COMPLETELY MODIFIED CHROMITES FROM THE ZVISHAVANE ULTRAMAFIC COMPLEX, SOUTH-CENTRAL ZIMBABWE

The Zvishavane Ultramafic Complex (ZUC) (formerly Shabanie Ultramafic Complex), is a meta-ultramafic/metamafic body that is exposed within the Zvishavane gneiss complex in south-central Zimbabwe. The complex hosts Africa’s largest mine and reserves of high-grade chrysotile asbestos. Rock types of the ZUC range from serpentinites, metadunites, metagabbros, and accessory chromites which have been completely altered to magnetites. A petrographic and chemical composition investigation of the chromite was undertaken to place some constrains on the origin of the ZUC.

All chromites occur within serpentinites. Although some magnetites occur as inclusions or marking the outlines of relict olivine crystals, they also occur in veinlets traversing the serpentinites. When plotted on 100*Fe³⁺/(Al+Fe³⁺) versus 100*Fe²⁺/(Mg+Fe²⁺) plot, the chromites plot in the magnesioferrite field, and in the ferrian chromite field when plotted on a Fe3+-Cr-Al ternary diagram. However, on wt% Al₂O₃ versus wt% Cr₂O₃ and 100*Cr/(Cr+Al) versus 100*Mg/(Mg+Fe²⁺) plots, the chromites all plot in the modified trend fields. The bulk of the chromites were metamorphosed at temperatures of ≤500^oC, and no samples experienced temperatures exceeding 550^oC. These temperatures are consistent with those independently obtained from geothermometry of enclosing silicate minerals. A plot of Fe³⁺/( Fe³⁺+Fe²⁺) versus wt% MnO is indicative of hydrous metamorphism of ZUC chromites. It is interpreted that modification of the ZUC chromites occurred during hydrothermal alteration which accompanied the formation of high-grade chrysotile asbestos which the ZUC is famous for.