Reconnaissance field studies were made at 50 historical mining districts in northern Nevada to determine the actual or potential contamination of surface or ground water by mine drainage water, mine-waste dumps, and mill tailings. The main focus of this report is on mining areas in the central Humboldt River Basin, from approximately Elko to Winnemucca, with coverage of adjacent or analogous mining areas. More than 650 new chemical analyses of mine water, mine waste and tailings, and leachates of those materials are described and interpreted to provide Federal land-management agencies and the public with concepts regarding sources of mining-related contamination and the extent of contamination. The new observations and geochemical data suggest that contamination is not nearly as common or widespread as postulated by some in the popular press. Out of the tens of thousands of small prospects, small mines, and some large mines on public lands, less than 1 percent of the historical sites pose significant threats to water quality, human health, or wildlife habitat.

Although there are tens of thousands of small prospects in northern Nevada, these small diggings generally release no more contamination than the same rocks prior to mining. Out of the hundreds of small- to medium-sized mines on public lands, only a few release acidic mine drainage or significant concentrations of metal contaminants. Large historical mines generally are on private property (patented mining claims) and were not sampled in this study, but only a small percentage are known to produce significant contamination. The approach in this investigation was to identify the largest accessible historical mine or mines in a district for observation and sampling and to seek the worst case of likely contamination. Field observations suggest that visible indicators of acidic mine drainage (such as red, iron-rich precipitates) are rare, and field measurements of pH and chemical analyses of several kinds of materials (dump rocks, tailings, surface water) indicate that only a few sites release acid or significant concentrations of metals. The most consistent observation at and near mining areas is neutral to weakly alkaline pH values (6.5 to 8.4) and water compositions attesting to significant amounts of carbonate derived from rocks, alluvium, and caliche-bearing soils. The alkalinity of surface water is sufficient to naturally mitigate local sources of acidic water by mixing; the neutralization processes induce reactions that greatly reduce the concentrations of most metals in the evolved water. There are, however, indications of elevated amounts of arsenic, molybdenum, and selenium that are mobile in weakly alkaline water. There is some risk for these elements to be cumulatively enriched in closed basins over many years, but the magnitude of that risk is beyond the scope of this investigation.

Acidic mine water loaded with toxic metals, asserted by the popular press to be common and typical of abandoned mines, actually is rare in northern Nevada. This study over 5 years, including several unusually wet spring seasons, found only 16 draining mine adits, and those had flow rates of less than 1 gallon per minute to about 20 gallons per minute. The small number of draining mine workings and the low flow relative to other mining areas are consistent with the vegetation and precipitation of the mine areas. The relatively low metal concentrations in the evolved mine water, compared to other mining regions, reflects the generally deep oxidation of sulfide minerals and the presence of carbonate minerals in wall rocks and soils that naturally mitigate acids. The findings here are similar to those of Price and others ("Water quality at inactive and abandoned mines in Nevada," Nevada Bureau of Mines and Geology, Open-File Report 95–4, 1995), who estimated that less than about 0.05 percent of mines in Nevada create acidic, metal-rich drainage. In areas where drainage, ponds, or puddles have a pH of less than 3, the water takes on high to extremely high concentrations of many potentially toxic metals.

Mill tailings are among the largest potential sources of contaminants, in part because they typically were placed in canyons and arroyos where they are vulnerable to storm events. Reconnaissance field studies in and near the Humboldt River Basin have identified 83 mills and associated tailings impoundments related to historical mining. The majority of the mill and tailings sites are not recorded in the literature. All tailings impoundments show evidence of substantial amounts of erosion. At least 11 tailings dams were breached by floodwater, carrying fluvial tailings 1 to 10 miles down canyons and across alluvial fans. Most of the tailings sites are dry most of the year, but some are near streams. Tailings that are wet part of the year do not appear to be reacting significantly with streamwater because physical factors such as clay layers and hardpan cement appear to limit permeability and release of metals to surface water. The major effect of mill tailings on surface-water quality may be brief flushes of runoff during storms events that carry acid and metals released from soluble mineral crusts. Small ephemeral ponds and puddles that collect in trenches and low areas on tailings impoundments tend to be acidic and extremely enriched in metals, in part through cycles of evaporation. Ponded water that is rich in salts and metals could be acutely toxic to unsuspecting animals. Rare extreme storms have the potential to cause catastrophic failure of tailings impoundments, carry away metals in stormwater, and transport tailings as debris flows for 1 to 10 miles. In most situations these stormwaters and transported tailings would affect wildlife but few or no people or domestic water wells. Because all identified historical tailings sites are several miles or more from the Humboldt River and major tributaries, tailings probably have no measurable effect on water quality in the main stem of the Humboldt River.

The processes that create acidic, metal-rich water in Nevada are the same as for other parts of the world, but the scale of transport and the fate of metals are much more localized because of the ubiquitous presence of caliche soils. The arid climate and associated soils of Nevada minimize the scale of acid-mine drainage that is common in other historical mining areas. Water quality in even the worst cases is naturally attenuated to near or below regulatory standard levels within 1 mile of the source. However, some unconventional processes that operate in Nevada need further study. Brief storms can dissolve mineral crusts on mine waste and tailings to create a "first flush" of runoff that could be harmful to wildlife or livestock. The prevailing alkaline water in this climate is capable of transporting significant concentrations of a suite of metals and metalloids that includes arsenic, molybdenum, selenium, and uranium either into local basins or sinks such as the Humboldt Sink, where they can become concentrated to toxic levels by plants or by evaporation. Careful study is needed of the multiple sources, transport, and fate of these metals.

Introduction

Mining for more than 140 years in Nevada has produced a rich history of people and towns, bonanzas and busts. One of the legacies is inactive and abandoned mines, estimated to number between 225,000 and 310,000 (Price and others, 1995). Some say that the abandoned mines are the source of many contaminants and leak "poisons into the West's water" (Watkins, 2000), but field investigations indicate that a very small percentage (possibly 0.05 percent, Price and others, 1995) are sources of significant acid-mine drainage and related contaminants. What is the effect of historical mines on the environment, especially on the precious water resources of these arid lands? Reconnaissance geochemical studies of 50 historical mining areas in and near the Humboldt River Basin were undertaken by the author to provide objective information to help guide Federal land management agencies (FLMA's) in their response to requirements of the Clean Water Act and other national laws for water and environmental quality. The goal of this investigation is to use knowledge of mineral resources to predict where geochemical hazards may be caused by historical mining (Plumlee and Nash, 1995; Plumlee and others, 1999); to observe and sample those predicted sites; and to evaluate the new information and chemical results to understand processes that do or do not mobilize metals in the surficial environment. Descriptions of mining districts in this report focus on the central part of the Humboldt River Basin, but discussions later in the report use field observations and geochemical information for other parts of the basin and adjacent mining areas of northern Nevada.

The study area comprises the Humboldt River Basin and adjacent areas (fig. 1), with emphasis on mining areas relatively close to the Humboldt River (fig. 2). The basin comprises about 16,840 mi²or 10,800,000 acres. The mineral resources of the Humboldt Basin have been investigated by many scientists over the past 100 years, but only recently has our knowledge of regional geology and mine geology been applied to the understanding and evaluation of mining effects on water and environmental quality. The investigations reported here apply some of the techniques and perspectives developed in the Abandoned Mine Lands Initiative (AMLI) of the U.S. Geological Survey (USGS), a program of integrated geological-hydrological-biological-chemical studies underway in the Upper Animas River watershed in Colorado and the Boulder River watershed in, Montana (Buxton and others, 1997; Nimick and von Guerard, 1998). The goal of my studies of sites and districts is to determine the character of mining-related contamination that is actively or potentially a threat to water quality and to estimate the potential for natural attenuation of that contamination. These geology-based studies and recommendations differ in matters of emphasis and data collection from the biology-based assessments that are the cornerstone of environmental regulations. If the reconnaissance studies or interpretations reported here raise questions or concerns, followup work should be done using trace-element sampling protocols and more precise analytical methods. Additional studies, such as biological populations, may also be warranted by my reconnaissance studies.

Acknowledgments. Discussions with the Bureau of Land Management (BLM) staff in district offices of northern Nevada provided helpful guidance regarding priorities on public lands. Steve Brooks, formerly of BLM's Winnemucca District Office, provided very helpful suggestions and technical information on abandoned mine sites. John Gray, Bill Miller, Lisa Stillings, and Alan Wallace1 of the USGS offered helpful guidance during this project, and Steve Smith and Helen Folger, USGS, provided very helpful reviews of draft versions of this report. Errors of fact or interpretation that are in this report are the responsibility of the author; comments from persons with new or divergent information will be appreciated at tnash@usgs.gov.

Study Methods

Field work for this study spanned more than 10 visits from September 1995 to June 2000. I also had visited many of the districts in previous years for investigations pertaining to mineral resource assessments. Sites visited and sampled were selected using published information, USGS records in the Mineral Resources Data System (MRDS), information on USGS 1:24,000-scale topographic maps, and by accessibility. Sample localities were recorded on the topographic maps and measured with a conventional GPS instrument having an accuracy of about ±100 to 150 ft based on tests at known sites. Analyzed samples are described in a spreadsheet file (Appendix 1), which also gives locations in latitude and longitude.

Solids Chemistry

Solids samples were collected from mine dumps, outcrops, and mill tailings. In most cases the intent was to collect a representative sample, but for some sites a select sample was collected to determine a special property. The most commonly employed sampling method for dumps and tailings involved collecting numerous small portions at 20 to 30 subsites to derive a composite sample. The fraction that passed through a 2-mm stainless steel sieve was retained for analysis. This is the standard protocol developed for USGS-AMLI investigations of mine dumps (Nash, 1999; Fey and others, 2000).

Solids samples were prepared for analysis under the direction of Paul Lechler, Nevada Bureau of Mines and Geology, Reno, and analyzed by two commercial laboratories. In one method the solid samples are dissolved in a mixture of four acids, then the concentrations of 35 major and trace elements are determined using inductively coupled plasma- atomic emission spectrometry (ICP-AES); this is considered a total analysis as the strong acids dissolve all but the most refractory minerals such as zircon. This method is essentially that of Briggs and Fey (1996), although slight variations in method may exist between laboratories. Solid samples were also analyzed by a method that uses weaker acids to dissolve most minerals and then uses an organic reagent to collect 15 metals of interest (Ag, As, Au, Bi, Cd, Co, Cu, Hg, Mo, Pb, Sb, Se, Te, Tl, and Zn). Analysis is by ICP-AES; results are very similar to those of Motooka (1990) but include five additional elements. This second analytical method has lower levels of determination and works well for some elements, such as Hg, Sb, Se, Te, and Tl, that are not effectively determined by the total digestion method. Total sulfur, determined by combustion, was done on some samples but discontinued because the method does not discriminate among forms of sulfur (such as sulfide and sulfate) and thus does not provide as much information as desired for topics such as acid-generating potential. Quality-assurance monitoring by Lechler shows that the precision and accuracy (deviation of the reported concentrations from accepted values) is less than 5 percent for most elements. Analytical results of 232 chemical analyses of solid materials are summarized in table 1; complete results are in a spreadsheet file (Appendix 2) that is included with this report.

The analytical methods just described are useful for characterizing the chemical composition of solid materials, but those analyses generally do not indicate what elements may be soluble (mobile) in the near-surface environment. Another method was used to estimate the mobility of metals, described in the "Leachate Chemistry" section.

