Litho- and chemostratigraphy of the Flatreef PGE deposit, northern Bushveld Complex

Abstract

The Flatreef is a world-class platinum-group element (PGE) deposit recently discovered down-dip from existing mining and exploration operations on the northern limb of the Bushveld Complex. Current indicated resources stand at 42 Moz PGE (346 Mt with 3.8 g/t Pt+Pd+Rh+Au, 0.32% Ni and 0.16% Cu) which, in the case of Pt, is equivalent to~10 years global annual production, making it one of the largest PGE deposits on earth. The grade and thickness of the Flatreef mineralised interval is highly unusual, with some drill core intersections containing up to 4.5 g/t Pt+Pd+Rh+Au over 90 m in drill core. Here, we document the down-dip and along-strike litho-and chemostratigraphy of the Flatreef and its footwall and hanging wall rocks, based on a diamond drill core database totalling > 720 km. At the base of the sequence intersected in the drill cores are up to 700-m-thick sills of ultramafic rocks (dunite, harzburgite, pyroxenite) emplaced into pelitic, dolomitic, and locally quartzitic and evaporitic rocks belonging to the Duitschland Formation of the Transvaal Supergroup. Next is an approximately 100-200-m sequence of lowgrade-sulphide-mineralised, layered mafic-ultramafic rocks containing abundant sedimentary xenoliths and, in places, several chromite seams or stringers. This is overlain by a~100-m-thick sequence of well-mineralised mafic-ultramafic rocks (the Flatreef sensu strictu), overlain by a laterally persistent mottled anorthosite layer at the base of > 1 km of homogenous Main Zone gabbronorite. Based on stratigraphic, lithological and compositional analogies to the layered rocks in the eastern and western Bushveld Complex, we correlate the Flatreef and its chromite bearing footwall rocks with the Upper Critical Zone, notably the interval between the UG2 chromitite and the Bastard Reef as found elsewhere in the Bushveld Complex. This includes recognition of a Merensky Reef correlative. The ultramafic rocks below the main chromitite seam (UG2 correlative) in the Flatreef footwall are correlated with the Lower Critical and Lower zones. However, compared to the western and eastern Bushveld limbs, the studied sequence is strongly enriched in sulphide and PGE, many of the rocks show elevated CaO, K 2 O, Rb and Zr contents, and lateral continuity of layers between drill cores is less pronounced than elsewhere in the Bushveld, whereas ultramafic units are locally considerably thickened. These compositional and lithological traits are interpreted to result from a range of processes which include contamination with calcsilicate and hornfels, intrusion of granitic magmas, and the influence of multiple structural events such as pre-to syn-emplacement regional-scale open folding and growth faults. Evidence for the existence of potholes also exists. In the shallow, up-dip portions of the project area, the entire magmatic sequence below the Main Zone becomes increasingly contaminated to the extent that individual units are somewhat more difficult to correlate between drill cores. This package represents the Platreef as expose...

Chromite chemistry of a massive chromitite seam in the northern limb of the Bushveld Igneous Complex, South Africa: correlation with the UG-2 in the eastern and western limbs and evidence of variable assimilation of footwall rocks

“…c Comparison of Fe 3+ /∑Fe UG-2 reference and UG-2E sample data from EPMA and Mössbauer spectroscopy tens of meters below the Merensky Reef. In the northern limb (at Turfspruit), the UG-2E matches that relationship with the main mineralization in the Platreef, which has been interpreted as being the Merensky Reef equivalent (Grobler et al 2019). Also, Mayer et al (submitted) document systematic 87 Sr/ 86 Sr i variations across the mineralized section of the Platreef that match similar variations in the main limbs of the BIC (Seabrook et al 2005).…”

“…The online version of this article (https://doi.org/10.1007/s00126-020-00964-y) contains supplementary material, which is available to authorized users. McDonald et al 2005;McDonald and Holwell 2007;Kinnaird and McDonald 2005;Yudovskaya and Kinnaird 2010;Grobler and Nielsen 2012;Yudovskaya et al 2017a, b;Grobler et al 2019). Deep drilling by Ivanplats on their Turfspruit project intersected a massive chromitite seam below high-grade mineralization.…”

“…Thus, the Platreef is loosely defined as a variably PGE-mineralized package of maficultramafic rocks, primarily pyroxenites, of variable thickness that contains chromitite layers, has a thickness that varies from less than 50 m and up to approximately 600 m, and contains metasedimentary xenoliths of the floor rocks (Van der Merwe 1976; Gain and Mostert 1982;Manyeruke et al 2005;Yudovskaya and Kinnaird 2010;Kekana 2014;Yudovskaya et al 2014). Grobler et al (2019) described the Platreef on the Turfspruit project as a suite of mafic-ultramafic rocks beneath the Main Zone, containing a Merensky Reef equivalent with an average thickness of 20 m, which is underlain by a UG-2 analogue chromitite seam, as well as other unmineralized mafic-ultramafic rocks.…”

Low-Sulfide Platinum–Palladium Deposits of the Paleoproterozoic Fedorova–Pana Layered Complex, Kola Region, Russia

“…Recent exploration in the Bushveld Complex has dramatically increased the significance of its contact-style mineralization, revealing the deep levels of the Platreef [80]. Considering recent exploration results from the Portimo, Fedorova-Pana and Monchegorsk complexes [60], contact-style low-sulfide Pt-Pd mineralization is obviously the most important PGE resource on the Fennoscandian Shield.…”

Igneous Rock Associations 23. The Bushveld Complex, South Africa: New Insights and Paradigms

“…More recently, new sections have been discovered to the south of the studied sections that display much less disrupted magmatic stratigraphy (Yudovskaya et al 2017b), and that can be more directly correlated with equivalent Upper Critical stratigraphy elsewhere. These sections have been distinguished from the Platreef by identifying it as "thick reef facies" and, to the south, as "Flatreef " (Grobler et al 2018) based on regional changes (local flattening) of the stratigraphy as contrasted to that observed to the north. This has allowed for the identification of UG2-and Merensky Reef-equivalent stratigraphy in the Northern Lobe (Yudovskaya et al 2017b;Grobler et al 2018), albeit with much more extensive footwall-derived contamination and sulphide-mineralization styles that are consistent with the Platreef setting.…”