Article

Geochemical signatures of discharge waters, Macraes mine flotation tailings, east Otago, New Zealand

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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.

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... 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. ...
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Article
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Article
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Article
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... 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). ...
Article
Full-text available
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.
... 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). ...
Article
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.
... 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 ). ...
Article
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 …
... 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. ...
Article
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.
Article
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.
Chapter
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.
Article
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Article
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Article
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.
Article
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.
Article
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.
Article
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.
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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.
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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.
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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.
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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%.
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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.
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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.
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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.
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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.
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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