Carbon and oxygen isotope compositions of the carbonate facies in the Vindhyan Supergroup, central India

The Vindhyan sedimentary succession in central India spans a wide time bracket from the Paleoproterozoic to the Neoproterozoic period. Chronostratigraphic significance of stable carbon and oxygen isotope ratios of the carbonate phase in Vindhyan sediments has been discussed in some recent studies. However, the subtle controls of facies variation, depositional setting and post-depositional diagenesis on stable isotope compositions are not yet clearly understood. The Vindhyan Supergroup hosts four carbonate units, exhibiting a wide variability in depositional processes and paleogeography. A detailed facies-specific carbon and oxygen isotope study of the carbonate units was undertaken by us to investigate the effect of these processes and to identify the least altered isotope values. It is seen that both carbon and oxygen isotope compositions have been affected by early meteoric water diagenesis. The effect of diagenetic alteration is, however, more pronounced in case of oxygen isotopes than carbon isotopes. Stable isotope compositions remained insensitive to facies only when sediments accumulated in a shallow shelf setting without being exposed. Major alteration of original isotope ratios was observed in case of shallow marine carbonates, which became exposed to meteoric fluids during early diagenetic stage. Duration of exposure possibly determined the magnitude of alteration and shift from the original values. Moreover, dolomitization is found to be accompanied by appreciable alteration of isotope compositions in some of the carbonates. The present study suggests that variations in sediment depositional settings, in particular the possibility of subaerial exposure, need to be considered while extracting chronostratigraphic significance from δ¹³C data.     

Geochemistry of Mesoproterozoic Deonar Pyroclastics from Vindhyan Supergroup of Central India: Evidences of felsic magmatism in the Son valley

Deonar Pyroclastics of Semri Group in the Vindhyan Supergroup originated as a result of violent and explosive intrabasinal submarine volcanism during the Mesoproterozoic period. These pyroclastics are rhyolitic to rhyodacitic in composition, comprised of banded, massive, pumiceous flow, breccia, vitric tuff, lapilli and volcanic bomb. The pyroclastic deposits represent welded and non-welded ignimbrites, exhibit typical eutaxitic texture. Mantle normalized multi-element patterns show enrichment in LILs and depletion in HFSFs. Ti, Nb and REE contents show close correlation with Zr, indicating their immobile character. HFSEs and Th/Nb, La/Nb and Zr/Nb values indicate contamination and these signatures represent mixing between mantle-derived rocks and the average continental crust. Deonar Pyroclastics reflect continental rift environment. Felsic magma plausibly generated by underplating of the mature Proterozoic crust of the Indian craton (which acted like a ‘heating lens’) resulted in extensive melting of metabasalt in the lower crustal levels. The high heat flow beneath the Indian shield accentuated heat generation which led to extensive partial melting of metabasalts. Thus, generation of rhyolitic magma occurred along the reactivated deep seated fractures and rifting of the craton, resulting in the explosive intra-basinal felsic vulcanicity in the Vindhyan basin.     

Taxonomy and affinity of Early Mesoproterozoic megascopic helically coiled and related fossils from the Rohtas Formation, the Vindhyan Supergroup, India

A well preserved assemblage of compressed, straight, circular to sinuously coiled megascopic and helical carbonaceous fossils and other varied megascopic morphoforms are known from the Early Mesoproterozoic Rohtas Formation, Semri Group within Vindhyan Supergroup exposed in Katni district of central India. These megascopic remains are preserved as impressions, compressions, partially mineralized remains, and/or epi-relief. Some of the forms are typical filamentous empty sheaths and others are trichomes, with cell like entities under various stages of degradation. This study, based on fresh collections and also of the topotype material of the helically coiled megascopic fossils, straight forms and related fossilized remains occurring as epi-relief from Katni indicate that the two morphotaxa are distinct entities and possibly appear to be prokaryotes. Grypania spiralis and Katnia singhii are most likely of cyanobacterial origin. Spirally coiled and circular fossils, with epi-relief, and which probably represents a tissue grade organism, are considered as Spiroichnus beerii Mathur, 1983. Linear sheet-like carbonaceous solitary form has been placed in the morphotaxon Proterotainia and described as P. katniensis n. sp. Certain rare circular, carbonaceous forms are considered as Chuaria sp. A few circular disc-like forms found in the assemblage are treated as dubiofossils.     

