Extreme ductile deformation of fine-grained salt by coupled solution-precipitation creep and microcracking: Microstructural evidence from perennial Zechstein sequence (Neuhof salt mine,Germany)

Microstructural study revealed that the ductile flow of intensely folded fine-grained salt exposed in an underground mine (Zechstein-Werra salt sequence, Neuhof mine, Germany) was accommodated by coupled activity of solution-precipitation (SP) creep and microcracking of the halite grains. The grain cores of the halite aggregates contain remnants of sedimentary microstructures with straight and chevron shaped fluid inclusion trails (FITs) and are surrounded by two concentric mantles reflecting different events of salt precipitation. Numerous intra-granular or transgranular microcracks originate at the tips of FITs and propagate preferentially along the interface between sedimentary cores and the surrounding mantle of reprecipitated halite. These microcracks are interpreted as tensional Griffith cracks. Microcracks starting at grain boundary triple junctions or grain boundary ledges form due to stress concentrations generated by grain boundary sliding (GBS). Solid or fluid inclusions frequently alter the course of the propagating microcracks or the cracks terminate at these inclusions. Because the inner mantle containing the microcracks is corroded and is surrounded by microcrack-free outer mantle, microcracking is interpreted to reflect transient failure of the aggregate. Microcracking is argued to play a fundamental role in the continuation and enhancement of the SP–GBS creep during halokinesis of the Werra salt, because the transgranular cracks (1) provide the ingress of additional fluid in the grain boundary network when cross-cutting the FITs and (2) decrease grain size by splitting the grains. More over, the ingress of additional fluids into grain boundaries is also provided by non-conservative grain boundary migration that advanced into FITs bearing cores of grains. Described readjustments of the microstructure and mechanical and chemical feedbacks for the grain boundary diffusion flow in halite-brine system are proposed to be comparable to other rock-fluid or rock-melt aggregates deforming by the grain boundary sliding (GBS) coupled deformation mechanisms.     

Creep of rocksalt

A review is presented of the fundamental flow properties and processes in experimentally deformed natural and synthetic halite single crystals and polycrystalline aggregates. Included in the summary are discussions of: (a) microstructures induced during steady-state creep; (b) creep-rupture of rocksalt; (c) experiments associated with “Project Salt Vault” and more recent field studies; and (d) brine migration. A representative steady-state flow law determined for natural aggregates and maximum natural deviatoric stresses deduced from subgrain sizes are applied briefly to considerations of creep in waste repositories and of salt dome dynamics. While the mechanical behavior of rocksalt is probably better understood than for all other rock types, further investigations, especially on load path, stress history and creep-rupture are clearly mandated. Furthermore, additional investigations of brine migration and of bench and field-scale deformations are needed, the latter incorporating realistic rocksalt flow properties into numerical simulations of natural rock-mass response.     

云南勐野井钾盐矿床特征,微量元素地球化学及成因探讨

勐野井钾盐矿床位于云南兰坪、恩茅拗陷带的南部。该带地处藏滇印支地槽褶皱系中段,点苍山、哀牢山断裂带以西,澜沧江断裂带以东。区内发育有巨厚的中新生代红色碎屑岩和含盐建造,本区的主要含盐层为晚白垩世至老第三纪勐野井组。 勐野井组有上下两套含盐层,中部为棕红色钙泥质粉砂岩,下含盐层在区域上是含膏层;上含盐层为石盐岩夹钾盐透镜体。下覆地层是上白垩统扒沙河紫红色石英砂岩,上覆为老第三系等黑组棕红色粉砂岩。矿区为一向斜构造呈北西—南东向分布,盐矿体呈穹窿状,石盐层平均厚196米,钾盐厚29米,NaCl平均含量71.67％ KCl 8.70％,为混有碎屑物的氯化物型钾盐矿床。 本文研究了富钾盐层剖面结构、碎屑物沉积特征、溴、铷、锶、硼等微量元素赋存规律及其在寻找钾盐矿床工作中的标志意义,并探讨了盐矿床的成因。全文蒙袁见齐教授审阅,并提出修改意见,在此表示感谢。     

