Chronicles of vadose zone diagenesis: cone‐shaped iron oxide concretions,Triassic Trujillo Formation,Palo Duro Canyon,Texas

Unusual cone‐shaped iron oxide concretions occur in the Late Triassic, lower fluvial sandstone member of the Trujillo Formation at Palo Duro Canyon in the Texas panhandle. In situ concretions are significant because they record both historical information about past processes that occurred within the geologic unit and present‐day information about the ability of the unit to conduct fluids. The dominant orientation of the concretions is cone‐apex up, body radiating down and out, with long axis perpendicular to bedding. Concretion morphologies are associated with the sedimentary texture and primary bedding structure of the host rock and the corresponding hydrologic regime (i.e. advection versus dispersion for iron‐transport behavior). Three lithofacies in the lower Trujillo member exhibit different cone forms. Field observations of cone orientation and morphology suggest vadose conditions for diagenetic precipitation of iron oxide cements, with timing potentially represented by the major pre‐Miocene unconformity.     

Fracture‐focused fluid flow in an acid and redox‐influenced system: diagenetic controls on cement mineralogy and geomorphology in the Navajo Sandstone

Differential cement mineralogy is influenced by depositional textures, structural deformation, pore fluid chemistry, and ultimately influences landscape evolution by introducing heterogeneities in erodibility. In Southern Utah, the region West of the Kaibab uplift known as Mollies Nipple (Mollies) in Grand Staircase‐Escalante National Monument exhibits a complex history of fluid–sediment interactions, which has resulted in a localized zone of anomalous diagenetic iron sulfate (jarosite) mineralogy in a well‐exposed dune–interdune deposit within the Navajo Sandstone. Mineralogy and geochemistry of cements within this region are examined using reflectance and imaging spectroscopy, field investigations, microscopy, and whole‐rock geochemical analyses. These data show that the in‐situ jarosite cement is localized to a plane along the highest ridge of the butte, providing an armor along with other secondary cements, which controls the butte's geomorphic evolution. The jarosite cement is associated with other mineralogies suggesting that the sulfate was one of the latest fluid‐related precipitates in the paragenetic sequence. It was preceded by a regional bleaching event, precipitation of clay cements, some localized concretionary iron oxide precipitation, and formation of deformation bands. At least one generation of dense iron oxide mineralization is associated with cataclastic brittle deformation predating the sulfate precipitation. Trace element geochemistry of cements shows certain metal oxide populations associated with extremely high (>2000 ppm) arsenic values. We interpret the combination of spatial mineral distribution, observed paragenetic sequence, and trace element geochemistry to suggest this region experienced acid sulfate diagenesis along fracture‐controlled fluid conduits related to weathering of proximal, unidentified, sulfides, or H₂S associated with deep source beds. Jarosite is highly soluble, and its presence suggests that abundant fluid flow has not occurred in this region since its formation. This terminal cement‐forming event must have occurred prior to sandstone exhumation and erosion to form the current extreme landscape at Mollies. This site exhibits the influence that fluid geochemistry, sedimentary mineralogy, and structural fabric have on geomorphic evolution.     

Precipitation patterns formed by self‐organizing processes in porous media

We compare laboratory and field examples of self‐organized mineral precipitates in porous media. Laboratory tests of silver chromate precipitation in glass beads and glass bead/gel mixtures produce structures such as periodic banding and mm‐size spherules. These are morphologically similar to the varied forms of iron oxide precipitates in the Jurassic Navajo Sandstone, Utah USA, that preserve records of former fluid redox boundaries in a porous and permeable sandstone. Experimental studies of periodic precipitates in porous media can provide valuable insight for understanding the diagenetic history of similar precipitates in natural environments.     

