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

摘要

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.