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Abstract
The stream catchment containing the tailings dam complex at the Macraes mine, east Otago, New Zealand has three chemically distinct water types. ( 1) Natural groundwater has pH beween 6 and 8, and sulphate levels locally elevated to 30 ppm. The total carbonate/calcium (Ca) ratio is c. 2, consistent with carbonic acid dissolution of basement schist calcite. Sodium (Na) is strongly elevated over chloride, and these levels are dominated by water‐rock interaction. (2) Decant pond waters lie on two tailings dams, one of which was inactive between 1993 and 1998. Their pH is strongly alkaline (8–9). Sulphate, Na, chloride, and Ca contents are higher than groundwater and these levels have risen steadily over time. Carbonate content is lower than groundwater. Arsenic (As) and iron (Fe) contents are high and variable; arsenic in the active tailings pile is commonly 5–15 ppm. (3) Mixtures of tailings water and natural groundwater (c. 1:1) form from seepage from the upper part of the main tailings pile only and mix beneath the main tailings dam to emerge down stream of the dam. The mixture pH (6.3) is slightly lower than that of the groundwater (mainly >7) whereas the sulphate content is high (>1500 ppm) because of the water component of the tailings. There is no resolvable time lag on the 1‐month scale between discharge of tailings water into the dam and emergence at the foot of the dam. Two distinct water types are identifiable in this setting. Chimney drain discharge is collected in pipes and discharged from those pipes. The tailings component of this water is chemically little different from that which left the tailings. Subsurface flow travels unconfined beneath the tailings dam and interacts chemically with the dam aggregate. Attenuation of Na and chloride, and addition of Ca, carbonate, and magnesium (Mg) occurs. Nearly all dissolved As and copper (Cu) was extracted from the subsurface flow during passage through the dam, whereas some As and all Cu is extracted from the chimney drain discharge. A small volume of a mixture of tailings water and groundwater has continued down stream of the tailings dam complex for c. 50 m in the basement schist as a contaminant plume which took c. 2 years to travel that distance in fractures within the schist. This contaminated water is chemically similar to that of the subsurface flow, and has high sulphate content but no detectable As or Cu.
... We focus on the main mine tailings impoundment (Figure 1), and provide comparisons between geochemical processes in the tailings and in unmineralised waste rocks. The geochemical processes in this mine tailings impoundment were initially examined in the early stages of mine development (first 8 years ;Craw 2000;Craw & Nelson 2000), and over a subsequent change in processing procedures (Craw 2003). This study expands on these earlier studies to provide a geochemical account of 23 years of active life of the main tailings impoundment, as a new tailings impoundment ( Figure 1) was commissioned in 2013. ...
... The mine site has had extensive analysis of groundwater and downstream surface waters since 1990, and a more intensive and regular water quality-monitoring programme since 1993. From 1993, water samples were collected and analysed on a 1-3 month schedule from over 80 points through the processing and tailings systems, from silt ponds downstream from various facilities, from groundwater bores around the site, and from surface waters in surrounding streams (Craw 2000(Craw , 2003Craw & Nelson 2000). The resultant time-series data sets, which have a total of 15000 analyses, form the basis of this study. ...
... Mineral processing between 1990 and 1999 involved separation of pyrite and arsenopyrite by flotation from crushed ore, followed by fine grinding and cyanidation at high pH (> 10). The sulphide concentrate was variably treated with sodium sulphite and hydrochloric acid during the cyanidation stage (Craw & Nelson 2000). The sulphide-rich cyanidation tailings were initially stored in a separate impoundment, but were subsequently combined with the rest of the tailings in what became the mixed tailings impoundment (Figure 1; Craw & Nelson 2000). ...
The main tailings impoundment at the Macraes mine was active for 23 years until it was decommissioned in 2013. Water compositions in the tailings complex were closely monitored for 20 years, providing a time series of compositional changes during the interaction between mine waters, sulphidic ore (pyrite and arsenopyrite) and greenschist facies schist host rocks. Most waters had circumneutral pH because of abundant (2–10%) calcite in host rocks. Relict Fe²⁺ caused a temporary pH decrease before re-neutralisation. Chlorite, albite and muscovite partially altered to kaolinite, smectites and illite. Dissolved sulphate reached as high as 8000 mg/L, but this was partially attenuated by the precipitation of gypsum in the tailings complex, while the alkalinity (as dissolved
) rose by three orders of magnitude in parallel with the increase in pH. Dissolved As decreased by six orders of magnitude, to < 0.01 mg/L, between the processing plant and the groundwater system below the tailings complex. Dissolved sulphate plumes emanate from tailings and waste rock piles, but are diluted downstream.
... As a result of the sparse population and strong winds, atmospheric sulfur emissions are low in areas outside of major cities and volcanic regions. In contrast to Europe, the eastern United States and China (Jådrysek, 2000;Jenkins, 2005;Mayer et al., 1995;Xiao and Liu, 2002), rainwater in southern New Zealand is dominated by marine aerosols (Craw and Beckett, 2004;Craw and Nelson, 2000;Jacobson et al., 2003;Litchfield et al., 2002). Aerosols are transported from the Tasman Sea by the Prevailing Westerly winds, with the occasional influence of the Southerlies from the Pacific east coast. ...
