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1.
The Jabali Zn–Pb–Ag deposit is located about 110 km east of Sana'a, the capital of Yemen, along the western border of the Marib-Al-Jawf/Sab'atayn basin. The economic mineralization at Jabali is a nonsulfide deposit, consisting of 8.7 million tons at an average grade of 9.2% zinc, derived from the oxidation of primary sulfides. The rock hosting both primary and secondary ores is a strongly dolomitized carbonate platform limestone of the Jurassic Shuqra Formation (Amran Group). The primary sulfides consist of sphalerite, galena and pyrite/marcasite. Smithsonite is the most abundant economic mineral in the secondary deposit, and is associated with minor hydrozincite, hemimorphite, acanthite and greenockite. Smithsonite occurs as two main generations: smithsonite 1, which replaces both host dolomite and sphalerite, and smithsonite 2, occurring as concretions and vein fillings in the host rock. At the boundary between smithsonite 1 and host dolomite, the latter is widely replaced by broad, irregular bands of Zn-bearing dolomite, where Zn has substituted for Mg. The secondary mineralization evolved through different stages: 1) alteration of original sulfides (sphalerite, pyrite and galena), and release of metals in acid solutions; 2) alteration of dolomite host rock and formation of Zn-bearing dolomite; 3) partial dissolution of dolomite by metal-carrying acid fluids and replacement of dolomite and Zn-bearing dolomite by a first smithsonite phase (smithsonite 1). To this stage also belong the direct replacement of sphalerite and galena by secondary minerals (smithsonite and cerussite); 4) precipitation of a later smithsonite phase (smithsonite 2) in veins and cavities, together with Ag- and Cd-sulfides.The δ18O composition of Jabali smithsonite is generally lower than in other known supergene smithsonites, whereas the carbon isotope composition is in the same range of the negative δ13C values recorded in most supergene nonsulfide ores. Considering that the groundwaters and paleo-groundwaters in this area of Yemen have negative δ18O values, it can be assumed that the Jabali smithsonite precipitated in different stages from a combination of fluids, possibly consisting of local groundwaters variably mixed with low-temperature hydrothermal waters. The carbon isotope composition is interpreted as a result of mixing between carbon from host rock carbonates and soil/atmospheric CO2.The most favorable setting for the development of the Jabali secondary deposit could be placed in the early Miocene (~ 17 Ma), when supergene weathering was favored by major uplift and exhumation resulting from the main phase of Red Sea extension. Low-temperature hydrothermal fluids may have also circulated at the same time, through the magmatically-induced geothermal activity in the area. 相似文献
2.
含矿流体混合反应与成矿作用的动力平衡模拟研究 总被引：7，自引：0，他引：7
本文在约定热液体系中成矿元素成矿速率(成矿过程中单位时间内单位体积所合成矿元素重量的变化)的基础上。借助于物质-热-化学-成矿四重全耦合的研究思路，构建了均匀热液体系、层状热液体系、岩浆侵入热液体系下成矿元素的迁移、富集、溶解与沉淀作用数值模型。模拟结果表明；(1)硫化物(H2S)和硫酸盐(SO42-)流体的混合反应是成矿热液体系中铅、锌、铁成矿元素成矿的重要控制因素；(2)均匀介质、岩浆侵入或地质构造的存在，对成矿元素在成矿流体运移的速度、流线、温度分布和成矿元素的溶解与沉淀分布都有着各自的特征．不同的成矿环境或成矿背景制约了成矿元素的迁移与富集以及矿体的产出定位。暗示成矿环境及成矿速率对热液体系中成矿元素的沉淀与溶解具重要作用；成矿流体的混合反应是成矿作用发生的重要机制之一。在成矿理论研究中必须充分考虑不同地质构造因素的约束。 相似文献
3.
