Distinguishing between local versus regional extension as a control on orogenic gold mineralisation: The new 2.4 Moz Castle Hill Camp, WA

Abstract

The significance of regional extension for orogenic gold mineralisation in the Archaean Eastern Goldfields of Western Australia is not well established and there is a lack of consensus on both the timing and nature of extensional deformation. The 2.4Moz Castle Hill gold camp, located in the Coolgardie Domain, is spatially associated with a NW-SE trending granitic pluton (the Kintore Tonalite). This pluton intruded prior to the gold mineralisation and is coeval with emplacement of the lower succession of the Kalgoorlie greenstone sequence. The Kintore Tonalite is located at a major change in the geometry of the Kunanalling Shear Zone, where the strike of the shear zone changes from the regional NW-SE trend, to ∼N-S. This change in the geometry of the Kunanalling Shear Zone is interpreted to highlight the presence of an early transfer fault intersecting the Kunanalling Shear Zone, or step-over zone on the Kunanalling Shear Zone, that was active at the time of greenstone emplacement. This early architecture created a crustal-scale pathway that facilitated the sourcing and emplacement of the tonalitic magma and also later gold-bearing fluids from lower crustal levels. Three main deformation events that reactivated this early architecture have been delineated, D1–D3. D1 is associated with NE-SW directed compression reactivating NW trending shear zones and lithological boundaries, causing reverse shearing, folding and development of a pervasive foliation. D2 is synchronous with the main gold event which occurred during NW-SE directed far-field compression, causing sinistral shearing on NW trending structures, the development of N-S striking faults, the formation of mineralised quartz veins associated and vertical axis rotation of the Kintore Tonalite. During the D2 stage gold was deposited with tellurobismuthite assemblages over the Castle Hill region, associated with synchronous extensional and contractional features at the southern and northern ends of the Kintore Tonalite, respectively. D2 extensional structures, related to mineralisation, are local features linked to development of sinistral movement on the N-S striking faults. D3 is associated with NE-SW directed compression and the development of N to NE trending dextral faults, dominated by brittle deformation. The primary control on the regional location of the gold system is interpreted to be early pre-existing crustal architecture, marked by the major strike change that hosts the tonalite. The geometry of structures developed within the step-over zone exerts a second order control on the location of deposits within the Castle Hill camp and fluids were focussed into areas of enhanced permeability during deformation. Individual deposits in the camp are hosted in mineralised structures that locally exhibit extensional kinematics but developed in an overall compressional regime.