The role of fluids in partitioning brittle deformation and ductile creep in auriferous shear zones between 500 and 700 °C

Abstract

The fabric, mineralogy, geochemistry, and stable isotope systematics of auriferous shear zones in various hydrothermal gold deposits were studied in order to discuss the role of fluids in rock deformation at temperatures between 500 °C and 700 °C. The strong hydrothermal alteration and gold mineralization indicates that effective permeability development goes ahead with high-temperature rock deformation. The economic gold enrichment is often hosted by breccias and quartz veins in the ductile shear zones, which either formed at fast strain rates or by low strain continuous deformation at slow strain rates. Both processes require (1) a close-to lithostatic to supralithostatic fluid pressure and/or (2) a strong rheology contrast of the deformed lithologies that is often developed during progressive hydrothermal alteration. Compartments of high fluid pressure are sealed from the rest of the shear zones by high-temperature deformation mechanisms, e.g. intracrystalline plasticity and diffusion creep, and compaction. In contrast, in mylonites with heterogeneous crystal plastic and brittle deformation mechanisms for the various minerals, an interconnected network of a grain-scale porosity forms an effective fluid conduit, which hampers fluid pressure build-up and the formation of veins. The auriferous shear zones of the various gold mines represent fluid conduits in the deeper crust, 100 m along strike and up to 1000 m down-dip. The hydrothermal fluids infiltrated may be responsible for low magnitude earthquakes in the Earth's lower crust, which otherwise deforms viscously.