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1.
The oxygen isotope compositions of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel, have been used to identify porewater types during diagenesis. Changes in porewater composition can be related to major geological events within southern Israel. In particular, saline brines played an important role in late (Pliocene-Pleistocene) dolomitization of these rocks. Diagenetic carbonates included early siderite (δ18OSMOW=+24.4 to +26.5‰δ13CPDB=?1.1 to +0.8‰), late dolomite, ferroan dolomite and ankerite (δ18OSMOW=+18.4 to +25.8‰; δ13CPDB=?2.1 to +0.2‰), and calcite (δ18OSMOW=+21.3 to +32.6‰; δ13CPDB=?4.2 to + 3.2‰). The petrographic and isotopic results suggest that siderite formed early in the diagenetic history at shallow depths. The dolomitic phases formed at greater depths late in diagenesis. Crystallization of secondary calcite spans early to late diagenesis, consistent with its large range in isotopic values. A strong negative correlation exists between burial depth (temperature) and the oxygen isotopic compositions of the dolomitic cements. In addition, the δ18O values of the dolomitic phases in the northern Negev and Judea Mountains are in isotopic equilibrium with present formation waters. This behaviour suggests that formation of secondary dolomite post-dates the tectonic activity responsible for the present relief of southern Israel (Upper Miocene to Pliocene) and that the dolomite crystallized from present formation waters. Such is not the case in the Central Negev. In that locality, present formation waters have much lower salinities and δ18O values, indicating invasion of freshwater, and are out of isotopic equilibrium with secondary dolomite. Recharge of the Inmar Formation by meteoric water in the Central Negev occurred in the Pleistocene, and halted formation of dolomite.  相似文献   

2.
The Yangla Cu deposit is the largest Cu skarn deposit in the Jinshajiang tectonic belt. Based on the detailed observation of crosscutting relationships, three mineralization stages (i.e., pre-ore, ore and supergene) have been identified in the Yangla deposit. The pre-ore stage is dominated by prograde skarn. The ore stage is characterized by the precipitation of hydrous silicate minerals, Fe-oxides, Fe-Cu-Mo-sulfides, quartz and calcite, whose mineral assemblages were formed in the early and late sub-ore stages. The early sub-ore stage is marked by retrograde alteration with the deposition of hydrous silicate minerals (e.g., actinolite, epidote and chlorite), Fe-oxides, abundant Fe-Cu-Mo-sulfides, quartz and minor calcite. Whilst, the late sub-ore stage, associated with silicic and carbonate alteration, is represented by widespread thick quartz or calcite veins with disseminated pyrite, chalcopyrite, galena and sphalerite. We present new carbon-oxygen (C-O) isotopic compositions of the ore-hosting marble and hydrothermal calcite of this deposit. The hydrothermal calcite in the Yangla deposit was precipitated from both the early and late sub-ore stages. Calcite I from the early sub-ore stage is anhedral, and occurs as spot in the skarn or locally replaces the skarn minerals. Calcite II from the late sub-ore stage is distinguished by being coarse-grained, subhedral to euhedral and its occurrence in thick veins. Calcite I contains lower δ13CPDB (−7.0‰ to −5.0‰) and δ18OSMOW (7.2‰ to 12.7‰) than Calcite II (δ13CPDB = −4.5‰ to −2.3‰; δ18OSMOW = 10.7‰ to 19.4‰). In the δ13CPDB vs. δ18OSMOW diagram, the Calcite I and Calcite II data fall close to the igneous carbonatite field and between the fields of igneous carbonatite and marine carbonates, respectively. This suggests a dominantly magmatic origin for the early sub-ore fluids, and there might have been increasing carbonate wall rock involvement towards the late sub-ore stage. The ore-hosting marble (δ13CPDB = −4.8‰ to −0.3‰; δ18OSMOW = 10.2‰ to 23.9‰) also shows a positive δ13CPDB vs. δ18OSMOW correlation, which is interpreted to reflect the decreasing alteration intensity during the interactions between the hydrothermal fluids and ore-hosting carbonates. Simulated calculation suggests that both the Calcite I and Calcite II precipitated at 350 °C to 250 °C and 250 °C to 150 °C, respectively. We proposed that CO2 degassing and water/rock interactions were likely the two major processes that precipitated the calcite and led to the observed C-O isotopic features of the Yangla Cu deposit.  相似文献   

