Chemical composition of seawater in Neoproterozoic: Results of fluid inclusion study of halite from Salt Range (Pakistan) and Amadeus Basin (Australia)

Data on chemical composition of brines in primary inclusions of marine halites and on mineralogy of marine evaporites and carbonates lead to the conclusion that during the Phanerozoic two long-term cycles of chemical composition of seawater existed. During each of those cycles, seawater dominantly a Na-K-Mg-Ca-Cl (Ca-rich) type changed to a Na-K-Mg-Cl-SO₄ (SO₄-rich) type. Recrystallised halite from the uppermost Neoproterozoic Salt Range Formation (ca. 545 Ma) in Pakistan, contains solitary inclusions indicating SO₄-rich brines. This supports the concept derived from the study on primary fluid inclusions from the Neoproterozoic Ara Formation of Oman; SO₄-rich seawater existed during latest Neoproterozoic time (ca. 545 Ma). In contrast, samples of recrystallised halite from the Bitter Springs Formation (840–830 Ma) in Australia contain inclusion brines that are entirely Ca-rich, indicating that basin brines and seawater were Ca-rich during deposition of central Australian evaporites. These combined data supported by the timing of aragonite and calcite seas suggest that during the Proterozoic, significant oscillations of the chemical composition of marine brines, and seawater, occurred, which are similar to those known to exist during the Phanerozoic. It is suggested that Ca-rich seawater dominated for a substantial period of time (more than 200 Ma), at 650 Ma, this was replaced by SO₄-rich seawater, finally returning to Ca-rich seawater at 530 Ma.     

Water-rock interaction and chemistry of groundwaters from the Canadian Shield

The chemical and isotopic compositions of groundwaters in the crystalline rocks of the Canadian Shield reflect different degrees of rock-water interactions. The chemistry of the shallow, geochemically immature groundwaters and especially of the major cations is controlled by local rock compositions, whereby dissolution reactions dominate. Conservative constituents, such as chloride and bromide, however, are not entirely a result of such reactions but appear to be readily added from leachable salts during the initial stages of the geochemical evolution of these waters. Their concentration changes little as major cations increase, until concentrations of Total Dissolved Solids (TDS) reach 3000 to 5000 mg 1^?1. The isotopic composition of these shallow waters reflects local, present day precipitations.In contrast to the shallow groundwaters, the isotopic and chemical compositions of the deep, saline waters and brines are determined by extensive, low-temperature rock-water interactions. This is documented in major ion chemistries, ¹⁸O contents and strontium isotopic compositions. These data indicate that the deep brines have been contained in hydrologically isolated “pockets”. The almost total loss of primary compositions make discussions on the origin of these brines very speculative. However, all brines from across the Canadian Shield have a very similar chemical composition, which probably reflects a common geochemical history. The concentrations of some major and most minor elements in these fluids appear to be governed by reactions with secondary mineral assemblages.     

Hydrogen,oxygen, helium and strontium isotopic constraints on the formation of oilfield waters in the western Qaidam Basin,China

Surveys and exploration for oil and gas have revealed many oil-field brines in the Tertiary strata of the western Qaidam Basin, China. The source and formation of these brines are unclear. Brine samples collected from petroleum wells in the oilfield area were analyzed for their general chemical composition and for hydrogen, oxygen, strontium and helium isotopes in order to trace their origin, formation, and resource distribution. Results show that the concentrations of resources such as K, B, Sr, Br and Li are unusually enriched in oil-field brines and have excellent potential for development and utilization in future. The geology, H, O, Sr and He isotopic composition, the chemical composition, and the geophysical measurements all show that the tertiary oil-field brines predominantly originated from the deep mixture of formation water and magmatic residual fluids in the western Qaidam Basin. The tectonic geology, intense and frequent magmatic-hydrothermal activity as well as high ³He/⁴He ratio indicate that the magmatic fluids possibly originate from the upper mantle-derived magmatic fluids. Thus, according to our interpretation of the formation and evolution of the oilfield brine, this source is adequate for the volume and composition to be explored and utilized latterly.     

