Pyrite In Rocks Research Articles

Controls on the Stratiform Copper Mineralization in the Western Syncline, Upper Peninsula, Michigan

The Western Syncline hosts reduced-facies, or Kupferschiefer-type, sedimentary rock-hosted stratiform Cu deposits (SSC) in the lowermost meters of the Nonesuch Formation, which is part of a thick section of clastic sedimentary rocks that comprise the upper fill of the Mesoproterozoic Midcontinent Rift of North America. Located in the Porcupine Mountains Cu district in Upper Peninsula, Michigan, these blind deposits were discovered in 1956, but are not yet developed, although recent renewed interest may result in near-term production. The deposits are distinguished by their relatively undeformed nature and lack of superposed hydrothermal events. Prior to lithification, chalcocite mineralization replaced diagenetic pyrite within two discrete tabular, albeit discontinuous, potential orebodies referred to as the lower Cu-bearing sequence (LCBS) and the upper Cu-bearing sequence (UCBS). The Top Cu Zone transgresses lithologic boundaries, suggesting that a limited volume of Cu-bearing fluids moved vertically upwards through the unlithified stratigraphy, since reductant pyritic rocks above this zone are essentially barren of Cu. The total Cu inventory that has a reasonable expectation of economic extraction is 3678 M lbs. of Cu with 15.3 M oz. of byproduct Ag. When a cutoff grade of 0.9% Cu over a minimum thickness of 2 m is applied to justify an underground room-and-pillar mine, the LCBS and UCBS are not continuous over the Western Syncline. Sedimentology is the first-order control of potential ore and its continuity; dark-gray shales and siltstones deposited under low-energy, anoxic conditions are preferred host rocks, whose thickness must be >2 m to be potential ore since host-rock thickness determines economic viability of extraction. Furthermore, stratigraphy influences the time constraints on mineralization as the lithification process impedes vertical permeability and thus the flow of Cu-bearing fluids upward through the unlithified section. Syn-sedimentary tectonic movements, likely along pre-existing buried faults, are a third-order control as the thickness of host rocks is enhanced under such conditions. Therefore, an understanding of the depositional and tectonic history throughout the Western Syncline is fundamental to understanding the limits of possible economic exploitation and to optimizing ore extraction.

Geological controls on evolution of evaporative precipitates on soil-free substrates and ecosystems, southern New Zealand

Soil-free bare substrates have formed by natural and human-induced processes in a semi-arid rain shadow, in the lee of actively rising mountains. These substrates have developed evaporative salt encrustations with a wide range of minerals controlled by local substrate permeability and mineralogy. Many of these small (hectare scale) sites also host endemic salt-tolerant ecosystems that are currently endangered by weed incursion. This study characterises the differing mineralogy and geochemistry of these rare ecosystem-hosting substrates. Impermeable substrates, especially clay-rich schist basement, are dominated by NaCl from marine aerosols in rain. The high Na at these sites further enhances impermeability of substrates by promoting surficial clay mobility, so that even Pleistocene-Recent sediments derived from schist basement develop evaporative crusts. The Na/Cl ratio of some of these substrates, especially sediments, has been increased by alteration of albite. However, mineralogically similar greywacke-derived sediments do not develop saline encrustations. Penetration of rainwater into substrates facilitates water-rock interaction reactions that yield entirely different evaporative salt mineralogy, as the NaCl component is overshadowed by constituents dissolved from the rocks. Schist basement and limestone-rich substrates develop carbonate-dominated evaporites, especially calcite, and associated waters are typically supersaturated with respect to carbonate minerals. Some sulphate-rich evaporites form where rock pyrite has been oxidised. Hydrothermally altered schist basement is locally enriched in Mg-bearing carbonates, sulphides and gold. Bare substrates on these sites are variably permeable and develop evaporative salts with carbonates, sulphates, ferric oxyhydroxide (some arsenic-bearing), and minor brucite, after extensive water-rock interaction. Most of the sites are alkaline, and pH locally exceeds 10, as a result of the combinations of evaporative processes and water-rock interactions. The specialist plants have evolved to tolerate the relatively high salinities and pH of these chemically distinctive sites.

