Controls on pit lake water quality at sixteen open-pit mines in Nevada

PERGAMON

Applied Geochemistry 14 (1999) 669±687

Controls on pit lake water quality at sixteen open-pit mines in Nevada Lisa Shevenell a, *, Katherine A. Connors b, Christopher D. Henry a a

Nevada Bureau of Mines and Geology, University of Nevada, Reno, MS 178, Reno, NV 89557-0088, USA b Mackay School of Mines, University of Nevada, Reno, MS 178, Reno, NV 89557-0088, USA Received 15 April 1998; accepted 13 September 1998 Editorial handling by D.D. Runnells

Abstract Thirty-®ve mines in Nevada currently have, or will likely have, a pit lake. The large bulk mineable deposits in Nevada mined below the water table are of several types, including Carlin-type Au, quartz-adularia precious metal, quartz-alunite precious metal and porphyry-Cu (-Mo) deposits. Of the 16 past or existing pit lakes at 12 di€erent Nevada mines, most had near neutral pH and low metal concentrations, yet most had at least one constituent (e.g., SO4) which exceeded drinking water standards for at least one sampling event. Water quality data indicate that, in general, poor water quality will not develop in Carlin-type Au deposits. Wall rocks in the geologic environment typical of these deposits, and in the speci®c pits sampled, contain substantial amounts of carbonate, which bu€ers the pH at slightly basic conditions and thereby limits the solubility of most metals. Similarly, the quartz-adularia precious metal deposits generally have geologic conditions that bu€er pH and naturally prevent the development of poor water quality. In both of these deposit types, certain elements such as As and Se that are mobile in neutral to basic waters may accumulate to levels near or exceeding drinking water standards. Pit lakes forming in quartzalunite precious metal deposits hosted in volcanic rocks or in porphyry-Cu (-Mo) deposits in plutonic rocks are of greatest environmental concern in Nevada, as both deposit types have relatively high acid-generating potential and low bu€ering capacity. However, the sampled Nevada pits in these deposit types indicate that the water may not be of poor quality. In addition, water quality in some pits may actually improve with time due to the increased waterrock ratio as the pit ®lls with water, as suggested by pit waters at one mine in a Carlin-type deposit (Getchell) that improved between 1968 and 1982. Although water quality in pits in each deposit type is generally good, local, site speci®c conditions (e.g., surface water in¯ow) and variations (e.g., evaporation rates) result in some pit lakes (e.g., Boss) in the quartz-adularia deposit type being of substantially poorer water quality than other lakes (e.g., Tuscarora) in the same deposit type. Despite underlying geologic controls based on deposit type, site speci®c variations in hydrogeologic conditions and surface geologic features can result in di€ering water quality in pit lakes in the same deposit types, and these factors may, in some cases, provide an overriding control on the geochemical evolution of speci®c pit lakes. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Mines in Nevada have produced a variety of metals, including Au, Ag, Cu, Fe, Pb, W and Zn. Arsenic and

* Correspondig author. Tel.: 702 784 6691; Fax: 702 784 1709; E-mail: [email protected]

Hg, potentially toxic elements commonly found with precious metal deposits, have also been produced. Other potentially toxic elements of local concern include B, Cd, Se and Tl. With continuing development of mining and processing techniques, it has been possible to exploit pro®table ore deposits with lower and lower concentrations of precious metals. Because very low concentrations (e.g., 0.02 ounces of Au per

0883-2927/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 8 8 3 - 2 9 2 7 ( 9 8 ) 0 0 0 9 1 - 2

670

L. Shevenell et al. / Applied Geochemistry 14 (1999) 669±687

ton) can be mined economically from bulk mineable deposits, large open pit mines have been and are currently being developed. Many of these mining operations in Nevada are extracting ore from below the water table, and hence, are withdrawing large volumes

of water in order to maintain dry operating conditions. Once these mines complete operations and stop dewatering activities, the pits will slowly ®ll with water, approaching an elevation of the pre-mining groundwater level. There are currently 35 open pit mining

Fig. 1. Map of Nevada showing locations of current and future open pit mines that will contain water upon cessation of mining activities. Pits discussed in this paper appear in larger text.

L. Shevenell et al. / Applied Geochemistry 14 (1999) 669±687

operations in Nevada (Fig. 1), many that will contain water upon cessation of mining, and some of which presently contain water. The environmental concerns related to dewatering and eventual ®lling of large open pits include the regional e€ects on water tables during and after pumping, the rate of pit lake ®lling, the ultimate water quality, the limnology, and potential impact of the lakes on wildlife. In formulating conceptual models and attempting to predict pit lake water chemistry, factors that must be considered include availability of metals, and the acidity and alkalinity of the ore deposit. Additional factors that must be evaluated in a thorough pit lake model include composition and ¯ux of in¯owing groundwater and surface water, oxidation, leaching, evaporation, and ¯uid residence times. Because many of these factors have not been evaluated at existing Nevada pit lakes, it is currently unknown if the pit lakes will be attractive nuisances drawing wildlife to an undesirable source of water, or if the lakes will ultimately be a valuable recreational resource. Although recent work has begun to address these issues (e.g., Davis and Ashenberg, 1989; Throop, 1991; Lyons et al., 1994; Kempton et al., 1995, 1997; Axler et al., 1996; Levy et al., 1996; Miller et al., 1996; Pillard et al., 1996; Atkins et al., 1997; Connors et al., 1997; Doyle and Runnells, 1997) considerably more information is required before these and other concerns can be adequately evaluated. This paper focuses on the level to which geologic characteristics of di€erent deposit types dictates ultimate pit water chemistry and provides a general, comparative view of pit lake water quality without investigation of detailed conceptual models (e.g., water balance) at individual mines. The problem of acid mine drainage and acidic pit water has long been recognized and the results of this type of geoenvironmental impact are far too evident in areas such as the Leadville district of Colorado or the Berkeley pit in Montana (Davis and Ashenberg, 1989; Plumlee et al., 1995a). The relative abundance of minerals that generate or neutralize acidity, mostly pyrite and calcite, determine whether or not a water in contact with a deposit will be acidic. In light of the recognized hazards, the most important environmental geochemical reaction involving pit water and exposed pit walls is that of sul®de mineral oxidation. Pyrite (FeS2), is a nearly ubiquitous mineral in metallic deposits, although not normally an ore mineral itself (Park and MacDiarmid, 1975; Cox and Singer, 1986). Pyrite is stable in the reduced environments in which many ore deposits form but breaks down in oxidizing environments, for example, where exposed at the earth's surface by natural erosion or by mining (Ficklin et al., 1992, 1994). Rapid acid generation is also caused by microbial oxidation of Fe + 2. These reactions result in release of H + , leading to decreased

