Large Fe isotope fractionations in sulfide ores and ferruginous sedimentary rocks from the Kuroko volcanogenic massive sulfide deposits in the Hokuroku district, northeast Japan
Large Fe isotope fractionations in sulfide ores and ferruginous sedimentary rocks from the Kuroko volcanogenic massive sulfide deposits in the Hokuroku district, northeast Japan
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DOI：
10.1016/j.gca.2020.12.009
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发表时间：
2021-02
影响因子：
5
通讯作者：
T. Otake;R. Yamada;R. Suzuki;Shunsuke Nakamura;A. Ito;K. Shin;Tsutomu Sato
中科院分区：
文献类型：
--
作者：
T. Otake;R. Yamada;R. Suzuki;Shunsuke Nakamura;A. Ito;K. Shin;Tsutomu Sato
缺氧海水可能在日本东北部北国地区火山成因块状硫化物(VMS)矿床的保存中发挥了重要作用，该地区是黑子型VMS矿床的典型所在地。在这项研究中，我们研究了这些矿床中硫化物矿石和上覆含铁沉积岩的铁同位素组成。这些数据与岩石学和地球化学数据相结合，使我们能够研究大型黑子型VMS矿床形成过程中的关键形成过程和条件。与负或正Ce异常有关的大的Fe同位素变化(约4‰(δ，56Fe)是黑子VMS矿床中形成的含铁沉积岩和黑子后的热液活动的特征。这表明铁(氢)氧化物是由北国盆地缺氧或亚缺氧型热液中溶解的Fe2+部分氧化而形成的。与黑子VMS矿床密切相关的铁质硅质岩中的正Eu异常表明其形成于高温热液。松明和福泽两个矿床中的富锌黑色矿石具有较低的δ56Fe值，这是由于热液与海水混合引发的黄铁矿快速沉淀所致。伊祖里和藤泽含铁硅质岩中δ56Fe值的正值变化可能是由于含铁沉积岩与黑色矿石同时沉淀，使热液流体的δ56Fe值变为正值。而富铜黄矿石与热液中溶解的铁相比，没有明显的Fe同位素分馏，可能是黄铁矿缓慢生长取代黑色矿石形成的。松明和福泽矿床硫化矿石丰度的差异可能反映了热液循环的持续时间。
Anoxic seawater may have played an important role in the preservation of volcanogenic massive sulfide (VMS) deposits in the Hokuroku district, northeast Japan, which is the type locality for Kuroko-type VMS deposits. In this study, we investigated the Fe isotopic compositions of sulfide ores and overlying ferruginous sedimentary rocks in these deposits. These data, coupled with petrographic and geochemical data, enable us to investigate the key formation processes and conditions during the formation of large Kuroko-type VMS deposits. Large Fe isotope variations of ca. 4‰ (δ56Fe) associated with negative or positive Ce anomalies characterize the ferruginous sedimentary rocks formed in the Kuroko VMS deposits and post-Kuroko hydrothermal activity. This suggests that iron (hydr)oxides were precipitated by the partial oxidation of dissolved Fe2+derived from hydrothermal fluids in anoxic or suboxic pools in the Hokuroku Basin. Positive Eu anomalies in the ferruginous cherts closely associated with the Kuroko VMS deposits indicate formation from high-temperature hydrothermal fluids. Zinc-rich black ores in both the Matsumine and Fukazawa deposits have lower δ56Fe values, due to rapid precipitation of pyrite triggered by the mixing of hydrothermal fluids with seawater. Positive shifts in δ56Fe values in the ferruginous cherts from the Ezuri and Fukazawa deposits may be explained by simultaneous precipitation of ferruginous sedimentary rocks with black ores, which modified the δ56Fe values of the hydrothermal fluids to positive values. However, Cu-rich yellow ores show no significant Fe isotope fractionation as compared with the dissolved Fe in the hydrothermal fluids, and were likely formed by slow growth of pyrite that replaced the black ores. The difference in the abundance of sulfide ores between the Matsumine and Fukazawa deposits may reflect the duration of hydrothermal circulation.