Geology of the Coromandel Harbour area

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Skinner, D.N.B. 1993 Geology of the Coromandel Harbour area. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences geological map 4 44 p. + fold. map

Abstract: The Coromandel Harbour map region consists of a Neogene volcanic sequence overlying a late Jurassic sedimentary basement. Both host epithermal vein systems from which over 113 000 oz (>3200 kg) of gold bullion with high silver content were produced. The Jurassic rocks (Manaia Hill Group) total more than 3500 m of a very low grade lithic-volcanic to lithic-feldspathic greywacke suite, altered toprehnite- pumpellyite facies (zeolite and quartz-prehnite subracies). Deposition progressed from a lower slope to mid fan environment (Paraunahi Sandstone to Waimana Formation), followed by a return to lower slope and inner fan (Kirita Sandstone), to outer fan (Matawai Formation), and basin plain sedimentation (Tokatea Hill Formation). Syntectonic mass flow deposits (Ahimia Conglomerate) were emplaced during early Rangitata Orogeny polyphase folding. Eruptions of the Coromandel Volcanic Zone in this map region commenced with early Mid Miocene (c. 14.5 million years ago) to early Late Miocene (c. 11 million years ago) andesites and dacites of Coromandel Group. These comprise (in younging sequence) Tuateawa Andesite, Beesons Island Volcanics, Whangapoua Andesite, Matarangi Andesite, Te Tutu Andesite, Maumaupaki Formation, and Mahinapua Andesite. Extensive faulting and epithermal mineralisation preceded the eruption of Maumaupaki Formation, which itself includes generalised porphyry dike swarms. Rhyolitic volcanism (Whitianga Group) commenced with the eruption of Carina Rock Ignimbrite from the early outer ring fault system of the Kapowai Caldera (10 million years ago). Epithermal mineralisation during the succeeding prolonged volcanic quiescence was interrupted by andesitic volcanism (Taurahuehue Andesite) on the flanks of the developing caldera (c. 9.5 million years ago). Within the caldera, lacustrine sedimentation was overtaken by massive eruptions of Wharepapa Ignimbrite accompanying caldera collapse (c. 8 million years ago). Comagmatic Rangihau Rhyolite intruded along inner ring faults, and was followed after erosion by extensive flows of post-caldera Ruahine Rhyolite and Taurauikau Andesite along and over the rim faults (c. 7.9 and 7.7 million years ago respectively). Hydrothermal alteration around the caldera rim accompanied sinterous quartz pipes and geyserite deposits. Two million years later (c. 5.9 million years ago), eruptions from north of the caldera margin (Pumpkin Rock Ignimbrite) infilled a paleo-Whitianga estuary. Since the Pliocene, only sedimentation of alluvial gravel terraces, drowned valleys, and coastal sand dune complexes has occurred. However, faulting is recognised as still active in the Hauraki Rift, a tectonic zone on the west of the Coromandel Peninsula, on which pre-rift arching began late in the Miocene. (auth)

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