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Exploration of the Wharekirauponga Epithermal Au-Ag Deposit,Hauraki GoldfieldA B Christie1, S D C Rabone2, R G Barker3 and R J Merchant4
ABSTRACT The Wharekirauponga prospect, located about 10 km north of Waihi in the Hauraki Goldfield, is a rhyolite-hosted,adularia-sericite, epithermal Au-Ag deposit, with a 40Ar-39Ar age of 6.3 Ma measured on vein adularia. Exploration between 1978 and 1993 consisted of aeromagnetic surveys, ground-based geological and hydrothermal alteration mapping,rock chip and soil geochemical surveys, ground magnetic surveys, CSAMT and DC resistivity surveys, and the drilling of 5505 m in 23 diamond drill holes. An approximately 3 km × 2 km area of low relief negative aeromagnetic anomalies, attributed to hydrothermal alteration, encompasses a c 500 × 250 m zone of +0.1 ppm Au in rock chips, in turn containinga c 350 × 50 m zone of mineralisation outcropping in a gorge of Wharikirauponga Stream. The Au-Ag occur predominantly as electrum in sheeted to stockwork quartz veins and disseminated in hydrothermally altered rocks.Maximum assays of drill core are 89 ppm Au (with 52 ppm Ag) and 72 ppm Ag (with 9.0 ppm Au), but the mineralisation is generally low grade. XRD analysis of more than 300 surface and drill core samples show that in the rhyolite, quartz+adularia alteration associated with the quartz veining and Au-Ag mineralisation, grades outward to quartz+ illite alteration,and to interlayered illite/smectite and kaolinite + smectite alteration assemblages in flanking rhyolitic tuffs. Inferred formation temperatures for these alteration assemblages are supported by fluid inclusion Th and Tm determined in vein quartz, which suggest fluid temperatures between 180 and 250°C, with apparent salinities up to 1.2 eq wt per cent NaCl. Fluid inclusions and oxygen isotopic analyses suggest boiling. Exploration to date has outlined a low-grade resource, but has so far not defined any upflow zones of the former geothermal system where higher Au grades may occur. The strike extent of the prospect has not been fully defined and it remains open to both the NE and SW where it passes beneath younger cover rocks.
Keywords: Wharekirauponga, Hauraki Goldfield, T12, T13, epithermal deposit, gold, silver, rhyolite, exploration.
CONCLUSIONS
WKP has had much effort expended in exploration and research, particularly in the application of XRD and fluid inclusion analyses from exploration and research viewpoints. The prospect exhibits a variety of favourable features to encourage exploration: it is a large mineralised system, which hosts a significant quartz vein stockwork zone within quartz-adularia altered host rhyolite; locally the veins carry high-grade mineralisation albeit over narrow intervals. The system is open in two directions and not well tested where it occurs in basal andesite host rocks, which are typically better host rocks for epithermal vein style mineralisation in the Hauraki Goldfield. The fluid inclusion and wall rock alteration data suggest that polyphasal boiling has occurred within the system; however, major upflow conduits are yet to be discovered. Although there isisotopic evidence of fluid mixing, specific mixing zones, also prime targets for gold mineralisation, have not been identified. The strongest mineralisation and vein development intersected to date is in the south of the gorge section (in DDHs 20 and 23). Low amplitude-long wavelength magnetic anomalies are extensivein this area, despite the presence of outcropping unaltered Whakamoehau Andesites. This suggests that Whakamoehau Andesite post-mineralisation cover here is thin and that alteration extends a considerable distance south of the gorge section. Significant exploration potential consequently exists in this direction beneath cover rocks (cf Golden Cross; Mauk andPurvis, this volume).
ACKNOWLEDGEMENTS Rabone, Barker and Merchant were at various times part of theteams of exploration geologists that worked on WKP. Research by Christie was funded by the Foundation of Research, Science and Technology under contracts to the GNS Mineral Resources Programme managed by Ian Graham. Julia Vodanovich and Carolyn Hume drafted the diagrams and Pat Browne reviewed the manuscript.
REFERENCES
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