The structure and tectonic evolution of the Broken Hill region, Australia

Abstract

One of the world largest silver-lead-zinc sulphide orebodies occurs at Broken Hill, New South Wales. Detailed mapping and recording of small-scale structures over a large part of the Willyama Orogenic domain around Broken Hill, has enabled the evolution of the area to be seen in terms of three superposed deformational and metamorphic events. The first event was by far the most intense and produced the dominant tectonite fabrics of the area. Intense deformation accompanied by syntectonic metamorphism to granulite facies produced widespread flat-lying gravity nappes whose probable root zone lies to the southeast of Broken Hill and whose downturned noses outcrop over extensive areas to the northwest. Later deformations of lesser intensity deform the earlier structures and form the major easily recognised folds of the region. The deformation can be divided between two orogenic episodes, one terminating at ca. 1500 Ma (Willyama) and one terminating at ca. 500 Ma (Delamerian). The Broken Hill orebody lies within the overturned limb of an extensive F 1 fold nappe and has therefore been bodily transported, along with its enclosing rocks, for a distance of the order of several tens of kilometres from its original point of deposition. The orebody shows some remobilisation into locally developed structures but does not occupy a unique tectonic position within the Willyama Domain.