Structural setting and paragenesis of the mineralised veins in the Waipori goldfield, southeast Otago, New Zealand

Abstract

The Waipori goldfield was discovered in 1861 on the southeast side of the Otago Schist belt in the area surrounding Lake Mahinerangi. Accurate historical records are not available but it is estimated the goldfield produced approximately 6000 oz of gold at 0.5 oz per ton. The general area is currently being explored for orogenic style gold deposits. The historic workings and vein systems are hosted in lower greenschist facies, textural zone TZIII, Caples Terrane schist. Mineralised veins are hosted in a variety of well defined, high angle (45o-90o), east-west, northsouth and northwest-southeast striking faults that cut the low-lying (8o-48o) metamorphic schist foliation at high angles. A conjugate set of faults, consisting of large regional faults and small discontinuous fault hosted quartz veins, occur across Waipori, striking north-south and east-west. Orientations of pre-existing post-metamorphic joints may control the orientations of a few normal faults across Waipori. Historically mined veins typically consist of variably mineralised white quartz veins and silicified breccias. Fault drag and warping of the schist foliation adjacent to veins indicates a normal sense of fault displacement. Quartz veins consist of euhedral prismatic syntaxial comb structure quartz that is commonly zoned and has grown approximately perpendicular to vein margins. The quartz veins contain pyrite, arsenopyrite and locally visible free gold grains. SEMEDS analysis has identified micro-particulate gold grains in arsenopyrite as well as minor amounts of scheelite, chalcopyrite, galena and silver. Stibnite and cinnabar are also observed in quartz and calcite veins at some localities. Gold was likely transported in a fluid solution attached to ligands as bisulphide complexes such as AuHS° or Au(HS)2-. Fluid temperatures are estimated to be <300°C. The gold is composed of a gold ± silver ± mercury alloy. Gold compositions form two populations at Cox’s, North O.P.Q., Nuggety Gully, and Waitahuna Heights lodes. The two populations likely represent measurements from the core and the edges/rims of gold grains or two separate types of gold; primary hypogene and secondary supergene. Paragenesis of vein systems involves two main stages. Steeply-dipping fault zones were infilled with gold ± stibnite ± scheelite ± cinnabar-bearing quartz and/or calcite veins in the first stage. This was then followed by fault reactivation in some lodes, producing fault gouge and fractures that are typically infilled with calcite. Later oxidation of sulphides such as arsenopyrite and pyrite has liberated refractory gold and some of this gold may have been remobilised and/or grown in situ to form supergene gold grains in the near surface environment. Steep normal faulting, prismatic quartz, open space filling textures, and minerals that are usually associated with shallow emplacement, cinnabar and stibnite, suggest the veins were emplaced at relatively shallow levels (<6 km), in an extensional setting, similar to other Otago deposits such as Nenthorn, Oturehua and Barewood. The Waipori deposits are correlated with a pulse of mineralisation that is thought to have occurred during a period of extension in the middle Cretaceous.