Zr/Hf ratio as an indicator of fractionation of rare-metal granites by the example of the Kukulbei complex,eastern Transbaikalia

The concept of granitic melt fractionation as the main process in the concentration of rare elements in granites calls for the development of a reliable method to determine the evolutionary sequences of granite series. We propose to use for this purpose a zirconium-hafnium indicator, the Zr/Hf weight ratio in granitic rocks (Zaraisky et al., 1999, 2000). By the example of three classic regions of rare-metal deposits, eastern Transbaikalia, central Kazakhstan, and Erzgebirge (Czech Republic and Germany), it was empirically shown that the Zr/Hf ratio of granites decreases during the fractional crystallization of granite magmas in the sequence granodiorite → biotite granite → leucogranite → lithium-fluorine granite. The reason is the higher affinity of Hf compared with Zr to a granite melt. This implies that the crystallization and settling of accessory zircon will cause the progressive enrichment of Hf relative to Zr in the residual melt. As a result, the Zr/Hf ratio decreases regularly in the series of sequential phases of granite intrusion related to a single magma chamber from granodiorite to biotite granite, leucogranite, and Li-F granite (from 45-30 to 10-2). Our experimental investigations supported the preferential enrichment of haplogranite melt in Hf and zircon crystals in equilibrium with melt in Zr (T= 800°C and P = 1 kbar). The Zr/Hf indicator was tested by the example of the wellknown Kukulbei rare-metal granite complex of eastern Transbaikalia (J3), which is unique in the degree of fractionation of initial granite melt with the formation of three phases of granite emplacement and vein derivatives. An important feature of the complex is its “short” differentiation trend. It was supposed that the granite magma of the first phase is parental, and the later phases forming small intrusive bodies in large massifs of biotite granites of the first phase are sequential products of its crystallization differentiation in a magma chamber. The biotite granites of the first phase are barren. The leucocratic granites of the second phase are accompanied by tin-tungsten greisen deposits (e.g., Spokoininskoe), and the upper part of cupola-like stocks of Li-F amazonite granites of the third phase host apogranite-type tantalum deposits (Orlovka, Etyka, and Achikan). In addition to three granite phases, the Kukulbei complex includes dikes of ongonites, elvans, amazonite granites, and chamber miarolitic pegmatites. All of the granitic rocks of the complex have similar isotopic ages of 142± 0.6 Ma. The Zr/Hf ratio decreases systematically from phase 1 (40–25), to phase 2 (20–10), and phase 3 (10–2). The ongonites, elvans, and pegmatites have similar Zr/Hf ratios (15-5), falling between the ranges of leucocratic muscovite granites and Li-F granites. Compared with other granite series, the granitic rocks of the Kukulbei complex show specific petrographic and geochemical features: they are strongly enriched in Rb, Li, Cs, Be, Sn, W, Mo, Ta, Nb, Bi, and F but depleted in Mg, Ca, Fe, Ti, P, Sr, Ba, V, Co, Ni, Cr, Zr, REE, and Y. From the early to late intrusion phases, the degree of enrichment and depletion in these element groups increases regularly. This is accompanied by a significant decrease (from 40 to 2) in Zr/Hf, which can be used as a reliable indicator of genetic relations, degree of fractionation, and rare-metal potential of granites. Granites with Zr/Hf values lower than 25 are promising for prospecting for Sn, W, Mo, and Be greisen deposits, whereas the formation of Ta deposits requires Zr/Hf values lower than 10.     