基于相化学研究老挝万象钾镁盐矿床形成的机制

老挝万象钾镁盐矿床是一个典型的海相碎屑盐缺硫酸盐型钾盐矿床, 该矿床形成于古近纪, 是古海水蒸发浓缩沉积形成。老挝万象钾镁盐矿床中缺乏硫酸盐和碳酸盐沉积物, 因此深入研究该矿床的形成机制很重要。本文研究探讨了该矿床形成时的古海水特点, 根据相化学, 分析成钾原始卤水的物理化学特性, 从矿体形成的化学基础来研究老挝钾镁盐矿床形成的机制。结果表明: 显生宙以来海水组分发生变化, 经海相非骨骼灰岩和钾盐蒸发岩矿物学研究, 发现这两种沉积岩长期以来连续变化, 在“文石海”是MgSO4型蒸发盐, 在“方解石海”是KCl型蒸发盐, 从白垩纪晚期、第三纪早期的底部石盐溴含量及矿物学特征表明, 此时处于“方解石海”, 古海水组分的特点是造成缺硫酸盐型钾盐矿床形成的物化基础; 通过NaCl-KCl-MgCl2-H2O和NaCl-KCl-MgCl2-CaCl2-H2O两个体系相图的分析认为, 当时所形成的成钾原始体系母液是高镁、低钾氯化物型的卤水, 在母液蒸发过程中, 由于原始海侵母液与残余高镁母液的掺杂作用, 致使结晶路线直接从氯化钠区到E点母液或光卤石与氯化钠共饱线上, 而没有通过氯化钠和氯化钾的共饱线, 因而在矿体中氯化钾相很少或几乎不存在, 由于外界CaCl2型水体的掺杂, 使成钾母液进入光卤石相区, 随着蒸发的进行, 最终形成溢晶石矿物。     

The behavior of lithium and its isotopes in oilfield brines: evidence from the Heletz-Kokhav field, Israel

Twenty-four brine samples from the Heletz-Kokhav oilfield, Israel, have been analyzed for chemical composition and Li isotope ratios. The chemical composition of the brines, together with geological evidence, suggests derivation from (Messinian) seawater by evaporation that proceeded well into the gypsum stability field but failed to reach the stage of halite crystallization. The present salinity of the samples (18-47 g Cl/L) was achieved by dilution of the original evaporitic brine by local fresh waters. Like brines from other sedimentary basins, the Li/Cl ratios in the Heletz-Kokhav samples show a prominent Li enrichment (five-fold to eight-fold) relative to modern seawater. The isotopic ratios of Li, expressed in the δ ⁶Li notation, vary from −26.3 to −17.9‰, all values being significantly higher than that of modern seawater (−32‰) irrespective of their corresponding Li concentration (1.0-2.3 mg/L). The isotopic composition of Li and the Li/Cl ratio in the oilfield brines were acquired in two stages: (a) The original evaporated seawater gained isotopically light Li during the diagenetic interaction between the interstitial Messinian brine and the basin sediments. A parent brine with an elevated Li/Cl ratio was formed. The brine was later diluted in the oilfields. (b) The δ ⁶Li values of the final brines were determined during epigenetic interaction with the Heletz-Kokhav aquifer rocks. At the same time, the Li/Cl ratio inherited from stage (a) remained largely unchanged. This work represents the first use of lithium isotopic composition to elucidate the origin and evolution of formation waters in sedimentary basins.     

