Deformational and metamorphic processes in the formation of mesothermal vein-hosted gold deposits - examples from the Lachlan Fold Belt in central Victoria, Australia

Gold-bearing quartz vein systems in metamorphic terranes are one of the most important types of lode gold resource. Major vein-type Au mineralisation of this style in central Victoria is restricted to narrow, structurally-controlled domains in a low grade metamorphosed quartz-rich turbidite sequence. Vein systems in these domains have developed in fault-related and fold-related dilatant fractures which were generated at supralithostatic fluid pressures during regional deformation and metamorphism. The timing of mineralisation, the nature of associated hydrothermal alteration, and the isotopic and chemical compositions of the fluids point to ore genesis involving large volumes of COH metamorphic fluids whose flow has been channelised along high-permeability fault zones. The development of auriferous vein systems has been controlled by the coincident development of structural and geochemical traps. The major structural traps are dilatant jogs on reverse faults, extension fracture arrays adjacent to faults, and saddle reefs and related structures in fold hinges. Gold precipitation is ascribed largely to structurally controlled mixing of secondary CH₄-bearing fluids with more oxidised primary gold-bearing fluids traversing the dilational fracture systems. The CH₄-bearing fluids have been produced by interaction of the primary fluid with graphitic slates adjacent to fault zones. Cyclic fluctuations in fluid pressure and shear stress accompanying episodes of fault motion are shown to control repeated episodes of fracture opening, fluid mixing, and mineralisation.