Precipitation Of Sulfide Minerals Research Articles

Sulfide Mineral Precipitation from Hydrothermal Fluids

Sulfide minerals precipitate from hydrothermal solution where their constituent metals and sulfide are more stable residing in the solid than in aqueous solution. An understanding of how and why sulfide minerals precipitate means understanding what chemical species hold metals and sulfide in solution, and what fundamental chemical and physical processes may liberate those reactants to precipitate in solids–processes such as pH increase, chemical reduction, dilution or cooling. In the natural setting, the fundamental chemical and physical drivers of sulfide precipitation are a response to a wide range of processes. For example, pH increases where acidic hydrothermal fluids react with feldspars or carbonates in wall rocks, or where a hydrothermal fluid boils out CO2 to the gas phase. The pH also increases where a brine is diluted by fresh water while maintaining equilibrium with a silicate wall-rock assemblage. Cooling follows from isoenthalpic boiling, which is caused by pressure drop as a fluid ascends in a hydrothermal system. Cooling also results from dilution by cold water or from heat conduction into cold wall rocks. Chemical reduction occurs where an oxidized fluid reacts with carbon or methane or ferrous iron. In this chapter, we explore the fundamental chemical and physical processes that drive sulfide mineral dissolution and precipitation as well as a limited set of more complex processes that control the fundamental drivers. The basic goal is to understand the chemistry of the simplest processes so as to improve our ability to deduce what actually happens in the natural world. Our basic approach is to run thermodynamic calculations of whole-system models of the processes causing sulfide precipitation and dissolution—models that take into account simultaneous equilibria involving assemblages of minerals at chemical equilibrium among themselves and with hundreds of aqueous species. We then interpret the models in terms of fundamental effects of pH …

Geophysical Imaging of Stimulated Microbial Biomineralization

Understanding how microorganisms influence the physical and chemical properties of the subsurface is hindered by our inability to observe microbial dynamics in real time and with high spatial resolution. Here, we investigate the use of noninvasive geophysical methods to monitor biomineralization at the laboratory scale during stimulated sulfate reduction under dynamic flow conditions. Alterations in sediment characteristics resulting from microbe-mediated sulfide mineral precipitation were concomitant with changes in complex resistivity and acoustic wave propagation signatures. The sequestration of zinc and iron in insoluble sulfides led to alterations in the ability of the pore fluid to conduct electrical charge and of the saturated sediments to dissipate acoustic energy. These changes resulted directly from the nucleation, growth, and development of nanoparticulate precipitates along grain surfaces and within the pore space. Scanning and transmission electron microscopy (SEM and TEM) confirmed the sulfides to be associated with cell surfaces, with precipitates ranging from aggregates of individual 3-5 nm nanocrystals to larger assemblages of up to 10-20 microm in diameter. Anomalies in the geophysical data reflected the distribution of mineral precipitates and biomass over space and time, with temporal variations in the signals corresponding to changes in the aggregation state of the nanocrystalline sulfides. These results suggest the potential for using geophysical techniques to image certain subsurface biogeochemical processes, such as those accompanying the bioremediation of metal-contaminated aquifers.

Biogeochemical cycling of iron in the Archean–Paleoproterozoic Earth: Constraints from iron isotope variations in sedimentary rocks from the Kaapvaal and Pilbara Cratons

Iron isotope compositions of low-metamorphic grade samples of Archean–Paleoproterozoic sedimentary rocks obtained from fresh drill core from the Kaapvaal Craton in South Africa and from the Pilbara Craton in Australia vary by ~ 3‰ in 56Fe/ 54Fe ratios, reflecting a variety of weathering and diagenetic processes. Depositional ages for the 120 samples studied range from 3.3 to 2.2 Ga, and Fe, C, and S contents define several compositional groups, including samples rich in Fe, organic carbon, carbonate, and sulfide. The δ 56Fe values for low-C org, low-C carb, and low-S sedimentary rocks are close to 0‰, the average of igneous rocks. This range is essentially the same as that of C org-poor late Cenozoic loess, aerosol, river loads, and marine sediments and those of C org-poor Phanerozoic–Proterozoic shales. That these δ 56Fe values are the same as those of igneous rocks suggests that Fe has behaved conservatively in bulk sediments during sedimentary transport, diagenesis, and lithification since the Archean. These observations indicate that, if atmospheric O 2 contents rose dramatically between 2.4 and 2.2 Ga, as proposed by many workers, such a rise did not produce a significant change in the bulk Fe budget of the terrestrial sedimentary system. If the Archean atmosphere was anoxic and Fe was lost from bedrock during soil formation, any isotopic fractionation between aqueous ferrous Fe (Fe aq 2+) and Fe-bearing minerals must have been negligible. In contrast, if the Archean atmosphere was oxic, Fe would have been retained as Fe 3+ hydroxides during weathering as it is today, which would produce minimal net isotopic fractionation in bulk detrital sediments. Siderite-rich samples have δ 56Fe values of − 0.5 ± 0.5‰, and experimentally determined Fe aq 2+-siderite fractionation factors suggest that these rocks formed from Fe aq 2+ that had similar or slightly higher δ 56Fe values. The δ 56Fe values calculated for Fe aq 2+ overlaps those of modern submarine hydrothermal fluids, but it is also possible that Fe aq 2+ had δ 56Fe values higher than those of modern hydrothermal fluids, depending upon the Fe aq 2+–Fe carbonate fractionation factor that is used. In contrast, C org-rich samples and magnetite-rich samples have strongly negative δ 56Fe values, generally between − 2.3‰ and − 1.0‰, and available fluid–mineral fractionation factors suggest that the Fe-bearing minerals siderite and magnetite in these rocks formed in the presence of Fe aq 2+ that had very low δ 56Fe values, between − 3‰ and − 1‰. Reduction of Fe 3+ hydroxide by sulfide, precipitation of sulfide minerals, or incongruent dissolution of silicate minerals are considered unlikely means to produce significant quantities of low- δ 56Fe Fe aq 2+. We interpret microbial dissimilatory Fe 3+ reduction (DIR) as the best explanation for producing such low δ 56Fe values for Fe aq 2+, and our results suggest that DIR was a significant form of respiration since at least 2.9 Ga.

