Iron isotope fractionation during sulfide liquid segregation and crystallization at the Lengshuiqing Ni-Cu magmatic sulfide deposit, SW China
Iron isotope fractionation during sulfide liquid segregation and crystallization at the Lengshuiqing Ni-Cu magmatic sulfide deposit, SW China
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冷水清镍铜岩浆硫化物矿床硫化物液体偏析和结晶过程中铁同位素分馏
DOI：
10.1016/j.gca.2019.07.015
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发表时间：
2019-09
影响因子：
5
通讯作者：
F. Huang
中科院分区：
文献类型：
--
作者：
Xin Ding;E.M. Ripley;W.Z. Wang;C.H Li;F. Huang
首次对冷水箐镍铜岩浆硫化物存款矿床中共生的磁黄铁矿和黄铜矿进行了铁同位素分馏研究。黄铜矿（Cp）的δ 56 Fe值为0.24‰ ~ 1.25‰，远高于共存的磁黄铁矿（Po，-1.25 ‰ ~-0.13 ‰）。冷水箐岩体中辉石的δ 56 Fe值为-0.03 ‰ ~ 0.18‰。硫化物矿化岩石中的磁黄铁矿，S的δ 56 Fe值为−0.3 ± 0.2‰，表明Δ 56 FePo-sil（磁黄铁矿-硅酸盐熔体）在−0.4 ± 0.2‰的范围内，而母硅酸盐岩浆的δ 56 Fe值为0.1‰，这是从演化最少的辉石δ 56 Fe值计算出来的。这种分馏与Schuessler等人（2007）对磁黄铁矿和流纹岩熔体实验确定的分馏相似，表明Δ 56 FePo-silof约为-0.4 ‰，代表冷水清侵入体中富含硫化物的矿化的近平衡值。在样品中以磁黄铁矿为主的硫化物分离，S的δ 56 Fe值低至−1.25‰。低δ 56 Fe值可以用非平衡过程来模拟，当R因子（硅酸盐熔体与共存硫化物液体的质量比）从1增加到1000时，不混溶硫化物液体和硅酸盐岩浆之间的分馏从−0.4 ‰减少到−1.4‰。低硫样品中硫化物沉积围岩中外来Fe的相对比例较高，控制了矿化岩石中硫化物组合的δ 56 Fe值偏低，冷水箐Fe同位素富集趋势与Fe-S键强度一致，但共存的磁黄铁矿和黄铜矿的差异与高温平衡不一致。冷水箐的Δ 56 FeCp-Po值（1.06-1.77‰）与Voisey's Bay和Eagle Ni-Cu矿床样品的测量值相似，表明在冷却的硫化物液体中，Fe同位素平衡可能持续到低于200 °C的温度。
We report the first measurements of Fe isotope fractionation between coexisting pyrrhotite and chalcopyrite from the Lengshuiqing Ni-Cu magmatic sulfide deposit, Southwest China. The δ56Fe values of chalcopyrite (Cp) range from 0.24‰ to 1.25‰, much higher than those in the coexisting pyrrhotite (Po, −1.25‰ to −0.13‰). The δ56Fe values of pyroxene separates from the Lengshuiqing intrusions are in a range from −0.03‰ to 0.18‰. Pyrrhotite in sulfide-mineralized rocks with greater than ∼2 wt.% S has δ56Fe values of −0.3 ± 0.2‰, indicating Δ56FePo-sil(pyrrhotite – silicate melt) in the range of −0.4 ± 0.2‰ with parental silicate magma δ56Fe of 0.1‰ as calculated from the least evolved pyroxene δ56Fe value. This fractionation is similar to that experimentally determined by Schuessler et al. (2007) for pyrrhotite and a rhyolitic melt, suggesting that Δ56FePo-silof approximately −0.4‰ represents a near-equilibrium value for sulfide-rich mineralization in the Lengshuiqing intrusions. Pyrrhotite-dominated sulfide separates in samples with less than ∼2 wt.% S show δ56Fe values as low as −1.25‰. The low δ56Fe values can be modeled by a non-equilibrium process, where the fractionation between immiscible sulfide liquid and silicate magma decreases from −0.4 to −1.4‰ as the R-factor (mass ratio of silicate melt to coexisting sulfide liquid) increases from 1 to 1000. We suggest that the relative proportion of externally introduced Fe from sulfidic sedimentary country rocks may be higher in low-S samples, controlling the low δ56Fe values of the sulfide assemblage in the mineralized rocks.The trend of Fe isotope enrichment at Lengshuiqing is consistent with Fe-S bond strengths, but the difference between coexisting pyrrhotite and chalcopyrite is not consistent with equilibration at high temperatures. The Δ56FeCp-Povalues at Lengshuiqing (1.06–1.77‰) are similar to those measured for samples from the Voisey’s Bay and Eagle Ni-Cu deposits, and suggest that in a cooling sulfide liquid Fe isotope equilibration may continue to temperatures less than 200 °C.
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