Magmatic process recorded in plagioclase at the Baogutu reduced porphyry Cu deposit,western Junggar,NW-China |
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| Affiliation: | 1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China;2. John de Laeter Centre, Dept. Applied Geology, TIGeR, Curtin University, Perth, WA 6945, Australia;3. Xinjiang Geoexploration Bureau for Geology and Mineral Resources, No. 6 Geological Party, Hami 839000, China;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. School of Earth, Atmosphere and Environment, Monash University, VIC3800, Australia;1. School of Environment, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand;2. Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan;3. Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan;4. Volcanic Risk Solutions, Massey University, Private Bag 11222, Palmerston North 4442, New Zealand;5. College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97330, USA |
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| Abstract: | Despite the fact that porphyry Cu deposits contain large amounts of Cu in one or more small stocks, few studies have discussed evidence for significant magma recharge in porphyry Cu deposits. A systematic elemental and Sr isotopic study of plagioclase crystals from mineralized diorite and granodiorite porphyry constrains the processes of crystallization and magma recharge at the Baogutu reduced porphyry copper deposit, western Junggar, NW-China. Large compositional changes in An (12–24 mol%) are observed along with strong positive correlations between An and FeO. Significant resorption textures are also preserved in plagioclase crystals as well as repeated oscillatory zoning in An and FeO, and complex Sr isotope variations. Three types of crystals with different core-to-rim Sr isotope variations are recognized. Type I crystals have core-to-rim increases in (87Sr/86Sr)i that could be explained by diffusion. For example, Sr isotope variations recorded in BCK2-1-2 crystal could be generated by diffusion with simulated maximum crystal residence times of 100–500 years with the proximate value of ∼300 years. Type II crystals with different variation trends in (87Sr/86Sr)i on opposite sides of the core could be produced either by multi-stage crystallization or by analytical uncertainty. Whereas, type III crystals with complex core-to-rim variations in (87Sr/86Sr)i, may record repeated magma recharge events. All these results suggest repeated recharging of the magma chamber by hotter, more mafic and less radiogenic Sr isotope melts. Therefore, more mafic melt injection recorded in plagioclase profiles may provide significant metal contributions to the magma chamber, which ultimately results in Cu mineralization at the Baogutu reduced porphyry copper deposit. |
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| Keywords: | Baogutu reduced porphyry copper deposit Plagioclase Magma recharge Diffusion |
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