Formation of the Wiesloch Mississippi Valley-type Zn-Pb-Ag deposit in the extensional setting of the Upper Rhinegraben, SW Germany 总被引：2，自引：0，他引：2
Katharina Pfaff Ludwig H. Hildebrandt David L. Leach Dorrit E. Jacob Gregor Markl 《Mineralium Deposita》2010,45(7):647-666
The Mississippi Valley-type (MVT) Zn–Pb–Ag deposit in the Wiesloch area, Southwest Germany, is controlled by graben-related
faults of the Upper Rhinegraben. Mineralization occurs as vein fillings and irregular replacement ore bodies consisting of
sphalerite, banded sphalerite, galena, pyrite, sulfosalts (jordanite and geocronite), barite, and calcite in the Middle Triassic
carbonate host rock. Combining paragenetic information, fluid inclusion investigations, stable isotope and mineral chemistry
with thermodynamic modeling, we have derived a model for the formation of the Wiesloch deposit. This model involves fluid
mixing between ascending hot brines (originating in the crystalline basement) with sedimentary formation waters. The ascending
brines originally had a near-neutral pH (around 6) and intermediate oxidation state, reflecting equilibrium with granites
and gneisses in the basement. During fluid ascent and cooling, the pH of the brine shifted towards more acidic (around 4)
and the oxidation state increased to conditions above the hematite-magnetite buffer. These chemical characteristics contrast
strongly with those of the pore and fracture fluid residing in the limestone aquifer, which had a pH between 8 and 9 in equilibrium
with calcite and was rather reduced due to the presence of organic matter in the limestone. Mixing between these two fluids
resulted in a strong decrease in the solubility of silver-bearing sphalerite and galena, and calcite. Besides Wiesloch, several
Pb–Zn deposits are known along the Upper Rhinegraben, including hydrothermal vein-type deposits like Badenweiler and the Michael
mine near Lahr. They all share the same fluid origin and formation process and only differ in details of their host rock and
fluid cooling paths. The mechanism of fluid mixing also seems to be responsible for the formation of other MVT deposits in
Europe (e.g., Réocin, Northern Spain; Trèves, Southern France; and Cracow-Silesia, Poland), which show notable similarities
in terms of their age, mineralogy. and mineral chemistry to the MVT deposit near Wiesloch. 相似文献
4.
The ultramafic-hosted Logatchev Hydrothermal Field (LHF) at 15°N on the Mid-Atlantic Ridge and the Arctic Gakkel Ridge (GR) feature carbonate precipitates (aragonite, calcite, and dolomite) in voids and fractures within different types of host rocks. We present chemical and Sr isotopic compositions of these different carbonates to examine the conditions that led to their formation. Our data reveal that different processes have led to the precipitation of carbonates in the various settings. Seawater-like 87Sr/86Sr ratios for aragonite in serpentinites (0.70909 to 0.70917) from the LHF are similar to those of aragonite from the GR (0.70912 to 0.70917) and indicate aragonite precipitation from seawater at ambient conditions at both sites. Aragonite veins in sulfide breccias from LHF also have seawater-like Sr isotope compositions (0.70909 to 0.70915), however, their rare earth element (REE) patterns show a clear positive europium (Eu) anomaly indicative of a small (< 1%) hydrothermal contribution. In contrast to aragonite, dolomite from the LHF has precipitated at much higher temperatures (~ 100 °C), and yet its 87Sr/86Sr ratios (0.70896 to 0.70907) are only slightly lower than those of aragonite. Even higher temperatures are calculated for the precipitation of deformed calcite veins in serpentine–talc fault schists form north of the LHF. These calcites show unradiogenic 87Sr/86Sr ratios (0.70460 to 0.70499) indicative of precipitation from evolved hydrothermal fluids. A simple mixing model based on Sr mass balance and enthalpy conservation indicates strongly variable conditions of fluid mixing and heat transfers involved in carbonate formation. Dolomite precipitated from a mixture of 97% seawater and 3% hydrothermal fluid that should have had a temperature of approximately 14 °C assuming that no heat was transferred. The much higher apparent precipitation temperatures based on oxygen isotopes (~ 100 °C) may be indicative of conductive heating, probably of seawater prior to mixing. The hydrothermal calcite in the fault schist has precipitated from a mixture of 67% hydrothermal fluid and 33% seawater, which should have had an isenthalpic mixing temperature of ~ 250 °C. The significantly lower temperatures calculated from oxygen isotopes are likely due to conductive cooling of hydrothermal fluid discharging along faults. Rare earth element patterns corroborate the results of the mixing model, since the hydrothermal calcite, which formed from waters with the greatest hydrothermal contribution, has REE patterns that closely resemble those of vent fluids from the LHF. Our results demonstrate, for the first time, that (1) precipitation from pure seawater, (2) conductive heating of seawater, and (3) conductive cooling of hydrothermal fluids in the sub-seafloor all can lead to carbonate precipitation within a single ultramafic-hosted hydrothermal system. 相似文献
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7.
