首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
The behaviour of niobium and tantalum in magmatic processes has been investigated by conducting MnNb2O6 and MnTa2O6 solubility experiments in nominally dry to water-saturated peralkaline (aluminium saturation index, A.S.I. 0.64) to peraluminous (A.S.I. 1.22) granitic melts at 800 to 1035 °C and 800 to 5000 bars. The attainment of equilibrium is demonstrated by the concurrence of the solubility products from dissolution, crystallization, Mn-doped and Nb- or Ta-doped experiments at the same pressure and temperature. The solubility products of MnNb2O6 (Ksp Nb) and MnTa2O6 (Ksp Ta) at 800 °C and 2 kbar both increase dramatically with alkali contents in water-saturated peralkaline melts. They range from 1.2 × 10−4 and 2.6 × 10−4 mol2/kg2, respectively, in subaluminous melt (A.S.I. 1.02) to 202 × 10−4 and 255 × 10−4 mol2/kg2, respectively, in peralkaline melt (A.S.I. 0.64). This increase from the subaluminous composition can be explained by five non-bridging oxygens being required for each excess atom of Nb5+ or Ta5+ that is dissolved into the melt. The Ksp Nb and Ksp Ta also increase weakly with Al content in peraluminous melts, ranging up to 1.7 × 10−4 and 4.6 × 10−4 mol2/kg2, respectively, in the A.S.I. 1.22 composition. Columbite-tantalite solubilities in subaluminous and peraluminous melts (A.S.I. 1.02 and 1.22) are strongly temperature dependent, increasing by a factor of 10 to 20 from 800 to 1035 °C. By contrast columbite-tantalite solubility in the peralkaline composition (A.S.I. 0.64) is only weakly temperature dependent, increasing by a factor of less than 3 over the same temperature range. Similarly, Ksp Nb and Ksp Ta increase by more than two orders of magnitude with the first 3 wt% H2O added to the A.S.I. 1.02 and 1.22 compositions, whereas there is no detectable change in solubility for the A.S.I. 0.64 composition over the same range of water contents. Solubilities are only slightly dependent on pressure over the range 800 to 5000 bars. The data for water-saturated sub- and peraluminous granites have been extrapolated to 600 °C, conditions at which pegmatites and highly evolved granites may crystallize. Using a melt concentration of 0.05 wt% MnO, 70 to 100 ppm Nb or 500 to 1400 ppm Ta are required for manganocolumbite and manganotantalite saturation, respectively. The solubility data are also used to model the fractionation of Nb and Ta between rutile and silicate melts. Predicted rutile/melt partition coefficients increase by about two orders of magnitude from peralkaline to peraluminous granitic compositions. It is demonstrated that the γNb2O5/γTa2O5 activity coefficient ratio in the melt phase depends on melt composition. This ratio is estimated to decrease by a factor of 4 to 5 from andesitic to peraluminous granitic melt compositions. Accordingly, all the relevant accessory phases in subaluminous to peraluminous granites are predicted to incorporate Nb preferentially over Ta. This explains the enrichment of Ta over Nb observed in highly fractionated granitic rocks, and in the continental crust in general. Received: 9 August 1996 / Accepted: 26 February 1997  相似文献   

