Precipitation Of Sulfide Minerals Research Articles

Integrating Tide‐Driven Wetland Soil Redox and Biogeochemical Interactions Into a Land Surface Model

AbstractRedox processes, aqueous and solid‐phase chemistry, and pH dynamics are key drivers of subsurface biogeochemical cycling and methanogenesis in terrestrial and wetland ecosystems but are typically not included in terrestrial carbon cycle models. These omissions may introduce errors when simulating systems where redox interactions and pH fluctuations are important, such as wetlands where saturation of soils can produce anoxic conditions and coastal systems where sulfate inputs from seawater can influence biogeochemistry. Integrating cycling of redox‐sensitive elements could therefore allow models to better represent key elements of carbon cycling and greenhouse gas production. We describe a model framework that couples the Energy Exascale Earth System Model (E3SM) Land Model (ELM) with PFLOTRAN biogeochemistry, allowing geochemical processes and redox interactions to be integrated with land surface model simulations. We implemented a reaction network including aerobic decomposition, fermentation, sulfate reduction, sulfide oxidation, methanogenesis, and methanotrophy as well as pH dynamics along with iron oxide and iron sulfide mineral precipitation and dissolution. We simulated biogeochemical cycling in tidal wetlands subject to either saltwater or freshwater inputs driven by tidal hydrological dynamics. In simulations with saltwater tidal inputs, sulfate reduction led to accumulation of sulfide, higher dissolved inorganic carbon concentrations, lower dissolved organic carbon concentrations, and lower methane emissions than simulations with freshwater tidal inputs. Model simulations compared well with measured porewater concentrations and surface gas emissions from coastal wetlands in the Northeastern United States. These results demonstrate how simulating geochemical reaction networks can improve land surface model simulations of subsurface biogeochemistry and carbon cycling.

Production and fluxes of inorganic carbon and alkalinity in a subarctic subterranean estuary

In this study, we focus on the biogeochemical processes that produce both dissolved inorganic carbon (DIC) and total alkalinity (TA) along a subarctic subterranean estuary (STE) located in the Gulf of St. Lawrence (Magdalen Island, Qc, Canada) in order to evaluate the DIC and TA fluxes as well as the buffering capacity of the exported groundwater to coastal waters. DIC and TA do not behave conservatively during mixing along the groundwater flow path and this implies the occurrence of internal redox reactions that control both their production and consumption. In addition, we show that the origin and composition of the organic carbon within the system alter the carbonate parameters by generating low pH conditions (5.9 - 7.2) and contributing to non-carbonate alkalinity (NCA) that accounts for more than 30% of TA. Whereas iron cycling plays a key role in the production of DIC in the fresh and low-salinity groundwaters, the precipitation of sulfide minerals neutralize the acidity produced by the metabolically produced CO2, in the saline groundwater where sulfate is available. The STE pCO2, computed from the DIC-pHNBS pair ranged from a few ppm to 16000 ppm that results in a CO2 evasion rate of up to 310 mol m−2d−1 to the atmosphere. Based on Darcy flow and the mean concentrations of DIC and carbonate alkalinity (Ac = TA - NCA) in the discharge zone, fluxes derived from submarine groundwater discharge were estimated at 1.43 and 0.70 mol m−2d−1 for DIC and Ac, respectively. Despite a major part of the metabolic CO2 being lost along the groundwater flow path, the SGD-derived DIC flux was still greater than the Ac flux, implying that groundwater discharge reduces the buffering capacity of the receiving coastal waters. This site-specific scale study demonstrates the importance of diagenetic reactions and organic matter remineralization processes on carbonate system parameters in STE. Our results highlight that subarctic STEs could be hot spots of CO2 evasion and a source of acidification to coastal waters that should be considered in carbon budgets.