3.
Calcitized Jurassic dolostones from central Mount Lebanon (Kesrouane Formation) are discussed utilizing petrographic, mineralogical and geochemical data. In particular, two sequential extraction methods for both major/trace elements and stable isotope analyses provide results that support and refine conventional bulk analyses data. The new data demonstrate that the major dedolomitization phase of the investigated Jurassic carbonates occurred as a result of the migration of karst‐related meteoric waters (characterized by soil‐derived carbon, and estimated δ18OV‐SMOW composition between ?7·2‰ and ?3·4‰) into previously dolomitized horizons within the limestone rock, during the final uplift and emergence of Mount Lebanon, after Palaeogene time. The study demonstrates that, in this case, the mechanisms of dedolomitization and their resulting fabrics are controlled primarily by the texture of the original dolomite rock. Pervasively dolomitized rocks, where the micritic matrix is entirely dolomitized, show calcitization mainly through dissolution/precipitation. By contrast, the rock textures that still include a considerable amount of limey micritic matrix – spared from dolomitization – are more prone to mole per mole and mimic replacement of the dolomite crystals by calcite.  相似文献   

4.
Carbonates often accompany lake and lake‐margin deposits in both modern and ancient geological settings. If these carbonates are formed in standing water, their stable isotope values reflect the aquatic chemistry at the time of precipitation and may provide a proxy for determining regional hydrologic conditions. Carbonate rhizoliths and water samples were collected from a playa lake in eastern Nevada. Pilot Valley (～43°N) is a closed‐basin, remnant playa from the Quaternary desiccation of palaeo‐Lake Bonneville. Water is added to the playa margin by free convection of dense brines to the east and forced convection of freshwater off the alluvial fan to the west. Both freshwater and saline springs dot the playa margin at the base of an alluvial fan. Water samples collected from seven springs show a range from ?16 to ?0·2‰ (Vienna Standard Mean Ocean Water), and are consistent with published values. The δ18Ocalcite values from rhizolith samples range from ?18·3 to ?6·7‰ (Vienna Pee Dee Belemnite), and the average is ?12‰ V‐PDB (1 ? σ SD 2‰). With the exception of samples from Little Salt Spring, the range in the δ18Ocalcite values collected from the rhizoliths confirms that they form in equilibrium with ambient water conditions on the playa. The initial geochemical conditions for the spring waters are dictated by local hydrology: freshwater springs emerge in the northern part of the basin to the east of a broad alluvial fan, and more saline springs emerge to the south where the influence of the alluvial fan diminishes. Rhizoliths are only found near the southern saline springs and their δ13Ccalcite values, along with their morphology, indicate that they only form around saltgrass (Distichlis sp.). As the residence time of water on the playa increases, evaporation, temperature change and biological processes alter the aquatic chemistry and initiate calcite precipitation around the plant stems. The range in δ18Ocalcite values from each location reflects environmental controls (e.g. evaporation and temperature change). These rhizoliths faithfully record ambient aquatic conditions during formation (e.g. geochemistry and water depth), but only record a partial annual signal that is constrained by saltgrass growth and the presence of standing water on the playa margin.  相似文献   

5.
Authigenic calcite and dolomite and biogenic aragonite occur in Holocene pan sediments in a Mediterranean‐type climate on the western coastal plain of South Africa. Sediment was analysed from a Late Pleistocene coastal pan at Yzerfontein and four Holocene inland pans ranging from brackish to hypersaline. The pans are between 0·08 and 0·14 km2 in size. The δ18OPDB values of carbonate minerals in the pan sediments range from ?2·41 to 5·56‰ and indicate precipitation from evaporative waters. Covariance of total organic content and percentage carbonate minerals, and the δ13CPDB values of pan carbonate minerals (?8·85 to ?1·54‰) suggest that organic matter degradation is a significant source of carbonate ions. The precipitation of the carbonate minerals, especially dolomite, appears to be mediated by sulphate‐reducing bacteria in the black sulphidic mud zone found in the brine‐type hypersaline pans. The knobbly, sub‐spherical texture of the carbonate minerals suggests that the precipitation of the carbonate minerals, particularly dolomite, is related to microbial processes. The 87Sr/86Sr ratios of pan carbonate minerals (0·7108 to 0·7116) are slightly higher than modern sea water and indicate a predominantly sea water (marine aerosol) source for calcium (Ca2+) ions with relatively minor amounts of Ca2+ derived from the chemical weathering of bedrock.  相似文献   

