New insights into volcanic processes from deep mining of the southern diatreme within the Argyle lamproite pipe,Western Australia |
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Authors: | Rayner Murray J Moss Stephen W Lorenz Volker Jaques A Lynton Boxer Grant L Smith Chris B Webb Kimberley |
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Institution: | 1.Argyle Diamonds, Rio Tinto, 1 William St, Perth, WA, 6000, Australia ;2.Mineral Services Canada, 501 - 88 Lonsdale Avenue, North Vancouver, BC, V7M 2E6, Canada ;3.Terram Vero Consulting Inc., 2685 Dundas Street, Vancouver, BC, V5K-1R1, Canada ;4.Physical Volcanological Laboratory, Department of Geography and Geology, University of Würzburg, Pleicherwall 1, D-97070, Würzburg, Germany ;5.Research School of Earth Sciences, Australian National University, 142 Mills Road, Acton, ACT, 2601, Australia ;6.PO Box 368, Maylands, 6931, WA, Australia ;7.Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol, BS8 1RJ, UK ;8.SRK Consulting, 1066 W Hastings St, Vancouver, BC, V6E 3X2, Canada ; |
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Abstract: | Underground mining and deep drilling of the richly diamondiferous ~1.2 Ga Argyle lamproite in Western Australia has prompted a re-evaluation of the geology of the pipe. Argyle is considered to be a composite pipe that formed by the coalescence of several diatremes and has been offset and elongated by post-emplacement faulting. Recent geological studies have recognised at least five distinct volcaniclastic lamproite lithofacies with differing diamond grades. The new data suggest that the centre of the southern (main) diatreme is occupied by well-bedded, olivine lamproite lapilli tuff with very high diamond grades (>10 ct/t). Characteristic features include a clast-supported fabric and high modal abundance of densely packed lamproite lapilli and coarse-grained, likely mantle-derived olivine now replaced by serpentine and/or talc. The persistence of small-scale graded and cross-bedding in this lithofacies to depths of ~1.5 km below the original surface prior to erosion suggests phreatomagmatic volcanism forming the diatreme was syn-eruptively accompanied by subsidence of the tephra, maintaining a steep-walled diatreme in the water-saturated country rock sediments. |
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