Iron precipitation in a natural CO2 reservoir: Jurassic Navajo Sandstone in the northern San Rafael Swell,UT, USA

Diagenetic iron (oxyhydr)oxide minerals are common precipitates expected in CO₂ reservoirs, and these minerals record fluid flow for application to carbon capture and sequestration (CSS). Multiple mineralogy and spectroscopy analyses on a pore to meter scale characterize a well‐exposed, lithologically controlled, iron (oxyhydr)oxides reaction front in the Jurassic Navajo Sandstone. Dolomite is the most prevalent cement (up to 23 area%), followed by only several percent or less of iron (oxyhydr)oxides, kaolinite, illite, and gypsum cements. Bulk geochemistry based on diagenetic mineralogies in the reaction front is compared with the water chemistry from nearby modern spring effluent emanating from the Navajo Sandstone to conclude that similar fluids (i.e., CO₂‐charged, saline waters containing hydrocarbons) are responsible for the precipitation. A second comparison of bulk geochemistry and diagenetic mineralogies of the reaction front with data from other abundant Navajo Sandstone concretions in the Spencer Flat region (in south central Utah) shows that reservoir fluids likely vary spatially and temporally in the porous and permeable Navajo Sandstone. CO₂ injection into porous and permeable, quartz arenite, saline aquifers will likely result in minor clay and abundant dolomite precipitation that will significantly decrease porosity.     

Bleached mudstone,iron concretions,and calcite veins: a natural analogue for the effects of reducing CO2‐bearing fluids on migration and mineralization of iron,sealing properties,and composition of mudstone cap rocks

This study investigates the Wangfu Depression of the Songliao Basin, China, as a natural analogue site for Fe migration (bleaching) and mineralization (formation of iron concretions) caused by reducing CO₂‐bearing fluids that leak along fractures after carbon capture, utilization, and storage. We also examined the origin of fracture‐filling calcite veins, the properties of self‐sealing fluids, the influence of fluids on the compositions of mudstone and established a bleaching model for the study area. Our results show that iron concretions are the oxidative products of precursor minerals (pyrite and siderite) during uplift and are linked to H₂S and CO₂ present in early stage fluids. The precipitation of calcite veins is the result of CO₂ degassing and is related to CO₂, CH₄, and minor heavy hydrocarbons in the main bleaching fluids. In our model, fluids preferentially enter high‐permeability fracture systems and result in the bleaching of surrounding rocks and precipitation of calcite veins. The infilling of calcite veins significantly decreases the permeability of fractures and forces the fluids to slowly enter and bleach the mudstone rocks. The Fe²⁺ released during bleaching migrates to elsewhere with the solutions or is reprecipitated in the calcite veins and iron concretions. The formation of calcite veins reduces the fracture space and effectively prevents fluid flow. The fluids have an insignificant effect on minerals within the mudstone. In terms of the chemistry of the mudstone, only the contents of Fe₂O₃, U, and Mo change significantly, with the content of U increasing in the mudstone and the contents of Fe₂O₃ and Mo decreasing during bleaching.     

An exhumed palaeo-hydrocarbon migration fairway in a faulted carrier system, Entrada Sandstone of SE Utah, USA

The Moab Anticline, east‐central Utah, is an exhumed hydrocarbon palaeo‐reservoir which was supplied by hydrocarbons that migrated from the Moab Fault up‐dip towards the crest of the structure beneath the regional seal of the Tidwell mudstone. Iron oxide reduction in porous, high permeability aeolian sandstones records the secondary migration of hydrocarbons, filling of traps against small sealing faults and spill pathways through the Middle Jurassic Entrada Sandstone. Hydrocarbons entered the Entrada Sandstone carrier system from bends and other leak points on the Moab Fault producing discrete zones of reduction that extend for up to 400 m from these leak points. They then migrated in focused stringers, 2–5 m in height, to produce accumulations on the crest of the anticline. Normal faults on the anticline were transient permeability barriers to hydrocarbon migration producing a series of small compartmentalized accumulations. Exsolution of CO₂ as local fault seals were breached resulted in calcite cementation on the up‐dip side of faults. Field observations on the distribution of iron oxide reduction and calcite cements within the anticline indicate that the advancing reduction fronts were affected neither by individual slip bands in damage zones around faults nor by small faults with sand: sand juxtapositions. Faults with larger throws produced either sand: mudstone juxtapositions or sand: sand contacts and fault zones with shale smears. Shale‐smeared fault zones provided seals to the reducing fluid which filled the structural traps to spill points.     