... The lake is impounded by schist bedrock in a depression between two tors, (Craw and Beckett, 2004). Rainfall is lower than at the coast,~500 mm/year (Allen et al., 1997;Bayly, 1967;Craw and Nelson, 2000), due to a rain shadow effect from the Southern Alps. The barren landscape is subjected to strong westerly winds, and the lake surface can lose up to 700 mm/year of moisture via evaporation (Allen et al., 1997;Bayly, 1967;Craw and Nelson, 2000). ...
... Rainfall is lower than at the coast,~500 mm/year (Allen et al., 1997;Bayly, 1967;Craw and Nelson, 2000), due to a rain shadow effect from the Southern Alps. The barren landscape is subjected to strong westerly winds, and the lake surface can lose up to 700 mm/year of moisture via evaporation (Allen et al., 1997;Bayly, 1967;Craw and Nelson, 2000). The lake dries up in the summer, where a halite crust with trace gypsum develops across the dry surface, and then refills in the following winter, which is when we sampled (June 2015). ...
Sulfide minerals commonly occur in sediments and basement rocks in southern New Zealand, as authigenic precipitates from groundwater below the oxygenated surface zone. There are two principal potential sources for sulfur in the groundwater system: weathering of sulfide minerals in the metamorphic basement and rainwater-derived marine aerosols. We present data for these two key sulfur sources: metamorphic sulfide and associated hydrothermal Au-bearing veins within the Otago Schist (average δ34S = −1.8 ± 2.4‰), and an inland saline lake (S derived entirely from rainwater, δ34S = 21.4 ± 0.8‰). We use these two end member δ34S values to estimate the contributions of these sources of sulfur in authigenic groundwater sulfide minerals and in waters derived from oxidation of these sulfide minerals, across a range of environments. We show that authigenic groundwater pyrite along joints in the Otago schist is derived primarily from metamorphic basement sulfur. In contrast, authigenic groundwater pyrite cementing Miocene-Recent aquifers shows a substantial marine aerosol component, and represents a distinct hydrogeological system. We suggest that marine aerosols represent a significant flux to the terrestrial sulfur cycle that has been present through the groundwater system in Otago over the past 20 million years.
... This waste rock is typically piled in the immediate vicinity, as permanent repositories that are revegetated, or as temporary stockpiles intended for replacement in exhausted excavations. Unmineralised waste rock has negligible environmental significance, and their runoff typically maintains high pH, with elevated levels of dissolved major ions derived from dissolution of silicate minerals and carbonates (Craw and Nelson 2000). Likewise, runoff from unmineralised rocks in excavations ( Fig. 3a) evolves to high pH as a result of the same water-rock reactions. ...
... Dissolved sulfate is remarkably persistent through mine water systems, and discharge waters commonly retain a strong dissolved sulfate signature (Fig. 4a). Likewise, water that has passed through waste rock that contains sulfide minerals also develops a strong sulfate-rich signature (Fig. 5a), reflecting its interaction with sulfides, carbonates, and silicates (Craw and Nelson 2000), and is distinctly different in composition from the rainwater, which is dominated by marine aerosols with a seawater composition (Fig. 5a). The sulfate concentrations of discharging waste rock waters decrease as the waste rock stacks are rehabilitated with caps and vegetation, thereby limiting water ingress, and the exposed sulfide grains become armoured with secondary minerals (Fig. 5a). ...
... The tailings are generated as a slurry of water and fine-grained (typically \100 lm) crushed rock. Tailings are generally stored in large impoundments behind dams constructed of waste rock, with internal drainage systems and low-permeability liners (Craw and Nelson 2000). The solid component of the tailings slurry settles into the impoundment, a lake (decant pond) forms on the surface of the impoundment, and water variably percolates through the impoundment (Craw and Nelson 2000;Craw 2003). ...
Orogenic deposits are an important source of gold around the world, with associated environmental impacts. New Zealand has a broad spectrum of these orogenic deposits, providing an ideal setting to develop a general conceptual model that can be used to predict potential environmental issues associated with their exploration and mining. This model provides a practical and quantitative framework for permitting and managing mine operations, with a focus on downstream water quality. The model has been quantified using data collected from natural mineralised occurrences, two active mines, and numerous historic mine sites. Mine waters in and around orogenic deposits almost invariably have a pH of 7–8. Minor localised acidification occurs in excavations and waste rock, but it is readily neutralised by the abundant calcite in the host rock. The ore can have strongly elevated levels of As and Sb; the proportions of these metalloids are controlled by geological factors, especially the crustal level of emplacement and the structure of the mineralised rocks. Agitation of sulfide mineral slurries during processing and pressure oxidation in the processing system can lead to dissolved metalloid concentrations of tens to hundreds of mg/L in mine tailings waters. The gold also commonly contains Hg, up to 40 wt%, and discharge of this Hg to the atmosphere during processing is possible, but Hg is not significantly mobilised from other mine rocks. High metalloid concentrations are the most significant environmental issue, but are decreased by adsorption to iron oxyhydroxide as the water percolates through mine rocks and tailings. Even so, additional treatment may be needed to lower metalloid concentrations for discharge to rivers and lakes.
... Detection limits varied for the many different sources, but were between 0.001 and 0.05 mg/L, significantly lower than the As concentrations that are of direct interest in this study. Major ion analyses for east Otago and Reefton waters have been published previously by Craw & Nelson (2000) and Hewlett et al. (2005), and additional major ion data have been compiled in the DAME database. The pH of mine wastes was determined in situ on associated waters, where present, using a field pH meter calibrated in the field with buffer solutions. ...
