Piercement Structures Formed by Metamorphic Mobilisation of the Broken Hill Orebody

The interfaces between the Broken Hill ore lenses and the enclosing wallrocks are, on a broad scale, parallel to metamorphic layering in the wallrock gneisses and quartzites. In detail, however, the concordant contacts are interrupted by piercement structures, formed by plastic injection of ore into the wallrocks during metamorphism.These discordant bodies vary from minor projections measuring less than I m long, to major ore shoots ("droppers") as much as 60 m long. The intrusive bodies consist of fragmented ore and wallrock in amatrix of recrystallised sulphides. Wallrock fragments have been altered to manganese-rich and calcium-rich mineral assemblages. The injected ore is enriched in sulphides compared with the parent ore, and there has been enrichment in a number of minor elements.Piercement structures formed during the waning phase of high grade metamorphism, when incompetent ore was injected plastically under pressure into the wallrocks, particularly at fold crests. Adjacent to the "droppers", retrograde schists developed as a result of shearing along the ore-wallrock interface.