Sedimentary Conditions of Evaporites in the Late Jurassic Xiali Formation, Qiangtang Basin: Evidence from Geochemistry Records

The Qiangtang Basin (QB), located in the central Tibetan Plateau, is a Jurassic marine basin and one of the most important prospective salt resource belts in China. In recent decades, many outcrops of gypsiferous?bed have been found in the Jurassic marine strata in the basin. Salt springs with abnormally high sodium (Na+) contents had been identified in the Late Jurassic Xiali Formation (Fm.) in the basin in the last years. However, to date, no potash or halite deposits have been identified in the QB. Gypsum outcrops and salt springs are very important?signs in the investigation of halite and potash deposits. Therefore, the Xiali Fm. is?a potentially valuable layer to evaluate for the possible presence of halite and potash deposits in the basin. However,?few studies?have explored the formation?conditions of evaporites in the unit. Here, we present detailed geochemical records from the Yanshiping section related to the study of the formation?conditions of evaporites in the Xiali Fm. of the QB. Climate proxies based on the obviously increased anion concentrations of SO42? and Cl? and the significant correlation?coefficients of Ca2+-SO42? (R = 0.985) and Na+-Cl? (R = 0.8974) reveal that the upper member of the Xiali Fm. (the upper Xiali Fm.) formed under an arid climate and evolved into the sulfate phase or early chloride phase. Provenance proxies based on the obviously increased K+ and Na+ ion concentrations and the significant correlation?coefficient of Na+-Cl? (R = 0.8974) suggest that the upper Xiali Fm. featured optimal provenance conditions for the possible formation of halite deposits. The regression and the semi-closed tidal?flat environment in the upper Xiali Fm. were favorable for the formation of potash and halite deposits. The low Mg2+ /Ca2+ values (mean value = 1.82) and significant Na+-Cl? correlation?coefficient (R = 0.8974) also suggest that the upper Xiali Fm. is the layer most likely to contain potential halite deposits. In addition, the macroscopic correlations of tectonism, provenance, paleoclimate, saliferous strata and sedimentary?environment between the QB and the adjoining Amu Darya Basin in Central Asia reveal that the two basins shared similar geologic settings that were favorable for the formation of evaporites during the Late Jurassic. Therefore, the upper Xiali Fm. is a valuable layer to explore for halite deposit and may be potentially valuable in the future exploration for potash deposits in the QB.     

云南兰坪-思茅盆地和老挝钾盐矿床物质来源新认识

云南兰坪-思茅盆地勐野井钾盐矿与泰国—老挝的钾盐矿的成矿关系被认为可能具有同源性,尤其深部热液可能是其重要的物质来源之一。针对该观点氢氧同位素证据缺乏、且深部热液到底是哪种热液尚不清楚的问题,文中根据老挝钻孔ZK2893中石盐包裹体水的氢氧同位素分析结果:δ~(18)O为-2.3‰~9.5‰,平均值为2.9‰,δD的范围为-78‰~-150‰,平均值为-108.6‰,大部分小于-90‰;在δD-δ~(18)O关系图上,数据点均在交代热液范围内,因此推断,老挝钾盐成矿的深部热液为大气降水与围岩形成的交代热液,围岩提供了重要的成矿物质。此项分析还表明老挝的交代热液温度主要集中在150℃左右,即包裹体形成的温度可能也在150℃左右,云南兰坪—思茅石盐包裹体的捕获温度为145℃左右,最高达170℃,二者比较接近,这为两地钾盐矿同源的可能性提供了新的证据。     

四川盆地三叠纪古盐湖已达钾石盐析出阶段——来自石盐流体包裹体化学组成的约束

石盐的流体包裹体成分可提供古流体组成的物理化学信息,用以探查卤水组成变化及环境演化规律等。四川盆地位于上扬子地台,其中的早-中三叠纪沉积建造是中国海相找钾的有利层位之一。获取石盐沉积时期的卤水成分信息,是深刻认识四川盆地古海水蒸发浓缩程度的重要途径。文章利用激光剥蚀电感耦合等离子体质谱法,对采自川东地区长平3井嘉陵江组的石盐流体包裹体开展了化学组成分析,结果显示古卤水化学类型为Mg_SO4型;流体包裹体中的ρ(K~+)与现代海水浓缩到钾石盐析出阶段的ρ(K~+)基本一致,可能揭示了盆地三叠纪时期古卤水已达到钾石盐析出阶段,对四川盆地沉积环境演化及钾盐成矿规律研究等具有重要的理论意义。     

