromitite with associated with high- grade PGE mineralisation.
• A mineralised domain within the Pyroxenite-Norite Zone is predominantly composed of disseminated sulphides within homogeneous pyroxenite/norite lithologies. Locally, fine-grained massive sulphide occur at contacts with hornfels rafts. Mineralisation is typically 1 g/t 3PE+Au, but locally can be 2 to 5 g/t 3PE+Au. Mineralisation also occurs along the contact between the Footwall Assimilation Zone and the Pyroxenite-Norite Zone.
In all of these zones, PGM-tellurides dominate, followed by PGE-arsenides and PGE-sulphides. Studies show that the abundance of PGE-arsenides, antimonides, Bi-Te minerals and PGE-sulphide minerals associated with the upper part of the T2 Unit corresponds to those of the upper part of the Merensky Reef found elsewhere within the Bushveld Complex. The zonation of PGM distribution suggests in-situ crystallisation where modal PGE mineral assemblages are controlled by the thermal gradient. The Bastard and T2 Unit mineralised zones contain an association of high-temperature primary magmatic Pt sulphides and Pt alloys that often form eutectoid intergrowths with base metal sulphides. This is an indication of crystallisation at around 1000°C. Chromitite is the only lithology which contains the sulphide laurite (RuS2; Ivanhoe Mines, 2017).
Base metals sulphides are mainly pyrrhotite, pentlandite, chalcopyrite and lesser pyrite. Their distribution and concentration varies, ranging from <1% to more than 25%. Rare core intersections may contain massive sulphides over tens of centimetres (Brits, 2016). Numerous sulphide textures are observed, the most frequent of which are large fractionated blebs, often associated with smaller disseminated mono-minerallic grains, suggesting several phases of sulphide formation. An early phase is dominated by irregular blebs of disseminated pyrrhotite and pentlandite followed by a later phase where chalcopyrite is dominant (Ivanhoe Mines, 2017).
The Northern Limb is separated from the rest of the Bushveld Complex by the Thabazimbi-Murchison Lineament, a pre-Bushveld, major, compressional tectonic boundary which influenced the subsequent tectono-thermal evolution of the complex. Folding in the Northern Limb has been controlled by two principal transpressional events that included movement along the Thabazimbi-Murchison Lineament (Nex, 2005). This resulted in two main open-fold geometries within the Transvaal sedimentary rocks. The first and dominant of these was caused by NE-SW sinistral transpression, producing a regional NNW trending low amplitude, sub-horizontal open folding. These F1 folds developed within both Archaean basement and Transvaal Supergroup and represent the earliest structures, formed contemporaneously as a result of mild ENE-WSW compression during the Limpopo-Murchison Orogeny at 2.78 to 2.64 Ga. Subsequent NW-SW transpressive inversion refolded the earlier F1 fold axes, producing basin and dome fold interference patterns (Friese, 2012).
Throughout the Northern Limb, there is significant brittle faulting and ductile shear zones, with a pattern of major, widely-spaced, ENE-trending shear zones dominating the regional trend. These combine to form large strike-slip duplex systems, hosting a complex array of riedel shears, normal faults, thrusts and dilational tension fractures that have been intruded in part by dykes and quartz-feldspar veins. These faults were reactivated during a major east-west crustal extension event associated with major brittle fracturing.
The major fault regimes after Brits and Nielsen (2015) and Friese (2012) include:
• NW to NNW trending, moderate to steeply dipping extensional faults that deformed the Transvaal Supergroup and Bushveld Complex by reactivation of those responsible for the Mesoarchaean ~2.98 to 2.96 Ga Pongola rift fault system;
• NE to NNE trending, steep to subvertical structures, predominantly SE dipping dextral strike-slip shear zones with associated NE directed, layer/bedding-parallel thrusting developed in shear zone-bounded domains.
• North-south striking, moderately west dipping extensional fault zones, with undulating gross geometry and an imbricate fan of combined normal dip-slip and sinistral strike-slip duplexes in their immediate hanging wall.
• WNW to WSW-trending extensional fracture/joint zones without significant displacement that were occupied by 1.9 Ga 'Soutpansberg' dolerite dykes that cross-cut all other structural discontinuities.
• Shallow NW dipping, SE-directed thrusts and associated ENE-trending, sub horizontal, low-amplitude regional F2 folds formed in pre- to syn-Rustenburg Layered Suite time as a result of mild SE-directed in situ compressive stress related to the Ubendian Orogeny at ~2.1 to 2.058 Ga.