671

pH. The concentration and ¯ux of dissolved O2 in the water and the mineralogy of the pit wall are the most important parameters in predicting the initial chemical consequences of rock-water interactions in the pits. Oxidation of less abundant sul®de minerals, including marcasite (also FeS2), chalcopyrite (CuFeS2), galena (PbS), sphalerite (ZnS), and realgar (AsS) can also generate acidity. Acidity can be neutralized by reaction with a variety of minerals, especially calcite (CaCO3) and dolomite (CaMg[CO3]2), and also by reaction with some silicate minerals. However, silicate neutralization reactions are generally slower than those involving carbonates and are, therefore, somewhat less e€ective at neutralization. Oxidation of pyrite, acid generation, and acid neutralization are all natural processes that occur in and around mineral deposits regardless of mining activity (Ficklin et al., 1992, 1994). It is hypothesized that the water quality that develops in a mine pit is strongly in¯uenced by the physical characteristics of a deposit, especially mineralogy and associated chemical components. This work presents a compilation and preliminary analysis of existing data on water quality at open pit mines in Nevada that began ®lling within the last two decades. These data are combined with information about the geochemical and mineralogical characteristics of the deposits being mined, and are used to determine which geologic environments are likely to result in poor water quality at ®lled open pits and which elements are of concern in these particular environments. Generalized geochemical characteristics among the pit lakes are evaluated here in the context of their individual geologic deposit types; however, detailed conceptual model development at individual pit lakes is beyond the scope of the current work. 2. Methodology In order to determine if geologic deposit type is a strong controlling factor in dictating Nevada pit lake geochemistry, data were compiled from all available sources to obtain water quality data to represent as many geologic deposit types as possible. Water quality data were limited to those deposits which currently have, or have had, water in open pit mines. Hence, not all deposit types in Nevada are represented in this work, and data from only 4 di€erent deposit types (and a sub-type) are presented below. Various data sets were used in this study, including the U.S. Geological Survey's Mineral Resource Data System and the Nevada Bureau of Mines and Geology's chemical data sets on ores (from over 4000 mines throughout the state) and country rocks, to determine which deposits are known to contain, or are likely to contain, anomalous concentrations of poten-

1990-93

1905-50

1930-40's 1979-82

None

1875-1905

Major oxidized cap mostly of chrysocolla

Cu

complete 30-45 m partial to 120 m

major supergene chalcocite cap

Cu, Au

Au, Ag

some

to 50-150 m depths

some

some

some

Extensively oxidized to >60 m < 5%

negligible

Pre-mining oxidation

Mo, Cu

Au, Ag

Au, Ag

Au

Au, Ag

Au, Ag

Au Au, W, Mo, As

Au

Au, Ag

Major commodity

7

1 2 5 6

8

11

1

20

8

5

1

22 24 33

79

10

11

No. of samples

33

350

*

no record

37

75

23

65

< 20

112 119 75

15 dry shallow/ dry

Depth (ft)

6/97

1997

1997

8/97

8/95

8/95

1995

1983 1983 1983

11/95

7/95 10/95

Year

6/1/92

1979

2/91 and 1/94

11/93

1990

1989

1993

1930's

4/19/68 4/19/68 4/19/68

mid 1970's

1994

Began ®lling

a,o

a,n

a,m

a,j,k

a,l

a,h

a,f

a,f,g

a

d,e

a,b,c

a

Reference

* Water levels ¯uctuated as water was periodically pumped between pits. a, NDEP, 1997; b, Zimmerman, 1997; c, Radtke et al., 1987; Getchell Mine Files, 1997; e, Barnes, 1997; Atkins et al., 1997; g, Osborne, 1991; h, Nolan, 1936; i, Sillitoe and Lorson, 1994; j, Goodrich, 1997; k, Shaver, 1991; l, Seedor€ et al., 1995; m, Byrns, 1997; n, Hershey, 1997; o, Maddry et al., 1987.

Sediment hosted epithermal deposit type: Manhattan, 22? 1985-91 Nye County

Porphyry copper/molybdenum deposit type: Cyprus Tonopah, Nye County 29 M lbs Mo 1988-90 Robinson district, White Pine County 350 1908-78 Liberty Pit 1948-68 Kimbley Pit 1951-66 Ruth Pit 1968-78 Yerington, > 60 1905-78 Lyon County

Quartz-Alunite deposit type: Ketchup Flat, 8.2 Nye County

1987-90

1987-89

1988-96

late 1800's to early 1900's 1860'ssporadic None?

1961-68 1938-60

1985-pres

1990-91

None?

1968-73

3.5

None

1987-93

Dates

Previous mining

4.3

(million tons)

Production

Quartz-Adularia deposit type: Adelaide Crown <1 north pit, Humboldt County Aurora Partnership, 1.5 Mineral County Boss, 0.65 Esmeralda County Tuscarora, 1 Elko County

Carlin deposit type: Big Springs, Elko County Cortez, Lander County Getchell Mine, Humboldt County North Pit Center Pit South Pit

Location

Table 1 Summary of water depths and numbers of samples collected at existing Nevada pit lakes 672 L. Shevenell et al. / Applied Geochemistry 14 (1999) 669±687

L. Shevenell et al. / Applied Geochemistry 14 (1999) 669±687

tially toxic elements (e.g., As, Se) and are likely to result in acid conditions or otherwise poor water quality. Additional ®eld examinations and sampling of rocks and waters were undertaken as part of this work to ®ll in some of the gaps in current knowledge. In the summer of 1995, the Nevada Bureau of Mines and Geology investigated several mines at which pits have at least partially ®lled with water since the close of mining (Table 1; Fig. 1). Samples were collected from 3 of the pits (the north pit at Adelaide Crown, Tuscarora pit, and the Aurora Partnership pit) in the summer of 1995 following one of the wettest winter-spring rainy seasons on record. Detailed methods of sample collection and analysis are reported in Price et al. (1995). The water samples were collected from these 3 pit lakes from a small rubber raft located in the center of the pit. A Te¯on bailer was lowered to 11.5 m below the surface, and to a depth of 13 m o€ the bottom of the lake to collect 2 separate samples at 2 di€erent depths. Waters for cation analyses were ®ltered through 1.6 mm ®lter papers to remove any arti®cially introduced turbidity, yet with the intention of retaining any truly colloidal material contained in the water column. Prior to sample collection, the polypropylene sample bottles used for cation analysis were soaked with 50% HNO3 for 2 days and then rinsed a minimum of 3 times with deionized (DI) water. Bottles used for anion analyses were also rinsed with DI water, and both bottle types were ®lled with DI water, which was emptied just prior to sample collection. All bottles were rinsed with ®ltered sample water prior to sample collection. All bottles were double bagged in zip-lock bags to avoid particulate contamination. The pH of the cation samples was adjusted with reagent grade HNO3 to a pH of R2 in the ®eld. Additional data were compiled from the Getchell Mine ®les for 3 pits that had ®lled between 1968 and 1982, but have since been pumped dry to allow additional underground mining activity at the site. Also included in the tabulation are data from the Yerington, Aurora Partnership, and Boss pits collected by PTI Environmental Services in August, 1995 (PTI, 1996). PTI (1996) reported data for 2 composite samples from each pit lake with each sample comprised of water collected in multiple positions within the pit lake. In addition, published data (Miller et al., 1996) from the Liberty and Kimbley pit in the Robinson district (Ruth, NV), and from the Yerington pit were incorporated into the database. The locations of pits for which data are available are noted in Fig. 1. Although pit lake data were compiled from the above sources, most of the data presented here were compiled from ®les at Nevada state oces, submitted by individual mining companies. Of the 12 mines containing water, all existing data that were submitted to the Bureau of Mining Regulation and Reclamation or

