Structural Architecture and Evolution of the Wharekirauponga Low-Sulfidation Epithermal Au-Ag Deposit, Hauraki Goldfield, New Zealand

Abstract

The Wharekirauponga (WKP) epithermal gold-silver deposit is situated in the southeastern Coromandel Peninsula, within the Hauraki goldfield. Historic exploration works at WKP identified hydrothermally-altered rhyolite flows and rhyolitic pyroclastic rock units cut by northerly-trending gold and silver-bearing quartz veins. Recent exploration drilling complemented the initial findings with the discovery of northeast-trending, northwest-dipping veins containing significant gold and silver mineralisation. This research aims to reconstruct the spatial and temporal evolution of the structures at WKP utilising both surface and sub-surface structural data, and correlating the findings with the regional scale paleo-structural regime. The interpretations of stereographic data, geopetal indicators and kinematic criteria analyses were supplemented with geochemical and geophysical studies to identify the critical controls on the development of ore shoots in the deposit. The northeast-trending Eastern Graben (EG)Vein and Teawaotemutu Stream (T-Stream) Vein occur as northwest-dipping quartz veins. The veins bound the northerly-trending, steep to subvertical veins of the quartz stockwork zone. Gold and silver are associated with electrum in the veins, with the thicker EG Vein outlining a preference for elevated (>1 ppm) gold values. Kinematic evidence indicates a sinistral oblique-slip movement of the fractures which subsequently hosted the EG Vein and T-Stream Vein. The present geometry of the northeasttrending and northerly-trending veins denotes a fracture mesh governed by an east-westoriented extensional mode. This fracture mesh likely manifests the reactivation of deep-seated northwest-trending basement lineaments identified from aeromagnetic data interpretations. The complex fracture network characterising the fracture mesh provided highly permeable pathways necessary for precious metal precipitation. Geopetal features such as vein sediments record a gentle, post-mineralisation tilting of the deposit towards the east-southeast. These features highlight the original formation of the veins as steep, sub-vertical to west-northwestdipping structures. The intersections between the northeast-trending EG Vein and T-Stream Vein and the northerly-trending quartz stockwork zone veins are hosts to steep, northnorthwest-plunging ore shoots. The vein deflections along the segments of the EG Vein are also potential hosts to similar plunging ore shoots. The deeper levels beneath the vein intersection zones are prime exploration targets for extended high-grade gold and silver mineralisation within the deposit.