Structure and mineralisation of gold-bearing quartz veins at Oturehua, Central Otago, New Zealand

Mackie, Claire.

North Rough Ridge is a northeast trending Quaternary asymmetric antiform which folds the regional unconformity of the Otago Schist and overlying sediments. The schist foliation of the ridge is a dome like structure, formed by several postmetamorphic generations of deformation. Historically mined gold bearing veins occurring near Oturehua lie at the northern end of the ridge. The host rock is lower greenschist facies textural zone m schist, which has interlayered laminated and massive lithologies. Approximately 50 veins occur in the vein swarm, with northwest strike, commonly at 3200 but with a range of orientations 275 - 324°. The veins formed in extensional sites along - 1 m wide normal fault zones and are steeply dipping, mainly to the northeast. Individual vein fault zones can be traced from- 50- 650 m, with variable thickness along strike. Vein thicknesses typically range from a few mm- 20 cm over 1-5 m along strike, with the greatest thickness seen- 1 m where fault zones steepen. Early Cretaceous mm - cm scale kink folds with northwest trending axes, subparallel to vein strike, occur in adjacent schist and are cross-cut by the mineralised veins. Quartz veins preferentially formed in weak kink folded schist. The quartz in the veins is dominated by intergrown quartz crystals and schist fragments, and some cavities with prismatic quartz crystals occur locally. Euhedral arsenopyrite occurs in quartz veins and impregnates immediate host rock up to 10 cm from veins. Arsenopyrite rich fault gouge of around 1 cm thick envelopes some veins. Veins and minor replacement of schist by ankerite and local silicification occur in host rocks, particularly in laminated lithologies. Gold occurs in sulphides and as < 0.5 mm free grains in the quartz veins. Gold analyses up to 75 ppm have been obtained from spot samples, and arsenopyrite grains have gold contents up to 1031 ppm. A broad alteration zone tens of metres wide comprising pyrite and ankerite surrounds the mineralised zones. Mineralisation occurred at - 293 - 350 °C. The mineralised structures at Oturehua appear to be similar to well-defined Otago quartz vein swarms such as seen at Nenthorn and Barewood, and have few similarities with the shallowdipping mineralised shear zone at Macraes mine.

v, 122, [5] leaves, : col. ill., maps ; 30 cm

2008Mackie