Structural control of gold‐scheelite mineralisation in a major normal fault system, Barewood, eastern Otago, New Zealand

Abstract

Abstract Gold and scheelite bearing quartz veins in Otago Schist at Barewood, eastern Otago, are located in a regional scale zone of faults, which is traceable for at least 20 km along strike. Individual mineralised faults in the zone can be mapped up to 10 km along their northwest strike. The faults have been normal faults since their inception. Early movement involved warping of adjacent schistosity up to 5 m from the faults, and shearing of the schist in the faults, which dip about 50° northeast. Later movement was entirely brittle, and was characterised by some shears and a set of steeply dipping (up to 80° northeast and southwest) conjugate fractures. Minor late‐stage movement occurred along moderately northeast dipping shears. Slickensides suggest that later stages of movement involved significant strike‐slip motion as well as a normal component. Two stages of quartz veins occur in many localities and both quartz vein types contain gold, scheelite, and sulphides. The early quartz veins are massive white quartz, which fills the fault zones. The later stage of quartz crosscuts early quartz or fills brittle fractures in adjacent variably sheared schist. Fluid inclusions in the early quartz have a density of c. 0.90 g/cm3 and a constant composition of about 1.6 wt% NaCl (equivalent). Later quartz fluid inclusions have densities between 0.82 and 0.90 g/cm3 and variable compositions between 0.5 and 2.8 wt% NaCl (equivalent). Mineralisation occurred during Cretaceous regional extension, and postdates the Hyde‐Macraes Shear Zone, which is a nearby major late‐metamorphic mineralised thrust system.