The origin of CH4-rich fluids in reduced porphyry-skarn Cu-Mo-Au systems
The origin of CH4-rich fluids in reduced porphyry-skarn Cu-Mo-Au systems
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还原斑岩-矽卡岩铜-钼-金体系中富含CH4流体的起源
DOI：
10.1016/j.oregeorev.2019.103135
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发表时间：
2019
影响因子：
3.3
通讯作者：
Jia-Hui Yan
中科院分区：
文献类型：
--
作者：
Wei Zhang;Anthony E. Williams-Jones;Cheng-Biao Leng;Xing-Chun Zhang;Wei Terry Chen;Chao-Jian Qin;Wen-Chao Su;Jia-Hui Yan
尽管地球上的大部分甲烷是生物来源的，但在各种地质环境中都发现了大量的非生物甲烷。此外，最近据报道在许多还原斑岩矿床的流体包裹体中存在甲烷。然而，人们对这种甲烷的成因仍然知之甚少。在这里，我们介绍了中国西北部塞勒特古勒还原斑岩-矽卡岩铜-钼-金矿床中岩浆溶解的富含 CH4 的水流体包裹体的研究结果。根据室温下存在的相，区分了四种类型的流体包裹体，即中密度两相水包裹体、低密度蒸汽包裹体、盐水包裹体和水液体包裹体。中等密度包裹体被捕获为单相，含有3.8-10.4mol/kg CH4，对应于ΔFMQ-1和ΔFMQ-0.5之间的氧逸度。这证明了流体的性质显着降低。 CH4 流体包裹体的 δ13C 值范围为 –29.4 ‰ 至 –19.1 ‰ (PDB)，这将这种甲烷与生物甲烷区分开来。事实上，富含 CH4 包裹体的高均质化温度（约 400°C）意味着 CH4 是直接从岩浆中溶出的。考虑到岩浆可能产生的环境，即在俯冲板块或其上方，我们认为甲烷是由俯冲带中含有机物的碳酸盐岩的反应产生的。尽管大多数斑岩系统中的铜、钼和金被解释为在氧化条件下迁移，但我们的结果表明还原的水性液体和蒸气也能够迁移相当浓度的这些金属。
Although most of the methane on Earth is of biogenic origin, significant abiogenic methane has been identified in a variety of geological environments. Moreover, the presence of methane has been reported recently in fluid inclusions from a number of reduced porphyry deposits. The genesis of this methane, however, remains poorly understood. Here, we present results of a study of magma-exsolved CH4-rich aqueous fluid inclusions in the Seleteguole reduced porphyry–skarn Cu–Mo–Au deposit, NW China. Four types of fluid inclusions, namely intermediate-density two-phase aqueous inclusions, low-density vapor inclusions, brine inclusions and aqueous liquid inclusions, have been distinguished on the basis of the phases present at room temperature. The intermediate-density inclusions were trapped as a single phase and contain 3.8–10.4 mol/kg CH4,corresponding to an oxygen fugacity between ΔFMQ − 1 and ΔFMQ − 0.5. This attests to the remarkably reduced nature of the fluid. The δ13C value of the CH4-hosted fluid inclusions ranges from –29.4‰ to –19.1‰ (PDB), which distinguishes this methane from biogenic methane. Indeed, the high homogenization temperature of the CH4-rich inclusions (~400 °C) implies that the CH4was exsolved directly from the magma. Given the environment in which the magma was likely generated, i.e., at or immediately above a subducting plate, we propose that the methane was produced by reactions involving organic matter-bearing carbonate rocks in the subduction zone. Although Cu, Mo and Au in most porphyry systems are interpreted to have been transported under oxidizing conditions, our results indicate that reduced aqueous liquids and vapors are also capable of transporting appreciable concentrations of these metals.