Tectonic hydrothermal gold mineralisation in the outboard zone of the Southern Alps, New Zealand

Abstract

Gold-bearing veins cut a belt of low-grade (pumpellyite-actinolite/greenschist facies) schist in the Ben Ohau Range to the east of the Main Divide, in the outboard zone of the Southern Alps continental collisional zone, New Zealand. The schist has been exposed along the currently active Ostler Fault system, which has had c. 5 km of reverse motion since the Pliocene. The veins consist of quartz, ankerite, calcite, chlorite, and pyrite, with minor chalcopyrite and galena. Hydrothermal chlorite contains about 490 ppm Zn, and the gold contains 3-5 wt% Ag. Hydrothermal alteration of host rock is minor apart from Sr enrichment (up to four times background). Fluid inclusions in quartz are aqueous with minor dissolved CO 2 and salts (<4 wt% NaCl equivalent), and homogenise at 236-270°C. The veins formed at 300 ± 20°C and 1000 ± 800 bars fluid pressure, probably under a hydrostatic fluid pressure regime. Oxygen and carbon isotopic data (δ 18 O = +10 to +14; δ 13 C = -6 to -10%o) are similar to data from economic metamorphogenic Au deposits of the nearby Otago Schist, but minor meteoric incursion may have occurred. Isotopic data are also similar to veins formed in the inboard zone of the Southern Alps orogen. The Ben Ohau veins demonstrate that gold can be concentrated in low-grade schists distant from the most active part of the hydrothermal system driven by continental collision.

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

“…Gold mineralisation of presumed Cenozoic origin is widespread in the central Southern Alps but is volumetrically trivial (Craw et al 1987;Johnstone et al 1990;Smith et al 1996). In the present study, anomalous gold concentrations up to 850 ppb occur associated with the quartz-ankerite generation of veins.…”

“…The associated iron oxyhydroxide veins may represent deposition in subsurface fractures of the type of material currently being deposited at the surface at the Copland hot springs (Barnes et al 1978). This style of gold mineralisation is different from the structurally earlier mesothermal gold veins of the Callery valley (Table 2; Craw et al 1987) and Ben Ohau Range east of the Main Divide (Smith et al 1996), which are inferred to have formed at >5 km depth. The occurrence of both mesothermal and epithermal systems as separate parts of a tectonically driven hydrothermal system in mountain belts has been described and predicted by Nesbitt et al (1986).…”

“…The fluids which deposited the later veins at shallow levels presumably had some meteoric component, but even these fluids have undergone some isotopic exchange with the host rock (cf. Jenkin et al 1994) and/or mixing with rising metamorphic fluid (Smith et al 1996). Carbon isotopes also reflect this partial exchange or fluid mixing (Fig.…”

Gold mineralisation near the Main Divide, upper Wilberforce valley, Southern Alps, New Zealand

“…Quartz is relatively rare in many late Cenozoic gold-bearing veins in the Southern Alps, especially the veins in low-grade hosts (Cox et al 1997;Templeton et al 1999), whereas quartz dominates Otago Schist veins (Williams 1974). However, late Cenozoic goldbearing quartz veins do occur near the Main Divide (Smith et al 1996). There is strong evidence in the upper Wilberforce valley for late Cenozoic hydrothermal quartz in extensional sites (Fig.…”

“…Ankerite, which is common in the Wilberforce veins, is characteristic of most Cenozoic veins in the Mt Cook area and northwest Otago Schist, but is rare in Mesozoic veins in Otago (Craw 1992;Smith et al 1996;Cox et al 1997;Templeton et al 1999). Quartz is relatively rare in many late Cenozoic gold-bearing veins in the Southern Alps, especially the veins in low-grade hosts (Cox et al 1997;Templeton et al 1999), whereas quartz dominates Otago Schist veins (Williams 1974).…”