Applied Geochemistry (v.23, #2)
Sulfide oxidation: Insights from experimental, theoretical, stable isotope, and predictive studies in the field and laboratory by Robert R. Seal; Wayne C. Shanks (101-102).
Impurities and heterogeneity in pyrite: Influences on electrical properties and oxidation products by Kaye S. Savage; Daniela Stefan; Stephen W. Lehner (103-120).
Macroscopic pyrite crystals originating from a variety of geologic settings were made into thick sections. Electrical properties were measured with a Hall system, and minor element composition was analyzed with laser ablation inductively coupled plasma mass spectroscopy (LA-ICPMS). Selected thick sections were oxidized in a moist air environment inside a glove chamber. The relative metal content of surface products formed during oxidation was analyzed by LA-ICP-MS. Natural pyrite exhibits a range of electrical properties corresponding to the content of the common minor elements Co, As and Ni. These properties are similar to those of synthetic pyrite doped with single elements. Pyrite enriched in Co is an n-type semiconductor with low resistivity and high carrier mobility, while arsenian pyrite tends to be p-type and have higher resistivity. The effect of Ni is weaker and tends to be obscured by Co and As in samples of mixed composition. Cobalt demonstrates the strongest effect on electrical properties. Enrichment of Co at oxidized pyrite surfaces is inversely correlated with its concentration in the underlying pyrite. Cobalt enrichment in oxidation products is also more pronounced along crystal defects such as fractures, and in crystals with heterogeneous distribution of trace elements. These observations might be explained by differences in the electronic structure of pyrite arising from the presence of impurities, and by the distribution of domains with different impurity compositions, facilitating electron transfer.
Kinetics of inorganic arsenopyrite oxidation in acidic aqueous solutions by M.A. McKibben; B.A. Tallant; J.K. del Angel (121-135).
In an effort to help evaluate the significance of common primary As-bearing minerals in releasing As into surface and ground waters, experiments were performed to determine rate laws for the irreversible inorganic aqueous oxidation of arsenopyrite by dissolved O2, Fe3+ and NO 3 - in low temperature acidic solutions. Batch reactor run conditions varied from pH 2–4.5 and 10–40 °C at ionic strength 0.01 M. A major constraint on defining and measuring the rate of arsenopyrite oxidation is the non-stoichiometry (incongruency) of the reaction in acidic solutions; As and S are not released completely into solution, apparently remaining behind as more slowly-dissolving solids in an Fe-depleted lattice. Therefore the rate of mineral oxidation (destruction) at low pH is best defined and measured by rates of change in total dissolved Fe concentrations, not by changes in dissolved As or S concentrations or Eh. The measured mineral dissolution rate therefore places an upper limit on actual inorganic As-release rates, providing a conservative basis for geochemical modeling that may over-predict, but not underestimate, As concentrations observed in natural settings.The molal specific rate laws for the oxidation of arsenopyrite by dissolved Fe3+ (pH 2) and O2 (pH 2–4.5), respectively, are: R sp,Fe 3 + ( moles mineral m - 2 s - 1 ) = - 10 - 5.00 ( M Fe 3 + ) 1.06 ± 0.11 R sp,O 2 aq ( moles mineral m - 2 s - 1 ) = - 10 - 6.11 ( M O 2 aq ) 0.33 ± 0.18 ( M H + ) 0.27 ± 0.09 The results indicate that Fe3+ oxidizes arsenopyrite at least 10 times faster than dissolved O2. Nitrate does not oxidize arsenopyrite. At low pH arsenopyrite oxidizes 3–4 orders of magnitude faster than (arsenical) pyrite and 4–5 orders of magnitude faster than realgar and orpiment. Therefore in rocks with low ratios of arsenopyrite to other As-bearing sulfides, including coals and ores, arsenopyrite oxidation can still be the dominant source of As-release from sulfide minerals. In natural settings undergoing long-term arsenopyrite oxidation at low pH (in the absence of secondary phases such as scorodite and Fe hydroxides), the rate of As-release may ultimately be controlled by the rate of dissolution of As–S compounds that form on the Fe-depleted arsenopyrite mineral surface. Rate data are therefore needed on native As, realgar and orpiment at low pH, as proxies for these compounds.
