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Mercury from mineral deposits and potential environmental impact James J. Rytuba

of the world’s production has come from just five mercury mineral belts. With the increasing recognition of environmental problems associated with the use and release of mercury from industrial and mining processes, production from mercury deposits has steadily decreased since the early 1980s. As a result of lower demand and prices, most of the largest mercury deposits are closed or continue to have limited production. However, mercury continues to be produced as a by-product from other ore deposit types, principally from several types of gold–silver and massive sulfide deposits. With the continuing effort to limit the release of mercury to the global atmospheric mercury pool and aquatic environments, by-product mercury production from several ore deposit types that are enriched in mercury is likely to increase in the future. This by-product source of mercury is likely to become increasingly important as the production from primary mercury deposits continues to decrease. This paper reviews the distribution, production, and mercury speciation of ore deposit types that contain significant mercury concentration and discusses the relative importance of these deposits for producing by-product mercury. The geochemical processes that contribute to Keywords Environmental impact Æ Mercury Æ environmental problems associated with mercury deposits Mining is reviewed and used to evaluate the potential environmental impact of mercury-enriched ores for which more limited information is available. Several authors have reviewed the environmental impact of mercury introduced during the amalgamation process in the recovery of gold Introduction and silver, most recently by Hylander (2001). Although both placer and lode gold deposits are a significant Mercury has primarily been produced from mercurybearing ores in which mercury is the primary and often source of mercury to the environment, this paper only ore metal constituent. The total world production of focuses on ore deposits in which mercury is a natural 20 million flasks (a flask equals 34.474 kg) of mercury has component and has not been introduced during ore largely come from such deposits and nearly three quarters processing. Abstract Mercury deposits are globally distributed in 26 mercury mineral belts. Three types of mercury deposits occur in these belts: silica–carbonate, hot-spring, and Almaden. Mercury is also produced as a by-product from several types of gold–silver and massive sulfide deposits, which account for 5% of the world’s production. Other types of mineral deposits can be enriched in mercury and mercury phases present are dependent on deposit type. During processing of mercury ores, secondary mercury phases form and accumulate in mine wastes. These phases are more soluble than cinnabar, the primary ore mineral, and cause mercury deposits to impact the environment more so than other types of ore deposits enriched in mercury. Release and transport of mercury from mine wastes occur primarily as mercury-enriched particles and colloids. Production from mercury deposits has decreased because of environmental concerns, but by-product production from other mercuryenriched mineral deposits remains important.

Received: 29 October 2001 / Accepted: 6 May 2002 Published online: 2 August 2002 ª Springer-Verlag 2002 J.J. Rytuba US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA E-mail: [email protected] Tel.: +1-650-3295418 Fax: +1-650-3295373

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Environmental Geology (2003) 43:326–338

Mercury deposits and mineral belts Mercury deposits are globally distributed in several mercury mineral belts that are composed of mercury deposits, occurrences, and areas of altered country rock that contain elevated concentrations of mercury (Fig. 1). Within each mineral belt the mercury deposits are co-genetic and reflect similar tectonic and volcanic processes that contributed to the concentration of mercury. For example, the California Coast Range mercury mineral belt formed

DOI 10.1007/s00254-002-0629-5

Fig. 1 The distribution of mercury mineral belts with significant mercury production (in red) occur in a relatively few areas of the globe. Three-fourths of the global mercury production has come from five mercury mineral belts (listed in decreasing mercury production): Almaden, Idrija, Amiata (1), Huancavelica (2), and the California Coast Range (3), which contains the New Almaden and New Idria deposits. Modified from Bailey and others (1973) Original article