6.
The Yinchanggou-Qiluogou Pb-Zn deposit,located in the western Yangtze Block,southwest China,is hosted by the Upper Sinian Dengying Formation dolostone.Ore bodies occur in the Qiluogou anticline and the NS-and NNW-trending faults.Sulfide ores mainly consist of sphalerite,pyrite,galena and calcite,with subordinate dolomite and quartz.Seventeen ore bodies have been discovered to date and they have a combined 1.0 million tons of sulfide ores with average grades of 2.27wt%Zn and 6.89wt%Pb.The δD_(H2O-SMOW) and δ~(18)O_(H2O-SMOW) values of fluid inclusions in quartz and calcite samples range from-68.9‰ to-48.7‰ and 7.3‰ to 15.9‰,respectively,suggesting that H_2O in the hydrothermal fluids sourced from metamorphic water.Calcite samples have δ~(13)C_(PDB) values ranging from-6.2‰ to-4.1‰ and δ~(18)O_(SMOW) values ranging from 15.1‰ to 17.4‰,indicating C and O in the hydrothermal fluids likely derived from a mixed source of metamorphic fluids and the host carbonates.The δ~(34)S_(CDT) values of sulfide minerals range from 5.5‰ to 20.3‰,suggesting that thermal chemical reduction of sulfate minerals in evaporates were the most probable source of S in the hydrothermal fluids.The ~(206)Pb/~(204)Pb,~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb ratios of sulfide minerals fall in the range of 18.11 to 18.40,15.66 to 15.76 and 38.25 to 38.88,respectively.The Pb isotopic data of the studied deposit plot near the upper crust Pb evolution curve and overlap with the age-corrected Proterozoic basement rocks and the Upper Sinian Dengying Formation hosting dolostone.This indicates that the Pb originated from a mixed source of the basement metamorphic rocks and the ore-hosting carbonate rocks.The ore geology and C-H-O-S-Pb isotopic data suggest that the YinchanggouQiluogou deposit is an unusual carbonate-hosted,strata-bound and epigenetic deposit that derived ore-forming materials from a mixed source of the underlying Porterozoic basements and the Sinian hosting carbonates.  相似文献   

7.
《Applied Geochemistry》2002,17(9):1241-1257
This study aimed to develop a methodology for assessing the hydrochemical evolution of a groundwater system, using fracture-filling and fracture-lining calcite. Fracture calcite in deep (to ca. 1000 m) granitic rocks of the Tono area, central Japan, was investigated by optical and electron microscopy, and chemical and isotopic analysis. Coupled with geological evidence, these new data imply 3 main origins for the waters that precipitated calcite: (1) relatively high-temperature hydrothermal solutions, precipitating calcite distinguished by δ18OSMOW from −3 to ca. 10‰, and with δ13CPDB from ca. −18 to −7‰; (2) seawater, probably partly of Miocene age, which precipitated calcite distinguished by δ13CPDB of ca. 0‰ and δ18OSMOW > ca. 20‰; (3) fresh water, with a variable δ13CPDB composition, but which precipitated calcite distinguished by δ13CPDB that was significantly < 0‰ and as low as ca. −29‰ and δ18OSMOW > ca. 17‰. Data for 14C suggest that at least some of the fresh-water calcite formed within the last 50 ka. The present day hydrogeological regime in the Tono area is also dominated by fresh groundwater. However, the marine calcite of probable Miocene age found at depth has shown no evidence for dissolution and many different calcite crystal forms have been preserved. Studies of other groundwater systems have correlated similar crystallographic variations with variations in the salinity of coexisting groundwaters. When this correlation is applied to the Tono observations, the calcite crystal forms imply a similar range of groundwater salinity to that inferred from the isotopic data. Thus, the present study suggests that even in presently low-salinity groundwater systems, calcite morphological variations may record the changing salinity of coexisting groundwaters. It is suggested that calcite morphological data, coupled with isotopic data, could provide a powerful palaeohydrogeological tool in such circumstances.  相似文献   

8.
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   

9.
The study focuses on the formation of lacustrine dolomite in late Miocene lakes, located at the East Mediterranean margins (Northern Israel). These lakes deposited the sediments of the Bira (Tortonian) and Gesher (Messinian) formations that comprise sequences of dolostone and limestone. Dolostones are bedded, consist of small‐sized (<7 μm), Ca‐rich (52 to 56 mol %) crystals with relatively low ordering degrees, and present evidence for replacement of CaCO3 components. Limestones are comprised of a wackestone to mudstone matrix, freshwater macrofossils and intraclasts (mainly in the Bira Formation). Sodium concentrations and isotope compositions differ between limestones and dolostones: Na = ~100 to 150 ppm; ~1000 to 2000 ppm; δ18O = ?3·8 to ?1·6‰; ?2·0 to +4·3‰; δ13C = ?9·0 to ?3·4‰; ?7·8 to 0‰ (VPDB), respectively. These results indicate a climate‐related sedimentation during the Tortonian and early Messinian. Wet conditions and positive freshwater inflow into the carbonate lake led to calcite precipitation due to intense phytoplankton blooms (limestone formation). Dry conditions and enhanced evaporation led to precipitation of evaporitic CaCO3 in a terminal lake, which caused an increased Mg/Ca ratio in the residual waters and penecontemporaneous dolomitization (dolostone formation). The alternating lithofacies pattern reveals eleven short‐term wet–dry climate‐cycles during the Tortonian and early Messinian. A shift in the environmental conditions under which dolomite formed is indicated by a temporal decrease in δ18O of dolostones and Na content of dolomite crystals. These variations point to decreasing evaporation degrees and/or an increased mixing with meteoric waters towards the late Messinian. A temporal decrease in δ13C of dolostones and limestones and appearance of microbial structures in close association with dolomite suggest that microbial activity had an important role in allowing dolomite formation during the Messinian. Microbial mediation was apparently the main process that enabled local growth of dolomite under wet conditions during the latest Messinian.  相似文献   