Hydrochemical characteristics and controlling factors for waters’ chemical composition in the Tarim Basin,Western China

This paper covers the chemical and isotopic composition of river water, groundwater from wells (15–25 m), saline spring water and stagnant surface water providing evidence for controlling factors of water composition and water evolution process in the Tarim Basin, Xinjiang, western China. Analytical data for major and minor ions of totaling 537 water samples were obtained from both years of teamwork and old reference materials. It is found that the ion background value ratio SO₄/Cl for river water (2.75) of the Tarim Basin is two times higher than that of the Qaidam Basin (0.88) and 18 times higher than seawater (0.14); K/Cl of these two basins (0.06 and 0.07) are all two times higher than seawater (0.02). This reveals that material sources of Lop Nur are relatively richer in potassium and sulfate, while poorer in chloride. Gradual changes of stable isotopic compositions in waters clearly indicate the effect of evaporation on water evolution of the basin. Besides evaporation and weathering of surrounding rocks, wide distribution of chloride type water, which commonly exist in saline springs/brines and seldom exist in other waters, indicates that hydrothermal Ca–Cl brines discharged from deep within the earth join water evolution of the basin.     

Geochemistry and stable isotopic signatures,including chlorine and bromine isotopes,of the deep groundwaters of the Siberian Platform,Russia

The chemical composition and the isotopic characteristics of formation waters from the Siberian Platform are presented. The study involved samples of formation brines from depths ranging from 100 to ∼4000 m at five different sites covering a large area of the Siberian Platform. Four water types were identified. The two main water types that were found are: (1) Ca–Cl brines that are believed to be the residual of an evaporated paleoseawater; and (2) Na–Cl brines that are derived mainly from halite dissolution. The origin of a third group of highly saline samples was not determined. However, the chemical and isotopic characteristics of this group of samples suggest that they were produced by various complex scenarios such as metamorphism, water–rock interaction, permafrost freezing and mixing. The last group of samples represents fresh and brackish waters across the area.     

Elemental composition of concentrated brines in subduction zones and the deep continental crust

It has been well established that fluids played an important part in determining chemical characteristics in many crustal terranes. Studies of fluid inclusions in eclogites have established that brines coexisted with the primary mineral assemblages during their metamorphic crystallization. These brines are currently multiply saturated in halide salts, carbonates, oxides, and sulfides. As a first step in quantitatively bounding the composition of the brines during metamorphism, the equilibrium compositions of the brines at room temperature were computed using the aqueous speciation codes EQ3/6. The results demonstrate that the brines are high density solutions (ca. 1.4 g/cm³) that have ionic strengths of approximately 8 mol, and are approximately 40% dissolved solids, by weight. These are predominately Na- and K-rich brines, with subordinate Ca and Mg. The approximate Na:K:Ca:Mg molar ratios are 4:2:0.5:0.2, but are sensitive to the assumed HCO₃⁻ concentrations. Charge balance is primarily maintained by the very high Cl concentrations. These brines bear resemblance to brines analyzed from fluid inclusions in metamorphic rocks reported by Roedder (Roedder, E., 1972. Composition of fluid inclusions. US Geol. Surv. Prof. Paper 440JJ, p. 164). Although these fluids have the potential of acting as significant metasomatic agents in subduction zones and deep crustal environments, their impact will be mineralogically discernible only if the fluid release and movement is channelized.     

柴达木盆地西部古近系和新近系油田卤水资源水化学特征及化学演化

在柴达木盆地西部一些背斜构造单元的古近系-新近系中,赋存储量巨大的油田卤水，有望成为第四系盐湖卤水的后续利用资源。通过对小梁山、南翼山、油泉子、开特米里克、油墩子、油砂山等典型构造区石油钻孔自喷的油田卤水以及地表盐湖卤水、晶间卤水取样分析，讨论了其水化学特征及资源分布，重点通过与海水、青海湖水蒸发曲线对比，分析了油田卤水的化学演化特征。结果表明，油田卤水中K、B、Li资源远超工业开采品位，Br、Sr等也达到工业开采品位，有优越的高品位综合开发利用前景；油田卤水资源元素富集规律表现为平面上以中部南翼山背斜最为富集，向北、向南相对降低，垂向上深部油田卤水比浅层晶间卤水、湖表卤水资源元素相对更为富集；油田卤水的化学演化主要受控于水岩反应、深部水的混合以及蒸发浓缩和盐岩的溶解作用，这些作用为卤水分异演化、富集成矿提供了良好的地球化学条件。     

Origin and mixing history of brines,Palo Duro Basin,Texas, U.S.A.

