Geochemical controls on ground water composition at the Cripple Creek Mining District, Cripple Creek, Colorado

Applied Geochemistry 18 (2003) 1–24 www.elsevier.com/locate/apgeochem

Geochemical controls on ground water composition at the Cripple Creek Mining District, Cripple Creek, Colorado L. Edmond Eary*, Donald D. Runnells, K.J. Esposito Shepherd Miller Inc., 3801 Automation Way, Fort Collins, Colorado 80525, USA Received 5 November 1999; accepted 30 September 2001 Editorial handling by R. Fuge

Abstract A combined approach involving evaluations of historical information, compositional trends, site mineralogy, and forward and inverse geochemical modeling was used to assess the effects of Au mining on ground water quality at the Cripple Creek Mining District. The District is located in a Tertiary volcanic diatreme complex surrounded by Precambrian granite. Historically, mining activity was underground whereas present-day mining occurs in surface mines. Between 1896 and 1941, a series of tunnels was excavated to drain the underground mining areas. The Carlton Tunnel, located about 900–950 m below the surface, is the primary ground water drain for the mining areas. Ground water flowing from the Carlton Tunnel has historically been of good quality. The geochemical processes controlling the quality of the Carlton Tunnel water were the focus of this study. Mineralogical and acid/base accounting data indicate that the diatreme is zoned vertically from an oxidized condition with acidic paste-pH, acidic ground water, and elevated metal concentrations near the surface to an alkaline condition with high pH, elevated SO4, and low metal concentrations at depth. The average travel time of water from the surface to the Carlton Tunnel is estimated to be at least 25a based on 3H determinations. Forward geochemical modeling results indicate that this travel time is sufficient for ground water to reach equilibrium with calcite, gypsum, and fluorite by the time it exits through the Carlton Tunnel. Equilibrium processes have effectively fixed the pH, alkalinity, and SO4 in the Carlton Tunnel water to near-constant levels for at least 24–70a based on comparisons to historically reported water compositions. Inverse geochemical modeling results indicate that there is sufficient neutralization capacity at depth in the diatreme to maintain the current good quality of the ground water flowing from the Carlton Tunnel for the forseeable future, assuming no significant changes in hydrogeochemical conditions. # 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction The Cripple Creek Mining District lies between the towns of Cripple Creek and Victor, on the southwestern flank of Pikes Peak in south-central Colorado (Fig. 1). Gold was discovered in the area in 1891 in small placer deposits. Subsequent discoveries of high-grade Au–Agtelluride veins led to extensive underground mining start-

* Corresponding author at present address: Enchemica, 2335 Buckingham Circle, Loveland, CO 80538, USA. Tel.: +1970-203-0179. E-mail address: [email protected] (L.E. Eary).

ing in the late 1890s and continuing with intermittent pauses until the late 1930s. After a period of relative inactivity, surface mining operations were resumed in the late 1980s to recover Au by cyanide heap-leaching from the lower grade ores rejected by previous underground mining operations. Currently, the Cripple Creek and Victor Gold Mining Company conducts mining operations in surface mines. The ore averages about 0.025 troy ounces/short ton, yielding from 200,000 to 240,000 troy ounces of Au/a. Total Au production from the District has been estimated at 21 million troy ounces, making it one of the leading Au-producing districts in the world (Pontius, 1996).

0883-2927/03/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(02)00049-5

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L.E. Eary et al. / Applied Geochemistry 18 (2003) 1–24

Fig. 1. Location map and sketch of relative positions of major drainage tunnels and shafts at the Cripple Creek Mining District.

Historically, the District was dewatered by a series of adits driven horizontally at various elevations below the underground mines. Locally, these adits are referred to as tunnels, although they are open on only one end. The Carlton is the deepest of the tunnels, and is located about 900–950 m below the top levels of the current surface mines. The Carlton is

now the primary ground water drain for the mining and surrounding areas, averaging 100 l/s. Since the early 1970s when regular monitoring began, water quality from the Carlton Tunnel has been characteristically slightly alkaline in pH, with low metal concentrations, and SO4 concentrations ranging from 1100 to 1400 mg/l.

L.E. Eary et al. / Applied Geochemistry 18 (2003) 1–24

The purpose of this study was to examine the geochemical processes controlling the quality of the ground water that flows from the Carlton Tunnel. The approach involved the determination of acid/base properties of the rock, assessment of primary and secondary mineralogical zonation, and a combination of forward and inverse geochemical modeling. The data developed from these studies were used to identify the specific water/rock reactions that control the composition of ground water that exits the mineralized zones through the Carlton Tunnel. 1.1. Geology and mineralogy The bulk of the Au ore in the District is located in an alkaline diatreme complex associated with Tertiary volcanism and regional uplift (Lindgren and Ransome, 1906; Jensen, 1998; Thompson, 1998). The diatreme was intruded into Precambrian granites and metamorphic rocks. The diatreme was subsequently intruded by phonolite and lamphophyre dikes and small stocks and sills of nepheline syenite. Most of the diatreme is comprised of a highly variable breccia called the Cripple Creek Breccia, comprised of clasts of phonolite, granite, granite gneiss, latite, and syenite within a variable groundmass. The breccia is vuggy in areas where hydrothermal alteration has resulted in low-grade potassic, argillic, sericitic, and pyritic alteration. Alteration of the original mafic minerals to carbonates has also occurred. The geology, mineralogy, and hydrology of the District were thoroughly described in a classic report by Lindgren and Ransome (1906). According to that report and more recent exploration data, the primary sulfide mineral throughout the deposit is pyrite. Pyrite occurs in vugs, veinlets, and disseminations in the Cripple Creek Breccia, but is rarely present in amounts greater than 5 wt.%. Small amounts of galena, sphalerite, marcasite, molybdenite, tetrahedrite, stibnite, pyrrhotite, and chalcopyrite are also present (Lindgren and Ransome, 1906), although current exploration drilling rarely encounters these trace minerals. Gold occurs in small zones in Au–Ag telluride veins, mainly as calaverite and sylvanite, and as widespread, low-grade disseminated native Au, generally associated with pyrite (Pontius, 1996; Jensen, 1998). The predominant gangue minerals are quartz, dolomite, fluorite, calcite, gypsum, anhydrite, and celestite, which are commonly found in vugs and veinlets in association with sulfides (Lindgren and Ransome, 1906). Other accessory minerals include rhodochrosite, Mn oxides, Ti oxides, magnetite, and apatite. Low-grade argillic and sericitic alteration is also common throughout the breccia and has left some sericite, kaolinite, and other clays along major fractures, vugs, and boundaries of breccia clasts (Lindgren and Ransome, 1906). Calcite and dolomite are the most common carbonates throughout the diatreme, occurring in direct association

