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Behavior of Mg and C-O isotopes during mafic magma-carbonate interaction at the Jinchuan Ni-Cu deposit, North China Craton
Chemical Geology ( IF 3.9 ) Pub Date : 2021-01-06 , DOI: 10.1016/j.chemgeo.2020.120044
Xin Ding , Edward M. Ripley , Benjamin S. Underwood , Ziyue Meng , Fang Huang

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% CO2 loss 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 CO2 from 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.



中文翻译:

华北克拉通金川镍铜矿床镁铁质岩浆-碳酸盐相互作用中Mg和CO同位素的行为

金川镍铜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​​的硫化物饱和是由与大理石脱碳有关的流体同化而氧化而触发的。这项研究使用Mg和CO同位素评估碳酸盐同化过程。δ的测量值18 O和δ 13 ℃下从围岩弹子碳酸盐矿物的范围从13‰至22‰,从-1.7‰至1.3‰,分别。从在接触区包体和混合岩石碳酸盐矿物具有较低的CO同位素比率,具有δ 18从11 O‰至16‰,δ 13从-4.7‰〜-2.3‰,分别℃。碳酸酯δ 26异岩和混合岩石的Mg值在-1.71‰至-1.36‰之间,与大理石的值(-1.74‰至-1.34‰)相似。全岩δ 26个与包体相关联的混合岩石样品的镁值从-1.69‰至-0.92‰变化。的δ 18个在边缘和金川入侵的中心O值辉石分别变化从5.1‰至7.8‰和5.3‰至6.5‰。尽管与国家岩石接触不同的距离,在这些样品中辉石的特征在于δ 26 Mg的值从-0.32‰至-0.16‰,其类似于地球地幔(-0.25‰±0.07‰)的典型值。

一氧化碳同位素变化与脱碳一致。瑞利分馏模型表明,异岩和杂种岩石的CO 2损失为40%至80%。接触区的镁同位素组成反映了异岩与铁镁质岩浆之间的反应同化。产品橄榄石和辉石继承δ 26反应白云石镁价值。释放的CO 2从异岩中进入岩浆的过程与增加了氧气逸度并可能触发硫化物饱和有关。但是,Mg同位素交换仅限于接触区,并且似乎与成矿过程无关。

更新日期:2021-01-06
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