High-temperature carbonate minerals have been observed in association with sulfide minerals below the platiniferous Johns-Manville (J-M) reef of the Stillwater Complex in a stratigraphic section that has been previously shown to be characterized by unusually Cl-rich apatite. The carbonate assemblage consists of dolomite with exsolved calcite in contact with sulfide minerals: chalcopyrite and pyrrhotite in the Peridotite Zone; and pyrrhotite with pentlandite, pyrite and chalcopyrite in Gabbronorite Zone I of the Lower Banded Series. A reaction rim surrounds the carbonate–sulfide assemblages, showing an alteration of the host orthopyroxene to a more calcium-enriched, Fe-depleted pyroxene. The calcite–dolomite geothermometer yields a minimum formation temperature as high as 950 °C for the unmixed assemblages. Iron and manganese concentrations exceed the range seen in carbonatite and mantle xenolith carbonates and are distinctly different from the nearly pure end-member carbonates associated with greenschist-grade (and lower) assemblages (e.g., carbonate veins in serpentinite) that occur locally throughout the complex. The association of high-temperature carbonates with sulfides beneath the J-M reef supports the hydromagmatic theory which involves a late-stage chloride–carbonate fluid percolating upwards, dissolving PGE and sulfides and redepositing them at a higher stratigraphic level. Characterization of the processes which form strategically important metal deposits, such as the J-M reef of the Stillwater Complex and the analogous Merensky reef of the Bushveld Complex in South Africa, could potentially lead to better exploration models and, more broadly, a deeper understanding of the cooling and compositional evolution of large bodies of ultramafic and mafic magma and of carbonatites, on both a local and a regional scale. 相似文献
8.
Simone Fontana Fadi Henri Nader Sadoon Morad Andrea Ceriani Ihsan Shakir Al‐Aasm Jean‐Marc Daniel Jean‐Marie Mengus 《Sedimentology》2014,61(3):660-690
An integrated approach consisting of fracture analysis, petrography, carbon, oxygen and strontium‐isotope analyses, as well as fluid‐inclusion micro‐thermometry, led to a better understanding of the evolution of fluid–rock interactions and diagenesis of the Upper Permian to Upper Triassic carbonates of the United Arab Emirates. The deposited carbonates were first marked by extensive early dolomitization. During progressive burial, the carbonates were affected by dolomite recrystallization as well as precipitation of vug and fracture‐filling dolomite, quartz and calcite cements. After considerable burial during the Middle Cretaceous, sub‐vertical north–south oriented fractures (F1) were cemented by dolomite derived from mesosaline to hypersaline fluids. Upon the Late Cretaceous maximum burial and ophiolite obduction, sub‐vertical east–west fractures (F2) were cemented by dolomite (Dc2) and saddle dolomite (Ds) derived from hot, highly saline fluids. Then, minor quartz cement has precipitated in fractures from hydrothermal brines. Fluid‐inclusion analyses of the various diagenetic phases imply the involvement of increasingly hot (200°C) saline brines (20 to 23% NaCl eq.). Through one‐dimensional burial history numerical modelling, the maximum temperatures reached by the studied rocks are estimated to be in the range of 160 to 200°C. Tectonically‐driven flux of hot fluids and associated diagenetic products are interpreted to have initiated during the Late Cretaceous maximum burial and lasted until the Oligocene–Miocene compressional tectonics and related uplift. The circulation of such hydrothermal brines led to partial dissolution of dolomites (Dc2 and Ds) and to precipitation of hydrothermal calcite C1 in new (mainly oriented north–south; F3) and pre‐existing, reactivated fractures. The integration of the obtained data confirms that the diagenetic evolution was controlled primarily by the interplay of the burial thermal evolution of the basin and the regional tectonic history. Hence, this contribution highlights the impacts of regional tectonics and basin history on diagenetic processes, which may subsequently affect reservoir properties. 相似文献
9.
