Connate Waters Research Articles

Fluid Inclusion, Rare Earth Element Geochemistry, and Isotopic (O and S) Characteristics of the Ardakan Barite Deposit, Yazd Province, Iran

The stratabound barite mineralization occurs in the Ardakan deposit as patches and veins in the dolomites and limestones of the Middle Triassic Shotori Formation. Rare-earth element (REE) geochemistry, O and S isotopes, and fluid inclusion data were used to identify the mode of barite formation. Barite is associated with subordinate fluorite and quartz and, to a lesser extent, with sphalerite, malachite, chrysocolla, and iron and manganese oxide-hydroxides. Barite contains a very low ∑REE concentration (14.80–19.59 ppm) and is enriched in light rare-earth elements (LREEs) relative to heavy rare-earth elements (HREEs). The low ∑REE content and the Ce/La ratio (4.0–6.5) indicate a hydrothermal (terrestrial) origin of the barite. Similar to barite, the ∑REE content in fluorite is low (0.14–6.52 ppm) and suggests a sedimentary setting. The Tb/Ca versus Tb/La diagram also indicates a hydrothermal origin of fluorite. The δ34S values in the barite (+27.9 to +32.4‰) indicate that the sulfur most likely originates from evaporites and/or connate waters from the Late Precambrian to the Lower Cambrian. The δ18O values (+15.9 to +18.1‰) in the barite show that the oxygen originated either from Late Precambrian–Lower Cambrian evaporites or from basinal brines with slightly higher δ18O values than the evaporites. The salinity and homogenization temperature ranges of the aqueous fluid inclusions in barite, fluorite, and quartz (0.88–16.89 wt% NaCl equivalent and 90–270 °C, respectively) reveal that the mineralizing fluids were formed from basinal brines with the participation of heated meteoric water. From this, it is concluded that the Ardakan barite deposit was formed by the meeting of heated, ascending sulfate-bearing meteoric water and cooler, Ba-bearing connate water trapped in the overlying Middle Triassic dolomites and limestones. The Ardakan deposit belongs to the structure-related class and the unconformity-related subclass of barite deposits.

Formation of barite in the Ab Torsh deposit, Kerman province, Iran: Insights from rare earth elements, O and S isotopes, and fluid inclusions

Barite can form in a variety of geological environments, as it occurs in a wide range of mineral deposits. To determine the origin and physicochemical conditions under which the Ab Torsh barite deposit formed, an extensive study was conducted using petrographic, Rare Earth Element (REE) geochemical, O and S isotopic, and fluid inclusion methods. Barite mineralization occurs at Ab Torsh as a stratabound vein in the Senonian carbonate rock units. Barite-quartz is accompanied by subordinate malachite, chrysocolla, Fe-Mn oxide-hydroxides, galena, azurite, fluorite, pyrite, and bornite. The ∑REE values are very low in barite (5.32–14.56 ppm), with chondrite-normalized patterns showing enrichment of Light Rare Earth Elements (LREE) relative to Heavy Rare Earth Elements (HREE). The low ∑REE content and REE element ratios (Ce/La, (La/La*)N, and (Gd/Gd*)N) indicate that seawater with a highly altered geochemical signature (connate water) acted as a Ba ore-forming fluid. The δ18O and δ34S values in barite (+10.4–+11.1 ‰ and +27.3–+27.8 ‰, respectively) and the δ34S values in galena (+6.3 and + 7.9 ‰) indicate that the sulfate (and thus sulfur) originated from sulfate-bearing connate waters and/or evaporites. Thermochemical Sulfate Reduction (TSR) was the most likely mechanism for the formation of the reduced sulfur in galena. The salinity and homogenization temperatures in the aqueous fluid inclusions of barite and quartz (2.7–19.3 wt% NaCl equivalent and 110–275 °C, respectively) indicate that basinal fluids containing a meteoric water component were the source of the mineralizing solutions. The fluid inclusion data demonstrate that two fluid mixing have occurred: one between the hot basinal brines and cold meteoric waters, and another between heated and cold meteoric waters. It is estimated that the hot fluids derived from a maximum depth of about 9 km. The Ab Torsh deposit is classified here as a structure (unconformity)-related barite deposit. It is concluded that intense faulting and brecciation of the host rocks caused by post-Cretaceous compressional tectonics probably provided the channels necessary for the upward migration of deep mineralizing fluids from a basinal brine source. Barite formed where these ascending hot, Ba-bearing hydrothermal fluids encountered cooler, sulfate-bearing connate waters trapped in the overlying Senonian strata and/or the descending cold meteoric waters that dissolved evaporite-bearing rock units.

