Diverse origins and timing of formation of basinal brines in the Gulf of Mexico sedimentary basin

A review of five different field areas in the Gulf of Mexico sedimentary basin (GOM) illustrates some of the potentially diverse chemical and physical processes which have produced basinal brines. The elevated salinities of most of the formation waters in the GOM are ultimately related to the presence of the Middle Jurassic Louann Salt. Some of these brines likely inherited their salinity from evaporated Mesozoic seawater, while other saline fluids have been produced by subsequent dissolution of salt, some of which is occurring today. The timing of the generation of brines has thus not been restricted to the Middle Jurassic. The mechanisms of solute transport that have introduced brines throughout much of the sedimentary section of the GOM are not entirely understood. Free convection driven by spatial variations in formation water temperature and salinity is undoubtedly occurring around some salt structures. However, the driving mechanisms for the broad, diffusive upward solute transport in the northern Gulf rim of Arkansas and northern Louisiana are not known. In the Lower Cretaceous of Texas, fluid flow was much more highly focused, and perhaps episodic. It is clear that many areas of the Gulf basin are hydrologically connected and that large‐scale fluid flow, solute transport, and dispersion have occurred. The Na‐Mg‐Ca‐Cl compositions of brines in the areas of the Gulf Coast sedimentary basin reviewed in this article are products of diagenesis and do not reflect the composition of the evaporated marine waters present at the time of sediment deposition. Large differences in Na, Ca, and Mg trends for waters hosted by Mesozoic versus Cenozoic sediments may reflect differences in: (i) the sources of salinity (evaporated seawater for some of the Mesozoic sediments, dissolution of salt for some of the Cenozoic sediments); (ii) sediment lithology (dominantly carbonates for much of the Mesozoic sediments, and dominantly siliciclastics for the Cenozoic sediments); or (iii) residence times of brines associated with these sediments (tens of millions of years versus perhaps days).     

The hydrogeochemistry of subsurface brines in and west of the Jordan–Dead Sea Transform fault

This study is based on 113 analyses of brines with Cl > 0.57 mol l^?1 (modern seawater), which were collected and analysed mostly during several decades of exploration for gas and oil in Israel. Based on critical evaluation of correlations of elements and ionic ratios and on spider patterns, six different brine events or source brines were identified in the Phanerozoic: the Triassic, Lower Cretaceous and the Mio/Pliocene brine families which were identified in boreholes Sdom‐1, Sdom Deep‐1 and Ha'on, and the Holocene Dead Sea brines. The Triassic brines are nowadays also encountered in under‐ and overlying rock units such as the Paleozoic Negev‐Yam Suf and the Jurassic Arad Groups, respectively. The southern Jordan–Dead Sea Transform (also known as the Rift) hosts the Mio‐Pliocene Sdom Deep and Sdom brine families. Brine bodies not sufficiently isolated by impervious sedimentary layers were flushed out during the Pliocene when the southern Valley drained north‐ and westwards through the Yizre'el Valley to the Mediterranean Sea. In the northern Rift Miocene to Pliocene seawater evaporated and infiltrated into the Rift sediments and into adjacent rocks. Further diluted by freshwater, it emerges as the Ha'on brine. Together with its derivatives, they form the Ha'on family. The derivatives of the Holocene Dead Sea brine family occur along the shoreline of the recent Dead Sea. Apart of all these evaporation brines, brines deriving from dissolution of evaporites locally occur in the area. The time‐bound chemical composition of paleoseawater is considered when discussing the ionic ratios of brines generated during different geological periods. Spider patterns of each brine family are compared and, where necessary, the relationship of brines to distinct families of brines is supported by inverse modelling.     

Relationship of brines in the Kinnarot Basin,Jordan‐Dead Sea Rift Valley

Element ratios and water stable isotopes reveal the presence of only two independent deep brines in the Kinnarot Basin, Israel: the evaporite dissolution brine of Zemah‐1 and the inferred Ha’on mother brine (HMB) with low and high Br/Cl ratios, respectively. HMB is considered to be a representative of the Late Pliocene evaporated Sedom Sea. The freshwater‐diluted evaporation brine emerges as Ha’on brine on the eastern shore of Lake Tiberias and is also identified in the pore water of lake sediments. HMB is converted into Tiberias mother brine (TMB) by dolomitization of limestones and alteration of abundant volcanic rocks occurring along the western side of the lake. The Ha’on and Tiberias brines, both characterized by high δD and δ¹⁸O values, are similar in Na/Cl and Br/Cl ratios but are dissimilar in Br/K ratios because these brines were subjected to different degrees of interactions with rocks and sediments. Excepting the brine from KIN 8, all brines from the Tabigha area including the nearby off‐shore Barbutim brine are related to the TMB. The brine KIN 8 and all brines from the Fuliya and Hammat Gader areas are related to the HMB. The brine encountered in wildcat borehole Zemah‐1 is generated by halite‐anhydrite/gypsum dissolution and is independent from the HMB system.     