2.
Partition coefficients (D) for Nb and Ta between rutile and haplogranite melts in the K2O-Al2O3-SiO2 system have been measured as functions of the K2O/Al2O3 ratio, the concentrations of Nb2O5 and Ta2O5, the temperature, in air and at 1 atmosphere pressure. The Ds increase in value as the K* [K2O/(K2O + Al2O3)] molar ratio continuously decreases from highly peralkaline [K* ∼ 0.9] to highly peraluminous [K* ∼ 0.35] melts. The D values increase more dramatically with a unit decrease in K* in peraluminous melts than in peralkaline melts. This compositional dependence of Ds can be explained by the high activity of NbAlO4 species in peraluminous melts and the high activity of KONb species (or low activity of NbAlO4 species) in peralkaline melts. A coupled substitution, Al+3 + Nb+5 (or Ta+5) = 2Ti+4, accounts for the Ds of Nb (Ta) being much greater in peraluminous melts than in peralkaline melts because this substitution allows Nb (Ta) to enter into the rutile structure more easily. The Ds of Ta between rutile and melt are greater than those of Nb at comparable concentrations because the molecular electronic polarizability of Ta is weaker than that of Nb. The Nb+5 with a large polarizing power forms a stronger covalent bond with oxygen than Ta+5 with a small polarizing power. The formation of the strong bond, Nb-O, distorts the rutile structure more severely than the weak bond, Ta-O; therefore, it is easier for Ta to partition into rutile than for Nb. These results imply that the utilization of the Nb/Ta ratio in liquid as a petrogenetic indicator in granitic melts must be done with caution if rutile (or other TiO2-rich phases) is a liquidus phase. The crystallization of rutile will increase the Nb/Ta ratio of the residual liquid because the Ds of Ta between rutile and melts are greater than those of Nb. Received: 28 December 1998 / Accepted 27 September 1999  相似文献   

3.
It is well established that the fractionation of Li–F granitic magmas at depth leads to the accumulation of flux elements such as F and Li, and metal cations such as Ta and Nb in residual melts. However, it remains to be determined whether magmatic fractionation is sufficient to concentrate Nb and Ta into economically significant quantities, and what role hydrothermal–metasomatic processes play in the formation of such ore deposits. In the literature, reliable data about the solubility of Ta and Nb in hydrothermal solutions is missing or incomplete. This study provides a quantitative experimental estimation of the possible contribution from hydrothermal processes in Ta enrichment in cupolas of albitized and greisenized Li–F granite. Experimental studies of Ta2O5 and columbite–tantalite (Mn,Fe)(Nb,Ta)2O6 solubility were carried out in fluoride solutions consisting of HF, NaF, KF, and LiF. At low fluoride concentrations (0.01 and 0.1 m), Ta2O5 solubility at 550°C and 100 MPa under Co–CoO oxidizing conditions is low (near 10?5–10?4 m) in all fluoride solutions (HF, NaF, KF, LiF). At high fluoride concentrations (1 and 2 m) the highest Ta2O5 concentrations (10?1 m) were detected in HF solutions. In KF, NaF, and LiF solutions, the Ta2O5 solubility is also high (10?3–10?2 m). The dependence of columbite–tantalite (Nb2O5-59 wt. %, Ta2O5-18 wt. %) solubility as a function of solution composition, T, and P has also been investigated. Tantalum and Nb concentrations have the highest values in HF solutions at reduced conditions (up to 10?3 to 10?2 m Ta in 1 m HF). In 1 m NaF solutions, the concentrations of Nb and Ta are, respectively, 2.5 and 3 orders of magnitude less than those in the 1 m HF solutions. Solubility of Ta and Nb in KF solutions has intermediate values. It is established that in NaF and KF solutions the dependence of solubility on pressure is distinctly negative. The Nb and Ta contents increase with increasing concentrations of HF and KF in solution, however, they do not change with increasing NaF concentration. In NaHCO3, Na2CO3, and HCl solutions columbite–tantalite solubility is low. Even in 1 m chloride solutions the content is within the limits of 10?5 m for Nb and 10?6 to 10?8 m for Ta. We conclude that hydrothermal transport of Ta and Nb is possible only in concentrated fluoride solutions.  相似文献   

4.