Alternating acid and alkaline diagenesis in the Shahejie Formation of E12 structure in the Laizhou Bay Sag, Bohai Bay Basin

The high-quality reservoirs in the Shahejie Formation of the Laizhou Bay Sag, Bohai Bay Basin, Eastern China formed in the Paleogene saline lacustrine environment and show complex diagenetic evolutionary processes caused by alternating acid and alkaline fluids. Through observations under an optical microscope and a scanning electron microscope, casting thin sections, and plane porosity measurements, this study investigated the diagenesis differences in the third member of the Shahejie Formation (Es3 Member) in the root and central portions of a fan delta in the E12 structure in the eastern Laizhou Bay Sag, respectively. The Es3 reservoirs in the well A-1 which have no any hydrocarbon shows, are relatively tight, and have high argillaceous content, poor particle sorting, strong mechanical compaction, and intense carbonate cementation due to long-term activities of alkaline fluids. Moreover, they were not altered by organic acid due to the far lateral distance from source rocks. By contrast, the Es3 reservoirs in the well A-2 with hydrocarbon accumulations, contain an anomalously high porosity zone due to the superimposition of five episodes of alternate acid-alkaline fluids. The diagenetic fluids controlling the formation of the high-quality Es3 reservoirs are sourced from connate saline pore waters, gypsum dehydration, salt diapir, and organic acid. Moreover, chlorite coats in the reservoirs in the well A-2 inhibit secondary overgrowth of quartz and, thus, protect primary pores. The chlorite coats and alkaline-acid dissolution alterations are critical to the formation of the high-quality reservoirs in well A-2.

Diagenesis of the Paleogene Sandstones in the DN2 Gas Field, Kuqa Foreland Basin and Its Link with Tectonics

AbstractWe investigated the diagenesis of sandstones from DN2 Gas Field of Kuqa Foreland Basin (KFB) to infer timing of fluid migration and discuss link between fluids and tectonics. Textures and chemical composition of authigenic minerals, fluid evidence from fluid inclusions and formation water measurements were used to fulfill the aim. Eodiagenesis occurred with the participation of meteoric waters and connate waters. Mesodiagenesis are related to high salinity fluids, which were attributed to originate from overlying Neogene Jidike Formation evaporite (main minerals including halite, anhydrite, glauberite, carnallite and thenardite). The onset of high salinity fluid migration was inferred to occur during the late Miocene (12.4–9.2 Ma) by using integration of Th measured in the present study and K‐Ar dating of authigenetic illites from previous study. The period is consistent with the crucial phase (13–10 Ma) witnessing the rapid uplift of southern Tianshan Mts and the stage when calcite and anhydrite veins formed in the studied strata. We thus argue that diagenesis related to high salinity fluids occurred as a response to Tianshan Mts's rapid uplift and related tectonic processes. The flow of high salinity fluids was probably driven by density gradient and channeled and focused by fractures formed contemporaneously.

Heat, noble gases and CO2 sources in geothermal fields of Mexico

The Trans-Mexican Volcanic Belt (TMVB) and the Gulf Extensional Province (Baja California) are the regions in Mexico with the highest heat flow and where the main geothermal energy resources are currently exploited. Here, previously published and newly collected heat and volatile species (He, Ar and CO2) data from 132 fluid samples of four geothermal areas belonging to TMVB (Los Azufres, Los Humeros, Acoculco and Cerritos Colorados) and two in Baja California (Cerro Prieto and Las Tres Vírgenes) were analyzed. The goal was to identify heat sources, understand how heat is transported into the geothermal reservoirs and how the multiple fluid sources (meteoric, magmatic and crustal) in the reservoir might impact the heat in each field. Distinct R/Ra ratios (where R is the measured 3He/4He ratio and Ra is the atmospheric ratio) are observed in the TMVB and Baja California geothermal fields, with those in the TMVB displaying significantly higher R/Ra values (7.14–7.27) than those in Baja California (1.21–6.62), indicating a higher contribution of mantle helium from possibly younger, active magmatic heat sources. Lower R/Ra values in Baja California might reflect local tectonic features, where reservoirs are primarily affected by crustal contributions from an old, subducted fossil slab related to the Farallon Plate. The analysis of heat to 3He ratios (or Q/3He) together with information provided by 4He/36Ar ratios suggest that heat and volatiles are transferred into the hydrothermal system by advection. The magma bodies beneath the geothermal areas of interest are sufficiently voluminous to provide aqueous fluids by exsolution to sustain this advective flow. The release of He and Ar from magma, during its degassing, appears to be controlled by their diffusivities, fractionating the pristine mantle 4He/36Ar ratio toward higher values, while advection transfers volatiles more rapidly than heat, fractionating the pristine mantle Q/3He ratios toward lower values. The Q/3He vs. 4He/3He relationship shows that boiling and dilution by meteoric water led to further fractionation of heat and volatiles, particularly in Los Azufres and Los Humeros fields, while Cerro Prieto has Q/3He ratios close to those at mid-ocean ridges or fractionated by circulation of low-heat, 4He-rich fossil waters. Distinct sources of heat and volatiles are also reflected in the CO2 content, the major dry gas component in these geothermal fields. Main sources of CO2 are mantle carbon (1%-51%) and limestone (34%–100%) from local sources and the subducting slab, although expected fractionation of the C isotopic composition and CO2/3He during magma ascent and degassing introduce uncertainties on the precise estimation of each source contributing C to the studied geothermal reservoirs. Yet, contribution of carbon from sediments might be minor (0%–35%), and mainly found in Cerro Prieto and Las Tres Virgenes fields located in Baja California, which are affected by the release of volatiles from the subducted Farallon plate and presence of connate waters in the reservoir of Cerro Prieto.