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with quartz and fluorite (Lindgren and Ransome, 1906). High contents of carbonate minerals are most often associated with areas of chlorite-epidote-talc alteration assemblages after original mafic silicates (Lindgren and Ransome, 1906). The carbonates occur as disseminations, veinlets, and veins that increase in frequency and abundance with depth (Jensen, 1998). In areas below about 500 m in depth, carbonates occur in thin (2.5 cm) to massive (60 cm) veins, indicative of pervasive hydrothermal alteration and replacement of primary mafic minerals. Recent studies of the deposit have indicated that the ore fluids contained significant CO2(g), leading to the alkalic character of the mineralogy and common presence of carbonate minerals in the deeper portions of the diatreme (Jensen, 1998; Kelley, 1998; Thompson, 1998). The top portion of the diatreme shows evidence of oxidation in the form of goethite pseudomorphs after pyrite and variable amounts of jarosite, Fe oxyhydroxides, gypsum, barite, celestite and black Mn oxides. Kaolinite, sericite, and montmorillonitic clays also occur in the upper oxidized zone. Carbonates are largely absent in the top portion of the diatreme. The authors believe that carbonates were either not formed in abundance in the upper portions of the diatreme or have been leached out during oxidation of primary pyrite. 1.2. Drainage tunnel system A serious impediment to underground mining in the early days was the flow of ground water into the workings. Consequently, the progress of underground mining was marked by the construction of successively deeper and deeper drainage tunnels. Two of the early, major drainage tunnels were the Moffat Tunnel, constructed in the late 1890s, and the El Paso Tunnel, constructed in 1902–1903 (Fig. 1). These two tunnels drained much of the shallow mine workings but mining rapidly progressed to depths below them, necessitating the construction of deeper drainage tunnels. The Moffat and El Paso Tunnels currently do not discharge any water. In 1907, a major horizontal tunnel was excavated to drain water collectively from underground workings beneath much of the District and allow mining to continue to greater depths. This tunnel, named the Roosevelt Tunnel, was started by sinking a shaft 235 m downward from the El Paso shaft-tunnel intersection, and then driving horizontally outward to the surface and inward to areas beneath existing underground mines (Fig. 1). The Roosevelt Tunnel was completed in 1918, with a total length of 7395 m at a portal elevation of 2470 m. Shortly after completion, the drainage rate from the Roosevelt Tunnel was as high as 530 l/s, and the water level in the underground workings and shafts dropped by 210 m in a matter of days, according to local newspaper accounts.

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At present, the average flow from the Roosevelt Tunnel is 1.0 l/s. The flow rate is seasonal, commonly with no flow during the winter. This seasonal flow originates from only the western end of the original Roosevelt Tunnel because the majority of the Tunnel, located to the east, is now under the influence of the Carlton Tunnel and water that now encounters the eastern portion of the tunnel continues downward to the Carlton Tunnel. The western end of the Roosevelt Tunnel receives water from an outlying and smaller mineralized area. The tunnel then passes through Precambrian granitic rocks, which are not effectively drained by the Carlton Tunnel because the trace of the Carlton Tunnel lies well east of the Roosevelt Tunnel at this point (CC&V, 2001). By the late 1930s, economic Au ore was still being found at depths below the Roosevelt Tunnel in deep workings associated with the Ajax, Portland and Golden Cycle shafts. Hence, work began in 1939 on a second major drainage tunnel with the primary intention of draining the Portland and Ajax shafts. This tunnel was named the Carlton Tunnel and, when completed in 1941, extended about 9760 m from the portal at an elevation of about 2100 m (Fig. 1). According to local newspaper accounts, water initially flowed from Carlton Tunnel at an estimated rate of 1250–7700 l/s. As a result, the water level in the Ajax shaft dropped by 150 m in less than 2 weeks. Water levels also dropped rapidly in the Portland Mine and associated workings. Currently, the average flow rate from the Carlton Tunnel is 100 l/s but varies seasonally. The rapid rates at which the tunnels drained the diatreme are suggestive of vertical fracture-control of water flow. Most of the Precambrian granite surrounding the diatreme is relatively impermeable compared to the fractured rocks of the ore-bearing diatreme. Hence, the diatreme tends to act as a ‘‘well,’’ with downward water movement occurring along near-vertical fracture systems and underground workings. Although the degree of connectivity of the fractures and underground working is not currently well known, the tunnels provided an efficient means for dewatering the diatreme. Prior to mining, the average elevation of ground water was about 2900 m in the southern portion of the diatreme, increasing to about 2960 m in the central portion, depending on the topography (Lindgren and Ransome, 1906) (Fig. 1). Currently, the ground water elevation is estimated to be just above the Carlton Tunnel at an approximate elevation of 2100–2200 m, although the precise elevation is probably locally variable because of fracture control of the hydrology. 1.3. Historical information on subsurface gas and water geochemistry In the early years of underground mining, when the mines were relatively shallow and easily ventilated, few problems with mine gases were encountered. However,

as the mines progressed deeper, numerous instances of ‘‘bad air’’ occurred (Lindgren and Ransome, 1906; Denny et al., 1930). Between 1903 and 1930, at least 35 fatalities were reported to be directly attributable to bad air (Denny et al., 1930). The primary mine gas of concern was CO2(g). Lindgren and Ransome (1906) provide 3 analyses of mine air, indicating ranges of 8–14.8% CO2 (g), 5.6–10.2% O2 (g), and the remainder as N2(g). A later study by Denny et al. (1930) reported ranges of 1.6–18.4% CO2 (g) and 0.4–19.4% O2 (g) from 23 locations in the underground workings. Denny et al. (1930) also experimentally investigated the source of the CO2 (g) in the underground mines. In their experiments, rock samples from 10 underground sites were crushed to 1.2-cm diameter, placed in 50-l carboys, covered with 3–6 l of tap water, pressurized slightly with compressed air, and sealed. Gas samples were extracted periodically and analyzed for composition. The experimental results showed a gradual depletion in O2 (g) to between 7.6 and 15% and an increase in CO2 (g) to as high as 10.5% after 100–250 days of reaction (Denny et al., 1930). The rock samples tested by Denny et al. (1930) contained from 0.5 to 3.2% sulfide-S and 1.1–16.8% carbonate-C. Rocks with a higher percentage of carbonate than total sulfide generally produced leachates with high HCO3 concentrations, indicative of neutral to alkaline pH conditions. Rocks that had a lower percentage of carbonate than total sulfide produced leachates with HCO3 concentrations below detection and elevated total acidities. Based on these experimental results, Denny et al. (1930) attributed the depletion of O2 (g) to the oxidation of Fe sulfide and the generation of CO2 (g) to the neutralization of produced H2SO4 by carbonate minerals. The buffering capacity of the diatreme rock is also evidenced by historical water compositions reported by Denny et al. (1930) from various mines (Table 1). The shallowest mines sampled by Denny et al. (1930), including the Jerry Johnson and Queen Mines, had water with high SO4 and high total acidity levels. These shallow mines are within the partially oxidized portion of the diatreme. These shallow mines are not known to be hydrologically connected to the drainage tunnels. However, the water compositions reported by Denny et al. (1930) provide early evidence of acid generation by sulfide oxidation in some shallow portions of the diatreme. In contrast, the deeper mines, including the Portland, Vindicator, and positions along the Roosevelt Tunnel, were reported by Denny et al. (1930) to have had alkaline water, with HCO3 levels ranging from about 180 to 350 mg/l and SO4 ranging from about 1000 to 1141 mg/l (Table 1). More detailed compositions for the bottom sump of the Portland Mine from 1925 and 1926 from Denny et al. (1930) also show high concentrations of base cations in association with the bicarbonate and sulfate (Table 2). Overall, the historical observations of Denny et al. (1930) and Lindgren and Ransome (1906) demonstrate

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L.E. Eary et al. / Applied Geochemistry 18 (2003) 1–24 Table 1 Historic compositions of mine waters adapted from Denny et al. (1930) Locationa

Alkalinity (mg CaCO3/l)

SO4 (mg/l)

Total acidity as H2SO4 (mg/l)

Jerry Johnson Mine Jerry Johnson Mine Queen Mine, 107 m level Queen Mine El Paso mine, 3 level stope Roosevelt Tunnel, main flow in tunnel Roosevelt Tunnel 1220 m from El Paso shaft Roosevelt Tunnel 976 m from El Paso shaft Roosevelt Tunnel 732 m from El Paso shaft Roosevelt Tunnel near Elkton shaft Vindicator Mine, 20 level, crevice in breccia Portland Mine, 915-m level, spring in granite Portland Mine, 915-m level, crevice in breccia Portland Mine, 915-m level, calcareous stalactite Portland Mine, 915 m level, SE side of mine

0 0 0 0 4 148 143 198 205 218 193 294 275 293 258

3100 6400 530 3610 1370 1270 341 566 713 1441 70 913 1004 1013 1140

2630 4630 450 3010 0 0 0 0 0 0 0 0 0 0 0

a

Locations organized by relative increase in depth.