This study presents a new set of quantitative experimental data on the partitioning of Ta, Nb, Mn, and F between aqueous F-bearing fluid and water-saturated, Li- and F-rich haplogranite melts with varying alumina/alkali content at T = 650–850 °C and P = 100 MPa. The starting homogeneous glasses were preliminary obtained by melting of three gel mixtures of K2O-Na2O-Al2O3-SiO2 composition with a variable Al2O3/(Na2O+K2O) ratio, ranging from 0.64 (alkaline) and 1.1 (near-normal) to 1.7 (alumina-rich). Ta, Nb, and Mn were originally present in glass only, whereas F was load in both the glass and the solution. The solutionto-glass weight ratio was 1.5–3.0. The compositions of quenched glass were measured by an electronic microprobe, and those of the aqueous solution, with the ICP-MS and ICP-AES methods. The F concentration in the quenched solution was calculated from the mass balance. Under experimental conditions the partition coefficients of Ta, Nb, and Mn between the fluid and the granitic melt (weight ratio fluid C Ta/melt C Ta = fluid/melt D Ta) are shown to be extremely low (0.001–0.008 for Ta, 0.001–0.022 for Nb, and 0.002–0.010 for Mn); thus, these metals partition preferentially into the melt. The coefficients fluid/melt D Ta and fluid/melt D Nb generally increase either with increasing alumina ratio A/NKM in the glass composition, or with rising temperature. The experiments also demonstrated that F preferentially concentrates in the melt; and the partition coefficients of F are below 1, being within the range of 0.1–0.7.

  相似文献   

5.
Most rare-metal granites in South China host major W deposits with few or without Ta–Nb mineralization. However, the Yashan granitic pluton, located in the Yichun area of western Jiangxi province, South China, hosts a major Nb–Ta deposit with minor W mineralization. It is thus important for understanding the diversity of W and Nb–Ta mineralization associated with rare-metal granites. The Yashan pluton consists of multi-stage intrusive units, including the protolithionite (-muscovite) granite, Li-mica granite and topaz–lepidolite granite from the early to late stages. Bulk-rock REE contents and La/Yb ratios decrease from protolithionite granite to Li-mica granite to topaz–lepidolite granite, suggesting the dominant plagioclase fractionation. This variation, together with increasing Li, Rb, Cs and Ta but decreasing Nb/Ta and Zr/Hf ratios, is consistent with the magmatic evolution. In the Yashan pluton, micas are protolithionite, muscovite, Li-mica and lepidolite, and zircons show wide concentration ranges of ZrO2, HfO2, UO2, ThO2, Y2O3 and P2O5. Compositional variations of minerals, such as increasing F, Rb and Li in mica and increasing Hf, U and P in zircon are also in concert with the magmatic evolution from protolithionite granite to Li-mica granite to topaz–lepidolite granite. The most evolved topaz–lepidolite granite has the highest bulk-rock Li, Rb, Cs, F and P contents, consistent with the highest contents of these elements and the lowest Nb/Ta ratio in mica and the lowest Zr/Hf ratio in zircon. Ta–Nb enrichment was closely related to the enrichment of volatile elements (i.e. Li, F and P) in the melt during magmatic evolution, which raised the proportion of non-bridging oxygens (NBOs) in the melt. The rims of zoned micas in the Li-mica and topaz–lepidolite granites contain lower Rb, Cs, Nb and Ta and much lower F and W than the cores and/or mantles, indicating an exotic aqueous fluid during hydrothermal evolution. Some columbite-group minerals may have formed from exotic aqueous fluids which were originally depleted in F, Rb, Cs, Nb, Ta and W, but such fluids were not responsible for Ta–Nb enrichment in the Yashan granite. The interaction of hydrothermal fluids with previously existing micas may have played an important role in leaching, concentrating and transporting W, Fe and Ti. Ta–Nb enrichment was associated with highly evolved magmas, but W mineralization is closely related to hydrothermal fluid. Thus these magmatic and hydrothermal processes explain the diversity of W and Ta–Nb mineralizations in the rare-metal granites.  相似文献   

6.
The experimental results of natural pyrochlore behavior in KF solutions in the presence of quartz at 550–850°C and 50–100 MPa are presented. It is shown that silicate matter (quartz) exerts a significant effect on pyrochlore solubility in aqueous solutions of fluorides of alkaline metals under hydrothermal conditions. This study of the fluid inclusions has revealed the occurrence of reactions of high-temperature hydrolysis of KF under the experimental conditions: KF + H2O = KOH + HF; in which case, the interaction with quartz SiO2 + 2KOH = K2SiO3 + H2O is followed by the formation of a silicate glass phase (an aqueous solution–melt). This phase of alkaline glass is a Nb concentrator (Nb2O5 up to 16 wt %). The coefficient of Nb distribution between the glass and the fluid is ≈500 (in favor of the glass). It is determined that the phase of the silicate solution–melt can serve as an effective concentrator of the ore component (Nb) at the last lowtemperature stages of crystallization of rare-metal granites.  相似文献   

