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The petrogenesis and mineralization of Zhaojinggou Nb–Ta deposit, Inner Mongolia: Evidence from geochronology, rock, and mineral geochemistry
Abstract
The Zhaojinggou Nb–Ta deposit is one of large rare metal deposits newly discovered in the northern margin of the North China Craton in recent years. This paper reports petrography, petrochemistry, columbite‐group minerals U–Pb chronology study of the amazonite granitic pegmatite (AGP) exposed in this deposit, and composition of columbite‐group minerals and biotite are obtained by electron probe microanalyzer and LA‐ICP‐MS. Eighteen analyses of columbite‐group minerals yielded weighted mean 206Pb/238U age of 116.9 ± 1.4 Ma. The crystallization temperature of biotite is 630–650°C, and the oxygen fugacity is 10−17–10−18 bars. The biotite has low MgO contents and high Rb, Rb/Sr, and FeOT/(FeOT + MgO) ratios. The AGP has extremely low MgO, Cr, Co and Ni contents, with Nb/Ta ratios range from 1.63 to 9.05 and Rb/Sr ratios range from 303.30 to 648.90, and obvious ‘M’ type tetrad effect of rare earth element indicating that the formation of the AGP is related to crust‐derived magma. The contents of Nb2O5 and FeO decrease, while the Ta2O5 and WO3 contents, Mn# and Ta# values increase gradually from the core to the rim of columbite‐group minerals. Some columbite‐group minerals have unobvious oscillatory zoning, and some have a clear bright zoing with high Ta contents on the rim, indicating that the genesis of Zhaojinggou Nb–Ta deposit is mainly magmatic crystallization differentiation, accompanied by hydrothermal autometasomatism in the late stage. The deposit was formed in an extensional tectonic background in the late Yanshanian, magma was formed by partial melting of Nb–Ta‐rich lower crust, undergoing high evolution. Mineralization is crystallization differentiation and hydrothermal self‐metasomatism
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In rare-metal granites, niobium and tantalum are generally hosted by Nb-Ta oxides. However, in SE China, the Nb-specialized Huangshan granites are a unique occurrence in which Nb is essentially hosted by LiFe micas. The Huangshan granites are part of the Early Cretaceous (Late Yanshanian) Lingshan granite complex and belong to the A-type granite series, with two facies differing by their mica compositions: medium-grained "protolithionite" granite and medium-grained lithian (lithium-rich) annite granite. The granites are characterized by elevated whole-rock Nb contents (average 144 ppm in "protolithionite" granite and 158 ppm in annite granite), quite low Ta contents (average 9 and 4 ppm, respectively), leading to very high Nb/Ta ratios (average 15.3 and 31.2). Niobium is mainly hosted in the micas, with an average Nb content of 1347 ppm in the lithian annite and 884 ppm in the "protolithionite," which is the highest ever reported in granitic mica. With an estimated endowment of ~80 kt Nb, the Huangshan granites represent a new style of potential Nb resource. Contrasting with the great rarity of columbite, there is abundant Hf-rich zircon, Y-rich fluorite, and Th-rich fluocerite included in the Huangshan micas. Such accessory minerals being typical of alkaline rhyolitic magmas and niobium enrichment in the Huangshan granites results from A-type melt. The extreme Nb enrichment in the micas results from the highly compatible behavior of Nb in this melt, combined with the high magma temperature (estimated at 790-800 °C) and possibly enhanced magma oxidation.
End members and species defined with permissible ranges of composition are presented for the true micas, the brittle micas and the interlayer-cation-deficient micas. The determination of the crys-tallochemical formula for different available chemical data is outlined, and a system of modifiers and suffixes is given to allow the expression of unusual chemical substitutions or polytypic stacking arrangements. Tables of mica synonyms, varieties, ill-defined materials and a list of names formerly or erroneously used for micas are presented. The Mica Subcommittee was appointed by the Commission on New Minerals and Mineral Names ("Commission") of the International Mineralogical Association (IMA). The definitions and recommendations presented were approved by the Commission.
In their last stages of evolution (i.e. at the magmatic-hydrothermal transition), peraluminous granitic melts exsolve a large amounts of fluids which can modify the chemical composition of whole-rock granitic samples and lead to the deposition of economicaly significant mineralization (Sn, W). Nb and Ta are lithophile elements considered to be " geochemical twins " because they have the same charge and a similar ionic radius. However, Nb/Ta ratios are highly variable in granites. Some authors have demonstrated that the Nb/Ta ratios decrease in granites during fractional crystallization [1]. Other studies have suggested that Nb and Ta could be fractionated in evolved peraluminous granites during the interaction with late magmatic fluids [2]. In this study [3], we demonstrate, using a compilation of whole-rock geochemical data available in the litterature that fractional crystallization alone is not sufficient to explain the distribution of Nb-Ta in most peraluminous granites. However, we notice that most of the granitic samples displaying evidence of interactions with fluids have Nb/Ta < ∼5. We propose that the decrease of the Nb/Ta ratio in evolved melts is the consequence of both fractional crystallization and sub-solidus hydrothermal alteration. We suggest that the Nb/Ta value of ~5 fingerprints the magmatic-hydrothermal transition in peraluminous granites. Furthermore, a Nb/Ta ratio of ~5 appears to be a good marker to discriminate mineralized from barren peraluminous granites.