The differentiated Mesozoic alkali dolerite Prospect Intrusion contains a wide range of secondary minerals, including carbonates (primarily calcite), laumontite, prehnite and heulandite, whose stability relationships imply a formation temperature of <200°C. The δ18O data for carbonates define a higher temperature (160 – 195°C) suite, and a lower temperature (51 – 73°C) suite. The δ13C, δ18O and 87Sr/86Sr isotope systematics for these carbonates suggest derivation of the higher temperature group from magmatic fluids, whereas the other group had a major meteoric component that probably originated from porewater in the country rock. Source fluids for prehnite were meteoric rather than magmatic in origin based on their δD and δ18O ratios. Early in the intrusion's emplacement, CO2-rich hydrothermal fluids formed a carbonate rind sealing the upper part of the hydrothermal system and produced the higher temperature carbonates (calcite) and laumontite. Later, cooler fluids with a meteoric component infiltrated vesicles and fractures, depositing the lower temperature carbonates (calcite, aragonite), heulandite and prehnite. 相似文献
10.
Most Mississippi Valley Type (MVT) Pb–Zn deposits have a common host-rock lithofacies sequence known as the hydrothermal dolomite (HTD) facies; however, the spatial and temporal relationships between dolomitization and Pb–Zn mineralization remain unclear. In this paper, we report a detailed cathodoluminescence (CL) study on the HTD facies of the No. II orebody in the Maoping Pb–Zn deposit in northeastern Yunnan Province, southwestern China. The results indicate that the dolomite in the Carboniferous limestone is part of a spectrum of hydrothermal mineral deposits, and the diagenesis and mineralization processes, in which the acidity of fluids played an important role, can be divided into six steps. A dolomite prototype formed during the first and second steps. In the third step, acid fluids produced by sulfide precipitation interacted with the dolomite prototype, resulting in dissolution and alteration, thus forming cavities. The fluid that formed during the first three steps flowed along open tensional faults. Dolomitization in the first step covered most of the region of later mineralization and gradually extended toward the center of the orebody. In the fourth and fifth steps, vugs and fractures were the main pathways for fluids, with only a small volume of fluid in the fifth step. Dedolomitization occurred in the sixth step and vugs were filled mostly by calcite. However, the fact that voids are present in only a small number of vugs is indicative of fluid exhaustion. Dolomite with growth zoning is characteristic of precipitation by ore-forming fluids accompanied by pulsed filling. There is no significant correlation between vugs and the locus of mineralization. Rather, the vugs indicate that the ore-forming fluids had passed through them, and the locus of massive ore formation was the center of the migrating fluid body. The saddle dolomite was deposited during pauses in sulfide precipitation. Cathodoluminescent zonation of HTD can be used as an aid in understanding the ore-forming process and for mineral prospecting in the Maoping deposit and other MVT deposits. 相似文献
11.
