Evolving metalloid signatures in waters draining from a mined orogenic gold deposit, New Zealand

Evolving metalloid signatures in waters draining from a mined orogenic gold deposit, New Zealand

Applied Geochemistry 31 (2013) 251–264 Contents lists available at SciVerse ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/lo...

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Applied Geochemistry 31 (2013) 251–264

Contents lists available at SciVerse ScienceDirect

Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem

Evolving metalloid signatures in waters draining from a mined orogenic gold deposit, New Zealand Joanna Druzbicka ⇑, Dave Craw Department of Geology, University of Otago, PO Box 56, Dunedin, New Zealand

a r t i c l e

i n f o

Article history: Received 15 February 2012 Accepted 11 January 2013 Available online 8 February 2013 Editorial handling by J. Webster-Brown

a b s t r a c t The Globe-Progress gold mine has recently (since 2007) been developed as an open pit on the site of historic underground mine workings on an orogenic-type Au deposit. Historic mines followed auriferous quartz veins with arsenopyrite and low Sb content. Ore in the modern mine includes later-stage cataclasite with abundant stibnite. Hence, rock exposed in the area has evolved from Sb/As ratio <1 to Sb/As ratio near to or greater than 1, with locally high Sb contents. Water that has interacted with these rocks in the mine and processing system developed Sb/As ratios that were similar to those of the rocks, as indicated by compositions of pit waters and leachates from static leach tests. Mine drainage waters are cir cumneutral (pH 6–8) with high HCO 3 and SO4 (both up to 300 mg/L). Dissolved As in drainage waters was typically near 0.5 mg/L or below, but reached 1 mg/L at times, and Sb reached 3 mg/L. The Sb/ As ratio of mine drainage waters has evolved from <1 in historic mines, to 30 below the modern mine. The rise in Sb/As was enhanced by a ferric chloride treatment system for process waters that preferentially removed more As than Sb. Arsenic behaved conservatively during downstream dilution, and the As flux from the modern mine, 30 mg/s, is similar to As fluxes from historic mines. In contrast, the Sb flux has increased from 1 mg/s from historic mines to nearly 100 mg/s below the modern mine. Excavation of the modern open pit, combined with the addition of a water treatment system, removed Fe from the mine waters that historically produced abundant Fe oxyhydroxide precipitates. Hence attenuation of metalloids by adsorption, which was widespread in the historic mines, has ceased. Changing mining methods and contrasting ore types can have a dramatic effect on the metalloid contents of circumneutral mine drainage waters, and these metalloid contents evolve with time through the mining process. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Orogenic Au deposits, which formed along active continental margins, occur at a number of locations around the Pacific Rim (Goldfarb et al., 2001). They are often hosted in carbonaceous rocks and, in many cases, the mining of orogenic-type Au deposits does not generate environmental impacts via acid mine drainage, as is often the case in epithermal-type deposits, because any acid generated is neutralised (Goldfarb et al., 1995; Craw, 2001; Ashley et al., 2003; Lee, 2003; Filella et al., 2009). The principal environmental consequence of the mining of orogenic Au deposits is As and Sb enrichment in mine waters. These metalloids are highly soluble in circumneutral waters and mine waters can dissolve and transport high concentrations of these potentially toxic elements (Masscheleyn et al., 1991; Shimada, 1996; Vink, 1996; Filella et al., 2002; Smedley and Kinniburgh, 2002; Smedley et al., 2002; Ashley et al., 2003; Craw et al., 2004; Wolthers et al., 2005; Bhattacharya et al., 2006).

⇑ Corresponding author. Tel.: +64 (3) 4740530. E-mail address: [email protected] (J. Druzbicka). 0883-2927/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2013.01.011

In orogenic Au deposits, As derives from typically abundant arsenopyrite (FeAsS) and dissolved As is generally the most significant environmental issue in these settings. The current trigger values for As indicate that freshwater ecosystems are affected at concentrations as low as 0.8 lg/L (ANZECC, 2000). Antimony, which is most commonly present as stibnite (Sb2S3), is increasingly seen as an environmentally significant water contaminant (Mori et al., 1999; Kelepertsis et al., 2006; Telford et al., 2009; Liu et al., 2010; Wu et al., 2011). The freshwater quality guideline values for Sb are not well established and the current provisional low reliability trigger value for Sb(III) is 9 lg/L (ANZECC, 2000). Antimony is typically only a minor component of orogenic Au deposits, and dissolved Sb is consequently generally present at lower concentrations (<0.1 mg/L) than As in circumneutral waters emanating from mines on orogenic Au deposits (e.g. Getaneh and Alemayehu, 2006; Milham and Craw, 2009a). Exceptions arise in some orogenic deposits where Sb is the most abundant commodity and Au is a byproduct (Ashley et al., 2003; Wilson et al., 2004; Filella et al., 2009). This study reports on a mining situation on an orogenic-type Au deposit, where dissolved As has historically been the most significant environmental issue, and Sb has been subordinate, associated

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with long-term underground mining. The study shows how changing mining practices, focussing on lower grade ore in a new open cut mine, has caused evolution of mine water compositions so that Sb dominates the dissolved metalloid load in mine drainage waters. This water composition evolutionary process has occurred over several years and in spite of, and partly because of, active water management and water quality treatment processes at the mine. 2. General setting The Reefton goldfield includes a set of orogenic Au deposits hosted by Cambrian to Ordovician metasedimentary rocks of the Greenland Group (Fig. 1A; Cooper, 1974; Rattenbury and Stewart, 2000; Christie and Brathwaite, 2003; Bierlein et al., 2004). These host rocks are weakly metamorphosed (lower greenschist facies) quartz-rich sandstones and argillite of a turbidite sequence. The abundance of carbonate minerals provides for the high acid neutralising capacity of the host rocks, typically 4–9% CaCO3 (equivalent) (Hewlett et al., 2005). Minor pyrite in the host rocks has only a minor effect on the high acid neutralising capacity (Hewlett et al., 2005). The Reefton goldfield was extensively worked historically by underground mining at numerous sites over the goldfield (Fig. 1A). The largest mines were at Waiuta in the south, and the Globe-Progress area near Reefton (Fig. 1A). These mines were accessed by deep shafts, with underground workings on numerous

levels down to ca. 878 m (Waiuta) and 431 m (Globe-Progress) below surface (Henderson, 1917; Rattenbury and Stewart, 2000). Mining began in 1886 and ceased in 1920 at the Globe-Progress site, while at Waiuta it continued until 1951 (Henderson, 1917; Christie and Brathwaite, 2003). Mining of Au in the Reefton goldfield had completely ceased in the 1950s, and all mine sites were abandoned without rehabilitation (Hewlett et al., 2005; Haffert and Craw, 2008). The Globe-Progress site, described in this study, was reopened in 2007 as an open pit mine around the historic underground workings (Milham and Craw, 2009a). The Reefton goldfield lies on the western side of the Southern Alps and is characterised by steep terrain covered in thick forest, most of which has regenerated naturally after mine-related deforestation. The region has a maritime temperate climate, with average annual temperature of 12.6 °C. Westerly winds prevail in the region, bringing abundant orographic rain from the nearby ocean (Fig. 1A). The mean annual rainfall is 2300 mm, with rainfall maxima in spring and autumn and minima in late summer and midwinter (Mew and Ross, 1994). Soil cover is typically thin due to steep relief and soil pH ranges between 5 and 6 (Hewlett et al., 2005). Streams draining the hills have moss-covered bouldery beds and numerous rock outcrops. 2.1. Modern Globe-Progress mine 2.1.1. Mine site and ore processing The Globe-Progress mine is located on a ridge (550 m above sea level) and the open pit is being developed to >200 m depth on the western side (Fig. 1B). The mine area is drained by one principal creek, Devils Creek, which is a boulder-strewn stream flowing on the western side of the mine (Fig. 1A and B). A significant part of

GW13B X

X GW11A

Fig. 1. (A) Map of the Reefton goldfield showing the location of the Globe Progress and Blackwater Mines (see inset for location in New Zealand). The square at the Globe Progress Mine location shows the extent of (B). (B) Location map showing waste rock and water management organisation in the mine as well as surface water sampling locations referred to in the text. NAG and PAG are waste rock piles and arrows represent pipelines. Points GW11A and GW13B show the locations of two groundwater monitoring bores referred to in the text.