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along a plate boundary as the boundary changed from a convergent to a transform boundary. An increase in the regional thermal anomaly associated with this tectonic transition started in the Miocene and resulted in the development of silica–carbonate-type mercury deposits early in the evolution of the thermal anomaly, and hot-springtype mercury deposits during its waning phase (Rytuba 1996). Fifty-one mercury deposits that have individually produced more than 1,000 flasks of mercury are distributed along the 400-km length of this mineral belt. Although there are two distinct types of mercury deposits in the mineral belt, the deposits are genetically related to the same regional tectonic and volcanic processes associated with development of a transform plate boundary. The mercury deposits adjacent to the Mediterranean Basin are not all co-genetic (Fig. 1), but rather consist of mercury belts that are small in aerial extent, and are formed at different times and in different geologic settings. The most important of these is the Almaden mercury mineral belt in central Spain where over one-third of the world’s mercury has been produced (Saupe´ 1990). This mineral belt consists of 11 mercury deposits located within a small area with dimensions of 10·20 km. The mercury ore bodies have extremely high grades, up to several weight percent mercury, mostly as cinnabar, but elemental mercury is present in all the deposits and is an important ore mineral in the Las Cuevas deposit. In North America there are ten mercury mineral belts (Fig. 2). The California Coast Range and Great Basin mercury mineral belts have accounted for more than three-fourths of the mercury production in North America. Mercury continues to be produced in the California and Great Basin belts as a by-product from gold–silver deposits genetically associated with some of the mercury deposits, and from sediment-hosted gold deposits. In North America, only the mercury mineral belts in Mexico continue to produce mercury from mercury deposits. Mexico also produces mercury from the reprocessing of precious metal mine tailings where mercury was used in the amalgamation process to recover gold and silver, such as in the Zacatecas area.

type deposits, and this deposit type is well developed in the California Coast Range mercury mineral belt (Fig. 2) where two of the largest deposits of this type are localized, New Almaden and New Idria. Silica-carbonate type deposits are associated with serpentinite that has been altered to an assemblage of silicate and carbonate minerals. The serpentinite was commonly emplaced along fault zones where its relative impermeability serves to localize carbon dioxide-rich fluids from which the mercury deposits formed. The mercury ores are hosted in silica–carbonate-altered serpentinite and adjacent sedimentary rocks. Hot-spring-type mercury deposits occur in most of the world’s mercury mineral belts and these deposits comprise most of the deposits in the Great Basin belt including McDermitt, the largest of this type (Fig. 2). This deposit type can be spatially associated with silica–carbonate type deposits where it may overprint the earlier formed silica– carbonate mercury ores. Hot-spring-type deposits formed in near-surface environments where meteoric-dominated geothermal systems developed in and adjacent to intermediate to felsic volcanic centers (Cox and Singer 1986; Rytuba 1996). Mercury ores are hosted in hot-spring sinter and associated sedimentary and volcanic rocks that have been silicified and altered to a clay alteration assemblage. Many of the hot-spring-type mercury deposits are geologically young and thermal fluids are often associated with these deposits, some of which are still actively depositing mercury and iron sulfides. Cinnabar is the main ore mineral in each of the three mercury deposit types with only a few exceptions. These exceptions include elemental mercury as the primary ore in some Almaden-type and silica–carbonate-type deposits; mercury sulfates and chlorides predominating in some silica–carbonate deposits; and corderoite (Hg3S2Cl2), schwartzite [(HgCuFe)12Sb4S13], and livingstonite (HgSb4S7) as the dominant mercury minerals in some hot-springtype deposits.

Mineral deposits enriched in mercury The presence of anomalous concentrations of mercury in many types of mineral deposits has long been recognized and used in the geochemical exploration for these ores (Rose Mercury deposit types and others 1979). Mercury geochemical dispersion patterns Three types of mercury deposits can occur in mercury in sediments, soils, and air have been successfully used to mineral belts: Almaden type, silica–carbonate type, and find mineral deposits, including several types of large gold hot-spring type. The Almaden-type primarily occurs in the and base metal deposits (McCarthy 1972; Sillitoe 1995). In Almaden mercury mineral belt in central Spain (Fig. 1), some of these mineral deposits, the mercury content has and only a small number of Almaden-type deposits occur been sufficiently high to permit recovery of mercury as a byoutside this belt. These deposits are primarily localized in product (Table 1). The mineral deposit types discussed in Silurian quartzite adjacent to submarine mafic craters (Hernandez 1985). One of these deposits, Las Cuevas, is Fig. 2 localized within a submarine caldera and some of the smaller deposits are localized in mafic dikes that are as- Distribution of mercury mineral belts in North America. Production of mercury from silica–carbonate type mercury deposits has sociated with these mafic volcanic centers (Rytuba and accounted for more than half of the 4 million flasks of total others 1988). These deposits are representative of the Al- production from North America. Two of the largest deposits of this maden type of mercury deposit (Cox and Singer 1986). type, New Almaden (A) and New Idria (B), occur in the California Silica-carbonate type deposits are more widely distributed Coast Range mercury mineral belt. Hot-spring-type mercury deposits, in the world’s mercury mineral belts. In North America, such as McDermitt (C) are the most common deposit type in the other mercury mineral belts. Mercury occurrence (yellow) represents a the largest production has come from the silica–carbonate single mercury deposit

b

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this study are those defined in Cox and Singer (1986), Bliss types: volcanogenic massive sulfide (VMS), sedimentary (1992), and Mosier and Page (1988). Significant by-product exhalative (sedex), hot-spring gold–silver, Comstock epithermal gold–silver vein, quartz–alunite gold (high sulfimercury has been produced from the following deposit


