Mercury Isotopes as Proxies to Identify Sources and Environmental Impacts of Mercury in Sphalerites

During the past few years, evidence of mass independent fractionation (MIF) for mercury (Hg) isotopes have been reported in the Earth’s surface reservoirs, mainly assumed to be formed during photochemical processes. However, the magnitude of Hg-MIF in interior pools of the crust is largely unknown. Here, we reported significant variation in Hg-MIF signature (Δ199Hg: −0.24 ~ + 0.18‰) in sphalerites collected from 102 zinc (Zn) deposits in China, indicating that Hg-MIF can be recorded into the Earth’s crust during geological recycling of crustal material. Changing magnitudes of Hg-MIF signals were observed in Zn deposits with different formations, evidence that Hg isotopes (especially Hg-MIF) can be a useful tracer to identify sources (syngenetic and epigenetic) of Hg in mineral deposits. The average isotopic composition in studied sphalerites (δ202Hgaverage: −0.58‰; Δ199Hgaverage: +0.03‰) may be used to fingerprint Zn smelting activities, one of the largest global Hg emission sources.


Figure 1. Variations of ∆ 199 Hg in different environmental samples (based on previously published data summarized in
) and sphalerites (this study). Black solid line indicates ∆ 199 Hg of 0, which represent no Hg-MIF. Gray dot lines represent the analytical uncertainty (∆ 199 Hg: ± 0.04‰).  Table S1) and sphalerites (B this study). The blue dashed line representing aqueous Hg(II) photoreduction 14 , has a slope of ~1.00. The black dashed line representing aqueous MeHg photodegradation 14 , has a slope of 1.36.
Sulphide mineral deposits are the most important Hg pool in the Earth's crust 25 . Due to the chalcophilic nature of its associations, Hg is found in abundance in hydrothermal deposits of sulphide minerals [e.g. cinnabar (HgS), sphalerite (ZnS), etc] 25 . Both Hg and zinc (Zn) belong to the IIB group in the element periodic table, and Hg has a close geochemical relationship with Zn 26 . The presence of anomalous concentrations of Hg have been observed in sphalerites 22,[25][26][27] , the most abundant form of Zn in hydrothermal deposits [28][29][30][31] . Extraction of Zn from sphalerites has received broad concerns due to the fact that Zn smelting is regarded as one of the largest anthropogenic Hg emission sources to the atmosphere 26,32,33 . From an economic geology viewpoint 26,[28][29][30][31] , four main formations of Zn deposits are categorized: sedimentary exhalative deposits (SEDEX), Mississippi Valley type (MVT), volcanic hosted massive sulphides (VMS) and intrusion related deposits (IR). Both SEDEX and MVT deposits formed from formation waters derived from sedimentary basins with high heat flows, which are characterized by the lack of igneous rocks 26,28,29,31 . The main difference between SEDEX and MVT deposits is in their depositional settings. SEDEX deposits form at or just below the seafloor 22,29,31 , whereas MVT deposits form in open spaces within carbonate platformal sequences 22,28,31 . VMS and IR deposits are common to igneous rocks, and have shown to be largely related to deep-seated intrusions of magmatic materials 26,30,31,34 . VMS deposits are mainly located in submarine divergent margins 26,30,31 , whereas IR deposits are typically found in carbonate rocks in conjunction with magmatic systems 26,31,34 . The total Hg concentration (THg) in sphalerites is highly variable, mainly controlled by deposit formations 26 . Changes in formation of Zn deposits indicate that sphalerites may be an important formation to investigate variations of Hg-MIF in deep geological settings. Meanwhile, knowing the Hg isotopic composition in sphalerites is essential to evaluating its environmental impact, including as a source signature of Hg emission from Zn smelting.
To date, only one study reported Hg isotopic distribution in sphalerites collected from Zn deposits worldwide 22 . Even though very limited number of samples (n = 7) were investigated, this study reported large variations of δ 202 Hg (− 1.41 to + 0.46‰) and minor Hg-MIF (Δ 199 Hg: − 0.12 to + 0.05‰) 22 . China has rich Zn resources and its Zn reserve ranks the second in the world 35 . In this study, sphalerites collected from 102 Zn deposits in China were measured for Hg isotopic compositions. Our data (Supplementary Table S2) show large ranges of δ 202 Hg (− 1.87 to + 0.70‰, n = 102) and Δ 199 Hg (range: − 0.24 to + 0.18‰, n = 102). The overall range of Δ 199 Hg are more than twice that reported by previous studies (Δ 199 Hg: − 0.12 to + 0.05‰, n = 7) 22 .