In the Earth's upper crust, where aqueous fluids can circulate freely, most mineral transformations are controlled by the coupling between the dissolution of a mineral that releases chemical species into the fluid and precipitation of new minerals that contain some of the released species in their crystal structure, the coupled process being driven by a reduction of the total free-energy of the system. Such coupled dissolution-precipitation processes occur at the fluid-mineral interface where the chemical gradients are highest and heterogeneous nucleation can be promoted, therefore controlling the growth kinetics of the new minerals. Time-lapse nanoscale imaging using Atomic Force Microscopy (AFM) can monitor the whole coupled process under in situ conditions and allow identifying the time scales involved and the controlling parameters. We have performed a series of experiments on carbonate minerals (calcite, siderite, dolomite and magnesite) where dissolution of the carbonate and precipitation of a new mineral was imaged and followed through time. In the presence of various species in the reacting fluid (e. g. antimony, selenium, arsenic, phosphate), the calcium released during calcite dissolution binds with these species to form new minerals that sequester these hazardous species in the form of a stable solid phase. For siderite, the coupling involves the release of Fe2+ ions that subsequently become oxidized and then precipitate in the form of FeIII oxyhydroxides. For dolomite and magnesite, dissolution in the presence of pure water (undersaturated with any possible phase) results in the immediate precipitation of hydrated Mg-carbonate phases. In all these systems, dissolution and precipitation are coupled and occur directly in a boundary layer at the carbonate surface. Scaling arguments demonstrate that the thickness of this boundary layer is controlled by the rate of carbonate dissolution, the equilibrium concentration of the precipitates and the kinetics of diffusion of species in a boundary layer. From these parameters a characteristic time scale and a characteristic length scale of the boundary layer can be derived. This boundary layer grows with time and never reaches a steady state thickness as long as dissolution of the carbonate is faster than precipitation of the new mineral. At ambient temperature, the surface reactions of these dissolving carbonates occur on time-scales of the order of seconds to minutes, indicating the rapid surface rearrangement of carbonates in the presence of aqueous fluids. As a consequence, many carbonate-fluid reactions in low temperature environments are controlled by local thermodynamic equilibria rather than by the global equilibrium in the whole system. 相似文献
12.
碳酸盐矿物是MVT型铅锌矿床最为常见的热液蚀变矿物,记录了成矿流体特征,其元素或同位素组成对示踪和定位矿体具有一定的指示意义,但由于该类矿床中碳酸盐矿物成因多样,单一的岩相学观察往往难以准确识别与成矿有关和无关的方解石,需要辅以地球化学手段来确定方解石成因。本文利用钻孔编录、岩相学观察、C-O-Sr同位素和原位微量与稀土元素组成分析等手段,探讨了青海多才玛超大型MVT铅锌矿床中与成矿有关和无关两期方解石的矿物化学特征。结果表明,多才玛矿床成矿前方解石(Cal1)空间上与矿体无相关关系,遍布于中二叠统九十道班组灰岩内,阴极发光呈暗棕色至棕黄色,无明显生长环带;成矿期方解石(Cal2)主要产于铅锌矿体内及其上盘围岩中,多呈砂糖粒状-皮壳状充填于围岩裂隙和溶孔内,阴极发光呈暗黄色至亮橙色,常发育生长环带。相对于Cal2, Cal1与围岩的C-O-Sr同位素组成更为接近,这表明水岩反应期间,Cal1组成受围岩影响更大,Cal2组成受影响更小;相比于Cal1,Cal2具有较高的Mn、Fe、Zn、Pb含量和U/Th、V/Cr值以及较低的LREE/HREE值、Mg、Sr含量和弱的Ce负异常,指示成矿... 相似文献
13.