Water Chemistry

Water samples were collected after testing for pH and conductivity with portable instruments. The methods are simplified from those described by Ficklin and Mosier (1999); no chemical determinations were made in the field. The pocket-sized conductivity meter (Corning CD-55), with an upper limit of 2,000 microsiemens per centimeter (μS/cm), responded consistently and showed no drift after calibration. The pH meter (Orion 250), with built-in temperature electrode, required frequent calibration during the day, and at most sites the calibration was checked on a standard solution after the field measurement. The field standards were buffered solutions of pH 4.0, 7.0, and 10.0. The field measurements of pH are considered to carry an uncertainty of about ±0.1 standard units, even though the meter reports to 0.01 unit. Water samples for chemical analysis were collected by a consistent technique adopted for reconnaissance investigations. The water was collected with a disposable 60-mL syringe, then pushed through a disposable 0.45-micrometer cellulose filter. The syringe and the 60- or 120-mL polyethylene bottle were rinsed twice in the sampled water collection. The filtered sample was acidified to a pH of about 2 at the site with 5 drops of ultrapure 1:1 nitric acid (HNO₃) per 60 mL. The acid stabilizes most metals but is not effective for mercury. Regrettably, no reliable mercury determinations can be made with this sampling method because it adsorbs onto the container (J.E. Gray, oral commun., 1997). At selected localities where results for Hg were needed, a special aliquot was collected: filtered into a glass bottle to which had been added an appropriate amount of dichromate solution to stabilize the Hg before analysis (Crock, 1996). At appropriate localities (with pH greater than 4) an unfiltered sample was collected for determination of alkalinity. Laboratory and field blank tests using deionized water indicated that contamination introduced by the sampling procedure and equipment is in the low parts per billion range (1–10 ppb), which is considered adequate in the search for metal concentrations orders of magnitude greater than the sampling error. Because the goals of this reconnaissance were to determine which metals are significantly enriched in surface water far above so-called "trace levels," costly and time-consuming trace-element protocols of the USGS or U.S. Environmental Protection Agency (USEPA) were not followed. The filtered and acidified water samples were analyzed by a commercial laboratory in Denver, Colo., within 20 days of collection, using ICP-MS (inductively coupled plasma-mass spectrometry); the general method is described by Crock and others (1999). Data for more than 60 elements are reported, generally to levels less than 1 ppb, but in this report only 10 to 15 elements of prime environmental concern will be discussed. Quality assurance is described in Nash (2000b). This analytical method is appropriate for determination of many metals of potential concern and expected to have a wide range in concentrations, and the method is especially good for trace elements in water such as As, Cu, Pb, and Zn (Crock and others, 1999). If water compositions reported here are a concern, followup work should be done using trace-element sampling protocols and more precise analytical methods.

Analytical results for surface-water samples were reported previously (Nash, 2000b) and are included in a spreadsheet file in this report (Appendix 3).

A summary of analytical results for 275 surface-water samples collected at 241 sites (table 2) shows the wide range of metal compositions present in samples from the study area. Some surface-water samples from draining mine adits or mill tailings impoundments have extremely high metal concentrations that contrast with most stream-water compositions and are notably higher than concentrations in surface water from areas with little mining (termed background). Acidic water tends to contain high to extremely high concentrations of base metals such as Cd, Cu, Fe, Mn, Pb, and Zn, but other metals such as Al, As, Se, and U can also be enriched. Evaporation causes some elements such as As, Co, Cr, and Ni in some unusual settings to become unusually concentrated. These and other trends in water chemistry are discussed in later sections.

Leachate Chemistry

The chemical analyses for solids described herein are not always appropriate for environmental characterization because they describe total metal rather than mobile (soluble) metal concentrations. To determine the mobility of metals, as well as the tendency of a material to generate acid, a passive leach test (Nash, 1999; Fey and others, 2000) was used. Solid materials from dumps, mill tailings, and outcrops were processed in the laboratory by a leach method that provides a measure of reactions in nature, such as during weathering or storm events. The method is elegantly simple: 100 g of rock sample is placed in a beaker with 2,000 mL of deionized water, stirred slightly, and an initial pH measured. After about 20 hours, the solution is stirred slightly to mix the leachate solution. At 24 hours the acidity (pH) and conductivity of the leachate are measured, and a 60-mL portion (aliquot) is taken with a syringe and passed through a 0.45-micrometer filter. The sample is acidified with 5 drops of ultrapure 1:1 nitric acid (HNO₃) to stabilize metals in the solution chiefly to minimize adsorption on the plastic bottle or formation of precipitates. The leachate is analyzed by the same ICP-MS method as for water samples collected in the field. This analytical method is well suited for these solutions with highly variable composition. The analytical results for 158 leachate samples, summarized in table 3, resemble those for surface water degraded by mine waste. However, unlike the water analyses that show the results of rock interactions, the leachate results show the potential to generate those compositions actual behavior is more complex and involves factors such as permeability, kinetics, and climate. The methods and applications are described in more detail elsewhere (Nash, 1999; Fey and others, 2000; Nash, 2002a).

Analytical results for the leachate solutions, determined by ICP-MS, are in a spreadsheet file included with this report (Appendix 4).

Geologic and Geographic Setting

Geology of the region has important ramifications. Carbonate-bearing units (limestone, dolomite, calcareous shale) are common in several mountain ranges in the region. These rocks provide excellent acid-neutralizing capacity, and clasts of these lithologies in stream deposits (alluvium) extend those properties to near-surface materials in contact with surface water. In addition, caliche (a soil component rich in calcium carbonate) is abundant in the matrix and as layers in most alluvium in the region, providing another highly reactive source of acid-neutralizing capacity. Structures, particularly relatively young basin-and-range-type normal faults, have uplifted the mountain blocks, promoting deep oxidation of ore deposits in many places and greatly diminishing the potential for acidic drainage because sulfide minerals were destroyed prior to mining. In a few rare places, the uplift is such that mechanical erosion exceeds chemical weathering, allowing sulfidic rocks to exist near the surface and create acidic drainage. This is perhaps best shown in the National and Hilltop districts described later. Placer deposits of gold in many of the districts demonstrate that gold-bearing deposits were exposed at the surface thousands of years ago, thereby providing one line of evidence for pre-mining conditions in the region. Outcrops of altered rocks, as well as the placer gold deposits, demonstrate that not all acidic, metal-rich surface water is caused by humans and mining.

Climate and topography are important factors in the development of mine drainage. Three aspects are briefly highlighted herein.

First, mine drainage can only happen where there is enough vertical relief to cause water to flow from mine workings. Precipitation that interacts with mined materials on flat terrain may enter ground water but would not be called mine drainage as defined here. In Nevada, mine drainage is present only in the hilly or mountainous parts of the region where precipitation can enter disturbed lands either directly or through fractures (ground-water flow) and then flow out on the surface. Mine shafts do not create surface drainage, but mine tunnels driven into mountains may collect ground water and carry it to the surface (indeed, many mine tunnels were constructed originally to provide an energy-efficient means of removing water from an underground mine, and after mining ceases these tunnels continue to carry water). Water pumped from mines is geochemically similar to mine drainage but will not be considered here.

Second, precipitation is highly variable across distances as short as a mile in Nevada, and these differences can be seen in the kinds and amount of vegetation (photos 1–6) and in the amount of water at a mine site. Generalized maps of precipitation in Nevada show the effects of elevation on increased amounts of precipitation, much of that as snow (fig. 3; Houghton and others, 1975). There should be no surprise that there is little or no mine drainage at lower elevations on the flanks of ranges. Evaporation exceeds precipitation in most of the study area, which reduces the magnitude of some hydrogeochemical effects in surface water, but it enhances others. There are two especially noteworthy problems: (1) Evaporation can concentrate some toxic metals to hazardous levels in this area the elements most prone to this are As, Mo, and Se; (2) orientation also is important because it influences the amount of sun exposure and evaporation. Large differences in precipitation and evaporation can be seen during and shortly after a spring storm. Just as aspen and fir trees tend to develop selectively in north-facing basins, more water is present in mine sites with a northerly aspect. This is evident in the larger amount of water in small creeks and in mine drainage of north-facing areas of the Battle Mountain, Hilltop, and National mining districts.

A third feature related to climate is the development of alkaline water and carbonate-rich aridsols and caliche in arid areas, including Nevada (Southard, 2000). The relationship of these soils to climate was first emphasized by Jenny (1941). Carbonate as caliche is a major constituent in alluvium and soils at lower elevations of the study area. The fine-grained and porous aspects of caliche make the carbonate more reactive and available to ephemeral near-surface water than calcium carbonate in limestone. Two aspects of the caliche may be important in mining areas. One, the surface water has high alkalinity and pH values greater than 8; this water has high acid-neutralizing capacity. Second, acid-mine water that is released during wet periods of the year is effectively neutralized by reactions with caliche as the water infiltrates this alluvium.

A new aspect of water management and water budget in northern Nevada is mine dewatering at more than a dozen major open-pit mines that is unprecedented in magnitude. Numerous issues relating to drawdown, discharge, and water quality are beyond the scope of this report; abundant information can be found in the Web site of Nevada Division of Water Protection (www.state.nv.us/cnr/ndwp), in particular the Humboldt River chronology (Horton, 2001). The magnitude of pumping is difficult for most people to comprehend. Some of the water is put to beneficial uses such as irrigating alfalfa, but much must be discharged into tributaries of the Humboldt River (photo 7). Briefly, individual mines pump more than 50,000 gallons per minute (gpm), and the total discharge into the Humboldt River in the late 1990's was more than 50 percent of the historical flow in the river. Later in this report, adit flows from historical mines will be described as ranging up to about 40 gpm.

Finally, it is important to note that the Humboldt River Basin is a closed internal basin. Metals carried in the river are not flushed out to the Pacific Ocean. The ultimate fate of the Humboldt River water is the Humboldt Sink, where it evaporates (fig. 1). The dynamics of this closed basin are beyond the scope of this report, but unusual geochemical enrichments can occur. For example, there is the potential for relatively soluble metals such as As, Mo, and Se, released in daily amounts that exceed no regulations, to accumulate in both concentration and total mass in the Humboldt Sink.

Ore Deposit Types and Their Environmental Signatures

Many types of ore deposits have been located or mined in the Humboldt Basin over the past 125 years. Economic geologists use numerous classification systems to compare and summarize geologic and economic attributes of deposits (Lindgren, 1933; Cox and Singer, 1986; Guilbert and Park, 1986). If one were to use one of these classifications, more than 40 mineral deposit types would be considered. These classifications subdivide deposits according to finer points of geologic age, ore genesis, mineralogy, and additional attributes that specialists need to know to properly evaluate the economic potential (likelihood of occurrence, ore grade, ore tonnage, and ore metallurgy) of a geologic terrain or a prospect. These classifications are too specialized for non-geologists and include much detail that is not necessary for a general understanding of mine-related geochemical effects on the environment. A much simpler system will be used here that emphasizes host rock type (lithology) and ore composition (table 4), both of which play major roles in the generation and dispersion of acidic water and chemical pollutants (Nash, 2001; Nash, 2002a).

Polymetallic Deposits

Many of the mines and prospects in the Humboldt Basin are of the polymetallic type in which silver was the main economic commodity of interest. These deposits also carry substantial amounts of base-metal sulfide minerals (such as galena [PbS], sphalerite [ZnS], and chalcopyrite [CuFeS₂]), allowing some of the mines to recover copper, lead, or zinc. Gold was a byproduct in some, and as the price of gold rose in the 1970's some of these deposits became targets for gold exploration. Many of the original mines of the region worked ores of this type, but these have not been of great interest in recent years unless they contain high gold concentrations. Although the polymetallic character is generally similar among this group, economic geologists recognize that there are many differences. Some examples of this type are small, simple veins filled by quartz and sulfide minerals, formerly amenable to mining by small underground operations as in the Tenabo and Battle Mountain districts. A few were larger and deeper vein systems, as at the Pansey Lee mine, described herein. Some are relatively large vein and replacement zones in limestone, as at the original Cortez silver mine or the Arizona mine near Unionville, also described herein. Many of these deposits are part of a larger system of veins related to an intrusive body (or even a porphyry copper-molybdenum system), well illustrated by the Battle Mountain district (Theodore and Blake, 1975) or the Rochester district (Vikre, 1981).

Polymetallic deposits have the potential to create acidic mine or rock drainage from the high amounts of sulfide minerals they contain, and they have the potential to release many toxic heavy metals in those acidic waters (Plumlee and others, 1999; Nash, 2002a). Local factors tend to determine the actual magnitude of contamination from polymetallic deposits; the most important factors are the amount of precipitation, amount of pre-mining oxidation, and the acid-neutralizing capacity of host rocks. Because these factors are highly variable from district to district, or even site to site, predictions based on the polymetallic model tend to be relatively unreliable without field data.