Fluid inclusion constraints on the origin of the brines responsible for Pb–Zn mineralization at Pine Point and coarse non‐saddle and saddle dolomite formation in southern Northwest Territories

The Pine Point region is a classic metallogenic mining camp that produced over 58 million short tons of Zn–Pb ore from approximately 40 base‐metal mineralized deposits hosted by Middle Devonian carbonates. The ore deposits are localized in paleokarstic features found in the epigenetic ‘Presqu'ile’ dolomite that preferentially replaced some of the upper barrier limestones. The main ore‐stage sulfides include galena, sphalerite, marcasite, and pyrite. A bulk fluid inclusion chemistry study was carried out on sulfide, coarse non‐saddle and saddle dolomite and calcite samples from the Pine Point and Great Slave Reef deposits, and unmineralized coarse non‐saddle and saddle dolomite samples from Hay West, Windy Point and Qito areas. Molar Cl/Br ratio data from Pine Point indicate the presence of four fluids at different stages of the paragenesis. The fluids trapped in sulfides and ore‐stage dolomites predominately consist of a Br‐rich fluid with a composition similar to that of evaporated seawater (fluid A), and a very Br‐enriched fluid of unknown origin (fluid B). Both these fluids are CaCl₂–NaCl (Na to Ca ratios of 1:10)‐rich brines and have compositions unlike the modern formation waters in the Devonian aquifers in the basin today. A third, relatively Cl‐rich (or Br‐poor), fluid (fluid C) was identified in two samples and may have acquired some chlorinity by dissolving halide minerals. Mixing between the Br‐rich fluid A and a dilute fluid also occurred in the later stages of the paragenesis, resulting in the formation of calcite and native sulfur. Saddle and coarse dolomites not associated with significant sulfide mineralization have a narrow range of halogen compositions similar to fluid A. There is no evidence of fluid B or C in the unmineralized samples. Relative to a modern‐day seawater compositions all the fluids have had some modification of their cation compositions. There is some weak evidence for interactions with clastic units or crystalline basement rocks. It is also possible however, that the evaporative brines could have formed from a relatively CaCl₂‐rich, NaCl‐depleted Devonian seawater, unlike the composition of modern‐day seawater.     

Long‐term chemical evolution and modification of continental basement brines – a field study from the Schwarzwald,SW Germany

Highly saline, deep‐seated basement brines are of major importance for ore‐forming processes, but their genesis is controversial. Based on studies of fluid inclusions from hydrothermal veins of various ages, we reconstruct the temporal evolution of continental basement fluids from the Variscan Schwarzwald (Germany). During the Carboniferous (vein type i), quartz–tourmaline veins precipitated from low‐salinity (<4.5wt% NaCl + CaCl₂), high‐temperature (≤390°C) H₂O‐NaCl‐(CO₂‐CH₄) fluids with Cl/Br mass ratios = 50–146. In the Permian (vein type ii), cooling of H₂O‐NaCl‐(KCl‐CaCl₂) metamorphic fluids (T ≤ 310°C, 2–4.5wt% NaCl + CaCl₂, Cl/Br mass ratios = 90) leads to the precipitation of quartz‐Sb‐Au veins. Around the Triassic–Jurassic boundary (vein type iii), quartz–haematite veins formed from two distinct fluids: a low‐salinity fluid (similar to (ii)) and a high‐salinity fluid (T = 100–320°C, >20wt% NaCl + CaCl₂, Cl/Br mass ratios = 60–110). Both fluids types were present during vein formation but did not mix with each other (because of hydrogeological reasons). Jurassic–Cretaceous veins (vein type iv) record fluid mixing between an older bittern brine (Cl/Br mass ratios ~80) and a younger halite dissolution brine (Cl/Br mass ratios >1000) of similar salinity, resulting in a mixed H₂O‐NaCl‐CaCl₂ brine (50–140°C, 23–26wt% NaCl + CaCl₂, Cl/Br mass ratios = 80–520). During post‐Cretaceous times (vein type v), the opening of the Upper Rhine Graben and the concomitant juxtaposition of various aquifers, which enabled mixing of high‐ and low‐salinity fluids and resulted in vein formation (multicomponent fluid H₂O‐NaCl‐CaCl₂‐(SO₄‐HCO₃), 70–190°C, 5–25wt% NaCl‐CaCl₂ and Cl/Br mass ratios = 2–140). The first occurrence of highly saline brines is recorded in veins that formed shortly after deposition of halite in the Muschelkalk Ocean above the basement, suggesting an external source of the brine's salinity. Hence, today's brines in the European basement probably developed from inherited evaporitic bittern brines. These were afterwards extensively modified by fluid–rock interaction on their migration paths through the crystalline basement and later by mixing with younger meteoric fluids and halite dissolution brines.     

