An unusual nickel sulfide breccia deposit from southern Zambia

What is it about?

The Munali nickel deposit occurs as a sulfide-cemented breccia (chaotically broken, veined and recemented) around the edge of a small gabbro body that is intrusive into metamorphosed sedimentary rocks of the Zambezi Belt. The sulfides occur together with an unusual mineral association of magnetite, apatite and carbonate, which together enclose blocks of coarse gabbro and of olivinite. Zircons extracted from the gabbro blocks have radiometric U-Pb ages of 862 million years, whereas zircons from the magnetite-apatite-carbonate breccia matrix have slightly younger ages of 860 to 856 million years.

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Why is it important?

This nickel sulfide deposit does not easily fit into the many classification schemes for nickel deposits. It is quite unusual in its shape (annular), position (at the top of a sill-like intrusion) and mineral composition (sulfides associated with apatite, magnetite and carbonates). Although its precise geological environment of formation is not yet well understood, it seems to have been generated during passive margin extension, which is not normally known for generation of nickel deposits. All these, and other aspects, should be investigated to allow for better exploration practices.

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