Skarn Deposits

These deposits have been important sources of tungsten, copper, and molybdenum in Nevada. Recent technologic changes in bulk mining and bulk processing have allowed some deposits to be mined for gold that was considered uneconomic 30 years ago. Chemically and geologically, these deposits have features in common with the polymetallic class, as shown in the Battle Mountain district. They merit consideration as a separate type because many of the historical mines had large production, and current mines tend to be open-pit operations that move even larger amounts of waste rock and ore. Skarn deposits have the potential to create large amounts of metal-rich acidic drainage, but most of this is immediately mitigated by carbonate-bearing host rocks (Hammarstrom and others, 1995). An additional mitigating factor in Nevada is the tendency for mined deposits to be deeply oxidized prior to mining; thus, there is much less sulfide in the mine and in waste dumps to create acid.

Skarn deposits were observed and sampled in the Battle Mountain, Buffalo Valley, Adelaide, Railroad, and Merrimac districts. These examples of skarn deposits are rich in many base metals, especially Cu, Fe, Pb, and Zn; the combination of pre-mining oxidation that destroyed sulfide minerals and abundant carbonate minerals precludes the formation of acidic water.

Epithermal Vein Deposits

These deposits have been important sources of precious metals in many Nevada districts, including National, Ten Mile, Gold Circle, and Tuscarora. New technology and higher prices for gold have allowed many broad stockwork and breccia zones to be mined by open-pit methods in the 1980's–1990's as at Sleeper, Florida Canyon, and Mule Canyon. These deposits are generally in volcanic rocks of Tertiary age, but some compositionally similar deposits (gold and associated metals) can form in altered sedimentary rocks. Called Comstock-type and hot-spring type by some classifications (Cox and Singer, 1986), these deposits tend to have low concentrations of pyrite and base metals in northern Nevada (Nash and others, 1995), whereas epithermal veins elsewhere can be rich in base metals and grade into the polymetallic class. The metallurgical restrictions of heap-leach technology that is used to process gold ores from most open pits tends to require low pyrite content; thus, most mined deposits are those that have undergone pre-mining weathering and oxidation that destroy pyrite. However, former underground mining and conventional milling of ores could accommodate sulfidic ores, as in the National, Gold Circle, and Tuscarora districts. Sulfidic waste and tailings from the older mines pose substantially greater problems than from the oxidized, low-sulfide, bulk-mined epithermal ores (Nash and Trudel, 1996). Character of wall rock alteration is an important factor in exacerbating or mitigating drainage signatures: green kinds of propylitic alteration provide added acid-neutralizing capacity that helps mitigate any acid produced (Nash, 2002a), whereas most clay-rich (argillic) or acid-sulfate alteration adds acid and metals and there is low acid-neutralizing capacity for natural attenuation of acidity.

Metal mobilities from these deposits vary greatly according to local climate and concentrations of sulfide minerals and base metals (Plumlee and others, 1999). In the San Juan Mountains of Colorado, very acidic, metal-rich mine drainage develops from deposits that were rich in base-metal sulfide minerals and exposed to the moderately wet climate (Nash, 1999; 2002a). Deposits in Nevada have generally lower content of base metals (except for deep parts of the Comstock lode), and in many Nevada deposits deep pre-mining oxidation destroyed most sulfide minerals. Where sulfide minerals are present and buffering capacity is low, as in the National district, highly acidic water develops and carries high concentrations of many metals. In most other Nevada locales, these epithermal deposits create few concerns, as will be described later. Studies of pit lakes in mined-out epithermal gold deposits (Shevenell and Connors, 2000) show near-neutral pH values and generally low metal concentrations, except where concentrated by evaporation.

Examples of epithermal deposits were studied and sampled in the National, Gold Circle, Tuscarora, and Safford districts.

Mercury Deposits

Deposits of mercury in volcanic or sedimentary rocks have been mined at several districts in northern Nevada. These deposits have many features in common with epithermal veins, described previously, but merit special treatment because of the problems associated with high mercury concentrations. The volcanic-rock-hosted mercury deposits of the Ivanhoe district contain even lower concentrations of Cu, Pb, and Zn than the epithermal veins but similar high concentrations of As, Sb, and Se. Called "hot spring" Hg deposits by some specialists (Rytuba, 1986), these deposits are associated with large amounts of hot-springs silica sinter but relatively low concentrations of pyrite.

Environmental concerns at mercury deposits are numerous and complex (Rytuba and Kleinkopf, 1995; Gray and others, 1999; Gray, 2003). The major concerns at the Nevada mines are (1) elevated Hg in unmined rocks and soils; (2) elevated Hg in soils created by mining and retorting the ore; (3) elevated Hg created by spills at the retort; (4) elevated Hg in the retort tailings (also called calcine); and (5) formation of methyl mercury from inorganic mercury compounds methyl mercury is extremely toxic (Crock, 1996). Acid drainage is created at some mercury mines but is not known or expected at the northern Nevada sites because sulfide mineral content is low and the mines are not wet.

Mercury mines investigated here do not appear to be releasing metals to the environment beyond the areas of disturbance. The mine workings, dumps, and retort tailings could pose health problems related to dust or ingestion, but formation of highly toxic methylmercury is unlikely at these sites in Nevada (Gray, 2003); such geochemical issues are beyond the scope of this study and specific investigations are recommended to address those questions.

Sediment-Hosted Gold Deposits

Deposits of gold in sedimentary rocks, often called "Carlin-type" for the first major mine of this kind in the Lynn district (fig. 2), have become the major focus of exploration and mining in northern Nevada. The deposits tend to be mined by large-scale open-pit operations, but in recent years some are mined underground as well. The major gold mines of northern Nevada were not accessible for study and sampling, but the small, mined-out Quito mine near Austin was studied briefly as an analog deposit. Gold is recovered by a variety of highly sophisticated techniques, from cyanide sprayed on heap leaches to complex, high-temperature autoclaves (pressure cookers). Because the mines process large tonnages of ore (several million tons per year), large amounts of waste and processed ore (leach piles or ground tailings) are produced and are subject to close monitoring. Many of the large and deep mines must pump water from the underground or open-pit mines, and that water is handled carefully by the mine operators before release on the surface. Important geo-environmental aspects (Hofstra and others, 1995; Plumlee and others, 1999; Bowell and others, 2000) include (1) rocks that have moderate to high acid-neutralizing capacity from carbonate minerals; (2) fine-grained pyrite in ore and alteration zones that commonly amounts to 5 percent and locally as much as 50 percent below the weathered zone; and (3) a trace-element suite that includes substantial amounts of As, Hg, Mo, Sb, Se, and Tl, and locally Zn and Cd (but other base metals are low). Observed waters in these deposits are alkaline, indicating predominance of the acid-neutralizing capacity of carbonates over acid from sulfides, and these waters have the potential to transport the Carlin suite of trace metals, especially the oxyanion forms of As, Mo, and Se.

In the 1990's a new variety of disseminated gold deposit was recognized as having more direct genetic association with a source pluton than the Carlin-type. Called "distal-disseminated gold" (Cox, 1992; Theodore, 1998), these deposits tend to have somewhat higher concentrations of base metals than the Carlin-type, closer to the gold skarn deposits with which they share many genetic similarities. In practice, these distal deposits have weathering and other geochemical properties that are so similar to the Carlin-type that distinction generally is not required. A few examples in the Battle Mountain and Hilltop districts will be mentioned.

Environmental concerns for these deposits are quite different from other deposit types in Nevada. (1) Acid-mine or rock drainage is generally acknowledged to be a minor problem. Although the unweathered rocks contain appreciable amounts of fine-grained pyrite, most acid created during post-mining oxidation is neutralized by nearby carbonate-bearing rocks or by limestone placed below waste dumps. (2) Alkaline mine or rock drainage (pH 6–8) may be a problem at these deposits. Arsenic, Hg, Mo, Sb, Se, and tend to be enriched in these ores, and these metals are relatively soluble under near-neutral pH conditions. Water analyses are sparse for these deposits and available data have been reviewed elsewhere (Hofstra and others, 1995; Plumlee and others, 1999). Zinc is relatively rich in some of these deposits and also can be transported at pH 6–8. (3) Oxidized ores, in which pyrite and other sulfide minerals are destroyed by pre-mining weathering, are amenable to simple cyanide heap leaching because the gold is "free" and potentially toxic trace metals such as arsenic are stable in iron oxide minerals. The oxide ores, as at the Marigold mine, are generally believed to pose no environmental hazards. (4) Pit lakes will form in most of these open-pit mines, some of which will be more than a mile in length and hundreds of feet deep. The nature of water flow into or out of pit lakes and the quality of the water are complex issues.

One example of this type of deposit, the Quito mine, was investigated and is described herein. It is much smaller than the major mining operations to the northeast, in Eureka County, but may provide some useful insights to those mining areas. Mill tailings at analog deposits at Manhattan (White Caps) and Northumberland in central Nevada were described by Nash (2003b).

Porphyry Cu-Mo Deposits

Porphyry deposits of copper and molybdenum, associated with shallow-level igneous stocks having a characteristic porphyritic texture (large crystals mixed with very fine ones), are the major source of Cu and Mo in the United States and the world. Mines range in size from large to huge (as at Bingham Canyon near Salt Lake City) and commonly are open pits. The geologic aspects with potential environmental implications include (1) large tonnages of rocks with high sulfide mineral and base-metal concentrations; (2) igneous rocks and alteration zones that can have low to moderate acid-neutralizing capacity; and (3) high fracture permeability that promotes percolation of surface water and encourages acid-generating and metal-liberating reaction. These and other aspects of porphyry deposits are reviewed elsewhere (Cox and others, 1995; Ludington and others, 1995). Metals of major concern include As, Cd, Cu, Fe, Mo, Pb, Sb, Te, and Zn from the Cu-Mo deposits. Porphyry deposits may produce a wide range in pH and metal concentrations (Bowell and others, 2000) depending upon degree of oxidation. The Mo-rich variety (termed "Climax-type") have the potential to produce acidic water that is rich in F and U.

Porphyry deposits are known in Nevada; two atypical examples have been mined in the Humboldt Basin at Copper Canyon and Copper Basin (Battle Mountain district) and a few prospects have been drilled. The two Cu-Mo mines in the Battle Mountain district are outside of the source porphyry intrusions and geochemically are more akin to skarn deposits (Doebrich and Theodore, 1996). Significant Mo and Cu porphyry prospects are known at Buckingham, near Battle Mountain (Loucks and Johnson, 1992), and at Mount Hope in southern Eureka County (Westra and Riedell, 1996). Poly-metallic vein deposits on the fringe of the Buckingham and Mount Hope Mo-Cu prospects were observed and sampled in this study, but the porphyry parts of these large systems are not exposed at the surface or mined.

Massive Sulfide Deposits

Only a few massive sulfide deposits have been mined in Nevada, including two examples in the study area and one to the north near Mountain City. These deposits are extremely rich in pyrite and can be mined for their content of sulfur, copper, zinc, or byproduct silver and gold. The deposits occur as layers or lenses of pyritic rock that contain high to very high concentrations of As, Cd, Cu, Fe, Pb, Zn, and other metals. Their chemical and mineralogical composition is similar to polymetallic deposits. The rock units containing these sulfide deposits have been metamorphosed, but their bulk composition has not changed greatly and the newly formed minerals contribute to a generally high acid-neutralizing capacity.

The high concentrations of pyrite and base-metal sulfide minerals in massive sulfide deposits can produce extremely acidic drainage, as in the Shasta district of California (Alpers and Nordstrom, 1991; Taylor and others, 1995; Plumlee and others, 1999). The combination of bad mining practices and wet climate can cause severe environmental degradation, but not all massive sulfides have this effect. The combination of dry climate and deep oxidation in the Basin and Range province of Arizona and Nevada greatly reduces the tendency for acid generation (Nash and others, 1996). Also, the green propylitic alteration of basaltic host rocks, common in this deposit type, introduces calcite and chlorite that provide high acid-neutralizing capacity. Water draining from many massive sulfide deposits is neutral to weakly alkaline, caused by the beneficial effects of altered wall rocks. Despite geologic differences, massive sulfide deposits have many geochemical features in common with polymetallic deposits: ore compositions alone would suggest huge problems, but natural mitigation by wall rocks can greatly reduce risks in some areas.