673

the Nevada Division of Minerals were compiled and a partial data listing appears in Price et al. (1995). Water quality data listings on the paper copies ®led at the Bureau of Mining Regulation generally did not include sampling location or procedure information. Because these data were collected and reported for compliance purposes, it is assumed that, unless otherwise noted, samples were collected in the same general locations from the surface of the pit lakes during each sampling event. This assumption appears reasonable due to the likelihood of individuals selecting the location with the easiest access to the pit lake from which to collect samples. In addition, many compliance-driven monitoring activities require ®ltration through 0.45 mm ®lter papers, and acidi®cation of the cation samples to a pH < 2 with HNO3, and refrigeration of samples until sample analysis, generally within 2 days for many constituents. Because this type of sample collection and preservation is standard in compliance-driven water quality data collection, it is assumed that these standard methods were employed during pit lake sampling at each of the mines discussed here. Table 1 summarizes the number of samples available from each pit lake, their deposit type, and recent recorded water levels at the pit lakes. Due to the extensive amount of data resulting from the tabulation of data from these varied sources, a complete tabular copy is not included here but can be obtained from the the Nevada Bureau of Mines and Geology anonymous ftp site (http://www.nbmg.unr.edu). 3. Geoenvironmental mineral deposit characteristics The near-surface parts of most sul®de-bearing deposits, including several examined in this study, have been variably oxidized. This natural oxidation destroyed much or all of the contained pyrite that would have generated natural acid waters. Mining changes the natural setting by increasing the local permeability of the rock, allowing greater access to rain, surface water, and groundwater. Enrichment, and therefore availability, of various chemical elements of environmental concern is another important in¯uence on pit water quality. Many metallic elements commonly enriched in mineral deposits, including Pb, Cu and Zn, are signi®cantly soluble only in acidic waters. However, several other trace elements of concern, including As, Hg and Se, which are often elevated in Nevada mineral deposits, can be soluble in neutral to alkaline waters. Naturally high concentrations of these elements, particularly As, have been reported in waters near undisturbed mineral deposits (Welch et al., 1996; Lico, 1992), as well as in many of the known mining districts in Nevada (Fig. 2). Fig. 2 shows the mining districts in Nevada known to have

674

L. Shevenell et al. / Applied Geochemistry 14 (1999) 669±687

Fig. 2. Mining districts in Nevada in which rock samples have been found to contain >200 ppm As in the bulk rock composition.

signi®cantly elevated As concentrations in the rocks (Asr200 ppm). In comparison, the average crustal abundance of As in unmineralized areas is only 1.8 ppm, ranging from 11 ppm in ultrama®c rocks to 15 ppm in shales (Levinson, 1974). An understanding of general geochemical characteristics that typify particular mineral deposit types can be used to make initial predictions about the quality of water that would result from interaction with rock at a speci®c mine or deposit. Exceptions to the predicted geochemical pattern are largely the result of site-

speci®c hydrogeologic conditions. Another important factor to recognize in the interpretation of the geologic controls on the chemical evolution of pit water is that, while each of the deposit types discussed here is distinct, they all share common features that set them apart from unmineralized rocks. That is, metallic mineral deposits are, by de®nition, a concentration of particular metals of economic interest; the various conditions that lead to these concentrations lead to systems that are perhaps more alike than di€erent when compared to unmineralized systems.

L. Shevenell et al. / Applied Geochemistry 14 (1999) 669±687

675

3.1. Carlin-type deposits

3.2. Porphyry copper-molybdenum

Carlin-type deposits, also termed sediment-hosted, disseminated precious metal deposits (Percival et al., 1988) or carbonate-hosted Au-Ag deposits (Cox and Singer, 1986), are the major source of Au production in Nevada. These deposits are generally large-tonnage, low-grade epigenetic Au deposits hosted predominantly in calcareous or dolomitic sequences (e.g., the Twin Creeks deposit), but also occurring in siliceous sedimentary rocks (e.g., the Betze (Goldstrike) deposit), and sometimes igneous rocks (e.g., the W side of the Betze pit). Host rocks for these deposits are often carbonaceous and generally contain abundant calcite. This deposit type is best known and most abundant in the Basin and Range province of Nevada and western Utah and is the most important Au deposit type currently being mined in the western United States (see Kuehn and Rose, 1995). The deposits typically contain relatively abundant disseminated pyrite and commonly lesser amounts of arsenopyrite (FeAsS), realgar (AsS), orpiment (As2S3) stibnite (Sb2S3), and cinnabar (HgS). The hydrothermal ¯uids that generated the deposits were acidic and were capable of dissolving calcite, so calcite is variably depleted within and immediately adjacent to the deposits. However, abundant calcite is generally preserved nearby. Sediment-hosted Au deposits have low potential for associated environmental problems relative to other mineral deposit types such as porphyry or massive sul®de deposits, particularly considering their large size (Hofstra et al., 1995). Trace elements generally enriched in or around the deposits include As, Sb, Hg, Tl and Ag, 2 Ba, W and Se. Oxide ore, which has been the principal exploited resource until recent years, has very little to no acid generating capacity and the acid consuming capacity of the host or surrounding rocks is commonly very high. Refractory ore, the unoxidized equivalent of the oxide ore, is generally variably decalci®ed, argillized, silici®ed, sul®dized carbonaceous sedimentary rocks containing disseminated Fe, As, Sb and Tl sul®de minerals. These minerals have high acid-generating capacity but are generally present in very small amounts (<5% by volume) in near-surface mines and are contained within or very near rocks with a high bu€ering capacity. In general this deposit type should have limited potential for downstream or o€-site environmental e€ects. However, elements soluble in neutral to basic waters and commonly elevated in these deposits include As, Sb, Hg, Se and Tl (Hofstra et al., 1995), and these may become elevated in pit waters, particularly during periods of drought when the e€ects of evapoconcentration are exacerbated.