Table 2 Historical compositions of water from the Portland Mine measured in 1925 and 1926, adapted from Denny et al. (1930) Constituent

1925

1926

SiO2 (mg/l) Al (mg/l) Fe (mg/l) Mn (mg/l) Ca (mg/l) Mg (mg/l) Na (mg/l) K (mg/l) Alkalinity (mg CaCO3/l) Cl (mg/l) SO4 (mg/l)

63 0 0 Trace 403 41 108 5 269 13 1107

56 0 0 Trace 351 28 163 7 289 15 1015

the effectiveness of the carbonates for neutralizing acidic water generated by sulfide oxidation as well as explaining the abundance of CO2 (g) in the underground mines.

2. Data and methods 2.1. Acid/base accounting data A total of 142 rock samples were obtained from 5 deep drillholes located on an approximate north–south line across the breadth of the diatreme. Sampling frequencies for rock types were adjusted to approximate the bulk volumes of the major rock units present in the diatreme. The samples were analyzed for total carbonate and sulfur forms (sulfide-S, sulfate-S, and total S) by modified static tests, following standard procedures

(EPA, 1978). The C and S data were converted to their equivalent acid/base accounting (ABA) units of acid generation potential (AGP), acid neutralization potential (ANP), and net neutralization potential (NNP) (where NNP=ANP-AGP), all in standard units of ton CaCO3/kT rock. The static ABA data were supplemented by over 33,000 total C and total S analyses of rock samples collected throughout the diatreme as part of ongoing exploration and mining (blast-hole drilling) programs. Net neutralization potentials were calculated from the total C and total S data assuming that carbonates are the only sources of C and that 60% of the total S is pyritic. The 60% value was determined from the static ABA tests. These assumptions are reasonable given that no significant amounts of organic C or C-bearing minerals other than carbonates have been identified and pyrite is the predominant sulfide mineral. The remainder of the S content is sulfate-S, reflective of the common presence of gypsum, anhydrite, celestite, and jarosite in the diatreme. Also, for 33 samples that were analyzed for NNP by both methods, the correlation between the NNP values determined by static testing and the NNP values calculated from the total C and sulfide-S was high (R2=0.95). The same 142 rock samples analyzed for static ABA values were also analyzed for paste-pH. The paste-pH was determined by inserting a pH electrode into slurries made from 1:1 mixtures of rock powder and distilled water. 2.2. Water-quality data Water-quality data from routine monitoring activities were available for monitoring wells, boreholes, springs

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and tunnel flows at weekly to monthly time intervals dating back to August, 1989. A few more data for the Carlton Tunnel were also available from US Geological Survey records for October, 1971–November, 1978. Water quality data from the monitoring wells, boreholes, and springs were used to represent the shallow ground water system. Shallow ground water occurs in localized perched zones above the largely unsaturated diatreme. Water quality data from the drainage tunnels were used to characterize the aqueous geochemistry present in the deeper portions of the diatreme where both unsaturated and saturated conditions occur. In accordance with the mine-monitoring plan, water samples were analyzed according to standard procedures. The pH and dissolved O2 (g) concentration were generally determined in the field at the time of sample collection. Samples were passed through 0.45-mm filters for dissolved measurements. It is likely that some degassing of CO2 occurred during the filtration step. Splits for metals were acidified and concentrations determined by inductively coupled plasma spectroscopy. Splits for anions were unaltered and concentrations determined by ion chromatography. Water samples from two monitoring wells, the tunnels, and local water supplies were also analyzed for 3H. The 3H analyses were done at the Tritium Laboratory of the Rosenstiel School of Marine and Atmospheric Science, Miami, Florida and reported in 3H units (TU) with an estimated error of 2 to 4 TU. 2.3. Geochemical modeling Geochemical modeling of the water compositions was conducted with the PHREEQC model (Parkhurst, 1995). Both ‘‘forward’’ and ‘‘inverse’’ modeling exercises were conducted using the MINTEQ.DAT thermodynamic database.

For the forward modeling, 11–15 water compositions from each of the shallow ground water monitoring locations (Table 3) were individually analyzed with PHREEQC to determine states of saturation with various solid phases. Likewise, for the Roosevelt and Carlton Tunnels, 41and 44 water analyses, respectively, from the monitoring database were individually analyzed with PHREEQC. The procedures for setting up input files for PHREEQC for the forward modeling were as follows. (1) Field-measured pH values were available for most of the water samples and were used. In the few cases that field pH values were not reported, laboratory-measured pH values were used. (2) Field-measured temperatures were used. (3) The common occurrence of measurable dissolved O2 (g) concentrations in tunnel water were used to estimate that redox conditions are generally oxidizing. Dissolved O2 (g) concentrations are only a qualitative indicator of redox conditions and probably reflect exposure of the water to air in the tunnels, but represent the only available information on potential redox conditions. (4) Solutions were adjusted for charge balance with SO4 because it is the dominant ion in all solutions. Nearly all solutions had a small deficit of anions (0–15%), hence, SO4 was added to achieve neutrality. (5) Metal concentrations that were below detection limits were not included in input files. For the inverse geochemical modeling, the goal was to identify specific dissolution and precipitation reactions that are most likely to be responsible for the observed changes in water composition with depth into the diatreme. The inverse modeling was applied for a hypothetical pathway from the shallow ground water zone to a level equivalent to that of the Roosevelt Tunnel and downward to the level of the Carlton Tunnel. The compositions of the solutions used to represent the 3 points along the vertical flowpath modeling are

Table 3 Summary of water quality data used for forward geochemical modeling Sample location Near surface locations Monitoring Well 2B Monitoring Well 2A Spring AG-1.5 Monitoring Well AG-1.0 Monitoring Well 3A Monitoring Well 3B Spring AG-2 Deep locations Roosevelt Tunnel Carlton Tunnel a b

Estimated midpoint of screened interval. Elevation of sampling point of spring.

Elevation (m)

No. samples

Dates

2904a 2893a 2793b 2808a 2789a 2784a 2709b

12 12 15 15 11 12 15

09/13/94 04/27/95 03/06/95 06/23/95 04/27/95 04/27/95 12/20/94

2470 2100

41 44

09/14/93 to 11/13/96 05/05/93 to 04/03/97

to to to to to to to

04/08/97 04/08/97 01/08/97 04/03/97 04/03/97 04/03/97 01/08/97

L.E. Eary et al. / Applied Geochemistry 18 (2003) 1–24 Table 4 Water compositions for the 3 points along flowpath from the shallow ground water zone to the Carlton Tunnel used for inverse geochemical modeling Constituent

Shallow Roosevelt Carlton ground water Tunnel Tunnel

No. of analyses in average 2 pH 2.42 – Alkalinity (mg CaCO3/l) Al (mg/l) 175 Ca (mg/l) 88 Cl (mg/l) – F (mg/l) 10.1 Fe (mg/l) 208 K (mg/l) 2.5 Mg (mg/l) 17.3 Mn (mg/l) 15 Na (mg/l) 5.8 SO4 (mg/l) 2098 9.1 SiO2 (mg/l) Zn (mg/l) 4.25 a

15a 7.69 75 0.044 269 6 5.5 0.012 3.5 53 1.36 39 849 11.8 0.22

13a 7.81 296 <0.05 547 23 2.5 0.006 5.2 32 0.45 81 1292 25.7 0.044

Samples collected over a one-year period.

provided in Table 4. The composition for the shallow ground water zone was estimated by averaging the analytical results from the two most acidic compositions obtained for ground water collected from a series of 4 boreholes completed in a sulfide-bearing portion of the upper 50 m of the diatreme. Water compositions representing the elevations of the tunnels within the diatreme were obtained by averaging 15 and 13 compositions from Roosevelt and Carlton Tunnels, respectively, collected over a one-year time period (Table 4). The averaging was done with PHREEQC using the solution mixing option.

