Origin of selective enrichment of Cu and Au in sulfide deposits formed at immature back-arc ridges: Examples from the Lau and Manus basins |
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| Institution: | 1. Institute of Geophysics, China Earthquake Administration, 100081, Beijing, China;2. Centre for Tectonics, Resources and Exploration, Department of Earth Sciences, School of Physical Sciences, University of Adelaide, SA 5005, Australia;3. School of Earth Sciences and Resources, China University of Geosciences, 100083 Beijing, China;1. State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China;2. Gold Geological Institute of CAPF, Langfang 065000, China;3. James Cook University, Townsville 4811, Queensland, Australia;1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Faculty of Land Resource and Engineering, Kunming University of Science and Technology, Kunming 650093, China;1. Department of Geography and Geology, University of Turku, FIN-20014, Finland;2. Ödemarksvägen 107, FIN-49220 Broby, Finland;3. School of Chemical Engineering, University of Adelaide, Adelaide SA 5005, Australia;1. Department of Mining Engineering, University of Tehran, Iran;2. Faculty of Health, Engineering and Science, Edith Cowan University, Perth, Australia;3. Department of Mining Engineering, University of Chile, Santiago, Chile;4. Advanced Mining Technology Center, University of Chile, Santiago, Chile;1. Department of Geochemistry and Ore-Forming Processes, A.N. Zavaritsky Institute of Geology and Geochemistry, the Uralian Branch of Russian Academy of Sciences, Pochtovy per. 7, Ekaterinburg 620075, Russia;2. Department of Civil and Environmental Engineering, University of Strathclyde, Glasgow G4 0NG, United Kingdom;3. ARC Centre of Excellence for Core to Crust Fluid Systems/GEMOC Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia;4. Department of General and Analytical Chemistry, University of Leoben, Leoben 8700, Austria |
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| Abstract: | It has long been recognized that magmatic fluids exsolved from the arc-like submarine magmas of immature back-arc basins can directly contribute metals such as Cu and Au to seafloor hydrothermal systems. The extent of this magmatic contribution, however, varies from basin to basin. In order to explain this variation, we make a comparative study of the behavior of Cu during magma differentiation in two immature back-arc ridges: Eastern Ridge (ER) of the Manus Basin and Valu Fa Ridge (VFR) of the Lau Basin. We investigate some of the factors that affect Cu behavior, including oxygen fugacity (fO2), water content, and crystallization pressure, by means of a geochemical model. Cu abundances show a continuous decreasing trend with magma evolution in the VFR lavas, whereas in the ER lavas Cu increases during the early stage of magma evolution, followed by a rapid decrease. The contrasting Cu behavior for the two lava suites is controlled on the first order by the fO2 of their primary magmas. The fO2 values of the primary ER magmas were modeled to be FMQ + 1.2 to FMQ + 1.8, which is sufficiently high to avoid the early sulfide saturation that typically accompanies Cu removal. By comparison, the fO2 values of the primary VFR magmas range from FMQ to FMQ + 1, falling within the range of mid-ocean ridge basalts. We attribute this difference in fO2 values between the primary ER and VFR magmas to variable input of sediment melt to their mantle sources. In addition, we show for the first time that Cu content does not increase significantly until the onset of plagioclase crystallization. This finding suggests that both high water contents and high pressure, which suppress plagioclase crystallization, are unfavorable for Cu enrichment in evolved oxidized magmas. We argue that back-arc ridges that develop shallow submarine magma chambers and have a large input of subducted sediment, have a strong potential to support ore-bearing magmatic–hydrothermal systems. |
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