Porphyry-Cu and porphyry-Mo deposits are very large, mineralogically and geochemically complex deposits hosted by felsic intrusive rocks (Table 2). The deposits are commonly zoned such that di€erent minerals are unevenly distributed through the deposit, and many porphyry-Cu deposits are spatially and genetically associated with other deposit types, notably basemetal (Cu and Fe) and precious-metal skarn. Pyrite, various Cu or Mo sul®des, and other base-metal sul®des are abundant throughout all these types. Calcite abundance is generally low in porphyry-Cu deposits, except in an outer halo of propylitic alteration and in the surrounding rocks, particularly if limestone was intruded by the felsic rocks. Porphyry-Cu deposits can be enriched in a wide range of metals and other elements, including Mo (although the Robinson district has little Mo), Au, Ag, Pb, Zn, As and Sb. Porphyry Mo deposits, including Cyprus-Tonopah (Hall deposit), are commonly enriched in Cu, Ag, Pb, Zn, and Au. A common feature of many porphyry-Cu/Mo deposits, including the Robinson district and CyprusTonopah, is a zone or layer enriched in chalcocite (Cu2S) developed by supergene enrichment. Much of the ore mined from the Liberty, Kimbley, and Ruth pits at Robinson was enriched in supergene chalcocite (Smith, 1976). 3.3. Quartz-adularia precious metal Quartz-adularia deposits are the second leading Au producer in Nevada after Carlin-type deposits. They contain variably low to moderate concentrations of pyrite and other sul®des. These deposits are generally hosted by volcanic rocks that initially contained little calcite; however, the neutral, low dissolved-solids hydrothermal solutions that generated the deposits also commonly deposited calcite. Quartz-adularia deposits include 2 distinct types: (1) those having high Ag/Au and moderate base metals and sul®des typi®ed by deposits of the historical Comstock district near Reno, Nevada, and (2) those having low Ag/Au and negligible base metals and sul®des such as Round Mountain in central Nevada (Fig. 1). The host rocks in both types of quartz-adularia deposits are generally intermediate to felsic volcanic rocks and associated volcaniclastic and sedimentary rocks. Alteration suites generally involve potassic alteration (adularization), argillic alteration (quartz, sericite, illite, with distal smectite), and silici®cation. These rocks generally have low to moderate bu€ering capacity. Some districts exhibit propylitic assemblages (chlorite, calcite, epidote, pyrite) distal to veining that may have higher acid-consuming potential. Trace elements enriched in the deposits can include As, Sb, Mo, Hg and Se, and,

Host rocks Generally calcareous or dolomitic sedimentary rocks; also siliceous sedimentary or igneous rocks.

Intermediate to felsic intrusive igneous rocks (from tonalite to monzogranite or syenite porphyry), as well as in associated breccia pipes intruding granitic, volcanic, calcareous sedimentary, and other reactive rocks.

Intermediate to felsic volcanic rocks and associated volcaniclastic and sedimentary rocks; may also extend into rocks underlying the volcanic sequence.

Felsic volcanic rocks, generally intrusions or lava domes comprising part of a composite stratovolcano complex; may also be hosted in older sedimentary rocks or crystalline rocks.

Deposit type

Carlin-type

Porphyry Cu-Mo

Quartz-Adularia(Low Sul®dation)

Quartz-Alunite(High Sul®dation)

Table 2 Generalized geologic characteristics of deposit types

Strongly zoned with central silici®cation and potassic alteration (adularia), grading laterally and upward into quartzsericite-illite, and distally into lower T smectite2zeolite minerals; many districts have broad halos of a distal propylitic assemblage, including chlorite, epidote, calcite and pyrite. Zoned: deep phyllic alteration with quartz-sericite-pyrite; peripheral zone of propylitic alteration with chlorite,2 epidote,2pyrite,2calcite; Intermediate-level argillic zone with montmorillonite-smectite-illite clays with pyrite; intermediate-level advanced argillic with central vuggy silica, grading outward into quartz-alunite (2pyropholite), quartz-kaolinite, and montmorillonite-illite-smecite alteration zones, all with pyrite.

Native Au or electrum associated with pyrite, chalcopyrite (and other Cu sul®des), tennantite, enargite, galena, sphalerite, and in shallow zones, cinnabar, orpiment and relagar. Ore is generally associated with the vuggy silica and quart-alunite alteration zones.

Alterations zones from bottom, innermost zones outward are: sodiccalcic (oligoclase or albite, actinolite, and sphene), potassic (K-feldspar, biotite, rutile, and pyrite or magnetite) to propylitic (oligoclase or albite, epidote or calcite, chlorite, rutile, and magnetite or pyrite). Phyllic (sericite, chlorite, rutile, and pyrite) and argillic (clay, sericite, chlorite, and pyrite) alteration zones may overprint early potassic assemblages.

Primary reduced ore: decalci®cation, argillization, silici®cation, sul®dization; oxide ore is a weathering product of primary reduced ore and generally has sul®des <1%.

Alteration

Native Au or electrum, with low to moderate sul®des; some systems contain abundant base metal sul®des including sphalerite,galena, chalcopyrite, pyrite and lesser precious metal sul®des; other systems have veins dominated by quartz and adularia with minor sul®de minerals. Telluride and selenides can be present.

Primary ore minerals include: chalcopyrite, pyrite, bornite, 2molybdenite; gangue and alteration minerals are potassic, grading outward to propylitic. Late stage assemblages include quartz, sericite, clay, pyrite,2alunite, 2pyrophyllite. Assemblages are often superimposed, and late veins are often present containing chalcopyrite, bornite, enargite, tetrahedirte, galena, sphalerite, and barite.

Disseminated Au; with quartz, dolomite, calcite, sericite, pyrite, marcasite, orpiment, realgar, kaolinite, illite/ smectite2barite; oxide ore has limonite/ geothite/hematite.

Ore zone mineralogy

676 L. Shevenell et al. / Applied Geochemistry 14 (1999) 669±687

L. Shevenell et al. / Applied Geochemistry 14 (1999) 669±687

in the higher sul®de type, Cu, Pb and Fe. While the higher sul®de type may have moderate potential for acid generation, the low sul®de type has little acid-generating potential. The near-neutral waters that accumulate in pits in these deposits may, however, contain elevated As, Sb, and possibly Se and Mo, elements that can be elevated in waters with pH values from acidic to alkaline (Plumlee et al., 1995b). Mineralization at Tuscarora illustrates the variation in mineral and chemical constituents in the quartz-adularia deposit type (Boden and Henry, 1997). Both the high and low total S types of deposits are present at Tuscarora: the latter with low Ag/Au, low pyrite concentrations (R1%), and negligible concentrations of other sul®des or base metals, and the former with high Ag/Au, slightly higher pyrite concentrations (12±5%), and moderate concentrations of other sul®des (galena and chalcopyrite) and base metals (a few % of both Pb and Cu). Both types are hosted within the same volcanic rock types that contained little initial calcite, but abundant calcite was added by the hydrothermal solutions that deposited the Au. Both types also have moderate As concentrations. The open pit sampled at Tuscarora for this study exploited the low Ag/Au type, but high Ag/Au type deposits were immediately adjacent. 3.4. Quartz-alunite precious metal The di€erence between quartz-alunite and quartzadularia deposits illustrates the importance of di€erences in mineral constituents. Pyrite can be moderately abundant in both of these deposit types, and both are hosted by volcanic rocks that were mineralogically similar before the hydrothermal alteration that deposited precious metals. However, the hydrothermal solutions that generate quartz-alunite deposits are highly acidic. These solutions dissolve almost all calcite and alter most silicate minerals to clay minerals to the extent that the remaining rock has little neutralization capacity. Therefore, mine drainage from quartz-alunite deposits is much more likely to be acidic and to carry high concentrations of metals. Associated minor elements generally include As, Cu, Te and Bi; Pb, Sb and Sn are also enriched at the Ketchup Flat deposit (John et al., 1991).

