Current mercury deposition at Ngawha Springs, New Zealand

Currently, cinnabar is depositing at Ngawha Springs, New Zealand from Hg transported as the element. Much of the deposition occurs after rain and all ore occurs in the uppermost few metres of the ground. Field conditions and laboratory experiments demonstrate that deposition requires oxygen and that the presence of chloride enhances the oxidation of Hg. The deposition of the cinnabar proceeds according to the reactions:

Hg²⁺ + 2e^- ⇌ Hg^°                                            E° = -0.851 V and 4H⁺ + O₂(g) + 4e^- ⇌ 2H₂O                       E° = +1.229 V. Thus.                                         2Hg^° + 4H⁺ + O₂ ⇌ 2Hg²⁺ + 2H₂O          E^° = +0.378 V.

The Hg²⁺ thus formed reacts rapidly with reduced-sulphur species from geothermal and biogenic sources to produce cinnabar.

Biogenic replacement of organic material by metal sulphides is an important deposition mechanism, because it is responsible for the bulk of the richest cinnabar occurrences after marcasite pseudomorphously replaces vegetation debris adjacent to the geothermal emanations. The destruction of the pseudomorphs by oxidation yields the high concentrations of cinnabar by removing all but the resistant Hg sulphide.

Investigation of the Hg species and their concentrations in gases, waters, soils, biota and rocks showed that Hg²⁺ compounds generally dominate in solid materials, whereas elemental Hg dominates in subterranean fluids and Hg adsorbed onto particulate matter dominates most surface waters. Organomercurials and cold, dilute-acid-extractable Hg do not dominate anywhere. The source of the anamalous Hg is two sedimentary facies which underlie the Ngawha basin: the basement and the subordinate olistostrome known as the Northland Chaos Breccia.

¹⁴C dating, mass-balance analyses and flux calculations reveal that the ore deposits are nearly 6900 years old, and that about 530 kg of Hg enter the Ngawha basin and environs each year. About 44% of the Hg diffuses into the air and about 5% is carried off by fumarole gases; of the remaining Hg only about 4.5% occurs in ore-grade concentrations.

The deposition and attrition mechanisms for the Ngawha deposits (which are closely compatible with many other Hg deposits) yield explanations for the geochemical characteristics of Hg deposits generally. Nearly all characteristics are inherent in the mechanisms developed to explain the Ngawha deposits. Consequently, methods for more efficient Hg exploration and geochemical data interpretation can be suggested, as may applications for pollutions abatement and geothermal anomaly detection.