Reactive transport modeling of column experiments for the remediation of acid mine drainage.

Reactive transport modeling was used to evaluate the performance of two similar column experiments. The experiments were designed to simulate the treatment of acid mine drainage through microbially mediated sulfate reduction and subsequent sulfide mineral precipitation by means of an organic carbon permeable reactive barrier. Principal reactions considered in the simulations include microbially mediated reduction of sulfate by organic matter, mineral dissolution/precipitation reactions, and aqueous complexation/hydrolysis reactions. Simulations of column 1, which contained composted leaf mulch, wood chips, sawdust, and sewage sludge as an organic carbon source, accurately predicted sulfate concentrations in the column effluent throughout the duration of the experiment using a single fixed rate constant for sulfate reduction of 6.9 x 10(-9) mol L(-1) s(-1). Using the same reduction rate for column 2, which contained only composted leaf mulch and sawdust as an organic carbon source, sulfate concentrations at the column outlet were overpredicted at late times, suggesting that sulfate reduction rates increased over the duration of the column experiment and that microbial growth kinetics may have played an important role. These modeling results suggest that the reactivity of the organic carbon treatment material with respect to sulfate reduction does not significantly decrease over the duration of the 14-month experiments. The ability of the columns to remove ferrous iron appears to be strongly influenced by the precipitation of siderite, which is enhanced by the dissolution of calcite. The simulations indicate that while calcite was available in the column, up to 0.02 mol L(-1) of ferrous iron was removed from solution as siderite and mackinawite. Later in the experiments after approximately 300 d, when calcite was depleted from the columns, mackinawite became the predominant iron sink. The ability of the column to remove ferrous iron as mackinawite was estimated to be approximately 0.005 mol L(-1) for column 1. As the precipitation of mackinawite is sulfide limited at later times, the amount of iron removed will ultimately depend on the reactivity of the organic mixture and the amount of sulfate reduced.

Microcosm studies for neutralization of hypolimnic acid mine pit lake water (pH 2.6).

Ten microcosms of 0.088 m3 water volume (0.3 m i.d. and 1.20 m height) were designed for neutralization studies representing hypolimnic ecosystem models for acid mine pit lakes. Sediment and water were collected from an acid lignite mine pit lake (Brandenburg, Germany) and filled into the microcosms. To determine the efficacy of controlled in situ organic carbon amendments as a possible neutralization method, sediment and water were treated with ethanol and Carbokalk with and without wheat straw. The water chemistry was monitored for 1 yr. At start-up and end of the experiments, the sedimentwas characterized. Iron and sulfate were removed with varying intensity from the water phase as a result of microbial iron and sulfate reduction together with a subsequent precipitation of unsoluble sulfide minerals to the sediment. The pH rose, and alkalinity generation and bacterial growth were observed. Neutralization rates were calculated using equivalents of accumulated total reduced inorganic sulfur together with the nonsulfidic reactive ferrous iron in the sediment. In the treated microcosms, the neutralization rates were between 6 and 15 equiv m(-2) a(-1). Carbokalk was most effective in stimulating growth of sulfate-reducing bacteria and probably also served as inoculum. With Carbokalk together with wheat straw, the pH increased from 2.6 to around 6.5 within the whole microcosm. The critical revision of the results indicates that the application of Carbokalk (approximately 3.9 kg m(-2)) together with the application of wheat straw (approximately 9.3 kg m(-2)) is most suitable for further experiments in outdoor enclosures (mesocosms). For that case, the prediction of the water quality for a lake water column after multiple lake turnover events is presented based on batch reaction simulation using the geochemical model PHREEQC.

