Abstract
New lithological and whole rock compositional data show that the main platinum-group element (PGE) horizons of the Flatreef succession show strong compositional similarities to the Merensky and Bastard reefs of the western Bushveld Complex (WBC), notably in terms of many immobile and incompatible minor and trace elements such as TiO2, Zr, Y, and REE. However, Al2O3, CaO, and Na2O contents are markedly lower in the Flatreef, whereas MgO contents are higher. In view of broadly similar silicate mineral compositions in the Flatreef and the WBC reefs, we suggest that the major element compositional differences between the rocks are largely due to higher modal proportions of orthopyroxene and olivine and lower proportions of plagioclase in the Flatreef. The thickness of the mineralised interval is much greater in the Flatreef than in the WBC (several 10 s of m vs ~ 1 m) and the abundance of sulfides in the Flatreef is typically somewhat higher (on average ~ 1.5% vs ~ 1%). These data, complemented by textural observations, are interpreted to reflect enhanced hydrodynamic crystal sorting accompanied by percolation of sulfide melt through incompletely solidified cumulates. Further genetic constraints are provided by metal data: The concentration of Ni (~ 3000 ppm) in the Flatreef is broadly similar to that in the Merensky Reef of the WBC, but Cu contents are markedly higher (average ~ 1500 ppm vs 700 ppm in the WBC). The concentrations of most PGE are slightly lower (Flatreef ~ 1.5–2 ppm Pt, ~ 100–150 ppb Rh; WBC MR 3.7 ppm Pt, 240 ppb Rh), but Pd has broadly similar contents (Flatreef ~ 1.2–2 ppm; WBC MR 1.54 ppm). The relatively high Cu content of the Flatreef is interpreted as a result of assimilation of Cu sulfides from the sedimentary floor rocks. The reason for the enrichment of Pd relative to Pt, especially in the basal rocks, remains unclear. It could reflect mobilisation of Pd via a fluid phase from the country rocks or the interior of the intrusion, relatively enhanced partitioning of Pd into the sulfides, or relative Pt depletion of the earliest magma pulses in response to Pt alloy fractionation triggered by contamination with reducing country rocks.
Highlights
Grobler et al (2019) used a ~ 700-km drill core database to document the stratigraphy of the “Flatreef”, a highly mineralised, flattened portion of the Platreef located downdip of previous mining and exploration operations in the northern lobe of the Bushveld Complex (Fig. 1 in Maier et al 2020)
The growing body of information including the new data presented in this paper suggests that the platinum-group element (PGE)-Ni-Cu ore deposits of the Flatreef formed through a combination of magmatic, hydrodynamic and hydromagmatic processes, starting with (i) the influx of several batches of PGE fertile basaltic magma
The magmas became saturated in an immiscible sulfide melt due to assimilation of country rock sulfides during emplacement resulting in somewhat higher Cu contents of the Flatreef than in the reefs of the remainder of the Bushveld
Summary
Grobler et al (2019) used a ~ 700-km drill core database to document the stratigraphy of the “Flatreef”, a highly mineralised, flattened portion of the Platreef located downdip of previous mining and exploration operations in the northern lobe of the Bushveld Complex (Fig. 1 in Maier et al 2020). In contrast to the relatively poorly layered Platreef, the Flatreef shows more regularly layered reefs of the type found in the western and eastern Bushveld Complex (WBC and EBC), including the Merensky Reef (MR) and UG2 chromitite. These findings suggested that many of the processes responsible for the formation of the MR and UG2 controlled the formation of the Flatreef (Grobler et al 2019). The geological setting and igneous stratigraphy of the Flatreef have been presented in detail by Grobler et al (2019) who correlated the most mineralised sequence of the Flatreef to the Upper Critical Zone (UCZ) of the WBC (Fig. 1). The Flatreef is much more mineralised (up to 4.5 ppm platinum-group element (PGE) over 90 m) than the Merensky and UG2 reefs of the WBC (each typically 5–8 ppm over ~ 1 m), and more contaminated resulting in abundant xenoliths and rafts of the floor rocks and reduced lateral continuity of layering
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call