... The grain size of the pharmacosiderite is micrometre scale. The shallow groundwater that pervades the schist basement and precipitates pharmacosiderite in mine adits has low dissolved potassium (1Á14 mg/L; Fig. 3), and low dissolved sulfate (B10 mg/L) (Craw & Nelson 2000). The groundwater obtains its dissolved constituents primarily from dissolution of schist minerals, including muscovite (K), albite (Na), and pyrite (S) (Craw & Nelson 2000). ...
... The shallow groundwater that pervades the schist basement and precipitates pharmacosiderite in mine adits has low dissolved potassium (1Á14 mg/L; Fig. 3), and low dissolved sulfate (B10 mg/L) (Craw & Nelson 2000). The groundwater obtains its dissolved constituents primarily from dissolution of schist minerals, including muscovite (K), albite (Na), and pyrite (S) (Craw & Nelson 2000). ...
Recent and historic gold mines and mine wastes in the South Island of New Zealand have been exposed under a wide range of climatic conditions for up to 100 yr. The studied sites are in rain shadow areas east of the Southern Alps, and in wet areas west of the mountains. Arsenic concentrations in mine wastes are up to 20 wt% locally. The arsenic is primarily derived from arsenopyrite in gold-bearing ore, although roasting and oxidation of ore occurred at some mines. Arsenolite was a byproduct of roasting at two sites, and this was also discharged into mine wastes. Five secondary As minerals, scorodite, kankite, pharmacosiderite, zykaite, and bukovskyite, have formed in different mines and mine wastes during surficial oxidation and remobilisation. The minerals formed under different chemical and climatic conditions. Ferrihydrite with combined arsenate (up to 20 wt% As) occurs at all localities. Scorodite is the most common secondary As mineral, replacing arsenopyrite and/or arsenolite, and forming cement in mine wastes. Dissolution and precipitation of scorodite, which is strongly pH dependent, controls maximum dissolved As concentrations on a regional scale. Kankite has formed instead of scorodite in a persistently moist microclimate east of the mountains. Pharmacosiderite formed instead of scorodite on the 50 yr time-scale in mine tunnels at pH 7–8. Pharmacosiderite is distinctly less soluble than scorodite in this groundwater setting at pH 7–8, and apparently restricts dissolved As to
... Macraes mine excavations have exposed fresh rock, and water that has been in contact with fresh and oxidising rock can be captured in groundwater drillholes and pit lakes that accumulate groundwater discharges in mineralised rocks. We have compiled relevant drillhole and pit lake water chemistry data from the mine area as well as background groundwater data (Fig. 1A-D), collected by the mining company OceanaGold Ltd for site environmental purposes (Craw 2000(Craw , 2003Craw & Nelson 2000;. ...
... The Na/Cl ratio of sulphide-oxidising waters is different from background groundwaters because of distinctly elevated dissolved Na (Fig. 12C). This difference arises because of dissolution of albite during sulphide oxidation and associated water-rock interaction, whereas background groundwaters retain a marine Na/Cl ratio inherited from aerosols in rain water (Craw & Nelson 2000). Likewise, dissolved Mg and K are strongly elevated in sulphide oxidation waters compared to background groundwaters (Fig. 12D). ...
Gold has been chemically mobilised by groundwater from host sulphide minerals in orogenic gold deposits of Otago. Mobilisation occurred near the Cenozoic Otago Schist erosional surface beneath a sedimentary cover. Initial Au mobilisation, on a scale of micrometres, occurred when solid solution and microparticulate gold in pyrite and arsenopyrite grains were liberated by sulphide oxidation to iron oxyhydroxide pseudomorphs. Larger-scale mobilisation involved leaching of gold from up to 100-m-thick zones which were the target of historic mining, with up to 10× enrichment of Au in reprecipitation zones. Gold in the supergene zones is commonly crystalline with octahedral shapes and nuggety forms which fill cavities and coat prismatic quartz crystals. This gold retains some or all of the Ag (typically 2-8 wt%) from the primary source gold. Oxidised groundwaters that have interacted with sulphides become enriched in dissolved sulphate, but retain high pH (7-8.5). Under these conditions, metastable thiosulphate ions can dissolve and transport Au and Ag to be precipitated later by either oxidation or reduction.
... The first type precipitated on and near the mineralized quartz veins over the last century since the mines were abandoned . This pharmacosiderite was observed to precipitate as thin encrustations from shallow groundwater (1-14 mg K/L; <10 mg SO 4 /L) pervading the host schists (Craw and Nelson 2000). The second type formed by crystallization of As-HFO for more than 50 years in mine waters with pH 7-8, ∼23 mg As/L, <10 mg K/L, and molar Fe/As ratio >1 ). ...
... The first type precipitated on and near the mineralized quartz veins over the last century since the mines were abandoned . This pharmacosiderite was observed to precipitate as thin encrustations from shallow groundwater (1-14 mg K L −1 ; <10 mg SO 4 L −1 ) pervading the host schists (Craw and Nelson 2000). The second type formed by crystallization of As-HFO for more than 50 years in mine waters with pH 7-8, ∼23 mg As L −1 , <10 mg K L −1 , and molar Fe/As ratio >1 ). ...