In their late stages of evolution, peraluminous granitic melts exsolve large amounts of fluids which can modify the chemical composition of granitic whole-rock samples. The niobium/tantalum (Nb/Ta) ratio is expected to decrease during the magmatic differentiation of granitic melts, but the behavior of both elements at the magmatic-hydrothermal transition remains unclear. Using a compilation of whole-rock geochemical data available in the literature, we demonstrate that fractional crystallization alone is not sufficient to explain the distribution of Nb-Ta in most peraluminous granites. However, we notice that most of the granitic samples displaying evidence of interactions with fluids have Nb/Ta < 5. We propose that the decrease of the Nb/Ta ratio in evolved melts is the consequence of both fractional crystallization and sub-solidus hydrothermal alteration. We suggest that the Nb/Ta value of ∼5 fingerprints the magmatic-hydrothermal transition in peraluminous granites. Furthermore, a Nb/Ta ratio of ∼5 appears to be a good marker to discriminate mineralized from barren peraluminous granites.
Rare-element pegmatites may host several economic commodities, such as tantalum (Ta-oxide minerals), tin (cassiterite), lithium
(ceramic-grade spodumene and petalite), and cesium (pollucite). Key geological features that are common to pegmatites in the
Superior province of Ontario and Manitoba, Canada, and in other large tantalum deposits worldwide, can be used in exploration.
An exploration project for rare-element pegmatites should begin with an examination of a regional geology map. Rare-element
pegmatites occur along large regional-scale faults in greenschist and amphibolite facies metamorphic terranes. They are typically
hosted by mafic metavolcanic or metasedimentary rocks, and are located near peraluminous granite plutons (A/CNK > 1.0). Once
a peraluminous granite pluton has been identified, then the next step is to determine if the pluton is barren or fertile.
Fertile granites have elevated rare element contents, Mg/Li ratio < 10, and Nb/Ta ratio < 8. They commonly contain blocky
K-feldspar and green muscovite. Key fractionation indicators can be plotted on a map of the fertile granite pluton to determine
the fractionation direction: presence of tourmaline, beryl, and ferrocolumbite; Mn content in garnet; Rb content in bulk K-feldspar;
and Mg/Li and Nb/Ta ratios in bulk granite samples. Pegmatite dikes with the most economic potential for Li-Cs-Ta deposits
occur the greatest distance (up to 10 km) from the parent granite.
Metasomatized host rocks are an indication of a nearby rare-element pegmatite. Metasomatic aureoles can be identified by their
geochemistry: elevated Li, Rb, Cs, B, and F contents; and by their mineralogy: presence of tourmaline, (Rb, Cs)-enriched biotite,
holmquistite, muscovite, and rarely garnet.
Once a pegmatite dike has been located, the next step is to assess its potential to contain Ta mineralization. Pegmatites
with the highest degree of fractionation (and thus the most economic potential for Li-Cs-Ta) contain blocky K-feldspar with
>3,000 ppm Rb, K/Rb < 30, and >100 ppm Cs; and coarse-grained green muscovite with >2,000 ppm Li, >10,000 ppm Rb, >500 ppm
Cs, and >65 ppm Ta. Pegmatites with Ta mineralization usually contain Li-rich minerals (e.g., spodumene, petalite, lepidolite,
elbaite, amblygonite, and lithiophilite) and may contain Cs-rich minerals (e.g., pollucite, Cs-rich beryl). The ore minerals
of Ta are commonly manganotantalite, manganocolumbite, wodginite, and microlite; Ta-rich cassiterite is also commonly present.
Tantalum mineralization tends to occur in albitic aplite, mica-rich (lepidolite, cleavelandite ± lepidolite), and spodumene/petalite
pegmatite zones.