Evolution of deformation and fault‐related fluid flow within an ancient methane seep system (Eocene,Varna, Bulgaria)

Faults are often important in fuelling methane seep systems; however, little is known on how different components in fault zones control subsurface fluid circulation paths and how they evolve through time. This study provides insight into fault‐related fluid flow systems that operated in the shallow subsurface of an ancient methane seep system. The Pobiti Kamani area (NE Bulgaria) encloses a well‐exposed, fault‐related seep system in unconsolidated Lower Eocene sandy deposits of the Dikilitash Formation. The Beloslav quarry and Beloslav N faults displace the Dikilitash Formation and are typified by broad, up to 80 m wide, preferentially lithified hanging wall damage zones, crosscut by deformation bands and deformation band zones, smaller slip planes and fault‐related joints. The formation of a shallow plumbing system and chimney‐like concretions in the Dikilitash Formation was followed by at least two phases of fault‐related methane fluid migration. Widespread fluid circulation through the Dikilitash sands caused massive cementation of the entire damage zones in the fault hanging walls. During this phase, paths of ascending methane fluids were locally obstructed by decimetre‐thick, continuous deformation band zones that developed in the partly lithified sands upon the onset of deformation. Once the entire damage zone was pervasively cemented, deformation proceeded through the formation of slip planes and joints. This created a new network of more localized conduits in close vicinity to the main fault plane and around through‐going slip planes. ¹³C‐depleted crustiform calcite cements in several joints record the last phase of focused methane fluid ascent. Their formation predated Neogene uplift and later meteoric water infiltration along the joint network. This illustrates how fault‐related fluid pathways evolved, over time, from ‘plumes’ in unconsolidated sediments above damage zones, leading to chimney fields, over widespread fluid paths, deflected by early deformation structures, to localized paths along fracture networks near the main fault.     

Microbiology of Concretions, Sediments and Mechanisms Influencing the Preservation of Submerged Archaeological Artifacts

Research conducted at deep-ocean ship wreck sites in the North Atlantic Ocean, the Mediterranean Sea and in the Gulf of Mexico have revealed that microorganisms play a significant role in both the preservation and loss of submerged artifacts. Research to be reported concentrates on the influence of microbiologically induced concretions on the survival of maritime artifacts. Rusticles, a common form of concretion, have been found to provide valuable information to archeology (Garzke et al. 1997, Proceedings of the Society of Naval Architects and Marine Engineers). The role of rusticles in the preservation of recalcitrant materials such as coal and glass fragments that become embedded within these growths, as well as forensic chemical analysis can be used to determine the nature of goods or munitions being carried by the ship at the time of its sinking, and will form examples.     

Iron mineralization and dolomitization in the Paran Fault zone,Israel: implications for low‐temperature basinal fluid processes near the Dead Sea Transform