Sulfuric acid speleogenesis and surface landform evolution along the Vienna Basin Transfer Fault: Plavecký Karst, Slovakia

Hypogene caves in the Plavecký hradný vrch Hill (Western Slovakia, Central Europe) were formed by waters ascending along faults in fractured Triassic carbonates related to the horst-graben structure at the contact of the Malé Karpaty Mountains and the NE part of the Vienna Basin. The Plavecká jaskyňa and Pec caves mostly contain horizontal passages and chambers with flat corrosion bedrock floors, fissure discharge feeders, wall water-table notches, replacement pockets, as well as a few other speleogens associated with sulfuric acid speleogenesis. The low-temperature sulfuric acid development phases of the Plavecká Jaskyňa are also indicated by the presence of sulfate minerals (i.e., gypsum and jarosite).Subaerial calcite popcorn rims were precipitated from water condensation at the edges of feeding fissures that were still active as thermal vents when the water table dropped. Hydrogen sulfide involved in the sulfuric acid speleogenesis was likely derived from anhydrites and/or hydrocarbon reservoirs with sulfate-saline connate waters in the fill of the adjacent Vienna Basin. It ascended to the surface along deep-rooted sub-vertical fault zones at the contact of the Vienna Basin with neighboring mountains. Three cave levels at 295 to 283 m asl in the Pec Cave, and five levels at 225 to 214 m asl in the Plavecká jaskyňa corresponded to phases of stable local erosional base levels in the bordering part of the Vienna Basin, most likely during periods of strongly decelerated and/or interrupted subsidence. Cave levels separated by vertical differences of only a few meters may also be related to the Pleistocene climatic cycles. The subhorizontal parts of the Pec Cave are probably of late Early Pleistocene age (˃0.99–1.07 Ma?). The two highest levels of the Plavecká jaskyňa developed during the early Middle Pleistocene (˃600 ka). Fine-grained sediments in the passage at 225 m asl with normal magnetic polarity contain jarosite. The middle level of the Plavecká jaskyňa at 220 m asl was formed in the mid-Middle Pleistocene, while the lower and lowermost levels formed in the late Middle Pleistocene (˃270 ka). The water table in the lowermost cave level probably dropped after the tectonic reactivation of the Podmalokarpatská zníženina Depression just in the front of a marginal horst structure of the Malé Karpaty Mountains.

Metallogenesis of Ni–Co–Fe-arsenide type mineralization in the Purísima-Rumicruz deposit, Jujuy, Argentina

The Purísima-Rumicruz vein system is a Ni–Co–Fe-arsenide type or Five-Element deposit located in the Cordillera Oriental, northwestern Argentina. The host rocks are Ordovician black shales of the Acoite Formation. Three vein types, were identified (a) sulfides/sulfoarsenides + carbonates ± quartz; (b) barite; and (c) crystalline quartz veins. Sulfide-carbonate veins are narrow and exhibit multi-episode infill and are NW-SE and E-W oriented. Four successive stages of mineral precipitation were established. The first stage, sulfide-rich, comprises galena, pyrite, chalcopyrite, bornite, and Zn-rich tetrahedrite-tennantite. A second stage corresponds to native silver, arsenides, Ni-sulfarsenides and pitchblende, indicative of a rise in the As/S ratio in the hydrothermal fluids. The third stage evidences a decrease in the As/S ratio with sulfide precipitation of silver sulfides, chalcocite, chalcopyrite, tetrahedrite-tennantite, bornite and millerite. Finally, a supergene event with covellite, digenite, limonite and Cu-carbonates occurred. Microthermometric studies of Stage 1 quartz yield Th between 185 and 276 °C and 5.2–12 wt% eq. NaCl. Salinities. Stage 3 calcite yield Th 110-240 °C and salinities in two ranges, 1–11 and 16.8–24 %wt. NaCl eq. The δ34S values in chalcocite and galena range between +5.8 and + 7‰, and the δ18OSMOW values from quartz, are between +6.2 and + 9.0‰. δ18OSMOW and δ13CPDB values in calcite samples range between +10.9 and +14.7‰, and −3.05 to −4.07‰, respectively. Isotope data indicate a cortical source of the fluids, probably from connate waters. Early Cretaceous rifting produced a thermal anomaly and direct faults and fractures. Deep-seated brines mixed with connate fluids of the Acoite Formation led to the formation of the Purísima-Rumicruz veins.