The sources and formation conditions of unconventional Zr–Nb–REE mineralisation (REE = rare earth elements) presently found in increasing number worldwide are still poorly constrained. One particular problem is the specific role of magmatic and hydrothermal processes active in various geological settings. Investigation of Zr–Nb–REE mineralisation at Khalzan Buregte and Tsakhir, Western Mongolia, enables to evaluate magmatic processes preceding economic mineralisation and, in a second step, to compare similar ore-forming processes developing in host rocks of contrasting rock composition (low- vs. high-silica rocks). The genesis of the Zr–Nb–REE mineralisation is re-assessed using field observations, whole rock analysis (chemical composition, quantitative modal analysis by X-ray diffraction) and by the application of various transmitted light and electron microscopic techniques. Coarse-grained intrusive bodies, dikes and volcanic rocks of alkaline, silica-saturated composition were found to be contemporarily emplaced at subvolcanic to volcanic levels forming four alkaline massifs within the Khalzan Buregte area. The whole rock composition of weakly altered magmatic rocks ranges from syenite to quartz monzonite and alkaline granite (alkali feldspar syenite to alkali feldspar granite according to their modal composition). Magmatic and at least two subsequent hydrothermal processes contributed significantly to the formation of economic concentrations of high field strength elements (HFSE) such as Zr, Hf, Nb, Ta, REE and Y in the Khalzan Buregte deposit and in the nearby Tsakhir prospect. Mixing of magma from at least three sources and the formation of potassium feldspar cumulates resulted in local enrichment of Zr, Nb and light rare earth elements (LREE) in the rocks up to sub-economic levels. There was no significant increase in Y and heavy rare earth elements (HREE) during magmatism.
Igneous rocks with high Ta concentrations share a number of similarities such as high Ta/Nb, low Ti, LREE and Zr concentrations and granitic compositions. These features can be traced through fractionated granitic series. Formation of Ta-rich melts begins with anatexis in the presence of residual biotite, followed by magmatic crystallization of biotite and muscovite. Crystallization of biotite and muscovite increases Ta/Nb and reduces the Ti content of the melt. Titanium-bearing oxides such as rutile and titanite are enriched in Ta and have the potential to deplete Ta at early stages of fractionation. However, mica crystallization suppresses their saturation and allows Ta to increase in the melt. Saturation with respect to Ta and Nb minerals occurs at the latest stages of magmatic crystallization, and columbite can originate from recrystallization of mica. We propose a model for prediction of intrusion fertility for Ta.
Compositions of 45 ferrotapiolite-tantalite pairs from 12-granitic pegmatites were analyzed by electron microprobe to establish empirically the shape, extent and position of the two-phase field in the columbite quadrilateral. The two-phase field derived from coexisting minerals approximates an isothermal section through a pseudoquaternary solvus in the columbite quadrilateral through most of the course of its boundaries, as defined by primary Nb-enriched ferrotapiolite-tantalite pairs from blocky zones and albitic units of their parent pegmatites. Extremely fractionated mineral pairs most enriched in Mn and Ta come from late associations replacing simpsonite; they crystallized at distinctly lower temperatures. Boundaries of the two-phase field are controlled by temperature, f(O2), structural state, impurities disturbing the AB2O6 stoichiometry, and probably pressure. -from Authors
Cathodoluminescence (CL) imaging and spectroscopy, as well as backscattered electron imaging, were used to assign the occurrence of several mineral phases and rock structures in altered nordmarkites and calcite-bearing granites from the Nb-Zr- REE deposits from Khaldzan Buregte and Tsakhir (Mongolian Altai) to three events: (1) intrusion of barren nordmarkites; (2) intrusion of small bodies of calcite-bearing granites with metasomatic alteration of the wall-rocks; and (3) alteration by F-rich fluids.
Unusual red and yellow CL caused by Fe ³⁺ and Mn ²⁺ emission centres were detected in microcline and albite. Fe ³⁺ centres were also established (along with others) in quartz, zircon, and possibly in fluorite.
Magmatic and metasomatic rock structures and internal structures of the minerals coexist in the samples. The primary magmatic features were in part preserved during alteration. In contrast, the internal and the centre structures may be changed during alteration even in non-replaced mineral phases. Euhedral minerals may be formed by secondary processes as shown for lath-shaped albite. The occurrence of pseudomorphs, the inheritance of elements during replacement, and the mechanical effects of secondary minerals on earlier mineral phases during metasomatic growth are proposed as criteria for the reconstruction of the mineral succession in altered rocks. Snowball structures may be formed as a result of metasomatic alteration rather than as a magmatic intergrowth.
