Structural geology and paragenesis of the Big River shear zone, Reefton goldfield, South Island, New Zealand

Rayner, Anthony James

The Reefton Goldfield produced over 67 tonnes of gold between 1870 and 1951 and was the largest gold producer on the West Coast of the South Island of New Zealand. Current Mining operations run by Oceana Gold Limited commenced in 2007 at the Globe-Progress deposit. Mining currently operates in four open cut pits; the Globe-Progress, General Gordon, Empress and Supreme and is expected to produce 455,000 ounces of gold over 7 years.
The Big River deposit was mined historically between 1882 and 1942, producing over 130,000 ounces of gold at an average grade of 34 g/t. Gold was recovered from a number of quartz rich reefs which were successfully worked with some grades reported as high as 63.7 g/t. The ore body was not exhausted however and although Big River was one of the largest producers in the goldfield there is no good description on the style and distribution of mineralisation. The Big River deposit is hosted within the Big River Shear Zone (BRSZ), which is hosted on the limb of the Big River anticline.
Early quartz veins, deposited during/ soon after peak metamorphism (~400°C) were deformed in association with the regional folding of the goldfield, which produced upright N-NE trending folds (F2). The quartz veins accommodated deformation through dynamic recrystallisation at temperatures of ~355°C, with a flow stress of 83.9 ± 25 Mpa.
Continued deformation is marked by the transition from ductile-brittle deformation of quartz veins below temperatures of 280°C. Brecciation of quartz veins is associated with the deposition of acicular arsenopyrite seams, pyrite veins and disseminated pyrite + arsenopyrite ± gold.
Further decrease in deformation temperature resulted in the brecciation of pyrite veins and deposition of galena and ankerite veins with associated sericite wallrock alteration. Pyrite vein deformation also resulted in symmetric micro-folding of veins associated with a pressure solution crenulation cleavage formation (S4) with axial plane-parallel white calcite veins. The calcite veins were deformed at temperatures between 250°C and 200°C, with a differential stress of 239 Mpa.
Chalcopyrite-sphalerite-pyrite bearing calcite veins were deformed at temperatures below 180°C, with a differential stress of 196 Mpa. Late stage gold + arsenopyrite + stibnite in-filled at shallow crustal levels associated with the exhumation of the BRSZ.
Fine–grained disseminated gold is associated with acicular arsenopyrite and pyrite hosted within breccia zones. Visible gold is inter-grown with arsenopyrite with associated stibnite which infill fractures. Cu-Pb-Zn ± Mo + Ag anomalies indicate a magmatic fluid component at some stage during the evolution of the BRSZ (371Ma?)
Early regional compression of Big River produced shear zone hosted mesothermal gold deposits with associated pyrite and arsenopyrite, hosted within upright to steeply inclined folds. As regional deformation continued, early structures were reactivated and new structures formed at new orientations discordant to previous structures. It is possible an influx of magmatic hydrothermal fluids associated with the rapid intrusion of the Karamea suite produced a relatively late stage of epithermal-style gold–stibnite ± Mo mineralisation. Late stage mineralisation (arsenopyrite + stibnite + gold) was introduced at shallow levels during Cretaceous exhumation.

165 pages : illustrations, maps ; 30 cm. CD-ROM 4 3/4in.

2011Rayner

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