3. Results and discussion 3.1. Subsurface acid/base characteristics The NNP data determined from the static testing of 142 samples and those calculated from about 33,000 analyses of total C and total S were compiled for all of the drillhole samples located across the diatreme and averaged over 30.5-m (100-ft) intervals. These data are plotted as 3-point moving averages as a function of elevation in Fig. 2. The data show that a minimum in NNP occurs at elevations above about 2800 m. NNP values increase to about 0.0 ton CaCO3/kT at elevations between 2800 and 2600 m, before increasing markedly to values from +20 to +100 ton CaCO3/kT at elevations below 2600 m (Fig. 2). These trends are evident for both the measured and calculated NNP values and are indicative of the

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increasing carbonate content with depth. These trends are consistent with qualitative data from logs of deep drillholes in the District that also indicate that the carbonates occur with increasing frequency in small to massive veins as depth increases (Jensen, 1998). The paste-pH data also show a distinct trend with depth, changing from acidic values at elevations above about 2740–2900 m to alkaline values at lower elevations (Fig. 3). The most acidic paste-pH values occur in the upper portion of the deposit above an elevation of about 2900 m. Ground water pH values measured at different points in the diatreme are consistent with the general trend of increasing paste-pH with depth, although the ground water pH data are generally 1–3 pH units lower in value. The difference between the two sets of data is likely due to the finer grain size and lower CO2 pressure involved in the measurements of paste pH. The paste-pH data are consistent with indications of oxidation in the upper part of the diatreme evident from the presence of goethite, jarosite, Mn oxides, and gypsum, combined with a paucity of carbonates (Lindgren and Ransome, 1906; Jensen, 1998). The acidic pH values are probably indicative of sulfide oxidation that took place as the interface between vadose and saturated conditions fluctuated over time prior to and after mining. The original ground water elevation prior to mining ranged from about 2740–2960 m, with a rough average of about 2900 m (Lindgren and Ransome, 1906). At elevations below about 2740 m, the paste-pH values are alkaline, indicative of the influence of the increasing abundance of carbonate minerals in the deeper parts of the diatreme and lack of oxidation (Fig. 3). Taken together, the negative NNP values and acidic paste-pH values at shallow depth imply that the original primary mineralogical zonation had a higher sulfide to carbonate ratio in the top portion of the diatreme than in the deeper portions. Otherwise, the paste-pH values in the oxidized zone would also be expected to be alkaline, as they generally are at greater depths, where the carbonate minerals exceed the sulfides and the resulting NNP values increase to large positive values. Oxidation of the more abundant Fe sulfides in the upper parts of the diatreme has depleted the carbonates, leaving behind acidic paste-pH values and acidic ground water pH values. Importantly, the increase in NNP with depth means that water infiltrating into the diatreme passes from acidic conditions near the surface to alkaline conditions at depth. Thus, water that penetrates to depths that are equivalent to those of the Roosevelt Tunnel and Carlton Tunnel encounters up to 600 m of alkaline, increasingly higher NNP rock during travel through the diatreme.[It is not possible to access water from within or below the main diatreme at an elevation intermediate between shallow, near-surface ground water and the Carlton Tunnel because of the lack of safely passable underground workings. Therefore, a surrogate source of water representing

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L.E. Eary et al. / Applied Geochemistry 18 (2003) 1–24

Fig. 2. Three-point moving averages for measured and calculated NNP values with elevation. The historical ground water range is from Lindgren and Ransome (1906).

an intermediate level was located in the Roosevelt Tunnel and was sampled near the portal. The Roosevelt Tunnel water currently originates from a mineralized area west of and separate from the main mineralization in the diatreme in which mining is now occurring. Because the Roosevelt Tunnel water contains a chemical signature that is characteristic of the mineralized area and is located about half-way between the surface of the mineralized diatreme and the Carlton Tunnel, it represents a valid intermediate point for geochemical assessments. The Roosevelt Tunnel water that is sampled drains from an eastern area that does not appear to be effectively drained by the Carlton Tunnel, whereas, farther east, the Roosevelt Tunnel no longer transmits water from the main diatreme because it is now effectively drained by the Carlton Tunnel (CC&V, 2001)]. 3.2. Major solute trends and equilibria Analytical data indicate that pH values are lowest in the monitoring wells and springs, but increase sharply with depth into the diatreme (Table 5). Near-neutral pH values occur in flows from the Roosevelt Tunnel and alkaline pH values occur in water from the Carlton Tunnel. Coincident with the increase in pH is an increase in alkalinity (Table 5). Solutions that reach the

level equivalent to the Roosevelt Tunnel have neutral pH but low alkalinity. However, along the flow path from the level represented by the Roosevelt Tunnel to the Carlton Tunnel, ground water continues to gain alkalinity, reaching concentrations of 200–300 mg/l. Similar to pH and alkalinity, the concentrations of Ca and SO4 increase with depth into the diatreme, reaching their highest levels at the elevations represented by the Carlton Tunnel (Table 5). Magnesium concentrations are variable with depth, generally ranging from 20 to 45 mg/l, with a high of 60 mg/l occurring in water from the elevation equivalent to the Roosevelt Tunnel (Table 5). In contrast to the other major solutes, F concentrations peak at an average of 15 mg/l in the shallow ground water system and decrease with depth (Table 5). Forward geochemical modeling results show that the potentials for calcite and dolomite dissolution exist to a depth between the elevations represented by the Roosevelt Tunnel and Carlton Tunnel. These results are described by trends in saturation indices, SI, (SI=log IAP/Keq, where IAP is the ion activity product and Keq is the equilibrium solubility constant) from negative to positive values with increase in pH (Fig. 4). (In Fig. 4 and subsequent SI plots, an increase in pH is approximately equivalent to an increase in depth— see Table 5) The solution compositions for the Carlton Tunnel are

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Fig. 3. Paste-pH and ground water pH values with elevation in the diatreme. The historical ground water range is from Lindgren and Ransome (1906). Table 5 Average concentrations and ranges (one standard deviation) of major analytes with depth for sampling periods in Table 3 Concentration (mg/l) Location

Depth (m)

Ca

Mg

SO4

F

Alkalinity

pH (s.u.)