The Sapucaia pegmatite is located in the well-known Eastern Brazilian Pegmatitic Province (EBPP), Minas Gerais, Brazil. Detailed mapping of the pegmatite body revealed five zones, showing distinct mineral assemblages: a border zone (BZ), an external wall-zone (EWZ), an internal wall-zone (IWZ), an intermediate zone (IZ), and a quartz core (Q). Phosphate masses found in the pegmatite contains two different assemblages: assemblage I is formed under oxidizing conditions, and assemblage II is formed under less strongly oxidizing conditions. In both assemblages, triphylite is the only primary phosphate, and several secondary species are produced by its alteration. In association I, three main transformation stages are observed: (i) the progressive oxidation of triphylite accompanied by the leaching of Li leads to the successive crystallization of ferrisicklerite and heterosite; (ii) a hydration stage transforms triphylite into hureaulite, barbosalite, and tavorite, and (iii) a final stage, corresponding to meteoric alteration, the latest highly hydrated phosphate and oxide species. In assemblage I, ferrisicklerite is replaced by minerals like jahnsite s.l. and frondelite s.l. during the hydration stage, and in both assemblages, a second generation of Mn-rich triphylite (triphylite II) is observed. Two unusual petrographic textures were also observed, showing reactions between triphylite and albite to form montebrasite and garnet rims. These textures formed during the albitization stage, and correspond to the reactions triphylite + albite -> ferrisicklerite + montebrasite + quartz, and triphylite + albite -> almandine + quartz, respectively. Finally, the geochemical trends in phosphates and silicates indicate that Sapucaia is a weakly fractionated and geochemically primitive LCT-type pegmatite. The degree of differentiation of the pegmatite increases from the border to the core, as shown by a decrease of Fe/(Fe + Mn) in olivine-type phosphates, which decreases from 0.75 in the IWZ, to 0.71 in the IZ. Muscovite and tourmaline chemical compositions also show evidence of increasing degree of differentiation; for example, the Ga and Nb contents of muscovite increases from 57 and 49 ppm in the IWZ, to 86 and 80 ppm in the IZ, respectively.
The Zhaojinggou rare-metal deposit is located in the Wulashan-Daqingshan region, central north margin of the North China Craton. Nb-Ta mineralizations are mainly hosted in the biotite alkali-feldspar granite (BAG) and albite granite (AG), with subordinate mineralization in the alkali-feldspar aplite dyke (AFA), the amazonite granitic pegmatite dyke (AGP), and the greisen (black + white colored greisen). In this paper, we present a systematic work of geochronology (CGM U-Pb combined with cassiterite U-Pb, monazite U-Th-Pb, and muscovite ⁴⁰Ar-³⁹Ar dating), whole-rock geochemistry, and Sr-Nd isotopes to understanding the timing of mineralization, petrogenesis, geodynamic setting, and ore-forming processes. Our geochronological data reveal two episodic Nb-Ta mineralization events in the Zhaojinggou deposit during the Early Cretaceous. The first and major ore-forming episode occurred at ∼130 Ma, forming a sequence of ore-bearing rocks of the BAG-AG-greisen-AFA. The second but subordinate ore-forming episode occurred at ∼114 Ma, forming the ore-bearing AGP. The whole-rock geochemistry indicates the granitic rocks (the BAG-AG-AFA and AGP) from the two episodes are highly evolved with analogous trace element and REE patterns, and have A-type affinity. The whole-rock εNd(t) values of these granitic rocks are relative similar, ranging from -9.6 to -7.6, with corresponding T2DM ages of 1490-1713 Ma. These features suggest that these A-type granites from the two discrete episodes are derived from the same magma reservoir by high-temperature partial melting of the Middle Proterozoic basement in an infracrustal extensional environment. These A-type granites experienced extremely fractional crystallization during emplacement. The whole-rock REE tetrad effects, element ratios (Nb/Ta, Zr/Hf and K/Rb), and Nb-Ta fractionation modeling indicate that both magmatic and hydrothermal processes are involved to form the two episodic Nb-Ta mineralizations. High degree of factional crystallization and magmatic-hydrothermal processes after high-temperature anataxis are crucial to form the Zhaojinggou rare-metal deposit. Moreover, these highly-evolved A-type granites (the BAG-AG-AFA and AGP) formed during the Early Cretaceous (at ∼130 Ma and ∼114 Ma respectively) are the important targets for rare-metals prospecting in the central north margin of the NCC.
