Evaporite Basin Research Articles

Complexation of metal ions in brines: application of electronic spectroscopy in the study of the Cu(II)-LiCl-H2O system between 25 and 90°C

The concentration and transport of metals in hydrothermal solutions depend on how metals ions combine with ligands to form complexes, and experimental methods are necessary to identify the important complexes. UV-Vis-NIR spectrophotometry was used to study the formation of Cu(II)-chloride complexes in LiCl brines up to very high chlorinities (18 m LiCl), at temperatures between 25°C and 90°C. The number of Cu(II)-chloride complexes necessary to account for the variation in spectra with varying chloride molality at each temperature was estimated using principal component analysis. The molar absorptivity coefficients and concentrations of each complex were then determined using a “model-free” analysis, which does not require any assumption about the chemistry of the system, other than the number of absorbing species present. Subsequently, the results from the “model-free” analysis were integrated with independent experimental evidence to develop a thermodynamic speciation model, where the logarithms of the equilibrium constants for Cu(II)-chloride formation reactions were fitted to the data using a non-linear least-squares approach. Maps of the residual function were used to estimate uncertainties in the fitted equilibrium constants.The results of this study are similar to published properties of distorted octahedral [CuCl(OH2)5]+ and [CuCl2(OH2)4]0 at all temperatures, but diverge for [CuCl3(OH2)3]− and distorted tetrahedral [CuCl4]2−. Moreover, the data suggest the presence of [CuCl5]3−, probably with D3h point group, at very high salt concentration. This study demonstrates that it is possible to determine apparent thermodynamic equilibrium constants for the formation of complexes of trace amount of metals in highly concentrated brines, such as those associated with many ore deposits. The results are dependent on the choice of activity coefficients for charged and neutral aqueous complexes, but this influence is relatively small compared with the experimental uncertainty. This study shows that Cu2+ chloro-complexes, predominantly [CuCl2(OH2)4]0 and [CuCl4]2−, will play a dominant role in nature where free oxygen is available (near-surface), and where chloride activities are very high (evaporitic basins; hypersaline soils).

Pb-isotope signatures of Italian alabasters: possible application to provenance studies of works of art

Gypsum alabaster is being used to realise works of art since ancient times. In Italy, a number of quarries exploiting Messinian outcrops (in particular in Sicily and Tuscany) are known since the Etruscan period, and the Opificio delle Pietre Dure of Firenze posed the question to geochemically characterise these in order to recognise the provenance area of raw materials. In this paper, a discrimination was attempted using Pb isotopes because of 1) the low residence time of this element in seawater, and 2) the supposed interaction between Tuscan alabasters and hydrothermal fluids of known Pb-isotope composition. Textures indicate that de- and re-hydration reactions after deposition actually affected alabaster involving fluids circulation. Pb-isotope data are scattered ( 206 Pb/ 204 Pb = 17.88-18.71, 208 Pb/ 204 Pb = 38.01-38.86, 207 Pb/ 204 Pb = 15.58-15.76) suggesting that: 1) fluids did not drive any compositional homogenisation; 2) the isotopic signature is inherited from the evaporitic sedimentary environment and reflects a complex hydrology of the Messinian evaporitic basins; 3) Pb isotopes are an unreliable marker because of the compositional overlap between Tuscan and Sicilian samples. However, at odds with the general trend of their large scattering, the collected data suggest a relative homogeneity at the local scale.

Brine-mineral reactions in evaporite basins: Implications for the composition of ancient oceans

The chemical evolution of several European Mesozoic and Tertiary evaporite basins was reconstructed by using mineral associations, primary fluid-inclusion analyses, and numerical simulations of evaporation scenarios. The solute proportion recorded in the fluid inclusions can be explained by the evaporation of present-day seawater as a major recharge. The sulfate depletion in the brines is responsible for the type of potash deposit formed, potassium-magnessium sulfates or sylvite. This sulfate depletion can be due either to dolomitization or to the addition of a CaCl 2 -rich solution to the basin. The sulfate depletion occurred in varying intensity in basins of the same age, as well as throughout the evolution of the same basin. Therefore, changes in potash mineralogy and sulfate depletion in fluid inclusions are not conclusive arguments in favor of secular variations in the composition of the ocean, as recently proposed by several authors.