(B) Fig. 1. (continued)

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Devils Creek waters upstream of the mine come from the Progress Creek, which drains Eocene coal-bearing strata (Fig. 1B, Hewlett et al., 2005). Below the mine, Devils Creek opens out to a gravel plain, where it is joined by a large tributary, Carton Creek, before discharging into the Inangahua River, a major regional drainage system (Fig. 1A). Ore at the modern mine contains variable sulfide mineral contents, and the Au is primarily hosted in the sulfide minerals. The ore has a wide range of metalloid concentrations (Fig. 2; after Milham and Craw, 2009a). A Au-rich concentrate is produced by flotation of the auriferous sulfide minerals, and this concentrate is transported 700 km to the Macraes Au mine on the eastern side of the mountains (Milham and Craw, 2009a). The waste slurry from the processing plant is piped to a tailings impoundment at the south-end of the mine (Fig. 1B). Arsenic was predicted to be the principal environmental issue at the new mine, based on observations from the historic workings (Hewlett et al., 2005) and observations elsewhere in the Reefton goldfield (Haffert and Craw, 2008). Consequently, waste rocks at the mine are categorised into two types: non-As generating (NAG) or potentially As generating (PAG; Fig. 2), and these waste rocks are stored separately (Fig. 1B). NAG goes to large-scale open piles, while PAG is directed to a smaller separate pile that is designed to store the potentially environmentally sensitive material with limited access to oxygenated surface waters, by encapsulating it in clay (Druzbicka et al., 2010). The ore concentrate is also rich in As, but this is transported off site for processing (above).

2.1.2. Water management Globe-Progress mine is located in an area with high rainfall, so the mine has surplus water on site that has to be released into the surrounding environment on a regular basis. Most mine waters drain into Devils Creek via a tributary, Oriental Creek, which has been incorporated into the north end of the mine site (Fig. 1B). Water from the processing plant is initially piped to the tailings impoundment (Fig. 1B) where the solids are allowed to settle. The supernatant is piped to the treatment plant where ferric chloride (FeCl3) is added to aid metalloid extraction. Treated water drains into Oriental Creek via a silt pond, Oriental Silt Pond (Fig. 1B). Seepage waters from under the tailings impoundment and the PAG cell are pumped back into the tailings impoundment, so these waters also leave the site via the treatment plant and Oriental Silt Pond to Oriental Creek.

Fig. 2. Arsenic and Sb data for mineralised rocks from the Globe Progress area including quartz vein and mineralised host rock (Christie et al., 2000) and modern mine exploration data (modified from Milham and Craw (2009a)). PAG waste rock data is >45 mg/kg for As and >213 mg/kg for Sb.

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The upper reaches of Devils Creek are piped beneath the mine site and returned to Devils Creek downstream of the site (Fig. 1B). Surface water runoff from the mine site is trapped and settled in another silt pond, the Devils Silt Pond, before it drains into Devils Creek (Fig. 1B). Excess water that accumulates on the pit floor in major rain events is currently pumped to the tailings impoundment and leaves the site via Oriental Creek after passing through the water treatment plant. However, for the period of interest in this study, up to the middle of 2010, the excess pit water was pumped to the Devils Silt Pond for settling of solids before it was drained to Devils Creek (Fig. 1B).

3. Methods This study focuses on the metalloid contents of Devils Creek waters during the initial construction and development of the modern Globe-Progress mine, between 2005 and 2010. The study makes extensive use of water quality data collected by the mining company, Oceana Gold Ltd., throughout that period, with that data set augmented by targeted sampling campaigns. Water samples were obtained regularly from a range of specifically-selected sites throughout this period. Waters unaffected by the Au mine activity were obtained from site PC01, in Progress Creek, a tributary of Devils Creek (Fig. 1B), for comparison to the impacted waters further downstream. Mine waters were sampled at the drainage points from the Devils Silt Pond (DSPD) and Oriental Silt Pond (OSPD) (Fig. 1B). Impacted Devils Creek waters were sampled below the confluence with Oriental Creek (DC01), and below the confluence with Carton Creek (DC08) close to the Inangahua River (Fig. 1A and B). The Oceana Gold water quality data for these five surface water monitoring sites were collected between March 2005 and February 2011. In general, As data were collected weekly, and Sb content was determined on a monthly basis. Only at DC01 site Sb was monitored weekly since late 2008. Additional compositional data were collected on a monthly (major ions) or weekly (Fe, pH, EC) basis. Pit waters were collected in July 2009 from puddles on the pit floor where rocks categorised as ‘mineralised’ or ‘unmineralised’ were present. Water samples were filtered through a 0.45 lm membrane filter and collected in plastic bottles. Analytical methods used included flame and electrothermal AAS, IC, ICP-MS, ICP-OES and titration. Early background water quality analyses were performed at Chemsearch Laboratory in the Department of Chemistry, University of Otago, a New Zealand accredited laboratory. The rest of the analyses were performed at Hill Laboratories, Hamilton, NZ, an internationally accredited laboratory. The methods for metalloid analyses were those outlined by APHA (1998, 2005): APHA 3113B/3125B for As and 3111B/3125B for Sb. Other water quality parameters used included pH (APHA 4500H+ B), electrical conductivity (EC, APHA 2510B), as well as major ions: Cl (APHA 4110C), 2+ SO2 (APHA 4110C/4110B), HCO 4 3 (APHA 2320B/4500CO2), Ca , 2+ + + Mg , Na , and K (all APHA 3111B/3125B). The metalloid detection limits for most of the analyses were 0.0005 mg/L for As and 0.0025 mg/L for Sb. Additional water sampling, specifically for this study, was undertaken on three occasions, in November 2009, February 2010, and February 2011. Sampling methods matched those described above. November 2009 sampling included collection of four samples along Oriental Creek between OSPD and the Devils Creek confluence, and these were analysed for dissolved Sb. February 2010 and 2011 samples were additionally filtered through 0.22 lm filters, as well as the standard 0.45 lm, for determination of dissolved constituents. These samples were analysed for total and dissolved As and Sb using an ICP-MS method (APHA 3125B) as well as for S by ICP-OES. Sulfur concentrations were later con-

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verted to sulfate. Samples for total concentrations were nitric acid digested prior to the analyses (APHA 3030 E). All analyses were performed by Hill Laboratories in Hamilton, NZ. The detection limits for the metalloids were 0.001 mg/L for As and 0.0002 mg/L for Sb, for both total and dissolved fractions. Due to the difference in the types of analyses, the detection limits for the February 2011 analyses were 0.02 mg/L for As and 0.004 mg/L for Sb, for both total and dissolved fractions. The analytical reproducibility was high, as confirmed by repeated dissolved fraction analyses for metalloids at DSPD, OSPD, DC01 and DC08 locations for the February 2010 sampling. The differences in the repeated analyses results were within 3–8%. Stream sediment samples were collected in February 2011 from three locations along the Devils Creek flow. The samples were collected in clean plastic bags, wet sieved to eliminate coarse organic matter and dried at 105 °C prior to analyses. The XRF analyses were performed by the CRL Energy SpectraChem Analytical Laboratory in Lower Hutt, New Zealand, an IANZ accredited laboratory. The instrument used was a Siemens/Bruker SRS303AS WavelengthDispersive X-ray Spectrometer, with a 3 kW generator. Samples were analysed as pressed-powders containing 10% Merck Hoechst wax binder. Arsenic standards used were GXR-1 to GXR-4 and GXR-6 (USGS manufactured reference materials). Three Sb standards were purpose made using stibnite and sand by the laboratory and a fourth was a certified VS-N Sb standard. The stream sediment analyses were used in the calculations of crude distribution coefficients for As and Sb in which the ratio of sediment to water concentrations were calculated (in L/kg). This was used to give an indication of the relative solubilities of the metalloids in the system. However, due to the dynamic nature of the environment for which these calculations were performed (a stream which regularly floods), they should not be treated on a par with true distribution coefficient (Kd) calculations which are usually derived for static systems. An inspection of Devils Creek banks was also performed in February 2011 between DSPD and DC01 sampling locations in order to identify any groundwater seeps. Only one such seep was identified to have sufficient flow to enable sampling and analysis for dissolved As and Sb. The Devils Creek discharge is monitored at the DC01 sampling site (ca. 1 km downstream of the mine) using a V-notch weir. The measurements are recorded automatically every 15 min from base flow discharges <100 L/s to heavy rain events that induce discharges >2000 L/s. Discharges at other sampling sites were measured using the Geopacks flowmeter (MFP51), with impeller counts recorded over 1 min, measured with a stopwatch. Results were converted to flow velocity (in m/s) using a calibration chart provided with the instrument. The cross-sectional area of the flow was determined with a tape measure. The measurements were re-