Sediment Conditioning with Organic and/or Inorganic Carbon Sources as a First Step in Alkalinity Generation of Acid Mine Pit Lake Water (pH 2−3)

Laboratory scale microcosm studies were conducted to determine the efficacy of controlled in situ saprobization of lake sediments as one opportunity for deacidification of artificial shallow lakes resulting from open cast lignite mining located in southeast Germany. Under lasting anoxic conditions iron and sulfate were removed from the lake water as a result of microbial iron- and sulfate reduction together with a subsequent precipitation of insoluble sulfide minerals to the lake sediment. The 2 L closed system microcosms were made up of a model of the sediment/water interface. They were filled with lake water and sediment and treated subsequently with different organic and/or inorganic carbon sources in combination with wheat straw. Just the water was characterized initially and finally 8 weeks later. The rise of pH together with acidity consumption was observed. In the microcosms treated with wheat straw and pyruvate, ethanol, Carbokalk, or Pfezi-granula the pH rose to 5.5 and 7.0, respectively, and iron- and sulfate reduction were observed. With wheat straw and ethanol or Pfezi-granula sulfate concentrations decreased from initial concentrations of 11.5−13.5 mmol L-1 to final concentrations of < 3 mmol L-1. However, the iron concentration decreased significantly to a value of 0.01 mmol L-1 exclusively with Pfezi-granula and wheat straw. For the most reactive microcosms geochemical equilibrium calculations suggest precipitation of different sulfide minerals. The critical revision of the results obtained from this study indicate that ethanol or Carbokalk together with wheat straw are suitable for further upscaling in larger-scale microcosms.

Brazil's premier gold province. Part II: geology and genesis of gold deposits in the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero

Orogenic, gold deposits are hosted by rocks of the Archean Rio das Velhas greenstone belt in the Quadrilatero Ferrifero region, Minas Gerais state, Brazil, one of the major gold provinces in the world. The gold deposits occur at the base of the mafic–ultramafic succession, with the most important orebodies controlled by E–W-striking, strike-slip faults. The main mineralization styles are (1) structurally controlled, sulfide replacement zones in banded iron formation (BIF); (2) disseminated sulfide minerals and gold in hydrothermally altered rocks along shear zones; and (3) auriferous quartz–carbonate–sulfide veins and veinlets in mafic, ultramafic, and felsic volcanic rocks, and also in clastic sedimentary rocks. The most common host rocks for ore are metamorphosed oxide- and carbonate-facies banded iron (± iron-rich metachert) formations (e.g., the Cuiaba, Sao Bento and Raposos deposits) and the lapa seca unit, which is a local term for intensely carbonatized rock (e.g., the giant Morro Velho mine with >450 t of contained gold). Metabasalts host most of the remaining gold deposits. Mineralogical characteristics and fluid inclusion studies suggest variations in the H2O/CO2 ratio of a low-salinity, near-neutral, reducing, sulfur-bearing, ore fluid. The presence of abundant CH4-rich inclusions is related to reduction of the original H2O–CO2 fluid via interaction with carbonaceous matter in the wallrocks. Oxygen fugacity was close to that of graphite saturation, with variations likely to have been influenced by reaction with the carbonaceous matter. Carbon-rich phyllites and schists, which commonly bound ore-bearing horizons, seem to have played both a physical and chemical role in localizing hydrothermal mineral deposition. Microtextural studies indicate that gold deposition was mainly related to desulfidation reactions, and was paragenetically coeval with precipitation of arsenic-rich iron sulfide minerals. Carbon isotope data are compatible with dissolution of CO2 from pre-existing mantle-derived carbonation zones, and indicate fluids of metamorphic origin. A major episode of hydrothermal fluid introduction into different rock types caused epigenetic gold formation and wallrock alteration at about 300 to 400 °C during the late stages of regional deformation and metamorphism of the greenstone belt. The age of gold mineralization is constrained to be younger than 2,698 and perhaps closer to 2,670 Ma.

Geology and fluid chemistry of the Fushime geothermal field, Kyushu, Japan

The Fushime geothermal field is located in a depression close to the coast line. The system is characterized by very high reservoir temperature (>350°C), and a high salinity production fluid. Geological analysis shows that the main reservoir in this field occurs in a fractured zone developed around a dacite intrusion located in the center of the field. High permeability zones recognized by drilling data are found to be associated with fault zones. One of these zones is clearly associated with a NW–SE trending andesite dike swarm which was encountered in some wells. Alteration in the system can be divided into four zones, in order of increasing temperature, based on calcium–magnesium aluminosilicate mineral assemblages: i.e., the smectite, transition, chlorite and epidote zones. The feed zone is located in the chlorite and epidote zones, which can be further divided into three sub-zones according to their potassium or sodium aluminosilicate mineralogy, from the center of the discharge zone: K-feldspar–quartz, sericite–quartz, and albite–chlorite zones. Chloride concentration of the sea-water is 19,800 mg/l, and Br/Cl mole ratio is 1.55. Based on geochemical information, the reservoir chloride concentration of this field ranges from 11,600 to 22,000 mg/kg. The Clres (Cl in reservoir), Br/Cl ratios and stable isotope data indicate that the Fushime geothermal fluid originated from sea-water and is diluted by ground water during its ascent. Some fluids produced from geothermal wells show low pH (about 4). It is thought that sulfide mineral (PbS, ZnS) precipitation during production produces this acidic fluid.

