The
Rahu and Ascot epithermal Au-Ag prospects are two km north of the
Karangahake deposit in the Hauraki Goldfield, and all occur in an approximately
6 km2 north-south elongated magnetic quiet' zone. The prospects are hosted by
andesitic flows, lithic-crystal tuffs and local sedimentary rocks; local silica
sinter occurs at Ascot. These host rocks are
intensely altered by quartz, adularia, albite, chlorite, pyrite, illite,
interstratified illite-smectite and calcite. Several minerals show zoned
distributions, such as widespread adularia that overlies more localised and
deeper albite. Illite is the only dioctahedral phyllosilicate in several drill
holes, but elsewhere it forms a shallow carapace underlain by interstratified
illite-smectite with 50 to 90 percent illite. Veins are common with quartz
dominating near the surface and calcite becoming more abundant at depth. Local
laumontite-calcite veins also occur. Pseudosecondary and secondary fluid
inclusions in quartz and calcite at Rahu and Ascot have homogenisation
temperatures (Th) that range from 162 to 268C and 128 to 243C, respectively.
Most inclusions have trapped dilute fluids with salinities of less than 1.9
weight percent NaCl equivalent, but 20 percent of the inclusions have salinities
of 2.0 to 10.5 weight percent NaCl equivalent. Calculated paleowater table
elevations suggest that the water table at Rahu occurred at 440 m above sea
level (asl). At Ascot, silica sinter formed
from discharging geothermal fluids (100C) at the paleosurface at ~135 m asl.
However the sinter also contains comb quartz with pseudosecondary inclusions
that have a Th average of 229C, corresponding to a paleowater table at ~430 m
asl during inclusion entrapment. Taken together, these results suggest that the
Ascot sinter was buried by approximately 300 m of cover material during a major
volcanic or tectonic event, and that subsequent hydrothermal fluids formed veins
and alteration at Rahu and deposited comb quartz at Ascot.