Structural characteristics, bulk porosity and evolution of an exhumed long-lived hydrothermal system

The geometry and spatial variability of fracture networks and matrix porosity of fault rocks are key parameters controlling the permeability and ultimately the fluid flux along fault zones. Detailed understanding of evolution and long-term sustainability of naturally porous and permeable fault rocks is thus of prime importance for predicting the occurrence and the successful exploration of natural fault-bound hydrothermal systems. This study presents continuous structural data and matrix porosity measurements collected from a cored drillhole across a long-lived and still active fault-bound hydrothermal system in the crystalline basement of the Aar Massif (Swiss Alps). Image analysis and He-pycnometry analysis for quantification of matrix porosity of tectonites showing variable ductile and brittle deformation intensity is combined with fracture porosity calculations to develop a bulk porosity profile across this hydrothermally active fault zone. In the investigated example, a central fault core that shows a several meter wide fault breccia with consolidated gouge material of increased porosity with maximum values of 9% (He-pycnometry) and >20% (image analysis) is adjoined by several large subsidiary faults and interconnected by a intensly fractured damage zone embedded in granitic to ultramylonitic host rock showing 0.1-6% porosity. The variable degree of ductile precursors forms a succession of subparallel sealing and high-porosity structures parallel to the fault zone bridged by a dense fracture network. Fluid flow is therefore directly related to the combined effect of fractures and enhanced fault-related matrix porosity, possibly dynamically changing with time due to fracturing and precipitation cycles. This suggests a key importance of matrix porosity within fault core rocks (breccia & fault gouge) for the transport of hydrothermal fluids.