Study on the Difference of Microwave Irradiation Effect Caused by the Particle Size Distribution of Rock Minerals

In the process of microwave-assisted rock breaking, there are many factors that affect the rock breaking effect, among which the particle size distribution of minerals has an important influence. In this paper, the samples composed of common minerals calcite and pyrite in rocks were used as the research object, and six kinds of mineral particle size ratio schemes were set up. By using COMSOL multi-physical field coupling analysis platform, a binary medium analysis model was established to study the distribution characteristics and evolution law of electromagnetic field, temperature field, stress field and plastic zone of the model after microwave irradiation. The results showed that the mineral particle size ratio had no significant effect on the size and distribution of electromagnetic field, while the central temperature of the sample increased with the increase of particle size ratio; with the extention of irradiation time, the larger the particle size ratio, the greater the first principal stress of the model, and the tensile zone appeared in pyrite; The larger the particle size ratio, the earlier the initiation time of plastic zone, the larger the area of plastic zone, and the more obvious the difference of morphological characteristics of plastic zone. When the size of minerals in the rock was uniform, the weakening effect of microwave irradiation on the rock was better.

Geochemical Assessment of the Ecological State of the Territory of a Cu–Ore Deposit by Thermodynamic Modeling of the Water–Rock–Gas System

Thermodynamic modeling with the HCh software complex was applied to analyze the probability of acid drainage formation during the evolution of a copper???ore deposit in low-temperature conditions (5??C) with various water levels. During the modeling, three main factors were taken into account: the percentage of pyrite in rocks, the availability of oxygen in the air, and the water content of the rocks (the relative rate of water exchange). It was found that the increase in the content of pyrite sulfur in the rock and an increase in the ratio of the rock???water mass (decrease in water exchange) lead to an increase in mineralization and a decrease in the pH of the drainage solution. The openness of the system to atmospheric gases (in particular, to O2) increases the acidification effect. The most environmentally unfavorable rocks at the deposit are mudstones, siltstones, and silt-sandstones.

Pyritization of Rocks in Black Shale/Host Rock Transition Zones: Evidence from the Bazhenov Formation, Western Siberia

A complex of lithological-geochemical studies was carried out in rocks of the Upper Jurassic–Lower Cretaceous Bazhenov Formation and their transition zones to host rocks: composition of rocks, ratio of organic carbon and sulfide sulfur (C/S values), and distribution of several redox indicators (degree of pyritization, content of the authigenic uranium, and values of Mn/Al and Mo/Mn ratios). Morphology of pyrite was examined with a scanning electron microscope. Two main types of pyrite were revealed: (a) framboidal pyrite formed during early diagenesis with the participation of bacterial activity; (b) cryptocrystalline pyrite formed during diagenesis (early stage included) and, presumably, mesocatagenesis, i.e., medium substage of catagenesis. It was established that some part of the cryptocrystalline pyrite predated the framboidal variety. Pyritization of rocks with the cryptocrystalline pyrite formation took place at the lower and upper boundaries of the Bazhenov Formation in the over- and underlying rocks, where the low-carbon pyritic rocks are formed. Intense pyritization is also observed in the high-carbon rocks near the top of the Bazhenov Formation, where the pyrite–kerogen rocks are formed. Both low-carbon pyritic and pyrite–kerogen rocks are characterized by C/S 2. The low-carbon pyritic and pyrite–kerogen rocks occur near boundaries of the lithologically different members (mainly biogenic rocks in the Bazhenov Formation and terrigenous varieties in the host rocks) that were deposited under different redox conditions existing in the water column and near the marine paleobasin floor. Boundaries of such members could serve as geochemical redox barriers at late stages of lithogenesis. The pyritic and pyrite–kerogen rocks were likely pyritized during diagenesis because of the deposition of pyrite on the geochemical redox barriers from fluids that contained iron sulfides and migrated from the high-carbon Bazhenov sequence. Presumably, pyritization continued during the catagenesis owing to thermogeochemical processes of the organic matter transformation.