Rates of zinc and trace metal release from dissolving sphalerite at pH 2.0–4.0 by Mark R. Stanton; Pamela A. Gemery-Hill; Wayne C. Shanks; Cliff D. Taylor (136-147).
High-Fe and low-Fe sphalerite samples were reacted under controlled pH conditions to determine nonoxidative rates of release of Zn and trace metals from the solid-phase. The release (solubilization) of trace metals from dissolving sphalerite to the aqueous phase can be characterized by a kinetic distribution coefficient, (D tr), which is defined as [(R tr/X(tr)Sph)/(R Zn/X(Zn)Sph)], where R is the trace metal or Zn release rate, and X is the mole fraction of the trace metal or Zn in sphalerite. This coefficient describes the relationship of the sphalerite dissolution rate to the trace metal mole fraction in the solid and its aqueous concentration. The distribution was used to determine some controls on metal release during the dissolution of sphalerite. Departures from the ideal D tr of 1.0 suggest that some trace metals may be released via different pathways or that other processes (e.g., adsorption, solubility of trace minerals such as galena) affect the observed concentration of metals.Nonoxidative sphalerite dissolution (mediated by H+) is characterized by a “fast” stage in the first 24–30 h, followed by a “slow” stage for the remainder of the reaction. Over the pH range 2.0–4.0, and for similar extent of reaction (reaction time), sphalerite composition, and surface area, the rates of release of Zn, Fe, Cd, Cu, Mn and Pb from sphalerite generally increase with lower pH. Zinc and Fe exhibit the fastest rates of release, Mn and Pb have intermediate rates of release, and Cd and Cu show the slowest rates of release. The largest variations in metal release rates occur at pH 2.0. At pH 3.0 and 4.0, release rates show less variation and appear less dependent on the metal abundance in the solid. For the same extent of reaction (100 h), rates of Zn release range from 1.53 × 10−11 to 5.72 × 10−10 mol/m2/s; for Fe, the range is from 4.59 × 10−13 to 1.99 × 10−10 mol/m2/s. Trace metal release rates are generally 1–5 orders of magnitude slower than the Zn or Fe rates. Results indicate that the distributions of Fe and Cd are directly related to the rate of sphalerite dissolution throughout the reaction at pH 3.0 and 4.0 because these two elements substitute readily into sphalerite. These two metals are likely to be more amenable to usage in predictive acid dissolution models because of this behavior. The Pb distribution shows no strong relation to sphalerite dissolution and appears to be controlled by pH-dependent solubility, most likely related to trace amounts of galena. The distribution of Cu is similar to that of Fe but is the most-dependent of all metals on its mole fraction ratio (Zn:Cu) in sphalerite. The Mn distributions suggest an increase in the rate of Mn release relative to sphalerite dissolution occurs in low Mn samples as pH increases. The Mn distribution in high Mn samples is nearly independent of pH and sphalerite dissolution at pH 2.0 but shows a dependence on these two parameters at higher pH (3.0–4.0).
Neutralization of sulfuric acid solutions by calcite dissolution and the application to anoxic limestone drain design by Danielle M.C. Huminicki; J. Donald Rimstidt (148-165).
Batch reactor (BR) experiments were conducted to measure the effect of hydrodynamics and gypsum coatings on calcite neutralization rates. A factorial array of BR experiments measured the H+ concentration change by calcite dissolution over a pH range of 1.5–3.5 and Na2SO4 concentrations of 0–1 M. The rate of H+ concentration change with time was determined by numerical differentiation of H+ concentration versus time. Regression modeling showed that for uncoated calcite, rates are only significantly affected by pH, r = - 10 - 2.32 a H + 0.76 . Whereas, for calcite coated with gypsum only time had a significant effect on calcite dissolution rates, r = −10−1.96 t −0.53.Because transport-limited dissolution rates for uncoated calcite are a function of the pH and Reynolds number, a model was developed to express the effects of these two variables on the rate of H+ consumption for a solution with a Darcy velocity, q, through a porous medium with a particle radius, r p, such that r ′ = 1.08 × 10 - 3 q 0.31 r p - 0.69 m H + 0.87 . This equation was integrated via a numerical model to simulate the performance of an idealized anoxic limestone drain (ALD). This model predicts the pH and alkalinity change along the length of an ALD. The model shows that the efficiency of an ALD is greater when the Darcy velocity is low and the particle radius is small.In addition, the growth of gypsum coatings causes the rate of H+ neutralization to decline as the square root of time as they form and block the H+ transport to the calcite surface. Supersaturation with respect to gypsum, leading to coating formation, can be avoided by diluting the ALD feed solution or by replacing limestone with dolomite.