Microstructural evolution of deformation-modified primary halite from the Middle Triassic Röt Formation at Hengelo, The Netherlands

The microstructure of halite from the subhorizontal, bedded Main Röt Evaporite Member at Hengelo, The Netherlands (AKZO well 382, depth interval of 420–460 m), was studied by transmitted and reflected light microscopy of gamma-irradiation decorated samples. Primary microstructures compare favourably with those found in recent ephemeral salt pans. Large, blocky, fluid-inclusion-poor halite grains and elongated chevrons are interpreted to have formed in the saline lake stage, while void-filling clear halite is interpreted to have formed during the desiccation stage of the salt pan. In addition, in all layers the grains are rich in deformation-related substructures such as slip bands and subgrains indicating strains of a few percent. The study of gamma-irradiation decorated thin sections shows that the main recrystallization mechanism is grain boundary migration. Grain boundary migration removes primary fluid inclusions and produces clear, strain-free new grains. Differential stresses as determined by subgrain size piezometry were 0.45–0.97 MPa. The deformation of the salt layers is probably related to Cretaceous inversion in the area.     

Origin of deformed halite hopper crystals, pseudomorphic anhydrite cubes and polyhalite in Alpine evaporites (Austria, Germany)

The Alpine Haselgebirge Formation represents an Upper Permian to Lower Triassic evaporitic rift succession of the Northern Calcareous Alps (Eastern Alps). Although the rocksalt body deposits are highly tectonised, consisting mainly of protocataclasites and mylonites of halite and mudrock, the early diagenetic history can be established from non-tectonised mudrock bodies: Cm-sized euhedral halite hopper crystals formed as displacive cubes within mud just during shallow burial. The crystals were deformed by subsequent compaction. Later, migrating fluids led to the replacement of halite by anhydrite retaining the shapes of deformed halite cubes. Polyhalite formed from subsequent enhanced fluid migration. Mudrock provided water by dewatering, while potassium and magnesium were dissolved from primary salt minerals. When these fluids interacted with sulphates, polyhalite precipitated. ⁴⁰Ar/³⁹Ar analyses date the polyhalite from within the retaining shapes of deformed halite hopper-shaped cubes from two localities to ca. 235–232 Ma (Middle Triassic). At this time, ca. 20–25 Ma after sedimentation, polyhalite crystallised at shallow levels.     

Textural evolution of synthetic anhydrite-halite mylonites

The rheology and textural evolution of mylonites for both crustal and mantle rocks are generally poorly understood. Stress and strain partitioning between shear zones and wall-rock, foliation development, flow mechanisms, and constitutive relations are little known. To address these problems, we have begun an experimental study in which anhydrite and halite aggregates are deformed in simple shear. Homogeneous mixes at intervals of 10 vol.% of anhydrite and halite powders of 90 to 180-μ m grain-size comprise the 1-mm-thick zone between the 35 °-precut surfaces in 2 by 4 cm right-circular cylinders of anhydrite. All specimens are confined under 200 MPa (P_c) and tested at 300 °C (T), producing an aggregate of near-zero porosity. All specimens are shortened at an axial displacement rate of 5×l0⁻⁶ cm/s to shear strains of about 2.0. Most stress-strain curves have a similar form. They are characterized by an initial linear response that is followed by a work-hardening region that leads to a plateau of constant stress whilst strain continues to accumulate. This apparent steady-state behavior is followed by a drop in stress to a second apparent steady-state stress. The strain at this stress drop and the magnitude of the drop increase with increasing anhydrite content. The second (lower) steady-state stress for all mixes approaches that of sheared pure halite. Optical examination of the fabrics shows that straining under the initial apparent steady-state is accommodated by both mineral phases. A foliation evolves as defined by mineral segregation and grain elongation of halite at 15 ° to 30 ° to the shear-zone walls. Anhydrite grains rotate until their (001) 010 slip system parallels the foliation. The anhydrite also exhibits twin lamellae, kinks and microfractures. Following the stress drop, rheologic behavior is dominated by dynamic recovery in halite, the recrystallized grain size increasing, consistent with the lower stress level.     

