Hydrothermal alteration within the alpine fault zone

ABSTRACT Hydrothermal alteration, hosted in brittle structures, has been documented along the length of the Alpine Fault Zone. The along-strike profile has been divided into two geographical groups based on differing fault zone character. The central section, Havelock Creek to the Whataroa River, and the northern section, Whataroa River to the Taramakau River, host different late-stage hydrothermal assemblages. Alteration occurs as veins, cemented breccias, and partially or wholly recrystallised fault rocks throughout the -1 km wide fault zone.Hydrothermal alteration, hosted in brittle structures, has been documented along the length of the Alpine Fault Zone. The along-strike profile has been divided into two geographical groups based on differing fault zone character. The central section, Havelock Creek to the Whataroa River, and the northern section, Whataroa River to the Taramakau River, host different late-stage hydrothermal assemblages. Alteration occurs as veins, cemented breccias, and partially or wholly recrystallised fault rocks throughout the -1 km wide fault zone. Geochemical investigations suggest that gouges, cataclasites, and mylonites within the hangingwall of the Alpine Fault Zone have been derived from a metabasite-poor, intermediate greywacke protolith of Torlesse affinity. Western Province derived fault rocks are not confirmed in the hangingwall. Cataclasites and gouges are retrogressed, hydrated equivalents of adjacent schist-derived amphibolite (upper greenschist) facies mylonite. CaO, MgO, Fe20 3\ and SrO are mobile while Ti02, Al20 3, and K20 are immobile within the Alpine Fault Zone. Most base metals show no distinctive enrichment or depletion trends. Cu and As occur in anomalously high concentrations at Dickson River and tributaries of Wainihinihi River, respectively. o13C and 00 results for vein and fault gouge calcites indicate a derivation from a meteoric fluid with a minor rock exchanged component (o C -7.0 to l.6%0; 00 10.4 to 27.4%0; n=lO). The analysed ankerite veins and breccias were derived from a mixed, meteoric rock-exchanged fluid (8C -7.6 to 3.4%0; 00 12.1 to 25.2%0; n=21). Along-strike variation in isotope signatures is negligible. Retrogression within cataclasites and fault gouges is pervasive. Green mylonites have been thoroughly retrogressed under semi-ductile conditions while localised retrogression has occurred proximal to fault/ fracture systems within the Alpine Fault Zone. Ankerite is the dominant alteration phase in the northern section while calcite is the major alteration phase in the central section. Alteration volume is greater in the northern section where three hydrothermal breccias and numerous vein networks have been reported. No hydrothermal breccias and minimal vein networks are reported from the central section of the Alpine Fault Zone. Throughout the entire strike length of the fault zone, increases in metallic mineralisation correspond to increases in dilatancy and fluid-rock ratios (i.e. cemented breccias contain highest amounts of metals). Upper greenschist facies rocks, particularly chlorite-bearing, argillaceous-type mylonites, appear to be the best sources of ankeritic carbonate. Lowered geothermal gradients may also enhance ankerite stability and promote ferromagnesian mineralisation to the north. Strain and seismicity are closely linked to hydrothermal mineralisation. Local fault-fracture networks, especially high-angle minor faults, have acted as permeable flow networks channelising fluids and providing favourable mineralising sites. Gouges and cataclasites have acted as fluid distributors and poor mineralising sites. Active and passive porosity and permeability have been assessed for the Alpine Fault Zone. Passive estimates of porosity from several outcrops and specimens have permitted the extrapolation of passive and active porosity and permeability at depth during and shortly after seismic events. Using this approach, brecciated and fractured mylonite are found to be the most permeable mediums during passive (interseismic) periods. During active (coand post-seismic) intervals, fluids are believed to be discharged from fluid sinks (cataclasites, gouges) and focused onto backbone networks (i.e. minor faults, large fractures) within fractured mylonites. Brittle strain distribution is highest to the north, possibly associated with the crustal-scale Alpine-Hope Fault junction. This increased fault/fracture distribution correlates with frequent shallow earthquakes and elevated levels of hydrothermal alteration. The Wainihinihi River area, lying near the fault junction, contains anomalously high arsenic which is thought to represent a deeper fluid source. It is suggested that enhanced shallow seismicity has created a crustal plumbing network which taps deeper, metal-rich metamorphically-derived fluids. The nearby Wilberforce Valley (Main Divide) is characterised by similar arsenic, isotope and seismicity signatures suggesting that the hydrothermal systems may be regionally related.