Formation waters in the Palo Duro Basin, Texas, U.S.A. fall into four major groups based on integrated chemical and isotopic characteristics: (1) interbed brines within the major Permian evaporite aquitard; these are the most chemically concentrated and¹⁸O-rich fluids in the basin, and are interpreted as evaporatively concentrated sea water which has been hydrologically isolated since the Permian; (2) brines below the salt on the eastern side of the basin have ClBr, divalent cation, and isotopic systematics indicating a mixture of evaporatively concentrated sea water and meteoric water of δD= −20‰; (3) brines below the salt on the western side of the basin have chemical and isotopic systematics suggesting a mixture of two pulses of meteoric water, one with δD= −20‰ and the other with δD= −55‰; and (4) waters above the salt have the isotopic composition of meteoric waters. Diagenetic alteration of the cation chemistry has occurred for brines within and below the salt. Aquifers below the salt on the eastern side are interpreted as having been charged with dense Permian evaporite brines which subsequently mixed in various amounts with a basin-wide pulse of Triassic meteoric water. On the western side the descending Triassic meteoric waters became saline by dissolution of halite and are currently mixing with a Tertiary pulse of meteoric water initiated by the Laramide uplift to the west. The hydrochemistry suggests flow on the western side of the basin and static conditions on the eastern side. An unrecognized, approximately N-S permeability restriction, or discontinuity in the potentiometric flow surface, is inferred for major aquifers in the central area of the basin.     

Evolution of isotopic composition and deuterium excess of brines in the Sichuan Basin, China

The isotopic composition and parameters for deuterium excess of brines, which were sampled in the Si-chuan Basin, show obvious regularities of distribution. The brine isotopic composition shows distinct two systems of marine and terrestrial deposits, with the Middle Triassic strata as the boundary. Brine hydrogen isotopic composition of marine deposits is lower while oxygen isotopic composition is higher than that of the SMOW, respectively, indicating that the brines were derived from seawater with different evaporating degrees at different times. From the Sinian strata, up to the Cambrian, Permian Maokou Formation and the Triassic Jialingjiang Formation, the δD values of brines tend to become relatively positive with the strata becoming younger. Brines of terrestrial deposits are considered to have been derived from precipitation and their isotopic composition is close to the globe meteoric water line (GMWL). Brines of transitional deposits between marine and terrestrial ones (the Upper Triassic Xujiahe Formation) have δD and δ18O values falling between the two end members of marine deposit brines and precipitation, indicating that the brines are a mixture of precipitation and vaporing seawater. Water samples from the brine-bearing strata of different ages show various deuterium excesses (d) with an evident decreasing trend as the age of strata gets older and older. Brine-bearing strata of the Triassic Leikoupo-Jialingjiang Formation, the Permian Maokou Formation, the Cambrian and Sinian strata are all carbonate rocks which have experienced intensive water/rock reaction and the deuterium excess essentially changes with time. All brine-bearing-strata surrounding the basin or faults, as well as those brine wells exploited for resources, have been obviously influenced by the precipitation supply. Therefore, the deuterium excesses of their brines have increased to different extents, depending on the amount of involvement of meteoric water. The variation and distribution of d values of the brines from different Triassic strata are related to the embedded depth of the strata. The deuterium excesses of brines become lower with increasing burial depth of the strata.     