Melt composition control of Zr/Hf fractionation in magmatic processes

Zircon (ZrSiO₄) and hafnon (HfSiO₄) solubilities in water-saturated granitic melts have been determined as a function of melt composition at 800° and 1035°C at 200 MPa. The solubilities of zircon and hafnon in metaluminous or peraluminous melts are orders of magnitude lower than in strongly peralkaline melt. Moreover, the molar ratio of zircon and hafnon solubility is a function of melt composition. Although the solubilities are nearly identical in peralkaline melts, zircon on a molar basis is up to five times more soluble than hafnon in peraluminous melts. Accordingly, calculated partition coefficients of Zr and Hf between zircon and melt are nearly equal for the peralkaline melts, whereas for metaluminous and peraluminous melts D_Hf/D_Zr for zircon is 0.5 to 0.2. Consequently, zircon fractionation will strongly decrease Zr/Hf in some granites, whereas it has little effect on the Zr/Hf ratio in alkaline melts or similar depolymerized melt compositions.The ratio of the molar solubilities of zircon and hafnon for a given melt composition, temperature, and pressure is proportional to the Hf/Zr activity coefficient ratio in the melt. The data imply that this ratio is nearly constant and probably close to unity for a wide range of peralkaline and similar depolymerized melts. However, it changes by a factor of two to five over a relatively small interval of melt compositions when a nearly fully polymerized melt structure is approached. For most ferromagnesian minerals in equilibrium with a depolymerized melt, D_Hf > D_Zr. Typical values of D_Hf/D_Zr range from 1.5 to 2.5 for clinopyroxene, amphibole, and titanite. Because of the change in the Hf/Zr activity ratio in the melt, the relative fractionation of Zr and Hf by these minerals will disappear or even be reversed when the melt composition approaches that of a metaluminous or peraluminous granite. It is thus not surprising that fractional crystallization of such granitic magmas leads to a decrease in Zr/Hf, whereas fractional crystallization of depolymerized melts tends to increase Zr/Hf. There is no need to invoke fluid metasomatism to explain these effects. Results demonstrate that for ions with identical charge and nearly identical radius, crystal chemistry does not alone determine relative compatibilities. Rather, the effect of changing activity coefficients in the melt may be comparable to or even larger than elastic strain effects in the crystal lattice.     

The TiO2/Ta ratio as an indicator of the degree of differentiation of tin granites

Tin deposits in Eastern Marche (French Massif Central) and North of Viseu (Portugal) are associated with final stages of differentiation of granitoid magmatism. Fractional crystallization causes increase in Sn, W, Ta, Rb, Cs, F and Li, and decrease in Ti, La, U, Th, Ba etc. That trend is very clearly expressed by tantalum, strongly correlated with tin, and by titanium. Hence the TiO₂/Ta ratio is a good indicator of granitic melt differentiation. Its value gradually ranges from 4 900 in the less differentiated granodiorite to <1 in the most differentiated granite from Marche area. That ratio could be a useful guide to suitable areas for tin exploration.     

Two types of magma sources of rare-metal alkali granites

The geological setting, age, and magma sources of rare-metal ores at the Khalzanburgetdei deposit in western Mongolia and the Khan Bogd occurrence in southern Mongolia are considered. The Khalzanburgetdei deposit, 392 Ma in age, is situated in the Lake Zone of the Early Caledonides of the Central Asian Foldbelt at a branch of a triple junction of grabens. The Khan Bogd occurrence, about 290 Ma in age, is located in the South Mongolian Hercynian Zone and related to a continental rift conjugated in space and time with Hercynian island-arc systems. To estimate features of rare-metal magma sources, the Nd and O isotopic compositions were used, as well as ratios of incompatible elements (Nb-U, Nb-Zr, La-Yb, Th-Ta, and Ce-Pb). Since the granitic magma was commonly saturated with these elements, their ratios may be used for estimation of magma source compositions, trends of magma fractionation, and accumulation of rare-metal minerals. A mixture of an OIB source and crustal material (Caledonian ophiolitic and island-arc complexes) served as a source of rare-metal granitic rocks of the Khalzanburgetdei deposit, while the Khan Bogd occurrence was related to a source of subduction-related basic rocks, probably, with participation of the depleted and enriched mantle and continental crust.     

Cosmic Dust as an Indicator for the Genetic Classification of Ganites

In the strdy of the source material of granites ,it is found that cosmic dust can be used as one of the indicators for identifying the genetic types of granites.It is suggested that granites can be grouped into two genetic types, dust-barren granite and dust-containing granite, corresponding to I-type and S-type respectively as defined by Chappell et al.     

