Mercury isotopic compositions of iron oxide‑copper‑gold (IOCG) hydrothermal systems: Deep Hg cycling in intracontinental settings

Abstract

Mercury isotopes display unique mass-independent fractionation (MIF), typically expressed as Δ199Hg, which relates to photochemical reactions occurring at Earth's surface. Pronounced Hg-MIF signals have been observed in various hydrothermal systems in both convergent margins and intracontinental settings, highlighting the recycling of Hg from marine or terrestrial reservoirs into shallow continental hydrothermal systems. However, the geochemical fate of Hg in deep continental environments of intracontinental settings remains poorly understood. Iron oxide‑copper‑gold (IOCG) hydrothermal systems typically involve the circulation of fluids from both deep magmatism and continental basins, providing an opportunity to investigate this issue. Here, we present the Hg isotopic compositions of ore minerals from five representative IOCG deposits in the Kangdian region, South China. All the studied ore samples display large mass-dependent fractionation with δ202Hg of −3.23 to 1.06‰, but a relatively narrow range of Δ199Hg of −0.17 to 0.11‰. These isotopic signatures support the hypothesis of binary mixing of Hg from two sources: (1) a magmatic endmember (∼two thirds) derived from the lithospheric mantle with low δ202Hg (−3 to −1‰) and near-zero Δ199Hg, and (2) a crustal source (∼one third) originating from the basement rocks with relatively higher δ202Hg (−1 to 1‰) and negative Δ199Hg (−0.2 to 0‰). Compared to the shallow Pb-Zn-Au-Sb hydrothermal systems which formed in intracontinental basins through the circulation of fluids in the upper crust, the Hg components (and by analogy, the ore metals) in the Kangdian IOCG system are dominated by a lithospheric mantle source. Our study sheds light on deep Hg cycling and confirms the Hg isotope tracer as a tool for revealing metal sources in hydrothermal ore systems.