Mass dependent fractionation signature of Hg
Previous studies on hydrothermal ore deposit samples have reported a large range of δ 202 Hg values, attributable to MDF during vapor phase transport and venting of hydrothermal fluids 18,20,21 36 reported a similar mean δ 202 Hg (− 0.74 ± 0.11‰, σ , n = 14) for world's third largest Hg mine (Wanshan, China). Syngenetic and epigenetic Hg are the two primary sources of Hg in hydrothermal deposits 37,38 . Syngenetic Hg enters the crust through volcanoes, hot spots, and oceanic spreading centres 18 . Values of syngenetic δ 202 Hg (mean: − 0.23 ± 0.19‰, σ , n = 3) have been reported for vent chimneys from the Guaymas Basin sea-floor rift, USA 18 . Epigenetic Hg originally comes from syngenetic Hg, whereas it has undergone biogeochemical cycling in the surface environment (e.g. emission, long-range transport and deposition), and re-entered the crust through sediment diagenesis processes 37,38 . Large variations of δ 202 Hg (> 10‰) have been reported in surface reservoirs (e.g., atmospheric, soils, sediments), whereas epigenetic Hg in sedimentary rock units in California Coast Ranges revealed relatively narrow δ 202 Hg ranges (− 0.93 to − 0.17‰) with a mean value of − 0.63 ± 0.24‰ (σ , n = 15) 20 , suggestive that epigenetic Hg is a mixture of Hg from surface reservoirs. Hydrothermal fluids percolate through crustal rocks which can leach, concentrate, and transport both syngenetic and epigenetic Hg 18,37,38 , and may be the reason for similar mean δ 202 Hg values between Zn and Hg ore deposits.

Mass independent fractionation signature of Hg
The overall range of 0.42% in Δ 199 Hg values in our samples is surprisingly large, being an order of magnitude higher than the analytical uncertainty for UM-Almadén (± 0.04‰, 2σ ). Even though some sphalerites showed large uncertainties of Δ 199 Hg (up to ± 0.10‰, 2σ ), possibly reflective of the heterogeneity of Hg in the samples, 83% of the samples have uncertainties within ± 0.04‰ (2σ ). Hg-MIF has been shown to be induced by MIE 9 during photoreduction of aqueous Hg(II) and photo-degradation of MeHg processes [14][15][16][17] . Other processes [e.g., elemental Hg(0) volatilization 10,11 , equilibrium Hg-thiol complexation 12 , dark Hg(II) reduction 13 ] have also been shown to generate Hg-MIF, which has been mainly explained by the NVE 8 . Among the various processes, photochemical reactions may be of greatest importance in observed MIF, as these reactions typically generate the largest Scientific RepoRts | 6:18686 | DOI: 10.1038/srep18686 Hg-MIF. Other processes produce Hg-MIF of almost one order of magnitude lower 5,6,14 . The ∆ 199 Hg/∆ 201 Hg of 0.93 ± 0.09 (2σ ) for the sphalerites (Fig. 2) is consistent with the aqueous Hg(II) photo-reduction reported by Bergquist and Blum 14 , suggesting that Hg-MIF in sphalerites may be caused by aqueous Hg(II) photo-reductions. Other processes which show ∆ 199 Hg/∆ 201 Hg of 1.5 to 2.0 10-13 , cannot explain the Hg-MIF observed in study (∆ 199 Hg/∆ 201 Hg of ~1).