Dissolved metals and associated constituents in abandoned coal-mine discharges, Pennsylvania, USA. Part 1: Constituent quantities and correlations by Charles A. Cravotta (166-202).
Complete hydrochemical data are rarely reported for coal-mine discharges (CMD). This report summarizes major and trace-element concentrations and loadings for CMD at 140 abandoned mines in the Anthracite and Bituminous Coalfields of Pennsylvania. Clean-sampling and low-level analytical methods were used in 1999 to collect data that could be useful to determine potential environmental effects, remediation strategies, and quantities of valuable constituents. A subset of 10 sites was resampled in 2003 to analyze both the CMD and associated ochreous precipitates; the hydrochemical data were similar in 2003 and 1999. In 1999, the flow at the 140 CMD sites ranged from 0.028 to 2210 L s−1, with a median of 18.4 L s−1. The pH ranged from 2.7 to 7.3; concentrations (range in mg/L) of dissolved (0.45-μm pore-size filter) SO4 (34–2000), Fe (0.046–512), Mn (0.019–74), and Al (0.007–108) varied widely. Predominant metalloid elements were Si (2.7–31.3 mg L−1), B (<1–260 μg L−1), Ge (<0.01–0.57 μg L−1), and As (<0.03–64 μg L−1). The most abundant trace metals, in order of median concentrations (range in μg/L), were Zn (0.6–10,000), Ni (2.6–3200), Co (0.27–3100), Ti (0.65–28), Cu (0.4–190), Cr (<0.5–72), Pb (<0.05–11) and Cd (<0.01–16). Gold was detected at concentrations greater than 0.0005 μg L−1 in 97% of the samples, with a maximum of 0.0175 μg L−1. No samples had detectable concentrations of Hg, Os or Pt, and less than half of the samples had detectable Pd, Ag, Ru, Ta, Nb, Re or Sn. Predominant rare-earth elements, in order of median concentrations (range in μg/L), were Y (0.11–530), Ce (0.01–370), Sc (1.0–36), Nd (0.006–260), La (0.005–140), Gd (0.005–110), Dy (0.002–99) and Sm (<0.005–79). Although dissolved Fe was not correlated with pH, concentrations of Al, Mn, most trace metals, and rare earths were negatively correlated with pH, consistent with solubility or sorption controls. In contrast, As was positively correlated with pH.None of the 140 CMD samples met all US Environmental Protection Agency (USEPA) continuous-concentration criteria for protection of freshwater aquatic organisms; the samples exceeded criteria for Al, Fe, Co, Ni, and/or Zn. Ten percent of the samples exceeded USEPA primary drinking-water standards for As, and 33% exceeded standards for Be. Only one sample met drinking-water standards for inorganic constituents in a public water supply. Except for S, the nonmetal elements (S > C > P = N = Se) were not elevated in the CMD samples compared to average river water or seawater. Compared to seawater, the CMD samples also were poor in halogens (Cl > Br > I > F), alkalies (Na > K > Li > Rb > Cs), most alkaline earths (Ca > Mg > Sr), and most metalloids but were enriched by two to four orders of magnitude with Fe, Al, Mn, Co, Be, Sc, Y and the lanthanide rare-earth elements, and one order of magnitude with Ni and Zn.The ochre samples collected at a subset of 10 sites in 2003 were dominantly goethite with minor ferrihydrite or lepidocrocite. None of the samples for this subset contained schwertmannite or was Al rich, but most contained minor aluminosilicate detritus. Compared to concentrations in global average shale, the ochres were rich in Fe, Ag, As and Au, but were poor in most other metals and rare earths. The ochres were not enriched compared to commercial ore deposits mined for Au or other valuable metals. Although similar to commercial Fe ores in composition, the ochres are dispersed and present in relatively small quantities at most sites. Nevertheless, the ochres could be valuable for use as pigment.