Sources and composition of fluids associated with fluorite deposits of Asturias (N Spain)

The fluorite deposits of Asturias (northern Iberian Peninsula) are hosted by rocks of Permo‐Triassic and Palaeozoic age. Fluid inclusions in ore and gangue minerals show homogenization temperatures from 80 to 170°C and the presence of two types of fluids: an H₂O–NaCl low‐salinity fluid (<8 eq. wt% NaCl) and an H₂O–NaCl–CaCl₂ fluid (7–13 wt% NaCl and 11–14 wt% CaCl₂). The low salinity and the Cl/Br and Na/Br ratios (Cl/Br_molar 100–700 and Na/Br_molar 20–700) are consistent with an evaporated sea water origin of this fluid. The other end‐member of the mixture was highly saline brine with high Cl/Br and Na/Br ratios (Cl/Br_molar 700–13 000 and Na/Br_molar 700–11 000) generated after dissolution of Triassic age evaporites. LA‐ICP‐MS analyses of fluid inclusions in fluorite reveal higher Zn, Pb and Ba contents in the high‐salinity fluids (160–500, 90–170, 320–480 p.p.m. respectively) than in the low‐salinity fluid (75–230, 25–150 and 100–300 p.p.m. respectively). The metal content of the fluids appears to decrease from E to W, from Berbes to La Collada and to Villabona. The source of F is probably related to leaching of volcanic rocks of Permian age. Brines circulated along faults into the Palaeozoic basement. Evaporated sea water was present in permeable rocks and faults along or above the unconformity between the Permo‐Triassic sediments and the Palaeozoic basement. Mineralization formed when the deep brines mixed with the surficial fluids in carbonates, breccias and fractures resulting in the formation of veins and stratabound bodies of fluorite, barite, calcite, dolomite and quartz and minor amounts of sulphides. Fluid movement and mineralization occurred between Late Triassic and Late Jurassic times, probably associated with rifting events related to the opening of the Atlantic Ocean. This model is also consistent with the geodynamic setting of other fluorite‐rich districts in Europe.     

Fluid evolution along multistage composite fault systems at the southern margin of the Lower Palaeozoic Anglo-Brabant fold belt, Belgium

Mineralised vein systems have been investigated at nine localities at the southern margin of the Anglo‐Brabant fold belt in Belgium. During the late Silurian to early Middle Devonian Caledonian orogeny, shear zones formed, inferred to be associated with granitoid basement blocks in the subsurface. The circulation of a metamorphic fluid, possibly originating in the Cambrian core of the fold belt, along these shear zones resulted in the formation of mesozonal orogenic mineralisation at the southern margin of the Anglo‐Brabant fold belt. The fluid had a composition dominated by H₂O–CO₂–X–NaCl–KCl. The shear zones form part of a greater fault zone, the Nieuwpoort–Asquempont fault zone, which is characterised by normal faulting that started before the Givetian and by the reactivation of the shear zones. Two fluid generations are associated with this normal faulting. First, a low salinity H₂O–NaCl(–KCl) fluid migrated through the Palaeozoic rocks after the Silurian. Based on the isotopic composition, this fluid could be a late‐metamorphic Caledonian fluid or a younger fluid that originated from the Rhenohercynian basin and interacted with Lower Devonian rocks along its migration path. Second, a high salinity H₂O–NaCl–CaCl₂ fluid was identified in the fault systems. Similar fluids have been found in southern and eastern Belgium, where they produced Mississippi Valley‐type Zn–Pb deposits. These fluids are interpreted as evaporative brines that infiltrated the Lower Palaeozoic basement, from where they were expelled during extensional tectonism in the Mesozoic.     

New and past geochemical data on fresh to brine waters of the Salar de Atacama and Andean Altiplano,northern Chile

The paper deals with the major chemistry and stable isotopes (hydrogen, oxygen, carbon, sulfur, strontium) of waters and solutes from the Salar de Atacama basin (Rio Pedro, Honar Creek and Laguna Chaxa) and Andean Altiplano (Laguna Miñique and Laguna Miscanti). The water inflows of the Salar are chemically quite different, the Rio San Pedro being of Na‐Cl type and the Honar Creek of Na‐HCO₃ type, in keeping with the sedimentary‐evaporitic and volcanic nature of the catchment rocks respectively. The δ³⁴S and δ¹⁸O values of sulfate and the ⁸⁷Sr/⁸⁶Sr ratio of strontium in the streams match those of drained rocks, whereas the δ¹³C values of dissolved carbonate are largely controlled by vegetation. The lagoons are evaporated meteoric water bodies, and the relative air humidity estimated from the slope of the isotopic evaporation line is in accordance with historical data on air humidity in the area. The Laguna Chaxa is Na‐Cl rich, and its isotopic composition are consistent with a mixed sedimentary‐volcanic provenance of sulfur and strontium solutes. The Laguna Miñique is Na‐SO₄ rich, and its sulfate δ³⁴S is nearly identical to that of Laguna Chaxa. The δ¹³C(HCO₃) values are quite different in the Laguna Chaxa and Laguna Miñique, with the former being notably enriched in ¹³C probably because of preferential uptake of ¹²C by the high biological productivity occurring in the lagoon. The limited set of new data is interpreted in the context of a much larger literature database. In particular, previous chemical data on inflows and brines in the Salar de Atacama were revisited, and compared with evaporation path models and mineral stability diagrams (boron, lithium and Mg‐minerals) computed using updated software and thermodynamic databases. The modeling shows that the removal of boron and lithium from sulfate‐rich brines possibly occurs, respectively, as ulexite and sulfate salts, and carnallite should be the final magnesium phase of the brine evolution.     