peated three times at each location and an average of these results was used. In cases where the flow was too low to enable the use of the flowmeter, visual estimates were performed by comparison to known flows. The estimates were performed by three individuals and an average of these observations was recorded. The receiving stream for the mine waters is the nearby Inangahua River, which has mean discharge of 16,000 L/s and flood discharges >200,000 L/s (Hewlett et al., 2005). The results of water analyses and discharge measurements from February 2011 were used to produce a budget for water volume and metalloid concentrations for the mine site (Table 1). Rainwater metalloid contents were assumed to be zero in this budget (Table 1). Inputs from surface runoff and groundwater between sampling points were estimated in order to balance the discharge budget (Table 1). Metalloid contents of groundwaters were estimated from borehole waters at GW11A (unmineralised rock) and GW13B (mineralised rock) (Fig. 1B), using compositions from January 2011 provided by Oceana Gold Ltd. PAG waste rocks from the Globe-Progress mine were analysed for As and Sb in the field using hand-held field-portable X-ray fluorescence spectrometry (XRF). An Innov-X Model Omega instrument was used and the method applied was in accordance with Method 6200 (EPA, 2007). Specific recommendations for rock surface analyses were also followed, as outlined in Ge et al. (2005). Fresh and clean rock surfaces were analysed and care was taken to ensure the surfaces were as flat as possible. The minimum detection limits in this method are affected mainly by surface roughness and moisture content. Metalloid detection limits in the field were estimated to be half of the lowest concentrations recorded: 45 mg/kg for As and 213 mg/kg for Sb (Druzbicka et al., 2010). Surfaces of waste rocks used in leaching experiments (see below) were also analysed by the same instrument. The analyses were repeated seven times for each rock and an average of the results was calculated. The detection limits estimated by the instrument for the seven samples in this study were 40 mg/kg for As and 150 mg/kg for Sb. The precision of this particular FPXRF unit was estimated by Druzbicka et al. (2010) and ranged from 1.6% to 13.4% relative standard deviation for As and Sb, depending on the concentrations determined. The comparison of FPXRF with laboratory XRF analyses results of the same samples revealed that the FPXRF method tends to underestimate the results by 20% at 0.1% As and 60% at 0.1% Sb level. Static leaching tests were performed on seven rock samples collected from the Globe-Progress waste rock stacks (NAG and PAG) which represented different styles and degrees of mineralisation. For leaching experiments, rock fragments of similar size (unbroken) were submerged in 500 mL of distilled water for 10 days. Samples for dissolved metal analyses were filtered through 0.45 lm membrane filter and preserved with HNO3. The leachates

Table 1 Mine water budget and flux calculations for As and Sb (total concentrations) on 21 February 2011. The proportions shown are those derived for DC01 calculations.

a

Measured flow (L/s)

Propn

As (mg/L)

Sb (mg/L)

Flux As (mg/s)

Flux Sb (mg/s)

Sb/As ratio

PC02 DSPD Oriental Creeka Seep into DC Rainwater Gw min. Gw unmin.

5.6 20 22 0.1 – – –

0.086 0.308 0.338 0.002 0.046 0.150 0.070

0.0000 0.0340 0.0990 0.1700 0.0000 0.0930 0.0031

0.0000 0.0540 3.6000 0.0270 0.0000 0.0130 0.0022

0.00 0.68 2.18 0.02 0.00 1.21 0.03

0.00 1.08 79.20 0.00 0.00 0.17 0.01

– 1.6 36.4 0.2 – 0.1 0.3

Calculated DC01 DC01

– 65

1.000 1.000

0.0584 0.0510

1.2372 1.5200

3.80 3.31

80.42 98.8

21.2 29.8

Calculated DC08 DC08

– 260

1.000 1.000

0.0165 0.0150

0.3104 0.3700

4.28 3.9

80.71 96.2

18.9 24.7

Oriental Creek was sampled 970 m downstream of OSPD, near the confluence with Devils Creek.

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were analysed for total and dissolved As and Sb using ICP-MS by Hill Laboratories in Hamilton. The methods and detection limits match those described above (method APHA 3125B, detection limits: 0.001 mg/L for As and 0.0002 mg/L for Sb).

in old workings, sampled from an exploration core (after Christie et al., 2000), are plotted in Fig. 2 where they are compared to the metalloid contents of ore and PAG waste rocks in the modern mine.

4. Historic mine sites

4.2. Historic mine waters in Devils Creek catchment

4.1. Ore types

The chemical compositions of waters in the Devils Creek catchment prior to the opening of the Globe-Progress mine are known from previous studies as well as from the historic background data gathered during the period of over 2 years by Oceana Gold Ltd. (Wilson, 2003; Hewlett et al., 2005; Craw et al., 2007). Two examples of representative water quality analyses (pre July 2007, across all five locations) are presented in Table 2. Progress Creek, upstream of the Globe-Progress site, originates from an historic coalfield (Fig. 1B) and has been affected by coal-related acid mine drainage (Hewlett et al., 2005). The pH at sampling point PC01 in this coalfield remained at a constant level of 4.5 throughout the 2005–2011 monitoring period, and has not been influenced by the recent mining operations (Table 2; Fig. 3A). Waters draining from the historic Globe-Progress mining area, measured at the future DSPD, OSPD and DC01 sites, were characterised by a pH between 6 and 8 (Fig. 3A–C; Table 2). All the Devils Creek catchment waters had low electrical conductivity (40–100 lS/cm, Fig. 3D–F, Table 2) and low major ion contents (Fig. 4A–F; Table 2) before the mine opened. Minor enrichment in Fe in the mildly acidic upstream PC01 waters (Fig. 5A) reflects the input of acid rock drainage from the coalfield (Hewlett et al., 2005). Likewise, the occasionally high (up to 10 mg/L) Fe in waters draining the historic Globe-Progress mine site at DSPD and OSPD are derived from mine tunnel drainage that contains abundant Fe oxyhydroxide precipitates, some of which are finer than the 0.45 lm filters used for water sampling (Kimball et al., 1995; Hewlett et al., 2005).

There are two distinct styles of mineralised rocks in the Reefton goldfield: quartz veins with associated altered host rocks, and cataclastic rocks associated with late-stage shears and fault gouge zones that locally cut across the earlier mineralised quartz veins (Christie and Brathwaite, 2003; Milham and Craw, 2009b). These two different mineralisation styles have distinctly different metalloid contents. The earlier quartz veins have abundant arsenopyrite, variable amounts of pyrite, and only minor Sb content that is in solid solution in arsenopyrite (Milham and Craw, 2009b). In contrast, the later cataclasites have abundant stibnite in association with pyrite and arsenopyrite. Pyrite in both mineralisation styles commonly contains As in solid solution as well. The ore mined historically in the Reefton goldfield consisted almost entirely of Au-bearing quartz veins with pyrite and arsenopyrite. Mining at Waiuta (Fig. 1A) focussed on a single large quartz vein, and little adjacent host rock was processed (Henderson, 1917). Processing residues are extremely As-rich, and As was a bi-product of the mines at times, with low Sb content (Sb/ As  0.01; Craw et al., 2007; Haffert and Craw, 2008). Historic mining at the Globe-Progress site was similarly focussed on quartz veins and immediately adjacent host rocks, and ore was rich in arsenopyrite and pyrite. Stibnite was also noted to occur in some old mine workings, locally up to 20–40% of rock mass, but the Au content was known to be very low and other sulfides were absent (Henderson, 1917). Typical As and Sb contents of mineralised rocks