The Duocaima carbonate-hosted Pb-Zn deposit is a newly found large deposit in the southern area of Qinghai Province.In this paper, the characteristics, genesis, significance to Pb-Zn mineralization of the widely developed breccias, and the ore-forming process have been carefully studied based on geological documentation of drilling holes, microscopic observations of petrography and microstructure and some stable isotope measurements.Based on the compositions of the clast and matrix, the breccias can be classified into three types: limestone clasts cemented by marl; limestone clasts with fine-grained calcareous materials; and limestone clasts cemented by hydrothermal calcite.The mineralization in the first type of breccia is weak, whereas it is strong in the latter two types of breccias.According to the locations of occurrence and structural characteristics of the breccias along with the relationship between the breccias and mineralization, part of the limestone clasts that are cemented by marl and outcrop in the contact zone between the Wudaoliang Formation(Nw) and the underlying Jiushidaoban Formation(Pj) are attributed to synsedimentary fault-genetic breccia, whereas the last of the limestone clasts that are cemented by marl and developed in the Jiushidaoban Formation(Pj) are attributed to the breccia generated by karst cave collapse; the limestone clasts with fine-grained calcareous materials and the limestone clasts cemented by hydrothermal calcite are attributed to breccia formed by hydrothermal dissolution.The breccia formed by karst collapse had consistently evolved for a long period of time, while the breccias with other origins were formed around the period of mineralization(i.e., about or slightly later than 20–16 Ma).The breccia generated by karst cave collapse and hydrothermal dissolution are somewhat related; the formation of the breccia from karst cave collapse provided open space for the later mineralization and reaction between hydrothermal fluids and host rocks, and the subsequent strong dissolution by hydrothermal fluids transformed some of the breccia formed earlier by karst cave collapse.Meanwhile, carbonate host rocks with breccias and brecciaed mineralization can be a potential sign of Mississippi Valley Type(MVT) deposits and important indicators for regional mineral exploration.The δ13CV-PDB, δ18OVSMOW, and 87Sr/86 Sr values of hydrothermal calcite in the Duocaima deposit range from 4.3‰ to 7.1‰, 14.9‰ to 20.1‰, and 0.707494 to 0.708185, respectively; the δ13CV-PDB, δ18OV-SMOW, and 87Sr/86 Sr values of the host limestones of the Jiushidaoban Formation range from 3.6‰ to 5.3‰, 18.0‰ to 20.5‰, and 0.707372 to 0.707945, respectively.The δ13CV-PDB and 87Sr/86 Sr values of hydrothermal calcite and limestone are similar, indicating single sources of C and Sr in this deposit, with the likely source being the limestone of the Jiushidaoban Formation.The minor scattering of the δ18OV-SMOW values suggests that different O isotope fluids underwent the isotope exchange reaction.The C-O-Sr isotope characteristics indicate that the host limestones experienced a dissolution and precipitation process during mineralization, which is beneficial to improving the porosity of host rocks and promoting the precipitation of metal sulfides.The δ34SV-CDT value of the breccia-type mineralization sulfides ranges from-30.4‰ to-0.3‰; that is, the δ34SV-CDT value is negative with considerable variation, illustrating that during the breccia-type mineralization process, the bacteriogenic reduction of sulfates provided the vast majority of sulfur, whereas the thermochemical reduction of sulfates was relatively unimportant.The brecciation that occurred as a result of karst cave collapse was mainly generated by the dissolution of groundwater; however, the brecciation related to hydrothermal dissolution and mineralization processes were caused by mixing of different fluids. 相似文献
14.
塔里木盆地热液活动地质地球化学特征及其对储层影响 总被引：67，自引：6，他引：61
塔里木盆地二叠纪时发生了强烈的岩浆-火山作用,与之相关的热液流体沿着断裂、裂缝以及不整合面活动,并与所经碳酸盐岩围岩发生反应,使围岩发生不同程度的溶蚀改造,主要表现在:1热液溶蚀和热褪色现象显著;2沉淀生成多种热液矿物组合,如萤石-石英组合、闪锌矿-绿泥石-方解石组合,重晶石-石英-黄铁矿-菱铁矿组合等;3热液作用区域碳酸盐岩成分发生了明显的变化,主要表现为Fe、Mn、Si等元素含量的升高,比正常灰岩高出几倍至几十倍。上述特点与岩溶作用和成岩溶蚀作用特征明显不同。对典型钻孔碳酸盐岩储层研究表明,无论灰岩还是白云岩,在热液作用下都会产生大量的微小溶蚀孔洞,储层物性得到明显的改善,因此热液作用是影响储层物性不可忽视的重要因素。 相似文献
15.