Petrography, Eh‐pH calculations and the stable isotope composition of oxygen are used to interpret geochemical processes that occurred during iron oxide mineralization and dolomitization along the Menuha Ridge segment of the Paran Fault, southern Israel, adjacent to the Dead Sea Transform (DST). Iron mineralization is strongly localized in the fault zone as ferruginous lenses, whereas Fe dolomitization spreads laterally into the Cenomanian‐Turonian carbonate host rock as stratabound beds. The average oxygen isotope fractionation between syngenetic quartz and iron oxides in the ferruginous lenses gives a temperature of 50 ± 10°C and δ¹⁸O SMOW water = ?3.5‰; consistent with an origin from metalliferous groundwater flow in the sedimentary basin. Ferroan dolomite initially formed under strongly reducing conditions, but this was followed by oxidation and pseudomorphic replacement of the dolomite by a mesh of fine‐grained iron oxides (simple zoned dolomites). This cycle of ferroan dolomite formation and replacement by iron oxides was repeated in complex zoned dolomites. Dolomite oxygen isotope compositions fall into two groups: a high δ¹⁸O group corresponding to the simple zoned dolomites and non‐ferroan dolomites and a low δ¹⁸O group corresponding to the complex zoned dolomites. Water‐rock calculations suggest that the epignetic dolomites formed under fluid‐buffered conditions: the high δ¹⁸O group are indicated to have formed at temperatures of ca. 25°C for waters with δ¹⁸O = ?4 to 0‰; the low δ¹⁸O complex zoned dolomites at 50–75°C for waters with the same isotopic composition. A kinetic calculation for a complex zoned dolomite‐bearing bed indicates that dolomitization must have occurred at high values of the dolomite saturation index. This requirement for high Mg supersaturation and the indication that epigenetic dolomitization is more protracted in stratigraphically deeper formations located closer to the DST is consistent with models proposing that Mg‐rich solutions originated in the Dead Sea Rift.     

Fluid dynamics and fluid‐structural relationships in the Red Lake mine trend,Red Lake greenstone belt,Ontario, Canada

The Red Lake mine trend, a deformation zone in the Archean Red Lake greenstone belt that hosts the world‐class Campbell‐Red Lake gold deposit, is characterized by abundant foliation‐parallel iron‐carbonate ± quartz veins with banded colloform‐crustiform structures and cockade breccias overprinted by silicification and gold mineralization. There is an apparent incompatibility between the cavity‐fill structures of the veins and breccias (typically developed at shallow crustal depths) and the upper greenschist to lower amphibole facies metamorphic conditions recorded in the host rocks (indicating relatively deep environments). This, together with the development of veins along the foliation plane, represents an enigmatic problem that may be related to the interplay between fluid dynamics and stress field. We approach this problem through systematic study of fluid inclusion planes (FIPs) in the vein minerals, including the orientations of the FIPs and the pressure–temperature conditions inferred from fluid inclusion microthermometry. We find that fluid inclusions in the main stage vein minerals (pregold mineralization ankerite and quartz and syn‐ore quartz) are predominantly carbonic without a visible aqueous phase, whereas many inclusions in the postore stage contain an aqueous phase. Most FIPs are subvertical, and many are subparallel to the foliation. High fluid pressure coupled with the high wetting angles of the water‐poor, carbonic fluids may have been responsible for the abundance of brittle deformation features. The development of subvertical FIPs is interpreted to indicate episodic switching of the maximum principal compressive stress (σ₁) from subhorizontal (perpendicular to the foliation) to subvertical (parallel to the foliation) orientation. The subvertical σ₁ is favorable for the formation of foliation‐parallel veins, as fractures are preferentially opened along the foliation in such a stress regime, the origin of which may be linked to the fluid source.     

A hydrogeologic model of stratiform copper mineralization in the Midcontinent Rift System, Northern Michigan, USA