The labyrinthine world of arsenic minerals has piqued the curiosity of many researchers in mineralogy, geochemistry, chemistry, and environmental sciences. Arsenic was known to the ancient civilizations; there are written Greek, Roman, and Chinese reports about minerals and substances of this element (Emsley 2001). The discovery of elemental arsenic is attributed to Albertus Magnus (1193–1280) who prepared it by reduction of As2O3. The common public association of arsenic and poison is the heritage of a long history of eliminating unwanted and unloved ones with compounds of this element. Mary Ann Cotton (1832–1873) was charged with murder of her mother, three husbands, a lover, eight of her own children, and seven stepchildren, all of them with an arsenic-based de-worming compound (Emsley 2005). Kořinek (1675) gave a vivid and frightening account on how a natural ferric sulfo-arsenate (bukovskýite) was used to poison the German armies of Albrecht Habsburg who invaded Bohemia in 1304. An arsenic derivative called lewisite (2-chlorovinyl-dichloroarsine) was used in the World War I (Emsley 2001). On the other hand, brightly-colored arsenic compounds were used in all imaginable products well into the 20th century. Arsenic whetted the appetite of many children as green arsenical chemicals were used as cake decorations and coatings of sugar sweets (Emsley 2005). The death of Napoleon Bonaparte has been regarded for a long time as a consequence of ingested or inhaled arsenical compouds (e.g., Aldersey-Williams 2011), however there are alternative interpretations (Lugli et al. 2011). Accidental mass arsenic poisoning occurred in Manchester in 1900 when many men drank beer contaminated with arsenic. The arsenic was tracked back to pyrite which was used to produce sulfuric acid which was employed in the manufacture of glucose for this batch of beer (Emsley 2005). Despite its toxicity, arsenic finds a few uses in …
... 500 m a.s.l) drained by steep-sided streams, including Deepdell Creek that drains to the Shag River (Fig. 1). The area has a cool temperate to semi-arid climate, with c. 600 mm/yr precipitation and 700 mm/yr evapotranspiration (Craw & Nelson 2000). Mean annual temperature is c. 12°C, with warm summers (up to 30°C) and cold winters (down to -10°C). ...
Small quantities (<1 m3) of arsenic-rich ore processing residues were piled outside the Golden Point battery, east Otago, by historic gold miners immediately before mining ceased. The residues originally contained concentrates (c. 10-fold) of gold-bearing pyrite and arsenopyrite, with abundant scheelite, and contain up to 10 wt% arsenic and 20 wt% iron. These concentrates were roasted to oxidise the sulfide minerals and release their contained gold. The gold was extracted by mercury amalgamation, and the residues are now enriched in mercury (up to 0.1 wt%) compared to original ore. Oxidation of the material has resulted in formation of crystalline scorodite (FeAsO4.2H2O) and bukovskyite (Fe2[AsO4][SO4][OH].7H2O), and amorphous iron oxyhydroxide. These three oxidised minerals have been locally (millimetre scale) dissolved and reprecipitated in the surficial environment, causing cementation of some residues. Minor relict pyrite remains in the residues, and oxidation of this maintains acid conditions (pH 2.2-3) in the residues. The scorodite and bukovskyite have been remarkably chemically stable in this acid oxidised environment over the >60 yr since the residues were abandoned. As long as the acidification continues, scorodite will remain stable.
... Percolating rainwater can facilitate oxidation and dissolution of As from the mine wastes and mine excavations and the dissolved As can be discharged into the environment with potentially toxic consequences for the downstream biota (Foy et al., 1978; Gebel, 1997; Loebenstein, 1993; Smedley and Kinniburgh, 2002; Turner, 1993). It is a worldwide occurrence that these elevated As concentrations decrease downstream via at least one of the three attenuation processes: (1) precipitation of secondary As minerals, such as scorodite (Ashley and Lottermoser, 1999; Borba et al., 2003; Craw and Nelson, 2000; Deutsch, 1997; Garcia-Sanchez and Alvarez-Ayuso, 2003; Krause and Ettel, 1988; Smedley and Kinniburgh, 2002; Vink, 1996; Williams, 2001), (2) chemisorption onto other solid phases, such as iron oxyhydroxides (HFO) (Belzile and Tessier, 1990; Foster et al., 1998; Hem, 1977; Jacobs et al., 1970; Majzlan et al., 2004; Pierce and Moore, 1982; Webster et al., 1994; Wilkie and Hering, 1996), (3) dilution with background waters. The environmental significance of dissolved As emanating from mines depends on the amount and spatial scale of these attenuation processes. ...
Mine and processing sites in the mesothermal gold deposits of the Reefton gold field, New Zealand, generate extremely high dissolved As concentrations (up to 59 mg/L). Attenuation of these waters takes place by at least one of the three mechanisms: (1) precipitation of the secondary arsenic mineral scorodite, (2) chemisorption onto iron oxyhydroxide (HFO) and (3) dilution with regional catchment water. The presence and effectiveness of these mechanisms vary among the three studied catchments. A strong physiochemical control on arsenic attenuation was identified due to a chemical gradient within the gold field itself and processing methods, which can generate site specific arsenic minerals, such as arsenolite. Precipitation of scorodite only occurs in the presence of dissolving arsenolite, which is a roasting by-product present at two of the studied sites. Abundant HFO is generated in the pyritic mesothermal part of the gold field, and here chemisorption onto HFO is the dominant attenuation process. In the non-pyritic part of the gold field, HFO is mainly produced as a result of ankerite dissolution but only where sufficiently exposed mineralised rock is present. In the absence of significant adsorption sites, dissolved As is attenuated only via less effective dilution and ecosystem guidelines are exceeded over kilometres downstream from the mineralised zone until drainage waters are diluted by regional catchment water. Catchment morphology was identified as a major control on dilution. Despite the presence of strong As point sources upstream, mine-related As contributes <10% to the regional As river load in all three catchments. On a regional scale As mobility across a wide range of pH regimes reveals a strong control of scorodite, which has already been observed locally.