Mobilization of ore fluids during Alpine metamorphism: evidence from hydrothermal veins in the Variscan basement of Western Carpathians,Slovakia

Hydrothermal polymetallic veins of the Gemeric unit of the Western Carpathians are oriented coherently with the foliation of their low‐grade Variscan basement host. Early siderite precipitated from homogeneous NaCl‐KCl‐CaCl₂‐H₂O brines with minor CO₂, while immiscible gas–brine mixtures are indicative of the superimposed barite, quartz–tourmaline and quartz–sulphide stages. The high‐salinity aqueous fluid (18–35 wt%) found in all mineralization stages corresponds to formation water modified by interaction with crystalline basement rocks at temperatures between 140 and 300°C. High brominity (around 1000 ppm in average) resulted from evaporation and anhydrite precipitation in a Permo‐Triassic marine basin, and from secondary enrichment by dissolution of organic matter in the marine sediments at diagenetic temperatures. Sulphate depletion reflects thermogenic reduction during infiltration of the formation waters into the Variscan crystalline basement. Crystallization temperatures of the siderite fill (140–300°C) and oxygen isotope ratios of the parental fluids (4–10‰) increase towards the centre of the Gemeric cleavage fan, probably as a consequence of decreasing water/rock ratios in rock‐buffered hydrothermal systems operating during the initial stages of vein evolution. In contrast, buoyant gas–water mixtures, variable salinities and strongly fluctuating P–T parameters in the successive mineralization stages reflect transition from a closed to an open hydrothermal system and mixing of fluids from various sources. Depths of burial were 6–14 km (1.7–4.4 kbar, in a predominantly lithostatic fluid regime) during the siderite and barite sub‐stages of the north‐Gemeric veins, and up to 16 km (1.6–4.5 kbar, in a hydrostatic to lithostatic fluid regime) in the quartz–tourmaline stage of the south‐Gemeric veins. The fluid pressure decreased down to approximately 0.6 kbar during crystallization of sulphides. U‐Pb‐Th, ⁴⁰Ar/³⁹Ar and K/Ar geochronology applied to hydrothermal muscovite–phengite and monazite, as well as cleavage phyllosilicates in the adjacent basement rocks and deformed Permian conglomerates corroborated the opening of hydrothermal veins during Lower Cretaceous thrusting and their rejuvenation during Late Cretaceous sinistral transpressive shearing and extension.     

Fluids,basin analysis,and mineral deposits

Sedimentary basins are the largest structures on the surface of our planet and the most significant sources of energy‐related commodities. With time, sedimentary successions in basins normally are subjected to increasingly intense diagenesis that results in differential evolution of basin hydrology. This hydrologic structure is in turn vitally important in determining how and where deposition of metals may occur. Fluids in all basins originate and flow as a result of sedimentological and tectonic events, so that fluid histories should reflect the control of both lithology and tectonism on ore deposition. Sandstone lithologies, in particular, reflect fluid‐flow events because they are normally the major aquifers in basins. However, early cementation results in occlusion of primary permeability in some facies (diagenetic aquitards) whereas in others, permeability develops due to the dissolution of unstable grains (diagenetic aquifers). Particularly for ore deposits in Precambrian basins, identification of paleohydrologic systems during basin evolution requires the integration of data derived from tectonics, sedimentology, stratigraphy, diagenesis, geochemistry and geology. Assessment of all these data is a prerequisite for the ‘holistic basin analysis’ needed to guide the search for basin‐hosted ores. Recent results from the Paleoproterozoic Mt Isa and McArthur basins in northern Australia serve as a template for exploring for mineral deposits in basins. Basinal fluids were saline, 200–300°C and evolved primarily from meteoric water in the Mt Isa Basin and from seawater in the McArthur Basin during burial to depths of 4–12 km. The δD_fluid and δ¹⁸O_fluid values in these brines were isotopically identical to those in the Zn‐Pb, Cu and U deposits. Geochemical changes of various lithologies during alteration support detrital minerals as the major source of the U, and volcanic units proximal to diagenetic aquifers as a source for the transition metals. Ages of diagenetic phases extracted from aquifer lithologies reveal that fluid migration from the diagenetic aquifers effectively covers the period of formation for U, Zn‐Pb and Cu mineralization, and that the deposits formed in response to tectonic events reflected in the apparent polar wandering path for the area. Sequence stratigraphic analysis and models of fluid flow also indicate that basinal reservoirs were likely sources for the mineralizing fluids. Thus, diagenetic aquifer lithologies were being drained of fluids at the same time as the deposits were forming from fluids that were chemically and isotopically similar, linking diagenesis and fluid events within the basin to the formation of the deposits.     