This paper presents a suite of two‐dimensional mathematical models of basin‐scale groundwater flow and heat transfer for the middle Proterozoic Midcontinent Rift System. The models were used to assess the hydrodynamic driving mechanisms responsible for main‐stage stratiform copper mineralization of the basal Nonesuch Formation during the post‐volcanic/pre‐compressional phase of basin evolution. Results suggest that compaction of the basal aquifer (Copper Harbor Formation), in response to mechanical loading during deposition of the overlying Freda Sandstone, generated a pulse of marginward‐directed, compaction‐driven discharge of cupriferous brines from within the basal aquifer. The timing of this pulse is consistent with the radiometric dates for the timing of mineralization. Thinning of the basal aquifer near White Pine, Michigan, enhanced stratiform copper mineralization. Focused upward leakage of copper‐laden brines into the lowermost facies of the pyrite‐rich Nonesuch Formation resulted in copper sulfide mineralization in response to a change in oxidation state. Economic‐grade mineralization within the White Pine ore district is a consequence of intense focusing of compaction‐driven discharge, and corresponding amplification of leakage into the basal Nonesuch Formation, where the basal aquifer thins dramatically atop the Porcupine Mountains volcanic structure. Equilibrium geochemical modeling and mass‐balance calculations support this conclusion. We also assessed whether topography and density‐driven flow systems could have caused ore genesis at White Pine. Topography‐driven flow associated with the Ottawan orogeny was discounted because it post‐dates main‐stage ore genesis and because recent seismic interpretations of basin inversion indicates that basin geometry would not be conductive to ore genesis. Density‐driven flow systems did not produce focused discharge in the vicinity of the White Pine ore district.     

Monitoring fluid chemistry in iron oxide–copper–gold‐related metasomatic processes,eastern Mt Isa Block,Australia

Most researchers in the Proterozoic eastern Mt Isa Block, NW Queensland, Australia, favour magmatic fluid and salt sources for sodic‐(calcic) alteration and iron oxide–copper–gold mineralization. Here we compare spatial, mineralogic and stable isotope data from regional alteration assemblages with magmatic and magmatic‐hydrothermal interface rocks in order to track chemical and isotopic variations in fluid composition away from inferred fluid sources. Tightly clustered δ¹⁸O values for magnetite, quartz, feldspar and actinolite for igneous‐hosted samples reflect high temperature equilibration in the magmatic‐hydrothermal environment. In contrast, these minerals record predominantly higher δ¹⁸O values in regional alteration and Cu–Au mineralization. This dichotomy reflects partial equilibration with isotopically heavier wallrocks and slightly lower temperatures. Increases in Si concentrations of metasomatic amphiboles relative to igneous amphiboles in part reflect cooling of metasomatic fluids away from igneous rocks. Variations in X_Mg for metasomatic amphiboles indicate local wallrock controls on amphibole chemistry, while variations in X_Cl/X_OH ratios for amphiboles (at constant X_Mg) indicate variable aH₂O/aHCl ratios for metasomatic fluids. Biotite geochemistry also reflects cooling and both increases and decreases in aH₂O/aHCl for fluids away from plutonic rocks. Decreased aH₂O/aHCl ratios for metasomatic fluids reflect in part scavenging of chlorine out of meta‐evaporite sequences, although this process requires already saline fluids. Local increases in aH₂O/aHCl ratios, as well as local decreases in δ¹⁸O values for some minerals (most notably haematite and epithermal‐textured quartz), may indicate ingress of low salinity, low δ¹⁸O fluids of possible meteoric origin late in the hydrothermal history of the region. Taken together, our observations are most consistent with predominantly magmatic sources for metasomatic fluids in the eastern Mt Isa Block, but record chemical and isotopic variations along fluid flow paths that may be important in explaining some of the diversity in alteration and mineralization styles in the district.     

DISCOVERY,PRODUCTION AND USE OF TIN‐BASED OPACIFIERS IN GLASSES,ENAMELS AND GLAZES FROM THE LATE IRON AGE ONWARDS: A REASSESSMENT*

Tin‐based opacifiers (lead stannate yellow and tin oxide white) were first used in glass production for a short period in Europe from the second to the first centuries bc , and then again throughout the Roman and Byzantine Empires from the fourth century ad onwards. Tin oxide was also used in the production of Islamic opaque glazes from the ninth century ad , and subsequently in enamels applied to Islamic and Venetian glasses from the 12th century ad onwards. A selection of published analytical data for the tin‐opacified glasses, enamels and glazes is summarized, and the methods used in their production are reassessed. The phase transformations occurring when mixtures of lead oxide, tin oxide and silica are fired are investigated with high temperature X‐ray diffraction (XRD) using a synchrotron radiation source, and these results are used to explain the observed differences in the glass, enamel and glaze compositions. Possible reasons for the use of tin‐based opacifiers in the second to first centuries bc , and for the switch from antimony‐ to tin‐based opacifiers in the fourth century ad are suggested, and the possible contexts in which tin‐based opacifiers might have been discovered are considered. The introduction of tin‐opacified glazes by Islamic potters in the ninth century ad is discussed in terms of technological transfer or independent invention.     

