Geochemistry of extremely modified chromites from the chrysotile asbestos-bearing Zvishavane Ultramafic Complex, south central Zimbabwe

Abstract

ABSTRACT The Zvishavane Ultramafic Complex (ZUC) is a metamorphosed ultramafic-mafic body 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, and metagabbros. Accessory chromites, which are altered to ferrian chromites, are hosted in metadunites and serpentinites. Chromium number, Cr# (100*Cr/(Cr + Al)), is extremely high, and ranges from 90.6 to 99.5. Low magnesium number, Mg# 100*Mg/(Mg + Fe2+), ranges from 5.1 to 8.4. Most ZUC ferrian chromites occur as inclusions or marking the outlines of relict olivine crystals, and in veinlets traversing the serpentinites. On 100*Fe3+/(Al + Fe3+) versus 100*Fe2+/(Mg + Fe2+) diagram, the chromites all plot in the magnesioferrite field, and in the ferrian chromite field on a Fe3+-Cr-Al ternary diagram. On wt% Al2O3 versus wt% Cr2O3 and 100*Cr/(Cr + Al) versus 100*Mg/(Mg + Fe2+) plots, chromites all plot in the modified trend fields. The bulk of the chromites were metamorphosed at temperatures of ≤500°C, and no samples experienced temperatures exceeding 550°C. A plot of Fe3+/(Fe3++ Fe2+) versus wt% MnO is indicative of hydrous metamorphism of ZUC chromites. It is interpreted that extensive modification of the ZUC chromites occurred during hydrothermal alteration, under conditions more oxidising than quartz-fayalite-magnetite but less than magnetite-hematite, which accompanied the formation of high-grade chrysotile asbestos for which the ZUC is famous. Four granitic plutons exposed within 50km of ZUC were likely the source of hydrothermal fluids responsible for extensive hydrothermal alteration, which likely led to the formation of high-grade chrysotile asbestos, and almost completely modifying the chromites in the process. Compositions of accessory chromites in chrysotile asbestos meta-ultramafic bodies elsewhere are likely to be extremely modified.