Supergene alteration of gold deposits has had important geochemical effects that changed the mineralogy and textures of near-surface exposures and can affect the economics of gold mines by enhancing gold liberation and increasing gold grades. This study links observed mineralogy with 655 water analyses from environments relevant to supergene processes in the Otago Schist goldfield of southern New Zealand, and these analyses are used as proxies for past supergene processes. There are two contrasting supergene alteration styles in otherwise identical host rocks and mineralisation zones in this area. Long-term groundwater alteration of sediments and underlying basement near to a regional unconformity developed progressively since the Cretaceous. This alteration has affected orogenic gold deposits exposed at the unconformity, and has resulted in supergene enrichment of gold with localised formation of centimetre scale gold nuggets. Alteration processes occurred under circumneutral pH conditions (pH 6–8.5), and was accompanied by extensive oxidation and clay alteration of basement rocks and sediments. Gold was mobilised as thiosulphate and bisulphide complexes and was reprecipitated by redox changes near the unconformity. The chemical processes of supergene enrichment were primarily abiotic, with minor bacterial facilitation of late stage gold mobility at the micron scale. In contrast, supergene alteration of eroding freshly-exposed mountain outcrops has occurred in the late Cenozoic without Au enrichment apart from liberation and minor particle size enhancement. The ambient pH of the associated waters was also circumneutral, but localised and temporary acidification (to pH ∼3) has occured on the metre scale or less in sulphidic outcrops. This supergene alteration was dominated by oxidation but some low redox minerals, senarmontite and secondary stibnite, have formed during the early stages of oxidation or subsequent reduction. Sulphide decomposition, with associated localised acidification and gold remobilisation was probably enhanced by bacterial processes in these exposed outcrops.
Large areas (square kilometre scale) of mine tailings have been deposited from placer gold mines in Central Otago, and are being deposited at Macraes orogenic gold mine in east Otago. Establishment of vegetation on these tailings involves at least some provision of plant nutrients from the rock. Phosphorus is the principal limiting nutrient, as the c. 1000 mg/kg P in accessory apatite, most abundant in micaceous schist, is only sparingly bioavailable on timescales of weeks to months. Nitrogen is an important limiting nutrient but schist, especially micaceous schist, typically contains 500–1000 mg/kg N, and this nitrogen is readily leachable with water on timescales of weeks to months. Arsenic uptake from tailings by pasture species is significant (< 90 mg/kg dry weight), but elevated As in tailings substrates (c. 1500 mg/kg) does not adversely affect plant health. Capping of tailings with variably oxidised schist is the most effective way of facilitating revegetation, and some addition of phosphatic fertiliser is desirable but other nutrients, including nitrogen, are adequately bioavailable in a schist cap and underlying tailings.
Ore mined at Macraes is processed to produce a sulphide mineral rich concentrate, which was originally finely ground and then cyanided to extract the gold. Refractory ore, mainly sheared micaceous schist, yielded recoveries below 50 % of the gold. Ore consisting of graphitic sheared schist is preg-robbing, and gold recovery was also low. Introduction of pressure oxidation processing before cyanidation has increased gold recovery. Pressure oxidation residues include ferric iron oxyhydroxide, jarosite, and iron arsenate, and associated waters have very high dissolved As. Dissolved As concentrations decrease by 6 orders of magnitude through the tailings impoundment system, via adsorption to iron oxyhydroxides and groundwater dilution.
Gold nuggets (centimetre scale) have formed in a supergene alteration zone on hydrothermal gold deposits, and occur intergrown with quartz and iron oxyhydroxide pseudomorphs after sulphide minerals, and along fractures in quartz and host rocks. The supergene alteration was driven by groundwater-driven water-rock interaction near to a regional unconformity beneath fluvial sediments, and involved clay alteration and oxidation that extended up to 50 m below the unconformity. Oxidation of pyrite and arsenopyrite produced temporary thiosulphate ligands that mobilised microparticulate gold encapsulated in the sulphide minerals. The nuggets have some crystalline form, and internally they consist of anhedral grains, elongated gold plates, and intimate intergrowths of gold and iron oxyhydroxide. Nugget surfaces have further micron scale overgrowths of microparticulate gold, gold plates, and gold crystals. Nuggets were eroded and recycled into nearby proximal Miocene quartz pebble conglomerates, where they concentrated in placers near the basal unconformity. Later recycling transferred gold into Pleistocene fluvial channels. Gold dissolution and redeposition as plates and crystals occurred on the exterior surfaces of placer gold particles, with little change in mass. All groundwater maintained high pH throughout the geological history because there was sufficient calcite in the basement rocks to neutralise any acid generated by pyrite oxidation. Hence, gold mobility in sediments was driven by thiosulphate complexes as for the in situ nuggets, albeit with lower dissolved sulphur concentrations. Despite aridification of the climate in the late Cenozoic, with resulting localised high dissolved chloride concentrations, chloride complexation did not contribute to gold mobility.