Genesis of brines in the basement of the Western Siberian Artesian Basin based on the use of genetic coefficients

This work discusses the possible ways that the chemical composition of the waters in the crystalline basement within several southeastern areas of West Siberia may be formed. Based on an analysis of the ratios of the principal components of solutions (genetic factors), the conclusion is made that some peculiarities of brines contained in the basement cannot be explained from the standpoint of their sedimentogenic-infiltration genesis. Some of the chloride-calcium solutions, as well as hydrocarbonate-sodium waters, probably formed as a result of the supply of endogenous fluids with HCl and CO₂ into the crystalline rocks of the basement.     

Composition of brines in halite‐hosted fluid inclusions in the Upper Ordovician,Canning Basin,Western Australia: new data on seawater chemistry

Analyses of primary and early diagenetic fluid inclusions in the halite from the Late Ordovician Mallowa Salt, Canning Basin, Western Australia indicate a Ca‐rich composition and high concentration of parent brines in the basin which were close to sylvite and carnallite precipitation. The salt‐bearing series in the sampled interval was overheated up to 62 °C. The recorded differences in gas compositions result from the input of several gas sources including dispersed organic matter in the salt series and hydrocarbon deposits in the underlying rocks. The high concentration of the brines in fluid inclusions does not allow quantitative reconstruction of the chemical composition of Late Ordovician parent seawater. Using the information from Early Cambrian and Late Silurian basins as a proxy, however, the new data indicate that Late Ordovician seawater was undoubtedly Ca‐rich and, in comparison with modern seawater, had a similar K content, considerably lower Mg content (c. 30%), approximately three times the Ca content and one‐third the SO₄ content.     

The geochemistry of brines and minerals from the Asse Salt Mine,Germany

The chemistry and stable isotopes (¹⁸O, D) of highly concentrated chloride brines and minerals from the Asse salt mine in the north of the Federal Republic of Germany were studied. Chemical data indicate the occurrence of three types of brines: (a) Mg-Cl type, of carnallitite origin with Li < 30 mg/kg; (b) Na-Cl type brines, of rock salt origin, with Li > 100 mg/kg; and (c) almost pure MgCl₂-type brines with Li > 100 mg/kg. The first group may be subdivided into brines with Li < 4.0 mg/kg and brines with Li between 18 and 30 mg/kg. Lithium is shown to be an efficient complementary tool in tracing the origin of the brines. The complex evolution of carnallitite-type brines is discussed in detail. Isotopic data of brines that were sampled directly from seepages (presumably unaltered) indicate that these brines are not a mixture with relatively fresh ground water from the overburden sediments. The stable isotope composition (¹⁸O and D) of hydration water in carnallite, kieserite and polyhalite sampled from the Asse mine were also studied. It is shown that water extracted from the so-called primary carnallite is isotopically different from water extracted from secondary carnallite. The isotopic fractionation factors for ¹⁸O and D between carnallite hydration water and mother solution were studied in the laboratory. Assuming that crystallization water of the so-called primary carnallite samples is not altered, the isotopic composition of the mother solution is evaluated.     

Palaeohydrodynamics of fluids in the Brent Group (Oseberg Field,Norwegian North Sea) from chemical and isotopic compositions of formation waters