Deformation at low and high stress-loading rates

In an extensional shear zone in the Talea Ori, Crete, quartz veins occur in high-pressure low-temperature metamorphic sediments at sites of dilation along shear band boundaries, kink band boundaries and boudin necks. Bent elongate grains grown epitactically from the host rock with abundant fluid inclusion trails parallel to the vein wall indicate vein formation by crack-seal increments during dissolutionprecipitation creep of the host rock. The presence of sutured high-angle grain boundaries and subgrains shows that temperatures were sufficiently high for recovery and strain-induced grain boundary migration, i.e. higher than 300 -350℃, close to peak metamorphic conditions. The generally low amount of strain accumulated by dislocation creep in quartz of the host rock and most veins indicates low bulk stress conditions of a few tens of MPa on a long term. The time scale of stress-loading to cause cyclic cracking and sealing is assumed to be lower than the Maxwell relaxation time of the metasediments undergoing dissolution-precipitation creep at high strain rates(10^-10 s^-1 to 10^-9 s^-1), which is on the order of hundred years. In contrast, some veins discordant or concordant to the foliation show heterogeneous quartz microstructures with micro-shear zones, sub-basal deformation lamellae, shortwavelength undulatory extinction and recrystallized grains restricted to high strain zones. These microstructures indicate dislocation glide-controlled crystal-plastic deformation(low-temperature plasticity) at transient high stresses of a few hundred MPa with subsequent recovery and strain-induced grain boundary migration at relaxing stresses and temperatures of at least 300 -350℃. High differential stresses in rocks at greenschist-facies conditions that relieve stress by creep on the long term, requires fast stress-loading rates, presumably by seismic activity in the overlying upper crust. The time scale for stress loading is controlled by the duration of the slip event along a fault, i.e. a few seconds to minutes.This study demonstrates that microstructures can distinguish between deformation at internal low stress-loading rates(to tens of MPa on a time scale of hundred years) and high(coseismic) stress-loading rates to a few hundred MPa on a time scale of minutes.     

塔里木盆地西部盐矿点卤水地球化学特征及成钾预测

在塔里木盆地的地质发展史上,发生了多次的海进和海退,带来了大量的成盐物质。而且,由于极度的干旱,沉积了大量的岩盐,有望在塔里木盆地找到钾盐。通过野外工作,发现在塔里木盆地西部有大量石盐和盐泉卤水出露。笔者基于地质概况和岩相古地理特征的介绍,通过对采自塔里木盆地西部卤水样品的化学分析,讨论了其地球化学参数及成钾可能性。根据卤水样的分析结果得出:大多数卤水都是从厚层的岩盐中淋滤出来的,岩盐主要是氯化物型,这种类型的岩盐表面上都没有达到钾盐沉积的阶段;其次,根据Br和K含量表明WSKL(乌克沙鲁)已经有钾盐沉积的显示,是一个值得注意的地方,应该把重点放在这个区域;最后,由于原始沉积的原因,塔里木盆地Br含量普遍很低,这一点通过实验结果和收集到的资料已经得到了证实,不能单纯地依据Br指数来判断岩盐的沉积阶段,必须寻找除海相成钾以外的其他的成钾模式。     

Auriferous lode of Hira-Buddini gold mine,Hutti-Maski schist belt,Dharwar craton: Mineralogy,alteration, types and mechanism of vein emplacement