The newly discovered Zhaojinggou TaNb deposit in the northern margin of the North China Craton contains ore reserves of more than 38 Mt. at an average grade of 0.013 wt% Ta2O5 and 0.012 wt% Nb2O5. The TaNb mineralization is mainly hosted in a peraluminous, albite granite body, which is about 400 m long and 20 to 100 m wide, with a surface exposure of ~0.05 km². Granitic pegmatites and greisen veins that range from several centimeters to meters in width, occur in the margin of the granite body, and wolframite-quartz veins intrude the wall rocks up to tens of meters away from the granite contact. LA-ICP-MS UPb dating for columbite of the granite yields the lower intercept ages between 131.8 ± 1.0 Ma and 134.6 ± 2.6 Ma on the Tera-Wasserburg concordia diagram, indicating that the mineralization occurred in the early Cretaceous, coeval with the peak stage of lithospheric thinning and destruction of the North China Craton. The columbite grains in the granite are commonly euhedral, and are surrounded or penetrated by irregular tantalite patches/rims containing Ta2O5 as high as 59.5 wt%. The Ta concentrations within single columbite crystals commonly increase slightly from the center outwards, consistent with a systematic decrease of Nb/Ta caused by crystallization of columbite from the silicic magma. Mica in the granite shows disordered bright and dark domains in their BSE images; bright domains are F-rich zinnwaldite crystallized from highly evolved melts that were relatively enriched in Fe, Li, Rb, Nb and Ta, whereas the dark domains are F-poor muscovite crystallized from later hydrothermal fluids that were relatively enriched in Al, Mn, Ba, Sn and W. It is thus likely that the tantalite patches/rims around the columbite grains crystallized from a Ta- and F-rich hydrosilicate melt in a magmatic-hydrothermal transitional stage. The wolframite-quartz veins adjacent to the granite body likely formed from a W-rich hydrothermal fluid.
We studied newly found high Nb–Ta alkaline rhyolites in the northern volcanic belt of the Great Xing’an Range, China. The LA–ICP–MS U–Pb weighted mean age is 114.07 ± 0.55 Ma, indicating that the rocks formed during the late Early Cretaceous and were the product of the late eruption of a Mesozoic volcano. The major element contents are characterized by high Si, rich K, low Fe, and poor Ca and Mg. In the total alkaline–silicon diagram, the sample points are in the alkaline rhyolite region. Meanwhile, rare earth elements show obvious Ce/Ce* positive anomalies and Eu/Eu* negative anomalies. In addition, trace elements are characterized by high Nb, Ta, and Yb, and low Sr. The two-stage Nd isotopic model age T 2DM of the depleted mantle is between 799–813 Ma, indicating that the diagenetic material originated from the depleted mantle or partial melting of newly formed young crustal materials. The source rocks melted at a relative shallow depth (<30 km), under lower pressure (<0.5 Gpa) and high oxygen fugacity; moreover, the residues in the source region were Ca-rich mafic plagioclase + amphibole + orthopyroxene. In the Nb–Y–3Ga and Nb–Y–Ce diagrams, the sample points are in the A1 type region. It can be concluded that the mantle-derived basaltic magma underplated and supplied the heat sources for partial melting of the metamorphic crustal rocks in an intraplate extensional tectonic environment related to a rift, mantle plume, and hot spot.
The Zhaojinggou Nb-Ta deposit is a newly explored large-scale deposit in Wuchuan County, Inner Mongolia. The Nb-Ta orebody is mainly hosted in the amazonite-bearing albite granite, and partly hosted in the granite aplite, greisen and amazonite granite pegmatite. In this paper, we use monazite and zircon LA-MC-ICP-MS U-Pb and biotite ⁴⁰ Ar- ³⁹ Ar dating methods to constrain the mineralization age. Monazite LA-MC-ICP-MS U-Pb geochronological dating of four samples from granites yields ages of 124±2 Ma (MSWD=2.9, n=27), 124±3 Ma (MSWD=2.0, n=46), 121±1 Ma (MSWD=3.3, n=43) and 124±2 Ma (MSWD=3.2, n=12). One zircon sample from albite granite yields age of 125±1 Ma (MSWD=1.6, n=18). Meanwhile, biotite ⁴⁰ Ar- ³⁹ Ar dating obtains the plateau age of 133.84±0.79 Ma (MSWD=3.31), isochron age of 134.55±0.79 Ma (MSWD=1.93) and reverse isochron age of 134.58±0.80 Ma (MSWD=1.99). Therefore, all these ages constrain that the ore-forming period of Zhaojinggou Nb-Ta deposit is Early Cretaceous. In conclusion, the Zhaojinggou deposit resulted from the tectonic-magmatic activities of extensional setting, and the Yanshanian Period is the main Nb-Ta metallogenic age in Inner Mongolia. © 2019, Editorial Department of Earth Science. All right reserved.
