Sulfide oxidation as a process for the formation of copper-rich magmatic sulfides
Sulfide oxidation as a process for the formation of copper-rich magmatic sulfides
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DOI：
10.1007/s00126-012-0420-9
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发表时间：
2013-10
影响因子：
4.8
通讯作者：
C. Wohlgemuth-Ueberwasser;R. Fonseca;C. Ballhaus;J. Berndt
中科院分区：
文献类型：
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作者：
C. Wohlgemuth-Ueberwasser;R. Fonseca;C. Ballhaus;J. Berndt
典型的岩浆硫化物以磁黄铁矿和镍黄铁矿为主，含少量黄铜矿，这些硫化物的整体原子Cu/Fe比通常小于1。然而，也有罕见的以富铜硫化物为主的岩浆硫化物矿点（例如，黄铜矿、蓝辉铜矿和黄铜矿，有时与金属Cu共存），原子Cu/Fe高达5。通常，这些类型的硫化物组合出现在中等至高度分馏的层状镁铁质-超镁铁质侵入体的上部，一个著名的例子是Skaergaard侵入体中上带的Pd/Au礁。提出的过程，以解释为什么这些硫化物是如此异常丰富的铜，包括分离结晶的Fe/（Ni）单硫化物和富铜流体的岩浆后的渗透。在这方面的贡献，我们探索和实验评估第三种可能性：富铜岩浆硫化物可能是岩浆氧化的结果。以FeS为主的含Ni/Cu硫化物在开放和封闭体系中均在不同氧逸度下达到平衡。我们的研究结果表明，硫化物熔体的Cu/Fe比的增加作为氧逸度的函数，由于优先转化为FeO和FeO 1.5的FeS，和电阻的Cu 2S被转换成氧化物组分，即使在氧逸度特性的硫化物/硫酸盐过渡（以上FMQ + 1）。这种现象将导致硫化物液体的金属/S比增加，并且还将降低其液相线温度。因此，在岩浆硫化物复合物中的硫化物液体下降线的任何建模都需要解决这个问题。
Typical magmatic sulfides are dominated by pyrrhotite and pentlandite with minor chalcopyrite, and the bulk atomic Cu/Fe ratio of these sulfides is typically less than unity. However, there are rare magmatic sulfide occurrences that are dominated by Cu-rich sulfides (e.g., bornite, digenite, and chalcopyrite, sometimes coexisting with metallic Cu) with atomic Cu/Fe as high as 5. Typically, these types of sulfide assemblages occur in the upper parts of moderately to highly fractionated layered mafic–ultramafic intrusions, a well-known example being the Pd/Au reef in the Upper Middle Zone of the Skaergaard intrusion. Processes proposed to explain why these sulfides are so unusually rich in Cu include fractional crystallization of Fe/(Ni) monosulfide and infiltration of postmagmatic Cu-rich fluids. In this contribution, we explore and experimentally evaluate a third possibility: that Cu-rich magmatic sulfides may be the result of magmatic oxidation. FeS-dominated Ni/Cu-bearing sulfides were equilibrated at variable oxygen fugacities in both open and closed system. Our results show that the Cu/Fe ratio of the sulfide melt increases as a function of oxygen fugacity due to the preferential conversion of FeS into FeO and FeO1.5, and the resistance of Cu2S to being converted into an oxide component even at oxygen fugacities characteristic of the sulfide/sulfate transition (above FMQ + 1). This phenomenon will lead to an increase in the metal/S ratio of a sulfide liquid and will also depress its liquidus temperature. As such, any modeling of the sulfide liquid line of descent in magmatic sulfide complexes needs to address this issue.