The auriferous lode in the Hira-Buddini deposit is confined to the sheared contact of amphibolite and felsic metavolcanic rock. Gold mineralization in the deposit is associated with sub-horizontal, sub–vertical, irregular and with few conjugate veins. These veins were emplaced during deformation in a ductile-brittle regime as inferred from the megascopic features and microstructures of the vein minerals. Fluid pressure was higher than the sum of the minimum principal stress and lithostatic load as well as the tensile strength of the shear zone. Crack-seal process appears to be the mechanism of vein formation. The microstructures of the vein minerals indicate a temperature of ~500ºC during the vein emplacement. In the auriferous lode, amphibolite and felsic metavolcanic rock have been subjected to intense alteration by the ore fluid with development of biotite-chlorite-tourmaline-calcite and muscovite (sericite)-chlorite-calcite-feldspar-biotite assemblages, respectively. Both the altered rocks contain significant amount of pyrite and chalcopyrite with native grains of gold and silver. Post-dating the fluid activity associated with gold mineralization, there is another stage of fluid activity manifested by the calcite veins and micro-veinlets.     

美国新墨西哥州钾盐矿床及其开发

新墨西哥州卡尔斯巴德 (Carlsbad)钾盐矿床是美国最早发现的古钾盐矿床 ,该矿床一直是美国钾盐的主要供给地。含钾蒸发岩系产出于得克萨斯州西部、新墨西哥州东南部Delaware盆地上二叠统海相地层中。含钾蒸发岩系可分为 4个建造。由下向上分别为 :①Castile建造 ,由石盐岩和其夹层硬石膏岩或石灰岩构成 ;②Salado建造 ,由钾盐、石盐岩、含泥石盐岩、硬石膏岩、杂卤石岩、白云岩、泥岩构成 ;③Rustler建造。由石盐岩、石膏岩、硬石膏岩、硅质岩、白云岩及石灰岩构成 ;④DeweyLake红层建造 ,由红色泥岩和砂岩构成。其中 ,Salado建造 ,厚 6 70m ,含 12个矿带 ,面积为 492 0km2 ,钾盐矿体主要由钾石盐和无水钾镁矾以及石盐等矿物所组成。次生的钾盐矿物有 :钾盐镁矾和钾镁矾等     

老挝他曲地区石盐流体包裹体特征、氢氧同位素组成及成盐物质补给方式

对老挝他曲地区钻孔中石盐流体包裹体特征、类型以及氢氧同位素组成进行了研究,用以重塑盆地成盐过程中的古环境,并在此基础上对盆地成盐物质的补给方式进行了探讨。结果显示,钻孔中塔贡组下盐段原生石盐岩主要包括人字形和漏斗形晶体两种类型。其中人字形石盐晶体在下盐段中广泛发育,漏斗形晶体数量较少,但通常与人字形石盐晶体产于同一层位,指示了下盐段沉积时盆地处于一种极浅水环境。石盐原生流体包裹体氢氧同位素组成明显偏离全球大气降水线,集中分布于其右下方,反映盆地成盐过程中处于强烈蒸发条件之下。氢氧同位素组成在纵向上的变化,可能揭示了在盐湖演化至钾盐沉积阶段大气温度具有升高趋势。白垩纪中期全球海平面达到极大值,而在呵叻盆地内部,持续的坳陷和强烈的蒸发作用将导致湖平面快速下降。这样,外海与盐盆之间则会产生水位差,由此产生的水力梯度将为外海海水通过障壁向盆地方向进行渗透提供潜在的动力。此外,早白垩世呵叻高原广泛发育的沙漠沉积,则为外海海水大规模的侧向渗透进入盐盆提供了可能的通道。因此,结合现有的资料,本文提出,除海侵补给外,外海通过障壁侧向渗透补给对老挝钾盐盆地成盐物质的供给也具有重要作用。     

The grain–fluid interaction as a self-stabilizing mechanism in fluvial bed load transport

A grain-scale model of fluvial bed load transport is described, with particular emphasis on the equilibrium between the saltating grains and the near bed flow, and its role in determining transport rate. The model calculates, explicitly, the modification of the velocity profile by the moving grains, together with the consequential reduction in surface fluid shear stress. As the surface fluid shear stress is reduced by the moving grains, so the entrainment rate decreases and the model reaches a steady state. The results provide insight into two important questions at a macroscopic level. First, they show that, in the absence of large static roughness, the dynamic roughness caused by the moving grains may be a significant contributor to flow resistance. Secondly, the model indicates the manner in which transport may be limited by a combination of the transport capacity of the flow and the availability of sediment for entrainment. Only in the case of high sediment availability does the fluid shear stress acting at the surface approach the critical entrainment value, reproducing the behaviour suggested by Bagnold (1956 ) and Owen (1964 ). This suggests that prediction formulae based on this assumption only describe the bed load transport system under particular conditions.     

