The Formation of Chromite and Titanomagnetite Deposits Within the Bushveld Igneous Complex

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Abstract

A brief analysis of the early tectonic history of southern Africa reveals that the intrusion of the Bushveld Complex is situated at the intersection of several lineaments. Based on gravity measurements, the Bushveld can be divided into five subcomplexes.

The differentiation of the Bushveld magma can be explained by two model systems: MgO — SiO2 — Fe3O4 — Cr2O3 and MgO — SiO2 — FeO — Fe2O3. However, these model systems can neither explain the formation of the chromitite seams of the Critical Zone nor the massive titanomagnetite layers of the Upper Zone.

fO2-Measurements on chromites indicate that the formation of chromitite seams is caused by periodical fluctuations of the fO2. A similar mechanism has been postulated before by various authors for the accumulation of massive titanomagnetite layers.

Detailed geochemical analyses of host rock magnetites are interpreted. Several field evidences indicate the existence of local, but enormous volatile activity and the formation of anomalous magnetite layers by an increase of the fO2, caused by contamination of the magma with sedimentary fragments.

A new model is presented, which explains the formation of massive chromite and titanomagnetite seams as a function of the fO2, as well as their extreme lateral consistency within the Bushveld Complex.