Geochemical features of rare-metal granites of the Zashikhinsky Massif,East Sayan

The paper presents detailed geochemical data on the rocks of the Zashikhinsky Massif and mineralogical–geochemical characteristics of the ores of the eponymous deposit. The rare-metal granites are divided into three facies varieties on the basis of the degree of differentiation and ore potential: early facies represented by microcline–albite granites with arfvedsonite, middle facies represented by leucocratic albite–microcline granites, and late (most ore-bearing) facies represented by quartz–albite granites grading into albitites. Microprobe data were obtained on major minerals accumulating trace elements in the rocks and ores. All facies of the rare-metal granites, including the rocks of the fluorite–rare-metal vein, define single compositional trends in the plots of paired correlations of rock-forming and trace elements. In addition, they also show similar REE patterns and spidergrams. The latter, however, differ in the depth of anomalies of some elements. Obtained geological, petrographic, and geochemical data suggest a magmatic genesis of the rocks of different composition and their derivation from a single magma during its differentiation. On the basis of all characteristics, the Zashikhinskoe deposit is estimated as one of the largest tantalum rare-metal deposits of alkaline-granite type in Russia.     

峨眉山二滩高钛玄武岩Zr/Hf分异的指示意义

对峨眉山二滩高钛玄武岩高场强元素的研究表明,Zr/Hf比值出现了明显分异,可分为高Zr/Hf组和低Zr/Hf组两组玄武岩。Zr/Hf比值的分异与岩浆结晶分异作用无关,而是由部分熔融和地幔源区的不同所导致,暗示高Zr/Hf组和低Zr/Hf组玄武岩分别具有不同的部分熔融条件和不同的地幔物质组分。     

Alkali feldspars as a main phosphorus reservoirs in rare-metal granites: three examples from the Bohemian Massif (Czech Republic)

Phosphorus-rich alkali feldspars were found in three peraluminous highly differentiated albite-topaz-Li-mica granites in the W and S parts of the Bohemian Massif. The average contents of P₂O₅ in K-feldspars (Křížovy kámen 0.57 wt%, Homolka 0.77 wt%, and Podlesí 0.83 wt%) are higher than the average contents of P₂O₅ in albites (Křížovy kámen 0.23 wt%, Homolka 0.23 wt%, and Podlesí 0.39 wt%). The analyses of feldspars indicate that partition coefficient of phosphorus between K-feldspar and albite range from 1.5 to 2.5. Measured data in K-feldspars suggest a statistically significant difference from the Al³⁺+P⁵⁺=2 Si⁴⁺ substitution mechanism for higher phosphorus concentration. The P-content in K-feldspars from the Podlesí-granite represent the highest P-content in natural feldspar known to date (up to 2.5 wt% of P₂O₅)- It is suggested that all studied granites retained nearly all phosphorus of granitic melts and that their alkali feldspars represent major reservoirs of phosphorus.     

Biotites and associated minerals as markers of magmatic fractionation and deuteric equilibration in granites

Analyses of 80 biotite, alkali feldspar, oligoclase, hornblende, Fe-Ti oxide separates from the coarse-grained granites of a late-hercynian epizonal diapir, the Ploumanac'h complex, Brittany, show that these minerals display a regular concentric cryptic layering related to fractional crystallization. The Ca, Mg, Ba, Sr, Cr, V content of minerals decreases as the Na, Fe, and Rb content increases. Biotites become more dioctahedral towards the outer residual syenogranite, with a correlative K deficiency. Trioctahedral biotites from the inner accumulative monzogranite are secondarily oxidized with a gain of Fe³⁺ and a loss of OH. This alteration id due to the percolation of exsolved fluids rich in H₂O and containing a small amount of CO₂, F, S, Cl. During this autometamorphic stage, trace elements like Rb, Sr are completely redistributed on the scale of hand specimens, with a restricted range of partition coefficients between biotite, perthite and oligoclase. This equilibration occurred at a temperature about 550 ° C and a fluid pressure about 1,000 bars, with f _H2_O probably less than 500 bars. A later stage of fluid circulation along fractures brings up a slight Li metasomatism. Biotites are a sensitive marker of both magmatic and postmagmatic stages of subsolvus or ‘wet’ plutonites.     