A Minimum-Impact Method for Measuring Corrosion Rate of Steel-Hulled Shipwrecks in Seawater

Current research on USS Arizona is focused on a minimum-impact technique for calculating corrosion rate of the battleship's steel hull by analysing physical and chemical properties of marine encrustation covering the exposed hull. An equation is derived that allows concretion thickness, density, and total iron content to be used to calculate corrosion rate of steel hull plate.
© 2006 The Authors     

The Roman wreck of c. AD 200 at Plemmirio, near Siracusa (Sicily): Interim report

A Roman wreck, named Plemmirio B^[1]i, lies on the rocky talus below the southern cliffs of Capo Murro di Porco, near Costa Bianca del Plemmirio (Penisola della Maddalena), prov. Siracusa, Sicily. The cargo of amphoras and iron bars, relatively well-preserved, is situated between 22 m and 47 m depth. Following brief examinations in 1974–1982, a University of Bristol expedition spent four weeks surveying the site in July-August 1983^[2] This paper presents an interim summary of these campaigns; a comprehensive report will be published after completion of excavations at the site^[3]. The archaeological deposit at Plemmirio B is characterized by concentrations of fragmented amphoras. There is no direct evidence for the structure of the ship, much of which may have disintegrated during the initial wreck process, but study of the artefact distribution suggests a vessel of fairly large capacity. The amphora consignment (which may have numbered no more than 200) comprised cylindrical African containers, 80% of which were form Africana 2A and 20% Africana 1^[4] w. The Africana 2A amphoras had internal resinous linings, and so may have contained a fish product rather than olive oil. Other amphoras exposed on the site were at least one Mauretanian Dressel 30 (Keay type 1) and two possibly intrusive amphoras of unclassified types. Ferrous concretions contained voids (hollow casts) which may once have been about 39 wrought iron bars, of two distinctive shapes. These, and several other concretions of unidentified forms, may represent a consignment of iron originally weighing approximately one tonne. Other finds from the wreck are two cooking pots, a small bowl or cup, a sounding lead, three fragments of tegula rooftiles, and two small stone blocks. One of the cooking pots confirms a date for the wreck based on the amphora association of circa AD 180–250, most likely in the first decade of the 3rd century. The amphora assemblage belongs to an important phase of commerce from North Africa represented by at least 20 known wreck cargoes; however, few of these sites have been scientifically recorded or are as closely dated as Plemmirio B, and the diversity of container types and cargo consignments on this wreck is of particular interest.     

Injection‐induced seismicity in Carbon and Emery Counties,central Utah

Utah is one of the top producers of oil and natural gas in the United States. Over the past 18 years, more than 4.2 billion gallons of wastewater from the petroleum industry has been injected into the Navajo Sandstone, Kayenta Formation, and Wingate Sandstone in Carbon and Emery Counties, central Utah, where seismicity has increased during the same period. Previous studies have attributed this seismicity to coal mining. Here, we present evidence for wastewater injection being a major cause of the increased seismicity. We show that, in the coal mining area, seismicity rate increased significantly 1–5 years following the wastewater injection, and the earthquakes, mostly with magnitudes <3.0, are concentrated in areas seismically active prior to the injection. Using simple analytical and numerical models, we show that the injection in central Utah can sufficiently raise pore pressure to trigger seismicity within 10–20 km of the injection wells, and the time needed for the diffusion of pore pressure may explain the observed lag of seismicity increase behind the commencement of injection. The b‐value of these earthquakes increased following the wastewater injection, which is consistent with these events being injection‐induced. We conclude that the marked increase in seismicity rate in central Utah is induced by both mining activity and wastewater injection, which raised pore pressure along preexisting faults.     