The use of environmental markers to distinguish marine vs. continental deposition and to quantify the significance of recycling in evaporite basins

Abstract Environmental markers, namely the bromine content of halite samples, the electrolyte content of primary fluid inclusions in halite and the isotopic composition of sulphates from two Tertiary evaporite sequences, provide complementary information on the depositional environment (marine vs. continental). The use of these markers, together with lithofacies and thicknesses of precipitated evaporites, enable the detection and quantification of evaporite recycling within evaporite basins. The information provided by the isolated use of each of these geochemical markers should be used with caution, as this could lead to erroneous interpretations of the depositional environment or may not detect significant evaporite recycling processes. The complementary information provided by geochemical markers enable quantification of solute input from recycling of previous precipitates within the basins themselves or from older evaporites. The input of recycled evaporites increases progressively, together with evaporite basin restriction to the open ocean.

Evaporitic Subtidal Stromatolites Produced by in situ Precipitation: Textures, Facies Associations, and Temporal Significance

ABSTRACT The transition between carbonate platforms or isolated carbonate buildups and overlying evaporites commonly is marked by assemblages of stromatolites and interlaminated carbonates and evaporites. Stromatolites display lamination textures that vary from peloidal and discontinuous on a scale of a millimeter to a few centimeters, to isopachous and continuously laminated on a scale of a centimeter to a few meters. The isopachous lamination texture may be composed of either: (1) micritic or radial-fibrous calcite, or (2) dolomite. Isopachous stromatolitic laminae are remarkably uniform, varying little in thickness over a given lateral distance, in contrast to stromatolites formed of peloidal laminae, which show marked variation in thickness over an equivalent lateral distance. These isopachous textures are uncommon on most open-marine carbonate platforms and apparently developed in transitional carbonate-to-evaporite settings because of increasing temperature, salinity, and anoxia related to water stratification, which would have created ecologic restriction and an opportunity for stromatolite growth. Stromatolites with isopachous lamination are here interpreted to have formed as a result of in situ precipitation of sea-floor-encrusting calcite and possibly dolomite, whereas the stromatolites composed of peloidal, discontinuous lamination are inferred to have formed by trapping and binding of loose carbonate sediment in microbial mats. While the presence of microbes in almost all near-surface environments nullifies use of the term abiotic to describe most precipitated minerals, we interpret growth of the isopachous stromatolites to have been dominated by chemogenic precipitation in the absence of microbial mats, and the growth of peloidal stromatolites to have been controlled by sedimentation in the presence of microbial mats. These transitional stromatolite facies are best developed atop Proterozoic and Paleozoic carbonate platforms that underlie major evaporite successions. However, inspection of Jurassic and younger evaporite basins, such as the Messinian of the Mediterranean region, shows that stromatolites with thin, isopachous lamination and radial-fibrous textures, though present, are rare. Instead, these facies may have been replaced by stromatolites with peloidal, clastic textures and by low-diversity diatomaceous and coccolith mudstones. Accumulation of the mudstones would have imposed two important effects: (1) Production of coccoliths would have helped extract calcium carbonate from seawater, thus lowering the growth potential for precipitation of sea-floor-encrusting stromatolites. (2) Settling of both coccoliths and diatoms would have created a sediment flux to the sea floor, which would have served to impede growth of precipitated stromatolites because of smothering of growing crystals.

Resedimentation of basin centre sulphate deposits: Middle Miocene Badenian of Carpathian Foredeep, southern Poland

Abstract The Middle Badenian anhydrite sections in the central part of the Carpathian Foredeep consist mainly of interbedded laminated anhydrite and anhydrite breccia. These facies display features characteristic of redeposited sediments such as distortions due to slumping, erosional boundaries, and graded-bedded laminae. Lamination in laminated anhydrite is rarely regular and often accompanied by disruptions. The predominance of clasts of laminated anhydrite combined with the observed brecciation of laminated anhydrite indicates that a short distance for transport and even in situ reworking were common. A high degree of convolution indicates that some reworked sediments were unconsolidated when they moved, but in many cases they were probably at least partly lithified. This is evidenced by the anhydrite breccia consisting of clasts of laminated anhydrite as well as the brecciation in situ. The resedimentation processes have not been related to regional bathymetrical differences as indicated by minor differences along seismic profiles. A great deal of redeposition within the basin centre of the Middle Badenian evaporite basin combined with the in situ brecciation suggests that the triggering mechanism for redeposition could be earthquakes. They generated mass flows and eventually turbidites, so that earlier deposited grains could be redeposited. This implies that in other evaporite basins the laminar facies might be at least partly related to redeposition and not necessarily to changes of surface brine concentration as usually supposed.