677

thermal calcite are moderately abundant. The deposit had undergone at least some oxidation before mining (Shawe et al., 1986).

4. Results and discussion For many of the mines discussed here, complete data listings of the deposit type, name of mine, depth of water sampled, date sampled, pH, temperature, various chemical constituents, calculated charge balance, and references to sample numbers or data sheets appear in Price et al. (1995). Other original data can be found on ®le at the Getchell Mine, and in Miller et al. (1996) and PTI (1996). Only selected data plots are included here. Table 1 identi®es the mines for which data are available in this work. Little information is available regarding water levels in the lakes through time, and, in some cases, the dates when the pits began to ®ll are uncertain. Hence, references to many of the data in Table 1 are personal communications with individuals most familiar with the particular sites. Note that spatial and temporal data are discussed here where available, but a complete data set is lacking for many of the pit lakes, and such evaluations cannot be made. With some notable exceptions, the vast majority of the sampled waters had pH between 6.5 and 8.5, the Nevada drinking water standard (Fig. 3). All 30 of the samples from pits located in quartz-adularia deposits, as well as all samples from the pit located in the geologically similar sediment-hosted epithermal deposit, met Nevada drinking water standards. In contrast, pH ranged from 3.7 to 8.3 in waters in the Carlin-type deposits; 13 samples had a pH <6.5. All but 2 of these acidic waters originated in the Getchell South pit lake.

3.5. Sediment-hosted epithermal The deposit at Manhattan is actually an atypical example of a quartz-adularia deposit that is hosted in slightly metamorphosed Paleozoic sedimentary rocks, somewhat similar to those that host Carlin-type deposits. The host rocks are shales and quartzites that contain less calcite than do the Carlin-type host rocks. As in other quartz-adularia deposits, pyrite and hydro-

Fig. 3. Measured pH in pit lakes in the mineral deposit types presented in this paper. Solid lines bound the range of Nevada drinking water standard.

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None of the samples exceeded the upper limit (pH = 8.5) of the drinking water standard. The Getchell data illustrate the spatial and temporal variability that can be expected for some geologic systems. Water at the surface of the South pit generally meets pH standards during certain times of the year. However, pH generally decreases with depth to below the standard (Fig. 4). Note that two di€erent pro®les were measured on the same date (1/26/82) and these two locations have slightly di€erent pro®les suggesting signi®cant variation both in time and laterally and with depth in the pit. Only one of 8 samples from the 3 pits developed in porphyry-Cu or porphyry-Mo deposits had an acidic pH (Fig. 3), and this low pH is not a result of waterrock interaction in the pit. In the Liberty pit in the Robinson district, large (but unknown) amounts of H2SO4 were used to leach Cu from ore stock piles. Some of the leach solution then drained into the pit. The resulting low pH and elevated metal concentrations are thought to result mainly from this leaching, not from groundwater reaction with wall rock and ¯ow into the pit. Because the water quality and low pH in this pit lake is not controlled solely by waterrock interactions, the Liberty pit is not considered further here. In contrast, the pits that ®lled naturally with groundwater all have neutral pH. The abundance of sul®des in porphyry-Cu deposits would suggest acidic water might be more common, however, carbonate rocks are common around all such deposits in Nevada and may be neutralizing any acidity that is generated. Water in several abandoned shafts and discharging from adits developed in quartz-alunite deposits commonly has low pH and high base metal concentrations (Price et al., 1995). These observations are consistent with the high acid generation and low neutralization potential of this deposit type. However, water quality data are available for only one pit developed in a

Fig. 4. Pro®les of pH versus depth at the formerly ®lled Getchell South Pit.

Fig. 5. Concentrations of Cu, Mn and Ni, and pH in the Ketchup Flat pit lake through time.

quartz-alunite deposit, the Ketchup Flat pit (Table 1; Fig. 5). Six samples collected between May 1995 and May 1997 show a decrease in pH from 8 to 4.1, possibly re¯ecting increased reaction with wall rocks. Base metal concentrations in the same samples were mostly relatively low and even decreased through time, despite the decreasing pH. In contrast to the Ketchup Flat system, temporal data from the Getchell Mine, a Carlin-type deposit, show that the pH of the pit waters can increase with time. Samples from the Getchell Center pit lake were collected on 7 separate dates from several depths and composited into one sample per date which was analyzed for major anions, cations and pH. Fig. 6 illustrates that pH steadily increased as the Center pit lake ®lled with water, while there was a corresponding decrease in SO4. During the early stages of pit ®lling, relatively shallow water was in contact with sul®des in the bottom of the pit, resulting in the observed low pH and elevated SO4. As the pit ®lled and the water came into contact with the oxidized zones in the pit, and as the water to rock ratio increased, the pH increased and SO4 decreased in the composite sample. Deeper parts of the lake in the unoxidized zone retained a

Fig. 6. SO4, Ca, and pH variations through time in the Center pit lake at the Getchell Mine.

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lower pH whereas those in the shallower parts were near neutral (Fig. 4). Nevertheless, the overall water quality in the Getchell Center pit lake generally improved with time (Fig. 6). The data also show that between 1968 and 1983, negligible changes in TDS occurred in the Getchell pit lakes, but other chemical constituents were observed to either increase or decrease. For instance, between 1968 and 1983, concentrations of Cl, F, Mg, As (with ¯uctuations), along with pH, increased in the pit lake, whereas concentrations of Ca, and Fe decreased along with SO4. Similar trends were observed in the South pit; sucient data are available to determine that, in addition to the elements seen to increase and decrease in the Center pit, F and HCO3 increased, and K and Na decreased. The increasing pH and HCO3, and decreasing SO4 with time shows the greater bu€ering e€ects as the pits ®ll and a greater proportion of the pit lake water is in contact with rock having a higher bu€ering capacity rather than the acid producing minerals generally located toward the bottom of the pits. As would be expected, more acidic and concentrated waters were present in the pits when they ®rst began to ®ll and the water to rock ratios were relatively low. These chemical changes through time were observed in both the South and Center pits at Getchell, but insucient data were collected to evaluate these changes in the North pit. At the Yerington pit, developed in a porphyry-Cu deposit, samples were collected at 3 and 2 di€erent depths, respectively, on 2 separate dates only 5 months apart (4/7/95 and 8/1/95). Negligible changes in the water chemistry occurred over this brief time period, indicating that concentrating e€ects of evaporation were negligible during the summer months of 1995. In addition, there is little strati®cation within the pit for these 2 dates. For instance, pH decreased from 8.25 at a depth of 0 m to 7.84 at 100 m depth. TDS increased only slightly from 630 mg/L at 0 m depth to 645 mg/L

Fig. 7. Pro®les of TDS versus depth at the formerly ®lled Getchell South Pit.