7.
Tang  Yong  Zhang  Hui 《中国地球化学学报》2015,34(2):194-200
Acta Geochimica - The partition coefficients of W, Nb, and Ta between the P-rich peraluminous granitic melt and the coexisting aqueous fluid were determined at 800–850 °C and...  相似文献   

8.
佛冈高分异I型花岗岩的成因:来自Nb-Ta-Zr-Hf等元素的制约   总被引：12，自引：8，他引：4  
陈璟元  杨进辉 《岩石学报》2015,31(3):846-854
华南南岭地区发育有大面积的与钨锡成矿相关的侏罗纪花岗岩,然而其中有些花岗岩的成因类型却难以确定。本文以佛冈岩体为例,结合前人已发表数据,对佛冈花岗岩体中Nb、Ta、Zr和Hf等元素的迁移特征及其原理进行探讨,并对佛冈花岗岩的成因类型进行了厘定。随着分异程度增加,佛冈花岗岩Nb和Ta含量增加,Nb/Ta(3.6~15.3)和Zr/Hf(17.3~38.9)比值降低并发生分异。随着Zr含量的降低,佛冈花岗岩的Zr/Hf比值降低,这一特征表明锆石的分离结晶作用使得佛冈花岗岩的Zr/Hf比值分异。Nb/Ta比值分异可能与角闪石和黑云母的分离结晶作用有关。随着Nb/Ta比值降低,Y/Ho比值增加,这一特征表明佛冈花岗岩Nb/Ta比值的分异也和岩浆演化后期的流体有关。佛冈花岗岩不含原生的富铝矿物,为准铝质到弱过铝质岩石。随着分异程度增加,佛冈花岗岩P2O5含量降低,表明它不是S型花岗岩。随着Y/Ho比值增加和Nb/Ta和Zr/Hf比值降低,佛岗花岗岩Ga/Al和Fe OT/Mg O比值增加,从典型I型花岗岩特征演化到类似A型花岗岩的地球化学特征。因此,我们认为佛冈花岗岩不是A型花岗岩而是高分异的I型花岗岩。区域上与成矿相关的流体和花岗质岩浆的相互作用和分离结晶作用,使得华南南岭地区的花岗岩地球化学特征复杂,所以其成因类型也变的难以确定。  相似文献   

9.
福建大坪花岗斑岩体位于永定县城南部的大石凹—蓝地火山喷发盆地,具斑状结构,基质呈霏细结构、细-微粒结构。文章运用电子探针(EMPA)和激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)等技术,对大坪岩体ZK10001钻孔中不同深度的岩石样品进行了矿物和岩石化学分析。分析结果表明,大坪岩体岩性为黑鳞云母碱长花岗斑岩,属于过铝质钙碱性花岗岩,具有较高的分异演化程度,Nb2O5和Ta2O5含量达到了花岗岩型稀有金属矿床的工业品位。铌钽矿物主要与黄玉、萤石的集合体呈稀疏浸染状、星点状产于基质间隙内,其次以星点状存在于石英和长石斑晶中。矿石矿物赋存特征与宜春钽铌矿床类似,可能具有一定的可选性和经济价值。大坪岩体的铌钽富集特征不同于斑岩型铜、钼矿床,与花岗岩型铌钽矿床也存在较大的差别,铌钽的富集过程经历了深部斑晶阶段和浅部基质阶段两阶段岩浆结晶分异作用,F等挥发分促进了铌钽在结晶残余岩浆中富集,在基质间隙间沉淀。大坪矿化岩体的发现暗示斑岩型铌钽矿床存在的可能性。  相似文献   

