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.

Late Cretaceous to Quaternary terrestrial sediments derived from fault scarps and actively growing folds in schist and greywacke basement are locally preserved in Otago and South Canterbury, New Zealand. Sediments are immature, sparsely carbonaceous, and generally poorly sorted and have undergone little oxidation during rapid erosion and deposition cycles. Mild oxidative alteration of sediment clasts in some horizons has resulted in pseudomorphous replacement of detrital biotite and metamorphic chlorite in greywacke clasts, and chlorite in schistose clasts, by a green, interlayered, ferrous iron-bearing smectitevermiculite mineral of variable composition. Smectitevermiculite compositions form a regular trend from low Si and Al, high Fe and Mg to moderate Si and Al, to low Fe and Mg. The Fe 2+ smectite-vermiculite also occurs as pools in the matrix and small veinlets, and as a cement intimately intergrown with fine-grained clastic matrix. The smectitevermiculite is a diagenetic product formed preferentially in sedimentary horizons which are reduced and moderately acidic (pH 3.5-4.5), neutral, or possibly alkaline.There is a close association between the Fe 2+ smectitevermiculite mineral and authigenic gold deposition; the •imectite-vermiculite is occasionally coated and impregnated with fine-grained (micrometre scale) gold. Gold precipitation probably occurs by electroless deposition from groundwaters that are undersaturated with respect to gold.

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