Carbon and oxygen isotopic variations in peritidal stromatolite cycles, Paleoproterozoic Kajrahat Limestone, Vindhyan basin of central India

Variations of carbon and oxygen isotopic ratios in response to cyclical sea level fluctuations have been documented from a Paleoproterozoic peritidal stromatolite succession. The upper division of the Kajrahat Limestone, Vindhyan Supergroup of central India consists of several shallowing upward stromatolite cycles identified by regular and systematic changes in stromatolite size. Normally, larger stromatolites are followed upward in the succession by smaller stromatolites and microbial laminites that occupy the top of the cycle. Desiccation cracks are found in all the facies indicating subaerial exposure. We investigated the stable isotope compositional variations across nine complete stromatolite cycles showing frequent subaerial emergence. Carbon and oxygen isotopic values of the limestones, in general, are comparable to contemporary marine values available from earlier studies but show regular depletion in response to shallowing of the water level. The δ¹³C and δ¹⁸O values of the limestones vary within an individual stromatolite cycle; depleted values characterize the topmost part of the cycles. The isotope pattern is explained by micritic carbonate deposition in different sub environments of the shallow marine domain having different salinity and variable duration of exposure. These variations also probably caused the observed scatter in δ¹³C and δ¹⁸O values of supratidal microbial laminites.     

Geochemical,and stable and radiogenic isotope records in Devonian and Early Carboniferous carbonates from Valle de Tena,central Pyrenees (Spain): evidence for their diagenetic environments

Mineralogical, textural and geochemical investigations were made to determine the post-depositional evolution of Devonian and Early Carboniferous carbonates from Valle de Tena. The carbonate association is made up of low-Mg calcite, which occurs as micrite, spar cements, neomorphic patches and spar filling veinlets. Non-stoichiometric dolomite and ankerite occur as cements (dolomite also as replacements) in the Middle Devonian, post-dating calcite types. All these phases pre-date tectonic stylolites, indicating compaction after stabilization of the carbonate minerals. Strontium concentrations indicate that Early Devonian and Early Carboniferous micrites initially precipitated as aragonite; Middle and Late Devonian micrites precipitated as high-Mg calcites. Both precursors were diagenetically stabilized to low-Mg calcites through interaction with meteoric waters in phreatic environments. Trace elements in dolomite and ankerite indicate precipitation from Sr-enriched meteoric water. All studied carbonates, except Middle Devonian limestones, precipitated in reducing environments, which favoured incorporation of Fe and Mn. Late calcite generations precipitated from more saline waters than micrites. Light ¹⁸O values in micrites suggest alteration mainly in meteoric-phreatic environments. The dolomites and ankerites precipitated from more ¹⁸O-depleted fluids than the calcites, suggesting a greater contribution from meteoric waters. Variations in ¹³C of micrites represent primary secular trends, according to published ¹³C variations. The ¹³C oscillations within each succession probably relate to sea-level oscillations. Strontium isotopes also point to a meteoric origin of diagenetic fluids. Model calculations suggest that O and Sr isotopes equilibrated between calcites and fluid at relatively low water/rock ratios, whereas C isotopic signatures are inherited from limestones.     

Stratigraphy, sedimentology and bulk organic geochemistry of black shales from the Proterozoic Vindhyan Supergroup (central India)

Four organic-rich shale units of the Proterozoic Vindhyan sedimentary succession have been scanned to reveal their origin and hydrocarbon potential. The wavy-crinkly nature of the carbonaceous laminae is suggestive of a microbial mat origin of the shales. These shales are thus different from Phanerozoic black shales which typically exhibit planar laminae. The hydrocarbon potential of the black shale units has been evaluated by Rock-Eval pyrolysis. Total organic carbon content of many of the shales exceeds 1%. The meanT _max for the black shales translate to a vitrinite reflectance range of 2.05-2.40% Rm based on standard conversion techniques. These shales have reached the catagenetic stage near the beginning of anthracite formation.     