Hydrochemistry of tropical springs using multivariate statistical analysis in Ithikkara and Kallada river basins, Kerala, India

Spring waters are the most important natural resource used for drinking by many people around the world, especially in arid and semi-arid areas. This resource cannot be optimally used and sustained, unless the quality of groundwater is assessed. The main objective of this study is to understand the quality and hydrochemistry of spring water sources of two important river basins of Southern Western Ghats—the Ithikkara and the Kallada river basins draining Kollam district of Southern Kerala as an example. Water Quality Index (WQI) values are used to assess the drinking suitability of spring water samples. Multivariate statistical approach (factor analysis and principal component analysis) allow driving hidden information from the data set about their possible influences of the environment on water quality. Multivariate analysis was performed on matrix of hydro-geochemical data. To study the influences of the environment on water quality, multivariate statistical approach is applied. A total of 57 samples were collected and analyzed for major cations and anions for pre-monsoon, monsoon and post-monsoon seasons of 2011–2012 periods from lowland and highland regions of the study area. The results of the WQI values show that majority of the samples fall in Excellent to Good category rendering these water samples suitable for drinking purposes. From the statistical analysis, it is inferred that in catchments rich with pyrite in rocks and clays, drainage of sulphuric acid may significantly contribute to acidity. The negative loading in pH and DO is due to anaerobic conditions in the spring source from the loading of high-dissolved organic matter, which results in the formation of ammonia and organic acids leading to a decrease in pH. The positive loading of SiO2 reflects the dissolution of silica minerals and high temperature. The evaluation of the saturation index of the water samples in highland regions indicted that water was oversaturated with respect to iron minerals and undersaturated with fluorite minerals. The chemistry of the spring waters favors kaolinization and subsequent leaching of mobile elements like alkali and alkaline earth elements. The study is helpful in understanding the relationship between individual ions and for the classification of groundwater types.

Agronomic Suitability of Biologically Produced PARP as a Source of Phosphorus for Maize Production

Phosphorus (P) and Nitrogen (N) deficiencies are recognized as the major constraint of agricultural productivity in developing countries including Zambia. While N deficits can be restored at least in part through the application of crop residues and manure, the restoration of soil P can only be achieved by use of phosphate fertilizers which are unaffordable by the small scale farmers. The aim of the study was to assess the availability of P for crop use from rock phosphate (RP), partially acidulated by acid produced by sulfur oxidizing bacteria. The RP was obtained from Keren Mining Limited at Sinda district, Zambia; the Pyrite rock (iron sulfur) and bacteria culture were obtained Nampundwe mine dump. The pyrite and PR were ground and mixed with the bacterial culture to produce the Partially Acidulated Rock Phosphate (PARP). A pot experiment was set up in a CRB design in a greenhouse on four soil types with four treatments of: soil alone (control), soil with GRP, soil with PARP and soil with super single phosphate (SSP) replicated four times. The results showed that the plant height, biomass yield and P uptake across the different P sources showed significant differences (p 90% effective within and across the four soils indicating that the PARP is reactive and suitable for direct application for crop production.

ОЦЕНКА ВЗАИМОСВЯЗИ САМОВОЗГОРАНИЯ ПОРОД С ДЕФОРМАЦИОННЫМИ ПРОЦЕССАМИ ПРИ КОМБИНИРОВАННОЙ РАЗРАБОТКЕ МЕСТОРОЖДЕНИЙ КОЛЧЕДАННЫХ РУД

For decades and centuries, spontaneous combustion of minerals occurs, which leads to significant material, economic and environmental damage. Under anthropogenic impact on the subsoil, conditions are created for the active development and opening of cracks as a result of deformation of the rock mass, under the influence of its open and underground parttime work. as a result, the emergence of conditions for the active penetration of atmospheric oxygen from the mine ventilation system and the surface atmosphere along the cracks formed as a result of deformation to sulfur-containing minerals of ores and rocks, prone to oxidation and spontaneous combustion. The mechanism of spontaneous combustion of pyritic rocks in the rock mass during the development of deposits is analyzed. It is proved that in the combined development of pyrite ore deposits, exogenous physicochemical processes must be considered in conjunction with geomechanical processes due to the development of mining operations beyond the open pit contour, aerogasdynamic determined by the method, scheme and ventilation system of an underground mine, as well as thermo-physical processes occurring as a result of exothermic oxidation of sulfides...

