New Zealand Journal of Geology and Geophysics abstracts

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D. W. SMITH
D. CRAW
P. O. KOONS

Geology Department
University of Otago
P.O. Box 56
Dunedin, 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 CO2 and salts (<4 wt% NaCl equivalent), and homogenise at 236-270deg.C. The veins formed at 300 +/- 20deg.C and 1000 +/- 800 bars fluid pressure, probably under a hydrostatic fluid pressure regime. Oxygen and carbon isotopic data (d¹⁸O = +10 to +14; d¹³C = -6 to -10[[perthousand]]) 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.

Keywords  gold; hydrothermal; quartz veins; Ben Ohau Range; Glentanner; fluid inclusions; isotopes; tectonics

New Zealand Journal of Geology and Geophysics, 1996, Vol. 39: 201-209

0028-8306/96/3902-0201 $2.50/0 (c) The Royal Society of New Zealand 1996

PDF file of entire paper: medium quality (2787K); (scanned from paper original: notes about this process)

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