A structural model developed for the deposit area involves three key deformation features (Ivanhoe Mines, 2017):
• Folding, which was pre-Bushveld intrusion, and comprised low amplitude, upright open folds that are defined by remnant metasedimentary interlayers and xenoliths that are oriented parallel to mineralised zones.
• Ductile shear zones that are 0.3 to 3 m wide, NW trending and dipping steeply at 60 to 70°, representing an oblique reverse sense of movement, with a variable dip direction, and possible antithetic riedel shear zones.
• Brittle faults that are 5 to 30 m wide, north trending, and moderately to steeply dipping at 50 to 70°, with an extensional, east block down normal sense of movement. The key brittle structure is the steeply inclined, easterly dipping Tshukudu Fault Zone which resulted in significant vertical displacement of the order of 60 m. It comprises a wide zone of imbricate fracturing in its hanging wall with intense brecciation within the fault zone.
The structural regime within the deposit area appears to be consistent with a large-scale strike slip duplex system compatible with the regional evolution of the Bushveld Complex (Friese, 2012). It is interpreted that the orientations of the ductile shear zones, the extensional Tshukudu fault zone, the observed folding and the granite dyke swarm can be explained by a long-lived strike-slip duplex configuration that has undergone transpressive inversion (Ivanhoe Mines, 2017).
Resources and Reserves
Published Ore Reserves and Mineral Resources at 24 May 2017 were (Ivanhoe Mines, 2017):
Probable Ore Reserves - 124.7 Mt @ 1.95 g/t Pt, 2.10 g/t Pd, 0.30 g/t Au, 0.14 g/t Rh, 4.40 g/t 3PE+Au, 0.17% Cu, 0.34% Ni;
Indicated Mineral Resources - 346 Mt @ 1.68 g/t Pt, 1.70 g/t Pd, 0.28 g/t Au, 0.11 g/t Rh, 3.77 g/t 3PE+Au, 0.16% Cu, 0.32% Ni;
Inferred Mineral Resources - 506 Mt @ 1.42 g/t Pt, 1.46 g/t Pd, 0.26 g/t Au, 0.10 g/t Rh, 3.24 g/t 3PE+Au, 0.16% Cu, 0.31% Ni;
The Indicated Resource contains 572 t Pt, 582 t Pd, 96 t Au, 37 t Rh, 1285 t 3PE+Au, 0.543 Mt Cu, 1.082 Mt Ni.
NOTE: Cutoff for resources is 2.0 g/t 3PE+Au where 3PE = Pt+Pd+Rh. Mineral Resources are between depths of 500 and 1350 m and are inclusive of Ore Reserves.
The Flatreef contains well mineralised, thickened layered magmatic strata lending itself to bulk underground mining, by long-hole and cut and fill mining.
This summary is largely summarised from Grobler, et al. (2019) and Ivanhoe Mines - 2017 - Ivanhoe Mines, Platreef 2017 Feasibility Study; An NI 43-101 Technical Report, prepared for Ivanhoe Mines, by OreWin Pty Ltd, Adelaide, South Australia, with geological sections submitted by Parker, H. and Kuhl, T., of Amec Foster Wheeler E&C Services Inc; 666p.
The most recent source geological information used to prepare this summary was dated: 2019.
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.
Grobler, D.F., Brits, J.A.N., Maier, W.D. and Crossingham, A., 2019 - Litho- and chemostratigraphy of the Flatreef PGE deposit, northern Bushveld Complex: in Mineralium Deposita v.54, pp. 3-28.|
Junge, M., Oberthur, T., Kraemer, D., Melcher, F., Pina, R., Derrey, I.T., Manyeruke, T. and Strauss, H., 2019 - Distribution of platinum-group elements in pristine and near-surface oxidized Platreef ore and the variation along strike, northern Bushveld Complex, South Africa: in Mineralium Deposita v.54, pp. 885-912.|
McDonald, I, Harmer, R.E., Holwell, D.A., Hughes, H.S.R. and Boyce, A.J., 2017 - Cu-Ni-PGE mineralisation at the Aurora Project and potential for a new PGE province in the Northern Bushveld Main Zone: in Ore Geology Reviews v.80, pp. 1135-1150.|
Yudovskaya, M., Belousova, E., Kinnaird, J., Dubinina, E., Grobler, D. and Pearson, N., 2017 - Re-Os and S isotope evidence for the origin of Platreef mineralization (Bushveld Complex): in Geochimica et Cosmochimica Acta v.214, pp. 282-307.|
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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.
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