Genesis of the Bayan Obo Fe–REE–Nb deposit: Evidences from Pb–Pb age and microanalysis of the H8 Formation in Inner Mongolia, North China Craton
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Dolostone
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Excellent dolomite exposures are observed in the eastern Salt Range (Pakistan), where the Cambrian Jutana Formation consists of two distinct units (i.e. oolitic – pisolitic unit and massive dolomite unit). Field observations revealed that the lower, oolitic-pisolitic unit mostly comprises medium to thick bedded, interlayered brown yellowish dolostone containing ooids/pisoids and faunal assemblages, and grey whitish sandstone with distinct depositional sedimentary features (i.e. trough-, herringbone- and hhummocky crossbedding). The upper massive dolostone unit consists of thick bedded to massive dolostone. These two units are separated by shale. Petrographic studies identified three dolomite types, which include: fine crystalline dolomite (Dol. I), medium-coarse crystalline dolomite (Dol. II) and fracture associated, coarse crystalline dolomite (Dol. III). Stable isotope studies indicate less depleted δ 18 O values for Dol. I (-6.44 to -3.76‰V-PDB), slightly depleted δ18O values for Dol. II (-7.73 to -5.24‰V-PDB) and more depleted δ 18 O values for Dol. III (-7.29 to -7.20‰V-PDB). The δ 13 C values of the three dolomite phases are well within the range of Cambrian sea-water signatures. Furthermore, δ 26 Mg-δ 25 Mg signatures (Dol. I; δ 26 Mg=-1.19 to -1.67, δ 25 Mg=-0.61 to -0.86 and Dol. II; δ 26 Mg=-1.34 to -1.59, δ 25 Mg=-0.70 to -0.83) indicate three phases of dolomitization in different diagenetic settings. First, an initial stage of dolomitization during the early Cambrian resulted from altered marine, Mg-rich fluids associated with the mixing zone mechanism. Second, a late stage of dolomitization was associated with burial during late Permian. A third dolomitization phase was related to post-Eocene times.
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Based on petrology and mineralogy, the geochemical characteristics of dolomite in the Cambrian Longwangmiao Formation in the eastern Sichuan Basin, were carried out. Results showed that dolomites mainly occurred in the middle and upper parts of Longwangmiao Formation, with most commonly as a crystalline dolomite and less commonly as a gain dolomite; the percentage content of dolostone in dolomite ranged from 48.6% to 75%, ratios of Mg/Ca were generally small, and positive and negative relationships between content of MgO and content of CaO were identified, respectively; the dolostones were generally characterized by high contents of Fe and Na while low contents of Sr and Mn, and only a part of dolostones had high content of Mn; compared with composition of contemporaneous seawater, most of dolostones were characterized by positive δ13C values and negative δ18O values. Based on the analyses of regional geological setting and geochemical characteristics, the dolomite in Longwangmiao Formation was mainly from the seepage-refluxion dolomitization, while evaporative-concentration dolomitization and burial dolomitization were not developed. In general, the dolomitization of Longwangmiao Formation was not complete. Furthermore, through the dolomitization model of Longwangmiao Formation, the distribution of sedimentary facies as well as the controlling effect of dolomitization on reservoir, the favorable reservoir was predicted, and was mainly developed around Qiyueshan fault zone.
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This study investigates the characteristics of the heterogeneous carbonate reservoir of Majiagou Formation in Jingxi area, Ordos Basin, China, to explain the diagenetic control and distribution of high‐quality reservoir intervals. The carbonates of the study area mainly consist of silt‐sized crystalline dolomite, microcrystalline dolomite, and microcrystalline limey dolomite. Both dolomite content and dolomite size gradually decrease both from top to bottom and from slope to basin. Intercrystalline pores exist in the silt‐sized crystalline dolomite, and vuggy pores and fractures mainly developed in microcrystalline (limey) dolomite. Furthermore, typical geopetal infills and three stages of calcite cements could be differentiated. Based on petrographical and geochemical analysis, the reservoir is dominantly controlled by seepage reflux dolomitization and telogenetic karstification. Four reservoir evolutions could be differentiated. The reservoir in the first model is dominated by intercrystal porosity occurring in silt‐sized crystalline dolostone. It experienced strong dolomitization, telogenetic karstification, and shallow as well as deep burial cementation. The reservoir in the second model consists of only a small amount of high‐angle dissolution fissures and vertical dissolution cavities in microcrystalline (limey) dolostone. It experienced relatively weak dolomitization, strong telogenetic karstification, and two important calcite cementation events. The reservoir in the third model is marked by many vuggy pores and dissolution fractures, occurring in microcrystalline (limey) dolostone. The strata experienced relatively weak dolomitization, strong telogenetic karstification, and two weak calcite cementation episodes. The reservoir in the fourth model is dominated by fractures and their related dissolution pores, which developed in microcrystalline (limey) dolostone. These dolostones experienced relatively weak dolomitization, telogenetic karstification, and calcite cementation. From a reservoir point of view, the reservoirs represented by the first and third models, which respectively are composed of silt‐sized crystalline dolomite occurring in the top of the whole dolostone succession and the dolomite affected by telogenetic karstification, are the most vital exploration targets.
