The
Karangahake deposit was the second largest producer of gold-silver bullion in
the Hauraki Goldfield, New Zealand. New
mapping of a 2 3 km (6 km2) area demonstrates that hydrothermal
alteration surrounds productive veins. Wallrocks proximal to veins are intensely
altered and alteration decreases toward the margin of the system.

At
Karangahake, distinct zonation of adularia, calcite, epidote, illite and
illite-smectite surrounds gold-bearing lodes with unaltered rocks containing
magnetite and plagioclase occurring on the margins. Adularia, illite,
illite-smectite, and calcite zonation defines hydrothermal alteration extending
2 km to the east of productive lodes. Epidote and adularia zonation occurs
proximal to veins. The adularia zone compares well to potassium/thorium studies
conducted in the area. Illite zonation is well developed to the east of the
veins grading into interstratified illite-smectite containing 5 to 40 per cent
smectite 1.5 km from the Maria Lode. Calcite is restricted to the east of the
study area occurring in 18 per cent off surface samples. Less altered rocks
containing plagioclase and magnetite occur distal to veins. The magnetite
occurrence compares well to aeromagnetic studies conducted in the area.

Adularia and coexisting
adularia and albite indicate the host rocks were highly permeable. Epidote,
illite, illite-smectite and previous fluid inclusion evidence implies
temperatures graded from >240C in the core of the system to approximately
150C on the periphery. The alteration association of quartz, adularia, calcite,
pyrite, chlorite, illite and illite-smectite reflects a near-neutral to weakly
alkaline water. Calcite occurrence and the absence of zeolite imply the fluid
contained significant dissolved CO2.