Geologic controls on groundwater salinity reversal in North Coles Levee Oil Field, southern San Joaquin Valley, California, USA

This paper documents a reversal in the groundwater salinity depth gradient in the North Coles Levee Oil Field in the San Joaquin Valley, California. Salinity, measured in mg/L, was mapped with water quality data from groundwater and oil and gas wells and salinity estimated from oil and gas well borehole geophysical logs using Archie's equation. The resulting three-dimensional salinity volume shows groundwater salinity increasing with depth through the Tulare and San Joaquin Formations to about 50,000 mg/L at 1100 m depth, then decreasing to 10,000–31,000 mg/L in the Etchegoin Formation at 1400 m depth. The high salinity zone occurs near the base of the San Joaquin Formation in sand lenses in shales that have been interpreted as representing a mudflat environment. The groundwater and produced water geochemistry show formation waters lie on the seawater dilution line, indicating the salinity structure is largely the result of dilution or evaporation of seawater and not due to water–rock interactions. Instead, changing depositional environments linked to decreasing sea level may be responsible for variably saline water at or near the time of deposition, leading to a salinity reversal preserved in connate waters. The steepness of the salinity reversal varies laterally, possibly due to post-depositional freshwater recharge allowed by thick sands, alternatively, by a change in connate water composition due to a lateral facies change present at the time of deposition. These results illustrate geologic and paleogeographic processes that drive the vertical salinity structure of groundwater in shallow alluvial basins.

Mineralogy and Fluid Inclusions of the Cunas Emerald Mine, Maripí, Boyacá, Colombia

The Cunas mine is currently one of the major producers of fine emeralds in Colombia; its emeralds typically display a magnificent green hue, which is highly appreciated in the world market. The mineralization is found in vanadium-rich black shales of the Muzo formation; emeralds occur in pockets within hydrothermal veins and breccias, consisting mostly of calcite, dolomite, albite, quartz, and minor pyrite, parisite-(Ce), and fluorite; hydrothermal alteration is pervasive and dominated by albitization and carbonatization. Emerald-hosted fluid inclusions are highly abundant and remarkably large and complex. Poly-phase inclusions are ubiquitous, occur both in emeralds and gangue minerals, and consist of two daughter crystals (typically halite and calcite or siderite; exceptionally parisite-(Ce)), a liquid brine, a CO2-N2-CH4-rich gas bubble, and occasionally minor liquid CO2. Vapor-rich inclusions were observed in quartz, and two-phase inclusions were identified in calcite and dolomite, thus suggesting a complex fluid evolution. Microthermometry analysis indicates the emerald-forming fluids were trapped at relatively low temperature ≈ 260-340°C and pressure ≈ 875-2400 kbar, with relatively high density —1.03 g/cm³—, and elevated salinity 39% NaCl eq. Wt.; other aqueous components detected include CaCl2, KCl, and FeCl2. Based on these data, we propose the emerald mineralization at the Cunas mine was originated by the mixing of two hydrothermal fluids of different sources; one fluid with high salinity derived from evaporite dissolution, responsible for the albitization of the host rocks; the second is a calcium-rich fluid evolved from connate waters, which was equilibrated by the interaction with calcareous and organic-rich wall rocks. As a result, emerald mineralization took place at structurally favorable sites where fluid mixing was promoted. The described geological and physicochemical features for the Cunas mine, are in agreement with an epigenetic sediment-hosted mineralization —Colombian-type— formed by the circulation and mixing of relatively low-temperature non-magmatic fluids.

Integration of stable isotopes, radiometric dating and microthermometry of saddle dolomite and host dolostones (Cretaceous carbonates, Kurdistan, Iraq): New insights into hydrothermal dolomitization

This paper integrates Mg-, C-, O-, Sr-isotopes, fluid-inclusions microthermometry, and radiometric dating of dolostones and vug-filling saddle dolomite and calcite of the Upper Cretaceous Bekhme Formation, the Kurdistan Region of Iraq, provide new insights into the origin, timing and geochemical evolution of hydrothermal dolomitizing fluids. Similar isotopic compositions and fluid-inclusion microthermometry of the dolostones and saddle dolomite suggest formation by similar fluids. The U–Pb ages of dolostones and saddle dolomite suggest that dolomitization (74.8 Ma) was accomplished in 300 Ka at near Earth's surface by flow of hydrothermal fluids followed by dolostone dissolution and precipitation of vug-filling saddle dolomite (73.8 Ma). Mg isotopic compositions of the dolomites (26MgDSM3=−0.81‰ to −0.76‰) are interpreted to indicate derivation of Mg from marine connate waters entrapped in overpressured basinal mudstones. Relatively high Sr-isotope ratios of the dolomites precludes the role of ophiolites as an important source of Mg in the basin.

