Introduction to the special issue on the Flatreef PGE-Ni-Cu deposit, northern limb of the Bushveld Igneous Complex

Abstract

More than 30 years ago, Cox and Singer (1986) suggested that magmatic platinum-group element (PGE)-Ni-Cu deposits are amongst the best understood of ore deposits, yet the origin of PGE mineralization in the Bushveld Igneous Complex (BIC) remains controversial after a century of study. In the northern limb of the BIC, the unravelling of ore formation proved particularly difficult due to relatively poor outcrop, which is typically affected by contamination of the intruding magmas with the host rocks and expressed in the form of abundant xenoliths, footwall rafts and disturbance of magmatic stratigraphy. In this thematic issue, we present contributions on the Flatreef, a recently discovered world-class PGE-Ni-Cu deposit constituting a downdip extension of the mineralized unit of the Platreef of the northern limb. Two deep shafts are currently being sunk, making the Flatreef one of the most significant new mine development on the Bushveld in several decades.

Strontium isotope stratigraphy through the Flatreef PGE-Ni-Cu mineralization at Turfspruit, northern limb of the Bushveld Igneous Complex: evidence of correlation with the Merensky Unit of the eastern and western limbs

“…The general stratigraphy of the RLS in the northern limb is described in detail in the introduction to this thematic issue (Maier et al 2020) and only the most relevant aspects are summarized here. There are a few key features to emphasize.…”

“…A problem with the term "Platreef" is that it has been used to describe both the complete sequence of magma-country rock interaction between the Main Zone and the country rocks (up to 400 m thick) as well as the localized PGE mineralization intervals within that package. To avoid this problem, we use "Platreef Unit" for the complete sequence and "Platreef" for the PGE-Ni-Cu mineralized intervals within that unit as suggested by Mitchell and Scoon (2012) and Maier et al (2020). In the Turfspruit area, the Platreef Unit sequences, which typically dip to the SW, have a change in slope and become subhorizontal at ca.…”

“…In the Turfspruit area, the Platreef Unit sequences, which typically dip to the SW, have a change in slope and become subhorizontal at ca. 700 m depth (Maier et al 2020). Such interval, known as the Flatreef, is also characterized by a decreasing amount of contamination and better preservation of magmatic stratigraphy (Grobler et al 2019).…”

“…The bottom portion ("Merensky 1" or M1) is dominantly pegmatoidal and consists of two different lithologies: an orthopyroxenite ("M1 Upper" or M1 U ) and a feldspathic harzburgite ("M1 Lower" or M1 L ). Although the "upper" and "lower" descriptors reflect the typical spatial relationship, the presence of olivine is the defining feature for the nomenclature and the M1 L occurs stratigraphically above the M1 U in some places (Maier et al 2020). The M1 U orthopyroxenite contains nonpegmatoidal patches (Fig.…”

“…If the MU-BU sequence is bounded by unconformities with the rest of the UCZ below and the Main Zone above, and local contamination was not the trigger and source of mineralization, there is not enough magma volume in the emplaced sequence to extract PGE at the grades measured. Therefore, PGE enrichment most likely occurred at depth in a staging chamber, as in the "model 2" summarized in Maier et al (2020).…”