Abstract

Geologic controls on development of high-flux hydrothermal conduits that promote epithermal ore formation are evaluated at large and small scales for geothermal systems of the Taupo Volcanic Zone, New Zealand. Most geothermal systems occur within a rifted volcanic arc (~150 km long) dominated by silicic volcanism, and they occur in association with major faults near caldera structures or within accommodation zones that transfer extension between rift segments. The geothermal systems are hosted in a thick sequence (1–>3 km) of young volcanic deposits that rest unconformably on weakly metamorphosed Mesozoic argillite and graywacke. Flow regimes and permeability controls in one extinct (Ohakuri) and six active (Broadlands-Ohaaki, Waiotapu, Rotokawa, Waimangu, Te Kopia, and Orakeikorako) geothermal systems show that in general, hydrothermal fluid flow is controlled by (1) heat from magmatic intrusions which drives convective circulation; (2) intergranular host-rock porosity and permeability; (3) fault-fracture network permeability produced by tectonism, volcanism, and/or diking; (4) pipelike vertical conduits produced by volcanic and hydrothermal eruptions; and (5) hydrothermal alteration and mineral deposition that may cause heterogeneity in the porosity and permeability of a fluid reservoir. Such controls influence fluid flow within three distinctive depth zones: (1) a feed zone (>2,000 m depth), (2) an epithermal mineralization zone (<200–2,000 m depth), and (3) a discharge zone (0–200 m depth). Within the deepest part of the feed zone, hydrothermal fluid flow is influenced by magmatic intrusions guided by faults, which localize convection cells, and the brittle-ductile transition at the base of the seismogenic zone, which limits downflow of meteoric water. Hydraulic connectivity through low-permeability Mesozoic rocks is favored along NNE- to ENE- and WNW- to NNW-striking structures given the NW-SE direction of maximum extension (~10 mm/yr). In the epithermal mineralization zone, high-flux structures extend upward from the feed zone and transmit fluids to shallow depths, analogous to a geothermal production well. The host stratigraphic interval is dominated by porous pyroclastic deposits and distributed flow can be widespread until the intergranular permeability is reduced by hydrothermal alteration or where dense, low-porosity, high-tensile strength rocks exist. Distributed fluid-flow accounts for large volumes of hydrothermal alteration extending 10 to >100 km3 that encloses geothermal reservoirs and high-flux fluid conduits. Fracture-dominated flow becomes important with decreasing porosity induced by hydrothermal alteration. In the discharge zone, the reduction in confining pressure, combined with mineral deposition and alteration, hydrothermal eruptions, and interplay of hot and cold waters create complex, but strongly localized flow paths that feed hot springs.

The permeability structure conducive to epithermal vein formation is analogous to a geothermal well: short in horizontal dimension (10s–100s m) but long in vertical dimension (>1,500 m) and possibly pipelike in shape. Episodic high-flux occurs over time scales of tens to thousands of years to accumulate sufficient amounts of gold and silver to form orebodies. During these episodes when faults and fractures are dilated, development of an upward-expanding column of boiling fluid promotes rapid ascent and high mass flow but also promotes silica and calcite precipitation, which can quickly reduce hydrothermal flow. Seismic activity and/or dike intrusion create and reactivate these high-flux pathways through extension and extensional shearing, caused by low differential stresses. The Taupo Volcanic Zone is highly prospective for epithermal-style mineralization, but the predominance of weak porous host rocks at shallow depths is prone to disseminated-style mineralization (e.g., Ohakuri). Structurally controlled mineralization forms in volcanic rocks where they have been embrittled by silicification through seismicity and fault displacement, caldera-forming eruptions, and dike intrusion.

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