WARREN, J. 1999. Evaporites. Their Evolution and Economics. x+438 pp. Oxford: Blackwell Science. Price £49.50 (paperback). ISBN 0 632 05301 1.

Evaporites. Their Evolution and Economics by J. Warren deals with very diverse aspects related to evaporites. Some have already been dealt with in similar previous books, others are a significant novelty. The first three chapters (1, Interpreting evaporite texture; 2, Brine evolution and mineralogy; 3, Evaporite basins and their stratigraphic evolution) describe well-known aspects commonly dealt with in the sedimentological literature. Evaporites are classified as primary, secondary and tertiary, ‘secondary’ corresponding to diagenetic (including sabkha textures and mineralogies) and ‘tertiary’ to evaporites in the exhumation stage. Brine classification into marine and continental, and theories about evolution of marine brines (global change v. mixture with basinal brines), are explained. Special attention is given to …

The impact of brine-rock interaction during marine evaporite formation on the isotopic Sr record in the oceans: evidence from Mt. Sedom, Israel

The effect of brine-rock interaction on the composition of strontium in evaporitic basins and its impact on the 87Sr/ 86Sr ratios in contemporaneous seawater are examined for the Sedom (Dead Sea Rift Valley, or DSR), the Messinian (Mediterranean) and the Louann (Gulf of Mexico) evaporites. For that purpose, mineralogical, chemical and isotopic (Sr, S) analyses were performed on the Sedom Fm. evaporites (halite, anhydrite and dolomite). 87Sr/ 86Sr ratios are distinctively lower in the Sedom evaporites (dolomites: 0.7082–0.7083; halites: 0.7083–0.7087) than in the contemporaneous late Pliocene seawater (≈0.709). At the same time the sulfur isotope ratios (δ 34S ≈ 20‰) are consistent with deposition from late Cenozoic seawater. This duality, together with the variation of strontium isotopes between the dolomites and halites can be explained by modification of the 87Sr/ 86Sr ratio in the lagoon water by influx of Ca-Chloride brines. The brines were formed by dolomitization of marine carbonates of the DSR Cretaceous wall rocks (where 87Sr/ 86Sr ∼ 0.7077). Brine-rock interaction can similarly explain the anomalous 87Sr/ 86Sr ratios in the Messinian and Louann evaporites. It is concluded that this process causes significant changes in the 87Sr/ 86Sr ratios of evaporitic lagoons. A water and strontium mass balance of the Sedom data is used to show the impact on the strontium oceanic budget. Extrapolation to larger evaporitic basins indicates that the combined global riverine and hydrothermal influx of strontium can be matched by halite or gypsum precipitating lagoon of 2–3.5 × 10 5 km 2. Examples for such evaporitic sites include the Messinian, Louann and Zechstein basins.