The Big Mike deposit, south of Winnemucca, is the largest example of a massive sulfide deposit in the study area (Rye and others, 1984). Other prospects are known, including the Black Beauty, which is probably of this type (LaPointe and others, 1991). The Rio Tinto mine near Mountain City was a major producer (Coats and Stephens, 1968), but it is located north of the Humboldt Basin. The sulfidic mill tailings from the Rio Tinto, near Mountain City (figs. 1 and 2), are a source of contaminants in the Owyhee River Basin; they are a National Priority List caliber site and the subject of ongoing discussion for reclamation under the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA, commonly called Superfund). The magnitude and extent of contamination from massive sulfide deposits in northern Nevada appear to vary according to local conditions of climate and mining history. Where dry, there are few problems. In Nevada, the mines themselves tend to pose few problems because wall rocks can counter acid production, but waste piles and tailings that are rich in pyrite can generate highly acidic, metal-rich water.

Acid Sulfate Deposits

Acid sulfate deposits are not known within the Humboldt River Basin, but examples at Goldfield, Nev. (fig. 1), and Summitville, Colo., are prominent in environmental studies of mines because their high sulfide content and lack of rock buffering tend to create extremely acidic drainage water (Plumlee and others, 1999; Nash, 2002a). Brief studies were made of the Goldfield district in 1999 to provide data to compare with deposit types within the study area, particularly the massive sulfide type. The remarkable feature at Goldfield, and elsewhere, is the intense alteration attributed to hot acid sulfate solutions that destroy nearly all minerals originally in wall rocks and leave a sponge-textured rock composed of fine silica, pyrite, and alunite (a potassium-aluminum-sulfate mineral). These altered rocks have the potential to create very acidic drainage water, as observed at Goldfield during an unusually wet June day in 1999, and persistently in the San Juan Mountains of Colorado.

No acid sulfate deposits or prospects are known in the Humboldt River Basin, but prospects in the Scraper Springs district (fig. 2) contain aluminum minerals characteristic of the acid sulfate alteration assemblage. My observations at Scraper Springs, described herein, did not reveal significant amounts of pyrite or other characteristics of the acid sulfate deposit type. My samples from the Goldfield district produced some extreme chemical values, as discussed in later sections.

Uranium Vein Deposits

Many kinds of uranium deposits in plutonic, volcanic, and sedimentary rocks have been identified in Nevada, but only a few were mined in a significant way (Garside, 1973). The behavior of uranium and associated metals is quite different among the various types of uranium deposits, but many of the potentially toxic metals tend to be mobile in either highly acidic or alkaline water. In the study area, the largest uranium deposits are of the vein-type, located a few miles south of Austin at the Apex mine. Here, and at other uranium vein deposits in Nevada, oxidized and reduced minerals are both present at various depths. The unoxidized, deeper parts having abundant pyrite and marcasite (both are FeS₂) have the greatest potential to create acid-mine drainage, which in turn can transport uranium and other metals (Wanty and others, 1999). The waste dumps at the Apex mine share many characteristics with those of the Los Ochos mine in Colorado (Nash, 2002a).

The environmental geochemistry of uranium mines is more complex than for other metal mines because radiation and decay products of uranium, such as radon and radium, also are involved. Regulatory standards are established for uranium and radionuclides in water and in mill tailings but have not been set for mine-waste dumps. Standards for released water and base metals are the same as those for metal mines. Mine reclamation generally focuses on exposure to radiation rather than chemical concentrations of uranium, but release of uranium and radionuclides to surface and ground water also is a concern, and water-quality standards must be met.

Mill Tailings Potential Sources of Contaminants

Introduction. Mills in historical mining districts of northern Nevada from about 1870 to about 1970 treated millions of tons of ore to recover metals of value and in doing so created large quantities of waste materials that can release acid or metals to the environment. Although the term "tailings" is often applied to any kind of mined material, regardless of ore grade or amount of processing, the term will be used here only for material that was ground and processed through a mill (photo 8). At most mines, more than 90 percent of the ore-grade material mined was processed, yielding mill tailings with grain size finer than beach sand and containing minerals that either escaped recovery or had too little value to recover. Other recovery methods, such as retorts for mercury or heap-leach pads for precious metals, left piles of crushed rock containing substantial amounts of ore-related metals.² These processing operations placed the tailings in nearby locations that were convenient but often unstable and subject to episodic contact with surface water. Because most mills used water in their chemical processing of ores, and most tailings emerged as a wet slurry, tailings tended to be placed in arroyos or stream channels that carry water. There were no Federal or State regulations against placing tailings in streams before 1935. The fine grain size of tailings tends to make them more reactive when exposed to water and more vulnerable to erosion than mine waste on dumps that has not been crushed or ground. In some situations mill tailings can be significant sources of pollution to surface and ground water; an extreme example in Nevada is the sulfidic tailings from the Rio Tinto mine that releases acid and metals to a tributary of the Owyhee River.

Although mills and tailings are part of abandoned mine lands, they have not received scientific study proportional to their potential effect on the environment: most geologic references and databases include less than 10 percent of mill or tailings sites and even less information on their size, physical situation, and composition. Milling facilities of active or recently active mining operations by major corporations are not included because they are subject to several regulatory processes for operation and closure; a few mill sites that were abandoned without reclamation during the last 10 years are included because they are of interest to land managers. During the course of geochemical studies in northern Nevada from 1995 to 2000, I made special effort to locate mills and tailings because they are inadequately recorded in the literature. I located in the field 83 mills or mill sites and derelict mill-tailings impoundments at 78 sites (Appendix 5). Transported (fluvial) deposits of significant size were observed at 13 sites downstream from mills. Samples from the tailings sites were collected onsite where possible.

The substantial erosion of tailings at many sites indicates that many tailings impoundments are at risk for failure during local extreme storms (flash floods). Tailings dam failures at 11 sites indicate that catastrophic failure of abandoned mill tailings is more common in Nevada than reported in the literature (Nash, 2002b). The chemical and physical consequences of these catastrophic failures exemplify the potential effect of such failures on wildlife and the landscape.

Properties of mill tailings. The engineering aspects of mills and tailings disposal are too complex to be reviewed here; the interested reader can pursue the extensive literature elsewhere. Some useful volumes include the review by Ritcey (1989) and research papers in two special volumes (Jambor and Blowes, 1994; ICARD, 2000). A few aspects can be described briefly for better understanding of general principles. (1) All mills had crushing and grinding equipment to reduce the particle size, which was required to allow contact with milling chemicals; grains typically are finer than beach sand. Stamp mills from the 1880's produced coarse-grained tailings, about 1 mm (0.04 inch) maximum diameter, whereas post-1920 flotation tailings are fine (about 0.25 mm or 0.01 inch) and some are very fine (less than 0.1 mm or 0.004 inch). These grain sizes can influence geochemical reactions and other properties. (2) Processing is specific for metals of interest: many mills focus on gold, whereas some concentrate copper or lead by gravity or flotation; other minerals are not affected and go out in the tailings. Pyrite typically is in the tailings, and some pre-1920 methods did not attempt to remove sphalerite (ZnS) because Zn was not valued. (3) Tailings are handled in many ways, almost always aided by gravity and almost always as thick mixtures with water. The mixtures vary, but those having about 70 percent water resemble wet concrete and can flow through pipes or sluices this is an advantage to processing the tailings but can lead to problems at the disposal site (pond), including dam failure (Ritcey, 1989). In Nevada, where water generally was in short supply, the water was recovered from tailings in settling tanks, and the semi-solid tailings would be stacked like hay. If wet, the tailings could flow to lowlands or constructed ponds (with dams), whereas the dry variety would accumulate in piles on a slope. These physical differences may explain the location of tailings today, whether it was by design or by convenience at the time.

Identification of mill and tailings sites. Mills vary greatly in size, style of construction, and preservation. The older mills tend to have stone foundations (photo 9), but concrete foundations were used after about 1900 (photo 10). Some mills are as small as a garage, and others are hundreds of feet wide; the larger mills generally have three to six levels up a hillside so that gravity could be used to move the materials from one operation to another. Some mills are remarkably intact and handsome structures, but many burned and have no superstructure. Mills generally were taken apart when the mining ceased, so equipment is rarely present today. Many photographs of mills from Colorado can be seen in another report (Nash, 2002a) as a guide to what the industry used over the past century.

In nearly all cases, mills were placed at a lower elevation than the mine. Some mills in Nevada were served by a gravity-driven tram, but in most cases the ore was delivered by mine car, wagon, or truck. In many cases, the mill was placed at the nearest site having water, but in some districts water was piped in from distant wells. Some mills were several miles from the mine, as in the Tonopah district where mills were sited about 12 miles to the west at Millers to obtain enough water. Some mills serviced just one mine, and others served an entire district. At mercury mines, mills and retorts tend to be small and serve a single mine. At some mill sites there is enough evidence remaining to deduce the kind of milling used, such as the heavy steel rods of the stamp mills at the Looney gold mine (Rochester district), or the cylindrical tanks used for cyanide at the Buckskin mill (National district). In many situations it is easiest to locate the mill and then search for the associated tailings. Identification of mill sites, however, is not the significant issue for land management because it is not the mill itself that causes concerns. Although many mills used cyanide, which is highly toxic, the cyanide is not likely to persist today unless in a sealed container. Cyanide is degraded by many natural processes, including oxidation and biode-gradation, and is unlikely to be significant in mills or tailings as old as those under discussion here. This issue will not be considered further, but the concerned reader should see other reports (for example, Smith and Mudder, 1999). Mercury used to recover gold can contaminate mill sites but has not been investigated here.

My reconnaissance studies of the historical mining districts of northern Nevada that are in or near the Humboldt River Basin disclosed 83 mills (or mill sites), 78 tailings impoundments, and 13 fluvial tailings sites (fig. 4). Also identified were 11 smelters (or slag sites). Very few of these tailings or slag sites are mentioned in electronic databases, and there is virtually no database for their size, physical situation, and composition. The sites are briefly described by Nash (2002b) and described later in the descriptions of mining districts. Chemical analyses of tailings and reactions in leach tests are in Nash (2000b).

Contaminant pathways from tailings. Contaminants from mill tailings migrate in the same ways as from mine waste dumps, but some mechanisms differ in detail. Because of the finer grain size of tailings, wind can carry small particles from tailings and has been recognized by the BLM to be a substantial concern at sites near the Getchell mine (Potosi district, fig. 2) and Paradise Peak mine (near Gabbs, fig. 1). I have seen evidence of the windborne particulates as dust clouds (photo 11) and as sand dunes at several localities but did not document the magnitude or areal extent of the contamination. Runoff from tailings will be described later at many sites, and seeps are present at a few sites (see photo 12). Streams also can react with mill tailings, as at the Rio Tinto tailings impoundment (photo 13). In the prevailing arid climate, efflorescent crusts (photo 14) tend to develop on tailings, and these are highly soluble according to leach tests and will create a flush of highly contaminated runoff in the first minutes of a rainstorm. Seeps to the surface are rare, in part because the geometry of most historical tailings piles is such that there is relatively little vertical dimension and thus little tendency for water to seep from the sides or gullies in the piles. Seeps of very acidic and metal-rich water were observed at the Rip Van Winkle tailings, described later. In general, we must assume that seeps can carry acid and metals from tailings after storms or snowmelt, and that infiltrating seepage water probably moves slowly through the pile and into underlying alluvium and ground water. The composition of these seeps is difficult to determine and is best done with wells drilled by machine or hand auger. More work of this type could be done to document the magnitude of this pathway and the composition of the waters that are mixing with shallow ground water.