The formation process of the Mengyejing potash deposit, Yunnan, China: evidence from geochemical and petrological characteristics

Two types of potash ores were sampled from the Mengyejing potash deposit (the MPD) which is the only ancient potash deposit ever found in China. Geochemical analyses, including Br, K, Mg and some heavy metals, were performed on those samples. The relatively consistent Br contents (141.42 to 763.67 ppm) in both bedded potash and veined potash ores show that the potash-forming brine was likely derived from seawater. K contents and Br/Cl ratios of the veined potash ores are relatively high in contrast to bedded potash ores. The K/Br ratios of veined potash ores exhibit a positive correlation with Br contents, which is opposite to that of the bedded potash ore of the MPD and potassium-salts from the evaporation of modern seawater. The contents of heavy metals in veined potash ores are much higher than those of bedded potash ores in the MPD and potash in other evaporite basins of the world. Sr and S isotopic compositions of these two types of potash ores suggest that the brines for precipitating veined potash were formed by mixing of connate waters (seawater or/and continental freshwater) and hydrothermal fluids. The geochemical and petrological characteristics of the veined potash ores indicate that the brine for forming veined potash orebodies caused by dissolution of previous bedded potash orebodies by hydrothermal fluids. The veined potash orebodies were formed through a cooling process by which sylvite precipitated first prior to halite.

Prediction of Oil and Gas Occurrence in the Jurassic Reservoirs of the Zone of Junction of the Yenisei–Khatanga and West Siberian Basins

Abstract —Based on a comprehensive analysis of the geologic structure, hydrogeology, and hydrogeochemistry, we predict oil and gas occurrence in the Jurassic reservoirs of the zone of junction of the Yenisei–Khatanga and West Siberian basins. We have established the regularities of formation of sandstones of the Sigovaya, Malyshevka, Vym, Nadoyakha, Sharapovka, and Zima reservoirs and examined their poroperm properties. The studied Lower and Middle Jurassic deposits, beginning from the Malyshevka reservoir, are characterized by abnormally high formation pressures with an anomaly ratio of up to 1.92. The Jurassic reservoirs bear weakly alkaline, moderately brackish to slightly saline groundwater of Cl Na, Cl–HCO3 Na, and HCO3–Cl Na compositions with TDS = 3.0–23.3 g/dm3. The low-mineralization connate waters determine an inverse vertical hydrogeochemical zoning in the sedimentary cover of the study region. The research data show favorable conditions for the generation and accumulation of hydrocarbon pools. It is shown that the main criteria for oil and gas occurrence are hydrogeochemical (contents of organic compounds, NH4, B, I, Br, naphthenic acids, and benzene; B/Br and HCO3/Cl coefficients), gas (total gas saturation of formation waters, degree of their gas saturation (Kg), and composition of water-dissolved gases), and hydrodynamic (formation pressures and their anomaly ratio (Ka)) indicators. Also, complex anomalies have been revealed in the Jurassic deposits, which trace the gas fields and revealed oil and gas occurrences. The Jurassic reservoirs are most promising for structure-lithologic traps in the peripheral zones of the basin and on the slopes of large positive structures. The Middle Jurassic deposits in the interior of the Yenisei–Khatanga Basin, poorly studied by deep drilling, might also be of interest as potential hydrocarbon pools.

Assessing the role of bitumen in the formation of stratabound Cu-(Ag) deposits: Insights from the Lorena deposit, Las Luces district, northern Chile