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Fig. 8. TDS (mg/L) versus depth in the 3 pit lakes at Getchell in September 1983.

at 100 m depth. Additional data indicate that the Yerington pit remains relatively uniform in composition both spatially and temporally, suggesting nearly constant mixing throughout the depth of the lake (Hershey and Miller, 1997; Hershey, 1997). In contrast, as noted above, the Getchell pits were highly strati®ed and had limited evidence for seasonal turnover. This is surprising considering that the Yerington pit lake is over 3 times deeper than were the former Getchell pit lakes. Many of the samples collected from pit lakes in all deposit types had total dissolved solids (TDS) in excess of the Nevada drinking water standard (500±1000 mg/ L), largely due to elevated SO4 concentrations. TDS increased with depth in two pro®les of the Getchell South pit (Fig. 7). In contrast, TDS was nearly constant or decreased with depth in two other pro®les from the Getchell South pit and from pro®les at the Getchell Center and North pit (Fig. 8). Concentrations of species such as As are generally independent of pH, whereas concentrations of metals such as Fe are strongly controlled by the pH of the water. Therefore, these two elements were plotted versus pH to investigate pH-concentration relationships (Fig. 9 and Fig. 10). Some of the neutral waters in Carlin-type, quartz-adularia, and sediment-hosted epithermal deposits have elevated Fe concentrations, although the vast majority of the pit lake samples had Fe below the drinking water standard. In the 3 porphyry-Cu pits, 6 of the samples had Fe below detection (<0.01 mg/L). Because most samples were collected for regulatory compliance purposes, it is assumed that most waters were ®ltered through 0.45 mm ®lter papers before acidi®cation. This type of ®ltration necessarily removed some colloidal material, but not the material in the smaller colloidal size fractions. Experience sampling both pit lakes and nearby groundwaters shows that a large part of the Fe in the waters may be colloidal, not dissolved. Filtration of groundwaters

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Fig. 9. Fe (mg/L) concentrations versus pH in pit lakes in the 5 ore deposit types presented in this paper. The solid horizontal line shows the maximum concentration allowed under Nevada Drinking Water Standards.

upgradient of the Boss, Bullfrog, Twin Creeks and Betze pits through 0.1 mm ®lters removed virtually all of the Fe from the waters (Connors et al., 1997). This suggests that much of the Fe measured in these pit lakes may be suspended colloidal material, likely in the 0.1 to 0.45 mm size fraction, and hence the analyses do not demonstrate pH dependence. However, this hypothesis requires testing because particulate Fe could have formed during sampling. During ®ltration, the water is exposed to air and any Fe + 2 present in the water would be rapidly oxidized to Fe + 3 giving the appearance of colloidal material on the ®lters when, in fact, the Fe may have been dissolved Fe + 2 in the water column prior to sampling. No analyses of Fe + 2/Fe + 3 pairs are available to evaluate if Fe was precipitated before or during sample collection, although on-going work is being conducted to evaluate this possibility. The vast majority of the water samples collected from pits have As concentrations below the Nevada drinking water standard of 0.05 mg/L (Fig. 9). All samples collected from the sediment-hosted epithermal deposit pit had As below 0.05 mg/L, and 5 were below detection (<0.005 mg/L). Similarly, As was within drinking water standards in the samples collected from the porphyry-Cu and quartz-alunite deposits (6 samples from one pit). In contrast, all 29 analyses from Carlin-type pits had detectable As, with 3 of these exceeding the Nevada drinking water standard (all 3 at the Getchell North pit; 2 samples plot on top of one another on Fig. 10 with As concentrations of 0.37 and 0.38 mg/L). Of the 8 samples from the Boss pit in the quartz-adularia deposit type, 6 had As exceeding drinking water standards (Fig. 10) suggesting As may be a concern in pits completed below the water table in this type of deposit.

Fig. 10. As (mg/L) concentrations versus pH in pit lakes in the 5 ore deposit types presented in this paper. The solid horizontal line shows the maximum concentration allowed under Nevada Drinking Water Standards.

Arsenic does not correlate with pH (Fig. 10) because it is not transported in aqueous solution in the same form as the base metals. Arsenic concentration is within the drinking water standard in the pit-lake waters from 5 of the 6 pits in Carlin-type deposits for which data are available. Only the North pit at Getchell had elevated As concentrations, and in the case of this pit, As was elevated in samples taken on 3 di€erent dates, yet was within the drinking water standard in 6 other samples (collected on 5/18/83). All samples collected from the Boss pit (quartz adularia) had elevated As concentrations (18 to 26 times drinking water standards), yet none of the samples from the Aurora Partnership or Tuscarora pits had elevated As concentrations. Despite the enrichment of As in many ore deposits and in Nevada mining districts (Fig. 2), As concentrations did not exceed drinking water standards in any of the porphyry-Cu, quartz-alunite, or sediment-hosted epithermal deposits sampled to date. Fig. 11 illustrates that As is consistently elevated in the Boss pit relative to the other pits in quartz-adularia

Fig. 11. Measured As in water from 4 pit lakes located in quartz-adularia types of deposits.

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deposits, and relative to the Nevada drinking water standard of 0.05 mg/L. In addition, the concentration of As is observed to increase with time. The other sampled pits in quartz-adularia type deposits have little potential for developing poor water quality with respect to As. This is at least in part due to higher in¯ow rates, both surface water and precipitation, and lower evaporation rates at the Tuscarora pit, for example, relative to the Boss pit. However, speci®c site conditions at Boss, discussed below, may in¯uence As contents as well as other constituents. Concentrations of other selected cations and anions are variable but mostly low (Fig. 12 through 15). Elevated Mn is observed in water samples from all of the deposit types, but for the quartz-adularia type deposits (Fig. 12), 18 of the 29 available samples have Mn <0.005 mg/L. Note that both the Carlin-type and sediment-hosted epithermal-type deposits have some samples with relatively high Mn concentrations (up to 4.5 mg/L in the Carlin-type Getchell Center pit, 1/26/ 82). Selenium concentrations are variable but generally low (Fig. 13). Selenium was below detection (<0.005 mg/L) in one sample from the quartz-alunite deposit type, and in all of the samples from the single sediment-hosted epithermal deposit. Sixteen of the 23 samples from the Carlin-type pits had Se below detection, and all analyses from lakes in this deposit type had Se contents below the Nevada drinking water standard. In contrast, the 6 samples from the Yerington porphyry-Cu pit lake exceeded drinking water standards, yet the samples from the Kimbley pit (also a porphyry-Cu deposit) were below detection. Selenium was particularly variable in samples from the quartz-adularia deposit type; of 34 samples, 11 were

Fig. 12. Measured Mn in pit lakes in the 5 ore deposit types presented in this paper. Solid line shows the value of the Nevada drinking water standard. Two samples from CyprusTonopah are not plotted on this graph (Mn = 11 and 38 mg/ L).

