Boiling and mixing of hydrothermal fluids in the Wharekirauponga rhyolite-hosted epithermal gold-silver deposit, Hauraki goldfield, New Zealand

Wharekirauponga is a low sulphidation, epithermal Au-Ag deposit hosted in a Late Miocene rhyolite dome complex. Electrum is present in sheeted to stockwork quartz veins and disseminated in the host rocks. Quartz + adularia alteration grades outward to quartz + illite, quartz + interlayered illite-smectite and quartz + kaolinite + smectite alteration zones. Boiling is indicated by crustiform and colloform banding, platy quartz pseudomor-phous, adularia, a wide range in fluid inclusion T-h (183-249 degreesC), and vapour-rich fluid inclusions. Estimated delta(18)O(water) values vary between -6.6 and +4.7 parts per thousand. Below elevations of c. 100 in asl, two isotopically different fluids are seen: a deep fluid with delta(18)O(water) -0 parts per thousand and another around -3 parts per thousand (mixed deep fluid + heated groundwater). At elevations >100 m, delta(18)O(water) values trend toward values (-6parts per thousand) typical of local meteoric water. However, some samples trend toward heavier values (up to +4.7 parts per thousand) due to boiling. The deep fluid contained either a magmatic (5-10 parts per thousand) component or delta(18)O-shifted meteoric water.