Magmatic-Hydrothermal Origin of Mercury in Carlin-style and Epithermal Gold Deposits in China: Evidence from Mercury Stable Isotopes

Carlin-style (sediment-hosted, "invisible" gold) and epithermal gold deposits are commonly enriched in mercury (Hg), but the source of this metal is not well-known. In the world, the association of Hg with gold is often related to magmatism; however, it is still unclear whether magmatism serves as a direct Hg source in gold deposits or only an energy source that drives hydrothermal circulation and leaches Hg from sedimentary country rocks. Here, we use Hg stable isotopes to directly indicate the source of Hg in a major Carlin-style gold deposit (Shuiyindong) and two small epithermal gold deposits (Xianfeng and Sishanlinchang) in China. In the three deposits, mineralized rocks (Au > 0.10 μg/g) are elevated in Hg levels by 1 to 3 orders of magnitude, compared to barren rocks (Au < 0.05 μg/g). The majority of the mineralized rocks in all three deposits show quite narrow ranges of δ202Hg (−1 to 0.5‰) and Δ199Hg (−0.1 to 0.1‰). The absence of significant mass-independent fractionation signals of Hg in mineralized rocks (Δ199Hg ≈ 0) is consistent with magmatic rocks, implying Hg was either leached from already crystalline magmatic rocks or directly mass transferred from a silicate magma via magmatic-hydrothermal fluid. Our reconnaissance study thus indicates a dominant origin of magmatic Hg for the three gold deposits.