Shear zone versus fold geometries at the Cannington Ag–Pb–Zn deposit: implications for the genesis of BHT deposits

Abstract

Existing structural analysis of drill-core and sulphide zonation were taken as evidence for a large-scale synform that dominates the geometry of the Cannington Ag–Pb–Zn deposit—a Broken Hill-type (BHT) deposit. Underground mapping in this study has found that northwesterly directed thrusting took place during peak-temperature D 1, and that S 1 gneissosity is continuous around an amphibolite mega-boudin. D 2 sillimanite–biotite shear zones contain asymmetric folds previously interpreted to be parasitic folds, but in this study they are interpreted as due to easterly-directed extension of S 1. S 1 and S 2 indicate that shear zones were continuous and not folded into the current deposit geometry. D 2 is associated with garnet–pyroxene alteration that proved to be a favourable host for low-temperature sulphide precipitation. Retrograde deformation post-D 2 controlled lode geometry. Such a concept of high-temperature shear zone formation can be applied to other BHT deposits such as the Broken Hill Main Lode and the Aggeneys–Gamsberg base metal deposits in South Africa. In the shear zone model, Cannington and other BHT deposits are situated within high-temperature thrust shear zones that formed in conjunction with regional large-scale folding.