Topography‐driven flow versus buoyancy‐driven flow in the U.S. midcontinent: implications for the residence time of brines

Topography‐driven flow is normally considered to be the dominant groundwater flow system in uplifted sedimentary basins. In the U.S. midcontinent region east of the Rocky Mountains, the presence of brines derived from dissolution of halite suggests that significant topography‐driven flushing has occurred to remove older brines that presumably formed concurrently with Permian evaporites in the basin. However, the presence of evaporites and brines in the modern basin suggests that buoyancy‐driven flow could limit topography‐driven flushing significantly. Here we used numerical models of variable‐density fluid flow, halite dissolution, solute transport, and heat transport to quantify flow patterns and brine migration. Results indicate the coexistence of large‐scale topography‐ and buoyancy‐driven flow. Buoyancy‐driven flow and low permeability evaporites act to isolate brines, and the residence time of the brines was found to be quite long, at least 50 Myr. The modern distribution of salinity appears to reflect near‐steady‐state conditions. Results suggest that flushing of original evaporatively‐concentrated brines occurred tens of millions of years ago, possibly concurrent with maximum uplift ca. 60 Ma. Simulations also suggest that buoyancy‐driven convection could drive chemical exchange with crystalline basement rocks, which could supply significant Ca²⁺, Sr²⁺, and metals to brines.     

Fracture mineralization and fluid flow evolution: an example from Ordovician–Devonian carbonates,southwestern Ontario,Canada

Petrography, geochemistry (stable and radiogenic isotopes), and fluid inclusion microthermometry of matrix dolomite, fracture‐filling calcite, and saddle dolomite in Ordovician to Devonian carbonates from southwestern Ontario, Canada, provide useful insights into fluid flow evolution during diagenesis. The calculated δ¹⁸O_fluid, ΣREE, and REE_SN patterns of matrix and saddle dolomite suggest diverse fluids were involved in dolomitization and/or recrystallization of dolomite. The ⁸⁷Sr/⁸⁶Sr ratios of dolomite of each succession vary from values in the range of coeval seawater to values more radiogenic than corresponding seawater, which indicate diagenetic fluids were influenced by significant water/rock interaction. High salinities (22.4–26.3 wt. % NaCl + CaCl₂) of Silurian and Ordovician dolomite–hosted fluid inclusions indicate involvement of saline waters from dissolution of Silurian evaporites. High fluid inclusion homogenization temperatures (>100°C) in all samples from Devonian to Ordovician show temperatures higher than maximum burial (60–90°C) of their host strata and suggest involvement of hydrothermal fluids in precipitation and/or recrystallization of dolomite. A thermal anomaly over the mid‐continent rift during Devonian to Mississippian time likely was the source of excess heat in the basin. Thermal buoyancy resulting from this anomaly was the driving force for migration of hydrothermal fluids through regional aquifers from the center of the Michigan Basin toward its margin. The decreasing trend of homogenization temperatures from the basin center toward its margin further supports the interpreted migration of hydrothermal fluids from the basin center toward its margin. Hydrocarbon‐bearing fluid inclusions in late‐stage Devonian to Ordovician calcite cements with high homogenization temperatures (>80°C) and their ¹³C‐depleted values (approaching ?32‰ PDB) indicate the close relationship between hydrothermal fluids and hydrocarbon migration.     

Origin of palaeo‐waters in the Ordovician carbonates in Tahe oilfield,Tarim Basin: constraints from fluid inclusions and Sr,C and O isotopes

Petrographic features, isotopes, and trace elements were determined, and fluid inclusions were analyzed on fracture‐filling, karst‐filling and interparticle calcite cement from the Ordovician carbonates in Tahe oilfield, Tarim basin, NW China. The aim was to assess the origin and evolution of palaeo‐waters in the carbonates. The initial water was seawater diluted by meteoric water, as indicated by bright cathodoluminescence (CL) in low‐temperature calcite. The palaeoseawater was further buried to temperatures from 57 to 110°C, nonluminescent calcite precipitated during the Silurian to middle Devonian. Infiltration of meteoric water of late Devonian age into the carbonate rocks was recorded in the first generation of fracture‐ and karst‐filling dull red CL calcite with temperatures from <50°C to 83°C, low salinities (<9.0 wt%), high Mn contents and high ⁸⁶Sr/⁸⁷Sr ratios from 0.7090 to 0.7099. During the early Permian, ⁸⁷Sr‐rich hydrothermal water may have entered the carbonate rocks, from which precipitated a second generation of fracture‐filling and interparticle calcite and barite cements with salinities greater than 22.4 wt%, and temperatures from 120°C to 180°C. The hydrothermal water may have collected isotopically light CO₂ (possibly of TSR‐origin) during upward migration, resulting in hydrothermal calcite and the present‐day oilfield water having δ¹³C values from ?4.3 to ?13.8‰ and showing negative relationships of ⁸⁷Sr/⁸⁶Sr ratios to δ¹³C and δ¹⁸O values. However, higher temperatures (up to 187°C) and much lower salinities (down to 0.5 wt%) measured from some karst‐filling, giant, nonluminescent calcite crystals may suggest that hydrothermal water was deeply recycled, reduced (Fe‐bearing) meteoric water heated in deeper strata, or water generated from TSR during hydrothermal water activity. Mixing of hydrothermal and local basinal water (or diagenetically altered connate water) with meteoric waters of late Permian age and/or later may have resulted in large variations in salinity of the present oilfield waters with the lowest salinity formation waters in the palaeohighs.     