Mechanisms of hydroxyl radicals production from pyrite oxidation by hydrogen peroxide: Surface versus aqueous reactions

Pyrite oxidation by hydrogen peroxide (H2O2) occurs in both natural and engineered systems. Hydroxyl radical (OH) is a key reactive intermediate for pyrite and coexisting substances oxidation. In acidic H2O2/pyrite systems, H2O2 decomposition by aqueous Fe2+ is documented to predominate for OH production, whereas here we show that H2O2 decomposition by surface Fe(II) sites contributes considerably to OH production under certain conditions. Pyrite oxidation by H2O2 under anoxic conditions was performed under different conditions (2–12 g/L pyrite, 0.025–1 mM H2O2 and pH 2–4), and OH and aqueous Fe2+/Fe3+ production as well as H2O2 consumption were measured during the oxidation. In order to evaluate the contribution of surface reaction to OH production, 1 mM 2, 2′-bipyridine (BPY) was added to inhibit H2O2 decomposition by aqueous Fe2+. The rate constants of OH production decreased by 44.4–65.6% with addition of 1 mM BPY, which suggests that both surface and aqueous reactions contributed to OH production. Regarding the surface reaction, density functional theory (DFT) calculation reveals that H2O2 was adsorbed onto the Fe(II) sites on pyrite surface and transformed to surface adsorbed OH which desorbed subsequently into the aqueous solution. On the basis of mechanistic understanding, a kinetic model was developed to assess the relative contributions of surface and aqueous reactions to OH production. The relative contribution of surface reaction is dependent on the ratio of pyrite surface concentration to aqueous Fe2+ concentration, which decreases with the progress of pyrite oxidation due to the increase in aqueous Fe2+. When the ratio is higher than the threshold value of 1.6 × 103 m2/mM, surface reaction becomes predominant for OH production. Typical systems necessitating consideration of surface reaction involve pyritic rocks and shale leaching and pollutants treatment by H2O2/pyrite. The mechanisms unraveled in this study supplement the fundamental of OH production from pyrite oxidation by both H2O2 and O2 in natural and engineered systems.

Hydrochemical and stable isotope indicators of pyrite oxidation in carbonate-rich environment; the Hamersley Basin, Western Australia

Sulphur (S) is a commonly occurring element in most aquifers, primarily in oxidised (sulphates) and reduced (sulphides) forms. Sulphides often constitute a risk to groundwater quality due to acid rock drainage, especially in catchments that are subject to mining excavations or groundwater injection. However, in semi-arid regions detection of the acid rock drainage risk can be challenging and traditional methods based on observations of increasing SO4 concentrations or SO4/Cl ratios in surface and groundwater, are not necessarily applicable. In addition, decreasing pH, usually accompanying pyrite oxidation, can be masked by the high pH-neutralisation capacity of carbonate and silicate minerals.Analysis of 73 surface and groundwater samples from different water bodies and aquifers located in the Hamersley Basin, Western Australia found that most of the samples are characterised by neutral pH but there was also a large spatial variability in the dissolved sulphate (SO4) concentrations that ranged from 1mg/L to 15,000mg/L. Not surprisingly, groundwater in aquifers that contained pyrite had high sulphate concentrations (>1000mg/L). This was associated with low δ34SSO4 values (+1.2‰ to +4.6‰) and was consistent with the values obtained from aquifer matrix pyritic rock samples (−1.9‰ to +4.4‰). It was also found that the SO4 concentrations and acidity levels were not only dependent on δ34SSO4 values and existence of pyrite but also on the presence of carbonate minerals in the aquifer matrix. The groundwater in aquifers containing both pyrite and carbonate minerals had a neutral pH and was also saturated with respect to gypsum and had high magnesium concentrations of up to 2200mg/L suggesting de-dolomitisation as the process buffering the acidity generated by pyrite oxidation.Based on the findings from this study, a classification scheme has been developed for identification of the acid rock drainage contribution to groundwater that encompasses a myriad of geochemical processes that occur in aqueous systems. The classification uses five proxies (SO4, SO4/Cl, SI of calcite, δ34SSO4 and δ18OSO4) to improve assessment of the oxidation of sulphide potential contribution to overall sulphate ion concentrations regardless of acidity levels of the aqueous system.