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Table 1 Mineral deposits enriched in mercury and mercury phases present

Mineral deposit type

Mercury phase and speciation

Mineral deposits that have produced by-product mercury Volcanogenic massive sulfide Sedimentary exhalitive (sedex) deposits Polymetallic base metal Hot-spring gold Comstock gold–silver High sulfidation gold–silver Sediment-hosted gold Antimony–mercury No by-product mercury production Antimony MVT Volcanogenic manganese Basaltic copper Volcanogenic uranium Bedded barite Low sulfide gold–quartz vein Porphyry copper

dation gold–silver), sediment-hosted gold, antimony–mercury, and polymetallic replacement and vein deposits. The largest by-product production has and continues to come from VMS and sedex deposits, especially those deposits that are enriched in zinc. The 27 major massive sulfide and 62 sedex deposits and mineral belts in the world are more numerous and widely distributed than the mercury deposit mineral belts (Fig. 3). Total mercury production from some of the more mercury-enriched VMS deposits, such as Boliden and others deposits in the Skellefte district, Sweden, and sedex deposits, such as Balmat, New York, (Fig. 3) has been comparable to that from moderate size mercury deposits of the hot-spring and silica–carbonate type. Although simple and disseminated antimony deposits typically have not produced by-product mercury, several antimony deposits in Tajikistan, the largest being Dhizhikrut, have and continue to produce mercury as a by-product. A new hydrometallurgic processing of antimony–mercury concentrates at the Isfara plant, Tajikistan, generates both electrolytic antimony and mercury. Other mineral deposit types contain elevated concentrations of mercury, greater than 10 lg/g, but, to date, these have not been a significant source of by-product mercury (Table 1). These deposit types include Mississippi Valley type (MVT), volcanogenic manganese, basaltic copper, volcanogenic uranium; simple antimony, porphyry copper, low-sulfide-gold-quartz, and bedded barite deposits. Among these deposits, the highest mercury concentration occurs in disseminated and simple antimony deposits and the Tajik antimony deposit. However, all of these deposit types can potentially contain sufficient mercury to be of environmental concern during the processing of ores and disposal of mine wastes. The geologic and geochemical processes that form a particular type of mineral deposit also concentrate mercury in phases that are specific to that mineral deposit type. As a result, the mercury-containing phases present in ores enriched in mercury are dependent on the type of mineral

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Hg solid solution in sphalerite (ZnS) Hg solid solution in sphalerite (ZnS) and cinnabar (rarely) Hg solid solution in sphalerite (ZnS) and cinnabar Cinnabar, Hgo, corderoite (Hg3S2Cl2) Cinnabar, corderoite Cinnabar Cinnabar, Hg in pyrite, As–Sb sulfides Cinnabar Hg solid solution in Sb sulfides, cinnabar Hg solid solution in ZnS Hg adsorbed on Fe–Mn oxides Hg–Cu amalgam Cinnabar Cinnabar Hg in sulfosalts, Au–Ag amalgam Cinnabar

deposit (Table 1). Mercury is not evenly distributed throughout these mineral deposits, but is enriched in parts of the deposit and commonly concentrated in a particular mineral phase. In VMS deposits, mercury primarily is present in solid solution within sphalerite (ZnS), which can contain up to 41.1 wt% mercury in its structure (Tauson and Abramovich 1980). The most mercury enriched VMS deposits occur in the Skellefte district, Sweden (Fig. 3) where over 85 deposits are present and commonly have ores containing from 10 to 340 lg/g of mercury (Allen and others 1997). Because mercury minerals are generally not present in VMS deposits, the high mercury content of these ores has often been overlooked. In other mineral deposit types that contain sphalerite, such as sedex and polymetallic replacement and vein deposits, mercury is also typically present in solid solution within sphalerite and only rarely as cinnabar (four polymetallic replacement deposits; Singer and others 1997). As a result of the close association of mercury with sphalerite in these deposit types, the mercury content of these sulfide ores is a function of the amount and mercury enrichment of sphalerite present. Schwartz (1997) has summarized the mercury content of zinc-rich ores and concluded that sedex deposits have the highest mercury content ranging from 27 to 1,198 lg/g mercury. The mean concentration of 92 VMS deposits is 32 lg/g, but the mercury content is strongly dependent on the age of deposit with Proterozoic age deposits having the highest mean mercury content of 196 lg/g (Schwartz 1997). Mercury is not equally distributed within the zinc-rich ores, but is selectively enriched in sphalerite deposited during distinct episodes in the ore-forming process. For example, in the Red Dog, Alaska, sedex deposit, the mercury content in sphalerite deposited early in the paragenetic sequence averages several tens of lg/g whereas late-stage sphalerite averages about 1,000 lg/g (Kelley and others 2000). Thus, the mercury content of sulfide concentrates from a particular deposit may vary considerably as various parts of the deposit are mined. In

