Magmatic process recorded in plagioclase at the Baogutu reduced porphyry Cu deposit, western Junggar, NW-China

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• 作者：MingJian Cao ; KeZhang Qin ; GuangMing Li ; YueHeng Yang ; Noreen J. Evans ; Rui Zhang ; LuYing Jin
• 关键词：Baogutu reduced porphyry copper deposit ; In situ Sr isotopes ; Plagioclase ; Magma recharge ; Diffusion
• 刊名：Journal of Asian Earth Sciences
• 出版年：15 March, 2014
• 年：2014
• 卷：82
• 期：Complete
• 页码：136-150
• 全文大小：5270 K

文摘

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 鈭?00 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.