Parent Brine of the Castile Evaporites (Upper Permian), Texas and New Mexico

ABSTRACT The Upper Permian (lower Ochoan) Castile Formation is a major evaporite sequence (10,000 km3) of calcite, anhydrite, and halite in west Texas and southeastern New Mexico. Traditionally the Castile brine has been considered to have been derived from seawater. This tradition has recently been challenged by two versions of the closed-basin drawdown model. They call for deposition from a mixed brine, in part marine and in large part nonmarine. They propose drawdown of as much as 500 m to form a major sink for ground water issuing from the surrounding Capitan reef complex. A large fraction of the solute in the brine body is inferred to have been recycled from older Permian evaporites on the surrounding shelf. Strontium-isotope analyses show no evidence that meteoric ground water was contributed to the Castile brine. From a stratigraphic, geographic, and lithologic array of 65 samples of anhydrite, gypsum, and calcite, 59 have an 87Sr/86Sr ratio of 0.706923 (sw of -225.0), a ratio that is the same as that of strontium in early Ochoan ocean water. If considerable (>15%) influx of meteoric water had occurred, enough continental strontium would have been introduced to have resulted in higher ratios. Low bromide values (20-40 ppm) in Castile halite, which have been used to argue for meteoric influx and for recycled salt, probably resulted from diagenesis. During shallow burial by halite, centimeter-size, bottom-grown crystals of gypsum were altered to nodular anhydrite. The rising water of dehydration caused the halite to recrystallize. During the recrystallization, some bromide was expelled. Despite the large volume of water that evaporated annually from its surface (52 km3/yr, assuming an evaporation rate of 2 m/yr), the Castile brine body never completely desiccated. The surrounding shelf was flat, hot, and generally dry. It probably could not have supplied a significant volume of meteoric spring water to the basin over tens of thousands of years. More likely, during the entire history of the evaporite sequence, influx was dominantly marine. Marine ground water flowed through the Capitan Formation into the evaporite basin along its southern and possibly western margin probably with a rate of flow that was usually fast enough to prevent major drawdown of the brine surface.

Evolution and geochemical signatures in a Neogene forearc evaporitic basin: the Salar Grande (Central Andes of Chile)

The Salar Grande is a 45-km-long (N–S axis), 4–5-km-wide, fault-bounded (pull-apart) Neogene forearc evaporitic basin, located in the Cordillera de la Costa of northern Chile whose sedimentary record is composed almost exclusively of a massive halite salt body. Underground waters coming from the east (now the Precordillera and Altiplano) were progressively enriched in solutes by interaction with volcanic rocks. These fluids drained into a lacustrine system located in the Depresión Central (in the Llamara–Quillagua area) and thick sequences of diatomites, carbonates, evaporites and clastic sediments were deposited. These lacustrine areas eventually dried out and became a salar with anhydrite and halite. During this stage (Late Miocene–Pliocene), evolved brines of the Llamara–Quillagua area migrated eastwards through structural paths, reaching the Salar Grande basin which acted as a final sink. A proof of this connection is the anhydrite level that links the western Llamara and southeastern Salar Grande areas. The accessory minerals and geochemical signatures in the rock salt are a record of the inputs arriving to the former lake and reflect variations in climate, and in tectonic and volcanic activity. The arrangement of halite textures along the sequence shows an evolution from a very shallow ephemeral saline lake (some cm–dm deep on average) to a salar where interstitial processes controlled by phreatic brines dominate. This evolution is coherent with a slight increase in the terrigenous content of the rock salt, and documents the increasing aridity of the climate as a response to the tectonic uplift of the Andean Ranges. In Pliocene times pore brines were definitively lost as a response to the downfall of the water table generated by the beginning of the exorheism due to the opening of the Loa River canyon towards the sea. The aim of this paper is to describe the Salar Grande through the study of its geological setting and its salt record. A detailed description of the salar will enable us to reach valuable conclusions about environmental conditions in the area during the Neogene, prior to the opening of the hydrologic system to the Pacific Ocean.

Giant polygons in the Realmonte Mine (Agrigento, Sicily); evidence for the desiccation of a Messinian halite basin