10.
The Maoniuping REE deposit, located about 22 km to the southwest of Mianning, Sichuan Province, is the second largest light REE deposit in China, subsequent to the Bayan Obo Fe-Nb-REE deposit in the Inner Mongolia Autonomous Region. Tectonically, it is located in the transitional zone between the Panxi rift and the Longmenshan-Jinpingshan orogenic zone. It is a carbonatite vein-type deposit hosted in alkaline complex rocks. The bastnaesite-barite, bastnaesite-calcite, and bastnaesite-microcline lodes are the main three types of REE ore lodes. Among these, the first lode is distributed most extensively and its REE mineralization is the strongest. Theδ34Sv-CDT values of the barites in the ore of the deposit vary in a narrow range of +5.0 to +5.1‰in the bastnaesite-calcite lode and +3.3 to +5.9‰in the bastnaesite-barite lode, showing the isotopic characteristics of magma-derived sulfur. Theδ13Cv-PDB values and theδ518OV-SMOW values in the bastnaesite-calcite lode range from -3.9 to -6.9‰and from +7.3 to +9.7‰, respectively, which fall into the range of "primary carbonatites", showing that carbon and oxygen in the ores of the Maoniuping deposit were derived mainly from a deep source. Theδ13Cv-PDB values of fluid inclusions vary from -3.0 to -5.6‰, with -3.0 to -4.0‰in the bastnaesite-calcite lode and -3.0 to -5.6‰in the bastnaesite-barite lode, which show characteristics of mantle-derived carbon. TheδDv-SMOW values of fluid inclusions range from -57 to -88‰, with -63 to -86‰in the bastnaesite-calcite lode and -57 to -88‰in the bastnaesite-barite lode, which show characteristics of mantle-derived hydrogen. Theδ18OH2OV-SMOW values vary from +7.4 to +8.6‰in the bastnaesite calcite lode, and +6.7 to +7.8‰in the bastnaesite-barite lode, almost overlapping the range of +5.5 to +9.5‰for magmatic water. The 4He content, R/Ra ratios are (13.95 to 119.58×10-6 (cm3/g)STP and 0.02 to 0.11, respectively, and 40Ar/36Ar is 313±1 to 437±2. Considering the 4He increase caused by high contents of radioactive elements, a mantle-derived fluid probably exists in the inclusions in the fluorite, calcite and bastnaesite samples. The Maoniuping deposit and its associated carbonatite-alkaline complex were formed in 40.3 to 12.2 Ma according to K-Ar and U-Pb data. All these data suggest that large quantities of mantle fluids were involved in the metallogenic process of the Maoniuping REE deposit through a fault system.  相似文献   

11.
Limestone consisting of finely to medium crystalline calcite mosaics is present in the upper part of the Winnipegosis Formation on the east‐central margin of the Elk Point Basin where the overlying Prairie Evaporite deposits have been removed. This type of crystalline limestone is interpreted as dedolomite, based on petrographic observations. The δ18O and δ13C values of the Winnipegosis dedolomite vary from ?12·8‰ to ?11·9‰ VPDB (Vienna Pee Dee Belemnite) and from ?0·5‰ to +1·7‰ VPDB, respectively; both values are significantly lower than those for the corresponding dolomite. The 87Sr/86Sr ratios of the dedolomite are significantly higher, between 0·7082 and 0·7087. The spatial distribution and geochemical data of the Winnipegosis dedolomite suggest that dedolomitization was related to an influx of fresh groundwater and dissolution of the Prairie Evaporite anhydrite during the latest Mississippian to the Early Cretaceous when the basin was subjected to uplift and erosion. The Winnipegosis dedolomite displays a series of replacement fabrics showing progressive calcitization of dolomite, including the occurrence of dedolomite restricted along fractures and adjacent areas, dolomite patches ‘floating’ in the dedolomite masses and massive dedolomite with sparsely scattered dolomite relicts. However, the characteristic fabrics resulting from dedolomitization documented in the literature have not been observed in the Winnipegosis dedolomite. Coarsely to very coarsely crystalline, subhedral to euhedral calcite cement is restricted in the dedolomite. The petrographic features, isotopic compositions and homogenization temperatures, coupled with the burial history of the Winnipegosis Formation, constrain the precipitation of the calcite cement from a mixing of basinal brines and fresh groundwater during Late Cretaceous to Neogene time. The more negative C‐isotopic signatures of the calcite cement (?5·3‰ to ?2·3‰ VPDB) probably reflect a hydrocarbon‐derived carbon.  相似文献   

