Structure and fluid migration in a late Cenozoic duplex system forming the Main Divide in the central Southern Alps, New Zealand

Abstract

The Alpine Schist immediately west of the Main Divide of the Southern Alps is a west‐dipping duplex system consisting of an imbricated stack of rock slabs, each c. 250–1000 m thick. The imbricated stack has low grade, little deformed pumpellyite‐actinolite facies semischists at the base, overlain by progressively higher grade and more deformed schists. The structurally highest slab mapped consists of multiply‐deformed biotite zone schist. The duplex lies on the hanging wall of the northwest‐dipping Main Divide Fault Zone, which separates semischist from structurally underlying greywacke. Rock slabs are internally disrupted by faults subparallel to layering, and consist of lozenge‐shaped blocks of 10–100 m. Fault zones separating rock slabs consist of <50 cm thick zones of weakly lithified cataclasite and soft gouge. Slickensides on fault surfaces plunge northwest. The duplex formed during late Cenozoic rise of the Southern Alps. Four generations of postmetamorphic veins cut rock slabs of the duplex system. The earliest veins contain quartz, chlorite, calcite, and sulphides and lie subparallel to foliation in the higher grade slabs only. A distinctive set of open‐space‐filling fissure veins in a lower greenschist facies slab contains euhedral quartz, adularia, bladed calcite, and chlorite. The whole duplex system is cut by steeply dipping shears and fractures containing quartz and ankerite infilling and breccia cementation. Late stage iron oxy‐hydroxide‐coated fractures, generally steeply dipping, cut all rock slabs. Pyrite and, locally, arsenopyrite accompanies quartz and ankerite veins in extensional sites of steeply dipping late stage fractures, and gold contents up to 850 ppb occur. This gold mineralisation occurred within c. 1 km of the surface and had epithermal affinities. Hydrothermal fluids mineralising the various fractures range in temperature between 350°C and near‐ambient temperatures. The quartz‐adularia‐calcite veins formed at 0.5–2.5 km depth before duplex formation, and later veins formed at shallower levels after duplex formation. Vein carbonates have δ 18O(smow) between 11.4 and 26.1%o, and δ13C(pdb) between ‐0.6 and ‐11.4%o. Calculated isotopic ratios for the mineralising fluids show little evidence for meteoric origin. The fluid may be meteoric water which has undergone isotopic exchange with host rocks, metamorphic water, or a mixture of these, and mineralisation occurred during rise of the fluids from depth.