Alpine olivine- and titanian clinohumite-bearing assemblages in the Erro-Tobbio peridotite (Voltri Massif, NW Italy)

The microstructures in the Erro-Tobbio peridotite indicate several stages of recrystallization of olivine + titanian clinohumite-bearing assemblages. The development of these assemblages is closely associated with serpentinite mylonites, in which they occur in shear bands and foliations and are inferred to have grown synkinematically, in veins, and as post-kinematic radial aggregates. In the peridotite wall-rock adjacent to these mylonites, the same assemblages have recrystallized statically at the expense of original olivine and pyroxenes, mesh-textured chrysolite and antigorite veins. In addition, the olivine-bearing assemblage occurs in widespread vein systems. The brittle deformation of the peridotite resulting in the development of these vein systems is closely related to ductile deformation of metagabbroic dykes in the peridotite. Although early metasomatism resulted in extensive rodingitization of the gabbros, some dykes show an eclogitic assemblage of Na-clinopyroxene + garnet + chloritoid + chlorite ± talc. These observations, the microstructures and the mineral chemistry all suggest that the assemblages in the ultramafic rocks and metagabbros developed during a prograde evolution towards high pressures (>13–16 kbar, 450–550° C), and during subsequent decompression. This metamorphic evolution is considered to be related to Late Cretaceous intraoceanic subduction in the Alps-Apennine system and closure of the Piedmont-Ligurian basin.     

Vein-plus-wall-rock melting mechanisms in the lithosphere and the origin of potassic alkaline magmas

A model is developed for the origin of ultrapotassic melts by melting of veined lithosphere; the veins are rich in clinopyroxene and mica, whereas the wall-rocks consist principally of peridotites. The veins originate by solidification of low-degree melts which are themselves the results of earlier, deeper, multistage processes ultimately due to the presence of a transition zone between large-scale channelled and porous flow regimes. The melting event producing the ultrapotassic magma begins in the veins due to the concentration of hydrous phases and incompatible elements, but spreads to include the surrounding wall-rocks by a combination of two mechanisms. The alkaline magma composition is thus a hybrid of vein (V) and wall-rock (W) components.

The melt hybridization mechanisms are: (i) Solid-solution melting: Minerals which from extensive solid-solutions are abundant in the vein assemblages (Cr/Al spinel, F/OH mica, amphibole and apatite). The breakdown of these phases take place over a temperature range between the solidus of the vein assemblage and the elimination of the more refractory end-members. This process bridges the temperature gap between the solid of vein and wall-rock, so that a melt component from the wall-rock is added to that from the vein before elimination of all vein minerals. Phlogopite forms the most effective of these sliding reactions, resulting in its stability at near-liquids temperatures in experiments. (ii) Dissolution of wall-rock minerals: The initial melt fraction in the vein infiltrates the surrounding wall-rock due to the dominance of surface energy minimization on melt flow at the intergranular scale. Following infiltration, dissolution of wall-rock minerals occurs at temperatures lower than their melting temperatures, thus imparting a refractory wall-rock component to the melt composition. Dissolution of olivine and/or orthopyroxene occurs preferentially, since these minerals are furthest from equilibrium with the strongly alkaline, vein-derived melt.

Remobilisation of several generations of veins explains the occurrence within a restricted space and time of rocks bearing chemical characteristics which are generally thought to indicate contrasting tectonic settings (e.g. central Italy). The ultrapotassic rocks are explained as being dominanyly vein-derived (i.e. high V/W ratio): further dilution of the V-component by wall-rock, supplemented by asthenospheric melt in advanced cases, leads to the production of more voluminous basaltic rocks bearing incompatible element signatures reminiscent of those of ultrapotassic rocks.     

