Polymetallic mineralised veins in ferroan/A-type Cretaceous leucogranite, Stewart Island, New Zealand

ABSTRACT

The 140 ± 1 Ma hypersolvus, ferroan, weakly peralkaline to weakly peraluminous North Red Head leucogranite in northwest Stewart Island is cut by quartz-pyrite-rich veins that contain a wide variety of Mo, Ag, Te, Bi, Au, Co, Cu, Pb, Zn, REE, Nb, Y, Th, U, Zr, Ti, Be and F-bearing minerals. Patchy hematite-pyrite alteration locally overprints leucogranite in the vicinity of the mineralised veins. Individual veins are up to 5 m thick and 200+ m long. U–Pb dating and trace-element geochemistry indicate a direct link between leucogranite crystallisation and exsolution of the vein-forming hydrothermal fluid. Mineralised veins developed along transpressional faults within the leucogranite soon after emplacement. Incipiently mineralised quartz ± pyrite veins at Waituna Bay and the northern end of West Ruggedy Beach several kilometres from North Red Head are probably part of the same hydrothermal system as the veins at North Red Head. Metal and alteration assemblages at North Red Head most closely resemble those in rare hydrothermal systems associated with oxidised fluorine-rich A-type granites.

KEYWORDS:

• A-type granite
• aeschynite
• bismoclite
• Freshwater Fault System
• gold
• hematite
• molybdenite
• silver
• Stewart Island
• Waituna Bay
• West Ruggedy Beach

Acknowledgements

Whole-rock chemical analyses were supplied by D Walls at the University of Otago, C Fraser at CRL Energy Ltd, SGS laboratories in Townsville and Perth and ALS laboratories in Brisbane. B Pooley provided polished thin-sections and K Lilly assisted with the acquisition of electron microprobe analyses. Transport in the field was provided by Rakiura Helicopters and Stewart Island Flights. Capable and enthusiastic assistance in the field was provided by R Smillie and P Stenhouse. Reviews by C Hart and an anonymous reviewer are gratefully acknowledged.

Associate Editor: Dr Nick Mortimer.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Mineralised outcrops on the southern side of North Red Head were discovered in February 2002 during fieldwork undertaken as part of the GNS Science QMAP project. Altered and mineralised rocks at Waituna Bay and the bladed quartz veins at West Ruggedy Beach were first recognised during student fieldwork in 1986 that was supported by the Benson Memorial Fund. Funding for this research was provided by the Ministry for Business, Innovation and Employment and the University of Otago.