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 [Sri = (87Sr/86Sr)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 Sri near the base of the Merensky Unit in the eastern and western limbs. The results show the existence of a significant shift (from Sri = 0.7060 to Sri = 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.
Highlights
The Bushveld Magmatic Province (BMP) in northern South Africa encompasses an extrusive sequence and an intrusive sequence (the Bushveld Igneous Complex (BIC); e.g., Kinnaird et al 2004)
The BIC intruded into the Rooiberg Group, Transvaal Supergroup, and Archean basement, and it is composed of three main suites: (1) the Rustenburg Layered Suite (RLS), a sequence of mafic to ultramafic layered rocks; (2) the Lebowa Granite Suite, a series of granites overlying the RLS; and (3) the Rashoop Granophyre Suite, which has been interpreted as the late felsic phase of the BIC (Kinnaird et al 2004; Kruger 2005)
Gabbronorites of the Main Zone are the dominant lithologies to −1187 m below the surface, followed by mottled anorthosites (MAN), forming the hanging wall 2 (HW2) of the Bastard unit (BU)
Summary
The Bushveld Magmatic Province (BMP) in northern South Africa encompasses an extrusive sequence (the Rooiberg Group) and an intrusive sequence (the Bushveld Igneous Complex (BIC); e.g., Kinnaird et al 2004). In the eastern and western limbs, the Merensky Reef is located near the top of the Upper Critical Zone and almost immediately below the base of the Main Zone. Recent studies (Yudovskaya et al 2017a, b; Grobler et al 2019) present compelling evidence for correlation between the upper parts of the Upper Critical Zone (from the UG-2 to the contact to the Main Zone) in all the three main limbs based on deep intersections of Platreef (referred to as Flatreef). We test the potential correlation of the Merensky Reef in the eastern and western limbs with the upper part of the Flatreef by documenting a detailed Sr isotopic stratigraphy through a drill core intersection of the Flatreef. The Sr isotopic data is complemented with data on the An content in plagioclase and whole rock major and trace element geochemistry
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