Surface-derived fluids in basement rocks:inferences from palaeo-hydrothermal systems

Abstract

Drilling into modern systems such as the KTB borehole in Germany indicates that the upper crust is pervasively fracturedand saturated with surface-derived fluids at hydrostatic pressures. The recognition of surface-derived fluids in palaeohydrothermal systems is limited to the effectiveness of geochemical tracers. At low total fluid fluxes, fluids tend to interact with wall rocks and become isotopically, chemically and thermally equilibrated. At high fluid fluxes, such as found in upper crustal mineralisation systems, fluid inclusion halogen systematics can be used to identify the origin of the fluids, even when the fluids have equilibrated isotopically and thermally with deeper parts of the hydrothermal system. Unlike the modern systems, fluid inclusions in vein fills in both mineralised and unmineralised systems show strong evidence for episodic fluid flow and fluctuating fluid pressures, even in upper crustal rocks.