江西永平铜矿区花岗岩热液蚀变与岩石成因:矿物化学、元素地球化学和Sr-Nd-Hf同位素制约

Hydrothermal alteration and petrogenesis of granites in the Yongping copper deposit,Jiangxi Province:Constraints from mineral chemistry,element geochemistry,and Sr-Nd-Hf isotopes

永平铜矿床是位于华南钱塘江—信江断裂坳陷带中的一个大型铜矿床。矿区内出露的岩浆岩主要有似斑状黑云母花岗岩体和少量石英斑岩脉,黑云母花岗岩岩体内及接触带发育斑岩型和矽卡岩型铜钼钨多金属矿化。对岩体中黑云母的电子探针成分分析表明,似斑状黑云母花岗岩中黑云母具有富镁贫铁高钛的特点,Fe/(Fe+Mg)为0.21～0.33,TiO_2含量为2.12%～4.28%;而石英斑岩脉中黑云母更加富镁贫铁,Fe/(Fe+Mg)为0.10～0.15,TiO_2含量为2.43%～3.86%。黑云母的化学组成表明花岗质岩浆形成时的氧逸度很高,与长江中下游地区其他含铜花岗质岩体相吻合。永平花岗岩体遭受了较强的热液蚀变作用,部分黑云母蚀变为绿泥石。石英斑岩中绿泥石具有富铁贫镁低钛的特征(Fe/(Fe+Mg)=0.72～0.76;TiO_2=0.02%～0.06%),其形成温度为139～224℃。而似斑状黑云母花岗岩中绿泥石相对贫铁富镁高钛(Fe/(Fe+Mg)=0.36～0.44;TiO_2=0.05%～0.36%),形成温度稍高,为229～346℃,与该矿床流体包裹体研究获得的成矿温度(220～ 400℃)基本吻合。Sr-Nd-Hf 同位素综合研究表明,永平似斑状黑云母花岗岩具有变化较大且相对较高的ε_(Hf)(t)值(-0.1～-10.3)和ε_(Nd)(t)值(-5.83～-7.95),反映岩石具有明显的壳幔混合成因特征。而石英斑岩的ε_(Hf)(t)值(-8.4～-12.5)和ε_(Nd)(t)值(-9.93～-10.2)均稍低于拟斑状黑云母花岗岩,反映其形成过程中幔源物质贡献相对较小。永平岩体的地壳端元很可能就是该区中元古代基底地层重熔的产物。

Yongping deposit is a large copper deposit located in the Qiantangjiang-Xinjiang intracontinental rift basin. The magmatic rocks in this deposit include porphyric biotite granite body and quartz porphyry dykes. Within and around the biotite granite body occurred porphyry-type and skarn-type Cu-Mo-W polymetallic mineralization. Biotite analysis by electron microprobe reveals that the biotites in the porphyric biotite granite are characteristic with high-Mg, low-Fe, and high-Ti, with Fe/（ Fe ＋ Mg） ratios of 0. 21 -0.33 and TiO2 contents of 2.12% -4.28%. The biotites in the quartz porphyry contain even higher Mg with Fe/（ Fe ＋ Mg） ratios of 0. 10 -0.15 and similar TiO2 contents （2.43% - 3.86% ）. The biotite chemistry suggests a very high oxygen fugacity for the granitic magma, which is similar to those copper mineralization related granites in the Middle-Lower Yangtze regions. Various degrees of hydrothermal alteration took place in the Yongping granites, which caused biotite altered into chlorite. The chlorites from the quartz porphyry show Fe-rich, Mg-poor, Ti-poor features, with Fe/（ Fe ＋ Mg） ratios of 0. 72 -0.76 and TiO2 contents of 0. 02% -0.06%, and the calculated＇temperatures are between 139℃and 224℃. In contrast, the chlorites from the porphyric biotite granite show lower Fe/（Fe ＋ Mg） ratio （0.36 - 0.44） and higher TiO2 contents （0.05% -0.36% ）, and the formation temperatures are higher from 229℃ to 346℃; ,which are in the same range as those mineralization temperatures （220 - 400℃ ） deduced from fluid inclusion study. A systematic Sr-Nd-Hf isotope study indicates that the porphyric biotite granite has a wider range and higher εHf（t） and εNd （t） values of - 0. 1 - - 10. 3 and - 5.83 - - 7.95, respectively, which suggest a crust-mantle mixing characteristic for the petrogenesis. The quartz porphyry show lower εHf（t） and εNd （t） values of -8.4 - -12.5 and -9.93 - -10.2, respectively, suggesting little or less contribution from the mantle source. Overall, the granitic rocks at Yongping are mainly products of remehing of basement rocks of Middle Proterozoic age in the area, with small but variable amounts of mantle contributions.