The Mineral and Geochemical Features of Sulfides in the Jade Hydrothermal Field of the Okinawa Trough in Off-Shore China

In this study, mineralogical and elemental geochemical characteristics of massive sulfide samples collected from the Jade hydrothermal field, located in the Izena depression in the central graben of the Okinawa Trough, were analyzed by means of optical microscopy, scanning electron microscopy, and electron probe microanalysis (EPMA). The results show that the mineralization in the Jade hydrothermal field can be divided into Zn-Cu-Pb-rich massive sulfides and Zn-Fe-rich massive sulfides. The former is composed of sphalerite, galena, chalcopyrite, pyrite, and anglesite, which is the product of the low-temperature alteration of galena. The latter is mainly composed of sphalerite, pyrite, marcasite, and traces of galena. Cu and Zn in pyrite may exist in the form of microinclusions, while Ag and Pb may exist in pyrite in the form of fine galena inclusions containing Ag. Fe and Cu may enter sphalerite in the form of ion replacement. Zn may enter chalcopyrite in the form of ion replacement. Consistent with the previous understanding, the metal elements in the hydrothermal liquid system in the Jade hydrothermal field mostly migrated as sulfur complexes, and when the hydrothermal fluid mixes with seawater, the physical and chemical conditions of the fluid change, resulting in sulfide mineral precipitation. However, the chemical structure of chalcopyrite is still controversial, which restricts the understanding of the substitution mechanism of trace elements during chalcopyrite precipitation.

Two-dimensional reactive transport model as a new approach for identifying the origins and contribution of arsenic in a soil and water system

Distribution of arsenic (As) in the environment can occur through various paths, depending on the interrelationship between soil and surface water, and groundwater. This study aimed to identify the origins and pathways of As among various potential origins in soil and water systems using two-dimensional reactive transport models (2D RTMs) and to evaluate the contribution of each origin to the geochemical distribution of As in the study area, which included a reservoir, two streams, and two abandoned mines. The 2D RTMs were prepared by combining hydraulic and geochemical reactive models. The MODFLOW-2005 package was used to simulate the water flow in the study area. The geochemical reaction and distribution of As were simulated using Geochemist's Workbench. The concentration of As in the reservoir was ∼50 μg/L, while that in the soils collected around the mine X and Y was 15.0–1853.2 mg/kg (median: 126.1 mg/kg) and 3.6–1629.2 mg/kg (median: 60.9 mg/kg), respectively. Hydraulic and geochemical input data were derived from field surveys and laboratory experiments. Based on the geochemical and hydrogeological input data, 2D RTMs determined that 32.0 kg of As accumulated into the reservoir over 18 years through surface water, which is from mines X (2.7 kg) and Y (29.3 kg). Hence, mine Y would be the primary origin of As to the reservoir. Based on 2D RTM, 2.4 kg of As entered the reservoir via groundwater (DO <0.1 mg/L). Also, precipitation of sulfide minerals (e.g., orpiment; As2S3) influences As dispersion and contribution under subsurface conditions. These modeling results suggest that 2D RTMs could be a new approach for identifying and assessing the contribution of potential As origins in soil and water system.

Dolomite cement microstratigraphy: A record of brine evolution and ore precipitation mechanisms, upper Knox Group, Tennessee and Kentucky, USA

AbstractTrace element changes in fluids associated with ore-forming events in sedimentary basins may be recorded by contemporaneous cements, especially zoned carbonate minerals (microstratigraphy). Cement analysis using advanced mapping and analytical techniques including scanning electron microscopy cathodoluminescence (SEM-CL), charge contrast imaging, high-resolution X-ray computed tomography (XCT), and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) documents geochemical changes associated with Mississippi Valley–type mineralization in solution-collapse breccias of the Cambrian–Ordovician Knox Group (Tennessee and Kentucky, USA). Dolomite cement zonation coincident with changes in Fe and Mn can be observed with optical microscope CL in bands as narrow as 5 µm, whereas panchromatic SEM-CL reveals microfractures and cement subzones coincident with changes in La and Ce concentrations in bands as narrow as 0.1 µm. XCT scans image a high-density (Fe-rich) dolomite zone at the onset of late sulfide precipitation. The transition from pre-ore to ore-stage cementation is marked by increased Fe, Mn, Zn, Cd, Ga, Pb, and Sr and decreased La and Ce concentrations. Fine-scale metal depletion cycles during this transition may record metal precipitation from brine in response to the availability of reduced sulfur. Except for Fe and Mn, post-ore dolomite zones generally have low metal concentrations. Thus, dolomite microstratigraphy tracks systematic changes in brine metal concentrations modified by episodes of localized sulfide mineral precipitation.