12.
The extensive Changba-Lijiagou Pb-Zn deposit is located in the north of the Xihe–Chengxian ore cluster in West Qinling. The ore bodies are mainly hosted in the marble, dolomitic marble and biotite-calcite-quartz schist of the Middle Devonian Anjiacha Formation, and are structurally controlled by the fault and anticline. The ore-forming process can be divided into three main stages, based on field geological features and mineral assemblages. The mineral assemblages of hydrothermal stage I are pale-yellow coarse grain, low Fe sphalerite, pyrite with pits, barite and biotite. The mineral assemblages of hydrothermal stage II are black-brown cryptocrystalline, high Fe shalerite, pyrite without pits, marcasite or arsenopyrite replace the pyrite with pits, K-feldspar. The features of hydrothermal stage III are calcite-quartz-sulfide vein cutting the laminated, banded ore body. Forty-two sulfur isotope analyses, twenty-five lead isotope analyses and nineteen carbon and oxygen isotope analyses were determined on sphalerite, pyrite, galena and calcite. The δ34 S values of stage I(20.3 to 29.0‰) are consistent with the δ34 S of sulfate(barite) in the stratum. Combined with geological feature, inclusion characteristics and EPMA data, we propose that TSR has played a key role in the formation of the sulfides in stage I. The δ34 S values of stage II sphalerite and pyrite(15.1 to 23.0‰) are between sulfides in the host rock, magmatic sulfur and the sulfate(barite) in the stratum. This result suggests that multiple S reservoirs were the sources for S2-in stage II. The δ34 S values of stage III(13.1 to 22‰) combined with the structure of the geological and mineral features suggest a magmatic hydrothermal origin of the mineralization. The lead isotope compositions of the sulfides have 206 Pb/204 Pb ranging from 17.9480 to 17.9782, 207 Pb/204 Pb ranging from 15.611 to 15.622, and 208 Pb/204 Pb ranging from 38.1368 to 38.1691 in the three ore-forming stages. The narrow and symmetric distributions of the lead isotope values reflect homogenization of granite and mantle sources before the Pb-Zn mineralization. The δ13 CPDB and δ18 OSMOW values of stage I range from-0.1 to 2.4‰ and from 18.8 to 21.7‰. The values and inclusion data indicate that the source of fluids in stage I was the dissolution of marine carbonate. The δ13 CPDB and δ18 OSMOW values of stage II range from-4 to 1‰ and from 12.3 to 20.3‰, suggesting multiple C-O reservoirs in the Changba deposit and the addition of mantle-source fluid to the system. The values in stage III are-3.1‰ and 19.7‰, respectively. We infer that the process of mineralization involved evaporitic salt and sedimentary organic-bearing units interacting through thermochemical sulfate reduction through the isotopic, mineralogy and inclusion evidences. Subsequently, the geology feature, mineral assemblages, EPMA data and isotopic values support the conclusion that the ore-forming hydrothermal fluids were mixed with magmatic hydrothermal fluids and forming the massive dark sphalerite, then yielding the calcite-quartz-sulfide vein ore type at the last stage. The genesis of this ore deposit was epigenetic rather than the previously-proposed sedimentary-exhalative(SEDEX) type.  相似文献   

