Brathwaite, R.L.; Christie, A.B. 1996 Geology of the Waihi area : part sheets T13 and U13. Scale 1:50 000. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences geological map 21 64 p. + 1 fold. map
Abstract: The Waihi map area includes part of the Coromandel Volcanic Zone, a Miocene to Early Pleistocene andesite-dacite-rhyolite, subaerial volcanic sequence which forms the Coromandel-Kaimai ranges. The ranges are flanked by the Hauraki Rift and the Tauranga Basin, both infilled by Pleistocene volcanogenic sediments of the Tauranga Group. The oldest volcanic rocks are correlated with the Waiwawa Subgroup of the Coromandel Group (Late Miocene). The lower part of the Coromandel Group and greywacke basement are not exposed. Waiwawa Subgroup is represented by andesite and dacite lava flows and tuff breccias of the Waipupu Formation (7.9-6.3 Ma), Kapukapu Andesite (7.8-7.0 Ma), Mangakara Dacite (6.9 Ma), Waitekauri Dacite, Whiritoa Andesite (6.7-5.6 Ma), Waiharakeke Dacite (~7.25 Ma); and dacitic ignimbrite, tuff, and siltstone of the Mataora Formation. Omahine Subgroup is represented in the north by Whakamoehau Andesite (6.7-6.6 Ma). Kaimai Subgroup andesites and dacites are most extensive in the south of the sheet. This subgroup is more variable in composition than the Waiwawa Subgroup, and contain relatively more dacite. It is represented by the Uretara Formation (5.6-4.3 Ma), Matangia Andesite (5.5 Ma), Ananui Andesite (4.5-3.9 Ma), and Ratarua Ignimbrite. K-Ar dating indicates a time break of about 1 Ma between the Waiwawa and Kaimai subgroups on the western side of the area. The oldest formations of the Whitianga Group are 7-6 Ma rhyolite flow-dome complexes (Maratoto Rhyolite) and associated ignimbrites and tuffaceous sediments (Edmonds and Rahu formations) which occur as erosional remnants and infaulted inliers in the north of the sheet area. Homunga Rhyolite (5.5-5.2 Ma) consists of four separate lava/dome complexes of biotite-bearing rhyolite on the eastern side of the sheet. An andesitic tuff breccia (Orokawa Breccia) is intruded by Homunga Rhyolite and overlain by Matangia Andesite. Domes of hypersthene-bearing Waimata Rhyolite (~4.1 Ma), Karewa Rhyolite, and Bowentown Rhyolite (~2.9 Ma) in the southern part of the sheet are younger than the biotite-bearing rhyolites. The Waihi Basin is infilled with Pliocene to Early Pleistocene lake sediments and ignimbrites of the Whitianga Group. At the base are Romanga Formation lacustrine sediments (Pliocene). Overlying ignimbrites are grouped into the Ohinemuri Subgroup consisting of Corbett and Owharoa ignimbrites (late Pliocene), and Waikino Ignimbrite (Early Pleistocene). Various lines of evidence suggest that the Waihi Basin may be a buried caldera. Late Pleistocene fluvial volcanogenic sediments of the Tauranga Group, mainly the Hinuera and Waitoa formations, form the upper part of the Pliocene and Pleistocene sedimentary sequence which fills still-active fault-angle depressions of the Hauraki Rift. Tectonic subsidence about 1 Ma ago produced the Tauranga Basin which is filled with sediments and ignimbrites of the Matua Subgroup of the Tauranga Group. Drowning of the Tauranga Basin during the last 12 000 years by Postglacial rise in sea level formed the Tauranga Harbour and associated sandspit of Athenree and Matakana Island. Late Pleistocene coastal terraces in the Waihi Beach-Athenree area resulted from rises in sea level. Eruptions of ash and pumice from the Taupo Volcanic Zone and Mayor Island in the last 100 000 years blanketed the area to a depth of 1 to 8 m. Identified tephras include the c. 60 000 year old Rotoehu Ash and c. 6340 year old Tuhua Tephra. Epithermal gold-silver and zinc-lead-copper bearing quartz vein deposits including the giant Waihi deposit are mainly hosted in Waiwawa Subgroup andesites, although some smaller deposits are in dacites of the Waiwawa Subgroup or rhyolite of the Whitianga Group. In a number of epithermal deposits reconnaissance K-Ar dating indicates hydrothermal alteration and associated epithermal mineralisation occurred within 1 Ma of the age of the volcanic host rocks. Coromandel Group andesites provide aggregate from several quarries. In the Katikati area warm groundwater is drawn from aquifers in the Tauranga Basin. Warm springs along the eastern boundary fault of the Hauraki Rift at Te Aroha supply a public bathing complex. The area is within a region of relatively quiet seismicity, but the Kerepehi Fault is recognised as active, and could generate earthquakes up to about magnitude 7. The ocean-facing Mataora and Waihi beaches and the entrances of Tauranga Harbour are at risk from tsunami. (auths)