Formation of orogenic gold deposits by progressive movement of a fault-fracture mesh through the upper crustal brittle-ductile transition zone

Orogenic gold deposits are comprised of complex quartz vein arrays that form as a result of fluid flow along transcrustal fault zones in active orogenic belts. Mineral precipitation in these deposits occurs under variable pressure conditions, but a mechanism explaining how the pressure regimes evolve through time has not previously been proposed. Here we show that extensional quartz veins at the Garrcon deposit in the Abitibi greenstone belt of Canada preserve petrographic characteristics suggesting that the three recognized paragenetic stages formed within different pressure regimes. The first stage involved the growth of interlocking quartz grains competing for space in fractures held open by hydrothermal fluids at supralithostatic pressures. Subsequent fluid flow at fluctuating pressure conditions caused recrystallization of the vein quartz and the precipitation of sulfide minerals through wall-rock sulfidation, with some of the sulfide minerals containing microscopic gold. These pressure fluctuations between supralithostatic to near-hydrostatic conditions resulted in the post-entrapment modification of the fluid inclusion inventory of the quartz. Late fluid flow occurred at near-hydrostatic conditions and resulted in the formation of fluid inclusions that have not been affected by post-entrapment modification as pressure conditions never returned to supralithostatic conditions. This late fluid flow is interpreted to have formed the texturally late, coarse native gold that occurs along quartz grain boundaries and in open spaces. The systematic evolution of the pressure regimes in orogenic gold deposits such as Garrcon can be explained by relative movement of fault-fracture meshes across the base of the upper crustal brittle-ductile transition zone. We conclude that early vein quartz in orogenic deposits is precipitated at near-lithostatic conditions whereas the paragenetically late gold is introduced at distinctly lower pressure.

Do bacterial viruses affect framboid-like mineral formation?

Abstract. Framboidal pyrite has been a matter of interest of many studies due to its abundance in a wide range of environments and being a marker of redox conditions. However, the clear origin of framboidal pyrite remains unresolved. Our studies are preliminary laboratory investigations on the influence of the shape and physicochemical properties of bacteriophages on the synthesis of framboid-like structures. This paper discusses the possible role of bacteriophages (bacterial viruses) in the precipitation of sulfide minerals (FeS and CuS) and their impact on the formation of framboid-like structures. Here, two bacteriophages (Escherichia phage P1 and Pseudomonas phage Φ6), which differ significantly in shape and physicochemical properties, were used. Our observations suggest that viruses may bind ions from the solution. Moreover, we showed that bacteriophages P1 can lead to the formation of finer mineral particles of FeS and CuS, whereas the framboid-like structures were found only in experiments with precipitation of FeS. However, the lipid-enveloped Pseudomonas phage Φ6 did not cause the formation of similar structures. It is assumed that Escherichia phage P1 can promote the formation of FeS-based framboid-like or spherical structures. The proposed four-step conceptualized mechanism facilitating the framboid-like structure synthesis via viruses is as follows: (i) binding of ions by capsids, (ii) bacteriophages behaving like a crystallization surface, (iii) destabilization of the colloid (ζ potential ± 0), and (iv) formation of fine agglomerates and subsequent formation of small crystallites. Further studies are required to find all factors that may be affected by bacteriophages during sulfide precipitation. In addition, it is important to consider viruses present in sedimentation environments, despite possible difficulties in laboratory culturing. The consideration of such viruses may make laboratory testing more valid in terms of sedimentation environments.

Microbial carbon, sulfur, iron, and nitrogen cycling linked to the potential remediation of a meromictic acidic pit lake.

