Data and supporting information for: 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 202Hg in cinnabar relative to its upper mantle
source. Two mechanisms contribute to this enrichment: one is the low
temperature, early diagenetic loss of volatile 198Hg0(g) to an extant gas
phase; the other is oxidation during cinnabar deposition. Loss of
198Hg0(g) results in 202Hg enrichment of Hg in residual organic matter in
source sediments. Evidence for this significant loss of 198Hg0(g) is
observed as large depletions in the δ202Hg 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 Hg0(org, aq) to the site of cinnabar deposition, where oxidation
of Hg0(org, aq) and H2S further enhances enrichment of 202Hg 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
Δ199Hg - Δ201Hg 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 Δ199Hg - Δ201Hg MIF plots have Δ199Hg /Δ201Hg 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 Hg0. In
open marine environments the dominant Hg species is likely to be Hg2+.
Δ199Hg /Δ201Hg slopes >1 deviating from these reduced sediment
trends appear to be controlled by the fugacity of H2S (fH2S), and variable
proportions of reduced Hg0 to oxidized Hg2+ in progenitor sediments,
reflecting their environments of deposition and redox state.