EGL Ref: 9215 23 June 2022 Page 8 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. 3.2.2. Regional Geology The Waihi operation is located on the eastern side of the Waihi Basin which comprises extrusive and intrusive volcanic rocks of various ages, and lacustrine (lake) deposits. These materials are mantled by tephras (volcanic airfall deposits) such as ignimbrites, tuff, volcanic ash, and colluvial and alluvial deposits. Brathwaite and Christie (Ref.5) 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 sedimentary or metamorphic basement rock is likely at considerable depth below the volcanic sequence. The Waihi Basin itself is a caldera (large volcanic centre) within the Coromandel Volcanic zone (Ref. 6, 7, and 8) as interpreted by Hayward (Ref. 2). The oldest volcanic formations in the Waihi area are andesites and dacites of the Late Miocene Waiwawa Subgroup of the Coromandel Group and includes the Waipupu Formation which are 7.9-6.3 million years old. 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. 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 Ruahorehore Dome on and against which the existing TSFs Storage 1A and 2 are predominantly located (Ref. 5). 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 have also been found around the Martha Open Pit. The overlying ignimbrites are grouped into the Ohinemuri Subgroup consisting of Corbett ignimbrites at 2.9 million years old and Owharoa ignimbrites (late pliocene) and Waikino ignimbrite at 1.5 million years old (early pleistocene). Eruptions of ash and pumice blanketed the area. Typical ash soils found in the area include the Waihi Ash series, the Hauparu Ash and the Rotoehu Ash (Ref. 9).
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