Chemical Composition of Dumps, Tailings, and Mineralized Rocks

Metal concentrations in mine-waste dumps, and of mill tailings, vary greatly by deposit type and by style of mining. The previous section described the general compositional trends by deposit type. In chemical terms, the concentrations of metals largely reflect the rarity of the metal, ranging from less than a part per million gold in some gold ores, to more than 40 percent iron in iron ores. Concentrations of metals required to constitute ore are related to their market value: because gold has a high unit value, not much gold is required to make it ore grade, whereas iron or lead, with low unit value, must be present in much higher concentrations. Not intuitive is the abundance of non-ore metals, such as As in any ore or Zn in a gold ore, that are not recovered in mining or milling and which collect on waste piles. Geology determines the concentration of these metals, and their concentrations generally are predictable by geologic setting and deposit type (Plumlee and others, 1999; Nash, 2002a).

Compositions of mined and waste rocks in mining districts reflect mining technology as well as the geologic influences outlined in the previous section. By economic definition, ore-grade rocks have the highest concentrations of metals in a district, whereas altered and mineralized waste rocks (sub-ore grade at the time of mining), having elevated but relatively lower concentrations of ore metals and associated minerals such as pyrite, are placed on dumps. Slightly altered rock, with very low (background) concentrations of metals and generally low sulfide content, is removed while gaining access to ores by excavating shafts or tunnels or by stripping in an open-pit mine. In most mines this variety of waste is stacked in piles that are spatially distinct from subeconomic mineralized ones. Stockpiles of ore-grade material are less common, but some are present at prospects and reopened mines that lack a nearby mill to treat the ore.

The composition of mill tailings reflects the ore type milled and varies according to changes in technology. Stamp mills in the 1800's, which used jig tables and mercury amalgamation to concentrate gold, released metal-rich tailings because the base metals were not removed by those milling methods. Zinc was a penalty at smelters until about 1920; thus, old mills did not attempt to concentrate sphalerite (ZnS). Pre-1920 tailings thus tend to be higher in Zn than the same type of ore processed after Zn became valuable. There also can be substantial variation in tailings mineralogy and composition from a given mill, shown in visibly differing colors of layers in the impoundment reflecting changes in the character of ore mined (such as oxidized near-surface ore compared to sulfidic ore from deeper zones), variability in ore mined on a custom basis (same mill but ore from different mines), or evolution in mill technique. Generally, lighter tailings color is suggestive of lower concentrations of heavy metals. Rusty coloration by iron oxide and sulfate minerals tends to indicate high metal concentrations, and many of these tailings can create acids when exposed to water. Gray coloration tends to reflect the presence of sulfide minerals, but there can be other geologic associations such as dark-colored host rocks. Most tailings contain only a small percentage of sulfide minerals because most mills were designed to remove these minerals, but even a small amount of sulfide mineral can create acid when exposed to water, and the acids will dissolve and carry base metals that may be present. An extreme example of this in Nevada is the sulfide-rich tailings from the Rio Tinto massive sulfide mine near Mountain City.

Representative samples of dumps, tailings, slag, and unmined altered rocks were collected and chemically analyzed to provide a generalized chemical description of the range in composition of these materials. Table 1 shows the median and maximum concentrations of elements of possible mineral-environmental interest. In this report the simple terms high and very high will be used to describe concentrations of elements relative to the database of 232 samples. The term "high" will be used to highlight concentrations above the group median; very high will be used to highlight concentrations much higher than the group median (the top decile).

The ranges in compositions of dump and tailings samples from nine deposit types in the Humboldt Basin are shown in figure 5. Clearly, there is a broad range in metal concentration for various samples of materials associated with these nine types of deposits. Although generalizations are risky, there are some general trends. Polymetallic, massive sulfide, and porphyry deposits tend to contain high concentrations of base metals, whereas mercury and epithermal vein deposits tend to be to be lowest in these metals. Uranium was enriched to levels sufficient to create significant radiation in only the uranium vein deposits, although some other environments not studied here are known to be enriched in uranium.

Compositions of mine-waste materials and mill tailings are generally in the same range, whereas altered rocks tend to contain lower concentrations of ore elements such as Ag and Cu (fig. 6). Compositions of individual samples span a large range, showing that one cannot assume that these mineralized materials are always high or low in metals.

Leach Tests of Dump and Tailing Samples

Leach tests were made to determine what metals are soluble, and thus mobile, in various deposit types and mined materials. The behavior of samples during the 24-hour passive leach tests is variable. For many samples, especially those with abundant iron oxides or jarosite (rusty-appearing minerals), the pH dropped to less than 3.5 in just a few minutes. In some samples the pH evolved to lower values during the 24 hours, and in a smaller number of samples (20 percent), the pH rose a few tenths of a unit, reflecting buffering by rock and gangue minerals. The behavior of metals as a function of pH is shown in figure 7, and leach test results are summarized in table 3.

Comparing results for dump, tailings, and altered rock samples, leachate concentrations generally are highest in dump samples and lowest in altered rock samples (fig. 8). One sample of smelter slag was tested, and it released little acid and metals, reacting only slightly; other leach tests on slags from Colorado and Arizona produced similar results (Nash, 2002a). There is wide variation in leachate concentrations within the groups of dump samples, tailings, or altered-rock samples. This wide variation is consistent with the range in dump compositions and differences in tailing composition from mill to mill or even from layer to layer within a tailings impoundment. The mobility of individual metals in leach tests is generally a function of pH, as in surface water (fig. 7; Nash, 2002a).

Trends for leachate compositions, grouped by deposit type, are shown in figure 9. Leachate compositions from samples of nine deposit types span a wide range, and the range within deposit types is large also. Generalizations are not reliable. Base metals and arsenic concentrations tend to be high in polymetallic, massive sulfide and acid sulfate deposit samples, but not exclusively so. I am more impressed by the overlap among the deposit types and the suggestion that waste from many deposit types can release significant levels of contamination.

Introduction to Mine-Related Waters

Surface water adjacent to metal mines can be relatively pristine or highly contaminated by effluent from mine workings and associated waste products. Much has been learned from the study of acidic water associated with coal mining, and in recent years the focus of much research and regulatory action has shifted to contaminants associated with abandoned or active metal mines, mills, and smelters (Plumlee and others, 1999). In this study, surface-water samples were taken from as many sites and situations as possible to obtain facts on water compositions and scale of metal transport from mine sites (fig. 10). The following is a highly simplified summary of some of the major principles, largely taken from reviews by Smith and others (1994), Plumlee (1999), and Plumlee and others (1999).

Acid Water

Why all the emphasis on acid-mine drainage and acid-rock drainage? In addition to being toxic to wildlife, acids are important for their ability to dissolve minerals and transport metals. Generally it is sulfuric acid at metal mines that initiates many of the concerns. Sulfuric acid is typically generated by weathering (oxidation) of pyrite or other sulfide minerals, often catalyzed by bacteria (Nordstrom and Alpers, 1999; Mills, 1999). Another source of acidity in near-surface water is the precipitation of Fe- and Al-oxyhydroxide minerals during which protons (hydrogen ions) are produced by hydrolysis reactions.

Acidic water has the ability to transport high concentrations of many base metals and also other elements of concern such as aluminum. This generalization is supported by theoretical models, laboratory reactions, and especially by water analyses in mining districts (Smith and others, 1994; Plumlee and others, 1999; Nash, 2002a). In numerous districts, and across many deposit types, there is a predictable relation: the more acid (lower pH), the higher the metal concentrations (fig. 11). Acidic water can be as clear and as colorless (photo 18) as background water because the metals are held in solution.

Water Composition and Deposit Types

The lithology of host rock and mineralogy of ores lead to some fairly predictable tendencies among deposit types, as discussed by Smith and others (1994) and Plumlee and others (1999). However, there is a wide range in composition within a deposit type, particularly the polymetallic deposits, and there are many exceptions to general "rules" for mine-drainage compositions. For the Humboldt Basin, the trends are summarized in figure 12. Highest concentrations of base metals tend to be from polymetallic and massive sulfide deposits (Big Mike is the example). Contrary to some popular assumptions, concentrations of As and Se are variable and approximately the same range for many deposit types.

Alkalinity of Surface Water

One of the most striking characteristics of surface water in arid regions of the Western United States is their high content of Ca-Mg-CO₃, which also is apparent in measurements of conductivity and alkalinity values. Waters in the Humboldt Basin and many parts of the Basin and Range province generally has conductivity of 300 μS/cm or more and alkalinity of more than 200 mg/L CaCO₃. Alkalinity, the capacity of water to accept hydrogen ions (H+ or protons) and not a descriptor of pH, is generally not a function of contamination but of normal rock and soil reactions with water. In common water with pH of about 5 to 8.3, bicarbonate is the dominant base anion and Ca and Mg are the dominant cations (Manahan, 1994; Langmuir, 1997). Alkalinity of water is measured by titration and expressed as equivalent amounts of CaCO₃ in milligrams per liter. Surface-water samples from the study area have alkalinities in the range of 0 to 390 mg/L CaCO₃ and a median of 220 mg/L. As in all water systems, alkalinities of samples collected in the Humboldt Basin decrease below pH 7 and are small to nil below pH 5.

The high alkalinities of most water in the study area are potentially significant for natural processes of acid neutralization. These alkaline waters have high capacity to neutralize acidic water when the two mix. If one walks down a stream contaminated by acidic drainage, measuring pH and conductivity every hundred feet, changes in pH and conductivity document mixing with inflowing surface water or subsurface springs. If the water is initially very acidic, with a pH of about 3, the water generally is clear (all metals are held in solution), but the water becomes cloudy red-brown and precipitates iron oxy-hydroxide minerals ("iron floc") a short distance after mixing as the rising pH causes iron and other base metals to precipitate. This is essentially titration, as done in the chemistry laboratory, of an acidic solution by a basic solution. This process is commonly seen in acidic drainages of Nevada and generally reflects mixing of water rather than reaction of an acidic solution with a rock such as limestone. Two environmentally important base metals that do not precipitate as quickly in the neutralization process are Zn and Cd, as will be discussed later.

Buffering and Acid Neutralization by Rocks

Natural or engineered mitigation of acidic mine drainage involves raising the pH of water by consumption of hydrogen ions. Limestone, composed chiefly of the mineral calcite, is most effective for consuming acid, but reaction with other minerals also consumes acidity, albeit more slowly than for carbonate minerals. Ion-exchange reactions, as with clay minerals, also helps. Acid-neutralization capacity refers to the amount of hydrogen ions that can be consumed per unit weight; chemists would express this as moles per gram. We can consider pure calcite to have very high capacity, whereas that for quartz is nil; for rocks, limestone (composed mostly of calcite) is high, shale (a mixture of clay, quartz and often some calcite) is moderate, granite is low, and sandstone is very low (unless calcite is present and then it can be quite high). Veinlets of calcite can make an otherwise low-capacity rock have high acid-neutralizing capacity.

Estimates of the acid-neutralizing capacity of geologic materials tend to underestimate the widespread occurrence of small amounts of calcite and other carbonate minerals, which are very reactive and overemphasize reactions involving feldspars and clay minerals (for example, Glass and others, 1982). Also, the estimates generally fail to acknowledge the large amount of calcite in alluvium as caliche or the abundance of narrow calcite-bearing veins because these are not easily seen or sampled at the surface. One easy and effective way to estimate rock and soil acid-neutralizing capacity is measuring pH at springs values in the range 7–8.5 are caused by reactions with carbonate somewhere in the flow path of the water. Water with alkaline pH can neutralize acidic water, as described previously, or the solids (calcite, biotite, clays) can consume the H⁺ by several kinds of reactions.

Reactions that Remove Metals

Mitigation of water quality requires that toxic metals be removed, either by sophisticated engineered systems that are expensive to build and maintain or by some simple reactions in nature. Many reactions occur (Smith and others, 1994; Smith, 1999), including precipitation, sorption, hydrolysis, and reduction. Sorption combined with coagulation of colloids is very effective in nature and generally occurs in response to an increase of pH; we see the result of these processes in the red-brown materials in streambeds and coatings on stream cobbles (iron-oxyhydroxides with entrained trace elements). Among the heavy metals of concern, Pb is the first to be removed by sorption as pH rises from 3 to about 4; Cu tends to be next at pH 4–5, then Zn at pH 5–6, and finally Cd and Ni at pH 6–7 (Smith, 1999). Arsenic is effectively removed by sorption if iron concentrations are high. In nature, these sorption reactions are most effective where Fe-rich colloids coagulate as pH rises in zones where acidic waters mix with those of near-neutral pH or contact acid-consuming rocks.