English

The Fisher site is a backfilled and reclaimed (in 1984) surface coal mine in western Pennsylvania, USA. A post-closure toe seep at the site discharged acid rock drainage generated in pyritic rock zones that were identified using geophysical techniques. In 1995, sodium hydroxide and bactericide solutions were injected through cased boreholes into the pyritic zones in a two-step process: sodium hydroxide followed by bactericide. Prior to the event, the toe seepage had been treated with the addition of sodium hydroxide followed by a series of settling ponds and wetland zones. Post- injection, the seepage exhibited net-alkaline chemistry and the sodium hydroxide amendment was discontinued. Based on the prevailing wisdom at the time, the effects of the injection event were expected to be temporary. Two decades later, the beneficial effects of the two-step injection event appear to persist and bond release for the site is pending. The seep chemistry has been monitored for over 25 years and the data trends suggest that the steady-state condition of net alkalinity in the seep water entering the ponds and wetland may be permanent. One current view is that the initial suppression of Acidithiobacillus ferrooxidans bacterial community with the sodium hydroxide and bactericide has been maintained by the seasonal infusion of anti-bactericidal organic acids derived from the robust vegetative cover. The situation appears to be self- sustaining. Others may view the data skeptically; that is, the cause and effect of bactericide application and the sustained benefits of the vegetative cover are not proven conclusively. Certainly, the observations suggest that additional focused study is warranted.

Geochemical changes during neutralisation of acid mine drainage in a dynamic mountain stream, New Zealand

Abstract The Mangatini Stream drains a coal mining area in the mountains of northwestern South Island of New Zealand. Abundant rainfall on pyritic rocks yields acid mine drainage (AMD) to the stream, which flows through a steep gorge at discharges that rapidly increase from 100 m3/s during frequent rain events. The AMD is treated with finely ground limestone, which is discharged as a slurry at a point in the middle of the gorge. The limestone slurry mixes and reacts with the AMD during flow ∼4 km downstream over ∼12 h. Neutralisation reactions increase stream pH from near 3 (untreated Mangatini Stream water impacted by AMD) to 5–6 in the first 250 m downstream, although mixing is commonly incomplete in this zone. Large stream discharge volumes in rain events dilute the neutralising material input, thus driving the pH back towards 4 downstream of treatment. More complete neutralisation is achieved 4 km downstream, even in major rain events, and pH can rise to >7. Partial neutralisation is sufficient to remove most of the dissolved Fe(III) (typically ∼30 mg/L) from the Mangatini Stream in the first 10 m, and remaining dissolved Fe is essentially all Fe(II), which decreases over time as it oxidises and precipitates. Dissolved Al in the Mangatini Stream (typically ∼50 mg/L) decreases steadily downstream over ∼100 m in the limestone mixing zone. Precipitated Fe and Al form amorphous oxyhydroxides that are transported as suspended solids and deposited on the stream bed with excess limestone in zones of low flow velocity. Dissolved Zn is removed from solution by adsorption to Fe oxyhydroxide when pH reaches ∼5, but dissolved Ni remains in solution despite the neutralisation process. Gypsum precipitation occurs throughout the limestone mixing zone, resulting in at least 30% decrease in dissolved SO 4 2 - . Minor ettringite forms in the first 100 m, but then probably redissolves. The limestone dosing system is an effective method of neutralising the effects of AMD and removing most dissolved metals in a steep mountain stream with frequent rain events where this dynamic environment places many constraints on treatment options.

Release of Aluminum from Clays in an Acid Rock Drainage Environment

Aluminum concentrations in low pH acid rock drainage (ARD) commonly exceed the freshwater aquatic toxicity threshold. An Al-containing ARD released from pyritic sandstone and shale fill along US. Interstate 80 in Center County, Pennsylvania has adversely impacted a former fishing stream. This ARD consistently contains low concentrations (<2 mg/L) of Fe and high concentrations (≈48 mg/L) of Al. Batch experiments conducted with kaolinite and H/Al-bentonite clays and clay soils collected from the Centre County, PA area suggests that synthetic brackish ARD enhances the release of Al from these clays, compared to synthetic acid (sulfuric) only and non-brackish ARD. This implies that highway deicing salts in road runoff could aggravate the leaching of Al from pyritic rocks used as fill or exposed in road cuts. Likewise, the use of ARD to enhance the production of natural gas from tight shales could result in the leaching of Al. Quantification of levels of Al released from field clays could be critical when considering remediation strategies.