Basin‐scale fluid movement patterns revealed by veins: Wessex Basin,UK

Calcite veins at outcrop in the Mesozoic, oil‐bearing Wessex Basin, UK, have been studied using field characterization, petrography, fluid inclusions and stable isotopes to help address the extent, timing and spatial and stratigraphic variability of basin‐scale fluid flow. The absence of quartz shows that veins formed at low temperature without an influence of hydrothermal fluids. Carbon isotopes suggest that the majority of vein calcite was derived locally from the host rock but up to one quarter of the carbon in the vein calcite came from CO₂ from petroleum source rocks. Veins become progressively enriched in source‐rock‐derived CO₂ from the outer margin towards the middle, indicating a growing influence of external CO₂. The carbon isotope data suggest large‐scale migration of substantial amounts of CO₂ around the whole basin. Fluid inclusion salinity data and interpreted water‐δ¹⁸O data show that meteoric water penetrated deep into the western part of the basin after interacting with halite‐rich evaporites in the Triassic section before entering fractured Lower and Middle Jurassic rocks. This large‐scale meteoric invasion of the basin probably happened during early Cenozoic uplift. A similar approach was used to reveal that, in the eastern part of the basin close to the area that underwent most uplift, uppermost Jurassic and Cretaceous rocks underwent vein formation in the presence of marine connate water suggesting a closed system. Stratigraphically underlying Upper Jurassic mudstone and Lower Cretaceous sandstone, in the most uplifted part of the basin, contain veins that resulted from intermediate behaviour with input from saline meteoric water and marine connate waters. Thus, while source‐rock‐derived CO₂ seems to have permeated the entire section, water movement has been more restricted. Oil‐filled inclusions in vein calcite have been found within dominant E‐W trending normal faults, suggesting that these may have facilitated oil migration.     

Geological setting, nature of ore fluids and sulphur isotope geochemistry of the Fu Ning Carlin-type gold deposits, Yunnan Province, China

Carlin‐type gold deposits in southern China are present in Palaeozoic to Mesozoic siliciclastic and carbonate rocks. The border region of Yunnan, Guizhou and Guangxi Provinces contains gold deposits on the south‐western margin of the Pre‐Cambrian South China Craton in south‐eastern Yunnan Province. The Fu Ning gold deposits host epigenetic, micron‐sized disseminated gold in: (i) Middle Devonian (D₁p) black carbonaceous mudstone at the Kuzhubao gold deposit and (ii) fault breccia zones at the contact between Triassic gabbro (β ) and the Devonian mudstone (D₁p) at the Bashishan gold deposit. The deposits are associated with zones of intense deformation with enhanced permeability and porosity that focused hydrothermal fluid flow, especially where low‐angle N‐S striking thrust faults are cut by NW striking strike‐slip and/or NE striking normal faults. Major sulphide ore minerals in the Fu Ning gold deposits are pyrite, arsenopyrite, arsenic‐rich pyrite, stibnite and minor iron‐poor sphalerite. Gangue minerals are quartz, sericite, calcite, ankerite and chlorite. Hypogene ore grades range from 1 to 7 g t^?1 Au and up to 18 g t^?1 Au at the Kuzhubao gold deposit and are generally less than 3 g t^?1 Au at the Bashishan gold deposit. Sub‐microscopic gold mineralization is associated with finely disseminated arsenic‐rich pyrite in the Stage III mineral assemblage. Two types of primary fluid inclusions have been recorded: Type I liquid–vapour inclusions with moderate‐to‐high liquid/vapour ratios, and Type II inclusions containing moderate liquid/vapour ratios with CO₂ as determined from laser Raman analysis. Temperature of homogenization (T_h) data collected from these primary fluid inclusions in gold‐ore Stage III quartz ranged from 180 to 275°C at the Kuzhubao gold deposit and 210 to 330°C at the Bashishan gold deposit. Salinity results indicate that there were possibly two fluids present during gold deposition, including: (i) an early fluid with 0.8–6.5 wt.% NaCl equivalent, similar to salinity in shear‐zone‐hosted gold deposits with metamorphic derived fluids; and (ii) a late fluid with 11.8–13.4 wt.% NaCl equivalent, indicating possible derivation from connate waters and/or brine sources. CO₂ and trace CH₄ were only detected by laser Raman spectrometry in gold‐ore‐stage primary fluid inclusions. Results of sulphur isotope studies showed that δ³⁴S values for pyrite and arsenopyrite associated with gold‐ore mineralization during Stage III at the Kuzhubao and Bashishan gold deposits are isotopically similar and moderately heavy with a range from +9 to +15 per mil, and also fall into the range of δ³⁴S values reported for Carlin‐type gold deposits. Sulphur isotopes suggest that the Fu Ning gold deposits were formed from connate waters and/or basinal brines. Fluid geochemistry data from the Fu Ning gold deposits suggest a Carlin‐type genetic model, involving fluid mixing between: (i) deep CO₂‐rich metamorphic fluids, (ii) moderately saline, reduced connate waters and/or basinal brines; and (iii) evolved meteoric waters.     

