EGL Ref: 9049 23 June 2022 Page 6 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. 3.0 STUDY AREA GEOLOGY 3.1. Waihi to WUG geological setting Brathwaite and Christie (Ref. 24) interpret the geology of the Waihi Basin as part of the Coromandel Volcanic Zone, a sub-aerial 8-million-year-old (late Miocene) to 1.5 million-year-old (early Pleistocene) andesite-dacite-rhyolite sequence that forms the Coromandel-Kaimai ranges. The basement rock (Manaia Hill Group sandstone, siltstone and conglomerate, Ref. 24) is assumed to be at considerable depth (~1 km). The Waihi Basin is a paleo-caldera (ancient large volcanic centre, like Lake Taupo) within the Coromandel Volcanic zone (Ref. 25) and the Coromandel Peninsula is located within an area of extensional tectonic faulting between the Taupo Volcanic Zone and the Hauraki Rift (i.e. Hauraki Plains and Firth of Thames). Faulting in the Waihi area trends approximately northeast-southwest. The oldest volcanic formations outcropping in the Waihi area are andesites and dacites of the Late Miocene Waiwawa Subgroup of the Coromandel Group and includes the Waipupu Formation andesites and dacites which are 7.9-6.3 million years old (Ref. 24). These andesites outcrop at Gladstone, Union, and Martha Hill, and hills further to the northwest. To the north of Waihi the Whitiroa andesite of the Waiwawa Subgroup comes in over the Waiwawa Formation andesite and dacites. The Whitiroa andesites are mapped out to the WUG site (Ref. 24). The host rhyolites flows and pyroclasitc deposits at the WUG site are assigned to the Maratoto Rhyolite of the Coroglen Subgroup of the Whitianga Group (Ref. 24, 27, 29). The rhyolites form a dome complex that is partially overlain by rhyolitic tuffs of the Edmonds Formation (Ref. 24, 30, 31). The rhyolites at WUG are interpreted to overlie the older andesites of the Waipupu Formation or Whitiroa andesite (Ref. 27, 31). The older andesites and rhyolites are overlain by the Whakamoehau andesite (Ref. 31). Dating (K-Ar) indicates a geological erosional time break of about 1 Ma (million years) between the andesites and dacites of the Waiwawa Subgroup and the eruption of andesites and dacites of the Kaimai Subgroup, which contains dacites belonging to the Uretara Formation which are 5.6-4.3 million years old. The dacites (Black Hill Dacite) of the Uretara Formation come in over the Waiwawa Formation andesites to the east of Waihi along the Ohinemuri River passing the TSF Development Site. Of similar age to the Kaimai Subgroup are rhyolites of the Minden Rhyolite Subgroup within the Whitianga Group, which includes domes of Homunga Rhyolite which is 5.5-5.2 million years old and forms the hills of Ruahorehore Dome on which the existing TSFs are predominantly against. The Waihi Basin caldera is infilled with Pliocene to early Pleistocene lacustrine (lake) sediments and ignimbrites of the Whitianga Group. At the base are lacustrine sediments of the Romanga Formation (4.5-3.0 million years old) part of the Coroglen Subgroup. Lacustrine sediments are also found around the current crest of the Martha Open Pit and in isolated locations at the TSF Development Site.
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