Table 2 Representative water quality data from five surface water monitoring locations in the Globe Progress Mine. Sampling date

pH

EC (mS/m)

Cl (mg/L)

SO4 (mg/L)

PC01 12-October-05 12-March-07 29-October-08 10-September-09 24-August-10

4.3 4.5 4.4 4.4 4.5

4.1 5.5 4.5 4.6 3.4

2.8 3.3 4.7 4.4 3.2

Ca (mg/L)

Mg (mg/L)

0.5 1.3 0.8 0.8 0.6

0.3 0.8 0.5 0.6 0.5

2.5 2.9 2.7 2.7 2.2

DSPD 12-October-05 12-March-07 29-October-08 10-September-09 24-August-10

6.4 6.7 7.1 6.6 7.1

5.6 15.9 36.6 58.8 73.8

3.2 3.4 4.5 5.5 5.6

10 34 100 180 260

12 46 72 120 140

2.8 9.4 26.0 42.0 53.0

3.6 10.2 22.0 43.0 58.0

OSPD 12-October-05 12-March-07 29-October-08 10-September-09 24-August-10

6.3 6.7 8.2 7.5 8.3

4.3 12.1 86.6 81.7 110.4

3.2 3.8 45 50 95

7 12 180 150 182

6 49 170 180 220

2.1 7.6 12.0 8.1 6.5

DC01 12-October-05 12-March-07 29-October-08 10-September-09 24-August-10

7.0 7.4 8.1 8.0 8.2

6.1 12.8 53.3 53.8 68.8

3.5 3.7 22 15 23

10 21 120 140 190

13 41 110 110 133

DC08 12-October-05 12-March-07 29-October-08 10-September-09 24-August-10

6.8 6.9 7.6 7.4 7.4

4.5 6.8 23.7 27.1 34.1

3.8 3.8 10 9.4 13.6

4 10 50 67 85

12 17 47 55 63

3.8 10.7 7.4 11.0 4.1

HCO3 (mg/L) 0.6 0.6 0.5 0.5 0.5

Na (mg/L)

K (mg/L)

Fe (mg/L)

As (mg/L)

Sb (mg/L)

0.33 0.48 0.53 0.46 0.34

1.43 2.45 1.30 1.30 1.15

0.0025 0.0025 0.0015 0.0010 –

0.0025 0.0025 0.0001 0.0003 –

2.8 3.4 7.4 18.0 34.0

1.23 3.38 2.80 4.90 6.00

1.80 9.97 0.68 0.16 0.24

0.0290 0.0530 0.0540 0.0180 0.1010

0.0025 0.0025 0.0300 0.2400 0.2800

2.6 8.2 11.0 9.6 7.5

2.3 3.6 120.0 150.0 230.0

1.33 9.70 23.00 17.00 21.00

1.87 7.79 0.12 0.11 0.07

0.0420 0.2790 0.4400 0.0470 0.0400

0.0025 0.0025 0.4800 1.6000 0.4500

3.3 7.6 15.0 26.0 32.0

3.5 8.4 13.0 25.0 35.0

3.0 3.8 60.0 43.0 65.0

0.90 1.43 12.00 6.30 7.90

0.85 3.41 0.42 0.15 0.13

0.0320 0.1220 0.2300 0.0260 0.0670

0.0025 0.0025 0.2200 0.5000 0.2500

2.3 3.6 8.8 13.0 14.8

2.1 3.2 7.5 12.0 15.9

3.1 3.7 22.0 22.0 31.0

0.55 0.69 4.30 2.90 3.80

0.44 0.24 0.33 0.14 0.18

0.0080 0.0080 0.0740 0.0110 0.0199

0.0025 0.0025 0.0720 0.1900 0.1040

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Fig. 3. Time series for the pH and electrical conductivity (EC) data for waters in the Devils Creek catchment from March 2005 to February 2011, during establishment of Globe Progress mine. The mine opened in July 2007 (dashed line). (A) The pH upstream of the mine (PC01) and at mine water discharge site (DSPD); (B) the pH at mine water discharge site (OSPD); (C) the pH downstream of the mine (DC01); (D) the EC upstream of the mine (PC01) and at mine water discharge site (DSPD); (E) the EC at mine water discharge site (OSPD); (F) the EC downstream of the mine (DC01).

Surface water As concentrations in the Devils Creek catchment were typically <0.05 mg/L before the modern mine opened, but were locally up to 0.13 mg/L at DC01 and up to 0.46 mg/L at OSPD (Fig. 6A–D and Wilson, 2003) because of input from historic mine tunnels (Hewlett et al., 2005). Antimony concentrations were very low and close to, or at, levels of minimum detection (typically 0.0025 mg/L). Groundwater draining from historic mine tunnels at the Globe-Progress site contained up to 60 mg/L As and 0.03 mg/L Sb (Wilson, 2003; Hewlett et al., 2005; Craw et al., 2007). The Sb/As ratios in surface waters, calculated from Hewlett et al. (2005) data, were between 0.0004 and 0.19 and those calculated from Wilson (2003, unfiltered samples) between 0.02 and 0.72. Typical Sb/As ratios for four sampling locations in this study (DSPD, OSDP, DC01 and DC08), obtained before the modern mine opened, ranged from 0.08 to 0.37 (Fig. 7A–D).

5. Modern Globe-Progress mine waters 5.1. Major ions and pH Three water quality analyses (post-July 2007, across all five locations) representative of the period since the opening of the modern mine are presented in Table 2. The pH of Globe-Progress

mine site drainage waters at DSPD and OSPD have remained circumneutral since the modern mine opened, with a general pH increase towards 8 (Fig. 3A and B; Table 2). This trend of broadly increasing pH persists downstream in Devils Creek to DC01 (Fig. 3C). The elevated pH trends have occurred in parallel with increasing alkalinity (as HCO3) at these sampling points (Fig. 4D– F). Both DSPD and OSPD waters show a steady increase in alkalinity over time from <50 mg/L up to 230 and 380 mg/L, respectively (Fig. 4D and E). Electrical conductivity has also increased considerably since mid-2007, reaching values of up to 1000 lS/cm at DSPD and up to 2000 lS/cm at OSPD (Fig. 3D and E). Substantially elevated amounts of dissolved SO2 (up to 300 mg/L) have developed in 4 DSPD and OSPD mine drainage waters (Fig. 4A and B; Table 2), compared to PC01 waters which have <15 mg/L SO4 (Table 2), and SO2 has become the dominant anion in the mine waters 4 (Fig. 4A, B and 5). Sulfate contents of Devils Creek remain high at DC01 (up to 240 mg/L; Fig. 4C), and the DC01 EC measurements are similar to those recorded at DSPD (Fig. 3F, Table 2). The Cl concentrations have remained unchanged (<10 m/L) at DSPD (Fig. 4A; Table 2), whereas Cl is elevated in OSPD waters (Fig. 4B; Table 2) because of addition of ferric chloride in the water treatment plant immediately upstream (Fig. 1B). Chloride at OSPD rose to 140 mg/L and elevated Cl persisted downstream to DC01

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257

 Fig. 4. Time series for the Cl, SO2 4 , HCO3 and Na data for waters in the Devils Creek catchment from March 2005 to February 2011, during the establishment of the Globe 2  Progress mine. The mine opened in July 2007 (dashed line). (A) The Cl and SO2 4 upstream of the mine (PC01) and at mine water discharge site (DSPD); (B) the Cl and SO4 at  the mine water discharge site (OSPD); (C) the Cl and SO2 downstream of the mine (DC01); (D) the HCO and Na upstream of the mine (PC01) and at mine water discharge 4 3  site (DSPD); (E) the HCO 3 and Na at mine water discharge site (OSPD); (F) the HCO3 and Na downstream of the mine (DC01).