湘西-黔东地区位于扬子陆块东南缘,在该地区碳酸盐岩地层中,目前已发现大、中、小型铅锌矿床及矿点200余处.为了解湘西-黔东地区铅锌矿床成矿作用过程,系统总结了区内主要铅锌矿床地质与地球化学特征,并对成矿机制进行探讨,建立成矿模式.区内铅锌矿床主要赋存于下寒武统碳酸盐岩中,分布明显受断裂及褶皱构造控制,矿体主要为层状、似层状或透镜状,矿物组成主要为闪锌矿、方铅矿、黄铁矿、方解石及少量萤石、重晶石和沥青,并伴随着广泛的以方解石化为主的热液蚀变.闪锌矿与方解石中的流体包裹体均一温度集中在120~200℃之间,盐度集中在8%~20%(NaCleqv)之间;成矿期方解石的δ13CPDB值范围为-4.89‰~1.50‰,δ18OSMOW值范围为13.37‰~25.09‰,略低于碳酸盐围岩;矿石硫化物δ34S值变化范围为22.3‰~36.1‰,以富含重硫为主;矿石硫化物铅同位素组成较为均一,变化范围较小,206Pb/204Pb在17.952~18.678之间,207Pb/204Pb在15.635~15.832之间,208Pb/204Pb在38.015~39.255之间.对地质和地球化学资料的综合分析表明,湘西-黔东地区铅锌矿床成矿流体为低温、中高盐度热卤水,主要来源于建造水和大气降水,成矿流体中的碳主要来源于碳酸盐围岩的溶解作用,硫来源于碳酸盐岩地层中硫酸盐热化学还原作用(TSR),铅锌主要来源于下伏地层,成矿时代为晚志留世-早泥盆世,属于比较典型的密西西比河谷型(MVT)铅锌矿床.综合以上分析建立了该地区铅锌矿床有机质参与下的多源流体混合成矿模式. 相似文献
16.
A carbonate buildup of Middle Triassic age, the Esino Limestone, outcrops in the Southern Calcareous Alps of Lombardy (N Italy). Along its margin and within the open subtidal facies, the Esino Limestone contains calcite cement-filled cavities of cm to m size. These features, known as evinosponges, may form pervasive networks within the host rock. The filling consists of concentric, isopachous layers of fibrous low-Mg calcite crystals characterized by strong undulose extinction and bent cleavages. The cement crusts are non-luminescent under cathodoluminescence, but both cements and host rock are cross-cut by micro-fractures filled with bright-luminescent calcite, related to late void-filling sparite. Mixing of different carbonates is reflected in stable isotope data. On the hand specimen scale, the oxygen and carbon isotope compositions of cements and host rock show little variation. When compared on a regional scale, the values cover a broad range from δ18O(PDB)=?5‰ to ?12‰ and from δ13O =0‰ to +3‰. The linear covariant trends defined by the oxygen and carbon isotope data for different sampling regions reflect the admixture of late, isotopically depleted calcite with an isotopically enriched non-luminescent calcite of early diagenetic origin. The Esino Limestone fibrous cements, which were probably precipitated in the marine or marine-meteoric phreatic environment, were affected by late diagenetic processes that caused mineral deformation and isotopic depletion through recrystallization and the admixture of a later calcite. These later calcites precipitated from penetrative fluids possibly related to Late Triassic volcanic activity and/or to the Late Cretaceous/Early Palaeogene alpine orogeny. 相似文献
17.
Two-dimensional reactive transport modeling of the Maqarin Eastern Springs site, a natural analogue for the alteration of a fractured limestone by high-pH Portland cement waters, has been performed using the CrunchFlow code. These 2D calculations included transport by advection–dispersion–diffusion along a single fracture and diffusion in the wall rock. Solute transport was coupled to mineral dissolution and precipitation. A limited sensitivity analysis evaluated the effect of different values of primary mineral surface areas, flow velocity and sulfate concentration of the inflowing high-pH solution.Major secondary minerals include ettringite–thaumasite, C–S–H/C–A–S–H and calcite. C–S–H/C–A–S–H precipitation is controlled by the dissolution of primary silicates. Ettringite precipitation is controlled by diffusion of sulfate and aluminum from the wall rock to the fracture, with aluminum provided by the dissolution of albite. Calcite precipitation is controlled by diffusion of carbonate from the wall rock. Extents of porosity sealing along the fracture and in the fracture-wall rock interface depend on assumptions regarding flow velocity and composition of the high-pH solution. The multiple episodes of fracture sealing and reactivation evidenced in the fracture infills were not included in the simulations. Results can qualitatively reproduce the reported decrease in porosity in the fractures and in the wall rock next to the fractures. Instances of porosity increase next to fractures caused by carbonate dissolution were not reproduced by the calculations. 相似文献
18.