13.
Unusually large biogenic sedimentary structures from the shallow quiescent-marine siliciclastics of the Upper Eocene Birket Qarun Formation in the Fayum area of Egypt display pronounced concretion formation around the trace fossils. The structures are massive, and vary morphologically, forming branched pillars (up to dm-scale), vertical (up to 180 cm height) amphora-like masses, and 3-D box-work “maze”. Bioturbation, mainly Thalassinoides attributable to the Glossifungites ichnofacies, mediated and modified the physical and chemical microenvironments influencing early diagenesis; i.e., burrows promote the precipitation of pervasive calcite-dominated cement. The inferred paragenesis, combined with the negative (light) carbon and oxygen stable-isotopic values of the bulk calcite (δ13CPDB from ?0.94 to ?4.98‰ and δ18OPDB from ?4.63 to ?7.22‰) and bulk dolomite (δ13CPDB from ?2.05 to ?8.23‰ and δ18OPDB from ?1.41 to ?11.20‰), imply that the pore-water carbon was derived directly from seawater and dissolution of metastable carbonate, which was mediated by bacterial decomposition of organic matter and mixing of meteoric ground water. Thereby, the carbonate cement precipitated mostly under eodiagenetic conditions near the sediment/water interface (<~3 m in depth). The distribution of these structures is confined to parasequence-bounding flooding surfaces (generally expressed as transgressive surfaces of erosion). Notably, sedimentological, ichnological and paragenetic data can be related to stratigraphic evolution such that geochemical and textural evidence is distinctly associated with (1) early cementation of the host sandstone during highstands of relative sea level, (2) the formation of firmgrounds during low relative sea level, (3) the development of a Glossifungites-demarcated discontinuity during initial relative sea-level rise, and (4) continued cementation with rising relative sea level. This was followed by burial diagenesis, evidence for which is derived from petrographic and isotopic data.  相似文献   

14.
Upper Pliocene dolomites (‘white earth’) from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations. The La Roda Mg‐carbonates (dolomite unit) occur as a 3·5‐ to 4‐m‐thick package of poorly indurated, white, massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds. The massive dolomite beds consist mainly of loosely packed 1‐ to 2‐μm‐sized aggregates of dolomite crystals exhibiting poorly developed faces, which usually results in a subrounded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin porous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca‐dolomite (51–54 mol% CaCO3), showing a low degree of ordering (degree of ordering ranges from 0·27 to 0·48). Calcite is present as a subordinate mineral in some samples. Sr, Mn and Fe contents show very low correlation coefficients with Mg/Ca ratios, whereas SiO2 and K contents are highly correlated. δ18O‐ and δ13C‐values in dolomites range from ?3·07‰ to 5·40‰ PDB (mean=0·06, σ=1·75) and from ?6·34‰ to ?0·39‰ PDB (mean=?3·55, σ=1·33) respectively. Samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in 18O and 13C between the two minerals. The correlation coefficient between δ18O and δ13C for dolomite is extremely low and negative (r=?0·05), whereas it is higher and positive (r=0·47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO3?/CO32? groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi‐arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence.  相似文献   

15.
The carbonate-hosted Pb–Zn deposits in the Sanjiang metallogenic belt on the Tibetan Plateau are typical of MVT Pb–Zn deposits that form in thrust-fold belts. The Jiamoshan Pb–Zn deposit is located in the Changdu area in the middle part of the Sanjiang belt, and it represents a new style of MVT deposit that was controlled by karst structures in a thrust–fold system. Such a karst-controlled MVT Pb–Zn deposit in thrust settings has not previously been described in detail, and we therefore mapped the geology of the deposit and undertook a detailed study of its genesis. The karst structures that host the Jiamoshan deposit were formed in Triassic limestones along secondary reverse faults, and the orebodies have irregular tubular shapes. The main sulfide minerals are galena, sphalerite, and pyrite that occur in massive and lamellar form. The ore-forming fluids belonged to a Mg2+–Na+–K+–SO2-4–Cl-–F-–NO-3–H2 O system at low temperatures(120–130°C) but with high salinities(19–22% NaCl eq.). We have recognized basinal brine as the source of the ore-forming fluids on the basis of their H–O isotopic compositions(-145‰ to-93‰ for δDV-SMOW and-2.22‰ to 13.00‰ for δ18 Ofluid), the ratios of Cl/Br(14–1196) and Na/Br(16–586) in the hydrothermal fluids, and the C–O isotopic compositions of calcite(-5.0‰ to 3.7‰ for δ13 CV-PDB and 15.1‰ to 22.3‰ for δ18 OV-SMOW). These fluids may have been derived from evaporated seawater trapped in marine strata at depth or from Paleogene–Neogene basins on the surface. The δ34 S values are low in the galena(-3.2‰ to 0.6‰) but high in the barite(27.1‰), indicating that the reduced sulfur came from gypsum in the regional Cenozoic basins and from sulfates in trapped paleo-seawater by bacterial sulfate reduction. The Pb isotopic compositions of the galena samples(18.3270–18.3482 for 206 Pb/204 Pb, 15.6345–15.6390 for 207 Pb/204 Pb, and 38.5503–38.5582 for 208 Pb/204 Pb) are similar to those of the regional Triassic volcanic-arc rocks that formed during the closure of the Paleo-Tethys, indicating these arc rocks were the source of the metals in the deposit. Taking into account our new observations and data, as well as regional Pb–Zn metallogenic processes, we present here a new model for MVT deposits controlled by karst structures in thrust–fold systems.  相似文献   

