Modeling 3D crustal structure in Lachlan Orogen, Victoria, Australia: Implications for gold deposition

Abstract

We use numerical simulations to identify sites of dilation and areas of high, shear strain and fluid flow that may be related to gold deposition in faults that transect the western sub-province of the Lachlan Orogen. Our results can explain how a late tectonic history consisting of a switch from east–west compression to north–south transpression contributes to the formation of gold deposits in association with late fault movements. The models simulate incremental east–west shortenings of 4% and a superimposed 1% north–south shortening on major crustal-scale (intrazonal) faults and suggest that strain and fluid flow was greatest in the shallow-dipping fault segments (first order faults) that lie within the mafic rocks of the lower crust. The areas above the shallow-dipping segments of intrazonal faults (second order) become sites for the initiation of later (third-order) faults and fracture networks, within the higher-level metasedimentary rocks, strain decreases in the steeper segments of the intrazonal faults. These second-order faults are inferred to act as highly permeable channel-ways for fluid discharge and it is their geometry and a shift from east–west reverse-dip-slip to north–south reverse-oblique-slip fault kinematics that controls the final distribution of gold mineralization in the folded metasedimentary rocks. Changing the direction of principle compression, has a dramatic effect on the extent and location of volumetric strain (dilation) and fluid flow with localized deformation within bedding-parallel veins dispersing fluid flow. Increased dilation facilitates the influx of gold-bearing fluids that in combination with a fault–fracture network, and geochemical factors, have led to significant amounts of localized gold mineralization.