Dissolved metals and associated constituents in abandoned coal-mine discharges, Pennsylvania, USA. Part 2: Geochemical controls on constituent concentrations by Charles A. Cravotta (203-226).
Water-quality data for discharges from 140 abandoned mines in the Anthracite and Bituminous Coalfields of Pennsylvania reveal complex relations among the pH and dissolved solute concentrations that can be explained with geochemical equilibrium models. Observed values of pH ranged from 2.7 to 7.3 in the coal-mine discharges (CMD). Generally, flow rates were smaller and solute concentrations were greater for low-pH CMD samples; pH typically increased with flow rate. Although the frequency distribution of pH was similar for the anthracite and bituminous discharges, the bituminous discharges had smaller median flow rates; greater concentrations of SO4, Fe, Al, As, Cd, Cu, Ni and Sr; comparable concentrations of Mn, Cd, Zn and Se; and smaller concentrations of Ba and Pb than anthracite discharges with the same pH values. The observed relations between the pH and constituent concentrations can be attributed to (1) dilution of acidic water by near-neutral or alkaline ground water; (2) solubility control of Al, Fe, Mn, Ba and Sr by hydroxide, sulfate, and/or carbonate minerals; and (3) aqueous SO4-complexation and surface-complexation (adsorption) reactions. The formation of AlSO 4 + and AlHSO 4 2 + complexes adds to the total dissolved Al concentration at equilibrium with Al(OH)3 and/or Al hydroxysulfate phases and can account for 10–20 times greater concentrations of dissolved Al in SO4-laden bituminous discharges compared to anthracite discharges at pH of 5. Sulfate complexation can also account for 10–30 times greater concentrations of dissolved FeIII concentrations at equilibrium with Fe(OH)3 and/or schwertmannite (Fe8O8(OH)4.5(SO4)1.75) at pH of 3–5. In contrast, lower Ba concentrations in bituminous discharges indicate that elevated SO4 concentrations in these CMD sources could limit Ba concentrations by the precipitation of barite (BaSO4). Coprecipitation of Sr with barite could limit concentrations of this element. However, concentrations of dissolved Pb, Cu, Cd, Zn, and most other trace cations in CMD samples were orders of magnitude less than equilibrium with sulfate, carbonate, and/or hydroxide minerals. Surface complexation (adsorption) by hydrous ferric oxides (HFO) could account for the decreased concentrations of these divalent cations with increased pH. In contrast, increased concentrations of As and, to a lesser extent, Se with increased pH could result from the adsorption of these oxyanions by HFO at low pH and desorption at near-neutral pH. Hence, the solute concentrations in CMD and the purity of associated “ochres” formed in CMD settings are expected to vary with pH and aqueous SO4 concentration, with potential for elevated SO4, As and Se in ochres formed at low pH and elevated Cu, Cd, Pb and Zn in ochres formed at near-neutral pH. Elevated SO4 content of ochres could enhance the adsorption of cations at low pH, but decrease the adsorption of anions such as As. Such information on environmental processes that control element concentrations in aqueous samples and associated precipitates could be useful in the design of systems to reduce dissolved contaminant concentrations and/or to recover potentially valuable constituents in mine effluents.
Sulfide oxidation and distribution of metals near abandoned copper mines in coastal environments, Prince William Sound, Alaska, USA by Randolph A. Koski; LeeAnn Munk; Andrea L. Foster; Wayne C. Shanks; Lisa L. Stillings (227-254).