Vietnamese (15th Century) Blue‐And‐White,Tam Thai and Lustre Porcelains/Stonewares: Glaze Composition and Decoration Techniques*

EDS, X‐ray fluorescence, Raman spectroscopy, thermal expansion–shrinkage measurement and scanning electron microscopy were applied to determine the elemental components, structural phases and glazing temperatures of the transparent glazes, blue underglaze and overglaze tam thai (including gold‐like lustre) decorations from the 15th‐century Vietnamese porcelains/stonewares found at the Chu Ðâu–My Xa kiln site and in the Cù Lao Chàm (Hôi An) cargo. The ancient technology for colouring the glazes is discussed. The various blue tones in the underglazed décor result from cobalt‐containing manganese ore, with the intentional addition of iron oxide. The overglaze copper‐green and the gold‐like lustre were obtained by dispersing copper in lead‐based glass. The red colour was made using hematite dispersed in lead‐rich flux.     

Molecular transport of methane, ethane and nitrogen and the influence of diffusion on the chemical and isotopic composition of natural gas accumulations

Laboratory experiments have been performed to determine diffusion coefficients of natural gas components (methane, ethane and nitrogen) and isotope fractionation effects under simulated in situ pressure (up to 45 MPa effective stress) and temperature conditions (50–200°C) in water‐saturated pelitic and coarse‐grained rocks. Effective diffusion coefficients of molecular nitrogen (0.39 × 10^?11 to 21.6 × 10^?11 m² sec^?1 at 90°C) are higher than those for methane (0.18 × 10^?11 to 18.2 × 10^?11 m² sec^?1 at 90°C). Diffusive flux rates expressed in mass units are generally higher for N₂ than for CH₄. Both methane and (to a lesser extent) nitrogen diffusion coefficients decrease with increasing total organic carbon (TOC) content of the rock samples because of sorption processes on the organic matter. This effect decreases with increasing temperature. Effective diffusion coefficients increase upon a temperature increase from 50 to 200°C by a factor of four. Effective diffusion coefficients and steady‐state diffusive flux decrease with effective stress. Stationary diffusive fluxes drop by 50–70% for methane and 45–62% for nitrogen while effective diffusion coefficients are reduced by 38% (CH₄) and 32–48% (N₂), respectively. Isotope fractionation coefficients of diffusive transport are higher for methane (?1.56 and ?2.77‰) than for ethane (?0.84 and ?1.62‰). Application of the experimental results to geological systems show that diffusive transport has only a low transport efficiency. Significant depletion of natural gas reservoirs by molecular diffusion is only expected in cases of very poor caprock qualities (in terms of thickness and/or porosity) and over extended periods of geological time. Under these circumstances, the chemical and isotopic composition of a gas reservoir will change and maturity estimates based on these parameters may be deceptive. To account for these potential effects, nomograms have been developed to estimate diffusive losses and apply maturity corrections.     

A comprehensive review of hydrocarbons and genetic model of the sandstone‐hosted Dongsheng uranium deposit,Ordos Basin,China