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Fig. 13. Measured Se in pit lakes in the 5 ore deposit types presented in this paper. Solid line shows the value of the Nevada drinking water standard.

below detection and 6 (all from the Boss pit; some points plot on top of one another on Fig. 13) exceeded drinking water standards. This variability is consistent with the fact that Se is relatively mobile in neutral to basic waters and can be enriched in this deposit type. As mentioned above, many of the available pit water samples have elevated TDS, and SO4 is largely responsible for these high TDS values. Both SO4 speci®cally and TDS in general commonly exceed Nevada drinking water standards in the pit lakes (Fig. 14). For comparison, a popular natural lake in Nevada (Pyramid Lake), which is heavily used for recreational purposes, has TDS averaging between 5400 and 5600 mg/L and has a pH of about 9.2. These data indicate that many of the pit lakes may generally have water quality somewhat better than some of the natural, terminal lakes in Nevada. However, the highest SO4 and TDS concentrations are in the Boss pit

Fig. 14. Measured TDS in pit lakes in the 5 ore deposit types presented in this paper. Solid line shows the value of the Nevada drinking water standard.

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Fig. 15. Measured Cl in pit lakes in the 5 ore deposit types presented in this paper. Solid line shows the value of the Nevada drinking water standard.

(quartz-adularia type), which also has unusually high Cl concentrations (Fig. 15). The Boss pit lake appears to be atypical, both of quartz-adularia deposits and of Nevada deposits in general. High SO4, Cl and TDS concentrations in the Boss lake probably result due to a set of complex factors including in¯ow of moderately saline water and high evaporation. Although in many cases, samples with elevated SO4 did not have any accompanying cation analyses, where analyses are available for the Boss pit, Na is the dominant cation, and was elevated, and SO4 the dominant anion, with approximately 3000 mg/L Cl. In the case of the

Fig. 16. Measured SO4 in water from 4 pit lakes located in quartz-adularia types of deposits.

Getchell, Kimbley, Manhattan, and Yerington pits, the few waters with elevated SO4, for which cation analyses were available, show that Ca is the dominant cation. Fig. 16 shows that, unlike most pit waters, water of the Boss pit is consistently elevated in SO4 above Nevada drinking water standards (250±500 mg/L). The concentration of SO4 also increases with time. The elevated SO4, particularly relative to other pit lakes in the quartz-adularia deposits, may result due to several factors, including: (1) elevated groundwater concentrations; (2) pit location relative to late Quaternary pluvial lakes; or (3) local waste disposal practices. Contributions due to local groundwater do not explain

Fig. 17. Location of the Boss pit in relation to late Quaternary pluvial lakes.

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the elevated SO4 concentrations because groundwater in the area has relatively low SO4 concentrations (322 mg/L), as well as low Na (350 mg/L) and Cl (211 mg/ L; Connors et al., 1997). However, both of the other factors may be signi®cant in the past and ongoing evolution of the Boss pit waters. Fig. 17 illustrates the location of the Boss pit in southwestern Nevada where it lies on the edge of a late Quaternary pluvial lake. Three other pits are located in the same part of Nevada (Aurora Partnership, Cyprus-Tonopah and Manhattan); however, none of these lakes are located within a Quaternary pluvial lake, and none have the elevated SO4 and Cl concentrations seen at Boss. It is hypothesized that excess SO4 and Cl in the Boss pit lake are contributed, at least in part, from periodic in¯ow of surface water which has ¯owed over the pluvial deposits dissolving evaporite minerals. Analysis of 1:24 000scale aerial photographs indicates that the Boss pit lies near the base of a slope draped by large coalescing alluvial fans within the paleo-lake basin. Although the photos reveal no stream channels leading into the pit, the upslope area would clearly drain toward the pit, and sheet wash during rains may introduce considerable pluvial sediment into the pit lake. This is supported by the elevated Na (1900 to 3100 mg/L) and Cl (2000 to 4200 mg/L) in the Boss pit waters, and further research is needed to determine which other elements, if any, may also be contributed by this mechanism. Local disposal practices at the Boss pit may have been another signi®cant in¯uence contributing to both SO4 (and thus TDS), as well as As. In recent years (1995±1996), some of the mine waste piles at Boss have been dumped into the pit (Fox, 1998). While most of the ore at Boss was oxidized, the speci®c chemistry of this waste rock is unknown. It is likely

Fig. 18. Cd, Co, Ni, and Sr at the Cyprus-Tonopah (porphyry-Cu) pit lake.

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Fig. 19. Al, Cu, Mn, and Zn at the Cyprus-Tonopah (porphyry-Cu) pit lake.

that this mine waste added to the readily leachable As, and possibly SO4. In addition, pluvial sediments rich in evaporite minerals may have been incorporated with the back®lled material and, if so, would signi®cantly increase available SO4, Cl and Na, further complicating the water-rock interaction system at the Boss pit. Data from most of the pit lakes investigated here indicate that pit lake waters in Nevada will generally have neutral to slightly basic pH and very low metal concentrations. Two exceptions to this trend are the Cyprus-Tonopah (porphyry-Mo; elevated metals) and Ketchup Flat (Fig. 5, quartz-alunite; low pH) pits. The Cyprus-Tonopah pit lake waters have elevated Al, Cd, Co, Cu, Mn, Ni, Sr and Zn (Fig. 18 and Fig. 19) with apparently increasing concentrations through time in all of these constituents except for Al and Cu. The increases may be in part the result of evapoconcentration. However, a major reason for these anomalous metal concentrations may actually be arti®cially imposed, in that the pit was not allowed to ®ll naturally between 5/92 and 1/94. The pit began to ®ll in February 1991 but was periodically pumped over 8 di€erent time periods with total pumpage ranging up to 2.8 million gallons (Goodrich, 1997). The pit was pumped dry in January 1994, and was allowed to ®ll thereafter. The sample with the highest metal concentrations was collected shortly after the pit began to re®ll following the last period of pumping in January 1994, during a time when there was relatively little water in the pit, and the water to rock ratio in the pit was lower. Although, none of the samples collected prior to November 1997 were ®ltered, in 1997, both ®ltered and un®ltered samples were collected for comparison. Metal concentrations in the 2 samples were very similar, indicating that the lack of ®ltration is not a factor in the elevated metal concentrations. More detailed studies are required at this site in order to understand the processes occurring and determine if