In rare-metal granites, niobium and tantalum are generally hosted by Nb-Ta oxides. However, in SE China, the Nb-specialized Huangshan granites are a unique occurrence in which Nb is essentially hosted by Li-Fe micas. The Huangshan granites are part of the Early Cretaceous (Late Yanshanian) Lingshan granite complex and belong to the A-type granite series, with two facies differing by their mica compositions: medium-grained "protolithionite" granite and medium-grained lithian (lithium-rich) annite granite. The granites are characterized by elevated whole-rock Nb contents (average 144 ppm in "protolithionite" granite and 158 ppm in annite granite), quite low Ta contents (average 9 and 4 ppm, respectively), leading to very high Nb/Ta ratios (average 15.3 and 31.2). Niobium is mainly hosted in the micas, with an average Nb content of 1347 ppm in the lithian annite and 884 ppm in the "protolithionite," which is the highest ever reported in granitic mica. With an estimated endowment of ~80 kt Nb, the Huangshan granites represent a new style of potential Nb resource. Contrasting with the great rarity of columbite, there is abundant Hf-rich zircon, Y-rich fluorite, and Th-rich fluocerite included in the Huangshan micas. Such accessory minerals being typical of alkaline rhyolitic magmas and niobium enrichment in the Huangshan granites results from A-type melt. The extreme Nb enrichment in the micas results from the highly compatible behavior of Nb in this melt, combined with the high magma temperature (estimated at 790-800 °C) and possibly enhanced magma oxidation.
Granites and pegmatites in the Strange Lake pluton underwent extreme enrichment in high field strength elements (HFSE), including the rare earth elements (REE). Much of this enrichment took place in the most altered rocks, and is expressed as secondary minerals, showing that hydrothermal fluids played an important role in HFSE concentration. Vasyukova et al. (2016) reconstructed a P-T-X path for the evolution of these fluids and provided evidence that hydrothermal activity was initiated by exsolution of fluid during crystallisation of border zone pegmatites (at ~450–500 °C and 1.1 kbar). This early fluid comprised a high salinity (25 wt% NaCl) aqueous phase and a CH4 + H2 gas. During cooling, the gas was gradually oxidised, first to higher hydrocarbons (e.g., C2H6, C3H8), and then to CO2, and the salinity decreased to 4 wt% (~250–300 °C), before increasing to 19 wt%, due to fluid-rock interaction (~150 °C). Here, we present crush-leach fluid inclusion data on the concentrations of the REE and major ligands at different stages of the evolution of the fluid.
The chondrite-normalised REE profile of the fluid evolved from light REE (La-Nd)-enriched at high temperature (~400 °C, Stages 1–2a) to middle REE (Sm-Er)-enriched at 360 to 250 °C (Stages 2b–3) and strongly heavy REE (Tm-Lu)-enriched at low temperature (150 °C, Stage 5). These changes in the REE distribution were accompanied by changes in the concentrations of major ligands, i.e., Cl⁻ was the dominant ligand in Stages 1, 2, 4 and 5, whereas HCO3⁻ was dominant in Stage 3.
Alteration of arfvedsonite to aegirine and/or hematite contributed strongly to the mobilisation of the REE. This alteration released middle REE (MREE) and heavy REE (HREE), which either partitioned into the fluid or precipitated directly as bastnäsite-(Ce), ferri-allanite-(Ce) or gadolinite-(Y). Replacement of primary fluorbritholite-(Ce), which crystallised from an immiscible fluoride melt and altered to bastnäsite-(Ce), was also important in mobilising the REE (MREE).
This paper presents the first report of the distribution of the REE in an evolving hydrothermal fluid. Using this distribution, in conjunction with information on the changing physicochemical conditions, the study identifies the sources of REE enrichment, reconstructs the path of REE concentration, and evaluates the REE mineralising capacity of the fluid. Finally, this information is integrated into a predictive model for REE mobilisation applicable not only to Strange Lake but any REE ore-forming system, in which hydrothermal processes were important.
The Yichun Ta-Nb deposit, which is located in Jiangxi Province, China, can be divided into four lithological zones (from bottom upward): two-mica granite, muscovite granite, albite granite, and lepidolite-albite granite zones. It remains controversial whether these distinct vertical zones were formed through late magmatic-hydrothermal metasomatic alteration or fractional crystallization of magma. To investigate the evolution mechanism of rock- and ore-forming fluid in this deposit, we studied fluid and melt inclusions in quartz and lepidolite in these four granite zones. These fluid inclusions are mainly composed of H2O-NaCl, and have homogenization temperatures ranging from 160°C to 240°C, with densities between 0.86 and 0.94 g/cm³ and salinities between 0.5 and 6.5 wt% NaCl equivalent. Raman spectroscopic analyses showed that the daughter minerals contained in silicate melt inclusions are mainly quartz, lepidolite, albite, muscovite, microcline, topaz, and sassolite. From the lower to upper granite zones, the albite contents in silicate melt inclusions increase, while the muscovite contents decrease gradually until muscovite is substituted by lepidolite in the lepidolite-albite granite zone. Additionally, the calculated densities of the silicate melt inclusions exhibit decreasing trends from bottom upward. The total homogenization temperatures of silicate melt inclusions, which were observed under external pressures created in the sample chamber of a hydrothermal diamond-anvil cell, decreased from 860°C in the lower lithological zone to 776°C in the upper lithological zone, and the initial melting temperatures of solid phases were 570-710°C. The calculated initial H2O contents of granitic magma showed an increasing trend from the lower (∼2 wt％ in the two-mica granite zone) to the upper granitic zones (∼3 wt％ in the albite granite zone). All of these features illustrate that the vertical granite zones in the Yichun Ta-Nb deposit formed through the continuous fractional crystallization of the granitic magma. Additionally, the low H2O contents and gradual enrichment of incompatible elements (F, B, Li, Ta, Nb, etc.) in the residual granitic magma favored the formation of a granite-type Ta-Nb deposit.