MW 2B MW 2A AG-1.0 AG-1.5 MW 3A MW 3B AG-2 Roosevelt Tunnel Carlton Tunnel

2904 2893 2808 2793 2789 2784 2709 2470 2100

92
52 94
43 146
52 246
67 267
67 262
26 216
50 286
36 499
39

26
14 27
14 43
10 47
9 43
10 39
3 36
9 60
12 34
4

373
193 452
203 810
246 882
236 839
221 768
88 704
224 1014
161 1305
103

0.6
0.3 1.8
0.7 15.5
6.1 8.9
3.9 6.9
2.4 4.4
1.4 6.0
2.0 5.7
1.2 2.6
0.3

– – – 10
18 57
23 119
19 28
19 46
21 270
180

4.28
0.33 3.67
0.30 3.98
0.22 5.79
0.42 6.36
0.22 6.71
0.18 7.30
0.51 7.35
0.67 8.05
0.22

oversaturated with calcite, indicating that conditions conducive to carbonate precipitation are reached in the lower portions of the diatreme, probably due to degassing of CO2. A definitive solubility control for Mg was not identified from modeling results. Tunnel water compositions

exceeded saturation with various forms of smectite-type clays and sepiolite and the formation of these minerals may affect Mg concentrations in the diatreme. An interesting aspect of the major solute chemistry is the increase in SO4 concentrations that is coincident with the increases in pH, alkalinity, and Ca (Table 5).

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Fig. 4. Saturation indices (SI) for (a) calcite and (b) dolomite for diatreme ground water as a function of pH.

These trends likely reflect the interaction of acidic water generated by pyrite oxidation in shallow portions of the diatreme with carbonates at depth. Alternatively, the trends may be indicative of the dissolution of SO4 minerals with increasing depth. The modeling results indicate that the ground water system does not reach equilibrium with gypsum solubility until reaching the level of the Carlton Tunnel (Fig. 5a), meaning that gypsum dissolution is possible all through the diatreme. Additionally, the modeling results show that equilibrium with fluorite occurs with an increase in pH, indicating that F concentrations are controlled by fluorite solubility as water infiltrates through the diatreme (Fig. 5b). Primary fluorite is a common accessory

mineral in veins in the diatreme. Based on these results, the observed decrease in F concentration with depth is probably caused by the coincident increase in Ca concentration (Table 5), thus shifting the equilibrium concentration of F to lower concentrations because of the precipitation of secondary fluorite. The flow ditch where ground water exits the diatreme through the Carlton Tunnel portal is lined with a light brown to whitish precipitate. Powder X-ray diffraction analyses of two precipitate samples indicate that it is comprised mostly of calcite (90–95%) with minor amounts of gypsum, quartz, and kaolinite (0–5%). The active precipitation of calcite and gypsum from the Carlton Tunnel water is consistent with the geochemical

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Fig. 5. Saturation indices (SI) for (a) gypsum and (b) fluorite for diatreme ground water as a function of pH.

modeling results; that is, calcite precipitation probably occurs as a result of CO2 (g) degassing as the tunnel water equilibrates with the atmosphere. Equilibrium modeling indicates that the water compositions in the Roosevelt and Carlton Tunnels are oversaturated with CO2 (g) by up to 100 times, compared to the ambient atmospheric partial pressure of 103.5 atm, depending on the pH. The CO2 (g) oversaturation is consistent with observations of effervescent water in the Carlton Tunnel, as well as the fact that elevated CO2 (g) levels in the deep underground workings were historically a serious hazard to miners (Denny et al., 1930). Based on these observations, the apparent strong oversaturation with calcite indicated by the for-

ward modeling is likely a relic of the degassing of CO2 (g) from water samples and subsequent shift in pH (Fig. 4a). The equilibria between calcite, gypsum, fluorite, CO2 (g), and, possibly Mg-bearing clays, effectively fix the concentrations of alkalinity (average=245
49 mg/l), SO4 (average=1275
156 mg/l), Ca (average=493
48 mg/l), F (average=2.6
0.3 mg/l), and Mg (average= 33
4 mg/l) in the Carlton Tunnel water. As a result, the concentrations of these analytes have remained approximately constant over the period of years of routine water quality monitoring. Another indication of long-term, steady-state chemical conditions is a historical report of water compositions for samples collected at a depth of about 900 m below

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the surface from the Portland Mine by Denny et al. (1930). These samples were reported to have SO4 concentrations from 1015–1107 mg/l, Ca concentrations from 351 to 403 mg/l, and Mg concentrations from 28 to 41 mg/l in 1925 and 1926 (Denny et al., 1930) (see Table 2). These concentration ranges are similar to those seen today in the Carlton Tunnel. 3.3. Metal solute trends and equilibria The dissolved metals most commonly encountered in the ground water include Al, Fe, and Mn. The concentrations of these metals generally decrease with increasing depth, in response to coincident increases in pH and alkalinity (Fig. 6a). Other metals, including Cu, Ni, and Zn, occur but their collective concentrations are generally more than an order of magnitude lower than the more common metals. Of these minor metals, Zn is the most mobile and is present in the highest concentrations. The concentrations of the minor metals also show a general decline with increasing pH, as exemplified by Zn (Fig. 6b). Forward modeling results indicate that different solids probably control the concentrations of the primary metals, depending on the pH. In general, metal-bearing SO4 solids dominate at low pH, and metal-bearing oxyhydroxides and carbonates dominate at pH greater than

5–6. This behavior is illustrated by Al, which has concentrations that are in reasonable agreement with the solubility of jurbanite at pH < 6, which is typical of the near-surface ground water sampled from the monitoring wells and springs (Fig. 7a). However, at pH > 6, the ground water becomes undersaturated with jurbanite and Al concentrations are more consistent with the solubility of microcrystalline gibbsite (Fig. 7b). Both jurbanite and gibbsite are commonly found in nature as a result of weathering of aluminosilicates by acidic to slightly acidic solutions (Van Breeman, 1973; Nordstrom, 1982; Karathanasis et al., 1988). A similar trend in solubility controls for Al has been observed for mine pit lakes (Eary, 1999). Dissolved Fe shows a pattern that is similar to Al, but the modeling results for SIs are more scattered. At pH < 6, the modeling results suggest that conditions of saturation to oversaturation with K jarosite [The solubility of potassium jarosite was represented by the data from Baron and Palmer (1996) (KFe3(SO4)2(OH)6 + 6H+=K+ + 3Fe3+ + 2SO2 + 6H2O; log Ksp= 4 11.0 at 25  C).] exist in the near-surface monitoring wells and springs (Fig. 8a). Saturation indices for other forms of jarosite in the PHREEQC database, that is Na jarosite and hydronium jarosite, show a pattern similar to Na jarosite except that they show overall slightly higher and lower SI values, respectively.

Fig. 6. (a) Summed concentrations of the common metals (Al+Fe+Mn) and (b) Zn concentration as functions of elevation. Brackets indicate one standard deviation about the mean.

L.E. Eary et al. / Applied Geochemistry 18 (2003) 1–24

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Fig. 7. Saturation indices for (a) jurbanite and (b) microcrystalline gibbsite as functions of pH. Detectable Al concentrations were not present in Carlton Tunnel.

At pH > 6, the modeling results show a trend toward undersaturation with jarosite and oversaturation with ferrihydrite (Fig. 8b). Ferrihydrite is a common secondary solid that forms by neutralization of acidic, Fe(III)bearing solutions. However, the solubilities of ferrihydrite and similar Fe(III) oxyhydroxides under neutral to alkaline pH conditions are ill-defined, being dependent on aging, crystallinity, redox conditions, solution composition, and analytical detection limits (Langmuir and Whittemore, 1971; Norvell and Lindsay, 1982; Hsu and Marion, 1985).

