Morphological evolution of gold nuggets in proximal sedimentary environments,southern New Zealand |
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| Institution: | 1. Guangzhou College South China University of Technology, Guangzhou 510800, China;2. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;3. Beijing Institute of Geology for Mineral Resources, Beijing 100012, China;4. Centre for Exploration Targeting, ARC Centre of Excellence for Core to Crust Fluid Systems, The University of Western Australia, Crawley, WA 6009, Australia;5. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS, Beijing 100037, China;6. Henan Academy of Land and Resources Sciences, Zhengzhou 450053, China;7. Department of Earth Sciences, University of New Brunswick, N.B, E3B 5A3, Canada;1. Geological Survey of Western Australia, Mineral House, 100 Plain St, EAST PERTH 6004, Australia;2. Centre for Microscopy Characterisation and Analysis, The University of Western Australia, 35 Stirling Hwy, CRAWLEY, WA 6009, Australia;1. University of the Witwatersrand, School of Geosciences, Private Bag 3, 2050 Wits, Johannesburg, South Africa;2. Rockwater Consulting, 10 Evergreen Lane, Constantia 7806, Cape Town, South Africa;1. CSIRO Mineral Resources, Australian Resources Research Centre, 26 Dick Perry Avenue, Kensington, WA 6151, Australia;2. Department of Applied Geology, Curtin University, Perth, WA 6845, Australia;1. CSA Global Pty Ltd, Level 2, 3 Ord Street, West Perth, 6005 Perth, Western Australia, Australia;2. Rosslyn Hill Mining Pty Ltd, Suite 1D, 21 Teddington Road, Burswood, 6100 Perth, Western Australia, Australia;3. John de Laeter Isotope Centre for Isotope Research, Curtin University, Kent St, Bentley, 6102 Perth, Western Australia, Australia;4. The Institute for Geosciences Research, Curtin University, Kent St, Bentley, 6102 Perth, Western Australia, Australia |
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| Abstract: | A rare collection of centimetre-scale proximal gold nuggets has been examined in the context of the Late Pleistocene to Holocene eluvial and colluvial sediments in which they occur, in a range of environments from arid temperate to periglacial. Liberation of supergene nuggets from their basement source in fault zones occurred progressively by physical erosion as the host basement rocks were uplifted between middle Miocene and Holocene. The host sediments are made up of poorly sorted angular greenschist facies schist debris. Some nuggets were recycled through several generations of these colluvial deposits, and additional nuggets were liberated to younger sediments during this time. The nuggets did not migrate laterally more than a few tens of metres during this repeated recycling, and were passively elevated vertically with the rising basement on to the crest and slopes of a 1600 m high mountain range. The most recently liberated nuggets retain almost all the morphological features of their supergene origin, including coarse (cm scale) crystal shapes, delicate crystalline internal structure, and imprints of oxidised pyrite crystals. Minor transport in colluvium has caused some abrasion and rounding of gold crystals. Repeated recycling progressively obscured the crystal shapes, although relict crystals are still recognisable on parts of most nuggets. Differential timing of liberation from basement has resulted in a wide range of rounding effects in groups of closely-coexisting nuggets. Variably crystalline gold overgrowths (micron scale) coat abraded surfaces of all or part of most nuggets, commonly intergrown with authigenic smectite clay minerals. These overgrowths developed from alkaline groundwater (pH 7–9) that had undergone extensive chemical interaction with labile minerals in the host colluvium. Abrasion during recycling removed some of the overgrowths, but this was replaced in the new sedimentary hosts. Physical and chemical processes affecting nuggets in these sediments have similarly affected eluvial and colluvial nuggets in a wide range of settings around the world, including Yukon (Canada), California (USA) and arid parts of Australia. |
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