ABSTRACT The Messinian Realmonte salt deposit consists of a 400-600 m-thick halite succession that can be divided into four main lith ologic units (base to top): A) cumulates of plates settled out from a stratified water column; B) plate cumulates in a shallowing-upward sequence containing six kainite layers near the top of the unit; C) cu mulates of skeletal hoppers with chevron overgrowths precipitated from a nonstratified water body; and D) skeletal halite and anhydrite. Spectacular vertical fissures cut through the upper part of unit B at its boundary with salt unit C. These fissures are spaced at intervals of up to 5 m apart, extend down to a depth of 6 m, and are filled by red mud. The salt beds affected by the fissures are commonly upturned (buckled) and are truncated and onlapped by the succeeding flat-lying halite beds of unit C. Additionally, the upturned layers are cut by vertical dissolution pipes that extend down to as much as a depth of 4 m below the fissured surface. To explain the observed superimposed structures we propose two mechanisms, both induced by the desiccation of the evaporite basin: (1) When the salt surface was exposed, the halite layers were buck led, broke into polygonal crusts, and formed tepee structures. The buckling was caused by a net volume increase due to thermal expan sion, and what is more important, precipitation of new halite induced by evaporative pumping of brines moving up from the groundwater table. The buckled salt layers were then affected by meteoric dissolu tion, which created vertical pipes. (2) The tepee structures and the associated zone of dissolution pipes are further crosscut by a new network of large polygons. These new polygonal sets are outlined by deep fissures in which red silt collected, blown by the wind or carried by episodes of surface flooding. This type of deep contraction crack is known to be produced by volumetric changes induced by annual temperature fluctuations. The documentation of contraction polygons and other exposure fea tures represents the first report of desiccation events during the de position of the Sicilian salt. No geological data are yet available to settle the questions if the salt of Sicily is correlatable with the halite present beneath the floor of the Mediterranean and if the desiccation of the Realmonte salt basin could have been induced by: a) local tectonic causes (uplift of the basin floor by thrust activity, b) simple evaporitic drawdown, c) basinwide drop of the Mediterranean sea level, or d) complex interaction among these factors.

Stable chlorine isotope evidence for non‐marine chloride in Badenian evaporites, Carpathian mountain region

Routine trace‐element geochemistry suggests that components in putative marine halite evaporites may be partly of nonmarine origin, but such interpretations are commonly ambiguous. Stable chlorine isotopes may provide a less‐ambiguous marker of chloride origin where δ37Cl departs from the range predicted for evaporite formation from seawater. Bedded halite with primary sedimentary textures preserves original δ37Cl values. Measurable change in δ37Cl can be generated by incongruent dissolution of halite, but only if less than half the original halite remains. Badenian (middle Miocene) halite from the Forecarpathian and from the East Slovakian and Transcarpathian basins has a δ37Cl range of – 0.2 to 0.8‰. Two phenomena cannot be explained by simple evaporation of 0.0‰ seawater. At Wieliczka, the Shaft Salt has distinctive δ37Cl values (– 0.2 to 0.0‰) relative to neighbouring salt beds (0.2 to 0.6‰), requiring a large, abrupt input of brine with negative δ37Cl. Halite with high (0.6 – 0.8‰) δ37Cl near the base of the East Slovakian and Transcarpathian evaporites requires a large input of chloride with positive δ37Cl into the basins. Expulsion of basin brine with non‐0‰δ37Cl into the evaporite basins may account for the nonmarine chloride sources.

Impact of salt on the structure of the Central North Sea hydrocarbon fairways

The Central North Sea lies within the North Permian evaporite Basin, which was largely flexural but with locally significant basement faults. While the main evaporite basin fill is halite, seismically reflective basin margin facies delineate the salt basin shape. The marginal Zechstein strata record Permian salt tectonics including listric faulting, slumping of kilometre-scale carbonate rafts and halite diapirism, all related to localized pre-Triassic gravity-driven collapse of the Zechstein basin margin. Triassic thickness in the Central North Sea (CNS) was directly influenced by salt, the most significant phase of salt tectonics in the history of the CNS in terms of volume of salt redistributed and dissolved. Lower–Middle Triassic ‘minibasins’ subsided into the salt over much of the CNS. Different initiating mechanisms operated around the basin–differential loading was important near sediment input points, and thin-skinned extension on the platforms balanced a pulse of basement extension in the Central Graben. Minibasin subsidence was accommodated by salt wall growth; salt was eroded during periods of base-level fall and redeposited locally and in the Southern North Sea. The minibasins touched down on basement progressively from the basin margins towards the Central Graben, the timing being a function of salt thickness. The diachronous halting of accommodation space generation combined with the effects of erosion led to a variety of field relationships between the Middle–Upper Triassic Skagerrak sands and the underlying salt–minibasin system. Regionally, the Skagerrak sands are thickest near input points and in the Central Graben, where accommodation space was supplemented by basement faulting. The platforms were subaerially exposed and eroded for much of the period between the late Triassic and late Jurassic. On these platforms, the distribution of the relatively thin Late Jurassic section was controlled by a combination of regional tilt and pre-existing topography created by differential erosion (including dissolution) of the salt walls versus grounded minibasins. The late Jurassic Fulmar shallow marine sands are thickest on the differentially eroded salt highs, which constituted several hundred foot-deep palaeovalleys. Rifting occurred in the Central Graben where tilts of detachments on top of major basement fault blocks caused thin-skinned extension in the overlying strata, either for the first time, or reactivating Triassic minibasin systems. The Fulmar sands are thickest where basement-controlled accommodation space was accentuated by detached fault systems (e.g. fallen diapirs) near sediment input points in the Central Graben. The Early Cretaceous was a time of tectonic stabilization following Late Jurassic basement rifting, although the perched detachment systems in the major basement graben continued to evolve. Accommodation space for Upper Jurassic and Lower Cretaceous turbidites was influenced by up-dip salt high collapse and down-dip ‘rim syncline’ growth. By the end of the Early Cretaceous these systems had stabilized and the surrounding basement highs were sufficiently blanketed to shut off salt dissolution. Chalk isopach variations pick out subtle regional tilts and local basement fault reactivations. Major flexural subsidence in the Tertiary tilted the basin again and precipitated local cover slip, e.g. extensional faults balanced by down-dip folds on the West Central Shelf and diapir rejuvenation on the graben margin. A variety of inversion structures are present in the CNS and are dominantly Cretaceous and Tertiary in age. Several styles of inversion structure occur including thin-skinned and thick-skinned compression, diapir rejuvenation, compaction and dissolution-related features. Salt governed the geometry of many of these structures and in turn the geometry of the Tertiary sand fairways.