Fig. 3 Distribution of the global volcanogenic massive sulfide (VMS) and sedimentary exhalitive (sedex) mineral belts and recently formed submarine sulfide deposits that are enriched in mercury (noted by Hg) or have produced by-product mercury (modified from Barrie and Hannington 2000; Sangster 1990) Original article


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parts of some gold-rich VMS deposits, such as Eskay Creek, Canada (Fig. 3; Roth and others 2000), mercury is also present as cinnabar, and more rarely as an amalgam with gold and silver as in the most mercury-rich deposit in the Skellefte district, Langsele, in Sweden (Nysten 1986). Mercury is commonly a trace constituent in several types of gold–silver deposits. In both hot-spring and quartz– alunite (high sulfidation) gold–silver deposits, mercury can be present in mercury phases that are similar to those present in hot-spring-type mercury deposits, primarily cinnabar, elemental mercury, and corderoite (Table 1). However, as in VMS and sedex deposits, mercury is not equally distributed throughout the gold ores, but is concentrated in parts of deposits. For example, at the Crofoot/ Lewis hot-spring-type gold deposit, Nevada, distinct mercury phases (cinnabar and corderoite) were present only in the most mercury-enriched part of the deposit and significant by-product mercury was produced when this part of the ore deposit was mined. The mercury content was much lower (0.1–219 lg/g) in the remainder of the deposit, such that distinct phases of mercury were not discernable (Ebert and others 1996). In sediment-hosted gold deposits, such as in the Carlin gold belt, Nevada, mercury is typically present as a trace constituent averaging 20 lg/g and locally as high as 640 lg/g (Li and Peters 1999). Distinct mercury-bearing phases are present and can include cinnabar, pyrite-enriched mercury, and arsenic and antimony sulfides in which mercury is present in solid solution. In ore deposit types that have not produced by-product mercury, a variety of mercury-bearing phases can be present and in some of these deposits mercury is present in solid solution in sulfide ore minerals (Table 1). More commonly, mercury is distributed throughout the ore body and not concentrated in a distinct mineral phase. In antimony deposits, mercury can be present as cinnabar and in solid solution within the antimony sulfide stibnite. In the oxidized parts of these ores, cinnabar and corderoite form as discrete phases within antimony oxides such as stibiconite and valentinite. In MVT deposits, mercury can be present in zinc-rich ores where it is present in solid solution in sphalerite. In manganese deposits, discrete phases of mercury have rarely been reported (two deposits, Singer and others 1997) and it is likely that mercury is sorbed onto iron and manganese oxides. Mercury is known to form inner sphere complexes with these phases. In basaltic copper deposits, such as in the Lake Superior basaltic copper deposits, Michigan, mercury is present as an amalgam in copper where concentrations range up to 20 lg/g in ores and several hundred lg/g in mine tailings from stamp mills (Kerfoot and others 2000). Mercury– gold–silver amalgams have also been reported from Comstock and hot-spring-type gold deposits as well as low sulfide gold quartz deposits. In both volcanogenic uranium and bedded barite deposits, cinnabar is the dominant mercury mineral present. Porphyry–copper systems typically do not have elevated mercury contents and only three porphyry copper deposit are reported to contain cinnabar (Dizon, Philippines, Andacolla, Chile, Tsagaan-Suvarga,

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Mongolia; Singer and others 1997). In the Sulphurets, Canada, porphyry copper contains an unusual association of mercury with gold–silver telluride phases. The association of quartz–alunite-type gold deposits with porphyry systems indicates that the more volatile mercury species are depleted in the high temperature porphyry part of the system and are concentrated in the superjacent, lower temperature quartz–alunite gold deposits.

