Using oxygen (δ18O, δ17O) and hydrogen (δ2H) isotopes in the Northern Limb of the Bushveld Igneous Complex to assess the extent of footwall contamination

Abstract

The Platreef in the Northern Limb is in the same stratigraphic position as the Critical Zone (i.e. Merensky Reef) in the Eastern and Western limbs of the Bushveld Igneous Complex (BIC). However, in some areas, the Platreef is in direct contact with Archean granitic gneisses and Transvaal Supergroup metasedimentary rocks. Although there is significant evidence of assimilation of Transvaal Supergroup metasedimentary rocks in the southern parts of the Platreef, there is no consensus on the extent of contamination and if contamination is proximal or distal to the Pt, Pd, Au mineralized zone/Merensky Reef equivalent. For this study, we report δ’¹⁸O, Δ’¹⁷O, and δ²H values of samples from the downdip portion of the Flatreef (part of the Platreef) on Turfspruit. The results show that δ’¹⁸O values range from ∼4 to 7 ‰ in intervals with little to no evidence of assimilation and can reach up to 17 ‰ in intervals with evidence of pervasive assimilation. The δ’¹⁸O values in areas with no evidence of assimilation are similar to previously reported values in the Eastern and Western limbs of the BIC. The Δ’¹⁷O values range from −0.200 to 0.050 ‰ throughout the stratigraphy. Most of the positive Δ’¹⁷O values are associated with intervals showing evidence of assimilation. The δ²H values (which range from −92 to −30 ‰) show no correlation between intervals with pervasive assimilation and those with little to no contamination. Samples from the Turfspruit area of the Northern Limb have δ’¹⁸O values of -5.53 to 17.14 ‰ which shows the extent of footwall assimilation of the Transvaal metasedimentary rocks. The triple oxygen isotope values show that the Northern Limb of the BIC records two separate episodes of crustal contamination: (1) a proximal contamination by local footwall lithologies; and (2) a deeper, more pervasive (with respect to the entire BIC) contamination, prior to injection of the BIC magmas into the middle/upper crust.