16.
This paper addresses the diagenesis of carbonate conglomerates in that it assesses the potential of conglomerates in refining the paragenetic history in complex structural areas, such as the Albanian foreland fold‐and‐thrust belt. Of major interest are stylolites (burial and tectonic) which are restricted to conglomerate fragments or which crosscut the conglomerate matrix. Based on the inferred age of stylolite development in relation to burial, uplift and tectonic history, and the Lower to Middle Miocene age of the conglomerates, the succession of diagenetic events was subdivided into several stages. The Poçem polymict transgressive carbonate conglomerate (Kremenara anticline, central Albania) was deposited in a shallow marine environment. These conglomerates are covered by intertidal rhodolithic packstones–grainstones. The stable‐isotope signature of these packstones–grainstones (δ18OV‐PDB = −1·0 to +0·7‰; δ13C = +1·0 to +1·4‰) plots is within the range of marine Early and Middle Miocene values. Shortly after deposition of the conglomerates, micritization, geopetal infill and acicular calcite cementation took place. A first calcite vein generation is interpreted as having formed from a Messinian brine during shallow burial. Burial stylolites developed during further burial in the Pliocene. These stylolites serve as an important diagenetic time marker. The post‐burial stylolite meteoric calcite vein cement probably precipitated during the following telogenetic stage. Karstification and calcite concretion precipitiation pre‐date overturning of the western limb of the anticline. Reopening of subvertical fractures and tectonic stylolites in the western limb of the Kremenara anticline, followed by oil migration, represents one of the latest diagenetic events. These fractures and stylolites provide major pathways for hydrocarbon production.  相似文献   

17.
Investigation of the palaeoclimatic conditions associated with Upper Jurassic strata in Portugal and comparison with published palaeoclimate reconstructions of the Upper Jurassic Morrison Formation in western North America provide important insights into the conditions that allowed two of the richest terrestrial faunas of this period to flourish. Geochemical analyses and observations of palaeosol morphology in the informally named Upper Jurassic Lourinhã formation of western Portugal indicate warm and wet palaeoclimatic conditions with strongly seasonal precipitation patterns. Palaeosol profiles are dominated by carbonate accumulations and abundant shrink‐swell (vertic) features that are both indicative of seasonal variation in moisture availability. The δ18OSMOW and δDSMOW values of phyllosilicates sampled from palaeosol profiles range from +22·4‰ to +22·7‰ and ?53·0‰ to ?37·3‰, respectively. These isotope values correspond to temperatures of formation between 32°C and 39°C ± 3°, with an average of 36°C, which suggest surface temperatures between 27°C and 34°C (average 31°C). On average, these surface temperature estimates are 1°C higher than the highest summer temperatures modelled for Late Jurassic Iberia using general circulation models. Elemental analysis of matrix material from palaeosol B‐horizons provides proxy (chemical index of alteration minus potassium) estimates of mean annual precipitation ranging from 766 to 1394 mm/year, with an average of approximately 1100 mm/year. Palaeoclimatic conditions during deposition of the Lourinhã formation are broadly similar to those inferred for the Morrison Formation, except somewhat wetter. Seasonal variation in moisture availability does not seem to have negatively impacted the ability of these environments to support rich and relatively abundant faunas. The similar climate between these two Late Jurassic terrestrial ecosystems is probably one of the factors which explains the similarity of their vertebrate faunas.  相似文献   

18.
Lower Messinian stromatolites of the Calcare di Base Formation at Sutera in Sicily record periods of low sea‐level, strong evaporation and elevated salinity, thought to be associated with the onset of the Messinian Salinity Crisis. Overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions, occasionally influenced by an influx of meteoric water. Evidence of bacterial involvement in carbonate formation is recorded in three dolomite‐rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre‐scale lamination and small fenestrae. The dominant microfabrics are: (i) peloidal and clotted dolomicrite with calcite‐filled fenestrae; (ii) dolomicrite with bacterium‐like filaments and pores partially filled by calcite or black amorphous matter; and (iii) micrite in which fenestrae alternate with dark thin wispy micrite. The filaments resemble Beggiatoa‐like sulphur bacteria. Under scanning electron microscopy, the filaments consist of spherical aggregates of dolomite, interpreted to result from calcification of bacterial microcolonies. The dolomite crystals are commonly arranged as rounded grains that appear to be incorporated or absorbed into developing crystal faces. Biofilm‐like remains occur in voids between the filaments. The dolomite consistently shows negative δ13C values (down to ?11·3‰) and very positive δ18O (mean value 7·9‰) that suggest formation as primary precipitate with a substantial contribution of organic CO2. Very negative δ13C values (down to ?31·6‰) of early diagenetic calcite associated with the dolomite suggest contribution of CO2 originating by anaerobic methane oxidation. The shale‐normalized rare earth element patterns of Sutera stromatolites show features similar to those in present‐day microbial mats with enrichment in light rare earth elements, and M‐type tetrad effects (enrichment around Pr coupled to a decline around Nd and a peak around Sm and Eu). Taken together, the petrography and geochemistry of the Sutera stromatolites provide diverse and compelling evidence for microbial influence on carbonate precipitation.  相似文献   