Daqing Shan in central Inner Mongolia had experienced a complex structural evolution during the Late Mesozoic. The Panyang Shan Thrust (PST), the Hohhot metamorphic core complex (Hohhot MCC), the Daqing Shan thrust-nappe system (DST) and high angle normal faults are the major structures formed during this stage. However, the temporal and special relationships among these events are still not clear. Based on field observations, we give a chronological sequence on them. Then we measured the Ar-40-Ar-39 ages of different minerals from the detachment fault zone of the Hohhot MCC and explain the tectonic implications of these ages based on the newly constructed structural evolution of the Daqing Shan area. The south-directed PST overlay pre-Cambrian rocks on Late Paleozoic-to-Mesozoic rocks. The PST was active at roughly the same time as the deposition of the Daqingshan Formation during the time from Late Jurassic to Early Cretaceous. The Hohhot MCC consists of metamorphic core complex, hanging wall unites and detachment faults. The metamorphic core complex is composed of pre-Mesozoic metamorphic rocks and Late Mesozoic granitoids. The hanging wall is made of pre-Cambrian low grade metamorphic rocks and syn-extensional sediments. There are three detachment faults outcropped representing different evolutionary phases of the detachment fault. The detachment faults from south to north are Hohhot fault zone, Xiaojing fault zone and Deshengying fault zone, which are the master detachment fault, early and deep detachment fault, and late and shallow fault, respectively. Lineation and fabrics of all these ductile shear zones indicate a common top-to-southeast shear in the detachment faults zone. Zircon ages of granitoids which intruded into the detachment fault zones indicate that the deformation of ductile shear zone had ended before similar to 131Ma, much earlier than the Ar-40-Ar-39 ages of the detachment fault zones. The DST is made up of a series of top-to-the-north or to-the-northwest reverse faults. These faults moved Archean crystalline rocks, Proterozoic gneissic granite, and Proterozoic low-grade metamorphic rocks on top of the autochthonous system composed of Paleozoic-to-Mesozoic rocks and the younger rocks. The DST cut the Hohhot fault zone and the mylonite was a part the hanging wall of DST. Both newly formed syn-deformational phyllonite in the fault zone and hornblende from a weakly deformed granodiorite dike that intruded into the thrust fault have Ar-40-Ar-39 ages of similar to 120Ma, representing the time when these fault were active. Then, the Daqing Shan area entered into a new south-north directed extensional stage represented by high angle normal faults. These brittle normal faults strike east-west and cut all the above structures. Combined with previous published papers, we estimate these normal faults were mostly active between 100Ma and 90Ma. Though Hohhot MCC is one of the most intensely studied MCCs in China, the cooling history and uplifting mechanism of its ductile shear zone is unclear. Here we adopts step heating Ar-40-Ar-39 method to date the cooling ages of different minerals from the ductile shear zone of Hohhot MCC, and 4 samples of hornblende, muscovite, biotite and K-feldspar give cooling ages ranging from 116Ma to 120Ma. A cooling curve is constructed according to the cooling ages and the related closure temperatures of the various minerals dated by Ar-40-Ar-39 method from this paper and Davis and Darby (2010), zircon U-Pb ages of the granitoids intruded into the detachment fault zone and apatite fission track ages of the fault zone. The cooling curve shows that the ductile shear zone experienced a rapid uplifting since 122Ma to 115Ma. Since the deformation of detachment fault zone had ended before similar to 131Ma, the Ar-40-Ar-39 ages do not represent the rapid lifting stage of the ductile shear zone which was caused by the detachment fault zones themselves. However, this period is roughly in consistent with the dating data from the DST fault zone. Combined with the field observations that the DST cut the ductile shear zones of Hohhot MCC, we propose the rapid cooling of the detachment fault zone from 122Ma to 115Ma is the result of the DST, in which the Hohhot MCC played as a part of its hanging wall.
