Structural setting of the Globe-Progress and Blackwater gold mines, Reefton goldfield, New Zealand

Abstract

Globe-Progress and Blackwater mines, the two largest mines in the Reefton goldfield of North Westland, have been placed into a regional structural context dominated by upright, tight folding of the host-rock Ordovician Greenland Group turbidites. The Greenland Group lacks distinctive lithostratigraphic marker horizons or bio-stratigraphic control, but mapping out areas of common structural facing from the determination of bedding-cleavage vergence along two transects has revealed a series of north-trending, generally shallow plunging folds. Undisrupted fold hinge closure is rarely seen, but the bedding-cleavage vergence boundaries are inferred to be fold hinge regions or in some instances post-folding faults. Vergence boundaries are often accompanied by changes in sedimentological younging. The technique defines the location of fold hinges, faults and, potentially, because of the structural control on mineralisation, gold-bearing quartz veins. The Globe-Progress gold-bearing quartz vein system occurs in a zone of relatively tight and shorter wavelength folds within a wider framework of more open, longer wavelength, east-verging folds. On the western flanks of Globe Hill, the Globe-Progress shear separates east-verging folded strata in the hanging wall from more symmetrically folded, commonly overturned strata in the footwall. The shear curves from an easterly strike to a southerly strike and steepens from 30–40° dips at depth to near-surface dips of 70–80°. Minor structures in the hanging wall indicate west over east-directed movement of the Globe-Progress shear. The Blackwater quartz vein system occurs parallel to steeply west-dipping, west-facing strata in a section of tight upright folds. The folds occur within the hinge area of a regional synclinal structure, which in turn is one of a series of 4–5 km long wavelength folds. © 2000 Taylor & Francis Group, LLC.