Other transport and deposition mechanisms operate at pH 6–8 for metal-oxyanions, such as arsenate, molybdate, and selenate. These toxic metals are quite mobile in near-neutral to alkaline pH water and pose a distinctly different set of environmental problems because raising pH only enhances their solubility. Sorption and dilution will help reduce the concentrations of these elements, but other natural processes do not appear to be effective in reducing them. If water quality does not meet standards for these oxyanions, the only remedial action may be engineered water systems. Zinc and Cd, which do not form oxyanions, also have fair solubility at neutral pH. In several watersheds, natural mitigation processes neutralize initially acidic water and decrease the concentrations of most metals below concern levels. However, concentrations of Zn, Cd, As, Mo, or Se remain high in some surface water at pH 6–8.

Frameworks for Evaluating Hydrogeochemistry

Specialists and generalists alike need a frame of reference for proper understanding of the hydrogeochemical information collected in this study. Unfortunately, there is no single, simple framework that meets all needs. Environmental scientists and regulators currently use two fundamentally different methods to evaluate water: some express water quality in terms of concentrations, such as parts per billion, and others express it as loads, such as pounds per day. The load of a metal or other constituent is the product of its concentration multiplied by the flow volume in the pipe or stream to yield a value with units such as pounds per day. Thus, the load calculation requires measurement of flow, such as cubic feet per second or gallons per minute, as well as concentration, such as parts per billion. The system of concentrations has been traditionally used by research biologists, hydrologists, and geochemists, whereas the system of loads is coming into increased use by land managers and regulators after the method of Total Maximum Daily Loads (TMDL) was introduced in 1972 in the Clean Water Act, as discussed herein.

For the evaluation of metal concentrations in water, some geochemists advocate the use of compendia on world- or content-scale water-quality analyses (for example, Livingstone, 1963; Forstner and Wittmann, 1979). Another approach is to compare the water in mining areas with the composition of water from nearby basins that have little or no mining this will be discussed next. A third approach is to use the framework defined by toxicologic studies and regulatory agencies such as the U.S. Environmental Protection Agency (USEPA) and State agencies. The regulatory standards of the Nevada Division of Environmental Protection (NDEP) will be introduced and discussed later in this section.

Geology-Based Reference Values

Background and baseline compositions of earth materials and water provide a realistic scientific framework for evaluating compositions present in mining areas. In this report "background" will be defined as a composition prior to the influence of humans or mining. Baseline compositions, which include contributions from humans and mining, should include a date of reference. For example, the composition of surface water and alluvium in the Battle Mountain or Eureka areas presumably has changed over the past 130 years (and may have been most degraded during unconstrained mining and smelting of the 1890's). Miller (2000) reported water compositions from streams and springs in four lithologies at sites believed to have little or no anthropogenic influence. Building on Miller's work, I collected samples in June 2000, from springs and headwater streams that appear to represent either background conditions in unaltered rocks (minimum influence of humans) or background conditions in visibly altered rocks (possibly small influence of prospecting), the results of which are summarized in table 5 and figure 13.

The composition of water logically reflects the rocks and minerals that react with the water as it flows on the surface or underground, and this was well demonstrated by Miller (2000) with analyses from headwater basins in four geologic settings. My samples are from diverse settings, and the number of samples for various lithologies is small (two to five). Because it is logical to suspect that various geologic settings would influence water compositions, background water compositions have been plotted by geology of the headwater basin that was sampled (fig. 14). There are a few trends, but these may not be significant considering the small number of samples averaged to make the datapoint and the lithologies are only crudely defined. Visibly altered bedrocks appear to be sources for higher concentrations of many elements. Arsenic tends to be enriched in water from volcanic and granitic terranes and Cu and Mo are enriched in some samples from granitic terranes. Two widespread shale units, the Vinini and Chainman Formations, are the carbonaceous variety that commonly are metalliferous, but the associated water compositions are not clearly metal-enriched unless the rocks are altered. Much more sampling will be required to demonstrate compositional relationships to source rocks, particularly the concentration of base metals.

Regardless of source lithology, the metal concentrations in basins having no mining and no visible alteration are very low far below regulatory water-quality standards. However, water compositions related to visibly altered rocks and scattered small prospects are significantly different from those in unaltered rocks (table 5; fig. 14). Another notable attribute of water from altered rocks is a much higher variation in metal concentrations: not only are mean concentrations higher, some individual samples can be tenfold to a hundredfold higher than those from similar unaltered lithologies. The concentrations of potentially toxic metals in water associated with altered rocks approach regulatory standards, and some individual headwater stream or spring samples do not meet those standards. Sulfate concentrations can be elevated in water that has reacted with unmined altered rock, but Cl and F concentrations are generally not elevated in unmined sources. Because large volumes of rocks in mountain ranges are altered, metal loads from these areas could be significant and should be taken into account in TMDL considerations, described herein. However, in the general framework of metal concentrations used in this report, the concentrations in water from altered rocks having no or minor mining are significantly lower than in streams, seeps, or runoff from mines and mine waste.

Biology-Based Standards

Toxicity and health effects of water are generally related to concentrations of contaminants (Manahan, 1994; Smith and Huyck, 1999), and these values are the scientific basis for regulatory standards adopted by the USEPA, NDEP, and other State agencies. These standards generally are based on beneficial use and toxicity to humans or aquatic life. Biologists debate the details of just which species should be protected by the water-quality criterion, such as the differing sensitivities of trout species to heavy metals (Besser, 2000), but such details are not relevant in this study. However, there are questions as to what general quality standards should be applied to surface water in the historical mining areas of Nevada. These waters are regulated by the Nevada Division of Environmental Protection (Nevada Division of Environmental Protection, 2002), but the system is not as easy to follow as in some States, such as Colorado, that list short segments of headwater and main-stem streams, their beneficial uses, and the relevant quality standards. The Nevada system will be discussed in a later section.

To provide a biology-based framework for evaluation of water quality, I will compare water compositions to the Aquatic Life Water-Quality Standard (ALWS), shown in table 6. These ALWS concentrations are used here as a general guideline and to allow comparison among districts, but in detail other regulatory standards may be appropriate. For example, the fine print of Nevada Division of Environmental Protection regulations (http://ndep.nv.gov/nac/445a-118.pdf) stipulates that the most stringent criterion be applied if there are multiple uses which means that the criteria for As or Pb in drinking water will apply, and the criteria for Cd, Hg, and Se for aquatic life will apply because they are more stringent than for drinking water.

To preview discussions that follow, analytical results for 275 water samples at 241 sites show that many kinds of mining-contaminated water at mine portals, from waste dumps or tailings impoundments, or in puddles and ponds, have degraded water quality. Relative to the criteria in table 6 (bold), the following percentages of sites have compositions that do not meet the water-quality standards: Zn, 44 percent; Fe and Cu, 42 percent; Se, 41 percent; Cd, 37 percent; Mn, 32 percent; As, 30 percent; Al, 22 percent; Mo, 16 percent; Pb, 8 percent; and U, 6 percent. From these numbers we can anticipate that Zn, Cu, and Fe are most likely to be present in concentrations that pose health threats, whereas concentrations of U and Pb are less likely to be concerns. But these statistics are only part of the story; the real questions are: how typical are the high metal concentrations in surface water, and how far do they persist beyond the mining property?

Flow Volumes of Surface Water

The volume and duration of water flow, significant parameters in the hydrogeochemistry of northern Nevada, can be difficult to visualize. Whether measured in gallons per minute (gal/min) or cubic feet per second (ft³/s), the flow volumes observed in northern Nevada span a huge range. Seeps or flows from an abandoned mine may be a few gallons per minute, and a creek that is too wide to jump may flow at about 2 ft³/s (900 gal/min). The Humboldt River at Battle Mountain has an average flow of about 40 ft³/s (18,000 gpm), increasing to more than 1,000 ft³/s at flood stage. Dewatering programs at the large gold mines use pumps capable of withdrawing more than 1,000 gal/min, and collectively the discharge can be in the range of 10,000 to 50,000 gpm (Barrick Goldstrike Mines Inc., 2000). In the late 1990's permitted discharges of mine water into the Humboldt River were approximately 70 percent of the mean flow of the Humboldt at Palisade (Horton, 2001).

Many streams in northern Nevada flow a few hours a decade, and a few have perennial flow. Maps can be misleading: the Reese River appears to be a major river about 65 mi long, but for most of that distance and on most days one can step across the barely flowing stream. Another major tributary of the Humboldt River, Rock Creek in northern Lander County, has vertical banks 10 to 20 ft high that attest to deep and raging floodwater a few times per century. The major springs that are shown on topographic maps generally flow at the surface for only a few hundred feet before disappearing into alluvial gravel. Whenever we speak of flowing water, it is important to try to visualize when it was flowing and how typical that flow may be. In my studies it was not possible to make systematic measurements of flow or to collect samples throughout the water year.

A nagging question behind all of the hydrogeochemical studies described here is: how typical is that water for this site or area? And, is it significant if the contaminated water flows only a few days a year? Another set of nagging questions deals with loads: are the loads from a mine drainage with flow of a few gallons per minute important at watershed scale? In the next section, metal loads for the Humboldt River, including pit-dewatering discharges at concentrations that comply with standards, will be shown to be much larger than for the metal-rich, low-volume mine seeps.

Metal Loads and Total Maximum Daily Loads

The Clean Water Act requires that State agencies develop standards based on loads rather than concentrations, and on the use of TMDL's in watershed planning. TMDL is explained elsewhere (http://www.epa.gov/owow/tmdl and http://www.nv.us/ndep/bwqp/tmdl) from which the following is extracted. A concept introduced in 1972 and refined in 1985, TMDL is a calculation of the amount of a pollutant that a water body can receive and still meet water-quality standards for health and beneficial uses. The TMDL definition starts with the water quality standard for the identified beneficial uses. The TMDL is the sum of the loads of a single metal or pollutant from all point and nonpoint sources and thus includes discharges from industrial facilities, farms, sewage facilities, and natural sources. Inclusion of natural nonpoint sources is important as this requires evaluation of discharges from unmined altered rocks in mining districts. The process of defining TMDL's is complex, but Section 303(d) of the Clean Water Act stipulates that TMDL's must be implemented on impacted watersheds sometime in the next decade.

Although toxicity and health effects of water are generally related to concentrations of contaminants (Manahan, 1994; Smith and Huyck, 1999), there has been increasing use of metal loads for watershed management (Black, 1996). Metal loads, defined as the metal concentration times flow volume and expressed in terms such as pounds or grams per day, provide an alternative way, and metal loads can be used to evaluate or rank mining-related water contamination. One complication is the need for a measurement of streamflow, which is a complicated task and one that the author can do only as a visual estimate for small flows in the 1- to 300-gal/min range. Examples of metal loads are in table 7, in which loads (in grams per day, g/day) are computed for four sources and the consequent downstream flows after mixing and attenuation. The loads are compared with those computed for the Humboldt River. All of these figures are approximate, with an uncertainty of about ±100 percent (for uncertainties in both analytical values and in Q, flow volume). The Humboldt River analyses (three) are for samples I collected from the main stem near Battle Mountain. The loading values are easiest to grasp when considered relative to other values in the same units (grams per day):

1. Metal load values decrease significantly downstream from the sources; the in-stream values are low compared to those at the source adits or seeps;

2. Metal load values for all metals at mine sources are low in comparison to those of the Humboldt River, chiefly because of the multiplier effect of flow volume.

A good example of the load value "paradox" is in the values for As: the concentration of As is relatively low (8 to 9 ppb) in the Humboldt River, well below the drinking-water standard, but the load value of 10 g/day is much higher than for the mine drainages having 1 to 4,300 ppb As. Likewise, for Cu and Zn the concentrations in the Humboldt River are very low (3 and 1 ppb, respectively), but the loadings are a hundredfold higher than the highly concentrated but low-flow mine drainages containing 0.3 to 6,100 ppb Cu and 2 to 7,300 ppb Zn (three atypical samples are excluded). The sampled mine waters do not flow directly into the Humboldt River. The loadings described here are simply compared to the Humboldt River, the area's largest flow of water, for comparison.

