Tectonic origin of the shape of the Broken Hill lodes supported by their structural setting in a high‐grade shear zone

Recent studies have claimed structural support for a syngenetic model of ore formation at Broken Hill. The structural features of the Line of Lode—foliation, lineation, boudinage, folding and shearing—are re‐evaluated and new data presented from several locations in and around the Line of Lode, including the Kintore Opencut and Readymix Quarries. Although deformation partitioning preserves areas of low strain, especially in the hangingwall, that exhibit primary features, the deformation history described shows a history of high‐strain on the Line of Lode. Gneisses in the wall rocks of the orebody show extreme extension in places, with destruction of primary layering and imposition of transposed tectonic fabrics. Sulfide bodies would have been softer than the wall rocks during deformation and any layering in the lodes is likely to be a result of tectonic processes rather than preserved bedding. The geometry of the orebody is described and its setting is revealed as a low‐strain site, a minor fold pair, that developed early in a major high‐grade shear zone. The orebody probably acquired its linear shape first as a result of mass transport of sulfides to this structural site and then by extension within the shear zone, an epigenetic process. Previous fold models for the Line of Lode are rejected, along with the application of regional stratigraphic units to the orebody footwall. Deformation of the Line of Lode before peak metamorphism is obscured by recrystallisation. Subsequent minor deformation occurred at both high metamorphic grade and under retrograde conditions to produce the range of features previously quoted in support of syngenesis. Sulfides were remobilised during both the post‐peak metamorphic high‐grade, and later low‐grade, deformation events.

Keywords:

• Broken Hill
• deformation history
• kinematics
• lineation
• ore genesis
• structure