Deep
hydrothermal fluids adjacent to the Alpine Fault move through the
brittle-ductile transition to mix with convecting fluids from near surface
environments. These tectonically driven fluids evolve mainly from prograde
metamorphism at mid crustal depth (ten kilometres) within the ductile regime
which has been elevated due to advection and drag on the fault plane. Through
the onset of brittle failure and cataclasis within Alpine Fault mylonites, fluid
overpressuring results in hydrofracturing and localized implosion. Along the
length of the Alpine Fault, hydrothermal alteration varies on most levels
(Table 1). There is an
observable increase in Fe-Mg carbonate between the Waitaha and Taramakau Rivers (between Ross and Aurthur's Pass,
respectively). Field studies concentrate to the north east of the Arahura River, noted for historical mineral
wealth. Late meteoric fluids move through cataclasite, brecciated mylonite, and
other open fractures. Chloritisation is the
major retrogressive
phase. Dolomitic hydrofracture veins (Figure 1) within coherent mylonites
contain iron and copper sulphides. Notably, the enrichment in Fe-Mg carbonate
towards the Alpine-Hope Fault intersection seems to represent a region of
enhanced permeability possibly related to shallow seismicity, increased regional
faulting, footwall variation, strike-slip dip-slip distribution, and/or
topographical anomalies (ie saddles). Isotope studies identifying an 'Inboard
Fault Gouge Calcite Zone' correlate to values from the Main Divide region at the
headwaters of the Arahura
River (Browning Pass-Wilberforce Valley) (Becker et aI, 2000). These correlations are
supported by rare arsenic anomalies (avg l06 ppm n = 5, Figure 2) found in both
regions which suggest that fluids may be interconnected at
depth.