Generally, the history of past sub-surface fluid movements is difficult to reconstruct. However, the composition of oil-field waters characterizes the origins and mixing processes that allow such a reconstruction. We have investigated present-day formation waters from Brent Group sedimentary rocks of the Oseberg Field in order to assess both their geochemical variations, and their origin(s). Water samples (sampled at the separator) produced from immediately above the oil–water contact and from the aquifer (water-saturated zone below the oil–water contact) were taken from 11 wells across the field. In addition, 3 trace water samples were extracted from oil produced from higher up in the oil column. The water samples were analysed for their chemical components and isotopic compositions. Conservative tracers such as Cl, Br, δD, and δ¹⁸O were used to evaluate the origin of the waters. All formation waters can be characterised as Na–Cl-brines. The separator samples are of aquifer origin, indicating that aquifer water, drawn up by the pressure reduction near the well, is produced from the lower few tens of metres of the oil-zone. By defining plausible endmembers, the waters can be described as mixtures of seawater (60–90%), meteoric water (10–30%), evaporated seawater (primary brines) (3–5%), and possibly waters which have dissolved evaporites (secondary brines). Alternatively, using multidimensional scaling, the waters can be described as mixtures of only 3 endmembers without presupposing their compositions. In fact, they are seawater, very dilute brine, and a secondary brine (confirming the power of this approach). Meteoric water was introduced into the reservoir during the end-Brent and early-Cretaceous periods of emergence and erosion, and partially replaced the marine pore fluids. Lateral chemical variations across the Oseberg Field are extremely small. The waters from closer to the erosion surfaces show slightly stronger meteoric water isotopic signatures. The primary and secondary brines are believed to come from Permian and Triassic evaporitic rocks in the deeply buried Viking Graben to the west, and to have been modified by water–rock interactions along their migration path. These primary basinal brines have not been detected in the oil–zone waters, suggesting that the brines entered the reservoir after the main phase of oil-migration. There are indications that these external fluids were introduced into the reservoir along faults. Present-day aquifer waters are mixtures of waters from different origins and hardly vary at a field-scale. They are different in composition to the water trapped in the present oil-zone. One of the oil-zone samples is a very dilute brine. It is thought to represent a simple mixture of seawater and meteoric water. Due to oil-emplacement, this geochemical signature was preserved in the waters trapped within the oil-zone. Another oil-zone water shows a very similar chemical signature to the aquifer waters, but the chlorine isotopic signature is similar to that of the dilute oil-zone water. This water is interpreted to represent a palaeo-aquifer water. That is, it was within the aquifer zone in the past, but was trapped by subsequent emplacement of more oil. These vertical differences can be explained by two features: (i) emergence of the Brent Group sedimentary rocks in the Early Cretaceous allowed ingress of meteoric water; (ii) subsequent rapid burial of Viking Graben rocks caused migration of petroleum and aqueous fluids into the adjacent, less deeply buried Oseberg Field.     

Fluid-rock interactions in a geothermal Rotliegend/Permo-Carboniferous reservoir (North German Basin)

Comprehensive data on the chemical composition of reservoir rocks and geothermal brines from the geothermal well doublet Groβ Schönebeck (North German Basin) drilled into a Rotliegend sedimentary and Permo-Carboniferous volcanic rock reservoir were sampled over the past years. They were characterized with respect to their major and minor elemental composition including various isotope ratios. The study considered the impact of drilling and reservoir operations on fluid composition and aimed at determining fluid–rock interactions to gain information on fluid origin and hydraulic pathways.The highly saline fluids (up to 265 g/L TDS) show δ ¹⁸O and δD of water (2.7–5.6 and −3.1–15, respectively) as well as δ ³⁴S of sulfate (3.6–5), and ⁸⁷Sr/⁸⁶Sr ratios (0.715–0.716) that resemble Rotliegend brines from an area located around 200 km in the west (the Altmark). Halogen ratios indicated that brines developed predominantly by evaporation of meteoric water (primary brine) together with halite dissolution brine (secondary brine). Indication for mixing with Zechstein brine or with younger meteoric water was not found.No geochemical distinction was possible between fluids deriving from different rock formations (dacites or sedimentary rocks, respectively). This is due to the evolution of the sediments from the effusive rocks resulting in a similar mineralogical and chemical composition and due to a hydraulic connectivity between the two types of rock. This connection existed probably already before reservoir stimulation as indicated by a set of faults identified in the area that could connect the Rotliegend formation with both, the volcanic rocks and the lower units of the Zechstein. Additional geochemical indication for a hydraulic connectivity is given by (1) the very high heavy metal contents (mainly Cu and Pb) in fluids and scaling that derive from the volcanic rocks and were that were also found in increased amounts up at the Zechstein border (Kupferschiefer formation). (2) The ⁸⁷Sr/⁸⁶Sr isotope ratios of fluid samples correspond to the ratios determined for the sedimentary rocks indicating that initially the fluids developed in the sedimentary rocks and circulated later, when faults structures were created by tectonic events into the volcanic rocks.     