Cueva de la Mora is a permanently stratified acidic pit lake and a model system for extreme acid mine drainage (AMD) studies. Using a combination of amplicon sequencing, metagenomics and metatranscriptomics we performed a taxonomically resolved analysis of microbial contributions to carbon, sulfur, iron, and nitrogen cycling. We found that active green alga Coccomyxa onubensis dominated the upper layer and chemocline. The chemocline had activity for iron(II) oxidation carried out by populations of Ca. Acidulodesulfobacterium, Ferrovum, Leptospirillium, and Armatimonadetes. Predicted activity for iron(III) reduction was only detected in the deep layer affiliated with Proteobacteria. Activity for dissimilatory nitrogen cycling including nitrogen fixation and nitrate reduction was primarily predicted in the chemocline. Heterotrophic archaeal populations with predicted activity for sulfide oxidation related to uncultured Thermoplasmatales dominated in the deep layer. Abundant sulfate-reducing Desulfomonile and Ca. Acidulodesulfobacterium populations were active in the chemocline. In the deep layer, uncultured populations from the bacterial phyla Actinobacteria, Chloroflexi, and Nitrospirae contributed to both sulfate reduction and sulfide oxidation. Based on this information we evaluated the potential for sulfide mineral precipitation in the deep layer as a tool for remediation. We argue that sulfide precipitation is not limited by microbial genetic potential but rather by the quantity and quality of organic carbon reaching the deep layer as well as by oxygen additions to the groundwater enabling sulfur oxidation. Addition of organic carbon and elemental sulfur should stimulate sulfate reduction and limit reoxidation of sulfide minerals.

Hydrothermal remobilization of subseafloor sulfide mineralization along mid-ocean ridges contributes to the global oceanic zinc isotopic mass balance

Hydrothermal activity on mid-ocean ridges is an important mechanism for the delivery of Zn from the mantle to the surface environment. Zinc isotopic fractionation during hydrothermal activity is mainly controlled by the precipitation of Zn-bearing sulfide minerals, in which isotopically light Zn is preferentially retained in solid phases rather than in solution during mineral precipitation. Thus, seafloor hydrothermal activity is expected to supply isotopically heavy Zn to the ocean. Here, we studied sulfide-rich samples from the Duanqiao-1 hydrothermal field, located on the Southwest Indian Ridge. We report that, at the hand-specimen scale, late-stage conduit sulfide material has lower δ66Zn values (−0.05 ± 0.15 ‰; n = 19) than early-stage material (+0.13 ± 0.15 ‰; n = 10). These lower values correlate with enrichments in Pb, As, Cd, and Ag, and elevated δ34S values. We attribute the low δ66Zn values to the remobilization of earlier sub-seafloor Zn-rich mineralization. Based on endmember mass balance calculations, and an assumption of a fractionation factor (αZnS-Sol.) of about 0.9997 between sphalerite and its parent solution, the remobilized Zn was found consist of about 1/3 to 2/3 of the total Zn in the fluid that formed the conduit samples. Our study suggests that late-stage subsurface hydrothermal remobilization may release isotopically-light Zn to the ocean, and that this process may be common along mid-ocean ridges, thus increasing the size of the previously identified isotopically light Zn sink in the ocean.

The influence of redox conditions on aqueous-solid partitioning of arsenic and selenium in a closed coal ash impoundment

A closed coal ash impoundment case study characterized the effects of field redox conditions on arsenic and selenium partitioning through monitoring of porewater and subsurface gas in conjunction with geochemical speciation modeling. When disposed coal ash materials and porewater were recovered for testing, oxidation led to lower arsenic and higher selenium concentrations in leaching test extracts compared to porewater measurements. Multiple lines of evidence suggest multiple mechanisms of arsenic retention are plausible and the concurrent presence of several redox processes and conditions (e.g., methanogenesis, sulfate reduction, and Fe(III)-reduction) controlled by spatial gradients and dis-equilibrium. Geochemical speciation modeling indicated that, under reducing field conditions, selenium was immobilized through the formation of insoluble precipitates Se(0) or FeSe while arsenic partitioning was affected by a progression of reactions including changes in arsenic speciation, reduction in adsorption due to dissolution and recrystallization of hydrous ferric oxides, and precipitation of arsenic sulfide minerals.

