Magnetite chemistry of the Far Northern Limb of the Bushveld Complex

Abstract

The boundary between the Upper Zone andMain Zone of the Bushveld Complex is considered to be marked by the first appearance of cumulus magnetite (Eales and Cawthorn 1996). Magnetite associated with the Upper Zone in the northern limb is titanium and vanadium rich (Ashwal et al. 2005). The Main Zone contains a low modal abundance of magnetite that differs from Upper Zone magnetite, with an intercumulus texture and lower concentrations of titanium and vanadium (Ashwal et al. 2005). Whether the Cu–Ni–platinum group element (PGE)–Au mineralisation of the Aurora project and the T Zone of the Waterberg project in the far northern limb is hosted in the Main Zone or Upper Zone remains uncertain. Furthermore, the relationship between the Aurora andWaterberg mineralisation is not fully understood. However, a possible link between the Aurora mineralisation to the south of the Hout River Shear Zone (HRSZ), and the T Zone mineralisation of the Waterberg project to the north of the HRSZ has been inferred previously (McDonald et al. 2017). Alternatively, a separate magmatic basin may have been present north of the HRSZ (Huthmann et al. 2017; Kinnaird et al. 2017; Huthmann et al. 2018). On the basis of mineral chemistry, the mineralisation at La Pucella of the Aurora project has been attributed to the Upper Main Zone of the Bushveld Complex (McDonald et al. 2017). However, the presence of cumulus magnetite in a La Pucella gabbroic unit is not consistent with Main Zone mineralisation (McDonald et al. 2017). To investigate whether the mineralisation of the Aurora project relates to the Upper Zone or Main Zone, we analysed magnetite grains in mineralised leucocratic rocks from Harriet’s Wish, Kransplaats and La Pucella sections of the Aurora project. Aurora magnetite compositions are compared to those from the mineralised T Zone of the Waterberg project. Magnetite grains were characterised in terms of cumulus or intercumulus textures, where the latter could reflect Main Zone magnetite (Ashwal et al. 2005). We compare the results to those previously reported for the Upper Zone and Main Zone magnetite from the Bellevue core (Ashwal et al. 2005). The analysed cumulus magnetite grains are enriched in vanadium, a characteristic of Upper Zone magnetite. However, these magnetites have relatively low concentrations of titanium, and high concentrations of Fe compared to Upper Zone magnetites from the Bellevue core (Ashwal et al. 2005). The relatively Ti-poor and Fe composition is attributed to the exsolution of ilmenite lamellae from former titanomagnetite/ulvöspinel, a feature observed previously in the Upper Zone (von Gruenewaldt et al. 1985). Therefore, the magnetite chemistry is most consistent with the Upper Zone. No significant differences in magnetite chemistry between the Aurora and Waterberg deposits were identified, supporting a possible connection of the two deposits.