Diagenetic alteration of a Mesozoic fluvial gold placer deposit,southern New Zealand |
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| Institution: | 1. Sorbonne Universités, UPMC-Paris 06, UMR CNRS 7193, ISTeP, F-75005 Paris, France;2. Géosciences Environnement Toulouse, Université Paul Sabatier Toulouse 3, 14 Avenue Edouard Belin, 31400 Toulouse, France;3. Sorbonne Universités, UPMC-Paris, 06, UMR 8222, Lab. Ecogéochimie des environnements benthiques (LECOB), Observatoire Océanologique, F-66650 Banyuls-sur-mer, France;4. Département des Sciences de la Terre, Université de Nantes, 2 rue de la Houssinière, 44072 Nantes;5. Sorbonne Universités, UMR CNRS 7207 (MNHN, CNRS, UPMC) Muséum national d''Histoire naturelle, CP 38, 8, rue Buffon, 75005, Paris |
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| Abstract: | Gold paleoplacers become progressively more affected by diagenetic processes with age and burial. Mesozoic paleoplacer deposits in southern New Zealand display intermediate stages of diagenetic transformation compared to little-affected Late Cenozoic paleoplacers and strongly-affected Paleozoic and Precambrian paleoplacers. The Mesozoic (Cretaceous) diagenesis resulted in near-pervasive alteration, cementation and lithification of the paleoplacer. Lithic clasts and matrix have been extensively altered to illite, ferrous iron-bearing smectite-vermiculite, and kaolinite, and the cement consists mainly of clays and calcite. Diagenetic pyrite, marcasite, vivianite, and Mn oxide also contributed to cementation. Alteration occurred under near-surface (<500 m depth) conditions with groundwater that had circumneutral pH, high alkalinity, and elevated dissolved K, Mg and Ca. Detrital albite remained unaffected by alteration. Detrital gold has been variably dissolved and redeposited, with widespread formation of gold overgrowths on the 1–10 μm scales, with 1–3 wt% Ag. Gold mobility was driven by reduced sulphur complexes in the low redox, high pH diagenetic environment. The overgrowth gold locally contributed to cementation of fine clastic grains, and has intergrown with diagenetic clays and Mn oxide. Post-diagenetic oxidation of the paleoplacer deposit has transformed much of the pyrite to ferric oxyhydroxide and deposited some ferric oxyhydroxide coatings on gold. These oxidation processes have had only minor effects on gold mobility and textures. Hence, the low redox conditions of diagenetic gold mobility were distinctly different from those typically associated with oxidation-related supergene gold mobility. Diagenesis can affect economics of paleoplacer mining by hindering rock disaggregation during processing, coating gold particles with secondary minerals, and increasing the clay content of the deposit, all of which can lower the efficiency of gold recovery. |
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| Keywords: | Alluvial Gold Clay Smectite Authigenic Groundwater |
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