Ivanhoe holds a 64% interest in Mining Right LP30/5/2/2/1/10067MR, while a Japanese consortium, comprising Itochu Corporation; Japan Oil, Gas and Metals National Corporation; and Japan Gas Corporation, holds a 10% interest, and local communities, local entrepreneurs, and staff hold the remaining 26% as a result of the Broad-Based Black Economic Empowerment transaction, implemented on 26 June 2014.
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Summary:
Deposit Types
Two main PGE deposit types occur within the Bushveld Complex:
• Relatively narrow (maximum 1 m wide) stratiform layers (reefs) that occur towards the top of the Upper Critical Zone typically 2 km above the base of the intrusion (Merensky reef-style), mainly found in the Western and Eastern Limbs. These narrow zones have been the principal targets for mining in the past; however, more recently wider zones with more irregular footwall contacts have been mined (termed potholes).
• Contact-style mineralisation at the base of the intrusion (Platreef-type) occurs mainly in the Northern Limb.
In general, within the Northern Limb, the Platreef comprises a variably layered, composite norite–pyroxenite–harzburgite intrusion that lies at the base of the Bushveld Complex, in contact with metasedimentary and granitic floor rocks. McDonald and Holwell (2011) reviewed the major literature on the Platreef and Northern Limb, and have concluded:
• The Platreef remains a complex and enigmatic deposit.
• Stratigraphic relationships with other stratiform deposits such as the Merensky and UG2 reefs have been suggested.
• The extent to which the Northern Limb was connected to the rest of the complex across the Thabazimbi–Murchison Lineament remains to be established.
• The Platreef represents a complex of sills intruded into basement granite-gneiss, Transvaal Supergroup sediments or pre-Platreef Lower Zone intrusions.
• Intrusive relationships of the Main Zone gabbronorites, into solidified and deformed Platreef, removes the Main Zone as a source of metals for the Platreef.
• Mineral chemistry, bulk geochemistry, and Sr, Nd, and Os isotope geochemistry of the Platreef are most consistent with an ultramafic (Critical or Lower zone) component.
• Platreef Nd values and 187Os/188Os initial isotope ratios overlap clearly with the Merensky Reef but not the UCZ.
• Conventional and mass-independent S isotopes suggest a primary mantle source of S that was overprinted by the addition of local crustal S where Platreef intruded pyrite-rich shales. Assimilation of S is viewed as a modifying process, not as the primary trigger for mineralisation.
Mineralisation
The Platreef Project is hosted within the Palaeoproterozoic (2.06 Ga) Bushveld Igneous Complex (BIC), which is the largest of the known layered igneous intrusions, covering an area > 65,000 km². The BIC hosts up to 75% of the world’s platinum resources (Naldrett et al, 2009).
The BIC includes an early bimodal volcanic sequence (the Rooiberg Group) that is followed by an intrusive layered series of ultramafic and mafic units known as the Rustenburg Layered Suite (RLS) and the Lebowa Granite and Rashoop Granophyre Suites. The RLS is 7 to 8 km thick and ranges in composition from dunite to diorite.
Hall (1932) divided the RLS into 5 zones in descending order:
• Upper Zone (UZ) — Gabbroic succession.
• Main Zone (MZ) — A succession of gabbronorites with occasional anorthosite and pyroxenite bands.
• Critical Zone (CZ) — The Lower Critical Zone (LCZ) consists of orthopyroxenitic cumulates, and the Upper Critical Zone (UCZ) comprises packages of chromitite, harzburgite, pyroxenite, norite, and anorthosite. The CZ hosts PGE–Au–Ni–Cu and chromite deposits in several different chromitite layers known as reefs. The most significant are the Merensky Reef and the Upper Group 2 (UG2) Reef of the Eastern and Western Limbs. These range on average from 0.4–1.5 m in thickness and the contained PGE (Pt, Pd, Rh, Au) content typically ranges from 4–10 g/t (Cawthorn, 2005).
• Lower Zone (LZ) — Upper and lower peridotites separated by a central harzburgite.
• Marginal Zone (MZN) — Norites with variable proportions of accessory clinopyroxene, quartz, biotite and hornblende, indicating magma contamination from the underlying metasediments. This unit is not always present.
In the East and West Limbs of the BIC, the RLS was intruded into the Magaliesberg Formation of the Proterozoic Transvaal Supergroup. In the North Limb, the RLS intrudes progressively older country rocks northward (Magaliesberg Formation, Malmani Subgroup and Duitschland Formation). In the East and West Limbs of the BIC, the CZ includes the Merensky Reef and UG2 chromitite that are exploited for PGE mineralisation. In the North Limb of the BIC, the mineralised horizons have been referred to as the Platreef. The North Limb hosts the Platreef Project.
Structure
Structurally, the Northern Limb is separated from the rest of the Bushveld Complex by the Thabazimbi-Murchison Lineament (TML). The TML is a pre-Bushveld, major, compressional tectonic boundary (suture zone) that formed as a result of the collision of the Pietersburg terrane and Kaapvaal shield around 2.97 Ga during the Murchison Orogeny (Friese, 2003, 2004). The Ysterberg-Planknek and Zebediela Faults play a significant role in the regional geology of the Northern Limb.
The tectono-thermal evolution can broadly be subdivided into pre- and syn-emplacement folding and multiple faulting events. Folding in the Northern Limb has been controlled by two principal transpressional events caused by movements along the TML in the south and the Palala Shear Zone.
According to Nex, (2005), this led to the formation of two main open-fold geometries within the Transvaal sediments. The first and most dominant folding event was caused by NE-SW sinistral transpression. This resulted in regional NNW trending low amplitude, sub horizontal open folding. These F1 folds developed within Archaean basement and Transvaal Supergroup and represent the earliest developed structures which formed contemporaneously as a result of mild ENE-WSW compression during the Limpopo-Murchison Orogeny at 2.78–2.64 Ga. Subsequent NW-SW transpressive inversion refolded the earlier F1 fold axis resulting in basin and dome fold interference patterns (Friese, 2012).
Significant brittle faults and ductile shear zones are known throughout the Northern Limb, and the major, widely-spaced, ENE-trending shear zones dominate the regional map pattern. These combine to form large strike-slip duplex systems, which host a complex array of riedel shears, normal faults, thrusts and dilational tension fractures that have been invaded in part by igneous dykes and quartz-feldspar veins. These faults are reactivated during a major E-W crustal extension event associated with major brittle fracturing.