19.
In topographic flat areas, sedimentary settings may vary from one outcrop to another. In these settings, calcite precipitates may yield macroscopically similar columnar features, although they are products of different sedimentary or diagenetic processes. Three columnar calcite crystal fabrics, i.e. rosettes, palisade crusts and macro-columnar crystal fans, have been differentiated near and at the contact between Upper Tournaisian dolomites and limestones along the southern margin of the Brabant-Wales Palaeohigh. Their petrographic characteristics, and geochemical and fluid inclusion data provide information on the (dia)genetic processes involved. Rosettes composed of non-luminescent columnar calcite crystal fans (1–5 cm in diameter) developed on top of one another, forming discrete horizons in repetitive sedimentary cycles. The cycles consist of three horizons: (I) a basal horizon with fragments from the underlying horizon, (II) a micrite/microspar horizon with incipient glaebules, (III) an upper horizon consisting of calcite rosettes, with desiccation features. The petrographical features and δ18O signatures of −10·0 to −5·5‰ and δ13C values of −5·5 to −3·2‰ support either evaporative growth, an evaporative pedogenic origin, or overprinting of marine precipitates. Palisade crusts, composed of a few to 10 mm long non-luminescent calcite crystals, coat palaeokarst cavities. Successive palisade growth-stages occur which are separated by thin laminae of micrite or detrital quartz, displaying a geopetal arrangement. Palisade crusts are interpreted as intra-Mississippian speleothems. This interpretation is supported by their petrographic characteristics and isotopic signature (δ18O = −8·7 to −6·5‰ and δ13C = −4·8 to −2·5‰). Macro-columnar crystals, 1–50 cm long, developed mainly perpendicular to cavity walls and dolomite clasts. Crystal growth stages in the macro-columnar crystals are missing. δ18O values vary between −16·4 and −6·8‰ and δ13C values between −5·2 and −0·9‰. These features possibly support a late diagenetic high temperature precipitation in relation to hydrothermal karstification.  相似文献   

20.
Cryoturbated facies are found at the boundary between soil horizons and Cretaceous chalk. Several types of secondary calcite appear in soil horizons: orange coloured and rounded (partially dissolved) nodules, deeply coloured angular aggregates, transparent isolated rhombs and polycrystalline nodules, needles. The carbon and oxygen isotope compositions of these calcites are correlated: δ13C = 4.9δ18OPDB + 15.9End members of this correlation are the orange rounded nodules (δ13C ? + 8%., δ18O ? ?1.5‰) and the transparent angular polycrystalline nodules (δ13C ? ?13; δ18O ? ?6).Partially dissolved nodules have formed under periglacial climatic conditions. Crystallisation would have occurred under the following (equilibrium) environmental conditions: δ18OSMOW (soil solution) ?7, δ13C (gaseousCO2) ? ?5.2, t ? ?2°C. Soil solution was enriched in 18O by evaporation and atmospheric CO2 was enriched in 13C as compared to present day. Transparent polycrystalline nodules are compatible with present day environmental conditions: δ18O (soil solution) ranging from ?9 to ?4 and δ13C (soil CO2) ranging from ?24.5 to ?23. These nodules crystallize between May and October at soil temperatures ranging from 10 to 25°C, from evaporated soil solutions. Angular coloured aggregates may form under present day winter conditions for temperatures between 0 and 10°C. However they may also result from present accretion of fragments of periglacial nodules.All recent secondary calcite results from CO2 degassing and evaporation of soil solutions. Degassing is controlled by the gradient of CO2 partial pressure within the soil profile. During winter this gradient is low and the resulting calcite precipitation is not significant. During summer a large difference in pCO2 appears between the root zone and deep soil horizons. The degassing accounts for an increase of about 2‰ in δ13C of the total dissolved inorganic carbon and of the related solid carbonate. Evaporation is the main driving force for secondary calcite precipitation.  相似文献   

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