Iron isotope behavior during fluid/rock interaction in K-feldspar alteration zone – A model for pyrite in gold deposits from the Jiaodong Peninsula, East China
Iron isotope behavior during fluid/rock interaction in K-feldspar alteration zone – A model for pyrite in gold deposits from the Jiaodong Peninsula, East China
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DOI：
10.1016/j.gca.2017.10.001
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发表时间：
2018-02
影响因子：
5
通讯作者：
Zhi-Yong Zhu;Shaoyong Jiang;R. Mathur;N. Cook;Tao Yang;Meng-Long Wang;Liang Ma;C. Ciobanu
中科院分区：
文献类型：
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作者：
Zhi-Yong Zhu;Shaoyong Jiang;R. Mathur;N. Cook;Tao Yang;Meng-Long Wang;Liang Ma;C. Ciobanu
热液矿床和伴生钾长石蚀变带中的铁同位素分馏机制仍然缺乏约束。采用多接收电感耦合等离子体质谱法对胶东半岛金矿中的一组含钾长石蚀变花岗岩、前寒武纪变质岩和黄铁矿的样品进行了分析。浸染状（J型）矿石中的黄铁矿δ 56 Fe变化范围为+0.01 ‰ ~+0.64‰，与寄主花岗岩的特征（+0.08 ‰ ~+0.39‰）重叠。相比之下，石英脉中的黄铁矿（L型矿石）的Fe同位素组成范围很广，从−0.78 ‰到+0.79‰。在J型黄铁矿中从未见到负值。含钾长石蚀变的寄主花岗岩的铁同位素特征明显重于块状硅酸盐地球。本区前寒武纪变质岩的Fe同位素组成在-0.16 ‰ ~+0.19‰之间，与大多数陆相岩石相似。大量样品中的主要和微量元素的浓度也进行了测定，以评估铁同位素组成和蚀变程度之间的任何相关性。我们注意到，在含SiO_2> 70wt%、Rb>75 ppm和总Fe_2O_3 <1.2wt%的岩石的钾长石渐进蚀变过程中，花岗岩的Fe同位素组成发生了系统的变化。Fe同位素特征随着蚀变程度的增加而变重。L型金矿床的Fe同位素组成极轻，可能是由于黄铁矿在开放断裂系统中沉淀时的瑞利分馏作用所致。两类矿石中黄铁矿的硫同位素组成也有差异。J型矿石中黄铁矿的δ 34 S值（11.2‰）比L型矿石中黄铁矿的δ 34 S值（7.7‰）系统地高出3.5‰。然而，Fe和S同位素特征之间没有相关性。硫的同位素分馏被用来限制黄铁矿沉淀的热液流体的fO 2的变化.这项工作表明，黄铁矿的Fe同位素组成显示出显着的响应，黄铁矿沉淀过程中的热液系统，铁同位素分馏发生在流体/岩石反应玲珑花岗岩的钾长石蚀变带.结果的意义是，矿化过程和相关的流体-岩石相互作用，这是普遍观察到的斑岩型铜-金-钼和其他热液矿床，可以很容易地使用铁同位素示踪。
Mechanisms for Fe isotope fractionation in hydrothermal mineral deposits and in zones of associated K-feldspar alteration remain poorly constrained. We have analyzed a suite of bulk samples consisting of granite displaying K-feldspar alteration, Precambrian metamorphic rocks, and pyrite from gold deposits of the Jiaodong Peninsula, East China, by multi-collector inductively-coupled plasma mass spectrometry. Pyrites from disseminated (J-type) ores show a δ56Fe variation from +0.01 to +0.64‰, overlapping with the signature of the host granites (+0.08 to +0.39‰). In contrast, pyrites from quartz veins (L-type ores) show a wide range of Fe-isotopic composition from −0.78 to +0.79‰. Negative values are never seen in the J-type pyrites. The Fe isotope signature of the host granite with K-feldspar alteration is significantly heavier than that of the bulk silicate Earth. The Fe isotopic compositions of Precambrian metamorphic rocks across the district display a narrow range between −0.16‰ and +0.19‰, which is similar to most terrestrial rocks. Concentrations of major and trace elements in bulk samples were also determined, so as to evaluate any correlation between Fe isotope composition and degree of alteration. We note that during progressive K-feldspar alteration to rocks containing >70 wt% SiO2, >75 ppm Rb, and <1.2 wt% total Fe2O3, the Fe isotope composition of the granite changes systematically. The Fe isotope signature becomes heavier as the degree of alteration increases. The extremely light Fe isotopic compositions in L-type gold deposits may be explained by Rayleigh fractionation during pyrite precipitation in an open fracture system. We note that the sulfur isotopic compositions of pyrite in the two types of ores are also different. Pyrite from J-type ores has a systematically 3.5‰-higher δ34S value (11.2‰) than those of pyrite from the L-type ores (7.7‰). There is, however, no correlation between Fe and S isotope signatures. The isotopic fractionation of sulfur is used to constrain a change in thefO2of the hydrothermal fluids from which pyrite precipitated.This work demonstrates that the Fe isotope composition of pyrite displays a significant response to the process of pyrite precipitation in hydrothermal systems, and that systematic fractionation of iron isotopes occurs during fluid/rock reaction in the K-feldspar alteration zone of the Linglong granite. The implications of the results are that processes of mineralization and associated fluid-rock interaction, which are ubiquitously observed in porphyry-style Cu-Au-Mo and other hydrothermal deposits, may be readily traceable using Fe isotopes.