Stable isotopic studies of mafic sills and proterozoic metasedimentary rocks located beneath the Duluth Complex, Minnesota

Thin (3–20 m), picritic to noritic sills are found in the Proterozoic metasedimentary footwall rocks of the Duluth Complex, Minnesota. The sills were emplaced prior to the major intrusions that comprise the Duluth Complex, and underwent contact metamorphism along with the country rocks. Oxygen, hydrogen, and sulfur isotopic compositions of the sills indicate a varied history of isotopic exchange between minerals, melts, and hydrothermal fluids in the high temperature environment below the major plutonic bodies of the Midcontinent Rift system. The pre-Duluth Complex sills exhibit a range in δ 18O values from 4.9‰ to 14.8‰, with values between 6‰ and 7‰ generally found in sill interiors. High δ 18O values near sill contacts with high 18O metasedimentary rocks of the pelitic Virginia Formation or Biwabik Iron Formation, coupled with a smooth sigmoidal isotopic profile centered at the contact, suggest that oxygen diffusion was an important exchange mechanism. The elevated δ 18O values near the center of the thickest sills are thought to reflect the emplacement of isotopically contaminated basaltic magma. Dehydration reactions in the pelitic rocks of the contact aureole liberated high 18O fluids that enhanced subsolidus diffusive exchange. Advective displacement of the diffusion profiles toward the sill interior is less than 40 cm, and suggests that layer parallel flow dominated in the dehydration of the contact aureole. Elevated, but uniform δ 18O values (9.7‰ to 10.5‰) in thinner sills suggests that oxygen diffusivity was increased relative to country rocks due to enhanced porosity, perhaps related to extensive development of microcracks. Although δD values of the pelitic country rocks record a history of dehydration, systematic variations of δD (−64‰ to −143‰) and H 2O (0.15 to 5.40 wt.%) content are not found in the sills. δ 18O values of coexisting plagioclase and pyroxene from the sills indicate a close approach to isotopic equilibrium, and are consistent with a diffusion-dominant exchange process at temperatures near 500°C. Results of diffusion modelling suggest a duration of isotopic exchange that may have extended from tens of thousands of years to 1.4 Ma, depending on local controls of porosity and permeability, as well as rates of fluid production in the contact aureole. Localized areas of 18O and D depletion in the sills (values as low as 4.9‰ and −143‰, respectively) denote exchange with meteoric water after interaction with the high 18O metamorphic fluids in the contact aureole. Although all of the elevated 18O samples in the contact environment may have suffered 18O depletion, most exchange with meteoric water appears to be spatially localized, and is thought to reflect highly channelized, fracture-controlled fluid flow. Sulfur isotopic values of the sills are variable (−2.7‰ to 11.2‰), and indicative of an evolution involving pre-emplacement contamination of basaltic magma, and sub-solidus exchange with an H 2S-bearing metamorphic fluid. Sulfur contents exceed 3.0 wt.% only within troctolitic to melatroctolitic sills, and δ 34S values of 7.8‰ to 8.3‰ are strongly suggestive of pre-emplacement contamination by sulfur derived from a Proterozoic sedimentary unit. High δ 18O and δ 34S rocks, particularly at sill margins, are consistent with either hydrothermal precipitation of fine-grained sulfide minerals, or isotopic exchange between magmatic sulfides and an H 2S-bearing metamorphic fluid.

Influence of an igneous intrusion on the inorganic geochemistry of a bituminous coal from Pitkin County, Colorado