C and O stable isotope variability in recent freshwater carbonates (River Krka, Croatia)

Three types of recent carbonate precipitates from the River Krka, Croatia, were analysed: (1) bulk tufa from four main cascades in a 34 km long section of the river flow through the Krka National Park; (2) a laminar stromatolite‐like incrustation formed in the tunnel of a hydroelectric power plant close to the lowest cascade; and (3) recent precipitates collected on artificial substrates during winter, spring and summer periods. Stable isotope compositions of carbon (δ¹³C) and oxygen (δ¹⁸O) in the carbonate and organic carbon (δ¹³C_org) were determined and compared with δ¹⁸O of water and δ¹³C of dissolved inorganic carbon (DIC). The source of DIC, which provides C for tufa precipitation, was determined from the slope of the line ([DIC]/[DIC₀]?1) vs. (δ¹³C‐DIC × ([DIC]/[DIC₀])) ( Sayles & Curry, 1988 ). The δ¹³C value of added DIC was ?13·6‰, corresponding to the dissolution of CO₂ with δ¹³C between ?19·5 and ?23·0‰ Vienna Pee Dee Belemnite (VPDB). The observed difference between the measured and calculated equilibrium temperature of precipitation of bulk tufa barriers indicates that the higher the water temperature, the larger the error in the estimated temperature of precipitation. This implies that the climatic signals may be valid only in tufas precipitated at lower and relatively stable temperatures. The laminar crust comprising a continuous record of the last 40 years of precipitation shows a consistent trend of increasing δ¹³C and decreasing δ¹⁸O. The lack of covariation between δ¹³C and δ¹⁸O indicates that precipitation of calcite was not kinetically controlled for either of the elements. δ¹³C and δ¹⁸O of precipitates collected on different artificial substrates show that surface characteristics both of substrates and colonizing biota play an important role in C and O isotope fractionation during carbonate precipitation.     

Carbon, oxygen, and strontium isotope geochemistry of carbonate rocks of the upper Miocene Kudankulam Formation, southern India: Implications for paleoenvironment and diagenesis

The carbon, oxygen, and strontium isotope compositions of carbonate rocks from the upper Miocene Kudankulam Formation, southern India, were measured to understand palaeoenvironment and carbonate diagenesis of this formation. Both carbon and oxygen isotope ratios of various carbonate phases including whole rocks, ooids, molluscan mold-fill and sparry pore-fill calcite cements are depleted in ¹⁸O and ¹³C compared to those of contemporaneous seawater, indicating that the Kudankulam carbonates underwent extensive meteoric diagenesis. Based on δ¹³C and δ¹⁸O values for sparry calcite cements (pore-fill and molluscan mold-fill) formed in the meteoric diagenetic realm (δ¹³C from −7.8‰ to −6.0‰ and −9.0‰ to −7.0‰; δ¹⁸O from −9.2‰ to −6.5‰ and −9.4‰ to −2.6‰, respectively), it is interpreted that the diagenetic system was open and was proximal to the vadose water recharge zone. The negative δ¹⁸O values of various carbonate components (about −9.4‰ to −4.1‰ for whole rocks; about −8.4‰ to −2.6‰ for ooids) suggest that during the late Miocene the paleoclimate of the study area was humid, unlike today, probably due to the intense Indian monsoon system. The carbon isotope compositions (−7.9‰ to −3.6‰ for whole rocks; −4.9‰ to −1.5‰ for ooids) are consistent with the interpretation that the paleo-ecosystem comprised a significant proportion of C₄ type plants, supporting a scenario of expansion of C₄ plants during the late Miocene in the Indian subcontinent as far south as the southern tip of India. The ⁸⁷Sr/⁸⁶Sr ratios of the Kudankulam carbonates (0.70920 to 0.72130) are much greater than those of the contemporaneous or modern seawater (between 0.7089 and 0.7091) and show a general decrease up-sequence. Such high Sr isotope ratios indicate significant radiogenic ⁸⁷Sr influx to the system from the Archean rocks exposed in the drainage area, implying that the deep-seated Archean rocks were already exposed in southern India by the late Miocene.     

Geothermal investigations in the Upper Vindhyan sedimentary rocks of Shivpuri area,central India

