Abstract

Four different quartz vein systems occur within 600 km 2 in east Otago. The vein systems have formed within fault systems which crosscut regional schistosity. The Macraes system formed from almost pure water at about 3 kbars and 350 degrees C (data from fluid inclusions and arsenopyrite geothermometer). The Bonanza system formed from immiscible H 2 O-CO 2 -NaCl fluids at about 1,000 bars. Nenthorn formed under near-surface conditions from boiling low-salinity water, with some CO 2 present. Barewood most closely resembles Macraes but is thought to have formed at slightly shallower levels. The veins formed at different times during Mesozoic-Cenozoic uplift of the Otago Schist belt. C and O isotope data and the high CO 2 content of some fluids suggest that the vein-forming fluids were metamorphogenic. A component of downward-percolating meteoric water was probably involved in shallower vein formation. Vein formation is considered to be an inevitable consequence of devolatilization of uplifting metamorphic rocks, with focusing of fluids into available structural features. Similar fluid flow has occurred within the last 1 m.y. in the northwestern extension of the Otago Schist, where Alpine fault uplift has induced gold mineralization on joint surfaces.

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