Predictability, unpredictability and dynamic fault behaviour during epithermal deposit formation

Abstract

Fault-vein architecture and scaling properties from the Cracow epithermal Au-Ag deposit, Australia, are compared with observations from active geothermal areas. Typical fault system features are revealed, such as segmentation, increased fracturing in step-over zones, and bifurcation at fault-vein tips. The ratio of maximum fault displacement (D) versus fault length (L) matches the ratio for global fault datasets. However, mineralisation occurs in a variety of fault locations (segments, stepovers, tip-zones etc) and this indicates geometry is not a predictor of where epithermal deposits may be found. Fault kinematics are also complex, with slickenfibres, stratigraphic offsets and wall rock vein orientations indicating the system was dominated by normal dip-slip, with transient oblique and strike-slip increments. Such complexity is to be expected because fault behaviour in active geothermal fields is remarkably varied, due to interaction with intrusions and fluctuating pore fluid pressures. In contrast, the correspondence with global D-L ratios suggests across-strike spacing of mineralised lodes may be predictable, even in kinematically complex systems. Furthermore, geophysical and borehole observations of active geothermal fields indicate there is potential for epithermal-type fault-vein systems to form at depths greater than 1 km.