Naturally acidic surface and ground waters draining porphyry-related mineralized areas of the Southern Rocky Mountains, Colorado and New Mexico

Abstract Acidic, metal-rich waters produced by the oxidative weathering and resulting leaching of major and trace elements from pyritic rocks can adversely affect water quality in receiving streams and riparian ecosystems. Five study areas in the southern Rocky Mountains with naturally acidic waters associated with porphyry mineralization were studied to document variations in water chemistry and processes that control the chemical variations. Study areas include the Upper Animas River watershed, East Alpine Gulch, Mount Emmons, and Handcart Gulch in Colorado and the Red River in New Mexico. Although host-rock lithologies in all these areas range from Precambrian gneisses to Cretaceous sedimentary units to Tertiary volcanic complexes, the mineralization is Tertiary in age and associated with intermediate to felsic composition, porphyritic plutons. Pyrite is ubiquitous, ranging from ∼1 to >5 vol.%. Springs and headwater streams have pH values as low as 2.6, SO4 up to 3700 mg/L and high dissolved metal concentrations (for example: Fe up to 400 mg/L; Cu up to 3.5 mg/L; and Zn up to 14.4 mg/L). Intensity of hydrothermal alteration and presence of sulfides are the primary controls of water chemistry of these naturally acidic waters. Subbasins underlain by intensely hydrothermally altered lithologies are poorly vegetated and quite susceptible to storm-induced surface runoff. Within the Red River study area, results from a storm runoff study documented downstream changes in river chemistry: pH decreased from 7.80 to 4.83, alkalinity decreased from 49.4 to

Hepburn Spa: cold carbonated mineral waters of Central Victoria, South Eastern Australia

Hepburn Spring is the major cold carbonated mineral spring of the ‘Spa’ country of Central Victoria, in South Eastern Australia. The spring occurs in a small tributary valley of the Loddon River, 15 km away from the crest of the Great Dividing Range. The waters are effervescent, have an alkalinity of around 1,500 mg/L (as HCO3−), are mildly acidic and have high iron concentrations. Hepburn Spring is one of nearly 100 small cold low flow carbonated mineral water springs that occur in the region. Hepburn Spring has been protected since 1865 in one of 34 ‘Special Mineral Spring Reserves’. By world standards the spring is small and not highly developed, but it retains much of its charm due to the bushland, forest setting. The bedrock of the region consists of indurated arkosic sandstones interbedded with carbonaceous shales and slates. The rocks are folded and cut by swarms of strongly developed meridional fissure fault systems. Hepburn Spring is situated on the fault and fold axes associated with the “Cornish line” and Gold mines have dewatered the spring on several occasions. After dewatering events the water level and flow recovered first and then the water composition and effervescence. The origin of the effervescent waters has attracted much attention; hypotheses include both a volcanic and a rock water reaction origin. The second hypothesis involves weathering of the carbonaceous and sometimes pyritic rock mass which contains only a few percent carbonate. This process produces high bicarbonate groundwater throughout Central Victoria. The Hepburn waters are a variant of these waters and it is suggested that controls on the carbonate solubility and redox conditions in the fissures flow systems results in effervescent waters as the waters ascend.

CCB-based encapsulation of pyrite for remediation of acid mine drainage

Acid mine drainage (AMD) from abandoned coal mines continues to be one of the most significant environmental problems. Remediation of AMD requires an addition of lime source to decrease the acidity, and grouting the entire mine and encapsulating the pyrite by calcium-rich additives is often employed. Utilization of alkaline coal combustion by-products (CCBs) has gained acceptance in such remediation applications because of their cost-effectiveness. A study was conducted to investigate the effectiveness of CCBs to abate acid mine drainage by encapsulation of pyrite. Geomechanical, hydraulic, and environmental tests were performed on grouts prepared with various ratios of CCBs as well as an alternative free lime source, lime kiln dust (LKD). The results indicated that the mechanical properties of grouts were dependent on their free lime contents. Hydraulic conductivities of pyrite–grout columns were relatively high due to the coating of the pyrite rock with the grout rather than the filling of all of the void spaces, as commonly experienced in field applications. The leaching tests indicated that the presence of high amounts of lime in a grout is not solely sufficient to improve the quality of AMD, since the rate of dissolution of a high lime content grout may be slow due to its rapid hardening. Therefore, it is recommended that grouts be selected with consideration of their hardening capacities, as well as the percentage of lime content present in the mixture.