where concentrations of up to 75 mg/L have been measured (Fig. 4C), above a background of <5 mg/L at PC01 (Table 2). Dissolved Na is elevated in mine waters at OSPD (up to 370 mg/L) and DSPD (up to 58 mg/L) (Fig. 4D and E) compared to background at PC01 (<5 mg/L). The DC01 Na concentrations reflect the averaged effects of both these mine waters and the concentrations recorded typically lie between DSPD and OSPD values (Fig. 4F). Changes in Cl and Na compositions in mine-affected waters are particularly well illustrated in the Piper diagram in Fig. 8. OSPD compositions have shifted towards the Na–Cl water type, while DSPD waters remained in the Ca–Mg–SO4 field. Groundwater in the mine area, included for comparison to these surface waters, is exclusively of Ca–Na–HCO3 type (Fig. 8). Iron contents of mine drainage waters, which were variable but commonly elevated, decreased when the mine opened in July 2007 (Fig. 5A–C; Table 2). The most pronounced change in Fe contents has occurred at OSPD, where the concentrations dropped almost instantaneously to <2 mg/L (Fig. 5B). This decrease in Fe is recognisable further downstream at DC01 (Fig. 5C), but is less pronounced than immediately below the mine site. 5.2. Metalloid concentrations and ratios Metalloid contents of mine waters fluctuated widely after the modern mine opened (Fig. 6A and B), and did not show the patterns

of steady increase that were observed in the major ion compositions. Arsenic concentrations were highest in 2008–2009 (up to 1.3 mg/L) at OSPD, whereas at DSPD As never exceeded 0.26 mg/L (Fig. 6A and B). Antimony concentrations at OSPD were as high as 3.1 mg/L in 2010 while DSPD has shown concentrations of up to 0.61 mg/L (Fig. 6A and B). Devils Creek waters downstream of the mine had up to 0.32 mg/L As and 1.8 mg/L Sb at DC01, while at DC08 As never exceeded 0.1 mg/L and the highest Sb concentration recorded was 0.36 mg/L (Fig. 6C and D). Waters collected from Oriental Creek below OSPD (Fig. 1B) in the targeted sampling campaign in November 2009 revealed an increase in Sb concentrations with downstream distance (Fig. 9). The Sb content of these mine drainage waters doubled over 1 km, from 0.04 to 0.08 mg/L (Fig. 9). A considerable difference between the As and Sb concentration patterns was observed along the flow of Oriental Creek. The As concentration, unlike Sb, first increased significantly from 0.03 to 0.25 mg/L. It then decreased gradually, as a result of dilution, to nearly 0.09 mg/L near the confluence with Devils Creek (Fig. 9). The sampling conducted in February 2010 revealed that the majority of As the DSPD location was present in the particulate rather than dissolved form. The total As concentration was 0.2 mg/L while the filtered samples contained 0.022 and 0.021 mg/L (Table 3). This suggests that 90% of total As in this locality is particulate, as it was removed by filtration through 0.45 and 0.22 lm filters. The reason for such a high particulate

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Fig. 6. Arsenic and Sb concentrations in mine waters over time. (A) DSPD; (B) OSPD; (C) DC01; (D) DC08. The dotted vertical lines show when the modern Globe Progress Mine was opened (July 2007).

Fig. 5. Time series for the Fe data for waters in the Devils Creek catchment from March 2005 to February 2011, during establishment of Globe Progress mine. The mine opened in July 2007 (dashed line). (A) The Fe upstream of the mine (PC01) and at the mine water discharge site (DSPD); (B) the Fe at the mine water discharge site (OSPD); (C) the Fe downstream of the mine (DC01).

As content at DSPD at that time was the release of untreated turbid pit waters at this locality. Some particulate As was also detected in the Oriental Creek where concentrations fell from 0.05 mg/L in the unfiltered sample to 0.036 and 0.035 mg/L in filtered waters (Table 3). This drop shows that 28–30% particulate As was present at Oriental Creek which receives treated mine waters. At downstream Devils Creek locations, DC01 and DC08, the proportion of the particulate As fraction fell to 25% and 20%, respectively, as a result of dilution. Antimony and SO2 4 , unlike As, were both present in dissolved form only, as the dissolved Sb and SO2 4 concentrations in filtered samples did not differ significantly from the total concentrations measured in unfiltered waters (Table 3). Metalloid concentrations of three stream sediment samples ranged from 300 to 770 mg/kg for As and from below detection limit (<20 mg/kg) to 160 mg/kg for Sb (Table 4). The Sb/As ratios calculated for the three samples were below 0.23 (Table 4). 5.3. Water–rock interaction The abundant rainwater that falls on the open pit at the modern Globe-Progress mine runs over large amounts of mineralised rocks

in the pit walls before accumulating in the bottom of the pit. The pools of water on the pit floor are generally turbid, with suspended fine grained, variably mineralised, rock particles. Hence, there is much opportunity for chemical interaction between water and rock in the pit on time scales of hours to weeks. This chemical interaction results in dissolution of metalloids. Typical pit waters have between 0.02 and 0.86 mg/L As and between 0.12 and 1.9 mg/L Sb (Fig. 10). To quantify these water–rock interaction processes with respect to different types of rocks, some static leaching tests were conducted on seven waste rock and ore samples from the modern mine (Table 5; Fig. 10). The experiments were designed to imitate a situation of prolonged rock–water contact within the pit or on a wet waste pile. NAG rocks included host rock and quartz vein materials with minor visible pyrite and arsenopyrite grains on the outside surfaces. Two samples of quartz veins containing dark sulfide seams and abundant pyrite have some characteristics of both NAG and PAG waste rock, and these are designated NAG/ PAG in Fig. 10. A strongly mineralised sample of sulfide-rich cataclasite and a sample of stibnite-rich quartz breccia are both typical of PAG or ore (PAG/ore samples in Fig. 10). Rock surfaces of these samples were analysed by field-portable XRF to quantify the amounts of As and Sb that was exposed to leaching waters. The rock surface compositions ranged from 235 to 10,700 mg/kg As and from below detection limit (150 mg/kg) to 27,200 mg/kg Sb (Table 5; Fig. 10). The lowest metalloid results were obtained from two NAG samples, both with weak pyritic mineralisation. The highest metalloid results came from the PAG/ore

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Fig. 9. Variations in metalloid concentrations with distance in Oriental Creek, downstream from the Globe Progress Mine and until the confluence with Devils Creek (1 km from the mine site). The initial increase in As concentration results from an historic adit water (with low Sb/As ratio) addition into Oriental Creek. The results were obtained from the November 2009 sampling round and include the uncertainty of measurement bars.

Table 3 As, Sb and SO2 4 total and dissolved concentrations at four locations in Globe Progress Mine, February 2010 sampling. Sulfate concentrations were calculated from sulfur results.

Fig. 7. Sb/As ratios in mine waters over time. (A) DSPD; (B) OSPD; (C) DC01; (D) DC08. The horizontal lines show Sb/As equal to 1. Hence, the results falling above the line indicate Sb being dominant over As. The dotted vertical lines show when the modern Globe Progress Mine was opened (July 2007).

Location

Analysis

As (mg/L)

Sb (mg/L)

SO2 4 (mg/L)

Sb/As

DSPD DSPD DSPD OSPD OSPD OSPD DC01 DC01 DC01 DC08 DC08 DC08

Total Dissolved Dissolved Total Dissolved Dissolved Total Dissolved Dissolved Total Dissolved Dissolved

0.200 0.022 0.021 0.050 0.036 0.035 0.085 0.066 0.064 0.015 0.012 0.012

0.60 0.58 0.60 0.90 0.87 0.94 0.62 0.57 0.61 0.15 0.16 0.16

243 240 240 216 213 216 183 180 174 72 69 69

3

18

7.3

10

Table 4 Arsenic and Sb concentrations in Devils Creek water (DC01) and in three sediment samples on 21 February 2011.

