Behavior of Mg and C-O isotopes during mafic magma-carbonate interaction at the Jinchuan Ni-Cu deposit, North China Craton
Behavior of Mg and C-O isotopes during mafic magma-carbonate interaction at the Jinchuan Ni-Cu deposit, North China Craton
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华北克拉通金川镍铜矿床镁铁质岩浆-碳酸盐相互作用过程中Mg和C-O同位素的行为
DOI：
10.1016/j.chemgeo.2020.120044
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发表时间：
2021-02
期刊：
影响因子：
3.9
通讯作者：
F. Huang
中科院分区：
文献类型：
--
作者：
Xin Ding;E.M. Ripley;B.S. Underwood;Z. Meng;F. Huang
金川镍铜存款矿床是世界上最大的岩浆硫化物矿床之一，但硫化物饱和的原因尚不清楚。有人认为，金川岩浆中的硫化物饱和度是由与大理石脱碳酸作用有关的流体同化作用引起的氧化作用引发的。本研究使用Mg和C-O同位素来评估碳酸盐同化过程。围岩大理岩中碳酸盐矿物的δ 18 O和δ 13 C的实测值分别为13 ‰~22 ‰和-1.7 ‰~1.3 ‰。接触带捕虏体和混合岩中的碳酸盐矿物具有较低的碳氧同位素比值，δ 18 O为11 ‰~16 ‰，δ 13 C为− 4.7 ‰~− 2.3 ‰。捕虏体和混合岩的碳酸盐δ 26 Mg值为-1.71 ‰~-1.36 ‰，与大理岩的δ 26 Mg值（-1.74 ‰~-1.34 ‰）相近。与捕虏体共生的混合岩样品的全岩δ 26 Mg值为-1.69 ‰~-0.92 ‰。金川岩体边缘和中心辉石的δ 18O值分别为5.1 ‰~7.8 ‰和5.3 ‰~6.5 ‰。尽管与围岩接触的距离不同，但这些样品中的辉石的δ 26 Mg值为− 0.32 ‰至− 0.16 ‰，与地幔的典型值（− 0.25 ‰ ± 0.07 ‰）相似。Rayleigh分馏模型表明捕虏体和混合岩的CO2损失为40%~80%。接触带的Mg同位素组成反映了包体与镁铁质岩浆的反应同化作用。产物橄榄石和透辉石继承了反应白云石的δ 26 Mg值。捕虏体释放的CO2进入岩浆的运输与氧逸度的增加和潜在的硫化物饱和度触发是一致的。然而，镁同位素交换仅限于接触带，似乎已经与成矿过程无关。
The Jinchuan Ni-Cu deposit is one of the largest magmatic sulfide deposits in the world, but the cause of sulfide saturation remains unclear. It has been suggested that sulfide saturation in the Jinchuan magma was triggered by oxidation through assimilation of fluids related to de‑carbonation of marbles. This study uses Mg and C-O isotopes to evaluate the process of carbonate assimilation. The measured values of δ18O and δ13C for carbonate minerals from country rock marbles range from 13‰ to 22‰ and from −1.7‰ to 1.3‰, respectively. Carbonate minerals from xenoliths and hybrid rocks in the contact zone have lower C-O isotope ratios, with δ18O from 11‰ to 16‰ and δ13C from −4.7‰ to −2.3‰, respectively. The carbonate δ26Mg values of xenoliths and hybrid rocks are from −1.71‰ to −1.36‰, similar to the values of the marbles (−1.74‰ to −1.34‰). The whole-rock δ26Mg values of hybrid rocks samples associated with xenoliths vary from from −1.69‰ to −0.92‰. The δ18O values of pyroxene at the margin and the center of the Jinchuan intrusion vary from 5.1‰ to 7.8‰ and from 5.3‰ to 6.5‰, respectively. Despite varying distances from contacts with country rocks, pyroxene in these samples are characterized by δ26Mg values from −0.32‰ to −0.16‰, which are similar to the typical value of the Earth's mantle (−0.25‰ ± 0.07‰).The C-O isotopic variations are consistent with de‑carbonation. Rayleigh fractionation models suggest from 40% to 80% CO2loss from xenoliths and hybrid rocks. The Mg isotopic compositions in the contact zone reflect reactive assimilation between xenoliths and mafic magma. Product olivine and diopside inherited the δ26Mg value of reactant dolomite. The transport of released CO2from xenoliths into magma is consistent with increasing oxygen fugacity and potentially triggering sulfide saturation. However, Mg isotope exchange was confined to the contact zone, and appears to have been unrelated to the ore-forming process.
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