On “Birthmarks” in rare-metal granites of the Losevka pluton,northern Kazakhstan

The Losevka pluton of rare-metal albite granite, which was explored as a possible source of columbite-zircon-malacon ore, is composed of quartz, sodic plagioclase, potassium feldspar, annite, protolithionite, lepidomelane, and Li-muscovite. The average chemical composition of this rock is as follows, wt %: 74.14 SiO₂, 0.04 TiO₂, 14.07 Al₂O₃, 1.05 Fe₂O₃, 0.78 FeO, 0.15 MnO, 0.09 MgO, 0.47 CaO, 4.65 Na₂O, 4.11 K₂O, and 0.03 P₂O₅. The accessory minerals are zircon, malacon, and cyrtolite (874 ppm); apatite (18 ppm); ilmenite (114 ppm); xenotime and monazite (119 ppm); and Nb-columbite (463 ppm). The black inclusions up to 15 cm in size, which are observed in this granite and called “birthmarks” by local geologists, consist of the same rock-forming minerals as the surrounding granite, but are enriched in MnO, MgO, CaO, TiO₂, and F and depleted in SiO₂ relative to the light granite. The black granite is also distinguished by much higher Sr and Ba contents and lower La, Rb, Y, Nb, REE, Cs, Ta, Th, and U contents. The black color is caused by enrichment in manganese oxides, manganoilmenite, and Mn-annite. All rock-forming minerals are pervaded by thin veinlets of Mn-oxides. In addition, bastnaesite, Y-and Th-fluorides, zircon, and malacon have been identified. Aggregates of black-colored minerals are not the products of the fractionation of the initial magma or immiscibility effects, because the structure of the albite-potassium feldspar-quartz-mica matrix is the same both in black and light granites. The percolation of a deep-sourced fluid enriched in Mn and F into a granitic melt might be a more probable origin.     

Controls on the fractionation of isovalent trace elements in magmatic and aqueous systems: evidence from Y/Ho,Zr/Hf,and lanthanide tetrad effect

 The parameters which control the behaviour of isovalent trace elements in magmatic and aqueous systems have been investigated by studying the distribution of yttrium, rare-earth elements (REEs), zirconium, and hafnium. If a geochemical system is characterized by CHArge-and-RAdius-Controlled (CHARAC) trace element behaviour, elements of similar charge and radius, such as the Y-Ho and Zr-Hf twin pairs, should display extremely coherent behaviour, and retain their respective chondritic ratio. Moreover, normalized patterns of REE(III) should be smooth functions of ionic radius and atomic number. Basic to intermediate igneous rocks show Y/Ho and Zr/Hf ratios which are close to the chondritic ratios, indicating CHARAC behaviour of these elements in pure silicate melts. In contrast, aqueous solutions and their precipitates show non-chondritic Y/Ho and Zr/Hf ratios. An important process that causes trace element fractionation in aqueous media is chemical complexation. The complexation behaviour of a trace element, however, does not exclusively depend on its ionic charge and radius, but is additionally controlled by its electron configuration and by the type of complexing ligand, since the latter two determine the character of the chemical bonding (covalent vs electrostatic) in the various complexes. Hence, in contrast to pure melt systems, aqueous systems are characterized by non-CHARAC trace element behaviour, and electron structure must be considered as an important additional parameter. Unlike other magmatic rocks, highly evolved magmas rich in components such as H₂O, Li, B, F, P, and/or Cl often show non-chondritic Y/Ho and Zr/Hf ratios, and “irregular” REE patterns which are sub-divided into four concave-upward segments referred to as “tetrads”. The combination of non-chondritic Y/Ho and Zr/Hf ratios and lanthanide tetrad effect, which cannot be adequately modelled with current mineral/melt partition coefficients which are smooth functions of ionic radius, reveals that non-CHARAC trace element behaviour prevails in highly evolved magmatic systems. The behaviour of high field strength elements in this environment is distinctly different from that in basic to intermediate magmas (i.e. pure silicate melts), but closely resembles trace element behaviour in aqueous media. “Anomalous” behaviour of Y and REEs, and of Zr and Hf, which are hosted by different minerals, and the fact that these minerals show “anomalous” trace element distributions only if they crystallized from highly evolved magmas, indicate that non-CHARAC behaviour is a reflection of specific physicochemical properties of the magma. This supports models which suggest that high-silica magmatic systems which are rich in H₂O, Li, B, F, P, and/or Cl, are transitional between pure silicate melts and hydrothermal fluids. In such a transitional system non-CHARAC behaviour of high field strength elements may be due to chemical complexation with a wide variety of ligands such as non-bridging oxygen, F, B, P, etc., leading to absolute and relative mineral/melt or mineral/aqueous-fluid partition coefficients that are extremely sensitive to the composition and structure of this magma. Hence, any petrogenetic modelling of such magmatic rocks, which utilizes partition coefficients that have not been determined for the specific igneous suite under investigation, may be questionable. But Y/Ho and Zr/Hf ratios provide information on whether or not the evolution of felsic igneous rocks can be quantitatively modelled: samples showing non-chondritic Y/Ho and Zr/Hf ratios or even the lanthanide tetrad effect should not be considered for modelling. However, the most important result of this study is that Y/Ho and Zr/Hf ratios may be used to verify whether Y, REEs, Zr, and Hf in rocks or minerals have been deposited from or modified by silicate melts or aqueous fluids. Received: 4 September 1995 / Accepted: 30 October 1995     