Use of Hg-MIF to trace metal sources in different types of Zn deposits
A dramatic variation in Hg-MIF was observed among different formations of Zn deposits. Hydrothermal fluids exposed to sunlight have been shown to generate Hg-MIF 18 . However, incorporation of Hg leached from sedimentary rocks with Hg-MIF may be more likely in sphalerites 22 . In our study, MVT (∆ 199 Hg: − 0.24 ~ + 0.14‰) and SEDEX (∆ 199 Hg: − 0.09 ~ + 0.18‰) deposits show large range of ∆ 199 Hg values (Fig. 1). MVT deposits are stratabound, epigenetic orebodies that occur in clusters in carbonate formations 28,31 . Sulphur and metals of MVT deposits are derived from low-temperature hydrothermal solutions formed by diagenetic recrystallization of the carbonates 28,31 . SEDEX is interpreted to have been formed by release of ore-bearing fluids into ocean water, where heavy, hot brines mixed with cooler sea water, result in the precipitation of stratiform ore 29,31 . The ore-bearing hydrothermal fluids for SEDEX deposits are deep formational brines formed during sediments diagenesis 26,29,31 . During sediment diagenesis at relative high temperatures, the metals (including Hg) liberated as pore fluid are assumed to have a considerable sulphur and metal (e.g. Hg) 26 content. Both SEDEX and MVT have no obvious spatial association with igneous rocks 29,31 . Leaching of sedimentary rocks by hydrothermal fluids then, are important sources of metals for both SEDEX and MVT deposits. As shown in Fig. 2, previous studies reported large Hg-MIF mainly in the surface of the crust, such as soil 39 (Fig. 1), which indicates that syngenetic Hg is probably the major Hg source. Similar insignificant Hg-MIF (mean ∆ 199 Hg: − 0.02 ± 0.02‰; range: 0 to + 0.04‰; σ , n = 3) has been reported for syngenetic Hg in vent chimney samples from the Guaymas Basin sea-floor rift (USA) 18 . VMS deposits are deep-seated intrusions of magmatic materials in submarine divergent margins (e.g. mid-ocean ridges and back arc rifts) 26,30,31 . Metals in VMS deposits are mainly incompatible elements which are concentrated in the fluid phase of a volcanic eruption 26,30,31 and transport of metals to VMS occurs via convection of hydrothermal fluids 30,31 . The heat supplied by the magma chamber (which sits below the volcanic edifice) can enrich the hydrothermal fluid in sulfur and metal ions 26,30,31 . Submarine volcanism and coeval chemical sedimentation may have provided a favorable setting for Hg transport and deposition. Mercury is found in abundance in VMS deposits associated with subaerial and submarine volcanism 22 . High levels of Hg concentration have been found in eclogite and peridotite in inclusions in kimberlite pipes 59 , which is thought to have a close relation with the formation of VMS deposits 22 . The IR deposits (such as skarn, manto, vein, etc) typically found in carbonate rocks in conjunction with magmatic systems, are characterized by mineral association of calcium and magnesium 22,31,34 . Similar to VMS, IR deposits have a close connection with igneous intrusions, and the ore-forming fluids are derived mainly from the igneous intrusions 22,31,34 . Ore bodies are commonly irregular in shape and may terminate abruptly at structural discontinuities 31,34 . Considering the close relation to deep-seated intrusions (e.g. volcanic and magmatic) 22,31,34 , mantle-derived Hg is believed to be most important source of Hg in VMS and IR deposits.