Neptunian dykes and associated features in southern Spain: mechanics of formation and tectonic implications 总被引：2，自引：0，他引：2
Neptunian dykes and sills in Jurassic carbonate platform strata of the Betic region of Spain occupy spaces created by wholly mechanical fracturing and displacement of the host strata, and later filled by pelagic sediments from above or by precipitation of calcite from circulating solutions. In some places, joint-bounded blocks of platform carbonates have been wholly removed, possibly by sliding down submarine slopes, leaving a staircase topography, commonly Fe-encrusted, that was subsequently filled by pelagic sediments. Other cavities that formed during Cretaceous times were developed by dissolution and current erosion in moderately deep submarine environments, and then filled by pelagic sediments from above. None of the cavities hosted by either platform or pelagic strata contain evidence for their formation by dissolution in a subaerial environment. The Jurassic and Cretaceous history of subsidence of the Betic margins is thus simpler than in versions requiring repeated emergence to form subaerial karstic cavities. 相似文献
19.
霍什布拉克铅锌矿床是新疆西南天山地区晚古生代碳酸盐岩容矿的重要矿床。本文采用高精度电感耦合等离子质谱(ICP-MS)对主要矿石矿物(黄铁矿、方铅矿、闪锌矿)和热液脉石矿物方解石进行了微量元素地球化学研究。研究表明,黄铁矿、方铅矿、闪锌矿、方解石具有较为相似的稀土元素特征,Eu正异常明显,弱负或弱正Ce异常指示成矿流体最初由相对氧化的条件演化为还原条件。与霍什布拉克岩体、容矿围岩的稀土元素组成进行对比后发现,硫化物及热液方解石稀土元素配分曲线部分与重结晶泥晶灰岩重合,结合前人同位素地球化学研究,认为容矿坦盖塔尔组碳酸盐岩为成矿提供了必要的成矿物质及成矿流体组分。Ga/In值显示闪锌矿为低温成矿;硫化物明显富集LREE,Hf/Sm、Nb/La和Th/La值小于1,显示铅锌成矿过程中成矿流体主体以富Cl为特征;Y/Ho、Zr/Hf和Nb/Ta值变化范围相对较小,表明成矿期流体性质相对较为稳定,基本未混入外来流体。 相似文献
20.
山东莱芜盆地碳酸盐岩热液溶蚀特征及水文地质意义 总被引：1，自引：0，他引：1
鲁中山区莱芜盆地在燕山期发生大规模岩浆侵入活动,岩浆岩体外围碳酸盐岩区分布有水源地、富水块段等岩溶发育富水区,且靠近岩体含水层多发生大理岩化。富含CO2、H2S等酸性成分的岩浆热液流体,能对碳酸盐岩产生明显的溶蚀作用。基于岩石样品化学组分分析、水文地质调查及数据统计分析、热液溶蚀地质点详细观测,构建热液溶蚀模式,对莱芜盆地及外围热液溶蚀特征及机理进行研究。结果表明:高温高压岩浆侵入使得接触带碳酸盐岩发生热液变质大理岩化;沿渗透性断裂、裂缝以及不整合面等运移的热液流体,对碳酸盐岩地层产生酸性热液溶蚀和混合溶蚀作用,形成不同规模溶蚀孔洞,构成地下水富集空间,为寻找城镇地下水后备水源地提供新方向。 相似文献