用Y／Ho比值指示俄罗斯乌拉尔南部晶质菱镁矿矿床的成因

乌拉尔省南部赋存有两种类型的晶质菱镁矿:1)白云岩地层中的层状矿体;2)白云质灰岩中的透镜状矿体。晶质菱镁矿矿体位于Riphean系列中下层的白云岩中,而在上层的白云岩单元中缺失。这两种类型的菱镁矿可通过矿体形态、晶体大小、石英和白云石含量不同来进行区分。第一种类型的菱镁矿储量巨大,菱镁矿呈粗粒结构,晶体粒径>10mm(最大达150mm);一般来说,矿体与白云岩围岩界限清楚,这种类型矿床以产在Riphean序列下部为特征。第二种类型的菱镁矿由于菱镁矿矿体穿插进入到白云岩围岩中,矿体很不规则,菱镁矿晶体也相对较小(1-5mm),这种类型的矿体主要产在Riphean中部层位中。这两种矿体都显示了交代成因的特征。但这两种菱镁矿矿石在一些主量元素和稀土元素的分布上具有不同的特征:与第二种类型相比,第一种菱镁矿具有较低的FeO,CaO和SiO2含量,与白云岩围岩(La／Lu>1)相比,具La／Lu<1的轻稀土亏损特征。第二种菱镁矿稀土分馏度较低,在稀土分配方面与白云岩围岩有差别。本文还特别讨论了Y／Ho值的重要性,因为该比值在菱镁矿和围岩中的类似性使得划分菱镁矿形成中的热液和成岩交代过程成为可能。因此我们认为,第一种类型菱镁矿,如具有高Y／Ho比值的Satka和Bakal矿床的形成属于沉积盆地发育过程中的早期成岩阶段;第     

Small-scale Hf isotopic variability in the Peninsula pluton (South Africa): the processes that control inheritance of source 176Hf/177Hf diversity in S-type granites

The ¹⁷⁶Hf/¹⁷⁷Hf composition of inherited and magmatic zircon in the 538 Ma S-type Peninsula pluton (South Africa) has been determined at different scales. In the smallest rock samples investigated (<0.5 dm³), as well as within individual thin sections, magmatic zircon crystals exhibit the same wide range in ε_Hf(538) as the pluton (8ε units). In addition, across a significant range of bulk-rock compositions, both the range and average of the magmatic zircon Hf isotopic composition do not vary significantly with compositional parameters that are expected to scale with the proportion of mantle-derived magma addition (e.g., Mg# and Ca). At all scales, the ε_Hf variability in the magmatic zircon fraction matches well with that portrayed by the time-evolved inherited zircon population [i.e., with the ε_Hf(538) range of the inherited zircon cores]. This evidence suggests that the ε_Hf heterogeneity of magmatic zircon is directly inherited from the source. However, the analysis of zircon core–rim pairs reveals that the ¹⁷⁶Hf/¹⁷⁷Hf composition of the inherited crystals does not directly transfer to their magmatic overgrowths. Small-scale modeling of zircon dissolution and re-precipitation in a static magma generates sub-mm melt domains having variable Zr content and Hf isotope composition. The composition of these domains is controlled by the size and isotope composition of the nearest dissolving zircon crystals and the cooling rate of the magma. These results suggest that in magma systems with a substantial inherited zircon load, zircon crystals within the same rock should record variable ¹⁷⁶Hf/¹⁷⁷Hf in the magmatic zircon fraction.     