Stratabound Cu-(Ag) deposits located in north-central Chile have been grouped into two NS parallel belts according to the age of their volcanic and volcanoclastic host rocks. Deposits from the Cretaceous belt are characterized by an unusual assemblage of Cu sulfides with migrated hydrocarbons, whereas the presence of organic matter has not yet been recognized in deposits from the Jurassic belt. Here we report the first evidence of pyrobitumen in a Jurassic stratabound Cu-(Ag) deposit and evaluate its role in ore formation. We present a micro-analytical study of pyrobitumen associated with Cu-(Fe) sulfides in the Lorena deposit, located in northern Chile. Our approach involved a combination of surface and drill core sample characterization, (FE)-SEM observations, EMPA and μ-XRF mapping of pyrobitumen. In addition, we determined the speciation of sulfur in pyrobitumen by using synchrotron μ-XANES. Two main events of hypogene mineralization were identified at Lorena deposit, i.e., a pre-ore and a main Cu ore stage. The pre-ore stage is characterized by globularly-shaped pyrobitumen and minor pyrite, and a widespread albite alteration of the andesite host rock. Pre-ore pyrite was replaced by chalcopyrite, bornite and chalcocite, providing evidence for a late Cu inception. In the main Cu ore stage, chalcocite is often associated with sharp-edged, angular pyrobitumen, most likely indicating remobilization or a later pulse of hydrocarbons. EMPA data show that pyrobitumen at Lorena is enriched in Fe (0.02–1.74 wt%), Cu (0.02–0.87 wt%), S (0.01–0.43 wt%) and Cl (0.01–0.4 wt%). Other elements detected include V, Co, Ni, Cr, Sb, As, Ag and Au. Both Ag and Au were only detected in a few spots, with Au reaching up to 600 ppm. Desktop and synchrotron μ -XRF mapping shows that S is homogeneously distributed within pyrobitumen, with lower S concentrations observed at grain rims. On the other hand, EMPA-WDS X-ray maps show that Cu precipitated on pyrobitumen grain surfaces, filling fractures and forming veinlets. In addition, textures showing the presence of micro- to nano-sized pyrobitumen inclusions within both pre-ore pyrite and main ore chalcocite grains point, unequivocally, to the interaction of hydrothermal fluids with hydrocarbons during the formation of this deposit. The high S concentrations measured in pyrobitumen and the presence of organosulfur compounds detected by μ -XANES can be attributed to an organic source. The pre-ore assemblage is interpreted as formed from a petroleum-aqueous fluid (connate waters?) mixture that migrated from the sedimentary source rock into the Jurassic host rocks where pyrite and pyrobitumen precipitated in the andesite. Late Cu-bearing hydrothermal fluids interacted with this pyrobitumen-pyrite assemblage to form Cu sulfides, either by replacement of pyrite or precipitation. We conclude that some of the S contained in pyrobitumen could have been released during interaction with the Cu-bearing hydrothermal fluid contributing, at least partially, to the S budget required for sulfide mineralization in the main ore stage. Our results provide evidence supporting pyrobitumen-fluid interaction as a key process in the formation of the Lorena deposit and possibly other stratabound Cu-(Ag) deposits in Chile.

Brines of the Siberian platform (Russia): Geochemistry and processing prospects

Complex researches have focused on chloride brines in the large artesian basins of the Siberian Platform. The highly concentrated ground brines are unique natural solutions that are fraught with unsolved mystery associated with their origin and the formation of their chemical composition. Most scientists believe that the most important process controlling the formation of these brines was interaction between palaeoseawaters and host rocks. The newest geochemical and stable isotope data (18О, 2H, 37Cl, 81Br, 87Sr/86Sr) showed that chloride sodium ground brines of the Siberian Platform originated as the result of halogenous rock leaching, whereas chloride calcium brines derive from buried bittern connate waters. The main evolutionary process was a long interaction in the water–rock system. This paper also provides information on the vast resource of multicomponent brines, so-called “liquid ores”, which are widespread over a large area of the Siberian Platform. It is an unconventional hydromineral raw material, whose use and exploitation has an extremely high potential as it may constitute a serious alternative to solid mineral deposits. The innovative methodology and pioneering methods for the processing and extraction of valuable components (especially Li) from brine are suggested.

Evaluation of the performance of oil as a membrane during low-salinity water injection; focusing on type and concentration of salts

Injection of low-salinity or smart water into the reservoir is one of the most common EOR techniques. This technique leads to more oil recovery in comparison to reservoir water and seawater injection. Several mechanisms explain the reasons for this increased recovery level, the most important of which is the wettability alteration towards water-wet conditions. However, in recent years, further mechanisms have been added to the previous ones as the main or complementary mechanism. One of these newly added mechanisms is the performance of oil as a semi-permeable membrane between connate and low-salinity waters. According to this mechanism, water molecules move from low-salinity water through oil to connate water due to the difference in their osmotic pressure (difference in chemical potential), which works as the driving force, until the system moves toward equilibrium in concentration. Introducing water molecules into connate water causes the swelling of connate water, and eventually moves the oil towards the pore throats. In this study, experiments have been performed on capillary tubes, while ignoring the effect of fluid-rock interactions to investigate the proposed theory. Experimental results confirmed the role of osmotic theory. Furthermore, a decrease in the salinity of the low-salinity water causes the oil-droplets' displacement to increase, whereas further reductions in the salinity of the injected water cause lower displacements. Therefore, there is an optimum value of low-salinity water concentration to obtain the highest possible oil drop displacement. The optimum values for the tested samples in this study were about 5000–10000 ppm. Additionally, it was found that among all of the essential seawater salts, potassium chloride leads to the highest oil drop displacement; and among oil samples, heptane showed better membrane-like performance in comparison to toluene and crude oil.

Discovery of Afifi, the shallowest and southernmost brine pool reported in the Red Sea

The previously uncharted Afifi brine pool was discovered in the eastern shelf of the southern Red Sea. It is the shallowest brine basin yet reported in the Red Sea (depth range: 353.0 to 400.5 m). It presents a highly saline (228 g/L), thalassohaline, cold (23.3 °C), anoxic brine, inhabited by the bacterial classes KB1, Bacteroidia and Clostridia and the archaeal classes Methanobacteria and Deep Sea Euryarcheota Group. Functional assignments deduced from the taxonomy indicate methanogenesis and sulfur respiration to be important metabolic processes in this environment. The Afifi brine was remarkably enriched in dissolved inorganic carbon due to microbial respiration and in dissolved nitrogen, derived from anammox processes and denitrification, according to high δ15N values (+6.88‰, AIR). The Afifi brine show a linear increase in δ18O and δD relative to seawater that differs from the others Red Sea brine pools, indicating a non-hydrothermal origin, compatible with enrichment in evaporitic environments. Afifi brine was probably formed by venting of fossil connate waters from the evaporitic sediments beneath the seafloor, with a possible contribution from the dehydration of gypsum to anhydrite. Such origin is unique among the known Red Sea brine pools.

