Applying the box plot method

Since footwall alteration zones are related to volcanic-hosted massive sulfide (VMS) deposits, their unique features provide likely targets for exploration. Of these, the chloritic footwall alteration pipe is the most characteristic of VMS deposits. This feature is identified geochemically by a strong relative enrichment in aluminium and magnesium, coupled with depletion in calcium and sodium, giving rise to chloritic rocks. During metamorphism such rocktypes are replaced by rocks containing magnesium-rich cordierite, phlogopite, orthoamphiboles or orthopyroxenes and aluminium-rich minerals, such as sillimanite and corundum. As such, the unusual geochemical features of the alteration zone, retained during the deformation and metamorphism, may be recognised in exploration by studying the geological and chemical composition of their rock formations.

The massive sulfide deposit in the eastern part of South Africa’s Namaqua Province, at Areachap, Kantienpan and the defunct Prieska Cu-Zn Mine are hosted in the Areachap Group, formed by volcano sedimentary layers of the mid-Proterozoic era. These deposits were affected by a complex deformation and metamorphic history.

Applying known lithogeochemical methods is especially complicated where the geology is not well understood, due to the poor rock exposure, as in the eastern part of the Namaqua Province.
The box plot presents a method that can readily be used to identify the alteration process, but it was designed for relatively un-metamorphosed environments. This study demonstrates that the box plot may also be applied to high-grade metamorphic terrains, replacing the low-grade metamorphic mineral assemblage with its high-grade equivalent. The trends in mineral composition themselves may be used in defining the box plot. These include, for example, the transition from annite to phlogopite and hornblende to gedrite or cummingtonite.

Conclusions based on using the graphic solution known as the isocon method demonstrated that primary footwall alteration zones in the Areachap Group’s VMS deposits are characterised by the predicted depletion and enrichment of elements, as expected. The whole rock compositions that were independently identified as the equivalents of altered rocks plot in the correct place in the box plot for high-grade regionally metamorphosed terrains, establishing the box plot as an effective and practical exploration tool for VMS deposits.

Dr Hennie Theart:

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