STRUCTURAL CONTROLS ON ACTIVE EPITHERMAL ENVIRONMENTS, TAUPO VOLCANIC ZONE, NEW ZEALAND

Geological controls on development of high fluid flux conduits are evaluated for active epithermal environments of the Taupo Volcanic Zone (TVZ), where over 20 discrete geothermal systems are associated with silicic volcanism and rapid crustal extension. The high fluid flux conduits we assess resemble those of vertical geothermal wells (>1000 m depth) where rapidly ascending, boiling fluid deposits Au-Ag bearing sulfides, banded silica and lattice calcite identical to minerals and textures in bonanza epithermal vein ore bodies.

The distribution of TVZ geothermal systems is controlled by rift architecture, with upflow zones localized within second order splays to major faults, fault linkage zones or tectonic/caldera fault intersections. These zones of high vertical permeability generally occur in accommodation zones between rift segments. The epithermal environment is largely hosted by porous and weak rocks where tectonic faults behave as barriers to transverse flow over interseismic periods. Distributed flow through fault-bounded compartments predominates, though exceptions occur where fluids pass through more competent rocks. Nonetheless, flow rates from hot springs (75° - 100°C) can be significant, in particular when discharging through hydrothermal eruption vents (5 -120 l/s), comparable to flow rates of geothermal wells (30 – 100 l/s).

Hydrothermal eruption vents appear to be the closest TVZ analogues to zones of high permeability associated with epithermal mineralisation. Judging from the rock-types of eruption breccias and the linear alignment of vents, such zones extend to 300 m depth where they become rooted in steeply dipping faults. In this environment, fault permeability is controlled by: (1) presence of a suitably competent rock type, (2) sufficient ground preparation through silicification to impart a tensile strength to otherwise weak rock, or (3) seismicity, inducing episodic and transient permeability consequent on fault reactivation. The absence of Au-Ag vein mineralisation in geothermal systems appears to be a function of rock properties rather than the absence of an appropriate stress regime or fluid chemistry.