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Lower Ordovician dolomites in the central Tarim basin are of various types and extensively distributed.Seven dolostone types could be documented based on fabrics and structures,which are crystalline dolomite,residual-grain dolomite,residual calcite dolomite,algal dolomite,cloudy centers and clear rims dolomite,clitellum dolomite and gypes dolomite.It's characterised by middle-high δ18O negative value(-3.2%~-8.7‰),low-middle δ13C negative value(-3%~-0.77‰),and high Z value of 118~123.Most diagenetic temperature calculated by empirical formulas using δ18O values are between 118 to 123℃,which indicates that the temperature degree of dolomitization is low,and so the kind of dolomite belongs to low temperature dolostone.These dolomites have also characteristics of low Mg/Ca and variable Fe/Mn ratio values.According to mineral composition analyzed by electron microprobe,dolomite monomineral can be divided into high Fe content and low Fe content species.87Sr/86Sr ratios of most samples are low(among 0.708 8~0.709 7),which are similar to seawater strontium isotope composition,and some are of high ratios between 0.710 1 and 0.710 9.These characteristics reveal that the origin of dolomite in the study area can be classified into penecontemporaneous dolomitization and burial dolomitization.According to Ordovican palaeogeographical evolution,the penecontemporaneous dolomitization arised in the seafloor environment,mainly in tidal flats or beach environment.And this kind of dolomite characterised by low diagenetic temperature,low 87Sr/86Sr ratios,and low Fe content dolomite minerals.The burial dolomitization can be interpreted by two diagentic models.One is seepage-reflux dolomitization during early shallow burial stage and characterised by low diagenetic temperature but high 87Sr/86Sr ratios,and high Fe content dolomite mineral.Another is deep burial dolomitization which occured in more strong reducible diagenetic enviornment during late deep burial stage.In general,the degree of dolomitization in burial diagentic enviorment is further enhanced and adjusted.The deep burial dolomite is characterised by high diagenetic temperature and high homogenization temperature of fluid inclusions,but low salinity.
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Several types of dolomitization are observed in the Lower Cretaceous carbonate sequence of the Tappehsorkh deposit, Irankuh Mining District, Iran: 1) regional and extensive early diagenetic dolomitization (D1), and 2) localized hydrothermal dolomitization (D2 and D3). D1 regional dolomite, which partly to completely replace limestone, is characterized by fine‐grained euhedral to subhedral dolomite rhombs. There is no evidence for Zn‐Pb sulphide mineralization associated with this type of dolomite. Medium to coarse‐grained D2 and D3 hydrothermal dolomites occur along the syn‐sedimentary Gushfil‐Baghabrisham normal fault, about 40 m above the Lower Cretaceous sequence within black siltstone, dolostone, and crystal lithic tuff and lava rocks. Hydrothermal dolomite cross‐cuts and brecciates the host rocks. This dolomite was replaced by quartz and sulphide minerals of the main ore stage. D1 regional dolomite has average values of ‐7.37‰ and 2.20‰ for δ 18 O and δ 13 C, respectively. The δ 13 C values of this dolomite fall well within the range of the Lower Cretaceous carbonates. D2 hydrothermal dolomite has average δ 18 O and δ 13 C values of ‐7.92‰ and 2.93‰, respectively. The respective δ 18 O and δ 13 C values of D3 hydrothermal dolomite are ‐12.27‰ and 2.39‰. The δ 18 O values of D2 and D3 dolomites are more negative than those of D1 regional dolomite; this can be due to their hydrothermal origin. Fluid inclusion studies on D3 dolomite and quartz show temperatures of 170‐260 °C. The concentrations of Fe and Mn in D1 regional dolomite are very similar to those of limestone, suggesting that they likely precipitated from the same fluid (seawater), whereas those of D2 and D3 dolomites are relatively high, in agreement with the hydrothermal origin for these dolomites. Precipitation of gypsum, which is ubiquitous in the study area, could have lowered the Ca/Mg ratio of seawater. While the extensive regional dolomitization formed from low‐temperature (evolved) seawater during the early diagenesis, D2 and D3 dolomites associated with the main stage of sulphide mineralization were formed by high‐temperature hydrothermal fluids moving along the Gushfil‐Baghabrisham Fault. Copyright © 2016 John Wiley & Sons, Ltd.
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