Processing waters contain up to 10 mg l−1 dissolved As at the Macraes mine, New Zealand, and this is all removed by adsorption as the water percolates through a large earth dam. Laboratory experiments were set up to identify which mineral is the most effective substrate for this adsorption of As. The experiments were conducted using infrared (IR) spectroscopy of thin mineral films adhering to a ZnSe prism. Silicates, including kaolinite, adsorbed only small amounts of As which was readily washed off. Hydrated Fe oxides (HFO) were extremely effective at adsorbing As, particularly the natural amorphous HFO currently being deposited from dam discharge waters at the Macraes mine. An adsorption isotherm determined for this natural material has the adsorption constant, Kads=(1.9±0.4)×104 M−1, and the substrate becomes saturated with adsorbed As when solution concentrations exceed about 50 mg l−1. Saturation is not being reached at the Macraes mine. Arsenic adsorbed on to natural HFO has a distinctive IR spectrum with the absorption peak varying from 800 cm−1 (alkaline solutions) to 820 cm−1 (neutral to acid solutions). Much of this adsorbed As is strongly bound and difficult to wash off. Arsenate ions adsorb in a bidentate structure which may be a precursor for scorodite crystallisation.
The Sutton Salt Lake is the only saline lake in New Zealand, and has formed in a windy cool‐temperate maritime climate. Consequently, the lake is distinctly different from most of the world's saline lakes that form in arid continental settings. Sutton Salt Lake forms annually in a shallow (5 m) bedrock‐floored depression c. 50 km from the nearest coast. The site receives c. 500 mm/year rainfall compared with coastal rainfall of near 1000 mm/year because of a minor rain‐shadow effect of coastal hills. Surface evaporation rate is high (c. 700 mm/ year) because of frequent strong winds. Sediments on the lake floor are derived by rain and wind erosion of the surrounding quartzofeldspathic schist bedrock, with a contribution from organic sources, particularly ostracods, and evaporative halite. The sediments have a higher proportion of phyllosilicates (muscovite, kaolinite, and chlorite) than the source rocks because of differential transport of these minerals into the lake depression. Lake water is entirely derived from rain, rather than groundwater, and the lake waters have had minimal chemical interaction with bedrock. Lake water pH is near 9 and pH of pore waters in drying lake sediments is near 8, compared with a pH near 7 for regional surface and ground waters. When full, the lake has salinity about one quarter to one third of that of sea‐water, and ion ratios are similar to sea‐water. The lake salinity is derived from marine aerosols in rainwater concentrated by c. 20 000 evaporation and refilling cycles in the lake depression.
Antimony, a toxic metalloid similar to arsenic, is present at variable levels in most gold-bearing rocks. Antimony is soluble in the surface environment, so antimony (Sb) mobilization in mine waters is an environmental issue around gold mines. The Reefton gold mine was originally developed in gold-bearing quartz veins; Sb concentrations were low (1,000 mg/kg), and the mine waters had low dissolved Sb (2S3). Processing of this ore has resulted in higher dissolved Sb in mine waters (0.1–1 mg/L), even after water treatment that removes most dissolved As (to 0.01 mg/L) by adsorption to suspended iron oxyhydroxide. Competition between As and Sb for adsorption sites on iron oxyhydroxide particles may have resulted in partial exclusion of the more weakly adsorbed Sb. The high rainfall (2,000 mm/year) at Reefton ensures adequate dilution of mine waters after discharge. The Macraes gold mine has no stibnite, and most Sb is in solid solution in the abundant arsenopyrite (Sb up to 2,000 mg/kg). Pit waters have both Sb and As dissolved up to 0.1 mg/L, partly because of evaporative concentration in a low-rainfall environment. Macraes tailings waters have high As (up to 3 mg/L) but negligible Sb (Tailings from this process have up to 3 wt% Sb, dispersed through As-rich iron oxyhydroxides that are formed in the autoclave. The Sb-rich tailings are strongly diluted (approximately 100:1) by the Macraes tailings, and adsorption of Sb to iron oxyhydroxides in the tailings piles ensures that there has been no increase in the Sb content of the tailings water since the Reefton concentrate has been added at Macraes.
The Macraes gold mine, in southern New Zealand, was developed in a low grade mesothermal deposit. A pressure–oxidation plant was commissioned in 1999 to oxidize pyrite and arsenopyrite in the ore and release gold from within the sulphide grains. Pressure–oxidation discharges include anhydrite, jarosite, and amorphous Fe arsenates with molar Fe/As of 2–7 and up to 0.5 mol% S. Discharge waters contain dissolved Fe(II). There were profound chemical changes within the mine tailings system over the transition to pressure–oxidation. Dissolved sulphate of tailing waters increased to ca. 4000 mg/l, and the waters rapidly became saturated with respect to gypsum. Gypsum was precipitated in many parts of the water pathways at the plant and an estimated 1–4 tonnes of gypsum/day was precipitated in the tailings of the dam structure. The pH of tailings lowered from >8 to 6 with pressure–oxidation, but then evolved towards 8 again. The observed pH was a result of two opposing chemical processes: acidification due to oxidation of dissolved Fe(II) and dissolution of calcite. Calcite forms ca. 5 wt.% of tailings and is sufficiently abundant to dominate the chemistry of tailings. Dissolved arsenic in waters of tailings decreased from >10 to <<1 ppm after pressure–oxidation began, because the amorphous Fe arsenates are less soluble than arsenopyrite. Adsorption of dissolved As onto Fe(III) oxyhydroxides is effective in the tailings system, and strongly limits As discharge. Any As discharge will move through the tailings of the dam system at <30 mm/year.