10.
An experimental study has been carried out to determine the partition coefficients of tungsten between aqueous fluids and granitic melts at 800 °C and 1.5 kb with natural granite as the starting material. The effects of the solutions on the partition coefficients of tungsten show a sequence of P > CO 3 2− > B > H2O. The effects are limited (generallyK D < 0.3) and the tungsten shows a preferential trend toward the melt over the aqueous fluid. The value ofK D increases with increasing concentration of phosphorus; theK D increases first and then reduces with the concentration of CO 3 2− when temperature decreases, theK D between the solution of CO 3 2− and the silicate melt increases, and that between the solution of B4O 7 2− and the silicate melt decreases. The partition coefficients of phosphorus and sodium between fluids and silicate melts have been calculated from the concentrations of the elements in the melts. TheK D value for phosphorus is 0.38 and that for sodium is 0.56. Evidence shows that the elements tend to become richer and richer in the melts.  相似文献   

11.
The partition coefficients KD=cfluid/cmelt of Cu, Sn, Mo, W, U, and Th between aqueous fluid and melt were measured in the systems haplogranite-H2O–HCl and haplogranite-H2O–HF at 2kbars, 750°C, and Ni–NiO buffer conditions using rapid-quench cold seal bombs, with many reversed runs. Concentrations of trace elements (1–1000 ppm) in the quenched aqueous fluid and in the glass were determined by plasma emission spectrometry (DCP). KD of F is close to 1 in the system studied. KD of Cu and Sn strongly increases with increasing Cl concentration due to the formation of chloride complexes in the aqueous fluid, while HF has no effect. However, in 2M HCl, KD of Cu approaches 100, while KD of Sn is below 0.1 under the same conditions. The partition coefficients of Mo and W are high if water is the only volatile present (Mo: 5.5, W: 3.5), but strongly decrease with increasing HCl and HF, due to the destabilization of hydroxy complexes. KD of U and Th is very low in the absence of complexing agents, but strongly increases with increasing HF concentration. KD of U also increases with increasing HCl concentration and with increasing CO2 concentration in the system haplogranite-H2O–CO2, indicating the stability of chloride and carbonate complexes of U at magmatic temperatures. The data suggest a stoichiometric ratio of Cl: U=3:1 and of F:U=2:1 in these complexes. Cl-rich fluids are responsible for the formation of porphyry Cu deposits, but are much less effective in the transport of Sn. F appears not to be essential for the concentration of Mo and W in fluids evolving from a granitic magma. The different complexing behavior of U and Th in aqueous fluids may account for their fractionation during magma genesis.  相似文献   

12.
The experimental study of an F-bearing silicic melt—U, Nb, Ta minerals—chloride-fluoride fluid system is focused on ascertaining the origin of uranium deposits spatially related to intraplate silicic volcanism. The first series of experiments on uranium solubility in silicic melts close in composition to ore-bearing rhyolite of the unique Strel’tsovka Mo-U ore field has been performed in order to determine more precisely the ore genesis. As starting solid phases, model homogeneous glass of the chemical composition (wt %) 72.18 SiO2, 12.19 Al2O3, 1.02 FeO, 0.20 MgO, 0.33 CaO, 4.78 Na2O, 3.82 K2O, 1.44 Li2O, and 2.4 F (LiF, NaF, KF, CaF2, MgF2); synthetic UO2 and UO3·0.33H2O; and natural columbite were used. The starting solutions contained 1.0 m Cl and 10−2 m F. The runs were conducted in a gas vessel at a pressure of 1000 bar and in a high-pressure hydrothermal vessel at 2000 bar. The O2 (H2) fugacity was set by Ni-NiO, Co-CoO, Fe3O4-Fe2O3, and Cu-Cu2O buffers. The equilibrium between melt and solution for major elements is reached during the first day, whereas 5–7 days are required for ore elements (U, Nb, Ta) to come into equilibrium. The solubility of Nb and especially Ta in Cl-F solutions equilibrated with F-bearing melt is extremely low. The solubility of U is much higher (10−4−10−5 mol/kg H2O). The energy dispersive spectroscopy of run products allowed us to establish that columbite dissolved incongruently with formation of U- and F-bearing pyrochlores. The performed experiments have shown that a silicic melt close to the rhyolitic magma of the Strel’tsovka caldera in composition is not able to generate postmagmatic ore-forming solutions containing more than 10−6−10−5 mol U/kg H2O under the relatively low pressure necessary for the existence of the first type of fluid. The amount of uranium that could have precipitated from this fluid in the zone of ore deposition is estimated at 216–9000 t. This estimate is two orders of magnitude lower than the total uranium resources of the deposits localized in the Strel’tsovka caldera. Thus, the upper crustal silicic magma chamber hardly was a source of uranium for Mo-U deposits of the Strel’tsovka ore field.  相似文献   