Evolution tectono-sédimentaire du Nord-Est du Maroc au cours du Trias supérieur : apport des données microtectoniques

Tectonic-sedimentary evolution of North-Eastern Morocco during Upper Trias : Microtectonic evidences. North-eastern Morocco occupies a hinge zone between two evaporitic basins of the Tethys margin : the Guercif basin in the west and the High Plateau basin in the south. The study of tectonic-sedimentary structures (brittle and viscoplastic strutures) enables to reconstruct and illustrate the control of extensional tectonics on sedimentation. Quantitative analysis led us to distinguish a major state of extensive stress, NNW-SSE orientes (with ENE-WSW permutation of stress), and an other NW-SE extension. The evolution of stress trends determinate here in is correlated with the sedimentary organisation during the Ladinian to Norian. This evolution occured before crustal extension in the Atlantic and Atlasic Moroccan domains.

The source rock horizons of the Ionian Basin (NW Greece)

The organic geochemical study of six geological sections and two wells in the Ionian Zone (NW Greece), allows us to distinguish five horizons of possible source rocks: The Vigla shales (Cenomanian–Turonian), the Upper Posidonia Beds (Callovian–Tithonian), the Lower Posidonia Beds (Toarcian–Aalenian), the marls at the base of the Ammonitico Rosso (Early Toarcian), and shale fragments incorporated within the Triassic breccias. These horizons have good hydrocarbon potential and an organic matter type I to II. In the deeper parts of the Botsara sub-basin, the oil window is located in the interval between 3700–5800 m depth. Consequently, the Triassic shales have already entered the gas window. The Lower and Upper Posidonia Beds, and the marls at the base of the Ammonitico Rosso are mature in terms of oil generation. The Vigla shales maturity corresponds to the early maturation stage. The timing of the principal oil-source horizons maturation is the Late Jurassic for the Triassic shale beds and the Serravalian for the Lower Posidonia Beds. The preservation of the organic matter in the Lower and Upper Posidonia Beds through Toarcian to Tithonian and in the marls at the base of the Ammonitico Rosso during Early Toarcian are directly related to the geometry of the syn-rift period of the Ionian Basin. The organic matter preservation in the Vigla shales is related to the sub-basins that were preserved by the continuation of halokinetic movements during the post-rift period. The geometry of the restricted sub-basins that were formed during the syn-rift and post-rift period of the Ionian Basin evolution favored water stagnation and the development of local euxinic conditions in the bottom waters; these conditions were accentuated during the anoxic events that are known to have affected the Tethys ocean during the Early Toarcian and Late Cenomanian. The organic rich shale fragments within the Triassic breccias were initially deposited as stratigraphic layers in sub-basins of the evaporitic basin. The establishment of evaporitic sedimentation in the entire basin favored the preservation of the organic matter. The processes that resulted in the formation of the evaporite dissolution collapse breccias caused also the fragmentation of the organic rich layers, which are present actually as organic rich shale fragments within the Triassic breccias.