Detecting Hydrothermal Pyritic Zones along Bald Eagle Ridge Using Induced Polarization

Acid rock drainage (ARD) is a major environmental problem in Pennsylvania. ARD occurs when iron sulfides, most commonly pyrite, oxidize to form sulfuric acid. This weathering reaction typically occurs slowly in undisturbed pyritic rock. When this rock is excavated, however, the greater surface area and availability of oxygen enable iron-oxidizing bacteria to flourish, increasing the reaction rate by many orders of magnitude (e.g., Singer and Stumm, 1970). Although the majority of ARD problems are due to coal mining, ARD can also be caused by highway construction. Typical characterizations can involve drilling test holes and carrying out acid-base accounting on the cores. Pennsylvania guidelines for highway construction require a drillhole every 500 ft (152 m). However, in areas of near-vertical beds or steeply dipping veins, sizeable pyritic zones can easily be missed. Induced polarization (IP) is a geophysical method that can be used to detect pyritic zones before excavation. The purpose of this article is to report the results of IP surveys of hydrothermal pyrite veins in the sandstones and shales of the Appalachians. IP is an electrical geophysical method developed by the mining industry to prospect for disseminated sulfide deposits. IP measures chargeability, the ability of a material to retain electrical charge. In general, rocks with pyrite contents of approximately one percent or greater have elevated chargeabilities. Although the potential value of IP in screening highway corridors for potential ARD problems has been known for some time (Jones et al., 1982), the technique is not widely used. To our knowledge, all of the published case studies (Jones et al., 1982; Anderson et al., 1990; Byerly, 1996; Butler et al., 2004) discuss the detection of pyritic zones in meta-sedimentary rocks. Although these studies were successful in locating some pyritic zones, they were unable to quantify the relationship between IP response …

English

Pyritic rocks with little or no carbonate mineral content generally produce acid mine drainage when exposed to the atmosphere and moisture. In the absence of carbonate minerals, it is often suggested that silicate minerals can provide some level of acid buffering. At the current time, databases of reaction kinetics are sufficiently detailed to allow calculations of the rates of silicate mineral dissolution reactions relative to the rate of pyrite oxidation. These types of calculations have been conducted for conditions of abiotic oxidation of pyrite by O2 to estimate the acid neutralizing capacity of silicate rock types in terms of specific mineral contents. Using the criteria of maintaining a pH > 5.0 for 10-years, these calculations yield the following results for some generalized igneous rock types.

The Paleoproterozoic carbonate-hosted Pering Zn–Pb deposit, South Africa. II: fluid inclusion, fluid chemistry and stable isotope constraints

