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

Abstract

The origin of PGE-Ni-Cu mineralization in the Platreef, northern limb of the Bushveld Igneous Complex (BIC), and the possible correlation with the Merensky Reef in the eastern and western limbs has been long debated. The Platreef and Merensky Reef share the same stratigraphic position in the uppermost part of the Upper Critical Zone (UCZ), near the transition to the overlaying Main Zone (MZ). However, discrepancies in interpretations have been difficult to resolve due to the effects of intense magma-country rock interaction throughout most of the northern limb succession. To address this problem, we generated a detailed stratigraphic profile of the initial strontium isotopic ratio [Sr i = ( 87 Sr/ 86 Sr) i ] in plagioclase across a Flatreef interval lacking macroscopic evidence of country rock assimilation. The in situ Sr isotopic ratios in plagioclase were determined using LA-MC-ICP-MS analysis on 37 samples from a drill core (UMT094) at the Turfspruit project. Strontium isotope stratigraphy is useful because of a well-documented shift in Sr i near the base of the Merensky Unit in the eastern and western limbs. The results show the existence of a significant shift (from Sr i = 0.7060 to Sr i = 0.7090) that matches the isotopic shift documented through the Merensky Unit in the eastern and western limbs. Thus, this new Sr isotope data indicates that the main mineralized interval of the Flatreef can be stratigraphically correlated to the Merensky Reef in the remainder of the BIC. In addition, we interpret these results as compelling evidence to suggest that the main mineralization processes in the Flatreef were likely similar to those operating in the eastern and western limbs and that interaction with local country rocks was not a necessary condition. Electronic supplementary material The online version of this article (10.1007/s00126-020-01006-3) contains supplementary material, which is available to authorized users.

Assessing the extent of local crust assimilation within the Flatreef, northern limb of the Bushveld Igneous Complex, using sulfur isotopes and trace element geochemistry

“…We have shown here with the δ 34 S, S/Se, CaO/Al 2 O 3 , and Ti/V values (Figs. 2, 3, and 4) and elsewhere with Sr isotopic values (Mayer et al 2020) that UMT094 intersected Upper Critical Zone magmatic rocks and associated PGE mineralization at the stratigraphic position of the Merensky Reef in which there is little to no evidence for proximal assimilation as shown by δ 34 S and 87 Sr/ 86 Sr i values similar to those of the eastern and western limbs of the BIC). Conversely, the lower part of the Platreef stratigraphy in UMT094, which includes the stratigraphic equivalent of the UG2, shows evidence for local assimilation in δ 34 S, S/Se, CaO/Al 2 O 3 , and Ti/ V (Figs.…”

“…There has been some debate in the literature as to whether or not the Platreef represents the lateral equivalent of the Upper Critical Zone of the eastern and western limbs of the BIC (Langa et al 2020;Grobler et al 2019;Kinnaird 2005;Sharman et al 2013;Mayer et al 2020;Maier et al 2020). Much of this debate has centered around differing interpretations regarding the extent of crustal assimilation in the Platreef magmas, in particular the role of the addition of crustal S to generate the atypical thickness of PGE mineralization in the Platreef compared to the rest of the BIC (Holwell et al 2007;Sharman et al 2013).…”

Geological setting and concentration of scandium in the Flatreef and eastern limb chromitites of the Bushveld Complex

“…Below the entire Flatreef sequence, sills of dunite, pyroxenite, and harzburgite may be found intruding the Transvaal Supergroup (Grobler et al, 2019;Yudovskaya et al, 2021). The package of rocks from the lower chromitite seam up to the noritic cyclic units above the Flatreef shows better mineral layering with less contamination than the Platreef, and can be well correlated with the interval from the UG-2 up to the Bastard Reef (above the Merensky Reef) in the western and eastern limbs of the BC (Grobler et al, 2019;Beukes et al, 2021;Mayer et al, 2021).…”

“…In this region, the FCU is also more variable, consisting of abundant pegmatitic mafic and ultramafic rocks, and some rock types appear to have been altered (Grobler et al, 2019). These rocks, where interaction of magma with sediment is highly evident, are termed the Footwall Assimilation Zone (FAZ) (Grobler et al, 2019;Mayer et al, 2021). Xenoliths found in this zone are often metamorphosed and recrystallized; for example, argillite is often altered to hornfels and dolostones or limestones form marble and localized skarn assemblages (Grobler et al, 2019).…”

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

“…The papers by Mayer et al (2020) and Beukes et al (2020) present the first Sr isotopic profiles of the Flatreef on the farms Turfspruit (Mayer et al) and Macalacaskop (Beukes et al). Both studies document a similar Sr isotopic stratigraphy and range in initial Sr isotope ratio as previously documented in the Merensky Reef interval of the western Bushveld by Kruger and Marsh (1985).…”