Importantly, both jarosite and Fe(III) oxyhydroxides have been mentioned in mineralogical descriptions of the shallow, oxidized zone of the diatreme (Lindgren and Ransome, 1906; Denny et al., 1930). These observations are consistent with the modeling results for oversaturated Fe(III) solids, although it may not be possible to represent dissolved concentrations exactly through calculations of solubility equilibrium. Manganese concentrations also decrease with depth, with the lowest values occurring at the Carlton Tunnel. Nearly all of the ground water compositions modeled

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Fig. 8. Saturation indices for (a) sodium jarosite and (b) ferrihydrite as functions of pH.

for the diatreme were strongly oversaturated with the various Mn oxyhydroxides (e.g., birnessite, manganite, nsutite) that might be expected to form at low temperatures. Typical modeling results for manganite are shown in Fig. 9a. Black Mn oxides are common as surface coatings on rocks from the oxidized zone of the diatreme. The observed water compositions and modeling results indicate that although conditions are favorable for the formation of Mn oxyhydroxides, they are not completely effective for limiting Mn concentrations to theoretical solubilities.

The modeling results also show that ground water infiltrating to the depth of the Carlton Tunnel eventually reaches saturation to oversaturation with rhodochrosite (Fig. 9b). Rhodochrosite is an accessory mineral in primary vein assemblages within the diatreme Lindgren and Ransome (1906). The observed decrease in Mn with depth may reflect, in part, solubility control by secondary rhodochrosite under the higher pH and alkalinities that exist for the deeper parts of the diatreme. Specific solubility controls for Zn responsible for the observed decrease in concentration with depth were not

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Fig. 9. Saturation indices for (a) manganite and (b) rhodochrosite as functions of pH.

clearly identified by equilibrium modeling. All of the tunnel samples were well undersaturated with smithsonite. Saturation indices for other possible hydrated Zn carbonates in the PHREEQC database show a trend that is similar to smithsonite but show only a slightly closer approach to saturation. The solutions were also found to be undersaturated with various Zn oxides and hydroxides, including zincite (ZnO) and Zn(OH)2, that have been hypothesized as possible controls for Zn concentrations in streams draining mineralized areas (Runnells et al., 1992).

Rather than Zn carbonates or oxides, more important solubility controls may be Zn silicates and/or Zn substitution in calcite. In studies of the effects of soluble SiO2 addition to mine waste-water by White et al. (1998), Zn2SiO4 has been determined to be the probable phase that controls Zn concentrations. Zinc silicates with the compositions approximated by ZnSiO3, Zn2SiO4, and hemimorphite [Zn4Si2O7(OH).2H2O] have also been suggested as proxies for representing the combined effects of adsorption, co-precipitation, and solid-solution processes on Zn concentrations in

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Fig. 10. Comparison of Zn concentrations in tunnel water to the solubility of Zn2SiO4 (solid lines) from White et al. (1998).

mineralized districts (Jenne et al., 1980; HudsonEdwards et al. 1996). In the diatreme, silica concentrations increase with depth, from an average of 8 mg/l in the shallow monitoring wells to 12 mg/l in water from the Roosevelt Tunnel to 26 mg/l in water from the Carlton Tunnel. Forward modeling results showed that the ground water compositions from the levels represented by the Roosevelt Tunnel and Carlton Tunnel were oversaturated with ZnSiO3. Also, a comparison of the Zn levels in water from the Roosevelt Tunnel and Carlton Tunnel shows a trend that is roughly consistent with Zn2SiO4 solubility for the range of SiO2 concentrations in the tunnel water (Fig. 10), although strict agreement is not observed. Another possible attenuation reaction for Zn is adsorption on and incorporation in calcite. Calcite has been reported to be an effective sorbent for Zn and

other divalent metals under neutral to alkaline pH conditions (McLean and Bledsoe, 1992; Zachara et al., 1993). Experimental studies by Zachara et al. (1993) indicate that Zn adsorption on calcite is maximized at pH >8, which is consistent with the pH range for the Carlton Tunnel. Samples of calcite precipitates from the Carlton Tunnel flow ditch were collected in September 1999 and analyzed for Zn contents (Table 6). Zinc concentrations in these samples ranged from 138 to 4570 mg/kg, indicating that Zn incorporation into calcite is an active mechanism that removes Zn from the Carlton Tunnel water. Because these various processes attenuate Zn in the diatreme, Zn concentrations in the Carlton Tunnel are low, ranging from 0.05 to 0.62 mg/l, with an average of about 0.10 mg/l. These concentrations are consistent with natural background concentrations in waters in

Table 6 Zinc concentrations in samples of precipitated calcite collected from the Carlton Tunnel flow ditch Sample No.

Zn (mg/kg)

Description

CT-02–02 CT-FM-04 CT-FM-01 CT-02–01 CT-FM-02 CT-FM-03 CT-FM-05

2914 3832 4570 138 4015 1320 1240

Unsolidified, actively forming precipitate on streambed Unsolidified, actively forming precipitate in streambed Semi-solidified precipitate on streambed Solidified precipitate on streambed Solidified precipitate on streambed Solidified precipitate on streambed Solidified precipitate on streambed

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unmined areas with sulfide ores contained in carbonate host rocks (i.e. 0.002 to 1.5 mg/l; Runnells et al., 1992). The forward modeling results for the Carlton Tunnel water indicate that Sr concentrations are controlled by celestite (SI range from 0.1 to 0.20) and strontianite (SI range from 0.05 to 0.60). Celestite has been reported to occur in veins and veinlets in the diatreme (Lindgren and Ransome, 1906) and is the probable source of the increasing Sr concentrations with depth. 3.4. Solute-flow rate relationships Observations made during historic tunnel construction indicate that ground water drained quickly from underground workings. The rapid flushing of water through the underground system to the Roosevelt and Carlton Tunnels clearly has the potential to affect their water quality, especially during periods of high infiltration, such as spring snowmelt or summer thunderstorms. Analytical data for Roosevelt Tunnel water show that its alkalinity tends to decrease as the flow rate increases (Fig. 11 a). Additionally, SO4 shows a trend of increased concentration with increased flow rate for the Roosevelt Tunnel (Fig. 11b). Consistent with these trends, pH values measured for the Roosevelt Tunnel show a trend of decreased values with increased flow rate (Fig. 11c). Accompanying these changes in acid-base indicators, the concentrations of Mn and Zn in the Roosevelt Tunnel water also increase as the flow rate becomes greater (Fig. 12). Overall, these concentration-flow rate relationships imply that the portion of water that rapidly infiltrates through the abandoned underground workings to the Roosevelt Tunnel is slightly more acidic than water derived from slow infiltration pathways. For the most part, however, the fast-flow, acidic water is neutralized by water/ rock interactions by the time it reaches the Roosevelt Tunnel because the lowest pH values reported for the Roosevelt Tunnel remain in the near-neutral range (5.5–8). In contrast to the Roosevelt Tunnel, the concentrations of alkalinity, SO4, and pH for the Carlton Tunnel water are not correlated to the flow rate. This observation indicates that the low levels of acidity reaching the level equivalent to that of the Roosevelt Tunnel do not penetrate the additional 370 m needed to reach the level of the Carlton Tunnel. In addition, concentrations of Mn and Zn in the Carlton Tunnel are only poorly related to flow rate (Fig. 13), and the absolute concentrations of Mn and Zn are approximately 10 times lower in the Carlton Tunnel compared to the Roosevelt Tunnel. This decline indicates that geochemical attenuation reactions have occurred for Zn and Mn, but these reactions do not completely fix their concentrations at constant levels in the Carlton Tunnel during periods of increased flow. The primary source of the low levels of metals and SO4 that reach the Roosevelt Tunnel is thought to be