Heat flow has been determined by combining temperature measurements in 7 boreholes with thermal conductivity measurements in the Upper Vindhyan sedimentary rocks of Shivpuri area, central India. The boreholes are distributed at 5 sites within an area of 15 × 10 km²; their depths range from 174 to 268 m. Geothermal gradients estimated from borehole temperature profiles vary from 8.0–12.7 mK m⁻¹ in the sandstone-rich formations to 25.5–27.5 mK m⁻¹ in the shale-rich formations, consistent with the contrast in thermal conductivities of the two rock types. Heat flow in the area ranges between 45 and 61 mW m⁻², with a mean of 52±6 mW m⁻². The heat flow values are similar to the >50 mW m⁻² heat flow observed in other parts of the northern Indian shield. The heat flow determinations represent the steady-state heat flow because, the thermal transients associated with the initial rifting, convergence and sedimentation in the basin as well as the more recent Deccan volcanism that affected the region to the south of the basin would have decayed, and therefore, the heat flow is in equilibrium with the radiogenic heat production of the crust and the heat flow from the mantle. The present study reports the heat flow measurements from the western part of the Vindhyan basin and provides heat flow information for the Bundhelkhand craton for the first time. Radioelement (Th, U and K) abundances have been measured both in the sedimentary rocks exposed in the area as well as in the underlying basement granite-gneiss of Bundelkhand massif exposed in the adjacent area. Radioactive heat production, estimated from those abundances, indicate mean values of 0.3 μW m⁻³ for sandstone with inter-bands of shale and siltstone, 0.25 μW m⁻³ for sandstone with inter-bands siltstone, 0.6 μW m⁻³ for quartzose sandstone, and 2.7 μW m⁻³ for the basement granitoids. With a total sedimentary thickness not exceeding a few hundred metres in the area, the heat production of the sedimentary cover would be insignificant. The radioactive heat contribution from the basement granitoids in the upper crust is expected to be large, and together with the heat flow component from the mantle, would control the crustal thermal structure in the region.     

Marine conditions in central Sweden during the early Preboreal as inferred from a stable oxygen isotope gradient

The marine benthic fauna and the δ¹⁸O_c of foraminifers and ostracods from six sites situated on a west–east transect through central Sweden have been analysed in order to estimate the palaeosalinity and palaeocirculation in this shallow‐marine environment. The measurements have been undertaken on material from the early Preboreal, when the Baltic Basin was in contact with the North Sea through straits in central Sweden. The δ¹⁸O_c values have a more negative value towards the east, indicating decreasing salinity. This was the result of limited possibilities for marine water to penetrate into the Baltic Basin and the mixing with freshwater from the melting Fennoscandian ice‐sheet. Four water masses existed in the area: a surface layer of freshwater, marine water from the North Sea, brackish–marine intermediate water on the Swedish west coast and brackish Yoldia Sea water in the Baltic Basin. The chronology is based on radiocarbon dates of marine fossils and, at one site, on the occurrence of the Vedde Ash (10 400–10 300 ¹⁴C yr BP). This is the first record from marine settings in Sweden. Copyright © 2001 John Wiley & Sons, Ltd.     

Petrography and geochemistry of septarian carbonate concretions from the Boom Clay Formation (Oligocene,Belgium)

The septarian carbonate concretions from the Boom Clay (Belgium) consist mainly of authigenic minerals such as micrite ( 70% bulk volume) and pyrite framboids ( 3%). These mineral phases occur between detrital grains and fossils. The septarian cracks are lined with calcite, which is sometimes covered with pyrite. The preservation of delicate sedimentological features in the concretion matrix (hardly compacted faecal pellets, burrows and uncrushed shells) points to an early origin of the concretions. Systematic geochemical variations from concretion centre to edge suggest that growth continued during shallow burial. The13C values (–17.5 to –20.5) of the concretionary carbonate show that bacterial sulphate-reduction processes were dominant. Sulphate-reduction-derived HCO3- was diluted by marine-related HCO3-, derived from dissolved bioclasts. A slight enrichment in 13C during growth is caused by the decreasing influence of sulphate reduction because of the progressive closure of the diagenetic system due to shallow-burial compaction. The 18O values (–0.5 to +1.0) of the concretionary carbonate point to a marine origin. The slightly 18O-depleted signature with respect to time-equivalent marine-derived carbonate relates to the incorporation of an 18O-depleted component, originating from sulphate and organic matter. The slight decrease in 18O during growth relates to an increasing influence of this component and to a decreasing influence of seawater-derived oxygen during early diagenesis.     