DC01 Sed_DC3 Sed_DC2 Sed_DC01

mg/L mg/kg mg/kg mg/kg

As

Sb

Sb/As

0.05 310 695 771

1.52 <20 160 81

30.4 – 0.2308 0.1055

samples had 20 mg/L As and Sb contents ranging up to 9 mg/L (Table 5; Fig. 10). The results provide no convincing evidence of particulate As or Sb presence in static leaching test waters. The highest particulate fraction of total As concentration was recorded for a NAG/PAG sample where it exceeds 16%. However, this may be due partly to analytical uncertainty. This finding suggests that the high particulate As at DSPD was the results of mine detritus presence at the time of sampling (above). Fig. 8. Piper diagram showing the distribution of surface (DSPD, OSPD) and groundwater (GW) major ion compositions after modern mine opening.

6. Discussion samples, with the cataclasite sample having the highest As and the stibnite-rich quartz breccia having the highest Sb (Table 5; Fig. 10). Leachate metalloid concentrations from these static tests were found to be lowest for NAG samples (<0.06 mg/L As and <0.01 mg/L Sb; Table 5; Fig. 10). The highest Sb content occurred in leachate from the stibnite-rich quartz breccia (Table 5; Fig. 10). Leachates from one NAG/PAG sample and both PAG/ore

6.1. Changes in some major ion and Fe compositions The changes in mine water composition that have taken place since the opening of the Globe-Progress mine stem from the obvious physical act of exposing large volumes of mineralised rock to the surface environment, the processing of the ore, and the subse-

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Excavation of the open pit at the modern mine intersected numerous historic mine tunnels that were channelling groundwater with abundant Fe oxyhydroxide precipitates (Hewlett et al., 2005; Craw et al., 2007). Hence, these Fe-bearing historic mine drainage waters were removed from the mine site at their sources, and this undoubtedly contributed to the decrease in the Fe content of waters at DSDP and OSPD, and subsequently Devils Creek at DC01, as the mine developed (Fig. 5A–C). The drop in Fe contents of waters was most rapid at OSPD, and this may have also been influenced by the water treatment plant processes which result in the removal of Asbearing Fe oxyhydroxide colloids from the water. 6.2. Metalloid dissolution pathways

Fig. 10. Rock surface (mg/kg) and leachate (mg/L) concentrations for seven waste rock and ore samples. The solid line represents a Sb/As ratio of 1. Note, the two lowest NAG rock results are below the minimum detection limit of the FPXRF method used and are not shown.

Considering the circumneutral and relatively O2-rich character of waters that interact with the mineralised rocks in the mine, as well as the lack of any observed intermediate Sb(III) oxides in the field, it is presumed that the dissolution of stibnite follows a direct ðVÞ oxidative dissolution to Sb O 3 , as presented by Eq. (1) (Ashley et al., 2003): 

þ  Sb2 S3 þ 18H2 O ¼ 3SO2 4 þ 2SbO3 þ 26e þ 36H

ð1Þ

(III)

quent treatment of raw processing waters. Dissolved SO2 is the 4 most prominent major ion that has increased as part of these processes (Fig. 4A–C). These increases in SO2 concentrations arise 4 from oxidation of the abundant sulfide minerals in the ore and variably mineralised waste rocks. Sulfide oxidation has occurred in both mine pit environment, leading to higher SO2 in DSDP, and 4 in the processing system that led to high SO2 4 in OSPD, and this has occurred in approximately equal amounts in these two different environments (Fig. 4A and B). In both instances sulfide oxidation has not caused the lowering of the pH and no increase in dissolved Fe was observed in waters as a result. Water alkalinity and pH rose in mine waters as a result of enhanced dissolution of carbonate minerals from the ore and host rocks (Hewlett et al., 2005). This water–rock interaction was sufficient to fully neutralise the weakly acid waters from the upstream coal-bearing strata (Fig. 3A and B), and enhance the alkalinity of the mine waters. In addition, the use of soda ash (Na2CO3) as a dispersant during the processing of the ore also contributed to alkalinity and to elevated Na in OSPD waters (Fig. 4E). Likewise, the increase in Cl content in the Oriental Creek (OSDP, only) is attributable to the addition of FeCl3 in the water treatment plant used to remove As and, to a lesser extent, Sb.

However, it is possible that production of Sb oxide does take place, for example in the treatment plant (which has not been inspected), in which case the dissolution pathway may be (Ashley et al., 2003): þ Sb2 S3 þ 3H2 O þ 6O2 ¼ 3SO2 4 þ 2Sb2 O3 ðvalentiniteÞ þ 6H

followed by the dissolution and oxidation of þ

ðIIIÞ Sb2 O3

ðVÞ

to Sb

ð2Þ O 3:



Sb2 O3 þ 3H2 O ¼ 2SbO3  þ6H þ 4e

ð3Þ

The breakdown of arsenopyrite in the supergene environment, according to Dove and Rimstidt (1985), follows reaction (4) to produce scorodite: þ FeAsS þ 14Fe3þ þ 10H2 O ! 14Fe2þ þ SO2 4 þ FeAsO4  2H2 O ðscoroditeÞ þ 16H

ð4Þ

Scorodite then tends to dissolve incongruently to produce arsenate and Fe(III) hydroxide (Dove and Rimstidt, 1985):

FeAsO4  2H2 O þ H2 O $ H2 AsO4 þ FeðOHÞ3 ðsÞ þ Hþ

ð5Þ

6.3. Metalloid budget Devils Creek base flow is measured at the V-notch at DC01. The Oceana Gold Ltd. database indicates that the flow is typically

Table 5 Arsenic and Sb concentrations in rocks and leachates. Water compositions represent the results of leaching tests performed on the corresponding rock samples (see text). Waste type

Analysis

WATER

ROCKS

As (mg/L)

Sb (mg/L)

As (mg/kg)

Sb (mg/kg)

NAG

Total Dissolved

0.065 0.06

0.0043 <0.004

562

<150

NAG

Total Dissolved

0.195 0.18

0.042 0.046

6218

622

NAG

Total Dissolved

0.029 0.03

0.0107 0.01

235

<150

NAG/PAG

Total Dissolved

0.42 0.35

0.26 0.43

1562

171

NAG/PAG

Total Dissolved

10.7 10.4

1.43 1.47

1310

627

PAG/ore

Total Dissolved

27 26

1.12 1.24

10739

4253

PAG/ore

Total Dissolved

3.5 3.3

9.10 9.6

386

27204

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between 50 and 100 L/s but can rise rapidly (hours) to >2000 L/s during rain events, and fall again almost as quickly. The average discharge, calculated over a 2-year period, is 230 L/s (Hewlett et al., 2005). The targeted water sampling campaigns in this study were deliberately conducted at times of low rainfall and consequently relatively low discharge (e.g., Table 1). Progress Creek at sampling site PC02, immediately upstream of the Globe-Progress mine (Fig. 1B), contributes about one tenth of Devils Creek discharge at DC01. Mine water discharges vary typically from 10 to 30 L/s for the OSPD and DSPD sampling sites, depending on mine operations and rainfall events. These discharges are influenced by the amounts of drainage from the adjacent silt ponds. A Devils Creek catchment water and metalloid budget was made using data acquired during the February 2011 sampling campaign (Table 1). The flows were measured where possible, and other contributions were estimated to balance the discharge budget. The budget aimed to match observed metalloid contents at the DC01 and DC08 sampling points downstream of the mine. The calculated budget matches the observed As concentrations, with the calculated value being 0.058 mg/L compared to the observed 0.051 mg/L at DC01. The measured As concentration at the DC08 sampling site is low because of dilution by the major tributary, Carton Creek (Fig. 1A), and was below the detection limit for the analytical method used in the February 2011 targeted sampling campaign. The As concentration at DC08 has been estimated for the budget (Table 1) using an average of four similar low-flow measurements at that point from the Oceana Gold database. The calculated As concentrations in the budget (0.017 mg/L) are similar to these averaged observed As concentrations (0.015 mg/L; Table 1). Matching calculated and observed Sb concentrations at DC01, by manipulating reasonable values of the poorly-known parameters of the budget, proved to be impossible while maintaining a match for As, as above. The calculated Sb concentration at DC01 (1.24 mg/L) is distinctly lower than the observed concentration (1.52 mg/L) at DC01. Calculated Sb (0.31 mg/L) is slightly lower than observed Sb (0.37 mg/L) at the DC08 sampling site as well (Table 1). From these calculations it is concluded that it is possible that some dissolved Sb has been added to the Devils Creek waters as they travel downstream. A probable reason for increasing Sb in the Devils Creek catchment is that minor Sb was dissolved from stream sediments, as implied by the increasing Sb downstream in Oriental Creek (Fig. 9). Occasional drainage of pit waters past the Devils Silt Pond to Devils Creek occurred during major rain events in the early history of the modern mine (above). These waters sometimes had high suspended solid contents when water volumes exceeded the carrying capacity of the silt pond, and some Sb-bearing solids from the pit may have been transported into Devils Creek at such times. Dissolution of Sb from this fine sediment probably occurred under lowflow conditions, as bottom waters and shallow groundwater in the stream bed moved slowly. Sediment discharge events no longer occur because the pit waters are now pumped (since mid-2010) to the tailings impoundment. Hence, it is no longer possible to verify this assertion under similar conditions. However, the analyses of Devils Creek stream sediment As and Sb concentrations reveal that some quantities of these metalloids continue to be present (Table 4). Calculations of crude distribution coefficients for the metalloids show that Sb is more readily mobilised. The ‘‘Kd’’ values calculated for As were on the order of 104 L/kg, while those for Sb were considerably lower, on the order of 102 L/kg and less. These observations were further confirmed by the Sb/As ratios for stream sediment samples, which are <1, opposite to the Devils Creek water ratios, and point towards greater Sb dissolution in this setting (Table 4). Similar Sb dissolution from sediments, on a much

