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

摘要

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 pseudomorphous, adularia, a wide range in fluid inclusion TH (183-249 °C), and vapour-rich fluid inclusions. Estimated δ~(18)O_(water) values vary between -6.6 and +4.7 %o. Below elevations of c. 100 m asl, two isotopically different fluids are seen: a deep fluid with δ~(18)O_(water) ～0 %o and another around -3 per thousand (mixed deep fluid + heated groundwater). At elevations >100 m, δ~(18)O_(water) values trend toward values (-6per thousand) typical of local meteoric water. However, some samples trend toward heavier values (up to +4.7per thousand) due to boiling. The deep fluid contained either a magmatic (5-10 per thousand) component or δ~(18)O-shifted meteoric water.