Nb/Zr fractionation on the Moon and the search for extinct Nb

A comprehensive Zr isotopic study was conducted on eleven lunar basalts and highland rocks to search for evidence of the extinct nuclide ⁹²Nb, which decays to ⁹²Zr with a half-life of 36 Ma. Internal isochrons were determined for two early highland rocks, 77215 and 60025. No resolvable Zr isotopic variations were detected in this wide range of lunar samples and thus there is no evidence for the former existence of live ⁹²Nb on the Moon. The Nb/Zr ratios of lunar ilmenites and bulk rock samples vary by only a factor of two to three relative to the chondritic Nb/Zr ratio. No evidence for larger Nb/Zr fractionation was found. This limited fractionation and late isotopic closure of the source region prevents the formation of measurable ⁹²Zr anomalies in high-Ti mare basalts. As a consequence, it is not possible to draw conclusions from the ⁹²Nb-⁹²Zr chronometer about the timing of early lunar differentiation and to constrain the role of ilmenite in the source region of high-Ti mare basalts. However, the fractionation is still sufficient to deduce an upper limit for the initial ⁹²Nb/⁹³Nb ratio of the solar system of <5 × 10⁻⁴.     

Geochemistry and formation conditions of rare-metal granites with various fluorine-bearing minerals (fluorite,topaz, and cryolite)

A comparative geochemical study of rare-metal granitoids with various fluorine-bearing minerals (fluorite, topaz, and cryolite) was carried out. It was shown that these rocks correspond to both plumasitic and agpaitic geochemical types. The fluorite-, topaz-, and cryolite-bearing granites of these geochemical types are distinctly different in geochemical parameters and the character of magmatic evolution. These differences are related to the composition of initial magmas and their sources. Rare-metal granitoids with fluorine-bearing minerals compose small massifs, stocks, and dike swarms. Their formation is independent of the composition and age of the country rocks or geologic structures where they occur. Original Russian Text ? V.S. Antipin, E.A. Savina, M.A. Mitichkin, 2006, published in Geokhimiya, 2006, No. 10, pp. 1040–1052.     

Experimental study of physical and chemical melting conditions of rare-metal granites at the Voznesenka ore cluster,Primorye region

The melting of two basic granite varieties in the Voznesenka Complex such as Yaroslavka biotite granite and Voznesenka Li–F granite was subject to experimental studies to analyze and to compare the conditions of their physicochemical formation. The experiments were conducted at 550–700°C and 50–500 MPa in pure water and in 0.1 and 1 m HF aqueous fluorine-bearing solutions. The melting temperature of Voznesenka Li–F granites was 60–70°C lower than that of Yaroslavka biotite granites. The temperature decreased by almost 100°C from the completion of biotite granite crystallization to the completion of Li–F granite crystallization.     

Zircon and whole-rock Zr/Hf ratios as markers of the evolution of granitic magmas: Examples from the Teplice caldera (Czech Republic/Germany)

Mineralogy and Petrology - Hafnium contents and Zr/Hf ratios were studied in zircons and their parent rocks from three magmatic suites associated with the Teplice caldera, Eastern Erzgebirge:...     

Geochemistry, geochronology and mineralization of the rare-metal granites in the Daping Ta-Nb deposit, Fujian Province, South China

<正>1 Introduction Rare-metal granites are widely distributed in South China.The Daping porphyritic granitic Ta-Nb deposit,located in the Yongding area of south Fujian province,South China,is a large rare-metal deposit recently discovered.Few studies have been made of its petrology,mineralogy,geochemistry,chronology and metallogeny.In recent years,several exploratory drillings have been done in this deposit.These drilling holes,from 380 to 600     

Geochemistry of fractionation of coherent elements (Zr and Hf) during the profound differentiation of peralkaline magmatic systems: A case study of the Lovozero Complex

The paper presents pioneering data on Hf distribution in peralkaline rocks, ores, and rock-forming and accessory minerals of the Lovozero Complex. Variations in the Zr/Hf ratio are determined in all rocks of the Lovozero alkaline massif. This ratio is proved to increase in the course of evolution of alkaline magma because of fractionation of alkaline pyroxene. The Hf distribution coefficient is evaluated for alkali-rich pyroxene, whose crystallization controls Zr and Hf fractionation during the differentiation of alkaline magma. These data and the equation of equilibrium and fractional crystallization are utilized in a model suggested for Zr and Hf fractionation in the course of evolution of the Lovozero intrusion.