The concept of loads reminds us to think of the total amount of metal released from a source. Although the details are complex, it is helpful to contemplate the differences between such scenarios as (1) a 1-gal/min seep of acidic, metal-rich (5,000 ppb As) water from a mine dump; (2) a large streamflow (thousands of gallons per minute) carrying relatively low metal concentrations (10 ppb As); and (3) a mountain of altered volcanic rocks (unmined) containing disseminated sulfide minerals that weather and release acid and metals to runoff and ground water. In this study, it was not possible to quantify the flow parameter for discharges, although visual estimates were made; especially, it was not possible to quantify the numerous small springs and seeps that hydrologists term "diffuse sources" in a mountain range. At places in the descriptions of mining districts, comments will be made on metal loads in an attempt to translate the geochemical information into the new system of loads and TMDL's.

Hydrogeochemical Investigations of Historical Mining Districts

Investigations made by the author from 1995 to 2000 were in 50 mining districts (21 outside of Humboldt River Basin), at selected historical sites that allowed access. The approach was to examine and sample the mines that had relatively large production because I assumed that the larger operations would have more effect than smaller prospects and mines they disturb more ground and create larger waste dumps, and most have associated mills and tailings. Mining districts in the western part of the Humboldt Basin were described in a previous report (Nash, 2001) and are only briefly described here. The following district descriptions emphasize the districts in the central part of the Humboldt Basin. Reference is made to some districts outside of the Humboldt River Basin for the information that they provide as analogs to mining areas within the basin.

Adelaide District (Gold Run)

Location. East side of Sonoma Range, 10 mi south of Golconda in, Humboldt County (fig. 15). Features are shown in figure 16.

Major commodities. Au, Ag, Cu, Pb, Zn

Mining history. District was organized in 1865; a burst of activity in 1897–1910 included building a narrow gauge railroad from Golconda, a smelter at Golconda and expansion of mines (Lincoln, 1923). The Adelaide and Adelaide Crown mines were the major producers prior to 1940 and were served by a major mill; the underground mines and mill did not reopen after World War II. An unsuccessful attempt was made to bulk mine the lode Ag-Au ores in the period 1989–91 by small, open-pit operation to recover Ag and Au by cyanide heap-leach methods.

Status of mining and exploration. No mines were active in the late 1990's, but there was modest exploration and property surveying.

Production. Production figures are lumped for several districts in the area; according to Willden (1964), the majority of the $1.4 million production from the area was from the Adelaide and Adelaide Crown mines. This is consistent with the size of the "new" (1930's?) electrified mill and volume of tailings. A substantial amount of rock was excavated in the 1980's open-pit operation that attempted to mass mine vein zones that earlier were selectively mined underground; the amount of gold recovered probably was quite small. Data compiled by Vanderburg (1938b) for 1907–36 for the Adelaide district show production was high in Cu, Pb, and Zn, and although Ag was much more abundant than Au, the values from Au were high. The total value for 1907–36 was $561,232 from lode deposits and $6,170 from placer deposits. Production at the Adelaide Crown open pit and heap leach was 3,068 ounces (oz) Au and 37,537 oz Ag in 1990, and about one-half of that in 1991; the operation has been idle since then (Bonham and Hess, 1996).

Geology. Information is inconsistent because there is no published detailed (1:24,000 scale) map of the western half of the district; the eastern half was mapped by Marsh and Erickson (1978) who show that the Adelaide mine is in thermally altered Cambrian Prebble Formation. A fairly large body of Late Cretaceous (104 Ma) granodiorite is exposed a mile east of the Adelaide mine and is inferred to extend below the metasedimentary rocks because of the observed metamorphism and also the magnetic signature of the area. The western part of the district contains Valmy Formation in the upper plate of a thrust that may correlate with the Roberts Mountains thrust (Cookro and Theodore, 1989). Ore in the district is restricted to the Prebble Formation, which comprises carbonaceous phyllite, calcareous shale and limestone, and minor quartzite. Several types of dikes cut the Prebble in the vicinity of deposits; the dikes range in composition from diorite to rhyolite, and some are highly altered. North-striking, high-angle faults are common in the district and are the major control on the western deposits as in the Adelaide Crown group.

Ore deposits. There are at least three ore types: (1) Copper skarn with byproduct Ag-Au-W; (2) epithermal veins and stockworks in altered sedimentary rocks, possibly with similarities to the Carlin sediment-hosted type (Cookro and Theodore, 1989); and (3) placer gold in several of the east-flowing streams.

The skarn deposits were the major producer of the district at the Adelaide and nearby mines where calc-silicate minerals such as garnet, vesuvianite, and diopside replace calcareous strata in the Prebble, along with sulfide minerals including pyrrhotite, pyrite, galena, chalcopyrite, molybdenite, and scattered scheelite (Ransome, 1909; Cookro and Theodore, 1989). Most of the mining of the skarn ores was prior to 1908 when F.L. Ransome visited and noted that the Adelaide shaft was 300 ft deep and workings in ore were at least 400 ft long.

The deposits in the western part of the district are associated with normal faults and silicification, rich in silver and gold, and considered to be epithermal in character (Cookro and Theodore, 1989). The alteration to jasperoid and the trace-element assemblage of As, Sb, Hg, Tl, Ag, and Au suggests a similarity to Carlin-type sediment-hosted deposits (Cookro and Theodore, 1989), but more definitive work needs to be done to set these deposits into current concepts for Nevada gold deposits. The silver-gold deposits were mined from many underground workings, in and near the north-striking Adelaide fault, over a distance of about 4 mi. Geochemical results of Cookro (1993) for more than 40 elements show wide variation locally in the belt of silver-gold mines, suggesting multiple types of geochemical enrichment, including the unconventional variety rich in Be-Li. The district may be geochemically zoned, albeit complexly. The presence of skarn-suite elements (Cu-Mo-Bi, and others) in the belt of "epithermal" silver-gold deposits suggests that skarn-type fluids may have migrated well west of the Adelaide mine and that local variations in fracturing and in sedimentary lithologies complicate the zoning pattern.

Chemical analysis of five typical samples collected for this study show the ores to be polymetallic in character. Dump samples of the skarn deposits have high concentrations of Ag, Bi, Mo, Pb, Se, Te, and W and very high concentrations of Cd, Cu, and Zn. Samples from the veinlike deposits have high concentrations of Ag, As, Cd, Mo, Sb, and Zn, which is not unusual for some epithermal deposits that have a polymetallic aspects. A sample of slag from athe smelter at Adelaide has high concentrations of Bi, Cu, Mo, Se, Tl, W, and Zn; this composition confirms that skarn ores were processed by the smelter.

Mineral-Environmental Conditions

Environmental geochemistry in this district is more complex than in most districts and not easily evaluated. On the negative side, this district has widespread mining and surface trenching and very little reclamation. Geochemical studies (Cookro, 1993) show that the ores and waste piles contain high levels of base metals and arsenic. On the positive side, there is very little surface water, many of the ores are oxidized, and carbonate-bearing rocks and gangue are abundant. Silicified zones are commonly rich in iron oxides, creating gossanlike rocks that look hazardous. These silicified zones tend to be low in carbonate after the hydrothermal alteration has destroyed calcite, but calcite-bearing rocks are not more than tens of feet away. If acidic water is created during storm events or in mine drainage, the acids will not move very far before they are naturally mitigated to neutral pH.

Mineralized rocks and ores. Analytical results for four dump samples and two tailings samples are similar to the much more extensive data of Cookro (1993) and the known mineralogy: values for Pb, Zn, and Ag are high, as are As and Sb. Skarn ores and waste rocks are rich in base metals, arsenic, and sulfide minerals in the deeper unoxidized zones. These rocks have the potential to generate metal-rich acid-rock drainage, but there is no evidence for that in the prevailing dry climate. Dumps produce no visible drainage trails of iron-oxide coatings or vegetation kill. Carbonate minerals in the metasedimentary rocks probably neutralize acids as soon as they develop. Leach tests on one sample showed a pH of 7.4 and low concentrations of base metals but a high level of Se.

The lode Ag-Au deposits in the Prebble Formation contain an "epithermal suite" of elements (As, Hg, Sb, Tl) but generally low levels of base metals (Cookro and Theodore, 1989); pre-mining oxidation of most ores and waste rocks destroyed sulfide minerals in most mined rocks. The ore-associated trace metals are fairly stable in oxide forms. Leach tests on two samples developed pH's of 5.1 and 5.7; the pH 5.1 leachate had high to very high levels of Cd, Cu, Se, and Zn.

Mills and tailings. The location and status of early mills could not be determined, but the newer mill (1930's–1950's? of the Adelaide Crown mine produced a fair volume of tailings. These tailings are in a lowland, but the impoundment is stable and does not appear to have been breached. The tailings samples contain relatively low levels of base metals, but about 20 ppm Ag is present.

A small amount of slag (tens of tons) is present where it was poured from an old smelter near the Adelaide mine shaft. Nothing remains of the smelter structure. The vitreous black slag is typical of slag from the late 19th century with varied textures that resemble vesicular lava flows; inclusions of limestone flux are common. Chemical analysis of the slag shows very high amounts of Cu, Pb, Zn, and As. Leach tests on similar slags show them to be relatively stable in near-neutral pH water. The slag poses no significant concerns in the current situation, but use of this material for construction purposes could cause ingestion of particles containing very high concentrations of base metals.

Surface water. Four samples (three streams, one pit lake) collected in 1996 and 1997 have pH values in the range 8.2–8.5 and low to moderate conductivity. Water chemistry of a pit lake (Adelaide Crown) is presented and discussed by Price and others (1995) who measured a pH of 7.0. My sample of water, ponded in a cut through a skarn deposit, contained high concentrations of As, Cu, Mo, and Se. A sample from a small pit lake in the Adelaide Crown property (polymetallic vein type) contained high As, but other metal concentrations were below the study average. A sample from a puddle on crushed material near the heap-leach facility contained high concentrations of Al and As.

Goldrun Creek, which flows through the area of skarn and polymetallic deposits and mines, has substantial flow and is an important resource for the area (wildlife, livestock, irrigation). The pH of 8.5 is normal, the conductivity of 650 μS/cm is higher than normal for the area, and concentrations of As, Mo, and Se are higher than the study average but below the ALWS and drinking-water standards. Other metal concentrations, including Zn, are low to very low. The water in this district appears to be effectively buffered by the sedimentary rocks, which may have prevented mobilization of the acid suite of base metals such as Cu and Zn.

Summary. The gold-bearing placer deposits in the district are evidence that mineralized rocks were exposed at the surface: ore-associated metals in addition to gold must have been dispersed down the paleostreams before the arrival of people and mining. The prevailing alkaline pH values near 8.2 suggest that the sedimentary rocks of the district, as well as alluvium, are effectively buffering pH and minimizing the mobility of most metals. Metalloids (As, Mo, Se) are enriched in surface water, but concentrations are below water-quality standards. There is virtually no chance for these waters to contaminate the Humboldt River because there rarely is flow that far at the surface in Goldrun Creek, a tributary to Ragan Creek. Dilution and adsorption reactions would mitigate metals in the ground-water regime. This is typical of streams leaving mining districts 5 to 15 mi from the Humboldt River.

Antelope Springs District

This mercury mining district, located 15 mi east of Lovelock (fig. 2), was active intermittently from about 1914 to 1970 and most productive in the 1940's, with a total of more than 12,000 flasks. Mineral-environmental descriptions and data can be found in Gray (2003) and Nash (2001).

Austin District (Reese River, here includes Birch Creek District)

Location. West side of Toiyabe Range, Lander County, 80 mi south of Battle Mountain (fig. 15). Features of part of the area are shown in figure 17.

Major commodities. Ag, U, minor Au, Cu, Pb, Zn, and turquoise

Mining history. Silver was discovered in 1862 along the route of the Pony Express, leading to a great land rush. The richest ore was mined in the 1860's, and mining declined through 1886. There was modest silver production after 1900. The uranium rush of the 1950's hit this area, and the largest uranium deposit in Nevada was located in 1953; the Apex (Rundberg) mine was active in the 1950's and explored again in the late 1970's, but no mill was ever built. Small, high-quality turquoise deposits have been mined intermittently since 1930.