A minesoil has developed over 5 years oxidative exposure on sulphide concentrate tailings (ca. 1 wt.% As) at the Macraes mesothermal gold mine, New Zealand. The minesoil has a dry crust which has formed due to evaporative drying. This dry crust is enriched in arsenic (ca. 5 wt.% As) as scorodite (FeAsO4·2H2O) because of upward mobility of dissolved arsenic during drying. Similar enrichment of arsenic has occurred along the walls of desiccation cracks which extend over 1 m into the minesoil. Capping of the tailings and minesoil with wet tailings (pH=8) results in dissolution of scorodite and remobilization of arsenic on the millimetre scale. Experimental capping of the minesoil with wet calcium carbonate remobilized some arsenic from scorodite on the centimetre scale, but much original arsenic enrichment was preserved after 400 days. A layer of gypsum (CaSO4·2H2O) and iron oxyhydroxide cementation developed at the interface between the minesoil and the experimental calcium carbonate cap, restricting water flow. This layer was ca. 1 mm thick after 400 days. Theoretical comparison between advection and diffusion in the minesoil suggests that diffusion is an important mechanism for chemical mobility on the 1–50-year time scale. However, advection can be important in secondary porosity of the dry crust of the minesoil and water penetrates this zone at a rate of 1.5 mm/day.
The Taieri Basin, in east Otago, New Zealand, is a tectonic depression bounded in part by active faults. The basin is floored and surrounded by Otago Schist that is locally overlain by Cretaceous-Tertiary sediments and volcanic rocks. Basin fill is predominantly Quaternary gravels, sands, and silts derived from the Otago Schist, and these sediments are at least 150 m thick in places. Aquifers are hosted in the Quaternary sediments, principally gravels and sands. A wedge of estuarine silts and sands (c. 8000-5000 yr BP) extends for 18 km to the downstream outlet of the basin, where it is 25 m thick. Irregularly interfingered fluvial gravels and sands, interbedded with silts, make up the principal aquifers at the upstream end of the basin. This complex fluvial sequence dips beneath the estuarine wedge. The estuarine wedge acts as a confining layer for groundwater in the underlying aquifers, and artesian water bores occur in the lower reaches of the basin. Alluvial fans on the western side of the basin are conduits for recharge from adjacent schist ranges. Alluvial fans on the southeastern side of the basin have their toes truncated by active faults, and recharge from these fans is structurally inhibited. Ca, Mg, and bicarbonate contents of Taieri Basin surface waters and ground waters indicate chemical interaction with calcite and chlorite of Otago Schist, in basement and/or aquifer sediment clasts. All waters have Na and Cl- contents dominated by rainout of NaCl in marine aerosols. Surface waters have lower Na and Cl- than groundwaters. Numerical modelling of groundwater flow at the upstream end of the basin can reproduce field-based inferences of flow patterns and together indicate southwestward flow beneath the estuarine wedge confining layer. This is significant because it implies low-concentration NO3+NO2 contamination of shallow aquifers at the upstream end of the basin will be carried to groundwaters in the southwest.
The concentration of arsenic has been determined in natural iron oxyhydroxides and oxide minerals extracted from soils at the Ashanti mine, Ghana, and was found to vary from 2 to 35,600 mg/kg. The highest concentration of arsenic in these phases occurred in neutral-pH oxidized clay-rich soils, up to 35,600 mg/kg, and in the oxidized surface portion of mine tailings. Where highly acidic soils occurred or reducing conditions were prevalent, lower concentrations of arsenic were measured in the iron oxyhydroxide and oxide fraction (up to 433 mg/kg). Lower concentrations of arsenic in these phases were also recorded from minerals in the organic-rich soils. The proportion of arsenic associated with amorphous iron oxyhydroxides was much greater than that associated with crystalline iron oxyhydroxide and oxide minerals within the studied sample set.
The kinetic behaviour of pyrite oxidation in carbonate-buffered solution was investigated in the laboratory. Oxygen concentration, surface area and temperature were varied while pH values were limited to the range of 6.7–8.5. The rate experiments were performed on crushed and sieved size-fractions of pyrite that were carefully cleaned and mixed with similar-size silica sand. Oxidation occurred in a moisture-suction device that maintained partially-water-saturated conditions. Dilute NaHCO3 solution and a CO2-O2-N2 gas mixture were passed continuously through the pyritic sand. The reaction rates were monitored by sulphate mass balance in the effluent solutions.The initial rate of oxidation was found to be a linear function of surface area. The rate-dependence on oxygen concentration is non-linear and the data fit a heterogeneous kinetic model in which the surface decomposition reaction, not sorption of oxygen, is the rate-determining step. This decomposition model explains the range of linear to non-linear models reported in the literature when different values of the adsorption constant are applied. The temperature dependence follows Arrhenius behaviour with an equivalent activation energy of about 88 kJ mole−1 in the temperature range of 3 to 25°C, showing that diffusion was not rate limiting. The oxidation rates of five pyrite specimens obtained from various locales exhibited maximum differences of only about 25%.