The oxidation of sulfide-rich rocks, mostly leftover debris from Cu mining in the early 20th century, is contributing to metal contamination of local coastal environments in Prince William Sound, Alaska. Analyses of sulfide, water, sediment, precipitate and biological samples from the Beatson, Ellamar, and Threeman mine sites show that acidic surface waters generated from sulfide weathering are pathways for redistribution of environmentally important elements into and beyond the intertidal zone at each site. Volcanogenic massive sulfide deposits composed of pyrrhotite and (or) pyrite + chalcopyrite + sphalerite with subordinate galena, arsenopyrite, and cobaltite represent potent sources of Cu, Zn, Pb, As, Co, Cd, and Hg. The resistance to oxidation among the major sulfides increases in the order pyrrhotite ≪ sphalerite < chalcopyrite ⩽ pyrite; thus, pyrrhotite-rich rocks are typically more oxidized than those dominated by pyrite. The pervasive alteration of pyrrhotite begins with rim replacement by marcasite followed by replacement of the core by sulfur, Fe sulfate, and Fe–Al sulfate. The oxidation of chalcopyrite and pyrite involves an encroachment by colloform Fe oxyhydroxides at grain margins and along crosscutting cracks that gradually consumes the entire grain. The complete oxidation of sulfide-rich samples results in a porous aggregate of goethite, lepidocrocite and amorphous Fe-oxyhydroxide enclosing hydrothermal and sedimentary silicates. An inverse correlation between pH and metal concentrations is evident in water data from all three sites. Among all waters sampled, pore waters from Ellamar beach gravels have the lowest pH (∼3) and highest concentrations of base metals (to ∼25,000 μg/L), which result from oxidation of abundant sulfide-rich debris in the sediment. High levels of dissolved Hg (to 4100 ng/L) in the pore waters probably result from oxidation of sphalerite-rich rocks. The low-pH and high concentrations of dissolved Fe, Al, and SO4 are conducive to precipitation of interstitial jarosite in the intertidal gravels. Although pore waters from the intertidal zone at the Threeman mine site have circumneutral pH values, small amounts of dissolved Fe2+ in the pore waters are oxidized during mixing with seawater, resulting in precipitation of Fe-oxyhydroxide flocs along the beach–seawater interface. At the Beatson site, surface waters funneled through the underground mine workings and discharged across the waste dumps have near-neutral pH (6.7–7.3) and a relatively small base-metal load; however, these streams probably play a role in the physical transport of metalliferous particulates into intertidal and offshore areas during storm events. Somewhat more acidic fluids, to pH 5.3, occur in stagnant seeps and small streams emerging from the Beatson waste dumps. Amorphous Fe precipitates in stagnant waters at Beatson have high Cu (5.2 wt%) and Zn (2.3 wt%) concentrations that probably reflect adsorption onto the extremely high surface area of colloidal particles. Conversely, crystalline precipitates composed of ferrihydrite and schwertmannite that formed in the active flow of small streams have lower metal contents, which are attributed to their smaller surface area and, therefore, fewer reactive sorption sites. Seeps containing precipitates with high metal contents may contribute contaminants to the marine environment during storm-induced periods of high runoff. Preliminary chemical data for mussels (Mytilus edulis) collected from Beatson, Ellamar, and Threeman indicate that bioaccumulation of base metals is occurring in the marine environment at all three sites.
Temporal variation and the effect of rainfall on metals flux from the historic Beatson mine, Prince William Sound, Alaska, USA by Lisa L. Stillings; Andrea L. Foster; Randolph A. Koski; LeeAnn Munk; Wayne C. Shanks (255-278).
Several abandoned Cu mines are located along the shore of Prince William Sound, AK, where the effect of mining-related discharge upon shoreline ecosystems is unknown. To determine the magnitude of this effect at the former Beatson mine, the largest Cu mine in the region and a Besshi-type massive sulfide ore deposit, trace metal concentration and flux were measured in surface run-off from remnant, mineralized workings and waste. Samples were collected from seepage waters; a remnant glory hole which is now a pit lake; a braided stream draining an area of mineralized rock, underground mine workings, and waste piles; and a background location upstream of the mine workings and mineralized rock. In the background stream pH averaged ∼7.3, specific conductivity (SC) was ∼40 μS/cm, and the aqueous components indicative of sulfide mineral weathering, SO4 and trace metals, were at detection limits or lower. In the braided stream below the mine workings and waste piles, pH usually varied from 6.7 to 7.1, SC varied from 40 to 120 μS/cm, SO4 had maximum concentrations of 32 mg/L, and the trace metals Cu, Ni, Pb, and Zn showed maximum total acid extractable concentrations of 186, 5.9, 6.2 and 343 μg/L, respectively.With an annual rainfall of ∼340 cm (estimated from the 2006 water year) it was expected that rain water would have a large effect on the chemistry of the braided stream draining the mine site. A linear mixing model with two end members, seepage water from mineralized rock and background water, estimated that the braided stream contained 10–35% mine drainage. After rain events the braided stream showed a decrease in pH, SC, Ca + Mg, SO4, and alkalinity, due to dilution. The trace metals Ni and Zn followed this same pattern. Sodium + K and Cl did not vary between the background and braided stream, nor did they vary with rainfall. At approximately 2 and 3 mg/L, respectively, these concentrations are similar to concentrations found in rainfall on the coasts of North America.High concentrations of total acid extractable Al and Fe were found at near-neutral pH in most of the waters collected at the site. Equilibrium solubility simulations, performed with PHREEQC, show that the stream waters are saturated with respect to Al, Fe and SiO2 solid phases. Because the “dissolved” sample fractions (acid preserved and filtered to 0.45 μm) show significant concentrations of Al and Fe it is presumed that these are present as colloids. The relationship between concentrations of Al and Fe, and rainfall was the opposite of that observed for the major ions Ca + Mg, SO4, and alkalinity, in that Al and Fe concentrations increased with increasing rainfall. Concentrations of Cu and Pb followed the same pattern. Adsorption calculations were performed with Visual MINTEQ, using the diffuse double layer electrostatic model and surface complexation constants for the ferrihydrite surface. These results suggest that 30–93% of Cu and 58–97% of Pb was adsorbed to ferrihydrite precipitates in the stream waters. Ni and Zn showed little adsorption in this pH range.Flux calculations show that the total mass of trace metals transported from the mine site, during the 60 day study period, was ranked as Zn (196 kg) > Cu (87 kg) > Pb(1.9 kg) ≈ Ni(1.9 kg). Nickel and Zn were transported mostly as dissolved species while Cu and Pb were transported mostly as adsorbed species. pH control on adsorption was evident when Cu and Pb isotherms were normalized by ferrihydrite flux. Decreased stream water pH due to periods of frequent and high volume rain events would cause desorption of Cu and Pb from the ferrihydrite surface, thus changing not only their speciation in solution but also their mechanism of transport.
Relationships between microbial communities and environmental parameters at sites impacted by mining of volcanogenic massive sulfide deposits, Prince William Sound, Alaska by A.L. Foster; L. Munk; R.A. Koski; W.C. Shanks; L.L. Stillings (279-307).
The relations among geochemical parameters and sediment microbial communities were examined at three shoreline sites in the Prince William Sound, Alaska, which display varying degrees of impact by acid-rock drainage (ARD) associated with historic mining of volcanogenic massive sulfide deposits. Microbial communities were examined using total fatty acid methyl esters (FAMEs), a class of compounds derived from lipids produced by eukaryotes and prokaryotes (bacteria and Archaea); standard extraction techniques detect FAMEs from both living (viable) and dead (non-viable) biomass, but do not detect Archaeal FAMEs. Biomass and diversity (as estimated by FAMEs) varied strongly as a function of position in the tidal zone, not by study site; subtidal muds, Fe oxyhydroxide undergoing biogenic reductive dissolution, and peat-rich intertidal sediment had the highest values. These estimates were lowest in acid-generating, intertidal zone sediment; if valid, the estimates suggest that only one or two bacterial species predominate in these communities, and/or that Archeal species are important members of the microbial community in this sediment.All samples were dominated by bacterial FAMEs (median value >90%). Samples with the highest absolute abundance of eukaryotic FAMEs were biogenic Fe oxyhydroxides from shallow freshwater pools (fungi) and subtidal muds (diatoms). Eukaryotic FAMEs were practically absent from low-pH, sulfide-rich intertidal zone sediments. The relative abundance of general microbial functional groups such as aerobes/anaerobes and gram(+)/gram(−) was not estimated due to severe inconsistency among the results obtained using several metrics reported in the literature.Principal component analyses (PCAs) were performed to investigate the relationship among samples as separate functions of water, sediment, and FAMEs data. PCAs based on water chemistry and FAMEs data resulted in similar relations among samples, whereas the PCA based on sediment chemistry produced a very different sample arrangement. Specifically, the sediment parameter PCA grouped samples with high bulk trace metal concentration regardless of whether the metals were incorporated into secondary precipitates or primary sulfides. The water chemistry PCA and FAMEs PCA appear to be less prone to this type of artifact. Signature lipids in sulfide-rich sediments could indicate the presence of acid-tolerant and/or acidophilic members of the genus Thiobacillus or they could indicate the presence of SO4-reducing bacteria. The microbial community documented in subtidal and offshore sediments is rich in SRB and/or facultative anaerobes of the Cytophaga–Flavobacterium group; both could reasonably be expected in PWS coastal environments. The results of this study provide evidence for substantial feedback between local (meter to centimeter-scale) geochemical variations, and sediment microbial community composition, and show that microbial community signatures in the intertidal zone are significantly altered at sites where ARD drainage is present relative to sites where it is not, even if the sediment geochemistry indicates net accumulation of ARD-generated trace metals in the intertidal zone.
