From Source Rock to Cinnabar – How the Giant Mercury Deposits in Earth’s Crust Formed

The largest concentrations of Hg on Earth exist as giant deposits of cinnabar (HgS). How such enrichments of Hg formed, based on its known crustal abundance has never been fully resolved, nor has the source(s) of Hg been unequivocally established. Hg isotopes were used to elucidate crustal processes leading to the concentration of Hg during thermal maturation of Hg and organic matter enriched sediments and cinnabar formation. Mass dependent fractionation (MDF) of Hg isotopes shows remarkable enrichment of 202 Hg in cinnabar relative to its upper mantle source. Two mechanisms contribute to this enrichment: one is the low temperature, early diagenetic loss of volatile 198 Hg 0⁠ (g) to an extant gas phase; the other is oxidation during cinnabar deposition. Loss of 198 Hg 0⁠ (g) results in 202 Hg enrichment of Hg in residual organic matter in source sediments. Evidence for this significant loss of 198 Hg 0⁠ (g) is observed as large depletions in the δ 202 Hg isotopic composition of proximal gas condensate liquids in high pressure – high temperature (HP/HT) reservoirs in the central North Sea (CNS). Migration of hydrocarbons and formation brines from Hg-enriched sediments transports reduced Hg 0 (org, aq) to the site of cinnabar deposition, where oxidation of Hg 0 (org, aq) and H 2 S further enhances enrichment of 202 Hg in cinnabar. The large changes in MDF are independent of mass independent fractionation (MIF) of mercury isotopes. Approximately 80% of the cinnabar samples examined in this study plot within ± 0.1‰ of the origin on a Δ 199 Hg - Δ 201 Hg MIF Hg isotope plot and have a Hg isotopic composition similar to that of continental flood basalts (CFB), consistent with an upper mantle source for Hg. MIF trends defined by coals and euxinic sediments on Δ 199 Hg - Δ 201 Hg MIF plots have Δ 199 Hg /Δ 201 Hg slopes ~1. These tend to be the most reduced Hg-enriched sediments, deposited in anoxic or euxinic environments in which the dominant Hg species is Hg 0 . In open marine environments the dominant Hg species is likely to be Hg 2+ . Δ 199 Hg /Δ 201 Hg slopes >1deviating from these reduced sediment trends appear to be controlled by the fugacity of H 2 S ( f H 2 S), and variable proportions of reduced Hg 0 to oxidized Hg 2+ in progenitor sediments, reflecting their environments of deposition and redox state.