soluble salts that are produced originally by sulfide mineral oxidation. These salts probably accumulate by evaporation on the rock surfaces of underground fractures and mine workings of that portion of the mineralized rock drained by the tunnel during dry periods. Characterization of mine wall salts by Denny et al. (1930) indicated that a supply of soluble salts existed, at least historically, in the underground workings. During periods of increased infiltration, these salts may be washed off the rock surfaces and transported in ground water to the level equivalent to that of the Roosevelt Tunnel, producing the changes in acid-base indicators, and Mn and Zn concentrations with increased flow rate shown in Figs. 11 and 12. This behavior is typical of the so-called ‘‘spring flush’’ that occurs in many other mineralized areas. While water infiltrating through the rock matrix will be buffered by the high carbonate content at depth, it is possible that connected vertical fractures and underground mine workings could short-circuit the protective natural system and allow slightly acidic water to reach the Carlton Tunnel. However, there are two lines of evidence that suggest that the travel time from the level equivalent to that of the Roosevelt Tunnel to the Carlton Tunnel for most of the water moving through the diatreme is slow, allowing time for water-rock reactions to buffer its composition. First, the concentrations of silica systematically increase with depth, indicating that a substantial increase in the amount of water-rock interaction occurs with the predominantly silicate matrix of the diatreme. Second, 3H concentrations in the Carlton Tunnel average 5.4 TU compared to 19.5 TU for the Roosevelt Tunnel and a range of 14–21 TU for modern water samples from shallow monitoring wells and the drinking water supply for the nearby town of Victor, Colorado (Table 7). Based simply on the difference in 3 H concentrations between water from the Carlton Tunnel and the shallower waters, a time equivalent to at least 2 half-lives for 3H (t1/2=12.43 a) or about 25a would be needed to achieve the decrease in 3H levels between the two tunnels. This time estimate may be a

Table 7 Tritium concentrations in 3H units (TU) in water from the Cripple Creek area for samples collected between July 1997 and December 1997 Sampling site

No. samples

Mean tritium (TU)

S.D. (TU)

Monitoring Well 1 Monitoring Well 2 Victora Roosevelt Tunnel Carlton Tunnel

5 5 5 2 5

21.0 18.8 14 19.5 5.4

2.4 1.9 2.4 – 1.5

a Town of Victor drinking water supply (derived from surface water sources).

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Fig. 11. Changes in (a) alkalinity, (b) sulfate, and (c) pH with rate of flow from the Roosevelt Tunnel.

minimum value given that atmospheric levels of 3H have decreased for the last 35a (Clark and Fritz, 1997). 3.5. Inverse modeling The forward modeling results discussed above identify the probable geochemical processes occurring in the diatreme controlling the composition of the Carlton

Tunnel water. However, they are not necessarily indicative of the capacity of the diatreme to regulate the water composition over the long-term under the current hydrogeochemical conditions. To estimate that capacity, inverse geochemical modeling was conducted. The inverse modeling was conducted for a hypothetical flowpath from the shallow ground water zone to the level equivalent to that of the Roosevelt Tunnel

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Fig. 12. Changes in (a) Mn and (b) Zn with rate of flow from the Roosevelt Tunnel.

(Segment 1) and from the level equivalent to that of the Roosevelt Tunnel to the Carlton Tunnel (Segment 2). Segment 1 is characterized by conversion of low-pH, shallow ground water with elevated metal concentrations to near-neutral pH water with low alkalinities and low but variable metal concentrations, as represented by the water from the Roosevelt Tunnel (Table 4). Segment 2 is characterized by conversion of the water from the Roosevelt Tunnel level to the water with alkaline pH, high alkalinities, high SO4, and low metal concentrations represented by the level of the Carlton Tunnel. The source solution for the shallow ground water zone was obtained by averaging the two most acidic compositions reported from a series of shallow boreholes completed in the upper 50 m of the diatreme (Table 4). The borehole data show substantially higher

levels of mineral acidity than present in the ambient groundwater. Hence, they are expected to represent an environmentally conservative estimate of the source solution that should cause the inverse model to calculate the maximum expected rate of consumption of acid neutralization capacity in the diatreme. The inverse modeling results for flowpath Segment 1 indicated 8 possible combinations of reactions that could explain the observed changes in chemistry. Seven of these 8 possibilities involved combinations of calcite and dolomite dissolution to provide neutralization of the acidity and the dissolution of gypsum to provide SO4. All of these models may be relevant; however, it is expected that pyrite oxidation must provide some substantial fraction of the SO4 to produce the acidic conditions generally present in the shallow ground water zone. Based on

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Fig. 13. Changes in (a) Mn and (b) Zn with rate of flow from the Carlton Tunnel.

these considerations, the models with gypsum as the only source of SO4 were not considered further. The eighth possibility provided a combination of reactions that included pyrite oxidation as the primary source of SO4 and calcite dissolution providing the neutralizing capacity (Table 8). According to this model, 0.46 g/l of calcite and 0.52 g/l of pyrite are dissolved in Segment 1. This means that a liter of water traveling from the shallow ground water zone to the level equivalent to that of the Roosevelt Tunnel, a vertical distance of roughly 500 m, would dissolve 0.46 g of calcite and 0.52 g of pyrite. The model also indicates that biotite, nepheline, montmorillonite, Mn oxide, and a Clbearing phase (artificially represented by halite) must

dissolve to produce K, Na, Al, Fe, Mn, and SiO2 in concentrations that match those found in water from the Roosevelt Tunnel. The solids that must precipitate to explain decreases in concentration over Segment 1 include chalcedony, fluorite, manganite, jurbanite, Na jarosite, and a Zn silicate (ZnSiO3) (Table 9). Also, a small amount of CO2 (g), produced by calcite dissolution, must be incorporated into the solution phase to achieve the observed increase in alkalinity. This list of precipitating minerals is consistent with those identified as probable solubility controls by forward geochemical modeling. For Segment 2, the inverse modeling also produced a total of 8 different reaction models that could explain

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L.E. Eary et al. / Applied Geochemistry 18 (2003) 1–24 Table 8 Inverse model results for Segment 1 Mineral

Formula

Mass dissolved (g/l)

Calcite Pyrite Pyrolusite Biotite Nepheline Montmorillonite Halite Fluorite ZnSiO3 Manganite CO2(g) Chalcedony Jurbanite Na-Jarosite

CaCO3 FeS2 MnO2 KMg3AlSi3O10(OH)2 NaAlSiO4 Mg0.48Fe0.22Al1.71Si3.81O10(OH)2 NaCl CaF2 ZnSiO3 MnOOH CO2 SiO2 AlOHSO4 NaFe3(SO4)2(OH)6

0.46 0.52 5.66 0.012 0.59 0.76 0.010

the observed changes in water composition. However, two of these possibilities did not involve either pyrite or carbonate dissolution to explain the required increases in SO4 and alkalinity observed over this segment. Five other models did not include pyrite dissolution as the source of SO4. Thus, these first 7 models were not considered further. The eighth reaction model included both pyrite dissolution to produce an increase in SO4 concentration and dolomite dissolution to produce the required increases in Ca, Mg, and alkalinity observed over Segment 2 (Table 9). A few water samples from the level equivalent to that of the Roosevelt Tunnel show oversaturation with dolomite but this condition occurs only occasionally during seasonal periods of low flow when pH and alkalinity are at maximum levels. The vast

Mass precipitated (g/l)

0.010 0.009 5.75 0.138 0.90 2.19 1.40

majority of the samples of water from the Roosevelt Tunnel are undersaturated with dolomite, meaning that the bulk of the water entering the column of rock above the Carlton Tunnel has the potential to dissolve dolomite in Segment 2. This reaction combination also requires the dissolution of biotite, gibbsite, halite, nepheline, pyrolusite, and Mg-nontronite, similar to the results for Segment 1. The model results also indicated that a portion of the carbonate alkalinity and Ca produced by the dissolution of dolomite reprecipitates as calcite (Table 9). This result is consistent with the results of the forward modeling, which indicated conditions of calcite oversaturation for the Carlton Tunnel water, and also with observations of calcite precipitation in the Carlton Tunnel flow ditch.