13.
The solubilities of zircon, rutile, manganocolumbite (MnNb2O6), manganotantalite (MnTa2O6), and the rare earth phosphates LaPO4, GdPO4, and YbPO4 in water-saturated haplogranitic melts containing 0–6 wt.% F were measured at 800° C and 2 kbar. The melt compositions investigated differ only in their F content, the proportions of Na, K, Al, and Si are identical in all experiments. While the solubilities of the rare earth phosphates are independent of F, the solubilities of all other minerals studied strongly increase with F. The TiO2 content of haplogranitic melt in equilibrium with rutile increases linearly from 0.26 wt.% without F to 0.47 wt.% for melts containing 6 wt.% F. Over the same range of F concentrations, the ZrO2 content of the melt in equilibrium with zircon increases with the square of the F content from less than 0.01 wt.% to 0.25 wt.%. The linear increase for rutile and the quadratic relationship for zircon suggest a complexing mechanism. Probably nonbridging oxygen atoms (NBO) expelled from coordination with Al by reaction with F form complexes with Ti and Zr, the ratio of NBO: metal cation being 1:1 for Ti, and 2:1 for Zr. Direct complexing by F is also a possibility. As titanium oxide phases and zircon are major sinks for HFS elements such as Ti, Nb, Ta, Zr, Hf, Th and REE in granites, their increased solubility in the presence of F favors the enrichment of these elements in residual mels. The Nb and Ta content of rutile in granitic pegmatites is due to extended solid solution of rutile with columbite group minerals, such as manganocolumbite and manganotantalite. The solubility of these components also increases with F, MnTa2O6 being more soluble than MnNb2O6. Rutile fractionation could therefore account for the increase in Ta/Nb frequently observed in highly differentiated granites. The solubility of the rare earth phosphates increases strongly from LaPO4 to GdPO4 to YbPO4, which explains the enrichment of heavy rare earth elements in highly evolved granites. In the presence of F, many HFS elements will be highly incompatible in granitic systems. Therefore, in a suite of granitic rocks generated by differentiation from the same source magma, a strong correlation should exist between HFS elements and F. However, because of the influence of F on the solubility of refractory phases such as zircon, a similar correlation could also result from different batches of magma containing different amounts of F equilibrating with the same refractory phase.  相似文献   

14.
The mechanism of Nb-, Ta-mineralizatio is discussed in the light of the properties of Nb-, Ta-complexes in different phases. Experiments show that Nb and Ta are essentially enriched in the melts when Nb-, Ta-bearing albite granites are completely melted (800–850°C) and in equilibrium with a HF-bearing vapor phase. It is also demonstrated from the experiments that the hydrolysis of Nb-, Ta-fluorine complexes in aqueous solutions takes place with increasing temperature reaching a maximum value in the vicinity of critical temperature and becoming stable under super-critical conditions. Under this circumstance, Nb-, Ta-complexes can be transported in the vapor phase. Ta exhibits a great ability of transport in the vapor phase as compared with Nb, while Nb is more soluble than Ta under hydrothermal conditions.  相似文献   

