May 2018 Project Martha – Historical & Archaeological Background 85 PHYSICAL ENVIRONMENT CONTEXT Waihi is part of a geographically defined district and it is the mineral-bearing geology of the area, containing the productive quartz reefs, which led to the discovery of the presence of gold by European settlers in 1878. This led to the huge enterprise of gold mining that gave birth to Waihi town and has sustained it to this day. The following geological information is summarised from Braithwaite and Christie 1996; McAra 1988; geological maps prepared by GNL and OGNZL; and, lecture notes given by A.H.V. Morgan (deceased), included in the Waihi Borough Council Diamond Jubilee Booklet 1902-1962. Geology The Waihi area is part of the Coromandel Volcanic Zone, a Miocene to Early Pleistocene andesite-dacite-rhyolite volcanic sequence which forms the Coromandel-Kaimai ranges. The surface geology of the Waihi area is relatively complex, with a distinct SW to NE division caused by the Waihi fault, which runs to the west of the town (Figure 67). The earliest andesite formations date to approximately 12 million years BP. It is these formations (mainly within the Waiwawa Subgroup), which host the quartz veins (or reefs) that are the reason for the mining activity in this area. Around Waihi the subgroup is represented by andesite and dacite lava flows and tuff breccias of the Waipupu Formation Subgroup (Figure 68). Overlying this reef-bearing quartz andesite and separated from it by an old land surface, is a younger series of andesites into which the veins do not extend. The wooded hills to the north and west of Waihi consist of these younger andesites (Omahine subgroup). Overlying this hard black younger andesite is a grey coloured rhyolite, which forms the hills east of Waihi and extends southwards to Mt Hikurangi (Coroglen Subgroup). The greater part of the Waihi Plains is covered by another rhyolite, grey to purple in colour and containing fragments of andesite and other rocks (Minden Rhyolite). Between the upstanding volcanic hills, sedimentary deposits of quaternary age overlie the most recent rhyolite formations (Onihemuri Subgroup) and are characterised by swampy low-lying ground. Sedimentary rocks are almost entirely absent within the Waihi area. In addition to the above there occur several outcrops of volcanic rocks of Late Miocene or early Pliocene age, the most conspicuous of which is Black Hill, which consists of a hornblende andesite (Kaimai Subgroup).
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