The Pering deposit is the prime example of Zn–Pb mineralisation hosted by stromatolitic dolostones of the Neoarchean to Paleoproterozoic Transvaal Supergroup. The hydrothermal deposit centers on subvertical breccia pipes that crosscut stromatolitic dolostones of the Reivilo Formation, the lowermost portion of the Campbellrand Subgroup. Four distinct stages of hydrothermal mineralisation are recognised. Early pyritic rock matrix brecciation is followed by collomorphous sphalerite mineralisation with replacive character, which, in turn, is succeeded by coarse grained open-space-infill of sphalerite, galena, sparry dolomite, and quartz. Together, the latter two stages account for ore-grade Zn–Pb mineralisation. The fourth and final paragenetic stage is characterised by open-space-infill by coarse sparry calcite. The present study documents the results of a detailed geochemical study of the Pering deposit, including fluid inclusion microthermometry, fluid chemistry and stable isotope geochemistry of sulphides (δ34S) and carbonate gangue (δ13C and δ18O). Microthermometric fluid inclusion studies carried out on a series of coarsely grained crystalline quartz and sphalerite samples of the latter, open-space-infill stage of the main mineralisation event reveal the presence of three major fluid types: (1) a halite–saturated aqueous fluid H2O–NaCl–CaCl2 (>33 wt% NaCl equivalent) brine, (2) low-salinity meteoric fluid (<7 wt% NaCl) and (3) a carbonic CH4–CO2–HS− fluid that may be derived from organic material present within the host dolostone. Mixing of these fluids have given rise to variable mixtures (H2O–CaCl2–NaCl ±(CH4–CO2–HS−), 2 to 25 wt% NaCl+CaCl2). Heterogeneous trapping of the aqueous and carbonic fluids occurred under conditions of immiscibility. Fluid temperature and pressure conditions during mineralisation are determined to be 200–210°C and 1.1–1.4 kbar, corresponding to a depth of mineralisation of 4.1–5.2 km. Chemical analyses of the brine inclusions show them to be dominated by Na and Cl with lesser amounts of Ca, K and SO4. Fluid ratios of Cl/Br indicate that they originated as halite saturated seawater brines that mixed with lower salinity fluids. Analyses of individual brine inclusions document high concentrations of Zn and Pb (∼1,500 and ∼200 ppm respectively) and identify the brine as responsible for the introduction of base metals. Stable isotope data were acquired for host rock and hydrothermal carbonates (dolomite, calcite) and sulphides (pyrite, sphalerite, galena and chalcopyrite). The ore-forming sulphides show a trend to 34S enrichment from pyrite nodules in the pyritic rock matrix breccia (δ34S = −9.9 to +3.7‰) to paragenetically late chalcopyrite of the main mineralisation event (δ34S = +30.0‰). The observed trend is attributed to Rayleigh fractionation during the complete reduction of sulphate in a restricted reservoir by thermochemical sulphate reduction, and incremental precipitation of the generated sulphide. The initial sulphate reservoir is expected to have had an isotopic signature around 0‰, and may well represent magmatic sulphur, oxidised and leached by the metal-bearing brine. The δ18O values of successive generations of dolomite, from host dolostone to paragenetically late saddle dolomite follow a consistent trend that yields convincing evidence for extensive water rock interaction at variable fluid–rock ratios. Values of δ13C remain virtually unchanged and similar to the host dolostone, thus suggesting insignificant influx of CO2 during the early and main stages of mineralisation. On the other hand, δ13C and δ18O of post-ore calcite define two distinct clusters that may be attributed to changes in the relative abundance in CH4 and CO2 during waning stages of hydrothermal fluid flow.

The Paleoproterozoic carbonate-hosted Pering Zn–Pb deposit, South Africa: I. Styles of brecciation and mineralization

The Pering deposit on the Ghaap Plateau, Northwestern Province, South Africa, was the largest of several Zn–Pb occurrences hosted by Neoarchean platform dolostones of the Transvaal Supergroup. With a Paleoproterozoic mineralization age, these occurrences are widely regarded as the oldest representatives of Mississippi Valley-type Pb–Zn deposits. Hosting an initial resource of 18 Mt at an average grade of 3.6 wt% Zn and 0.6 wt% Pb, the Pering deposit was mined from 1984 until its final closure at the end of November 2002. In this study, available geological and grade distribution maps were evaluated and complemented by the examination of mining-related outcrops, drill core, and a large set of ore and host rock samples. Four different styles of brecciation can be distinguished at the Pering deposit: (1) pyritic rock matrix breccia; (2) chemical wear breccia; (3) mosaic breccia; and (4) crackle breccia. Geological and mineral paragenetic observations on these different breccia types suggest that the formation of the Pering deposit commenced with an initial stage of hydrothermal karstification. Large volumes of pyritic rock matrix breccia formed by wall rock collapsing into the open space attributed to carbonate dissolution. This stage of hydrothermal karstification acted as ground preparation for the subsequent mineralization event. By the upward advance of the hydrothermal karstification process, fluid reservoirs in the previously undisturbed dolostone host rock succession were tapped, ultimately leading to fluid mixing. Hydrothermal sulphides are the most abundant where fluid mixing was most effective, i.e. along the outer and upper margins of the breccia bodies, and in stratabound zones along permeable host rock units. Chemical wear brecciation and formation of large volumes of fine-grained replacive sphalerite mineralization mark the early stage of hydrothermal Zn–Pb mineralization associated with this fluid mixing. The fine-grained stage of sulphide mineralization was succeeded by very coarse-grained open-space-infill mineralization. The latter is very uniform across the entire deposit and typically cements mosaic and crackle breccia, but also fills remaining open space within chemical wear brecciated portions of the deposit.