新疆东天山玉海铜(钼)矿床流体包裹体和稳定同位素研究

玉海铜(钼)矿床成矿岩体为石英闪长(玢)岩,矿化呈细脉状、细脉-浸染状和稀疏浸染状。围岩蚀变主要为钾硅酸盐化、石英-绢云母化、青磐岩化和黏土化蚀变。矿床类型为斑岩型。铜(钼)矿化主要发育于钾硅酸盐化阶段、石英-绢云母化阶段和青磐岩化阶段。流体包裹体可划分为气液两相包裹体、含子晶三相包裹体和CO_2包裹体3种类型。钾硅酸盐化阶段的均一温度为307~423℃,盐度w(NaCleq)为4.18%~10.11%,密度0.62~0.77g/cm~3,属于高温、中-低盐度流体;石英-绢云母化阶段均一温度为172~336℃,盐度为w(NaCleq)为3.23%~8.55%,密度0.70~0.93 g/cm~3,属于中温、低盐度流体;晚期青磐岩化阶段均一温度155~296℃,盐度w(NaCleq)为3.71%~9.08%,密度0.80~0.96 g/cm~3,属于中低温、低盐度流体。从早阶段到晚阶段,成矿流体温度逐渐下降,各成矿阶段成矿流体盐度均小于11%,但钾硅酸盐化阶段成矿流体盐度稍高。石英-绢云母化阶段成矿流体δD=-91.6‰~-72.1‰,δ~(18)OH_2O=-1.8‰~6.3‰;青磐岩化阶段成矿流体δD=-97.1‰~-68.3‰,δ~(18)OH_2O=-6.3‰~2.2‰;成矿流体具有岩浆水和大气降水混合特征,但青磐岩化阶段大气降水含量更高。硫化物的δ~(34)S值为-3.5‰~2.8‰,硫来自石英闪长(玢)岩。     

Stable isotope systematics of surface water bodies in the Himalayan and Trans-Himalayan (Kashmir) region

Stable hydrogen (δD) and oxygen (δ¹⁸O) isotope ratios of the headwaters of the Indus and its tributaries, surface ice in glaciers, saline and fresh water lakes and thermal springs in the Himalayan and Trans-Himalayan (Kashmir) region are reported. The δ5D-δ¹⁸ relationship for the river samples shows a slope of 9.12 +-0.29 which agrees well with the estimate of 8.99 ±0.33 based on a simple Rayleigh fractionation model. The unique signature of a higher deuterium excess (d) of the ‘Western Disturbance’ is preserved in these samples. An altitude effect of -0.9 per mil/km is observed in the δ¹⁸O of Indus waters. At a lower altitude (Beas) the altitude effect is almost double, indicating that the altitude effect decreases with elevation in this region.     

Cretaceous climate oscillations in the southern palaeolatitudes: New stable isotope evidence from India and Madagascar

Palaeotemperatures for the Cretaceous of India and Madagascar have been determined on the basis of oxygen isotopic analysis of well-preserved Albian belemnite rostra and Maastrichtian bivalve shells of from the Trichinopoly district, southern India, and Albian nautiloid and ammonoid cephalopods from the Mahajang Province, Madagascar. The Albian (possibly late Albian) palaeotemperatures for Trichinopoly district are inferred to range from 14.9 °C to 18.5 °C for the epipelagic zone, and from 14.3 °C to 15.9 °C for the mesopelagic zone, based on analyses of 65 samples; isotopic palaeotemperatures interpreted as summer and winter values for near-bottom shelf waters in this area fluctuate from 16.3 to 18.5 °C and from 14.9 to 16.1 °C, respectively. The mentioned palaeotemperatures are very similar to those calculated from isotopic composition of middle Albian belemnites of the middle latitude area of Pas-de-Calais in Northern hemisphere but significantly higher than those calculated from isotopic composition of Albian belemnites from southern Argentina and the Antarctic and middle Albian belemnites of Australia located within the warm-temperate climatic zone. Isotopic analysis of early Albian cephalopods from Madagascar shows somewhat higher palaeotemperatures for summer near-bottom shelf waters in this area (20.2-21.6 °C) in comparison with late Albian palaeotemperatures calculated from southern India fossils, but similar winter values (13.3-16.4 °C); however, the latter values are somewhat higher than those calculated from early Albian ammonoids of the tropical-subtropical climatic zone of the high latitude area of southern Alaska and the Koryak Upland. The new isotopic palaeotemperature data suggest that southern India and Madagascar were located apparently in middle latitudes (within the tropical-subtropical climatic zone) during Albian time. In contrast to the Albian fossils, isotope results of well-preserved early Maastrichtian bivalve shells from the Ariyalur Group, Trichinopoly district, are characterised by lower δ¹⁸O values (up to −5.8‰) but normal δ¹³C values, which might be a result local freshwater input into the marine environment. Our data suggest that the early Maastrichtian palaeotemperature of the southern Indian near-bottom shelf waters was probably about 21.2 °C, and that this middle latitude region continued to be a part of tropical-subtropical climatic zone, but with tendency of increasing of humidity at the end of Cretaceous time.     