Bioclast-controlled patchy barite cementation – Origin and impact on reservoir properties in deeply buried Upper Jurassic sandstones, North Sea

Reservoir non-destructive patchy barite cement occurs in volumes up to 16 vol% in deeply (>5150 m) buried shoreface sandstones in the Hejre field, Danish Central Graben, North Sea. Barite typically occurs as an accessory cement in the sandstones of the Upper Jurassic Heno Formation; however in the Hejre field, barite appears more abundant in the crestal-most well than in other wells. The purpose of this study is to describe the characteristics and the origin of patchy barite cement besides implications for reservoir properties. Petrographic investigations by optical and scanning electron microscopy supplemented by isotopic analysis by SIMS (secondary ion mass spectroscopy) were applied to reveal the barite cementation process. Porosity and permeability of the sandstones were measured by conventional core analysis. Barite cement occurs in the Hejre field with three morphologies: crystals arranged in rosettes, crystal fillings of bioclasts, and multiple-oriented intergrown small crystals in veins. SIMS showed similar sulphur and oxygen isotopic compositions for these barite morphologies, and this composition resembles evaporite-derived barite cement. A hydrothermal origin of the fluid is supported by the associated precipitation of pyrite, chalcopyrite, sphalerite and galena, and a high Sr/Ba ratio in barite. Conventional core analysis revealed major reductions in porosity and permeability when barite cement occurs as fracture filling or in combination with calcite cement in shell lags. However, these occurrences are rare and have a minor influence on the overall reservoir properties. Minor reduction of porosity and permeability characterizes sandstones with barite cement in rosettes of radiating crystals. This growth pattern was probably promoted by slow seep of sulphate-rich fluids which, by mixing with barium-bearing formation water became super-saturated with respect to barite as they moved towards lower temperature and pressure zones. Faults acted as pathways for transport of sulphate-rich fluids possibly from evaporite deposits, whereas barium may have originated from compactional fluids leached out of underlying volcaniclastic rocks. Thermochemical sulphate reduction led to precipitation of sulphide minerals, minor sulphur fractionation and fluid acidification, whereby calcareous bioclasts dissolved. Barite precipitated when the fluids entered these large- or over-sized pores with small obstacles causing prolonged fluid-rock interaction. The bioclast-replacing and patchy barite cement caused only limited reduction of reservoir porosity and permeability compared with barite-free sandstones from the Heno Formation found at a similar burial depth.

Seawater Chemistry and Hydrothermal Controls on the Cenozoic Osmium Cycle

AbstractOsmium isotope ratios are a key tool to track changes in global weathering and carbon cycle evolution through time. Long‐term changes in seawater Os isotope records over the Cenozoic have been used to argue for changes in weathering from increased uplift, leading to long‐term global cooling. Sulfide mineral precipitation during hydrothermal circulation traps the majority of mantle‐derived Os within mid‐ocean ridge (MOR) hydrothermal systems. Building from evidence that the amount of sulfide mineral precipitation in the subseafloor of MOR systems is related to the amount of sulfate and calcium in seawater, we show that as seawater chemistry changed through time, so too has the extent of hydrothermal sulfide formation and the global Os cycle. With currently estimated changes in seawater Ca/SO4 ratios, the observed progressive increase in seawater Os isotope values through the Cenozoic can be linked to changes in seawater chemistry—instead of a major shift in continental weathering processes.

A reservoir model for geothermal energy production from the Middle Devonian Slave Point Formation

Clarke Lake is a depleted gas field developed in carbonate platform deposits of the Slave Point Formation (Middle Devonian) in northeastern British Columbia, Canada. The field displays anomalously high reservoir temperature (≥100 °C) and strong water drive, making it a candidate for repurposing as a source of geothermal power. Porous and permeable reservoir developed near the platform margin through the hydrothermal alteration of host limestone to dolomite. A depositional model provides a basis for mapping the dolomite reservoir properties. Facies analysis combined with porosity and permeability measurements are applied to flow simulations to test how reinjected water would affect geothermal power production.Nine depositional facies and two diagenetic facies are identified: the former based on bioclast assemblages, rock types, texture and composition; and the latter based on rock fabric and the extent of alteration to dolomite. These facies were deposited within lagoonal, reef-flat, shoal, reef margin and foreslope settings associated with a rimmed carbonate platform and can be separated into stratigraphic successions that influenced dolomitization: an initial shoal unit, S1, three subsequent reef units, R1, R2, R3, and terminal shoal units of D1, D2 and D3. Shoal units were deposited in the transgressive systems tracts, whereas reef growth units were deposited in the highstand systems tracts. Dolomitized lagoonal, reef flat, reef margin and shoal lithologies show enhanced porosity and permeability due to dissolution of stromatoporoid bioclasts, forming mouldic and vuggy porosity. Diagenetic facies show high permeability due to fractures but reduced porosity as a result of precipitation of porosity-occluding dolomite, fluorite, and sulphide minerals. High quality reservoir zones occur primarily at the reef margin, caused by fabric-selective hydrothermal alteration of carbonate sediments near the contact with shales of the Horn River and Muskwa formations.Numerical modelling of geothermal injection and production scenarios shows that thermal breakthrough from injection wells to producing wells presents a negligible risk to estimated power productions, with efficiencies being reduced by 10–15% compared to scenarios that assume no thermal breakthrough. Permeable, fractured diagenetic and reef front facies are suitable targets for geothermal fluid production.

