Timing relationships and structural controls on the location of Au-Cu mineralization at the Boddington gold mine, Western Australia

Abstract

The Boddington gold mine is hosted in Archean volcanic, volcaniclastic, and shallow-level intrusive rocks that form the northern part of the Saddleback greenstone belt, a fault-bounded sliver of greenstones located in the southwestern corner of the Yilgarn craton, Western Australia. Total Au content of the Boddington gold mine (past production plus in situ resource) exceeds 400 metric tons, making the Boddington gold mine one of the largest Au mines currently operating in Australia.Geologic mapping and radiometric dating indicate that five phases of igneous activity occurred during development of the Saddleback greenstone belt. Basaltic, intermediate, and minor felsic volcanism occurred between approximately 2714 and 2696 Ma and again at approximately 2675 Ma. An older suite of ultramafic dikes was emplaced between approximately 2696 and 2675 Ma and a younger suite was emplaced between approximately 2675 and 2611 Ma. Granitoid plutons crystallized at approximately 2611 Ma and cut all the other Archean rocks in the Saddleback greenstone belt.Regional upper greenschist facies metamorphism accompanied the earliest phase of ductile deformation (D 1 ). Sericite-quartz + or - arsenopyrite-altered shear zones developed during subsequent ductile deformation (D 2 ). Crosscutting relationships indicate that D 1 and D 2 predate approximately 2675 Ma. Further ductile shear zones characterized by quartz-albite-sericite + or - pyrite alteration developed during D 3 , after approximately 2675 Ma. Narrow brittle faults (D 4 ) with biotite + or - clinozoisite alteration halos, active between approximately 2675 and 2611 Ma, cut the three generations of ductile shear zones.Rare quartz-albite-fluorite-molybdenite + or - chalcopyrite + or - pyrrhotite veins developed prior to D 1 and the regional metamorphism. These veins are not associated with any Au mineralization or significant Cu. Quartz + or - pyrite + or - molybdenite + or - Au veins and crosscutting clinozoisite-biotite + or - actinolite + or - quartz-chalcopyrite-pyrrhotite + or - galena + or - molybdenite + or - scheelite Au veins developed during movement on the D 4 faults between approximately 2675 and 2611 Ma. Mineralized veins crosscut the three generations of ductile shear zones but are not foliated. Movement on the D 4 faults controlled the location of mineralization within the Boddington gold mine. Higher grade mineralization occurs along the D 4 faults and coplanar pyroxenite dikes and where the faults intersect older shear zones, and quartz veins. Widespread lower grade stockwork mineralization is concentrated in the general vicinity of the D 4 faults. The orientation of veins within stockworks is consistent with vein development during sinistral strike-slip movement on the D 4 faults. Au-Cu + or - Mo + or - W mineralization at the Boddington gold mine, therefore, occurred late in the tectonic evolution of the Saddleback greenstone belt.The timing of mineralization at the Boddington gold mine is analogous to many other structurally late Au deposits in the Yilgarn craton, e.g., Mount Magnet, Mount Charlotte, and Wiluna. Movement on the D 4 faults and mineralization may have been coeval with the emplacement of granitoid intrusions at approximately 2611 Ma. Whereas these granitoids are unaltered and therefore unlikely to have been the source of significant volumes of hydrothermal fluids, they may have provided the thermal energy necessary to drive circulation of auriferous hydrothermal fluids through D 4 faults that may also have accommodated their intrusion.Previous workers at the Boddington gold mine have inferred that mineralization is genetically linked to subvolcanic intrusions emplaced between approximately 2714 and 2696 Ma. However, this inference is inconsistent with the crosscutting relationships of structures and mineralized veins which indicate that mineralization occurred between approximately 30 and 80 Ma after emplacement of these rocks.