Addition of crustal sulfur to the Jinchuan magma or oxidation of the magma associated with carbonate assimilation has been considered to be the main cause of sulfide saturation in the magma by two opposing groups of researchers. To address this controversy, we have carried out an integrated mineralogical and C-S-Sr-Nd isotope study of the Jinchuan magmatic Ni-Cu-platinum group element (PGE) sulfide ore deposit. Pure marble, olivine marble, serpentine marble, and hybrid rocks occur in the contact zone. The δ¹³C_carb values of the Jinchuan sulfide-bearing ultramafic rocks containing calcite xenocrysts are from –4.7 to –2.9‰, which are similar to or slightly lower than those of the marbles and associated hybrid rocks (–3.9 to 0.8‰) but significantly higher than those of the calcite-barren intrusive rocks (–9.3 to –8.0‰). This, together with the calcite xenocrysts and calcium-silicate minerals in hybrid rocks and some intrusive rocks, indicates that carbonate assimilation took place during magma-carbonate interaction. Only less than several hundred ppm of the CO₂ gas produced in the reaction zone could be added to the Jinchuan magma. Such a trace amount of additional CO₂ was rapidly swamped by the much greater mass of FeO in the magma, resulting in little change in the FeO/Fe₂O₃ of the magma, and thereby negligible change of magma redox state. This is evident from similar calculated fO2 values for the calcite-bearing (~QFM+0.7) and calcite-barren (~QFM+0.6) intrusive rocks. The new results rule out the possibility that sulfide saturation in the Jinchuan magma resulted from in situ carbonate assimilation. The Sr-Nd isotope data from this study and previous studies are consistent with up to 20% bulk contamination with siliceous crustal materials at depth, followed by minor amounts of in situ carbonate assimilation by the Jinchuan magma. Our new sulfur isotope data expand the range of δ³⁴S for the Jinchuan deposit significantly. The new range is from −7.6 to 3.0‰, with an average of −1.8‰, which is generally lower than the mantle value (0 ± 2‰). The new result supports the premise that crustal sulfur was involved in the genesis of the Jinchuan deposit. Very high, crustal-like S/Se ratios (as high as 8080) for some of the samples from the deposit provide additional support for the interpretation. The country rocks in the vicinity of the Jinchuan deposit analyzed to date have δ³⁴S values varying from −4.0 to 11.3‰, with an average of 2.9‰, which is higher than both the mantle value and the average value of the Jinchuan deposit, suggesting that the Jinchuan magma acquired some crustal sulfur at depth, likely concurrent with the siliceous assimilation. Numerical modeling of δ³⁴S-S/Se of sulfide ores and country rocks further illustrates that the observed variations of δ³⁴S and S/Se ratios are related to the assimilation of S-rich rocks located at depth, followed by progressive dilution of the contaminated δ³⁴S-S/Se signature. Based on the new results, we conclude that contamination with siliceous crustal materials as well as addition of crustal sulfur at depth played a critical role in triggering sulfide saturation in the Jinchuan magma.

中文翻译：

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各类地壳污染在金川岩浆镍铜铂族元素矿床成因中的作用：新的矿物学和CS-Sr-Nd同位素约束

两个对立的研究小组认为，金川岩浆中​​地壳硫的添加或与碳酸盐同化相关的岩浆氧化是岩浆中硫化物饱和的主要原因。为了解决这一争议，我们对金川岩浆镍铜铂族元素（PGE）硫化物矿床进行了综合矿物学和CS-Sr-Nd同位素研究。接触带中出现纯大理石、橄榄石大理石、蛇纹石大理石和混合岩。含方解石异晶的金川含硫化物超镁铁质岩石的δ ¹³ C_{碳水化合物}值为 –4.7 至 –2.9 ‰，与大理岩及伴生杂晶岩的 δ 13 C 碳水化合物值相似或略低（–3.9 至 0.8 ‰），但显着高于不含方解石的侵入岩（–9.3 至–8.0 ‰）。这与混合岩和一些侵入岩中的方解石异晶和硅酸钙矿物一起表明，在岩浆-碳酸盐相互作用过程中发生了碳酸盐同化。_{反应区产生的CO 2}气体只有不到几百ppm能够加入到金川岩浆中​​。如此微量的额外CO ₂很快被岩浆中质量大得多的FeO所淹没，导致岩浆中FeO/Fe ₂ O ₃几乎没有变化，从而导致岩浆氧化还原态的变化可以忽略不计。从含方解石 (~QFM+0.7) 和方解石贫瘠 (~QFM+0.6) 侵入岩的相似 fO2 计算值可以明显看出这一点。新结果排除了金川岩浆中​​硫化物饱和是由原位碳酸盐同化作用引起的可能性。本研究和之前研究的 Sr-Nd 同位素数据与深层硅质地壳物质高达 20% 的大量污染一致，随后金川岩浆发生少量原位碳酸盐同化。我们的新硫同位素数据显着扩大了金川矿床的 δ ³⁴ S范围。新的范围为-7.6~3.0‰，平均值为-1.8‰，普遍低于地幔值（0±2‰）。新结果支持了地壳硫参与金川矿床成因的前提。矿床中的一些样品具有非常高的类似地壳的 S/Se 比率（高达 8080），为解释提供了额外的支持。目前分析的金川矿床附近围岩的δ ³⁴ S 值在-4.0~11.3‰之间变化，平均值为2.9‰，高于地幔值和金川矿床的平均值，这表明金川岩浆在深处获得了一些地壳硫，可能与硅质同化同时发生。^{δ 34}的数值模拟硫化物矿石和围岩的 SS/Se 进一步说明，观察到的 δ ³⁴ S 和 S/Se 比值的变化与位于深处的富硫岩石的同化有关，随后受污染的 δ ³⁴ S-S/Se逐渐稀释签名。基于新的结果，我们得出结论，硅质地壳物质的污染以及深部地壳硫的添加在引发金川岩浆中​​的硫化物饱和方面发挥了关键作用。