Environmental impacts of mercury-enriched ore deposits The release of mercury to the environment from mineral deposits enriched in mercury can impact humans and biota through direct and indirect pathways. Direct pathways include ingestion of tailings and soils contaminated with mercury and respiration of mercury vapor and enriched particles. Ingestion is primarily a concern for children that display pica activity. Indirect pathways that impact humans are more important and include consumption of fish and, more rarely, edible plants that have been contaminated with methylmercury that has formed from biochemical transformation of mercury released from a mineral deposit. Mercury may be released from ore deposits in several ways. Mercury-mineralized areas emit mercury to the atmosphere that may significantly contribute to the atmospheric pool of mercury, or be regionally re-deposited (Gustin and others 2002). Mine wastes and contaminated soils are a potential source of particulate mercury and soluble ionic mercury species that can be transported from the mineralized site and converted to methylmercury in downstream aquatic environments. At mercury-enriched ore deposits developed by underground working, acid mine drainage may be an important source of mercury and methylmercury. Finally, various ore-processing methods, such as roasting and smelting of ores, may release mercury species to the atmosphere. Studies of mercury emissions to the atmosphere from mineralized areas have focused on primary mercury deposits (Ferrara and others 1998) and associated geothermal areas; consequently, little is known about emissions from other ore deposit types that are enriched in mercury. Ferrara and others (1991) have shown that in the Amiata mercury mineral belt, Italy (Fig. 1), soil degassing is the main source of mercury release to the atmosphere. In other large mercury mineralized areas, such as the New Idria district in California (Fig. 2), most of the mercury emissions to the atmosphere (up to 90%) come from mineralized locations with slightly elevated mercury content (up to 5 lg/g; Coolbaugh and others 2002). About a tenth of the total emissions comes from mine wastes and mining-disturbed areas with high mercury content (Gustin and others 2001). Gustin and others (2001, 2002) have estimated that emissions from the Great Basin mercury belt and associated mineralized areas in Nevada are about a third of the total mercury released by coal-fired power

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plants in the US. In these mineralized areas, a significant amount of mercury can be concentrated in the foliar portion of plants because plants uptake mercury primarily through their leaves (Gustin and others 2001). Levels of mercury up to 9.8 lg/g occur in Pinus and Cytisus growing in mercury-contaminated soils in the Amiata mercury mineral belt (Ferrara and others 1991). Inorganic mercury species predominate in mercury-enriched plants that have grown in contaminated soils in the southwest Alaska mercury mineral belt (Bailey and Gray 1997). The leaf parts contain the highest mercury concentration, up to 970 ng/g, and stems the lowest, 210 ng/g (Bailey and others 2002). Methylmercury concentrations in leaves can be significant, up to 11 ng/g, and is species dependent, with highest values in willow and lowest in alder (Bailey and others 2002). Mercury mine wastes consist of several types of material with varying mercury content and speciation that reflect the type of mercury ore deposit and processing method used. Mercury mine sites are typically small in aerial extent and consist of a furnace and a condensing system. The ores have been mined by both underground and open pit methods and were commonly processed in a rotary furnace or retort that heated mercury sulfide above its stability temperature. Elemental mercury vapor was released along with sulfur dioxide and other stack gases and dust, and this vapor was vented into a condensing system. Condensing systems consisted of a series of U-shaped pipes in which the mercury vapor was cooled. Liquid mercury was collected in a watercooled basin at the base of each U of the condensing column. Mercury-enriched dust was also collected in a cyclone prior to entering the condenser, but considerable dust, termed condenser soot, collected on the inside surfaces of the condenser. This material was periodically removed and reprocessed, or discarded along with the mine tailings, commonly termed calcines. Secondary mercury minerals formed from the mercuryenriched vapor within the furnace or retort, and in the condensing systems (Kim and others 2000). These minerals are very fine grained (micrometer and nanometer particles) and formed on surfaces of rock fragments within the furnace (or retort), as well as in the soot within the condensing system. The speciation of these secondary mercury phases is distinct from those present in the primary mercury ores, and typically more soluble. Metacinnabar, the high-temperature polymorph of mercury sulfide, is the dominant secondary mercury mineral in calcines derived from silica–carbonate-type mercury deposits because, during the roasting process, impurities introduced into the mercury sulfide crystal structure impede its reversion back to cinnabar upon cooling to ambient conditions (Kim and others 1998). In calcines derived from hot-spring-type deposits where chloride is enriched, secondary mercury phases consist of mercury chloride minerals, such as corderoite, in addition to metacinnabar and mercury sulfates and oxides (Kim and others 2000). In addition to these secondary minerals, some primary mercury minerals remain in the calcine because of encapsulation within silicate and carbonate