Although the effects of igneous dikes on the organic matter in coal have been observed at many localities there is virtually no information on the effects of the intrusions on the inorganic constituents in the coal. Such a study may help to elucidate the behavior of trace elements during in situ gasification of coal and may provide insights into the resource potential of coal and coke affected by the intrusion. To determine the effects of an igneous intrusion on the inorganic chemistry of a coal we used a series of 11 samples of coal and natural coke that had been collected at intervals from 3 to 106 cm from a dike that intruded the bituminous Dutch Creek coal in Pitkin, CO. The samples were chemically analyzed for 66 elements. SEM-EDX and X-ray diffraction analysis were performed on selected samples. Volatile elements such as F, Cl, Hg, and Se are not depleted in the samples (coke and coal) nearest the dike that were exposed to the highest temperatures. Their presence in these samples is likely due to secondary enrichment following volatilization of the elements inherent in the coal. Equilibration with ground water may account for the uniform distribution of Na, B, and Cl. High concentrations of Ca, Mg, Fe, Mn, Sr, and CO 2 in the coke region are attributed to the reaction of CO and CO 2 generated during the coking of the coal with fluids from the intrusion, resulting in the precipitation of carbonates. Similarly, precipitation of sulfide minerals in the coke zone may account for the relatively high concentrations of Ag, Hg, Cu, Zn, and Fe. Most elements are concentrated at the juncture of the fluidized coke and the thermally metamorphosed coal. Many of the elements enriched in this region (for example, Ga, Ge, Mo, Rb, U, La, Ce, Al, K, and Si) may have been adsorbed on either the clays or the organic matter or on both.

Fluid inclusions of the Ilkwang Cu−W-bearing breccia-pipe deposit, Kyongsang Basin

The copper-tungsten mineralization of Ilkwang mine, Kyongsang Basin was associated with crystallization of a small stock of granodiorite and quartz monzonite, explosive brecciation, and exsolution of magmatic hydrothermal fluids. Fluid inclusions in quartz phenocrysts of granitic rocks and hydrothermal quartz in breccia matrix trapped aqueous fluids that had caused brecciation and precipitation of coppertungsten ore mineals. Four groups of fluid inclusions are identified: Type I (vapor-rich and low salinity), Type II (liquidrich and low to moderate salinity), Types IIIa and IIIb (liquidrich and high salinity) and Type IV (CO2-bearing, vapor-rich, and low salinity). The earliest high-saline fluid (Type IIIa) and the following CO2-bearing fluid (Type IV) were exsolved from crystallizing melt and trapped at relatively high pressure (> 1 kbar). As water was partitioned from melt to aqueous fluid, the volume expansion caused overpressuring of the magmatic hydrothermal system and led to an explosive hydrothermal brecciation. The pressure quenching related to the brecciation was followed by crystallization of the matrix producing early-hydrothermal quartz and tourmaline. Additionally, a subsequent decrease in pressure produced boiling of aqueous fluid, generating Types I and IIIb inclusions. They were associated with sulfide-mineral precipitation at 493–343°C and 450–120 bars. The separation of vapor phase including acid volatile species CO2 from the liquid phase might have raised pH of the hydrothermal fluid, which was favorable for the formation of sulfides. The late-magmatic, high-salinity fluids (Type IIIb) were mixed with circulating meteoric fluids and then were cooled and diluted to form lower-temperature and lower-salinity hydrothermal fluids that were trapped as inclusions during the final stage of hydrothermal activity. The fluid-inclusion data are comparable with those from breccia-pipe deposits of porphyry-copper systems. It is suggested that hydrothermal activities and favorable structure factors in the Ilkwang mining district played a role for the generation of favorable P-T conditions and helped provide sufficient energy to form a breccia zone.

Evaluation of proposed precipitation mechanisms for Mississippi Valley-type deposits

The mechanism of precipitation is an important aspect of any genetic model for Mississippi Valley-type deposits. Yet most of the precipitation mechanisms for minerals in the Mississippi Valley-type association have serious flaws. Solution mixing would require an unlikely series of solutions to account for the various minerals in the ores, and it does not account for the universal occurrence of organic matter in the ores nor for the oxidation state of sulfur in pyrite in the ores. Sulfate reduction addresses some of these problems, but is inconsistent with kinetic data and could not be reversed to account for the oscillations between precipitation and dissolution of sulfide minerals in the ores. Carbon dioxide effervescence does not address the precipitation of most minerals in the ores, and all of the evidence for effervescence may be explained in other ways. Cooling of the mineralizing solution could precipitate many minerals, but fluid inclusion data suggest that, in many deposits, the solution did not cool significantly as any particular stage formed. A credible genetic model also must explain why all of the minerals precipitated at the same sites; any combination of the above mechanisms which suggests that unrelated mechanisms occurred at the same sites by coincidence is unlikely. The most reasonable scenario is that a hot, thiosulfate-bearing mineralizing solution reacted in various ways with organic matter at the sites of mineralization to precipitate the ore minerals. The organic matter acted as a reductant, source of carbon dioxide, source of organic acids, and a substrate for bacterial metabolism of thiosulfate in various stages of mineralization. Thus organic matter links all stages of the mineralization to the same sites.