THE NATURE OF ABYSSAL CHLORIDE BRINES

Highly mineralized, largely sodium-chloride type, brines in deep artesian basins are similar in composition to the mineralized abyssal hydrothermal waters of present day volcanic regions as well as the material impounded in liquid inclusions of minerals. It is also well established that present brines are very similar to hydrothermal ore-forming solutions. Some highly concentrated brines can be explained by derivation from salt basins but there is no adequate explanation for the worldwide distribution of concentrated brines. Exchange reactions merely change composition, not concentration. Some hydrocarbons found in minerals as well as in brines have originated from CO₂, H₂O, and H₂ liberated from volcanoes and magma chambers. The abyssal brines may be the product of magmatogenic hydrothermal activity or the brines themselves may be the source of ore-forming hydrothermal solutions. If the latter is true, there is still no explanation of the high mineralization of brines. Many brines are deafly similar to sea water but the salts in sea water are largely produced by volcanic activity. It is suggested that many of the brines are formed in deep unexposed parts of artesian basins from highly concentrated hydrothermal solutions migrating from magmatic chambers. --J. A. Redden.     

Isotopic composition of sulfur in modern metalliferous hydrotherms in Cheleken

Iron and zinc sulfides, deposited by petroleum brines, are believed to have been derived from terrigenic sedimentary rocks, judging by the isotopic composition of their sulfur, and not from any deposits of these metals in the depths. This view is supported by isotopic analysis of sulfur in brines inside and outside the oil field and in waters of mud volcanoes. Isotopic composition of native (and apparently young) lead, deposited in two of the wells, may likewise represent an epigenetic concentration of the originally dispersed metals in the brines.     

Geochemistry and genesis of brine emanations from Cretaceous strata of the Mamfe Basin, Cameroon

The geochemistry of 5 salt springs in the southwestern Mamfe Basin was investigated in order to infer the mineral content of their source and to relate the genesis of the springs to the local geology. Field observations revealed that, they are cold springs (23–28 °C), and are composed of secondary brines that are neutral to alkaline with pH values ranging from 7 to 8.7. Results of chemical analysis show that the springs contain major ions that form evaporite minerals, as well as chalcophile elements. The dominant cation is Na⁺ (>96%), and the dominant anion is Cl⁻ (>99%). Based on correlation coefficients between ions that form evaporites and field occurrence of efflorescences of halite, it is suggested that the ancient evaporites in the Mamfe Basin are composed entirely of carbonate and chloride salts. Meteoric and convective fluid flow processes are responsible for the dissolution of ancient evaporites and subsequent migration of brines to the surface from underground. The brines migrate through permeable strata with migration pathways resulting from a combination of fracture porosity created by post––Cretaceous tectonism and intergranular porosity enhanced by the chemically aggressive migrating brines.     

Physicochemical parameters and geochemical features of fluids of precamrbian gold deposits

This paper analyzes literature data on physicochemical parameters and chemical composition of fluids of Precambrian endogenous gold deposits. The average values and ranges of temperature, pressure, and salinity of fluids from the Archean and Proterozoic gold deposits are estimated. It is revealed that fluids of Archean deposits are dominated by methane, while those of Proterozoic deposits, by nitrogen. It is proposed that the accumulation of nitrogen in the atmosphere is related to the intense nitrogen degassing from the Earth’s interior. The highest pressures of endogenous fluids in this period could reflect specifics of deep geodynamics of the planet in the Proterozoic. The large gold deposits (>100 tons) are characterized by narrower range of physicochemical parameters as compared to small deposits. The contribution of heated chloride brines in the formation of majority of large Proterozoic deposits is established.