Intracontinental and arc-related hydrothermal systems display distinct δ202Hg and Δ199Hg features: Implication for large-scale mercury recycling and isotopic fractionation in different tectonic settings

Abstract

Mercury isotopes display both mass-dependent and mass-independent fractionation and allow the tracing of pathways and storage of surface-derived Hg in the lithosphere. While the subduction-related orogenic recycling of Hg from marine reservoirs into hydrothermal systems in continental arc settings has been documented recently, the source of Hg in intracontinental hydrothermal systems remains unclear. We measured Hg isotopes in two intracontinental anorogenic/postorogenic Late Mesozoic hydrothermal gold deposits in the South China craton and the Central Asian orogenic belt of northern China, respectively. The ore and sulfide samples from the studied systems have positive δ202Hg (0.70 ± 0.39‰, 1SD, n = 49) and negative Δ199Hg values (−0.12 ± 0.05‰, 1SD, n = 49). These values are different from their country rocks and regional geological environment (volcanic arc granites, marine sedimentary rocks) which have positive Δ199Hg values, but similar to that of their Precambrian supracrustal basement rocks of largely non-marine continental materials. We conclude that Hg in the intracontinental hydrothermal systems was leached from basement rocks by upper crustal basinal fluid circulation driven by regional heat flow, likely due to lithospheric thinning and upwelling of the asthenosphere in the Late Mesozoic. The intracontinental hydrothermal systems and their continental sources with positive δ202Hg and negative Δ199Hg values are complementary to volcanic-arc and marine sedimentary rocks with opposite δ202Hg - Δ199Hg compositions. The distinct Hg isotopic features of hydrothermal systems in different tectonic settings, in particular the indelible Δ199Hg signature, allow the tracing of large-scale material cycling in the Earth.