Linked thermal convection of the basement and basinal fluids in formation of the unconformity‐related uranium deposits in the Athabasca Basin,Saskatchewan, Canada

World‐class unconformity‐related U deposits in the Athabasca Basin (Saskatchewan, Canada) are generally located within or near fault zones that intersect the unconformity between the Athabasca Group sedimentary basin rocks and underlying metamorphic basement rocks. Two distinct subtypes of unconformity‐related uranium deposits have been identified: those hosted primarily in the Athabasca Group sandstones (sediment‐hosted) and those hosted primarily in the underlying basement rocks (basement‐hosted). Although significant research on these deposits has been carried out, certain aspects of their formation are still under discussion, one of the main issues being the fluid flow mechanisms responsible for uranium mineralization. The intriguing feature of this problem is that sediment‐hosted and basement‐hosted deposits are characterized by oppositely directed vectors of fluid flow via associated fault zones. Sediment‐hosted deposits formed via upward flow of basement fluids, basement‐hosted deposits via downward flow of basinal fluids. We have hypothesized that such flow patterns are indicative of the fluid flow self‐organization in fault‐bounded thermal convection (Transport in Porous Media, 110, 2015, 25). To explore this hypothesis, we constructed a simplified hydrogeologic model with fault‐bounded thermal convection of fluids in the faulted basement linked with fluid circulation in the overlying fault‐free sandstone horizon. Based on this model, a series of numerical experiments was carried out to simulate the hypothesized fluid flow patterns. The results obtained are in reasonable agreement with the concept of fault‐bounded convection cells as an explanation of focused upflow and downflow across the basement/sandstone unconformity. We then discuss application of the model to another debated problem, the uranium source for the ore‐forming basinal brines.     

Metamorphic and basin fluids in quartz–carbonate–sulphide veins in the SW Scottish Highlands: a stable isotope and fluid inclusion study

Metalliferous (Fe–Cu–Pb–Zn) quartz–carbonate–sulphide veins cut greenschist to epidote–amphibolite facies metamorphic rocks of the Dalradian, SW Scottish Highlands, with NE–SW to NW–SE trends, approximately parallel or perpendicular to regional structures. Early quartz was followed by pyrite, chalcopyrite, sphalerite, galena, barite, late dolomite–ankerite and clays. Both quartz–sulphide and carbonate vein mineralisation is associated with brecciation, indicating rapid release of fluid overpressure and hydraulic fracturing. Two distinct mineralising fluids were identified from fluid inclusion and stable isotope studies. High temperature (>350°C) quartz‐precipitating fluids were moderately saline (4.0–12.7 wt.% NaCl equivalent) with low (approximately 0.05). Quartz δ¹⁸O (+11.7 to +16.5‰) and sulphide δ³⁴S (?13.6 to ?1.1‰) indicate isotopic equilibrium with host metasediments (rock buffering) and a local metasedimentary source of sulphur. Later, low‐temperature (T_H = 120–200°C) fluids, probably associated with secondary carbonate, barite and clay formation, were also moderately saline (3.8–9.1 wt.% NaCl equivalent), but were strongly enriched in ¹⁸O relative to host Dalradian lithologies, as indicated by secondary dolomite–ankerite (δ¹⁸O = +17.0 to +29.0‰, δ¹³C = ?1.0 to ?3.0‰). Compositions of carbonate–forming fluids were externally buffered. The veins record the fluid–rock interaction history of metamorphic host rocks during cooling, uplift and later extension. Early vein quartz precipitated under retrograde greenschist facies conditions from fluids probably derived by syn‐metamorphic dehydration of deeper, higher‐grade rocks during uplift and cooling of the Caledonian metamorphic complex. Veins are similar to those of mesothermal veins in younger Phanerozoic metamorphic belts, but are rare in the Scottish Dalradian. Early quartz veins were reactivated by deep penetration of low‐temperature basin fluids that precipitated carbonate and clays in veins and adjacent Dalradian metasediments throughout the SW Highlands, probably in the Permo‐Carboniferous. This event is consistent with paragenetically ambiguous barite with δ³⁴S characteristic of late Palaeozoic basinal brines.     

Syntectonic infiltration by meteoric waters along the Sevier thrust front,southwest Montana