Effects of mineral precipitation on the sulfur isotope composition of hydrothermal solutions

The effect of precipitation of mineral phases on the sulfur isotopic composition of hydrothermal minerals is quantitatively evaluated in terms of a Rayleigh distillation process. The precipitation of either sulfate or sulfide minerals is considered as the removal mechanism for dissolved sulfur from a finite solution reservoir. The isotopic composition of sulfur in hydrothermal minerals is strongly controlled by the fraction of sulfur remaining in the solution as well as by the relative proportions of oxidized to reduced sulfur compounds. When sulfate is precipitated from a SO 2− 4-dominant solution with minor amounts of H 2S, its δ 34S-value can change, with time, from slightly greater than that of the original solution in early stages to significantly lower than that of the original solution in late stages. Conversely, when sulfide minerals are deposited from a H 2S-dominant solution with minor amounts of SO 2− 4, their δ 34S-values may resemble the δ 34S-value of the original solution in early stages, but may change to values remarkably higher than that of the original solution in late stages. Sulfur and oxygen isotopic data for barite from the Bad Grund Pb-Zn deposit in the Harz Mountains, Germany, illustrate the application of the reservoir effect, and a few barites with low δ 34S-values are interpreted to represent the product of precipitation of the gangue mineral in late stages.

Fluid inclusion gas chemistry in east Tennessee Mississippi Valley-type districts: Evidence for immiscibility and implications for depositional mechanisms

Analyses of fluid inclusion gases from Mississippi Valley-type districts in east Tennessee reveal the presence of several distinct aqueous solutions and vapors that were part of the mineralizing process. Inclusion contents were released by crushing 5 to 25 mg mineral samples and by decrepitating individual inclusions; all analyses were obtained by quadrupole mass spectrometry. Most analyzed inclusion fluids consist of H 2O with significant amounts of CH 4 (0.3 to 2.9 mol%), CO 2 (0.1 to 4.7 mol%), and smaller amounts of C 2H 6, C 3H 8, H 2S, SO 2, N 2, and Ar. In general, inclusion gas abundances are greatest for sphalerite from the Mascot-Jefferson City district, lower for the Sweetwater district, and lowest for the Copper Ridge district. Compositional similarities in the inclusion fluids from the three districts imply that mineralization probably formed from fluids that permeated the entire region, rather than from completely separate fluids at each site. Saturation pressures calculated for these fluid compositions range from 300 to 2200 bars. Burial depths for the host unit have been estimated to be about 2 to 3 km in the east Tennessee area during Devonian time, the age of mineralization indicated by recent isotopic ages. Pressures at these depths, whether hydrostatic or lithostatic, would not have been adequate to prevent phase separation. Thus, our gas analyses represent either a mixture of vapor-rich and liquid-rich inclusions, or liquid-rich inclusions that trapped excess vapor. A lack of visible vapor-rich inclusions, high gas contents in individual fluid inclusion gas analyses obtained by decrepitation, and a positive correlation between decrepitation temperature and gas content for individual inclusions strongly suggest that the samples contain liquid-rich inclusions that trapped varying amounts of excess vapor. This excess gas probably accounts for the anomalously high homogenization temperatures and the wide range of homogenization temperatures observed in fluid inclusions in these ores. The vapor phase could have formed either by phase separation resulting from over-pressured aqueous fluids migrating into a region of hydrostatic pressure, or by incorporation of a pre-existing gas cap at the sites of deposition into the invading aqueous fluid. Exsolution of a vapor phase from the mineralizing brines should cause precipitation of carbonate and sulfide minerals, but reaction path modelling indicates that the resulting sparry dolomite:sphalerite ratios would be too high to form an ore-grade deposit. On the other hand, if the vapor phase was from a pre-existing sour gas cap that was intercepted by a Zn-rich brine, large amounts of sphalerite would precipitate in a fairly small region. Preliminary mass balance calculations suggest that a gas cap of dimensions similar to the individual districts in east Tennessee could have contained enough H 2S to account for the total amount of sphalerite precipitated.

Cap Rock Geology of the Hazlehurst Salt Dome, Copiah County, Mississippi

ABSTRACT Hazlehurst Dome is a shallow, sparsely explored, salt dome near the center of the Mississippi Interior Salt Basin. Two recent sulfur exploration wells cored marine false carbonate cap rock, true calcite cap rock, and anhydrite cap rock. The marine false carbonate cap rock contains calcite, dolomite, glauconite, and marine microfossils. This unusual dolomite-enriched unit has a 13C value of -8.2(PDB), suggesting a predominantly marine source for the carbon. The true calcite cap rock contains alternating light and dark bands of calcite, the darker bands containing locally abundant sulfide minerals. The range of carbon isotope values for the banded true calcite cap rock, from -45.1 to -50.20(PDB), indicates that methane was the principal source of carbon. This cap rock is locally crosscut by steeply dipping calcite veins, which are symmetrically banded, with alternating bands of dark-brown calcite (containing fluid inclusions of liquid hydrocarbons), white calcite, and sulfide minerals. Carbon isotope values for the veins are similar to the true calcite cap rock itself. An exotic suite of cap rock minerals, consisting of sphalerite, galena, strontianite, celestite, and barite, is locally present in both the horizontally banded cap rock and its veins. The proximity of Hazlehurst Dome to present-day oil field brines enriched in Sr-Ba-Zn-Pb suggests a genetic link between the brines and cap rock minerals containing the same elemental suite. Carbon isotope data from calcite in both true calcite cap rock and veins indicate that sulfate-reducing bacteria played a major role in the precipitation of the calcite and sulfide minerals. The presence of liquid hydrocarbons in the late-stage calcite veins implies that liquid hydrocarbons moved into the cap rock late in its developmental history, and sulfate-reducing bacteria preferentially utilized methane, thus preserving the liquid hydrocarbons. This initial study of the Hazlehurst calcite cap rock suggests that the dome warrants further evaluation for hydrocarbon potential. The anhydrite cap rock has yet to be completely penetrated and only the upper section was sampled by the sulfur exploration wells. The anhydrite cap rock contains as much as 3% sulfide minerals crudely layered in the interlocking anhydrite matrix.