15.
Available experimental data on the solubility of Nb2O5 and the stability constants for particles of an aqueous solution in the Nb–O–H–F system were processed. As a result, a set of thermodynamic properties for 25°C and 1 bar was obtained, in addition to the equation parameters for the HKF model (Helgeson–Kirkham–Flowers) for hydroxo and hydroxofluoride niobium complexes. F– ion is the most important factor governing the concentration of dissolved Nb: neutral hydroxo complex Nb(OH)5(aq) is formed at a low HF concentration, whereas an increase in HF results in an increase in the first Nb(OH)4F(aq) and second Nb(OH)3F2(aq) fluoride complexes. The Nb(OH)5F– oxofluoride anion determines oxide solubility in alkali F-bearing fluids. Neutralization of acidic fluoride solution can be the main factor leading to niobium deposition.  相似文献   

16.
The formation of ore-bearing granites in the Yenshanian movement in southeast China and the geochemical characteristics of some RE-bearing granites have been studied through multivariate statistical analysis and physicochemical approach. The main conclusions have Been drawn as follows: (1) The granites are believed to be products of anatexis of the crustal materials. The formation temperature of granitic magma is estimated at about 600°C and the crystallization temperatures range from 600° to 500°C. The temperature of Li-, Fand H2O-rich residual magma in the latest stage of magmatism is probably below 500°C. (2) A rock series from early lepidomelane-granite through protolithionite-and zinnwalditegranite to lithioniteor lepidolite-granite is considered as a result of actual crystallization. (3) The mineral paragenesis and fades zonation of granite plutons are mainly controlled by Ph2o, μNa2O and μK2O in the magma. (4) During the magmatic evolution the ore-forming elements (REE, Nb, Ta, etc.) are variable in geochemistry. REEs, similar to mafic components, were highly concentrated at the early stage of the magmatic evolution and deposited under favourable conditions in the zinnwaldite-and protolithionite-granites; Nb and Ta have a preference for felsic and volatile components, thus are mainly concentrated at later stages of the magmatic evolution. Nb ore deposits are formed in the lithionite and lepidolite granites, for Ta is intimately associated with Na2O, Li2O, F and H2O.  相似文献   

17.
Mineral/melt trace element partition coefficients were determined for rutile (TiO2) for a large number of trace elements (Zr, Hf, Nb, Ta, V, Co, Cu, Zn, Sr, REE, Cr, Sb, W, U, Th). Whilst the high field strength elements (Zr, Hf, Nb, Ta) are compatible in rutile, other studied trace elements are incompatible (Sr, Th, REE). In all experiments we found DTa > DNb, DHf > DZr and DU > DTh. Partition coefficients for some polyvalent elements (Sb, W, and Co) were sensitive to oxygen fugacity. Melt composition exerts a strong influence on HFSE partition coefficients. With increasing polymerization of the melt, rutile/melt partition coefficients for the high field strength elements Zr, Hf, Nb and Ta increase about an order of magnitude. However, DNb/DTa and DHf/DZr are not significantly affected by melt composition. Because DU ? DTh, partial melting of rutile-bearing eclogite in subducted lithosphere may cause excesses of 230Th over 238U in some island arc lavas, whereas dehydration of subducted lithosphere may cause excesses of 238U over 230Th. From our partitioning results we infer partition coefficients for protactinium (Pa) which we predict to be much lower than previously anticipated. Contrary to previous studies, our data imply that rutile should not significantly influence observed 231Pa-235U disequilibria in certain volcanic rocks.  相似文献   