261

larger spatial scale, was reported by Ashley et al. (2007) downstream from the Hillgrove Sb–Au mine, NSW, Australia. 6.4. Metalloid fluxes Metalloid fluxes past the DC01 V-notch discharge measurement point can be calculated for dates where both As and Sb data were available. There were 42 suitable sets of measurements in the available data set between April 2008 and February 2011. The results ranged from 1 to 168 mg/s for As, and 0.52 to 450 mg/s for Sb. Average As and Sb flux values were 33 and 95 mg/s, respectively. Similar flux calculations performed for the February 2011 results show that 4 mg/s As and nearly 100 mg/s Sb were draining down Devils Creek past DC01 and DC08 on the day of sampling (Table 1). The Sb concentration calculated for the purpose of the metalloid budgeting was lower than the one observed. Thus, as a result, the calculated Sb flux is considerably lower than the observed Sb flux (Table 1). Hewlett et al. (2005) estimated the historic As flux down Devils Creek to be between 25 and 32 mg/s, whereas the historical Sb flux calculated for the catchment, including historic mine tunnels, was 0.9 mg/s. They concluded that these metalloid loadings come from the historic mine area, but that over 90% of the metalloids were derived from natural sources, primarily mineralised rocks in the catchment. The average As flux associated with development of the modern mine is essentially the same as that determined by Hewlett et al. (2005) for the historic mine site. The As flux determined in the budget for low-flow conditions (4 mg/s at DC01; Table 1) is considerably lower than the typical values for the modern and historic Devils Creek catchment. The modern low As flux under low-flow conditions is not surprising because much of the mine drainage water was, and is, treated with FeCl3 to remove As before being released into Oriental Creek (Fig. 1B). This treatment process clearly became more effective as the mine developed, as early high As at OSPD gave way to generally low As at the same point in 2009 (Fig. 6B). The increase in the efficiency of FeCl3 treatment is most likely due to the adjustment of the method by the mine over time. Farther downstream, the low-flow As flux changes little between DC01 and DC08, despite the increase in stream discharge associated with incoming tributaries (Table 1; Fig. 1A). Comparison of As concentrations between DC01 and DC08 over the time period of this study suggests that dilution between these sites ranged from a factor of 1.4 to a factor of 18. Most of this dilution resulted from incoming Carton Creek water (Fig. 1A), and was subject to the vagaries of rainfall and runoff. Dilution, with near-constant As flux, appears to have been the principal process for attenuation of As concentrations in Devils Creek downstream of the modern mine. Attenuation of As by adsorption to Fe oxyhydroxides, with associated decrease in As flux, is a common phenomenon at many mine sites (Fukushi et al., 2003; Gault et al., 2005; Lee et al., 2005; Asta et al., 2010a; Hiller et al., 2012), and this occurred extensively at the historical Globe-Progress mine site (Hewlett et al., 2005; Craw et al., 2007; Haffert and Craw, 2008). However, the Fe oxyhydroxide precipitates and the small amounts of Fe that were in waters of the Devils Creek catchment before development of the modern mine have been effectively removed during development of the mine (Fig. 5A–C), precluding adsorption as an As attenuation mechanism in the modern mine waters. Similarly, As attenuation can occur by precipitation of variably crystalline arsenate minerals (Krause and Ettel, 1989; Papassiopi et al., 1996; Harvey et al., 2006; Drahota and Filippi, 2009), and this process also operated at the historical Globe-Progress mine site (Hewlett et al., 2005; Craw et al., 2007). However, solubility of arsenates is high in circumneutral waters, and even higher in the increasingly alkaline waters that have evolved during the development of the modern Globe-Pro-

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gress mine (Fig. 3A–C; Krause and Ettel, 1989; Harvey et al., 2006; Haffert and Craw, 2010; Paktunc and Bruggeman, 2010). The solubility of scorodite (FeAsO42H2O) for instance, which is considered a common control on dissolved As contents of mine waters, has been experimentally shown to increase over three orders of magnitude with a pH increase from 7 to 8 (Bluteau and Demopoulos, 2007). The mine water treatment process at the modern Globe-Progress mine is less effective for Sb than for As (Milham and Craw, 2009a), and Sb contents of mine drainage waters leaving the mine via Oriental Creek (Fig. 1B) are commonly relatively high (Fig. 6B). Oriental Creek contributes 98% of the modern flux of Sb in Devils Creek at the time of this study (Table 1). The rest of the Sb flux derives from the DSPD discharge point (1%) and from mineralised groundwater. In the budget, the calculated Sb concentrations and fluxes do not match the observed ones. This suggests an additional source of Sb along the Devils Creek flow which has not been accounted for, and contribution from dissolution of Sb from the stream bed is suggested (above; Table 4). The naturally-sourced flux of Sb in these waters (Hewlett et al., 2005) is negligible compared to Sb that was added as a result of the modern mining activity at the time of this study. Hence, while the As flux has been relatively stable, or has at times decreased, with the development of the modern mine, the Sb flux has increased considerably. The more successful As removal by FeCl3 treatment, compared to Sb, has been observed in experimental studies before (Kang et al., 2003). Craw et al. (2004) have described As and Sb adsorption onto hydrated Fe oxide (HFO) occurring to a similar degree for both metalloids at the Hillgrove mine site in Australia. It is probable that the differences in these results are attributable to the differences in the sorption mechanisms involved. The treatment process includes the formation of Fe oxyhydroxides within the As- and Sb-rich waters while the naturally-driven sorption, as described at Hillgrove, may be occurring on already pre-formed HFO. The results obtained by Kang et al. (2003) indicate that at pH 6–8, Sb(V) removal achieved was only between 30% and 20%, while removal in excess of 90% can be expected for As(V) in similar conditions (pH < 7) (Edwards, 1994). Kang et al. (2003) also report that at optimal pH of 5.0, the dose of FeCl3 required to satisfactorily remove Sb(V) from water is at least around nine times the amount required for the removal of As(V). Therefore, it is inferred that the Sb removal in the Globe Progress treatment plant could have been more successful had considerably higher amounts of coagulant been used. 6.5. Metalloid ratios The Sb/As ratio of mine waters was historically <1, and this ratio first rose above 1 in July 2007, the month that the new mine opened. At that time, the Sb/As ratio at the OSPD was 3.4 (Fig. 7B). However, it was not until May 2009 that the ratio began to increase markedly (Fig. 7A–D). Since then the Sb/As ratios have typically ranged from 8.2 to 25.6 for the four sampling locations affected (Fig. 7A–D). Three periods can be distinguished which illustrate how the Sb/As ratio developed in mine waters over time. These periods include the time before the Globe-Progress mine opened (pre-July 2007), a transition period of almost 2 years until May 2009, and the latest period which shows the highest Sb/As ratios in waters (Fig. 11). The Sb/As ratios calculated from the targeted water sampling campaigns ranged from 3 to 18 in February 2010 (Table 3) and from 1.6 to 36.4 in February 2011 (Table 1). In the latter sampling campaign, a groundwater seep into Devils Creek retains the low historic Sb/As ratio of 0.2 (Table 1). The static leaching tests show that the Sb/As ratio of resultant leachates is similar to the Sb/As ratio of the rocks that are involved in the water–rock interaction (Fig. 10). Hence, rocks with Sb/As