Sulphur isotopic fractionation between sulphate and sulphide in hydrothermal ore deposits: disequilibrium vs equilibrium processes

ABSTRACTCorrelative fractionation relationships of sulphur isotope data for coexisting sulphate and sulphide pairs from hydrothermal ore deposits on δ38S versus Δ34S diagrams are deciphered theoretically. Taking into account dissolved H2S and SO42‐ in hydrothermal fluids during precipitation of both sulphate and sulphide minerals, a 4‐species closed system is suggested for describing the conservation of mass among all sulphur‐bearing species on the δ‐Δ diagrams. The covariation in the δ34S values of both sulphate and sulphide is ascribed to isotopic exchange between oxidized and reduced sulphur species during mineral precipitation. The isotopic exchange could be a thermodynamic process due to simple cooling of high temperature fluids, which results in an equilibrium fractionation, or a kinetic process due to mixing of two sulphur reservoirs, which leads to a disequilibrium fractionation. The δ34S value of total sulphur in a hydrothermal system could change due to the precipitation of minerals, or due to the escape of H2S and/or SO42‐. Sulphur isotope data for anhydrite and pyrite pairs from the Luohe porphyrite iron deposit in the Yangtze River Valley is used to illustrate the mixing responsible for the disequilibrium fractionation.

Solubility of gold in NaCl-and H 2S-bearing aqueous solutions at 250–350°C

A total of 108 silica capsule experiments were performed in order to determine the solubility of gold in aqueous solutions containing both NaCl and H 2S at temperatures of 250, 300, and 350°C, at pressures dictated by vapor/liquid coexistence. The starting materials in the capsules were H 2 O + S° + gold wire + NaCl (0 to 4 m) ± Na 2 SO 4. The pH, a H 2 S ( aq), aH 2( aq), ƒ H 2(g) ,ƒ O 2(g) , and ƒ S 2(g) values of the solutions were controlled by the sulfur hydrolysis reaction (4 S ° + 4 H 2 O(1) = 3 H 2 S( aq) + HSO − 4 + H +) and the sulfide/sulfate reaction. The quenched run products were analyzed for Au (0.1 to 66 ppm), ΣH 2S, ΣSO 2− 4, and pH. The calculated solution compositions at 250–350°C fall in the following ranges: pH = 1.9 to 5.0, log aH 2 S( aq) = −2.0 to −0.7, log a HSO 4 − = −3.8 to −1.4 , and log a H 2( aq) = −7-0 to −4.7. The results of our experiments indicate that gold solubility is independent of the activity of Cl − and H + in the solutions, indicating that chloride complexes are not important. The gold solubility, however, increases with increasing H 2S(aq) activity, indicating that gold dissolved largely as a bisulfide complex according to the reaction: Au(s) + 2H 2S(aq) = HAu(HS) 0 2 + 1 2 H 2(aq) . The equilibrium constant determined from our experimental data for the above reaction is constant over a temperature range of 250 to 350°C at log K = −5.1 ± 0.3. The above equilibrium constant, together with that for a reaction involving Au(HS) − 2 obtained by Shenberger and Barnes (1989), determines the dissociation constant of HAu (HS) 0 2: HAu( HS) 0 2 = H + + Au( HS) − 2. The log K value becomes −5.3 ± 0.5 at 250°C, −5.6 ± 0.6 at 300°C, and −6.2 ± 0.6 at 350°C. Therefore, HAu(HS) 0 2 is the dominant gold-sulfide species at pH below about 5.5, while Au(HS) − 2 becomes dominant at higher pH conditions. The results from our study suggest that the solubility of gold in ore-forming fluids in equilibrium with pyrite and/or pyrrhotite at 250–350°C is typically between 0.1 ppb to 1 ppm Au, transported mostly as bisulfide complexes; gold-chloride complexes do not become important unless the fluid is H 2S-poor (e.g., <10 −4 m H 2S(aq) at 250°C), chloride-rich (>0.5 m 2Cl), and of low pH (<4.5). Precipitation of gold from ore-forming solutions may occur by increasing a H 2 ( aq), such as by reactions with organic matter or ferrous-bearing minerals, or by decreasing a H 2 S( aq), such as by precipitation of sulfide minerals or by mixing of H 2S-poor fluids. Simple cooling or heating of fluids without changing the H 2(aq) and H 2S(aq) contents is not an effective mechanism of gold precipitation when gold is transported largely as a bisulfide complex. Effects of oxidation or of boiling on gold precipitation, however, cannot be easily evaluated from the available thermodynamic data alone.