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the concentrations will decrease to levels seen in 1992 as the pit again ®lls with water. In the Ketchup Flat pit, the data from the 6 available analyses show that only Mn, Fe, SO4 and TDS exceed drinking water standards. This pit is located in a quartz-alunite type deposit which would be expected to evolve poor water quality based on geologic setting (e.g., see Price et al., 1995). The only indication of ultimate poor water quality in this pit at present is the decreasing pH, most recently measured at pH = 4.1 (5/ 20/97). Insucient data are available from this pit lake to evaluate if poor water quality will be present in the future, and monitoring should continue. It should be noted that the greatest number of samples available for this evaluation were collected at the 3 Getchell pits. Some samples from these pits have had concentrations of various elements that exceed drinking water standards. Other pits do not necessarily exhibit elevated concentrations, but in some cases, very few samples are available and samples may simply not have been collected in a portion of the pit containing elevated concentrations. Although the data presented here suggest that some of the Getchell pit waters were of fairly poor quality relative to other pits, this may simply re¯ect that greater temporal and spatial data are available at Getchell than at the other pits, and hence, a greater proportion of the Getchell pits is characterized. This paper presents preliminary, generalized information regarding the types of water quality issues which may be expected in speci®c hydrogeologic environments in pit lakes forming in Nevada. However, the presented data were not collected with a particular scienti®c goal or to test speci®c hypotheses, but rather for regulatory compliance purposes. Samples were collected by numerous individuals and analyzed by several di€erent laboratories. Current information does not indicate if replicate samples were collected from the same locations in the individual pits during subsequent sampling events. However, it is assumed that samples were collected in the same general locations during each sampling event due to the likelihood of individuals selecting the location with the easiest access to the pit lake from which to collect samples. The data presented here were not all subject to stringent QA/QC requirements, and it is recommended that future pit lake samples be collected under a careful scienti®c or quality assurance plan such that spatial and temporal chemical variability within the pit lakes can be more thoroughly characterized. 5. Conclusions Although insucient data are available to fully characterize the 16 current or past pit lakes in Nevada,

the data presented here allow identi®cation of general trends and characteristics that can be expected. The pit lake data from Getchell (Carlin-type deposit) suggest that the water quality will generally improve with time. Wall rocks at many pits from which waters have been sampled contain substantial amounts of calcite, which tends to bu€er the pH at neutral or slightly basic conditions. Care must be taken to look closely at the geology, mineralogy, and alteration of the deposits, and depending on the nature of the mineralization and alteration in which a particular pit is located, di€erent geoenvironmental e€ects may occur within the same overall deposit. For example, propylitic alteration zones, which commonly contain calcite, generally will not evolve acidic waters, although propylitic zones around some quartz-alunite deposits can contain enough pyrite that available calcite is destroyed during weathering. Propylitic zones are common in the periphery of many major igneous-hydrothermal ore deposits and, fortunately, tend to be of greater areal extent than the potential acid-generating argillic alteration zones at the centers of the deposits. Despite the likely bu€ering e€ect on pit waters in most Nevada mines, pit-lake geochemistry should be monitored with time. The geochemistry and biogeochemistry of pit lakes is likely to change due to evaporation, variable kinetics of water-rock interactions, changing in¯ux and composition of groundwater and surface waters as pits ®ll to steady-state levels, and changing climate on seasonal and decadal time-scales. Pit lakes in Carlin-type deposits may have initial low pH waters while the lakes are shallow and in contact with unoxidized, relatively sul®de-rich zones. These waters likely will become increasingly neutral to alkaline as the pit lakes ®ll and waters come into contact with the oxidized and more carbonate-rich zones, and as the water-to-rock ratio increases. These pit lakes will likely have elevated As and SO4, but relatively low concentrations of other elements of concern. Pit lakes in porphyry-Cu deposits have been known to evolve acidic waters; however, the majority of samples available in this study showed near neutral pH. Elevated Se and SO4 concentrations were observed in at least one pit lake in a porphyry-Cu deposit (Yerington). Some pits in porphyry-Mo deposits (Cyprus-Tonopah) may have decreasing pH with time and elevated concentrations of Al, Cd, Co, Cu, Mn, Ni, Sr and Zn. Pit lakes in the quartz-adularia type deposits are likely to have elevated concentrations of As at neutral to slightly basic pH. These pit lakes may also have elevated F, Cl, Se and SO4 concentrations that increase with time, particularly in the southern portions of Nevada due to evapoconcentration. This is observed at the Boss pit lake where evaporative demands are large (11470 mm/yr; Shevenell, 1996). The one pit lake in a sediment-hosted epithermal deposit (Manhattan) is

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actually a variation of a quartz-adularia type deposit, but hosted in sedimentary rocks. Like most of the other quartz-adularia deposits, the water quality in this pit lake was generally good, and this pit similarly exhibited elevated SO4 and Fe, but decreasing Mn. Although limited data are available for pit lakes in quartz-alunite type deposits, they may develop poor water quality based on data from shafts and adits in some portions of the state, and this is consistent with predictions based on the geologic environment. The one pit lake of this type for which data are available is Ketchup Flat and this lake exhibits decreasing pH with time yet also shows decreasing Ni and Cu concentrations with time, with otherwise low metal concentrations (except for elevated Mn). Additional studies are required at this pit lake as insucient data are available to explain these apparently contradictory results (decreasing pH with accompanying decreases in metals concentrations). Pit lakes can be expected to be both spatially and temporally variable. The pit lakes with the greatest amount of available data (Getchell) show that chemical strati®cation can occur within a pit lake, with higher TDS, lower pH waters occurring at depth. Samples collected at this lake also show a general improvement of water quality with time as the pH of the lake increased between 1968 and 1983. The water quality of the 16 pit lakes in Nevada was generally good with near neutral pH and low metal concentrations. However, elevated As, Fe, Mn, and SO4 can be expected from the majority of future Nevada pit lakes due to the similarity in the geologic types of deposits at the di€erent pits. Hydrologic, climatic and local geologic controls can be very important in dictating the ultimate pit lake water quality. Although both the Boss and Tuscarora pits are in quartz-adularia deposits, the water quality of the two are signi®cantly di€erent. The quality of the Boss pit water is considerably worse than that in the Tuscarora pit, in part, because evaporation is approximately double at the Boss pit in comparison to that at Tuscarora, and a signi®cant amount of surface water in¯ow occurs at Tuscarora which serves to dilute the waters in the pit lake. In addition, the Boss pit is located on a Quaternary pluvial lake which appears to contribute in¯ow of additional SO4, Na and possibly other constituents to the lake. The chemistry of the Boss pit illustrates that ore deposit type does not necessarily dictate ultimate pit water chemistry since other local, site-speci®c variables (e.g., evaporation and amount and quality of surface water and groundwater in¯ows) can be the dominant factors in pit lake water evolution. Considerably more information is required at the existing pit lakes to fully understand the evolution of water quality in each of the di€erent ore deposit types.

685

There is an urgent need for consistent, well documented studies to thoroughly evaluate the hydrogeologic controls on the spatial and temporal variations in water quality at these pit lakes. A better understanding of the current pit lakes will assist in predicting the future water quality at open pits that have not yet begun to ®ll with water.

Acknowledgements The authors wish to thank all of the mining companies that allowed site access or directly provide data for this work. The ®rst author would also like to gratefully acknowledge the Bureau of Land Management (Nevada State Oce), Getchell Gold Corporation, and the Western Governor's Association for providing partial funding for this work. The authors thank Dr. Don Runnells, Dr. David Keith, and an anonymous reviewer for Applied Geochemistry for their helpful comments on the manuscript. The ®rst author would also like to gratefully acknowledge Ron Hershey (Desert Research Institute) and Gary Goodrich (Westec) for their helpful discussions and for providing their unpublished data from the Yerington and Cyprus-Tonopah pit lakes.

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