The evolution of the Lorraine evaporite basin: implications for the chemical and isotope composition of the Triassic ocean

During the Upper Triassic times about 75 m of halite with minor anhydrite was deposited in the Lorraine basin. Samples of halite and sulfates were systematically collected from a borehole intersecting the sequence. The chemical evolution of the original brine was investigated by X-ray microanalysis of primary inclusions trapped in halite. The original brines were of the Na–Mg–K–Ca–Cl type, being consistent with the mineral association of halite and minor anhydrite. Their solute content can be reproduced by evaporation of present day ocean composition in an open basin. However, some modifications are required. The inflow of a small excess of CaCl 2 into the basin (1.2 to 1.9% of the total recharge) is required to explain the low SO 4 values analyzed in fluid inclusions. The same inflow helps to explain more satisfactorily the low Na and high Cl contents. The back-reaction of the brine with previous Ca-sulfate that forms polyhalite could explain the low and variable K values analyzed. The excess of Ca with respect to present day seawater, and the formation of SO 4-poor evaporite sequences is the rule rather than the exception in the geological record. We have not been able to assess the ultimate origin of the Ca excess in the basin. Nevertheless, the great variation of the SO 4 content within short vertical distances (centimeter scale), suggests that these variations are due to processes taking place within the evaporite basin, rather than to changes in the composition of the ocean at a global scale. The isotopic composition of sulfur and oxygen was determined using samples of sulfates from the same borehole. The δ 34S values of 21 samples analyzed show a narrow variation: δ 34S=+15.5±0.4‰. The δ 18O values of eight samples analyzed show a wider range of variation: δ 18O=+13.0±1.1‰. None of the processes which modify the δ values, such as sulfate reduction or additional sulfate sources (continental, recycling of sulfates from the margins), were clearly identified. The values of δ 34S SW=+15.7 to +14.5‰ and δ 18O SW=+13.7 to +12.2‰ for the sulfate dissolved in the Triassic ocean explain the δ values of the sulfates interbedded in the halite of the Lorraine basin, and most analyses reported in the literature.

Origin of polyhalite deposits in the Zechstein (Upper Permian) Zdrada platform (northern Poland)

Polyhalite deposits in the Zechstein (Upper Permian) of northern Poland occur in the Lower Werra Anhydrite. In the Zdrada Sulphate Platform, the polyhalite appears to be a very early replacement of anhydrite. The replacement was caused by the halite‐precipitating brines which contained potassium and magnesium ions. The formation of polyhalite was preceded by the syndepositional anhydritization of the original gypsum deposit which has often preserved its primary textures. This anhydritization on the platform and its slopes was a reaction of the precipitated gypsum in a hydrologically open evaporite basin, with brines of salt basins adjacent to the sulphate platform. These brines, when nearly saturated with respect to halite, and potassium and magnesium rich, reacted with anhydrite to precipitate polyhalite along the slopes of the Zdrada Platform. The oxygen and sulphur isotopic compositions of sulphate evaporites indicate that marine solutions were the only source of sulphate ions supplied to the Zechstein basin, and that anhydrite was transformed to polyhalite by reaction with marine brines more concentrated than those that precipitated precursor calcium sulphate minerals.

La série évaporitique keupérienne de Lorraine-Champagne (France). Analyse et implications génétiques des discontinuités sédimentaires

In the Lorraine-Champagne basin, the Carnian salt deposit (~ 150 m thick) presents numerous discontinuities located at the limit between successive ‘event sequences’. Each sequence begins with the flooding of the basin and a shallow sedimentation of claystones and halite beds (maximum of a few metres thick). A trend towards a subaerial environment announces the second phase marked by an emersion (tepees) and a long period of desiccation (giant polygonal fissures). The sediments (claystones and sulfates) laterally deposited in marginal flats show the same discontinuities. Similar features are known in the Permian, Triassic and Miocene evaporite basins of the U.S.A., West Europa and Morocco.