Implications to the geochemical cycling of Hg
A conceptual model for the geochemical cycling of Hg-MIF in different geochemical Hg pools is shown in Fig. 3. Photochemical reactions in the aquatic systems (e.g. ocean, water drops in cloud) play the foremost role in the generation of Hg-MIF 6,45 . Photoreduction of Hg(II) and MeHg impart negative Hg-MIF (Δ 199 Hg < 0) in the produced Hg(0), and therefore cause positive Hg-MIF (Δ 199 Hg > 0) in residual Hg(II) in the water phase 14 . The ocean is one of the largest Hg(0) sources to the atmosphere 1 and gaseous Hg (Hg 0 g ) represents the majority of atmospheric Hg pool 60,61 . It has a long atmospheric residence time of 0.5 to 2 years, allowing for hemispheric-to-global mixing and for transport of this metal far beyond the regions where it was emitted 1 . The biogeochemical cycling of Hg in the Earth's surface may be capable of distributing the Hg-MIF in a global scale. Tectonic movements allow for the recycling of the Earth's crustal materials, which transport Earth's surface materials to the interior crust 19 . The Hg-MIF signals observed in different formations of Zn deposits, as well as other geological Hg pools (e.g., coals, rocks, and mineral deposits), have been interpreted as reflecting the insertion of Hg-MIF generated from the Earth's surface to the interior crust. The magnitude of Hg-MIF in different geochemical reservoirs may be explained by the mixing of epigenetic and syngenetic Hg. Recycling of the Earth's crustal material has continued for billions of years, therefore, the magnitudes of Hg-MIF in Hg pools of the interior crust may allow for temporal lags between Hg-MIF generation on the Earth's surface and ultimate dilution by the syngenetic Hg. Our understanding of many key issues related to the geological cycling of Hg (e.g. the residence time and depth of the subducted Hg in the interior of the crust), may be enhanced by Hg-MIF signatures in future studies. Also, Hg-MIF may be useful in economic geology, particularly in the field of determination of metal sources in sulphide mineral deposits.

Isotopic signature of Hg in sphalerites and its environmental implications
Based on the reserve of Zn (RZn) in each deposit, and the THg and Hg isotopic composition of its sphalerite (Supplementary Tables S2 and S3), the average isotopic compositions of Hg (δ 202 Hg average and Δ 199 Hg average ) in the 102 Zn deposits may be described by:  26,32,33,62 . In a plot of ∆ 199 Hg vs. δ 202 Hg for sphalerites, Hg ores and coals (Fig. 4), most sphalerites overlap with Hg ores. ANOVA tests for δ 202 Hg (P = 0.87) and Δ 199 Hg (P = 0.57) show insignificant difference between sphalerites and Hg ores. However, most coal samples are outside the ranges of δ 202 Hg and Δ 199 Hg values for Zn and Hg ore deposits. ANOVA tests between coals and Zn/Hg ores showed significant difference in Δ 199 Hg (P = 0.03), but insignificant differences in δ 202 Hg (P = 0.80). This study implies that Hg isotopes may be useful to discriminate Hg and Zn mining from coal combustion on local, regional and global scales. Using Hg isotope to trace Hg emissions from Zn smelting requires a better understanding of how smelting processes may induce Hg isotope fractionation. Hg isotope fractionation has been observed during coal combustion 63 and ore roasting 22,23,36,64 , resulting in isotope signatures different from the parent materials. Zn smelting requires roasting of sphalerites for desulfurization, which produces waste slag and flue gas containing gaseous Hg(0) 32,33 . Roasting of sphalerites inevitably leads to Hg(0) volatilization 32,33 , and elemental Hg(0) volatilization has shown to cause relative negative δ 202 Hg in the produced Hg(0) 10,11 , which may lead to relative positive δ 202 Hg in Zn slags. Sonke et al. 22 . demonstrated MDF of + 0.4‰ in δ 202 Hg between Zn slags (δ 202 Hg: − 0.24 ± 0.71‰, 2ó, n = 4) and  Table S1). This image is drawn by R. Yin. sphalerite (δ 202 Hg: − 0.65 ± 1.33‰, 2σ , n = 4) during Zn smelting. This study does not attempt to investigate Hg isotope fractionation that is likely to occur during zinc smelting and atmospheric transport. To reveal the true Hg isotopic signature of Chinese Zn smelting, more research on Hg isotope fractionation during hydrometallurgical processing is needed.

Methods
Sample information. Details of sample location, collection, preparation and Hg concentration (THg) analysis of 100 samples have been described by Yin et al. 26 . Two additional samples collected from the Lanuoma deposit (M-24) and Zaxikang deposit (M-24) in eastern Tibet were prepared similarly 22 . Relevant information (e.g., name and type) of all the deposits are summarized in Supplementary Table S3.