Cuspate ore-wall rock interfaces, piercement structures and the localization of some sulfide ores in deformed sulfide deposits

Univ. Witwatersrand, Geol. Dep., Johannesburg, South Africa
Trojan Nickel Mine, Zimbabwe
Tsumeb Corp., Namibia

Studies on three deformed massive sulfide deposits (Trojan nickel deposit, Zimbabwe; Matchless copper deposit, Namibia (South-West Africa); Broken Hill silver-lead-zinc deposit, Australia) show that the interfaces between massive sulfide rock and silicate wall rock are characterized by irregularities and projections of sulfide rock into the wall rock. The structures range from simple cusps, in which ore layering remains parallel to the ore-wall rock interface and to wall-rock layering, to veins which occupy brittle fractures in the wall rocks. In size, the features range from minor projections less than 1 m across to major piercement structures which may exceed 50 m in length. The structures can usually be related to specific phases of rock deformation. In most cases, the contained sulfide rock is a tectonic breccia with a foliation parallel to margins of the structure.The cusp and related structures reflect competency contrast during rock deformation. Simple cusps develop in a similar manner to certain types of mullions. However, where the competent wall rock is relatively brittle, ductile sulfides take advantage of brittle fractures and other zones of weakness to penetrate into the wall rock and develop discordant vein structures. The mechanism by which the sulfides move is plastic flow, although there is some evidence for the activity of a fluid phase.The process has economic significance in the generation of locally thickened parts of ore-bodies.

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