18.
We report new geological, mineralogical, geochemical and geochronological data about the Katugin Ta-Nb-Y-Zr (REE) deposit, which is located in the Kalar Ridge of Eastern Siberia (the southern part of the Siberian Craton). All these data support a magmatic origin of the Katugin rare-metal deposit rather than the previously proposed metasomatic fault-related origin. Our research has proved the genetic relation between ores of the Katugin deposit and granites of the Katugin complex. We have studied granites of the eastern segment of the Eastern Katugin massif, including arfvedsonite, aegirine-arfvedsonite and aegirine granites. These granites belong to the peralkaline type. They are characterized by high alkali content (up to 11.8 wt% Na2O + K2O), extremely high iron content (FeO1/(FeO1 + MgO) = 0.96–1.00), very high content of most incompatible elements – Rb, Y, Zr, Hf, Ta, Nb, Th, U, REEs (except for Eu) and F, and low concentrations of CaO, MgO, P2O5, Ba, and Sr. They demonstrate negative and CHUR-close εNd(t) values of 0.0…−1.9. We suggest that basaltic magmas of OIB type (possibly with some the crustal contamination) represent a dominant part of the granitic source. Moreover, the fluorine-enriched fluid phases could provide an additional source of the fluorine. We conclude that most of the mineralization of the Katugin ore deposit occurred during the magmatic stage of the alkaline granitic source melt. The results of detailed mineralogical studies suggest three major types of ores in the Katugin deposit: Zr mineralization, Ta-Nb-REE mineralization and aluminum fluoride mineralization. Most of the ore minerals crystallized from the silicate melt during the magmatic stage. The accessory cryolites in granites crystallized from the magmatic silicate melt enriched in fluorine. However, cryolites in large veins and lens-like bodies crystallized in the latest stage from the fluorine enriched melt. The zircons from the ores in the aegirine-arfvedsonite granite have been dated at 2055 ± 7 Ma. This age is close to the previously published 2066 ± 6 Ma zircon age of the aegirine-arfvedsonite granites, suggesting that the formation of the Katugin rare-metal deposit is genetically related to the formation of peralkaline granites. We conclude that Katugin rare-metal granites are anorogenic. They can be related to a Paleoproterozoic (∼2.05 Ga) mantle plume. As there is no evidence of the 2.05 Ga mantle plume in other areas of southern Siberia, we suggest that the Katugin mineralization occurred on the distant allochtonous terrane, which has been accreted to Siberian Craton later.  相似文献   

19.
Electronic microprobe analyses for olivine, clinopyroxene and Cr-spinel in picrites, which we have discovered recently in the Emeishan continental flood basalt province (ECFBP), show that the olivine is rich in Mg, and that Cr-spinel is rich in Cr. Based on the olivine-melt equilibrium, the primary parental melt compositions are calculated. The high-Mg olivine-hosted picrite can be regarded as parental melt. Thus, the melting temperature and pressure are estimated: T=1600℃ and P=4.5 GPa. It suggests that the picrites are connected with the activity of mantle plumes. Their major element composition is comparable to many other CFBs by their high Fe8, (CaO/Al2O3)8 and low Na8, indicating a high pressure. All rocks display a similar chondrite-normalized REE patterns, i.e., enrichment of LREE, relative depletion of HFSE and absence of negative Nb and Ta but depletion in P and K. Some incompatible element ratios, such as La/Ta, La/ Sm, (La/Nb)PM, (Th/Ta)PM, are in a limited range, show that they were derived  相似文献   

20.
内蒙古赵井沟大型铌钽矿床地质特征及成因   总被引：8，自引：2，他引：6  
内蒙古武川县赵井沟矿床是近年来在内蒙古中部地区找到的一处大型铌钽矿床.铌钽氧化物储量为8000余吨(钽氧化物含量大于50％),其中,ω(Nb2O5)和ω(Ta2O5)的平均含量为0.011％和0.012％.铌钽矿化主要在早二叠世碱长花岗岩类侵入杂岩体内,呈浸染状和条带状产出,并且构成似层状、脉状和透镜状矿体.研究表明,碱长花岗岩、碱长花岗细晶岩和碱长花岗伟晶岩锆石U-Pb同位素年龄值分别为(277.14±0.82) Ma、(277.0±2.1) Ma和(276.6±2.1) Ma.鉴于铌钽矿化呈浸染状在含矿侵入杂岩体内产出,推测赵井沟矿床的成矿作用与海西期构造-岩浆活动有关.古大陆块体伸展构造条件下,富钠质钙-碱性岩浆作用为铌钽矿床的形成提供了动力和物质来源,而断裂构造为成矿物质沉淀聚集创造了空间条件.赵井沟矿床属富钠的过铝质花岗岩型铌钽矿床.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号