Applications of Light Stable Isotopes(C,O, H) in the Study of Sandstone Diagenesis: A Review

This article reviews the applications of light stable isotope, including carbon, oxygen and hydrogen, in thestudies on origin and formation temperature of authigenic carbonate, quartz and clay minerals. Theoretical knowledge andanalytical methods for major light stable isotopes are introduced in detail. Negative and positive δ¹³C values indicatesignificant differences on the origin of carbonate cements. The δ¹⁸O value is an effective palaeotemperature scale forauthigenic minerals formation. Various fractionation equations between δ¹⁸O and temperature are proposed for carbonatecements, quartz cements and clay minerals, whose merit and demerit, applicable conditions are clarified clearly. Clumpedisotope analysis can reconstruct the temperature of carbonate precipitation with no requirement on the δ¹⁸O of initial waters,which makes temperature calculation of carbonate cements formation more convenient and accurate. Hydrogen and oxygenisotopes mainly reflect the origin of diagenetic fluid for clay mineral formation, providing reliable evidence for diageneticenvironment analysis. This work aims at helping researchers for better understanding the applications of light stable isotopein sandstone diagenesis.     

Feldspar Alteration and Diagenetic Characteristics of the Parsora Sandstones, Son Basin, India

Moderate to poorly sorted immature Parsora sandstones rich in K-feldspar show much of the feldspar during early diagenesis transformed to kaolinite after prolonged interaction with acidic pore solutions. The kaolinitic epimatrix formed and was later partially or wholly converted as an orthomatrix producing chert-phyllosilicate assemblage. Ferric oxide, bleached biotite, kaolinite and quartz cement denote an oxy-acidic early diagenetic environment. Late diagenesis involved neoformation of primary or secondary matrix, illitisation of montmorillonite and muscovite authigenesis. The high pressure-temperature regime required for these transitions resulted from tectonic activity during Triassic-Jurassic times. Carbonate-chlorite appeared late in the sediments denoting an alkaline-reducing condition at the late part of the diagenesis. Finally, the secondary porosity developed through carbonate dissolution was later filled up with allochemical ferric iron cement receiving ions from the percolating meteoric water.     

Multiphase partial and selective dolomitization of Carnian reef limestone (Transdanubian Range,Hungary)

Partially dolomitized carbonate successions provide a good opportunity to understand the commonly multistage process of dolomitization. Petrographic methods, fluid inclusion microthermometry and stable isotope measurements were applied to reconstruct the diagenetic evolution and dolomitization of a partially dolomitized Carnian reef limestone from the Transdanubian Range, Hungary. The diagenetic history began with reef diagenesis and formation of dolomite micro‐aggregates in microbial fabric elements; this was followed by the development of euhedral porphyrotopic dolomite crystals through overgrowths around the previously formed dolomite micro‐aggregates during the earliest burial stage. Increasing burial resulted in the extension of the dolomite patches via formation of finely crystalline replacement dolomite. From the Late Norian, when the Carnian reef carbonates reached the depth of 1·0 to 1·8 km, the diagenetic evolution continued in an intermediate to deep‐burial setting. Contemporaneously, an extensional regime was established, leading to fracturing. The progressive burial resulted in the recrystallization of the pre‐existing dolomite with increasing temperature, while saddle dolomite cement was precipitated in fractures. In connection with the Alpine Orogeny, intense denudation took place during the Late Cretaceous, accompanied by fracturing. Similar tectonically controlled denudation and fracturing occurred in several stages during the Cenozoic. As a result of these processes, the studied Carnian carbonates were raised to a near‐surface position or became subaerially exposed, leading to dedolomitization of the last dolomite phase and precipitation of calcite cement in cavities and fractures. This study revealed that by investigating partially and selectively dolomitized rock types, it is possible to document and understand those stages of the multiple dolomitization process which can barely be detected in the completely dolomitized rock bodies. Recognition of the dolomitization phases could provide the basis for the analysis of their relations with the depositional, diagenetic and tectonic processes, and stages of basin evolution.     