The Dongsheng uranium deposit, the largest in situ leach uranium mine in the Ordos Basin, geometrically forms a roll‐front type deposit that is hosted in the Middle Jurassic Zhiluo Formation. The genesis of the mineralization, however, has long been a topic of great debate. Regional faults, epigenetic alterations in surface outcrops, natural oil seeps, and experimental findings support a reducing microenvironment during ore genesis. The bulk of the mineralization is coffinite. Based on thin‐section petrography, some of the coffinite is intimately intergrown with authigenic pyrite (ore‐stage pyrite) and is commonly juxtaposed with some late diagenetic sparry calcite (ore‐stage calcite) in primary pores, suggesting simultaneous precipitation. Measured homogenization temperatures of greater than 100°C from fluid inclusions indicate circulation of low‐temperature hydrothermal fluids in the ore zone. The carbon isotopic compositions of late calcite cement (δ¹³C_VPDB = ?31.0 to ?1.4‰) suggest that they were partly derived from sedimentary organic carbon, possibly from deep‐seated petroleum fluids emanating from nearby faults. Hydrogen and oxygen isotope data from kaolinite cement (δD = ?133 to ?116‰ and δ¹⁸O_SMOW = 12.6–13.8‰) indicate that the mineralizing fluids differed from magmatic and metamorphic fluids and were more depleted in D (²H) than modern regional meteoric waters. Such a strongly negative hydrogen isotopic signature suggests that there has been selective modification of δD by CH₄±H₂S±H₂ fluids. Ore‐stage pyrite lies within a very wide range of δ³⁴S (?39.2 to 26.9‰), suggesting that the pyrite has a complex origin and that bacterially mediated sulfate reduction cannot be precluded. Hydrocarbon migration and its role in uranium reduction and precipitation have here been unequivocally defined. Thus, a unifying model for uranium mineralization can be established: Early coupled bacterial uranium mineralization and hydrocarbon oxidation were followed by later recrystallization of ore phases in association with low‐temperature hydrothermal solutions under hydrocarbon‐induced reducing conditions.     

Fluid inclusion and stable isotope constraints on the genesis of the Jian copper deposit,Sanandaj–Sirjan metamorphic zone,Iran

The Jian copper deposit, located on the eastern edge of the Sanandaj–Sirjan metamorphic zone, southwest of Iran, is contained within the Surian Permo‐Triassic volcano‐sedimentary complex. Retrograde metamorphism resulted in three stages of mineralization (quartz ± sulfide veins) during exhumation of the Surian metamorphic complex (Middle Jurassic time; 159–167 Ma), and after the peak of the metamorphism (Middle to Late Triassic time; approximately 187 Ma). The early stage of mineralization (stage 1) is related to a homogeneous H₂O–CO₂ (XCO₂ > 0.1) fluid characterized by moderate salinity (<10 wt.% NaCl equivalent) at high temperature and pressure (>370°C, >3 kbar). Early quartz was followed by small amounts of disseminated fine‐grained pyrite and chalcopyrite. Most of the main‐ore‐stage (stage 2) minerals, including chalcopyrite, pyrite and minor sphalerite, pyrrhotite, and galena, precipitated from an aqueous‐carbonic fluid (8–18 wt.% NaCl equivalent) at temperatures ranging between 241 and 388°C during fluid unmixing process (CO₂ effervescence). Fluid unmixing in the primary carbonaceous fluid at pressures of 1.5–3 kbar produced a high XCO₂ (>0.05) and a low XCO₂ (<0.01) aqueous fluid in ore‐bearing quartz veins. Oxygen and hydrogen isotope compositions suggest mineralization by fluids derived from metamorphic dehydration (δ¹⁸O_fluid = +7.6 to +10.7‰ and δD = ?33.1 to ?38.5‰) during stage 2. The late stage (stage 3) is related to a distinct low salinity (1.5–8 wt.% NaCl equivalent) and temperatures of (120–230°C) aqueous fluid at pressures below 1.5 kbar and the deposition of post‐ore barren quartz veins. These fluids probably derived from meteoric waters, which circulated through the metamorphic pile at sufficiently high temperatures and acquire the characteristics of metamorphic fluids (δ¹⁸O_fluid = +4.7 to +5.1‰ and δD = ?52.3 to ?53.9‰) during waning stages of the postearly Cimmerian orogeny in Surian complex. The sulfide‐bearing quartz veins are interpreted as a small‐scale example of redistribution of mineral deposits by metamorphic fluids. This study suggests that mineralization at the Jian deposit is metamorphogenic in style, probably related to a deep‐seated mesothermal system.