Table 9 Inverse model results for Segment 2 Mineral

Formula

Mass dissolved (g/l)

Dolomite Pyrite Manganite Biotite Nepheline Mg-Nontronite Gibbsite(mc) Chalcedony Halite Calcite Montmorillonite Fluorite ZnSiO3 Rhodochrosite

CaMg(CO3)2 FeS2 MnOOH KMg3AlSi3O10(OH)2 NaAlSiO4 Mg0.165Fe2Al0.33Si3.67O10(OH)2 Al(OH)3 SiO2 NaCl CaCO3 Mg0.48Fe0.22Al1.71Si3.81O10(OH)2 CaF2 ZnSiO3 MnCO3

5.93 0.28 3.06 0.018 0.19 2.78 9.02 14.6 0.028

Mass precipitated (g/l)

2.51 18.65 0.006 0.0004 4.0

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Other required precipitation reactions include fluorite, ZnSiO3, rhodochrosite and montmorillonite. The formation of small amounts of ZnSiO3, fluorite and rhodochrosite is needed to produce the decrease in Zn, F, and Mn concentrations observed for Segment 2. In contrast to Segment 1 where manganite was predicted as a means for Mn loss, Mn loss is best explained by rhodochrosite formation for Segment 2. The large amount of montmorillonite formation (Table 9) is needed in the modeled system to produce the decrease in Mg concentration observed from the level equivalent to that of the Roosevelt Tunnel to the Carlton Tunnel coincident with the dissolution of significant amounts of dolomite. Clays are present throughout the diatreme as argillic alteration products (Lindgren and Ransome, 1906), but data on specific stoichiometries are not available. The model results indicative of montmorillonite formation may be best considered as being representative of loss of Mg by formation of any number of Mg-bearing layered silicates. 3.6. Estimation of system capacity and buffering longevity Given the knowledge of the volumetric rate of flow through the diatreme, the inverse model results can be used to estimate the current rate at which carbonate neutralization capacity is being consumed, according to the observed changes in ground water compositions with depth. A comparison of this rate to an estimate of the amount of neutralization capacity present in the diatreme is presented in the calculation below as a way to approximate the potential longevity of the system for maintaining the water quality of the Carlton Tunnel water at current levels. This estimation is applied to the portion of the diatreme represented by Segment 2, which leads from the elevation equivalent to that of the Roosevelt Tunnel to the elevation of the Carlton Tunnel. The calculation described below is subject to the major assumption that the current hydrogeochemical conditions in the diatreme do not change. In addition, the following 5 basic assumptions are made. (1) All SO4 is produced by pyrite oxidation. (2) Reprecipitation of calcite in the Carlton Tunnel is ignored. (3) The volume and rate of water movement between the Roosevelt and Carlton Tunnels remains the same as historically observed. (4) The difference in chemical compositions between the level equivalent to that of the Roosevelt Tunnel and the Carlton Tunnel is generally representative of the entire diatreme. (5) The rate of acid generation in the upper portions of the diatreme is constant with time. The inverse modeling results for Segment 2 indicate that 5.93 g/l of dolomite must be dissolved (Table 9). Given an average flow rate from the Carlton Tunnel of 100 l/s, the rate of carbonate consumption can be calculated as follows



   g dolomite eq CaCO3 L 5:93 2 100:3 L s eq dolomite       s kg lbs 1  2:2  3:15 x 107 a 1000 g kg   ton 4 tons CaCO3  1 ¼ 4:1X10 a 2000 lbs

ð1Þ

The result is that an estimated 4.1  104 tons of CaCO3 equivalents are consumed per year by water flowing downward from the elevation equivalent to that of the Roosevelt Tunnel to the elevation of the Carlton Tunnel. This rate of carbonate consumption can be compared to the reservoir of CaCO3 equivalents present in the rock mass between the two tunnels. Based on the trends in NNP with elevation shown in Fig. 2, an average NNP value of about 50 tons CaCO3/kT can be assigned to the rock mass between the level equivalent to that of the Roosevelt Tunnel and the Carlton Tunnel. The difference in elevations between the two tunnels is 370 m. The surface expression of the diatreme is roughly elliptical, with a long dimension of about 6100 m and short dimension of 4570 m. The exact dimensions of the subsurface form of the diatreme are not well established. We make the assumption that if the diatreme is about half as large at depth as at the surface, it would have an elliptical shape with a long dimension of about 3050 m and a short dimension of 2285 m at depth. Using the formula for the volume of an elliptical cylinder of V=abh, where a and b are the long and short dimensions of the cylinder, h is the height of the cylinder, an assumed porosity of 0.10, rock density of 2650 kg/m3, and NNP of 50 tons CaCO3/kT, the mass of available carbonate neutralizing capacity present in the rock between the elevations that are equivalent to those of the two tunnels can be calculated as:   kg ð370 mÞð3049 mÞð2297 mÞð3:14Þð0:90Þ 2650 3 m      lbs ton ton CaCO3  1 50  2:2 1000 ton kg 2000 lbs ¼ 1:1  109 ton CaCO3

ð2Þ

The division of the mass of neutralization capacity at 1.1  109 ton CaCO3 from Eq. (2) by the estimated rate of consumption of 4.1  104 ton CaCO3/a yields a time of a few tens of thousands of years. This result shows that the neutralization capacity of the rock vastly exceeds the annual rate of consumption under the presentday hydrogeochemical conditions. Hence, the longevity of the geochemical processes to maintain the composition of the Carlton Tunnel water can be estimated to be a very long time.

L.E. Eary et al. / Applied Geochemistry 18 (2003) 1–24

4. Conclusions The results of the site characterization data and geochemical modeling indicate that sulfide oxidation and neutralization by carbonate minerals are the dominant geochemical processes that affect ground water compositions in the Cripple Creek Mining District. Geochemical modeling results indicate that a large excess of neutralization capacity exists in the rock mass below an elevation equivalent to that of the Roosevelt Tunnel. Thus, the effects of acid generation from oxidation reactions in the upper parts of the diatreme are being effectively ameliorated by neutralization reactions with increasingly abundant carbonate minerals in the deeper portions of the diatreme. These results are consistent with observations of the increase in alkalinity of the water that occurs with increasing depth into the diatreme, reaching conditions of calcite, gypsum, and CO2 (g) oversaturation in the Carlton Tunnel. The net effects of these buffering processes are nearly constant pH, Ca, Mg, SO4, and alkalinity concentrations in the Carlton Tunnel water. Based on comparisons to historical water quality data from underground mines and more recent monitoring results, average compositions of water exiting the diatreme through the Carlton Tunnel have not changed substantially over the last 70a. This situation is predicted to continue into the foreseeable future.

Acknowledgements The authors thank Cripple Creek and Victor Gold Mining Company, especially John Hardaway, Gordon Seibel, and Peter O’Connor, for their assistance and discussions. Useful discussions with Bill Schafer and Mark Williamson are also gratefully acknowledged. The authors also thank reviewers J. W. Ball and D. Langmuir for their constructive comments on this paper.

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