The Kohuamuri siliceous sinter as a vector for epithermal mineralisation, Coromandel Volcanic Zone, New Zealand

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• 作者：Ayrton Hamilton ; Kathleen Campbell ; Julie Rowland ; Patrick Browne
• 关键词：Siliceous sinter ; Epithermal ; Mineralisation vector ; Hauraki goldfield ; Palaeoenvironmental reconstruction ; Coromandel Volcanic Zone ; New Zealand
• 刊名：Mineralium Deposita
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：52
• 期：2
• 页码：181-196
• 全文大小：
• 刊物类别：Earth and Environmental Science
• 刊物主题：Geology; Mineral Resources; Mineralogy;
• 出版者：Springer Berlin Heidelberg
• ISSN：1432-1866
• 卷排序：52

文摘

The Kohuamuri siliceous sinter is the largest known fossil hot-spring system in the Hauraki Goldfield, a 200 × 40 km volcanic terrain with at least 50 adularia-illite epithermal deposits formed 16.3–5.6 Ma within the Coromandel Volcanic Zone, New Zealand. The sinter is associated with rhyolite and ignimbrite of the Whitianga Caldera (Miocene–Pliocene) and consists of two deposits, the Kohuamuri deposit itself, a large in situ outcrop (47,000 m2) and its associated sinter boulder field (4500 m2), and the Kaitoke deposit 900 m to the southwest, comprising boulders in a landslide situated on a normal fault. The well-preserved macroscopic and microscopic textures at Kohuamuri are similar to actively forming and ancient hot-spring deposits elsewhere, derived from deep circulating, magmatically heated, near-neutral pH alkali chloride fluids oversaturated in amorphous silica and that discharge at the Earth’s surface at ≤100 °C. Lithofacies, petrography, mineralogy, as well as trace element concentrations of the Kohuamuri/Kaitoke deposits were used to locate likely palaeo-thermal conduits from the deep reservoir and to reconstruct the palaeoenvironmental setting of the siliceous sinter as an aid to assessing the economic potential of the ancient geothermal system. Both deposits contain the high-temperature (>75 °C) geyserite lithofacies, with the Kohuamuri deposit also exhibiting textures affiliated with cooler middle and distal sinter apron areas, as well as geothermally influenced marsh facies. Trace element analysis of sinter lithofacies revealed concentrations and zonations of Au, Ag, base metals (Pb, Cu, Zn) and pathfinder elements (As, Sb) associated with epithermal deposits, elevated in the proximal vent area, and providing evidence of possible Au and Ag ore mineralisation at depth. The methodology used in this study could be utilised globally to identify and assess as yet unidentified epithermal deposits.