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Centre for Earth and Environmental Science Research

The volcanotectonic architecture of epithermal Au-Ag mineralisation at the Waihi Epithermal System, Coromandel Volcanic Zone, New Zealand.

Executive Summary
ResearchersPenelope Small, CEESR
Dr. Scott Bryan, CEESR
Prof. Andy Rankin, CEESR
Dr. Jamie Wilkinson, Imperial College, London
Mr. Lorrance Torckler, Newmont Waihi Gold
Funding Body/SourceNewmont Waihi Gold and Kingston University
Duration2007 - 2009
Project SummaryThe volatile and metal contents of specific magmas and potentially their mode of emplacement play important roles in the development of epithermal ore-forming systems. In addition, fault activity appears linked to magma emplacement and volcanism, such that the location and timing of epithermal mineralisation at Waihi and elsewhere in the Coromandel Volcanic Zone may be intimately related to discrete volcano-tectonic episodes such as graben inception or extension. This study is evaluating the volcano-tectonic architecture and investigating genetic relationships between the andesite-rhyolite volcanic rocks and epithermal Au mineralisation in the Waihi Epithermal System to constrain better, relationships between magmatism, structure and mineralisation. An important outcome of this work will be identifying key characteristics of the syn-mineralisation igneous suites to aid regional targeting.

Fig. 1 Aerial view of Waihi with current research focused at Favona. Image courtesy of Newmont.

Background

Most low-sulphidation epithermal gold deposits, including nearly 60 percent of the world's bonanza veins, are associated with bimodal (basalt-rhyolite) volcanic suites erupted in a broad spectrum of extensional tectonic settings, with the defining characteristics of the epithermal deposits related directly to the magmatic sources (Sillitoe & Hedenquist, 2003; Tomkins & Mavrogenes, 2003). Within metallogenic provinces such as the Hauraki gold field of the Coromandel Volcanic Zone, many igneous rocks are barren or metal-poor, whereas a few have produced mineralisation, and this variability may ultimately be source-controlled (Tomkins & Mavrogenes, 2003). Understanding magma processes from generation to emplacement, and the processes controlling the compositions of magmatic volatile phases (e.g., Halter et al., 2002) are key to understanding the transfer of metals to the volatile phase and hydrothermal system, and formation of economic mineralisation.

This project is focusing on the structure, and magmatic and fluid processes responsible for the localisation and development of zones of epithermal ore mineralization in the Waihi Epithermal System. Gold was discovered in the district in 1869, which has yielded >8.1M oz. from seven mining centres within a 15 km. radius of Waihi. The largest of these, the Martha Gold Mine at Waihi (Fig. 1; http://www.newmont.com/en/operations/australianz/waihigold/index.asp ), has produced >6.3M oz. of gold and has 1.13M oz. of gold in resources scheduled for mining over the next decade. Gold deposits in the Waihi Epithermal System are generally found in classical low sulphidation epithermal fissure vein systems developed in dilational structural environments within Miocene volcanic rocks. Individual veins within these deposits can attain 2 km strike length, 700 m vertical range and 30 m widths and comprise polyphasal crustiform/colloform banded quartz carrying precious metal mineralisation in the form of electrum and acanthite (Torckler, 1999).

This project builds on a 2 month research consultancy by Project Supervisor Scott Bryan in 2004 that provided a review of the Volcanic Setting of Waihi District epithermal Au mineralisation for Newmont Waihi Operations.

REFERENCES

Halter WE, Pettke T, Heinrich CA (2002) The origin of Cu/Au ratios in porphyry-type ore deposits. Science, 296: 1844-1846.

Sillitoe RH, Hedenquist JW (2003) Linkages between volcanotectonic settings, ore-fluid compositions, and epithermal precious metal deposits. In: Simmons SF, Graham I (eds) Volcanic, geothermal, and ore-forming fluids; rulers and witnesses of processes within the Earth. Special Publication (Society of Economic Geologists, 10, pp 315-343.

Tomkins AG, Mavrogenes JA (2003) Generation of metal-rich felsic magmas during crustal anatexis. Geology, 31: 765-768.

Torckler L (1999) Waihi District Mining & Exploration History, excluding current Normandy prospects. Unpublished Report, Normandy Pty Ltd.

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