Rapid alteration of fractured volcanic conduits beneath Mt Unzen

The nature of sub-volcanic alteration is usually only observable after erosion and exhumation at old inactive volcanoes, via geochemical changes in hydrothermal fluids sampled at the surface, via relatively low-resolution geophysical methods or can be inferred from erupted products. These methods are spatially or temporally removed from the real subsurface and thus provide only indirect information. In contrast, the ICDP deep drilling of the Mt Unzen volcano subsurface affords a snapshot into the in situ interaction between the dacitic dykes that fed dome-forming eruptions and the sub-volcanic hydrothermal system, where the most recent lava dome eruption occurred between 1990 and 1995. Here, we analyse drill core samples from hole USDP-4, constraining their degree and type of alteration. We identify and characterize two clay alteration stages: (1) an unusual argillic alteration infill of fractured or partially dissolved plagioclase and hornblende phenocryst domains with kaolinite and Reichweite 1 illite (70)-smectite and (2) propylitic alteration of amphibole and biotite phenocrysts with the fracture-hosted precipitation of chlorite, sulfide and carbonate minerals. These observations imply that the early clay-forming fluid was acidic and probably had a magmatic component, which is indicated for the fluids related to the second chlorite-carbonate stage by our stable carbon and oxygen isotope data. The porosity in the dyke samples is dominantly fracture-hosted, and fracture-filling mineralization is common, suggesting that the dykes were fractured during magma transport, emplacement and cooling, and that subsequent permeable circulation of hydrothermal fluids led to pore clogging and potential partial sealing of the pore network on a timescale of ~ 9 years from cessation of the last eruption. These observations, in concert with evidence that intermediate, crystal-bearing magmas are susceptible to fracturing during ascent and emplacement, lead us to suggest that arc volcanoes enclosed in highly fractured country rock are susceptible to rapid hydrothermal circulation and alteration, with implications for the development of fluid flow, mineralization, stress regime and volcanic edifice structural stability. We explore these possibilities in the context of alteration at other similar volcanoes.

Gold occurrences of the Woumbou–Colomine–Kette district, eastern Cameroon: ore-forming constraints from petrography, SEM–CL imagery, fluid inclusions, and C–O–H–S isotopes

The Woumbou–Colomine–Kette district is located in the East Metallogenic Province of the Republic of Cameroon. Northeast- to E-striking splays of the regional Pan-African crustal-scale Sanaga Shear Zone control widespread gold mineralization. Gold occurs within local shear zones developed along granite-gneiss contacts, as well as within the margins of the Pan-African intrusions. Field investigations coupled with mineralogical and textural studies reveal two stages of gold mineralization. Stage 1 led to pyrite, chalcopyrite, sphalerite, pyrrhotite, bismuthinite, tellurobismuthite, galenobismuthite, and native gold deposition with quartz, sericite/muscovite, chlorite, calcite, and ankerite as gangue minerals. Stage 2 included the precipitation of pyrite, hematite, tellurobismuthite, and native gold. A SEM–CL study distinguished the two stages of quartz, as well as an early remnant barren quartz depositional stage. Two fluid inclusion assemblages (FIAs) with four types of fluid inclusions can be observed. The FIA 1 is aqueous-carbonic (±N2±CH4), with carbonate daughter minerals, which is related to stage 1 of the gold mineralization and composed of low salinity (~7.5 wt% NaCl equiv) H2O-CO2-NaCl (type 1), CO2-rich (type 2), and H2O-rich (type 3a) fluid inclusions. The inclusions provide evidence of episodic phase separation, with unmixing at P~2 kbar, T~300 °C, and at a paleodepth of ~7 km. The FIA 2 consists of aqueous (type 3b) fluid inclusions with a salinity of 0.2 to 11.7 wt% NaCl equiv and trapping temperatures between 205 and 245 °C. The δ18Oquartz (+11.0 to +11.7‰), δDfluid inclusions (−46.2 to −40.9‰) and δ13Cfluid inclusions (−4.9 to −4.1‰) for gold-bearing quartz veins suggest a metamorphic source for the ore-forming fluids, although some contribution from the mantle and/or magmatic source(s) cannot be ruled out. The δ34S (+6.5 to +7.0‰) for gold-bearing pyrites suggests a metamorphic source for the sulfur that transported gold and was involved in the precipitation of sulfide minerals. These data are consistent with a mesozonal orogenic gold deposit model for the Woumbou–Colomine–Kette district, which is an important part of the emerging greenfields exploration region for Pan-African orogenic gold and with the more significant targets defined by granite-gneiss contacts along NE- and E-striking shear zones.