Structural, petrographic, and isotopic data for calcite veins and carbonate host‐rocks from the Sevier thrust front of SW Montana record syntectonic infiltration by H₂O‐rich fluids with meteoric oxygen isotope compositions. Multiple generations of calcite veins record protracted fluid flow associated with regional Cretaceous contraction and subsequent Eocene extension. Vein mineralization occurred during single and multiple mineralization events, at times under elevated fluid pressures. Low salinity (T_m = ?0.6°C to +3.6°C, as NaCl equivalent salinities) and low temperature (estimated 50–80°C for Cretaceous veins, 60–80°C for Eocene veins) fluids interacted with wall‐rock carbonates at shallow depths (3–4 km in the Cretaceous, 2–3 km in the Eocene) during deformation. Shear and extensional veins of all ages show significant intra‐ and inter‐vein variation in δ¹⁸O and δ¹³C. Carbonate host‐rocks have a mean δ¹⁸O_V‐SMOW value of +22.2 ± 3‰ (1σ), and both the Cretaceous veins and Eocene veins have δ¹⁸O ranging from values similar to those of the host‐rocks to as low as +5 to +6‰. The variation in vein δ¹³C_V‐PDB of ?1 to approximately +6‰ is attributed to original stratigraphic variation and C isotope exchange with hydrocarbons. Using the estimated temperature ranges for vein formation, fluid (as H₂O) δ¹⁸O calculated from Cretaceous vein compositions for the Tendoy and Four Eyes Canyon thrust sheets are ?18.5 to ?12.5‰. For the Eocene veins within the Four Eyes Canyon thrust sheet, calculated H₂O δ¹⁸O values are ?16.3 to ?13.5‰. Fluid–rock exchange was localized along fractures and was likely coincident with hydrocarbon migration. Paleotemperature determinations and stable isotope data for veins are consistent with the infiltration of the foreland thrust sheets by meteoric waters, throughout both Sevier orogenesis and subsequent orogenic collapse. The cessation of the Sevier orogeny was coincident with an evolving paleogeographic landscape associated with the retreat of the Western Interior Seaway and the emergence of the thrust front and foreland basin. Meteoric waters penetrated the foreland carbonate thrust sheets of the Sevier orogeny utilizing an evolving mesoscopic fracture network, which was kinematically related to regional thrust structures. The uncertainty in the temperature estimates for the Cretaceous and Eocene vein formation prevents a more detailed assessment of the temporal evolution in meteoric water δ¹⁸O related to changing paleogeography. Meteoric water‐influenced δ¹⁸O values calculated here for Cretaceous to Eocene vein‐forming fluids are similar to those previously proposed for surface waters in the Eocene, and those observed for modern‐day precipitation, in this part of the Idaho‐Montana thrust belt.     

Fluid sinks within the earth's crust

The Urach 3 research borehole in south‐west (SW) Germany has been drilled through the sedimentary cover, and the gneisses of the Variscian crystalline basement at 1600 m below the surface (Black Forest basement) has been reached. An additional 2800 m has been drilled through the fractured crystalline rocks, and the borehole has been used for a number of hydraulic tests in the context of a ‘hot‐dry rock’ (HDR) project exploring for geothermal energy. The fracture system of the basement is saturated with a NaCl brine with about 70 g L^?1 dissolved solids. Water table measurements in the borehole cover a period of 13 years of observation, during which the water table continuously dropped and did not reach a steady‐state level. This unique set of data shows that the hydraulic potential decreases with depth, causing a continuous flow of fluid to the deeper parts of the upper continental crust. The potential decrease and the associated downward migration of fluid is an evidence for the progress of water (H₂O)‐consuming reactions in the crystalline rocks. Computed stability relations among relevant phases at the pressure temperature (PT) conditions in the fracture system and documented fossil fracture coatings in granites and gneisses suggest that the prime candidate for the H₂O‐consuming reaction is the zeolitization of feldspar. The potential of the gneisses to chemically bind H₂O matches the estimated amount of migrating H₂O.     

Reactive transport and thermo‐hydro‐mechanical coupling in deep sedimentary basins affected by glaciation cycles: model development,verification, and illustrative example

Deep sedimentary basins are complex systems that over long time scales may be affected by numerous interacting processes including groundwater flow, heat and mass transport, water–rock interactions, and mechanical loads induced by ice sheets. Understanding the interactions among these processes is important for the evaluation of the hydrodynamic and geochemical stability of geological CO₂ disposal sites and is equally relevant to the safety evaluation of deep geologic repositories for nuclear waste. We present a reactive transport formulation coupled to thermo‐hydrodynamic and simplified mechanical processes. The formulation determines solution density and ion activities for ionic strengths ranging from freshwater to dense brines based on solution composition and simultaneously accounts for the hydro‐mechanical effects caused by long‐term surface loading during a glaciation cycle. The formulation was implemented into the existing MIN3P reactive transport code (MIN3P‐THCm) and was used to illustrate the processes occurring in a two‐dimensional cross section of a sedimentary basin subjected to a simplified glaciation scenario consisting of a single cycle of ice‐sheet advance and retreat over a time period of 32 500 years. Although the sedimentary basin simulation is illustrative in nature, it captures the key geological features of deep Paleozoic sedimentary basins in North America, including interbedded sandstones, shales, evaporites, and carbonates in the presence of dense brines. Simulated fluid pressures are shown to increase in low hydraulic conductivity units during ice‐sheet advance due to hydro‐mechanical coupling. During the period of deglaciation, Darcy velocities increase in the shallow aquifers and to a lesser extent in deeper high‐hydraulic conductivity units (e.g., sandstones) as a result of the infiltration of glacial meltwater below the warm‐based ice sheet. Dedolomitization is predicted to be the most widespread geochemical process, focused near the freshwater/brine interface. For the illustrative sedimentary basin, the results suggest a high degree of hydrodynamic and geochemical stability.