The Quito gold deposit was discovered in the early 1980's at the site of a known stibnite deposit, 10 mi southeast of Austin. This sediment-hosted deposit was mined by an open pit, with substantial logistical difficulties in the rugged mountains, from 1986 to 1989.

Status of mining and exploration. The silver prospects have been dormant for many years in a period of depressed silver prices. The Quito gold mine and heap-leach facility have been closed and reclaimed. Stockpiled uranium ore at the Apex mine has remained untouched in 15 years of low prices. Exploration in the district is minor.

Production. Records for the early days of silver mining are incomplete but estimates are about $20 million, with possibly $500,000 to $1,000,000 after 1900. At an average price of $1.20 an ounce for silver from 1863 to 1886, actual silver production would have been about 17 million ounces. The value of uranium was more than $100,000 and of turquoise, probably less than $50,000.

Geology. Geology of the silver deposits was described in detail by Ross (1953) and the district setting revised by Stewart and McKee (1977). Oldest rocks in the district are early Paleozoic metasedimentary rocks, which were intruded by the large Austin pluton of Jurassic age and intermediate composition. Thin patches of Tertiary volcanic rocks overlie the older rocks. The many silver veins cut the pluton and nearby metasedimentary wall rocks, and the uranium veins likewise are in quartzite and phyllite at the southern margin of the pluton. The Quito gold deposit is in Paleozoic sedimentary rocks that are highly deformed by thrust and normal faults.

Ore deposits. Three types of ore deposits have been important in this district: (1) silver-rich quartz veins associated with the Jurassic pluton; (2) uranium (pitchblende and autunite) veins in metasedimentary rocks; and (3) gold disseminated in siliceous and calcareous rocks. The gold deposits have been called "Carlin-type" (sediment-hosted gold) but the other deposits are of uncertain classification. The silver veins are not like most epithermal veins in other parts of Nevada; rather, the silver minerals are in very coarse-grained quartz that has a texture suggestive of a hotter and deeper "mesothermal" setting, consistent with the probable deep setting of the Jurassic pluton. Tourmaline in the silver veins also is suggestive of the deeper plutonic setting, such as present in polymetallic quartz veins in plutons of Montana and Arizona. The veins at Austin were deeply oxidized and the silver enriched above the water table. There is no model for these veins in Cox and Singer (1986).

Chemical analyses of seven samples from mine dumps and tailings related to the Ag veins show them to be characterized (relative to other deposits and districts in this study) by high concentrations of As, Cd, Cu, Mo, Pb, and Zn, and very high concentrations of Ag and Sb.

The uranium veins and disseminations are in metasedimentary rocks and dikes adjacent to the Austin pluton and some ore is in the pluton itself (Garside, 1973; J.T. Nash, unpub. data, 1977). Most of the early mining was in near-surface zones containing autunite and iron oxide minerals, but the prospecting in the 1970's was chiefly in deeper reduced zones containing pitchblende with pyrite. The uranium deposits are of uncertain age and origin because most of the geologic observations were for autunite, which is probably redistributed from older primary pitchblende (Plut, 1979). Sulfide- and pitchblende-rich ores, abundant on stockpiles, were not exposed in workings or dumps in 1977, when I examined them, and are not described in older publications. The pitchblende has not been reliably identified. The primary sulfidic ores may have been similar to uranium deposits of the Midnite mine, near Spokane, Washington (Ludwig and others, 1981).

New chemical analyses of seven dump samples from the Apex mine show them to be enriched in many metals in addition to uranium. Relative to the median concentrations for all dump samples in this study, the Apex dump samples have high concentrations of As, Bi, Cd, Cu, Mo, Pb, Sb, Se, and Tl; concentrations of Mo, Sb, Se, and Tl are notably high. Leach tests on six samples of uranium-mine waste yielded pH values in the range of 4.0 to 6.8. The more acidic leachates carried high concentrations of Al, As, Cd, Cu, Se, and Zn and very high concentrations of U. The potential for acidic water to mobilize U from these waste piles may be significant and, for this study, is unique to these materials.

Mineral-Environmental Conditions

Mineralized rocks and ores. The silver, gold, and uranium deposits of the district have very different rock and mineral compositions. One major difference is the composition of the Austin pluton and adjacent siliceous metasedimentary rocks: both of these rock types are predicted to have low acid-neutralizing capacity (Glass and others, 1982). The Paleozoic rocks in the Quito gold mine area, however, are rich in carbonate minerals and are predicted to have very high acid-neutralizing capacity. The silver veins are richest in base metals that can be toxic if transported, the gold ores are rich in a small suite of elements (As, Sb, Se, Zn) that can be a concern, and the uranium deposits are rich in uranium and radionuclides, which pose a distinct set of environmental concerns. Deep oxidation of the silver ores prior to mining (and the tendency to mine those ores only in the oxidized zone) means that the mine workings and waste dumps contain oxide minerals rather than sulfides. Here, potentially toxic metals such as As and Pb tend to be stable in oxidized minerals (or are adsorbed on iron oxide minerals), and there is a decreased tendency to form acid-rock drainage.

The uranium ores pose concerns that are unique in the study area. The mined rocks are very rich in uranium. Judging from radioactivity measured by a scintillometer (an indirect measurement of uranium and radioactive decay products, or radionuclides as a general term), the radioactivity of mined rocks (waste or stockpiles) ranges from not much higher than some volcanic rocks of the region to extremely high. Some of the rock piles with highest radioactivity appear to be stockpiles of material mined in the late 1970's these are less weathered and contain more pyrite than the rocks on piles thought to be from mining in the 1950's. Many of the piles have higher radioactivity than ores in other districts that were hauled to mill for processing. Based on experience with uranium ores at other mines but having no training in health physics, I estimate that about one-half of the waste piles do not meet radioactivity standards. The caveat in this case is exposure: are people being exposed to the radioactivity of these waste piles? Is the exposure short-term or long-term? Further work is needed to assess the exposure risk to humans. Caution is suggested to residents. In particular, these rocks should not be hauled away for use in construction where people will be living on them or breathing their dust.

The uranium ores and waste dumps are surprisingly rich in As, Bi, Cd, Cu, Mo, Pb, Sb, Se, and Tl, all of which are potentially toxic. Leach tests of six samples of mine-waste dumps and stockpiles produced results that are unique among the samples in this study. The leachate pH values were in the range of 3.6 and 6.8, not unexpected considering the pyrite and iron oxides in these rocks. However, the composition of the leachates was much richer in metals than anticipated. The leachates were high in the following metals: As (6 samples), Cu (4), Fe (1), Mo (1), Pb (6), Sb (5), Se (4); and U was very high in all six leachates (the highest values observed in this study). The concentrations of uranium (15 to 135 ppb) were higher than concentrations obtained from samples of uranium dumps from two uranium districts in Colorado using the same leach and analytical methods (Nash, 2002a).

Mills and tailings. This was one of the largest of the early mining camps with 11 mills erected prior to 1868; Vanderburg (1939) noted that many of the mills were speculative and unproductive. Numerous "firsts" were made here, including the Reese River process for roasting silver ores with salt to improve the recovery of silver and gold (this process creates the distinctive red tailings seen at many mills in the region). The numerous mills described in the literature are difficult to reconcile with features seen today because they commonly burned or were dismantled, and tailings commonly were reprocessed as technology improved. Because the veins were mined underground and ores were hand sorted underground and again at the mill, the volume of tailings produced is much smaller than expected for a district that produced $20 to $50 million in silver. Another reason for the discrepancy in observed tailings is the common practice of retreating or removing early (pre-1900) tailings that contained substantial amounts of silver that the early processes failed to recover. Today, five mills are evident but several pre-1900 mills were not recognized. A significant volume of tailings is present only in Pony Canyon on the west edge of Austin, west of the Clifton tunnel that extends under the mines on Lander Hill. Two mills are at the Clifton tunnel (photo 25), an older one built of wood and a newer one of metal, probably built in 1935. A large concrete mill foundation can be seen in New York Canyon, 3 mi north of town, near the True Blue mine; no tailings could be located at this site. Two relatively modern mills are located at the western mouth of Slaughterhouse Gulch, a mile north of town, but they appear to have had only small production.

Tailings in Pony Canyon west of Austin blend in with alluvial sands shed from the granitic rocks; thus, their volume and extent are difficult to estimate. In recent years an attempt was made to recover silver in a heap-leach operation on the mill tailings. Most of the heap-leach operation was reclaimed by the BLM in 1999. A pond in the middle of the tailings area that was left after reclamation collects water with a pH of 8.0 and high conductivity; concentrations of Cu, Sb, and U are elevated, and the concentration of As is notably high at 77 ppb.

The Pony Canyon tailings are possibly at risk to erosion in an extreme storm. The canyon setting and geometry seem favorable for focusing stormwater, and a substantial part of the tailings remain in the flood plain after reclamation. Many of the questions for this site relate to regulatory criteria that deal with effects on human or wildlife health. Erosion of tailings in a storm event probably would have little effect on property as there are no structures downstream. These tailings are relatively unreactive, judging from their composition, and stormwater carrying tailings probably would have only a transient effect on wildlife or ground-water quality upon infiltration.

Surface and ground water. The higher elevations of the Toiyabe Range receive far more precipitation, especially as snow (more than 16 inches), than the valleys (about 8 to 16 inches; Houghton and others, 1975). The main productive area of the district near the town of Austin is intermediate in terms of precipitation, but there is little surface water, and only a few mine portals drain water. During the main period of mining, deeper operations that were reached by shafts had significant water problems that required costly pumping. A drainage tunnel was driven from the west after most of the mines ceased operating (Ross, 1953). No samples of water were collected, but there were no signs of acid-mine drainage (such as Fe-Mn-oxide coatings). No water drains from uranium mine workings, and surface water in that area does not contact waste rocks; however, storms and snow do affect these dumps, and some of that moisture infiltrates dumps and enters ground water.

Trees and flowing surface water testify to the high amount of precipitation in the Quito mine area at an elevation of 8,500 ft. The open pits, which have been reclaimed, contain no ponds and produce only a small amount of seepage. One seep in the floor of the east pit had a pH of 6.0 (photo 24), and seepage through the waste dumps has pH values of 7.9 and 8.0. The acidic seep carried high concentrations of Cd, Cu, Fe, and Zn relative to the average values for this study. The seeps through the mine-waste dumps carried very high Se concentrations (32 and 48 ppb). The streams leaving the Quito mining area are well buffered at pH 8.2–8.4 and have metal concentrations well below the study average and below the ALWS.

A small amount of water flows episodically in Pony Canyon, west of the village and west of the mining and milling area. In June of 2000, a small pond of water collected in the area of mill tailings along Pony Canyon, in the area reclaimed in 1999 by BLM. The pond water had a pH of 8.0 and carried high As and U concentrations but low metal concentrations. This water may reflect multiple sources and processes, including effects of past heap-leaching and evaporative concentration. Water in the canyon generally infiltrates the porous alluvium east of the mill tailings but during some storm events must flow a mile or more to the west. The water well

SIR 2004–5236

Hydrogeochemical Studies of Historical Mining Areas in the Humboldt River Basin and Adjacent Areas, Northern Nevada

By J. Thomas Nash

Table of Contents

Solids Chemistry

Water Chemistry

Leachate Chemistry

Polymetallic Deposits

Skarn Deposits

Epithermal Vein Deposits

Mercury Deposits

Sediment-Hosted Gold Deposits

Porphyry Cu-Mo Deposits

Massive Sulfide Deposits

Acid Sulfate Deposits

Uranium Vein Deposits

Acid Water

Water Composition and Deposit Types

Alkalinity of Surface Water

Buffering and Acid Neutralization by Rocks

Reactions that Remove Metals

Geology-Based Reference Values

Biology-Based Standards

Flow Volumes of Surface Water

Metal Loads and Total Maximum Daily Loads

Adelaide District (Gold Run)

Mineral-Environmental Conditions

Antelope Springs District

Austin District (Reese River, here includes Birch Creek District)

Mineral-Environmental Conditions