Sulfur enrichment of humic substances in a Delaware salt marsh sediment core

Humic sulfur, operationally defined as the sulfur extracted with humic substances in 0. l N NaOH solution, comprises up to 51% of the total sulfur inventory in a sediment core taken from a Delaware Spartina alterniflora marsh. Pyrite sulfur is the next largest fraction, except at the near-surface sediments, where zerovalent sulfur concentrations are significant. X-ray photoelectron spectroscopy indicates that the humic sulfur consists of sulfoxides or sulfones and, in a more reduced state, organic sulfides and/or organic polysulfides. A subsurface decrease in the humic acid C:S atomic ratio to 56 ± 2 suggests that the upper 4 cm of marsh sediment is the locus for humic sulfur formation. S. alterniflora detritus and microbial biomass cannot fully account for observed sulfur enrichment of humic C:S atomic ratios. Therefore, the enrichment of humic substances by sulfur is probably via reaction of reduced sulfur compounds with organic matter. A humic sulfur formation rate of 10.6 μmol S · cm −3 · a −1 is calculated for the surface sediments and leads to an areal production of 18 μmol S · cm −2 · a −1 of humic sulfur. Humic sulfur formation and preservation is enhanced by the limited availability of iron for the rapid precipitation of iron sulfide minerals and the apparent resistance of organic sulfur compounds towards reoxidation to sulfate, especially in the upper 9 cm of marsh sediment where inorganic sulfur compounds are rapidly oxidized.

Sulfur isotopes and paragenesis of sulfide minerals in the Silurian Waldron Shale, southern Indiana

A diverse assemblage of sulfide morphologies and a wide range of sulfur-isotope compositions indicate that precipitation of sulfide minerals was an ongoing process during both early and later stages of diagenesis in the Silurian Waldron Shale in Indiana. Five distinct forms of sulfide minerals have been identified in samples from seven locations in southern Indiana: (1) pyritized brachiopod shells, (2) pyritized burrows, (3) pyrite cubes, (4) prismatic marcasite, and (5) cockscomb marcasite. Petrographic evidence is used to determine a probable sequence of sulfide precipitation. Isotope compositions range from about -40‰ to +40‰, and each sulfide morphology exhibits a distinctive range of δ34S values. These results are consistent with a model in which sulfate reduction and subsequent precipitation of sulfide minerals occurred in a system that became progressively more closed with respect to sulfate. The sequence of sulfide mineralization reflects the chemical evolution of pore waters during diagenesis of the Waldron Shale. Redox conditions in the sediment were initially aerobic to dysaerobic, sulfate reduction occurring only in reducing microniches where brachiopod shells and then organic-walled burrows were pyritized. Sediments became more fully reducing with burial, when euhedral sulfides were formed. The transition from pyrite to marcasite precipitation suggests a decrease in pore-water pH during later diagenesis and may also indicate fluctuations in redox conditions.

Geochemical Modeling of Scale Formation, and Formation Damage during Production from Sulfate and Carbonate Mineral-Bearing Reservoirs

The physical and chemical processes that affect reservoir fluids during production can be modeled by methodologies similar to those used for modeling clastic diagenesis. That these processes may result in formation damage and scale formation make them of interest to production geologists and engineers. Pathway modeling, based upon a series of critical divides, predicts which reactions are likely to occur between formation, production tubing, and reservoir fluids. Thermodynamic equilibria modeling calculates direction and magnitude of possible reactions. Integration of these approaches with observations of patterns of scale formation, production line, and formation damage yield a model capable of predicting the magnitude and direction of reactions that may produce negative impacts on reservoir production. Critical divides characterizing these processes in carbonate and sulfate mineral-bearing reservoirs include: (1) presence or absence of sulfate-bearing minerals within the production volume; (2) presence of iron within production line or formation; (3) ratio of concentration of bicarbonate to hydrogen sulfide; (4) capacity of aqueous and solid phases to buffer formation fluid pH; and (5) magnitude of pressure and temperature drops during production. The model qualitatively predicts: (1) likelihood of sulfide, sulfate, or carbonate mineral precipitation during production; (2) souring of the reservoir; and (3) corrosion ofmore » production tubing. The model has been developed from production histories for Weber Sandstone reservoirs, Colorado and Wyoming, and has been applied to examples of reservoir production from Tensleep and Minnelusa reservoirs in Wyoming.« less