Gypsum Amendment Induced Rapid Pyritization in Fe-Rich Mine Tailings from Doce River Estuary after the Fundão Dam Collapse

Mine tailings containing trace metals arrived at the Doce River estuary, after the world’s largest mine tailings disaster (the Mariana disaster) dumped approximately 50 million m3 of Fe-rich tailings into the Doce River Basin. The metals in the tailings are of concern because they present a bioavailability risk in the estuary as well as chronic exposure hazards. Trace metal immobilization into sulfidic minerals, such as, pyrite, plays a key role in estuarine soils; however, this process is limited in the Doce River estuarine soil due to low sulfate inputs. Thus, to assess the use of gypsum amendment to induce pyritization in deposited tailings, a mesocosm experiment was performed for 35 days, with vinasse added as carbon source and doses of gypsum (as a sulfate source). Chemical and morphological evidence of Fe sulfide mineral precipitation was observed. For instance, the addition of 439 mg of S led to the formation of gray and black spots, an Fe2+ increase and decrease in sulfides in the solution, an increase in pyritic Fe, and a greater Pb immobilization by pyrite at the end of the experiment. The results show that induced pyritization may be a strategy for remediating metal contamination at the Doce River estuary.

Mineralogy and fluid characteristics of the Waschleithe Zn skarn – a distal part of the Schwarzenberg mineral system, Erzgebirge, Germany

The Waschleithe skarn is situated in the northern sector of the Schwarzenberg district in the western Erzgebirge (Germany), a district which hosts several large polymetallic skarn deposits. The Waschleithe is a Zn-dominant skarn comprising abundant dark Mn-rich hedenbergite, minor grandite garnet and magnetite related to the prograde stage as well as epidote, quartz, fluorite, amphibole, chlorite, ilvaite and Mn-rich pyroxenoids related to the retrograde stage of skarn formation. Ore minerals are related to the early retrograde stage and include sphalerite, galena, chalcopyrite, pyrite as well as minor amounts of scheelite and cassiterite. Fluid inclusion homogenization temperatures range from ~385 to 360 °C and from ~320 to 285 °C for the prograde and retrograde skarn stage, respectively. Thus, late-stage cooling is proposed as the major controlling factor for sulfide mineral precipitation. Fluid salinities for both stages are very low (<3% eq. w[NaCl]) and all observed fluid inclusion assemblages show homogeneous liquid–vapor ratios. Formation pressures >25 MPa suggest a minimum depth of formation of approximately 3 km. Oxygen fugacity during prograde skarn formation was initially low and increased towards the late prograde stage. The late-Variscan Eibenstock granite is identified as the most likely source of the skarn-forming fluids. The recognition of the Waschleithe skarn as a distal Zn skarn has major implications for the understanding of the Schwarzenberg district mineral system and provides valuable information for exploration targeting.

