The origin of CH4-rich fluids in reduced porphyry–skarn Cu–Mo–Au systems

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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 CH4 was 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.