Assessment of Groundwater Salinity in Aloha Pliocene Aquifer of Marbella (Málaga)

In order to study the origin of the salinity and the salinization mechanisms that operate in the Pliocene Aloha Aquifer, hydrochemical and isotopic environmental techniques have been used, due to their known effectiveness and usefulness in the identification of water-rock interaction processes, location of recharge zones and its origin. Analyzing major ions, trace elements, ionic ratios and isotopic ratios of the elements of the water molecule, stable isotopes (δ18O and δ2H) and δ18O and δ34S of sulphate. Previous hydrochemical studies of the Pliocene Aloha Aquifer indicate high concentrations of Cl- and Na+ as well as HCO3, SO4 and Ca that should be indicated the interaction with waters of highersalinity, but also with rocks (longer residence time in the aquifer). Therefore, as a starting hypothesis, two possible origins of the high salinity are considered: mixture with seawater or connate waters stored in Pliocene formations. Finally, the study confirmed that the salinity of the Aloha Aquifer is of a localcharacteristic and not due to seawater intrusion.

Dolomitization and hydrotectonic model of burial dolomitization of the Furongian-Lower Ordovician carbonates in the Tazhong Uplift, central Tarim Basin, NW China: Implications from petrography and geochemistry

Abstract The nature of the dolomitizing fluids and genesis of the Furongian-Lower Ordovician dolomites and dolomite cements are interpreted on the basis of petrography, geochemistry (trace elements, REEs, O-C-Sr isotopes) and fluid inclusion microthermometry. These carbonate strata were deposited on a shallow marine platform in the Tazhong Uplift in central Tarim Basin and were extensively dolomitized. Based on field investigations and petrographic examination, two types of fabric-retentive dolomitic textures (dolomicrites (DM) and microbial dolomudstone (DMS)) and three types of fabric-obliterated dolomitic textures (very fine- to fine-crystalline planar-e(s) dolomite (RD1), fine- to medium-crystalline planar-e(s) dolomite (RD2) and medium- to coarse-crystalline nonplanar-a, planar-e(s) dolomite (RD3)) are identified. Two types of dolomite cements (medium- to coarse-crystalline dolomite cement (DC1) and coarse- to very coarse-crystalline saddle dolomite cement (DC2) are identified. The REE patterns of the analyzed dolomites and calcite cements are characterized using seawater normalization standard. The DC2 dolomite cement and later-stage calcite cement (LSCAL) display positive Europium anomalies (δEu) and positive Cerium anomalies (δCe). Whereas the DM, DMS, RD1, RD2, RD3 and early-stage calcite cement (ESCAL) samples display positive δCe, light REE (LREE) enrichment and negative δEu. The DM and DMS were formed through seawater flux and mesosaline brine derived from evaporative seawater during early-burial seepage-reflux dolomitization and microbial dolomitization, respectively. The preferential occurrence of RD1 along low-amplitude stylolites suggest that their formation may have linked to pressure dissolution by which Mg-ions were probably released for replacement dolomitization during late early-burial to early shallow-burial dolomitization. Moreover, the tightly packed and curved crystal faces of RD2 and RD3 with irregular overgrowth rims and increasing crystal sizes presumably resulted from the recrystallization and replacement upon the RD1 and/or from the cortical overgrowth and dissolution of remaining precursor limestones by the influx of dolomitizing fluids during late shallow-burial dolomitization. The average range of the homogenization temperatures (Th) of CD2 is 132.4 °C–198.7 °C; and mostly higher than the estimated ambient temperature (95 °C–150 °C) for the Furongian-Lower Ordovician strata, implying that the growth of DC2 in the study area was the result of hydrothermal dolomitization. The comparable isotopic (δ18O, δ13C and 87Sr/86Sr) values between RD2, RD3, DC1, DC2 dolomites and coeval Neoproterozoic to Early Phanerozoic seawater values, in addition to the relatively high Th and salinity levels in these dolomites, indicates that their dolomitizing fluid were derived from radiogenic 87Sr-enrich coeval seawater during late shallow-burial dolomitization and hydrothermal dolomitization. The hydrotectonic driving model of burial dolomitization of the Furongian-Lower Ordovician carbonates in the Tazhong Uplift resulted from compressional tectonics that affected the Tarim Basin during Cambrian to Middle Ordovician and Late Ordovician period. This model is characterized by fluid flow in response to significant thermal heating, pressure-loading and enhanced fluid expulsion by squeezing, and the resultant higher-temperature Mg-rich connate waters and hydrothermal basinal brines are conveyed by convective-advective flows along permeable horizons, fractures and overthrust fault planes, resulting in extensive replacement dolomitization of carbonate materials and recrystallization of early formed dolomites; forming irregular networks of dolomites and calcite cements in the host carbonate rocks.

