The Granny Smith gold deposit: the role of heterogeneous stress distribution at an irregular granitoid contact in a greenschist facies terrane

Abstract

The Granny Smith gold deposits formed late in the structural history of the Yilgarn Block at a high crustal level in a largely brittle structural regime. Gold mineralisation is located along a N-S striking fault which wraps around the contact of a small granitoid intrusion. In different sections of the fault, mineralisation may be developed in the granitoid, in the adjacent sedimentary sequence and/or along the contact between them. In the granitoid, gold mineralisation is in conjugate networks of thin carbonate-quartz veins and their alteration halos. Small displacements along veins are common. In contrast, veins and faults in the sedimentary rocks are subparallel to bedding. Spatial variations in the conjugate vein orientations indicate that the local stress field was heterogeneous and controlled by the shape of the granitoid contact. The greatest variations in vein and implied stress orientations occur in zones where the contact is most irregular. These are also the areas of richest mineralisation. Fluid flow was thus focused in a regional-scale low mean-stress region created by the geometry of the granitoid intrusion. Its irregular contact caused deposit scale variations in fluid flow and resulted in heterogeneous gold grades along the contact zone.