Isotopic and textural discrimination between hypogene, ancient supergene, and modern sulfates at the Questa mine, New Mexico by Andrew R. Campbell; Virgil W. Lueth (308-319).
Chemical and mineralogical changes due to pyrite weathering are of interest with respect to understanding long-term physical stability of mine rock piles at the Questa mine, New Mexico. The ability to discriminate between ancient and modern processes is important for establishing the extent of modern weathering within the piles. Initial inventories of sulfur minerals and representative isotope compositions in rocks from orebodies, the hydrothermal alteration zones associated with orebodies, hydrothermal alteration scars, and mine rock piles were determined. Ore body sulfides have δ 34SCDT of 0 ± 4‰, typical for sulfides formed by magmatic processes in stockwork Mo systems. Pyrite from alteration scars has a wide range of δ 34S values from 0.0‰ to −13.6‰. Sulfate from the ore body has markedly positive δ 34S (5–10‰) accompanied by positive δ 18 O SO 4 values (6–15‰) reflecting equilibrium formation from magmatic fluids. Sulfates from alteration scars have δ 34S values over a broad range, similar to alteration scar pyrites, from −10.6‰ to 0‰ and δ 18 O SO 4 of 0 ± 3‰. Sulfates with fine grained, delicate, and euhedral mineral habits suggesting recent formation within the mine rock piles, have δ 34S values similar to orebody pyrite and alteration scars but more negative δ 18 O SO 4 values (−3‰ to −10‰). Sulfates from all three sources occur in these piles, and their stable isotope values have proven useful in differentiating them and their environments of formation (i.e., hypogene, ancient supergene, and recent weathering). Correlating the isotopic compositions with textures allows petrographic assessment for the origins of sulfate minerals in the rock piles, but this must be applied with caution because some sulfate mineral recycling has occurred.
Environmental geochemistry of a Kuroko-type massive sulfide deposit at the abandoned Valzinco mine, Virginia, USA by Robert R. Seal; Jane M. Hammarstrom; Adam N. Johnson; Nadine M. Piatak; Gregory A. Wandless (320-342).
The abandoned Valzinco mine, which worked a steeply dipping Kuroko-type massive sulfide deposit in the Virginia Au–pyrite belt, contributed significant metal-laden acid-mine drainage to the Knight’s Branch watershed. The host rocks were dominated by metamorphosed felsic volcanic rocks, which offered limited acid-neutralizing potential. The ores were dominated by pyrite, sphalerite, galena, and chalcopyrite, which represented significant acid-generating potential. Acid–base accounting and leaching studies of flotation tailings – the dominant mine waste at the site – indicated that they were acid generating and therefore, should have liberated significant quantities of metals to solution. Field studies of mine drainage from the site confirmed that mine drainage and the impacted stream waters had pH values from 1.1 to 6.4 and exceeded aquatic ecosystem toxicity limits for Fe, Al, Cd, Cu, Pb and Zn. Stable isotope studies of water, dissolved SO 4 2 - , and primary and secondary sulfate and sulfide minerals indicated that two distinct sulfide oxidation pathways were operative at the site: one dominated by Fe(III) as the oxidant, and another by molecular O2 as the oxidant. Reaction-path modeling suggested that geochemical interactions between tailings and waters approached a steady state within about a year. Both leaching studies and geochemical reaction-path modeling provided reasonable predictions of the mine-drainage chemistry.