Shallow-level, late-stage gold mineralisation in Sawyers Creek, Shotover valley, northwest Otago, New Zealand

Abstract

A set of mainly west and northwest striking normal faults in Sawyers Creek, Shotover valley, crosscuts post-Oligocene Moonlight Generation structures. These faults have provided access for hydrothermal fluids to shallow crustal levels. Silicification and carbonate alteration of breccias and wall rock has occurred, to form reef zones up to 3 m wide. Gold is found with pyrite and arscnopyrite within the siliceous portions of the reef, especially at fault intersections. The hydrothermal fluid was water with minor dissolved CO₂ Mineralisation occurred at about 150-200°C at 2–3 km depth. Wall-rock interaction with the fluid caused replacement of chlorite by ankeritic carbonate. The composition of the ankerite is governed by the composition of the chlorite it replaced. Fluid reaction with chlorite caused the fluid to become progressively more alkaline which eventually resulted in precipitation of gold. Similar fluid activity, in similar structural settings, is found throughout northwest Otago.

Keywords:

• gold
• Shotover
• fluid inclusions
• isotopes
• structural analysis
• alteration
• ankerite