新疆准噶尔北缘玉勒肯哈腊苏铜(钼)矿床流体包裹体和稳定同位素研究

玉勒肯哈腊苏中型斑岩型铜(钼)矿主要赋存在闪长玢岩中,有少量矿化产在北塔山组火山岩及似斑状黑云母石英二长岩中。矿化呈细脉状、细脉-浸染状和浸染状。围岩蚀变主要为钾化、硅化、绢云母化、石膏化、磁铁矿化、绿泥石化、绿帘石化。矿床的形成经历了斑岩期、剪切变形期和表生期,铜和钼矿化主要形成于斑岩期的硫化物-钾硅酸盐阶段和辉钼矿阶段。石英和方解石中的流体包裹体可划分为H2O-NaCl型和H2O-CO2(±CH4/N2)-NaCl型。硫化物-钾硅酸盐阶段的成矿温度为141~500℃,主要集中在200~340℃;流体的w(NaCleq)为2.96%~14.97%;流体的密度为0.60~0.98 g/cm3。碳酸盐阶段的流体以中-低温度(140~320℃)和低盐度〔w(NaCleq)为2.74%~10.61%〕为特征。硫化物的δ34S值集中于-4.5‰~-0.1‰,峰值为-3.5‰,表明硫来自深源岩浆。石英和方解石的δ18OSMOW值为9.1‰~13.2‰,δ18OH2O值为2.05‰~6.28‰,δD值为-120‰~-97‰,表明主成矿阶段的成矿流体主要是岩浆水,混合有大气降水;碳酸盐阶段的流体主要为大气降水,混合有岩浆水。成矿时代为中泥盆世〔(373.9±2.2 Ma)〕,成矿作用与闪长玢岩的侵入有关。温度和压力的降低导致流体沸腾,同时,水-岩交换反应、流体成分的改变等在铜钼成矿过程中起着主导作用。     

西藏邦铺斑岩矽卡岩矿床成矿流体特征——来自流体包裹体及C_H_O同位素的制约

选取西藏冈底斯斑岩成矿带东段的邦铺矿床斑岩矿区2条勘探线上的11个钻孔,进行了详细的岩芯编录和矿物组合、脉体穿切关系研究,将该矿床内与斑岩成矿相关的脉体划分为A、B、D脉3种类型。通过对矽卡岩矿区的详细地表及平硐观察,发现了石榴子石、阳起石、绿帘石等一系列代表流体演化特征的矿物。邦铺矿床具有典型斑岩型矿床的蚀变分带特征,从中心向外依次表现为黑云母化-硅化-绢云母化-青磐岩化,泥化呈"补丁状"无规则分布在绢云母化和青磐岩化之上。矽卡岩化则以典型矽卡岩矿物的出现为特征。A脉中绝大多数包裹体均一温度为320~550℃,盐度主要集中在两个区间内,分别为17.0%~22.0%(气液两相包裹体)和30.8%~67.2%(含子晶包裹体);B脉中绝大多数包裹体均一温度为380~550℃,盐度主要集中在1.6%~10.1%、23.2%~24.5%(气液两相包裹体)和30.8%~67.2%(含子晶包裹体)3个区间内;D脉中绝大多数包裹体均一温度为213~450℃,盐度为7.3%~11.6%。流体包裹体研究表明,与斑岩成矿的相关流体具有从高温、高盐度向低温、低盐度演化的特征;形成A、B脉的流体发生了强烈的沸腾作用,由此导致的压力波动是Mo、Cu沉淀的主要原因。16件与斑岩成矿相关的石英δDV-SMOW=-107.1‰~-185.8‰,δ18OV-SMOW=9.5‰~14.5‰;15件与矽卡岩成矿相关的石榴子石、绿帘石、石英及方解石δDV-SMOW=-184.7‰~-126‰,δ18OV-SMOW=3.9‰~12.9‰;4件斑岩成矿后期的方解石δ18OV-SMOW=-1.6‰~10.4‰,δCV-PDB=-6.5‰~-3.4‰;6件与矽卡岩成矿相关的方解石δ18OV-SMOW=1.8‰~11.9‰,δCV-PDB=-5.1‰~4.6‰。C_H_O同位素分析数据表明,邦铺整个斑岩-矽卡岩成矿系统流体主要经历了岩浆脱水去气和大气降水加入这两大地质过程。