Experimental pressure solution compaction of synthetic halite/calcite aggregates

Experimental observations are reported of weakening of sediment-like aggregates by addition of hard particles. Sieved mixtures of calcite and halite grains are experimentally compacted in drained pressure cells in the presence of a saturated aqueous solution. The individual halite grains deform easily by pressure solution creep whereas calcite grains act as hard objects and resist compaction. The fastest rate of compaction of the mixed aggregate is not obtained for a 100% halite aggregate but for a content of halite grains between 45% and 75%. We propose that this unusual compaction behavior reflects the competition between two mechanisms at the grain scale: intergranular pressure solution at grain contacts and grain boundary healing between halite grains that prevent further compaction.     

老挝东泰钾盐矿床地球化学及其沉积后变化初步研究

呵叻高原是世界上最大的钾盐沉积矿床分布区之一,研究区位于呵叻高原北部的沙空那空盆地东部边缘,钻探资料揭示该地区钾盐资源相当可观。该含盐建造包含上、中、下3个盐段,每个盐段都由一套蒸发岩_碎屑沉积旋回组成,钾盐层主要赋存于下盐段上部。剖面和地层学分析表明,研究区盐构造类型为低结构低成熟度的盐背斜构造,盐体几何形态为一低结构低成熟度的盐背斜,盐背斜轴部矿层厚度较两翼矿层厚得多。岩盐中微量元素Br、K含量及Br×1 000/Cl系数的变化指示成盐卤水经过了阶段性的浓缩和淡化过程,并且成盐作用越来越弱。ZK04孔较低的Br含量〔w(Br)=18×10〈sup>-6〈/sup>〕表明下膏盐层在成盐过程中受到了非海相流体的影响。中膏盐层基底石盐中高K低Br特征表明,下膏盐层残余高浓度卤水影响了其成盐过程,并且该基底石盐很可能又经历了溶解和重结晶过程。     

Formation and chemical evolution of magnesium chloride brines by evaporite dissolution processes—Implications for evaporite geochemistry

The evolution of magnesium chloride brines with high bromide contents via a multistage reaction and dissolution process has been studied in brine seeps of a German potash mine. The observed chemical trends and phase equilibria can be modeled and interpreted in terms of a NaCl solution (cap rock brine) infiltrating into a potash zone characterized by the metamorphic mineral assemblage kieserite + sylvite + halite + anhydrite. Establishment of a persistent, stable equilibrium assemblage and constant fluid composition in the invariant point IP1 of the six component (Na-K-Mg-Ca-Cl-SO₄-H₂O) system of oceanic salts is prevented by the perpetually renewed input of NaCl-brine and by the intermittent exposure of incompatible kieserite. Instead, the solutions develop towards the metastable invariant point IP1(gy), with the mineral assemblage carnallite + polyhalite + sylvite + halite + gypsum, where gypsum takes the place of anhydrite (stage I). The temporary exposure of kieserite and the ensuing formation of polyhalite effectively buffer the solutions along the metastable polyhalite phase boundary during stages II and III. Eventually, in stage IV, polyhalite becomes depleted and admixture of more NaCl brine leads to low sulfate solution compositions, which are now only constrained by carnallite + sylvite + halite, and the once hexary system degenerates to a quaternary one (Na-K-Mg-Cl-H₂O) in point E. Bromide in brines shows equilibrium partitioning with respect to the wall rock minerals. The pattern of evolving brine compositions may serve as a model for similar brine occurrences, which in some cases may have been misinterpreted as remains of fossil, highly concentrated and chemically modified seawater. Similar magnesium chloride brines of salt lakes (e.g., Dead Sea, Dabusun Lake) show subtle differences and are constrained by fewer mineral equilibria (more degrees of freedom), and their low sulfate contents are due to gypsum precipitation, driven by calcium chloride input from dolomitization reactions. Finally, the observed reaction sequence is generalized, and a model for the formation of magnesium sulfate depleted, chloride-type potash salts and bischofite deposits by leaching of sulfate-type evaporites is proposed.