External fluids cause alteration and metal redistribution in the granite-hosted Tangziwa Sn[sbnd]Cu deposit, Gejiu district, China

The granite-hosted Tangziwa SnCu deposit in the Gejiu district shows variable alteration of the granite with associated mineralization. In this paper, we used whole rock chemistry mass balance calculations and Sr, Nd, Pb, Li, and B isotope data to constrain the source of metals and fluids in different alteration stages. The δ7Li and δ11B values of unaltered and weakly altered (Type I) granite samples fall in narrow ranges of −0.70 to 3.61‰ and −14.87 to −12.82 ‰, respectively, which is typical for granites. The Li and B isotopic composition of moderately (Type II) to strongly altered (Type III-1 and Type III-2) granite samples are shifted toward higher δ7Li (up to 13.61‰) and lower δ11B values (as low as −31.32‰). These extreme values reflect the addition of Li and B via an external fluid. Petrographic observations and isocon mass balance calculations demonstrate that early stage, magmatic fluids altered biotite and plagioclase to muscovite and clay minerals, respectively, and led to loss of Ca, Na, Fe, and Cs. The later stage, external fluids caused the destruction of feldspar and the precipitation of calcite, anhydrite/gypsum, fluorite, and sulfide minerals (pyrite, chalcopyrite, arsenopyrite, and stannite). This alteration is associated with an increase in the contents of Ca, S, Fe, Co, Cu, Sb, Cd, and Sn and a reduction of the contents of Pb, Sr, Cs, and LREEs. The high contents of Cu and S, which are not typically abundant in granite, could be added by the external fluids characterized by high δ7Li and low δ11B. The weakly and moderately altered granites have lower Sn contents than the unaltered granite. This indicates that late magmatic or external fluids may have leached Sn from weakly and moderately altered granite and that sulfide minerals in the strongly altered granites eventually scavenged this mobilized Sn.

Identifying redox transition zones in the subsurface of a site with historical contamination

Reactive iron mineral coatings found throughout reduction-oxidation (redox) transition zones play an important role in contaminant transformation processes. This research focuses on demonstrating a process for effectively delineating redox transition zones at a site with historical contamination. An 18.3 meter core was collected, subsampled, and preserved under anoxic conditions to maintain its original redox status. To ensure a high vertical resolution, sampling increments of 5.08 cm in length were analyzed for elemental concentrations with X-ray fluorescence (XRF), sediment pH, sediment oxidation-reduction potential (ORP), total volatile organic carbon (TVOC) concentration in the sample headspace, and abundant bacteria (16S rRNA sequencing). Over the core's length, gradients observed ranged from 3.74 to 8.03 for sediment pH, −141.4 mV to +651.0 mV for sediment ORP, and from below detection to a maximum of 9.6 ppm TVOC concentration (as chlorobenzene) in the headspace. The Fe and S gradients correlated with the presence of Fe and S reducing bacteria. S concentrations peaked in the Upper Zone and Zone 1 where Desulfosporosinus was abundant, suggesting precipitation of iron sulfide minerals. In Zone 2, Fe concentrations decreased where Geobacter was abundant, potentially resulting in Fe reduction, dissolution, and precipitation of minerals with increased solubility compared to the Fe(III) minerals. Using complementary geochemical and microbial data, five redox transition zones were delineated in the core collected. This research demonstrates a systematic approach to characterizing redox transition zones in a contaminated environment.

Geochemical transformations beneath man-made ponds: Implications for arsenic mobilization in South Asian aquifers

The role of man-made ponds on arsenic mobilization was examined in Bangladesh. Here, we describe a field experiment that shows how recharge from a newly constructed pond creates a reactive front that moves downward into the underlying aquifer, but only advances slowly, less than 8 cm/year. We found that pond recharge introduces organic carbon that likely drives the reduction of sulfate and solid-phase iron. However, over the six-year period of the study the pond did not drive arsenic contamination of the underlying groundwater. An electron balance indicates that significant precipitation of ferrous iron and sulfide minerals may immobilize arsenic despite the shift towards more reducing conditions, explaining the very low observed aqueous arsenic concentrations. We additionally found that the amount of solid-phase electron acceptors available in the shallow sediments strongly retards the advance of a reduced sediment front. Our results suggest that labile organic carbon introduced by man-made ponds is efficiently mineralized in the sediments immediately below the pond bottom and thus is unlikely to drive arsenic mobilization deeper within the aquifer. We suggest that the excavation of man-made ponds removes young surficial sediments, leaving aged and less reactive sediments beneath the pond, so that recharge through excavated ponds does not mobilize arsenic at the high rates observed in recharge through natural wetlands and river banks.