Structurally-controlled hydrothermal fluid flow in an extensional tectonic regime: A case study of Cretaceous Qamchuqa Formation, Zagros Basin, Kurdistan Iraq

Shallow water carbonate rocks of the Lower Cretaceous Qamchuqa Formation host a significant reserve of oil in the NNE Arabian plate-Southwestern Zagros Sutured Belt in the Kurdistan Region, Iraq. Such carbonates consist of several dolostone bodies with unique and complex arrays of saddle and zebra dolomite textures especially in fractured intervals. Integration of field, core description, microstructural analysis, petrographic, stable C, O and Sr isotopes, and fluid inclusion analyses reveals two main phases of multiple fluxes of hydrothermal fluid circulation during the Zagros Orogeny at the end of Cretaceous and Cenozoic times. An early regional non-focused hydrothermal dolomitization and a subsequent fault- and fracture- controlled hydrothermal fluid flow, which led to replacement and cementation by a range of saddle dolomite and zebra textures. Several generations of matrix dolomite, saddle dolomite and calcite cement were identified. The host dolostone has been affected by shortening, folding, fracturing, and thrust faulting. Saddle dolomite pipes, in particular, were found to be associated with en-échelon folding. The morphology and areal extent of the zebra dolomite are controlled by the pore geometry of the host dolostone in relationship to fracturing and faulting.Both matrix and saddle dolomite cements have comparable, overlapping stable isotopic values ranging from 0.39‰ to 3.57‰ and −0.12 to 3.11‰ VPDB, respectively for δ13C and δ18O values range from −10.53‰ to −4.47‰ and −12.87 to −7.79‰ VPDB, respectively. 87Sr/86Sr ratios of pervasive matrix dolomite range from 0.707702 to 0.707839, fall close to late Cretaceous seawater values. Slightly more radiogenic values of saddle dolomites ranging from 0.707759 to 0.708024 resulted from mixing of basinal brines with connate waters. Isotopic signatures of later calcite cements show more radiogenic Sr ratios from 0.707755 to 0.708198 and negative δ13C values 1.76 to −7.45‰, possibly related to late diagenetic processes due to incursion of meteoric waters during subaerial exposure.Fluid inclusion data of pervasive matrix dolomite and saddle dolomites show homogenization temperatures (Th) with a wide range from 71.3 to 169.7 °C and 82.3 to 188.2 °C, and salinity between 14.4 to 25.4 and 15.6 to 27.9 wt% NaCl eq., respectively. These values demonstrate the effect of hydrothermal fluid flow of the alteration of host dolomite and formation of saddle dolomite in fractures and faults.

Cerro Prieto Geothermal Field (Baja California, Mexico) – A fossil system? Insights from a noble gas study

Results of a survey of noble gases, water stable isotopes (δD and δ18O), and chlorine (Cl) and bromine (Br) contents measured in geothermal waters from the high-enthalpy, liquid-dominated Cerro Prieto Geothermal Field (CPGF) are reported. Helium isotopic ratios (3He/4He) help to identify two distinct fluids: a magmatic fluid, with 3He/4He (R) ratios of up to 7.3 times that of the atmospheric ratio (Ra = 1.384 × 10−6), and a 4He-rich fluid, with 3He/4He ≤ 3.5 Ra. The helium isotopic systematics point to limited modern freshwater recharge in the CPGF, and thus to the presence of a nearly fossil geothermal system. U-Th/4He-estimated minimum residence times for the 4He-rich fluid component vary from 1.5 to 1.8 Ma, corresponding to the age of the Colorado River paleo-delta sediments, suggesting the presence of connate waters. Cl and Br concentrations indicate that this connate water component is seawater with minor halite dissolution. Together with Cl and Br, the isotopic signature of the water (δD, δ18O) indicates the presence of a third old fluid of meteoric origin, from Late Quaternary to Holocene, corresponding to recharge during colder and wetter periods than today. This is identified as being old groundwater from the Colorado Delta unconsolidated sediments, which cap the Cerro Prieto reservoir. Our results suggest little direct recharge to the CPGF. The spatial distribution of helium isotopes in the reservoir shows that connate waters dominate the central portion of the field and mix with the magmatic fluid containing mantle He, with both fluids rising at the center of the basin where the heat source is located. Deep magmatic fluids containing mantle helium enter laterally into the reservoir, through the main lateral strike-slip faults (i.e., the Cerro Prieto and Imperial faults).