Fig. 11. Arsenic and Sb concentrations for three sampling locations: DC01 (triangles), DSPD (circles) and OSPD (squares). Three periods in mine water composition changes are marked by the symbol colours and lines showing approximate cut-off ratios. Concentrations pre July 2007: blue, July 2007–May 2009: red, May 2009–February 2011: black. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

near 1 yield waters that have Sb/As near 1 (Fig. 10). This is apparent on the bulk rock scale, where the mineralised rocks at the modern Globe-Progress mine straddle the Sb/As = 1 line (Fig. 2), and the pit waters that have interacted with these rocks also straddle the Sb/As = 1 line (Fig. 10; Milham and Craw, 2009a). The higher amounts and proportions of Sb in the modern Globe-Progress mine rocks compared to the historic mines (Fig. 2) are clearly partially responsible for the evolution of mine water metalloids to higher Sb/As ratios (Fig. 11). Hence, mine rock runoff, which principally appeared at DSDP, evolved towards higher Sb/As ratios (Fig. 7A). This evolution is a direct consequence of development of the open cut mine into lower grade rocks with higher Sb compared to the underground historic mines that targeted Au-rich quartz veins only. It appears that, to a small degree, As was leached preferentially from rocks, relative to the in situ rock sample concentrations. This is indicated by the results being slightly skewed towards the As side of the graph (Fig. 10). It is also evident that all but one leaching test results fall distinctly below the 1:1 (Sb:As) line, on the As side of the graph. This is contrary to the important observation of Sb/As ratios in the modern mine being greater than 1. The reason for the situation observed in the experiment probably lies both in the initial compositions of the rocks involved in the leaching experiments, and in the known faster dissolution rates of arsenopyrite relative to stibnite (Walker et al., 2006; Asta et al., 2010b; Biver and Shotyk, 2012). Rocks in the leaching experiments were allowed to react for a very short period of time (only 1 week). Pit water chemistry, on the other hand, has developed over a longer time-frame and its results lay on the Sb side of the graph. In addition, these waters are in contact with very fine-grained rocks at the bottom of the actively mined pit which has a further effect on sulfide dissolution rates. Mine water from the water treatment plant leaves the site at OSPD, and this is the other main contributor to evolving Sb/As ratios in the Devils Creek catchment (Fig. 11). These OSPD waters initially developed their metalloid contents by the same chemical water–rock interactions described above, in various parts of the mine system. Like the DSPD waters, these mine waters destined for the treatment plant will have evolved towards higher Sb/As ratios in the growing open pit. However, these waters have also

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undergone FeCl3 treatment that preferentially removes more As than Sb (Milham and Craw, 2009a), further increasing the Sb/As ratio of the treated waters draining from OSPD (Fig. 7B). Hence, the high Sb/As ratios apparent at OSPD are partly a result of the distinct lowering of As contents in 2009, as well as the high dissolved Sb from water–rock interaction (Fig. 6B). Arsenic and Sb enrichment in mine waters is the known environmental consequence of orogenic Au mining. Typically in these settings As levels exceed Sb concentrations, as was the case in the historic Gold Progress Mine waters. The typical Sb/As ratios calculated from historic records were found to be <0.5 which agrees well with the data published in the literature concerning various other Au mining sites. For example, in Bralorne Mine, Canada, the As concentration is 5.5 times that of Sb in portal water (1.7 mg/L As, 0.3 mg/L Sb, Sb/As = 0.18) and it is >9 times higher in mine pool water (3.3 mg/L As, 0.3 mg/L Sb, Sb/As = 0.09) (Desberats et al., 2011). Antimony/As ratios of low-sulfide Au-quartz deposits in Alaska documented by Goldfarb et al. (1995) are usually below 1 (except four samples), with values typically ranging from 0.1 to 0.3. Concentrations of As and Sb in mine waters in Ethiopia, described by Getaneh and Alemayehu (2006), reach up to 0.3 mg/L As and 0.04 mg/L Sb. The Sb/As ratio in this locality is always <1 and ranges from 0.09 to 0.58. Similarly, in the Macraes Mine, located in the lower South Island of New Zealand, As concentrations are mostly considerably higher than those of Sb (Sb/As < 1) in mine discharge waters (Milham and Craw, 2009a). For example, the Sb/As ratio of pit floor water is 0.27 (0.073 mg/L As and 0.02 mg/L Sb) (Milham and Craw, 2009a). In contrast, mine waters from the Beatrice Gold Belt in Zimbabwe are consistently enriched in Sb with respect to As. Arsenic concentrations in shaft waters and receiving streams range from 0.05 to 0.17 mg/L and Sb concentrations are between 1.5 and 2.3 mg/L (Ravengai et al., 2005). The typical Sb/As ratio of these waters is between 10 and 20. Studies of the Macleay River catchment in Australia which is affected by discharge from the orogenic Sb–As–Au (–W) Hillgrove mines have been conducted by Ashley et al. (2003, 2007). In this case Sb concentrations also dominate over As in the analysed mine waters. The concentrations recorded reach 55 mg/L Sb and 7.2 mg/L As (Ashley et al., 2003) and the Sb/ As ratios are typically >1 and range from 0.8 to 67 (Ashley et al., 2007). In comparison, waters from the modern Globe Progress Mine described in this paper exhibit Sb/As ratios up to 36 (Table 1).

7. Conclusions Compositions of mine waters at the newly-opened Globe-Progress gold mine have evolved on a time scale of years to become  enriched in Sb, SO2 4 , Na and Cl , and depleted in Fe compared to historic mine waters in the same area. The mine water pH was circumneutral and has become more alkaline, with pH near 8 and elevated HCO 3 . The As concentrations of the modern mine waters are elevated but variable. The compositional evolution of waters with respect to As and Sb contents, and more particularly the Sb/As ratio, has resulted from a transitional change from waters affected only by historical mining to the progressive introduction of modern mine signatures. Historical mines were As-dominated with only minor Sb, no free stibnite, and the Sb/As ratio of rocks and mine waters was <1. Progressive open pit excavation and exposure of mineralised rocks in the modern mine has made more fresh rock available for the abundant rain to facilitate the oxidative dissolution of metalloids. Large volumes of lower grade cataclastic ore with elevated Sb contents, including stibnite, have been exposed and mined. Similar chemical processes have affected the same rocks during processing through the mine

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system. Processing waters were treated with FeCl3, which preferentially removed more As than Sb, further elevating the Sb/As ratio of resultant mine waters that drained downstream. The Globe-Progress mine provides a dramatic example of how changing mining methods and ore types can influence the metalloid compositions of receiving waters. Once the mine waters flowed into the principal drainage channel, Devils Creek, the As concentrations behaved conservatively and As flux was approximately constant despite downstream dilution. Arsenic flux was similar in historic and modern mine drainage. Minor dissolution of Sb from stream sediments has further raised the Sb/As ratio of the mine drainage. Antimony flux was substantially increased from the modern mine compared to historic mine drainage. However, downstream dilution of Sb has occurred to a similar extent to As. Both As and Sb are diluted to trivial levels in the Inangahua River immediately downstream of the mining area.

Acknowledgements This research was funded by Foundation for Research, Science and Technology and University of Otago. We would like to thank Oceana Gold Ltd. for providing the necessary data and insight into mine site management. Sam Taylor, Dave Hodson and Anicia Henne helped with fieldwork and data acquisition on site. Melanie Rosak collected samples in February 2010 and Duncan Ross provided additional data. Two anonymous reviewers are thanked for their constructive comments.

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