minerals. The concentration of mercury in calcines ranges from 10 to 800 lg/g, with higher concentrations occurring in calcines generated by retorts, which are inherently more inefficient than rotary furnaces. The highest concentration of mercury in mine wastes, up to several weight per cent, occurs in condenser soot and cyclone dust that has not been reprocessed. The mercury species in these wastes can include elemental mercury, metacinnabar, and mercury sulfates and chlorides (Kim and others 2000). Other mercury mine wastes include waste rock and low-grade ore with mercury concentrations that range from 100 to 1,500 lg/g, and mercury phases similar to that present in the primary ore. Mercury retorts and furnace operations released significant mercury vapor and dust that accumulated in soils, with concentrations of up to 30 lg/g mercury occurring in surface soils. At very long-lived mines, such as Almaden, Spain, mercury concentrations in soil can be as high as several hundred lg/g . The speciation of mercury in these soils is likely to be similar to that found in condenser soot. Soil profiles developed on serpentinite and clastic sedimentary rocks indicate that mercury species deposited on the soil surface by stack gases have been dissolved and transported downward such that anomalous mercury values extend to depths of about 50 cm (Rytuba 2002). In temperate climates where the source of mass loading is from mercury mine wastes, transport of mercury from mine sites occurs primarily in the particulate form, and most of the mercury flux occurs during peak flow events (Ferrara and others 1991; Whyte and Kirchner 1999). In arid climates, however, the transport of mercury from mine sites is limited and watersheds are not usually impacted at distances greater than 1 km from the mine (Gray and others 1999). Mercury mine wastes deposited adjacent to streams were removed by flood events, thus providing renewed space for continuous mine waste disposal during the life of the mine. The release of mercury from mine wastes is dependent on particle size because mercury concentration in calcines increases with decreasing particle size such that the fine-grained fraction of calcines may have twice the mercury concentration of the coarsest fraction (Lowry and others 2001; Shaw and others 2001). Colloidal transport of mercury from mine wastes is an important transport mechanism. Laboratory-column leaching experiments of calcines indicate that mercurybearing (>2 lg/g) colloids in the 50–400-nm size range consist of crystalline alunite–jarosite, hematite, and an amorphous silica–aluminum phase (Lowry and others 2001; Shaw and others 2001). Mercury is not present as sorbed species, but as colloidal size particles of cinnabar, metacinnabar, montroydite, and mercuric chloride (Kim and others 2001). These fine-grained mine wastes and mercury-bearing colloids can contaminate stream sediments and overbank deposits with mercury (up to 1,500 lg/g) for great distances downstream from mine sites (Gray and others 2000; Rytuba 2000). Downstream from the Idrija mine in Slovenia (Fig. 1), the world’s second-most productive mercury mineral belt, sediments in the River Soca and in the Gulf of Trieste are contaminated


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with inorganic mercury more than 50 km from the mine area (Horvat and others 1999). Sediments contaminated with tailings are a concern because they are a source of ionic mercury that can become methylated in downstream aquatic environments. For example, inorganic mercury phases in mine tailings from the Idrija mine introduced into Gulf of Trieste contribute to methylation of mercury in bottom seawater sediments (Horvat and others 1999). Mercury and methylmercury in mine drainage Mine drainage may be an important source of mercury and methylmercury from mercury-enriched ores that have been mined primarily by underground mining methods. The concentration of mercury and methylmercury in mine drainage is dependent on the type of ore deposit associated with the drainage, and whether the mine drainage has reacted with mine wastes. The reaction of mine drainage with mine wastes increases the concentration of both mercury and methylmercury in mine drainage and is an important process that controls the release and transport of both mercury species from mine sites (Rytuba 2000). The highest mercury concentration, up to 300,000 ng/l, occurs in mine drainage that has reacted with mine wastes from silica– carbonate-type mercury deposits (Fig. 4 population I) and exceeds the 12-ng/l USEPA aquatic life criteria. Mercury is primarily present in the filtered fraction of mine drainage as particles less than 0.45 lm, indicating that colloids and dissolved mercury species leached from the mine wastes predominate in the water. In contrast, mercury concentrations are considerably lower in mine drainage at the point of discharge from underground mine workings where the drainage has not reacted with mine wastes (Fig. 4 field II). The total mercury concentration has a similar range in values for both silica–carbonate and hot-spring-type deposit. Mercury can be present as dissolved species and colloids (particles