Massive dolomitization of a late Miocene carbonate platform: a case of mixed evaporative brines with meteoric water, Nijar, Spain

Late Miocene platform carbonates from Nijar, Spain, have been extensively dolomitized. Limestones are present in the most landward parts of the platform, in stratigraphically lower units and topographically highest outcrops, suggesting that dolomitizing fluids were derived from the adjacent Nijar Basin. The dolomite crystals range from <10 to ≈100 μm existing as both replacements and cements. Na, Cl and SO4 concentrations in the dolomites range from 200 to 1700 p.p.m., 250–650 p.p.m., and 600–7000 p.p.m., respectively, comparable with other Tertiary and modern brine dolomite values, and also overlapping values from mixing‐zone dolomites. Sr concentrations range between 50 and 300 p.p.m., and the molar Sr/Ca ratios of dolomitizing fluids are estimated to range between 7× seawater brine to freshwater ratios. The δ18O and δ13C of the dolomites range from −1·0 to +4·2‰ PDB, and −4·0 to +2·0‰ PDB, respectively. 87Sr/86Sr values (0·70899–0·70928) of the dolomites range from late Miocene seawater to values greater than modern seawater.Mixtures of freshwater with seawater and evaporative brines probably precipitated the Nijar dolomites. Modelled covariations of molar Sr/Ca vs. δ18O and Na/Ca vs. δ18O from these mixtures are consistent with those of the proposed Nijar dolomitizing fluids. Complete or partial dolomite recrystallization is ruled out by well preserved CL zoning, nonstoichiometry and quantitative water–rock interaction modelling of covariations of Na vs. Sr and δ18O vs. δ13C. The possibility of multiple dolomitization events induced by evaporative brines, seawater and freshwater, respectively, is consistent with mineral‐mineral mixing modelling.The basin‐derived dolomitizing brines probably mixed with freshwater in the Nijar Basin or mixed with fresh groundwater in the platform, and were genetically related either to deposition of the Yesares gypsum or the Feos gypsum. Dolomitization occurred during either the middle Messinian or the early upper Messinian. Nijar dolomitization models may be applicable to dolomitization of other late Miocene platform carbonates of the western Mediterranean. Moreover, the Nijar models may offer an analogue for more ancient evaporite‐absent platform carbonates fringing evaporite basins.

Alkali alteration styles and mechanisms, and their implications for a ‘brine factory’ source of base metals in the rift‐related McArthur group, Australia

McArthur Group alkali metasomatism occurred in several phases, none of which fit a classic model of alkali‐mineral zonation towards an evaporitic basin centre as previously proposed. They comprise: (i) trough‐margin alteration along the Emu Fault Zone, formed by gravity‐driven meteoric‐evaporitic brines sourced from adjacent shelves, and focused up to 3.5 km out into the adjacent deeper water clastic succession of the Barney Creek Formation; these formed zoned albite‐microcline assemblages, variably overprinted by B‐bearing illite; (ii) ore‐related alteration at the HYC Zn‐Pb‐Ag deposit, which overprinted trough‐margin alteration in places, adding disseminated ankerite, adularia and base‐metal sulfides to some feldspathised beds; (iii) early diagenetic alkali metasomatism that was associated with descending/advecting brines sourced from overlying evaporite sequences, and transmitted through porous clastic sediments (e.g. parts of the Lynott Formation, Mara Dolomite and Tatoolla Sandstone); and (iv) extensive inversion‐related K‐feldspar metasomatism of volcaniclastic beds towards the basin centre. The descending/advecting brine model (iii) has implications for sedimentary Zn‐Pb ore genesis in the basin, because it offers a mechanism to efficiently deliver batches of saline fluid into the deep diagenetic environment, resulting in alkali alteration and metal leaching, and the production of ideal Zn‐Pb‐transporting ore fluids. A variation in the flux of saline diagenetic water, caused by eustatically and tectonically driven aridity cycles in sedimentary facies, could have imposed strong episodicity on the formation of related Zn‐Pb deposits. This speculative model is relevant to metals exploration on the premise that each ‘brine factory’ potentially gave rise to a related cycle of Zn‐Pb ore deposition at the same or an overlying stratigraphic level.