[T32] 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 formation of these giant cinnabar deposits.

Hg isotopes provide compelling evidence for a genetic association of Hg in cinnabar deposits being sourced from sediments deposited during global LIP events. HgS deposits in the Terlingua mining district of SW Texas, and the New Idria Hg mine in the California Coast Range are examples of Hg recycling from organic-rich marls and tuffaceous black shales of Cenomanian-Turonian age, having a Hg isotopic signature confirming a genetic relationship to the OAE-2 LIP event.

Mass dependent fractionation (MDF) of Hg isotopes show remarkable enrichment of δ²⁰²Hg in cinnabar relative to its upper mantle source. Two mechanisms contribute to this enrichment: one is the low temperature, early diagenetic loss of volatile ¹⁹⁸Hg_(g) to an extant gas phase; the other is oxidation during cinnabar deposition. Loss of ¹⁹⁸Hg_(g) results in δ²⁰²Hg enrichment of residual Hg in source sediments. Evidence for this significant loss of ¹⁹⁸Hg_(g) is observed as large depletions in δ²⁰²Hg in proximal gas condensate liquids in high pressure – high temperature (HP/HT) reservoirs in the central North Sea (CNS).

Supporting evidence is also provided by the ratios of Δ¹⁹⁹Hg / Δ²⁰¹Hg on mass independent fractionation (MIF) isotope plots, where cinnabar from Huancavelica (Peru) has a slope of 1.97 (± 0.033; 1SE), indistinguishable from measured slopes observed in Hg⁰_(g) vaporization experiments (2 ± 0.3; 1SE). The large changes in MDF of Hg isotopes appears to be independent of MIF of Hg isotopes, which generally preserve their primary mantle Hg isotopic signature.

Hydrocarbon migration 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 δ²⁰²Hg in cinnabar.