Previous studies of pyrite oxidation kinetics have concentrated primarily on the reaction at low pH, where Fe(III) has been assumed to be the dominant oxidant. Studies at circumneutral pH, necessitated by effective pH buffering in some pyrite oxidation systems, have often implicitly assumed that the dominant oxidant must be dissolved oxygen (DO), owing to the diminished solubility of Fe(III). In fact, Fe(III)(aq) is an effective pyrite oxidant at circumneutral pH, but the reaction cannot be sustained in the absence of DO. The purpose of this experimental study was to ascertain the relative roles of Fe(III) and DO in pyrite oxidation at circumneutral pH.
The Macraes gold-tungsten deposit occurs in a low-angle thrust system in biotite grade Otago Schist. Native gold, scheelite, pyrite and arsenopyrite are found in and adjacent to quartz veins and silicified schist of lenticular reef zones, where the thrust system cuts through graphitic pelitic schist. Mineralization is confined to a shear zone, up to 80 m thick, which is closely sub-parallel to the regional schistosity. Chemical alteration is dominated by silicification, with some addition of Cr and depletion of Sr and Ba. Alteration extends only about 5 m from major veins. Oxygen becomes isotopically heavier away from veins due to temperature decrease as hot fluids penetrated into cooler (250C?) rock. Graphite within the shear zone rocks has reflectance of 6–7% (in oil), similar to graphite in medium-high grade Otago Schist, and is presumed to be metamorphic in origin. This graphite has acted as a reducing agent to cause precipitation of gold where the thrust system, acting as a conduit for metamorphic fluids, intersects the graphitic schist. The metals were derived from the underlying schist pile which may include an over-thrust oceanic assemblage containing metal-enriched horizons.
Fine grained (ca. 15 μm), arsenopyrite-bearing mine tailings have been exposed to drying and oxidation for 4 a pending relocation. The tailings are still partly covered by a pond of decanted pore waters. The water table in drying tailings has lowered by 1–3 m and desiccation cracks up to 2 cm wide have formed on the 1 m scale, extending through the unsaturated zone. Tailings in the unsaturated zone have similar pore water contents to saturated tailings: typically 16–32 wt% water. Saturated tailings retain alkaline pH (ca. 10) from the mine cyanidation plant, but pH lowers progressively towards ca. 7 near the surface, or near desiccation cracks, in the unsaturated zone. The redox state of the tailings changes in parallel with pH, with an empirical relationship: Eh(mV)=−55 pH+290. Water in the remnant decant pond reflects this relationship also. Unsaturated tailings have variable but low permeabilities, typically 10−3 to 10−4 m/day, and more permeable horizons have allowed incursion of oxygenated air and/or rain water from desiccation cracks. Sulphide grains in all tailings examined are unaltered. Sulphides and solutions in the tailings are out of thermodynamic equilibrium predicted from the redox–pH conditions, due to kinetic constraints. Incursion of rain water locally facilitates deposition from pore waters of insoluble Fe oxide and arsenate minerals, thus fixing As in the dry unsaturated tailings.
Cu, Ag and Cr concentrations in natural water may be lowered by mild chemical reduction involving ferric hydroxide-ferrous ion redox processes. V and Mo solubilities may be controlled by precipitation of ferrous vanadate or molybdate. Concentrations as low as 10−8.00 or 10−9.00 M are readily attainable for all these metals in oxygen-depleted systems that are relatively rich in Fe. Deposition of manganese oxides such as Mn3O4 can be catalyzed in oxygenated water by coupling to ferrous-ferric redox reactions. Once formed, these oxides may disproportionate, giving Mn4+ oxides. This reaction produces strongly oxidizing conditions at manganese oxide surfaces. The solubility of As is significantly influenced by ferric iron only at low pH. Spinel structures such as chromite or ferrites of Cu, Ni, and Zn, are very stable and if locally developed on ferric hydroxide surfaces could bring about solubilities much below 10−9.00 M for divalent metals near neutral pH. Solubilities calculated from thermodynamic data are shown graphically and compared with observed concentrations in some natural systems.
A regional unconformity cut into the Otago Schist belt in southern New Zealand is overlain by auriferous terrestrial sediments. Formation waters from the overlying sediments have reacted with the schist basement, resulting in a thick (up to 20 m) zone of intensely kaolinitized basement immediately below the unconformity. Primary rock textures are preserved throughout this alteration zone. Two distinct types of alteration zone have developed. Non-oxidizing alteration resulted in kaolinitization of muscovite, but some albite and chlorite was preserved even in the most altered rocks. Chlorite locally altered to ferrous-iron-bearing smectite—vermiculite during non-oxidizing alteration, and this unusual mineral occurs in basement and overlying sediments. Oxidizing alteration has resulted in degradation of almost all schist minerals, leaving kaolinite and goethite, with relict muscovite and quartz. A distinctive 12 Å interlayered clay mineral occurs in altered schist and overlying sediment at one locality. Oxidizing alteration is responsible for localized gold mobility in basement gold deposits. Non-oxidizing fluids have mobilized detrital gold in auriferous terrestrial sediments. The formational waters were mobilized during late Cenozoic regional deformation of the unconformity and overlying sediments.
A baseline survey of water quality and trace metals in the upper Manuherikia and Idaburn rivers
- W W Ahlers
- K A Hunter
Chemical evolution of surface and ground waters, Macraes mine area, east Otago
- D Craw
- M Nelson