STRUCTURAL CONTROL OF GOLD-SCHEELITE MINERALIZATION IN A MAJOR NORMAL-FAULT SYSTEM, BAREWOOD, EASTERN OTAGO, NEW-ZEALAND

Gold and scheelite bearing quartz veins in Otago Schist at Barewood, e astern Otago, are located in a regional scale zone of faults, which is traceable for at least 20 km along strike. Individual mineralised fau lts in the zone can be mapped up to 10 km along their northwest strike . The faults have been normal faults since their inception. Early move ment involved warping of adjacent schistosity up to 5 m from the fault s, and shearing of the schist in the faults, which dip about 50-degree s northeast. Later movement was entirely brittle, and was characterise d by some shears and a set of steeply dipping (up to 80-degrees northe ast and southwest) conjugate fractures. Minor late-stage movement occu rred along moderately northeast dipping shears. Slickensides suggest t hat later stages of movement involved significant strike-slip motion a s well as a normal component. Two stages of quartz veins occur in many localities and both quartz vein types contain gold, scheelite, and su lphides. The early quartz veins are massive white quartz, which fills the fault zones. The later stage of quartz crosscuts early quartz or f ills brittle fractures in adjacent variably sheared schist. Fluid incl usions in the early quartz have a density of c. 0.90 g/cm3 and a const ant composition of about 1.6 wt% NaCl (equivalent). Later quartz fluid inclusions have densities between 0.82 and 0.90 g/cm3 and variable co mpositions between 0.5 and 2.8 wt% NaCl (equivalent). Mineralisation o ccurred during Cretaceous regional extension, and postdates the Hyde-M acraes Shear Zone, which is a nearby major late-metamorphic mineralise d thrust system.