Two types of greenstone belts occurring within Archeozoic and Proterozoic respectively have been identified in the Daqingshan - Wulashan district, which belongs to central part of Inner Mongolia autonomous area. The average Au contents of Archeozoic and Proterozoic greenstone are 2.8 × 10-9 - 32 × 10-9, 2.6 × 10-9 - 34 × 10-9 respectively. Compared with the other types of rocks, the greenstone is characterized with high Au content, and always occur the large or middle scale gold deposits. The gold deposits of the study area may be classified as five types: (1) stratoid veinlet-disseminated type, such as the Motianling gold deposit; (2) sedimetamorphic rocks stratabound type, such as the Maoduqing and Youlougou gold deposits; (3) K-feldspar-quartz vein type, such as the Wulashan gold deposit; (4) altered rock-quartz vein composite type, such as the Donghuofang gold deposit; (5) quartz vein type, such as the Houshihua gold deposit. The former two types belong to the Proterozoic greenstone type gold deposit, and the latter three types belong to the Archeozoic greenstone type gold deposit. According to the Xie Xuejin's theory of geochemical prospecting and the geological features of the study area, six geochemical block had been choosed as ore-finding prospective area: (1) the Xihezi - Xindigou - Hongpan Au geochemical block; (2) the Yinhao - Erdaowa - Shanggaotai Au geochemical block; (3) the Amawusu - Xishanwan - Bainaimiao Au-Cu geochemical block; (4) the Heiaobao - Saiwusu - Chagancilao Au geochemical block; (5) the Jinpen - Mamitu - Manzhouli Au-Cu geochemical block; (6) the Wulashan - Yushuwan Au geochemical block. It is estimated that the Au productivity reach to 67550t within the six geochemical blocks, which indicates that there is good metallogenetic material foundation and ore-finding prospective potential in the study area.
The columbite-group minerals from the Koktokay No. 3 granitic pegmatite, Altay, NW China were studied in the paper by using the electron microprobe analysis and back-scattered electron imaging. The results indicate that the columbite-group minerals in this district are mainly Mn-enriched, which are mainly manganocolumbite and manganotantalite. From the border zone to the core of the pegmatite, the ratio of Ta/(Nb+Ta) varies almost vertically with nearly constant ratio of Mn/(Fe+Mn). The chemical compositions of the columbite-group minerals from the earlier magmatic stage vary limitedly, while those from the later magma-hydrothermal transitional stage vary over a wide range. The back-scattered electron images show that the compositional zonation is not clear in columbite-group minerals from the magmatic stage (zones I ∼ IV), while those from the magma-hydrothermal transitional stage (zones V ∼ VII) exhibit a complex compositional zonation. Indeed, the zonation patterns vary from the progressive, via the oscillatory to the replacement structure during the consolidation of the granitic pegmatite. The study indicates that the composition variations of the columbite-group minerals from different zones of pegmatite and their compositional zonation are due to hydrothermalism during the crystallization of columbite-group minerals, and the zonation patterns are mainly controlled by the degree of hydrothermalism.
Western Sichuan is an important area of rare metal resources in China. Keeryin pegmatite type rare metal ore deposit is located at the center of the Songpan - Garze orogenic zone, western Sichuan. To study it can trace the evolution of the Songpan - Garze orogenic zone. In this paper, 40Ar/39Ar plateau age of 176.25±0.14 Ma of muscovite from the muscovite - microcline pegmatite in Genze intrusive rock and the age of 152.43± 0.60 Ma from Dangba lithium deposit are gotten. Considering the genesis of the granites and published K-Ar, Rb-Sr, U-Pb isotope ages, it is concluded that in the late of Indosinian epoch and the early of Yanshanian epoch, large scale magmations happened in Keeryin region, and lasted a long time. But rare metal mineralization took place in steady and close geologic environment in the late of magmations. These geological phenomena showed that the Songpan - Garze orogenic zone evolved into stable period until the beginning of Himalayan movement. The stable period is ready for ore-forming of many type deposit.
The Longwo and Baishigang plutons, locating in the eastern end of the EW-trending Fogang granite belt, are typical examples of granitic rocks formed by crust-mantle interaction in Nanling region. Zircon LA-ICP-MS U-Pb dating yields an age of (169.1 ± 2.5) Ma for the Longwo pluton and an age of (157.8 ± 2.3) Ma for the Baishigang pluton, indicating that both of them were formed in early Yanshanian. The major and trace element contents of biotites are measured by employing EMPA and LA-ICP-MS techniques, and their petrogenetic significances are discussed. The EMPA analyses of major elements demonstrate that biotites of the Longwo pluton are distinguished from that of the Baishigang pluton by higher abundances of TiO2, MgO, and lower concentrations of FeO, Al2O3 and volatile constituents (e.g., F and Cl), indicating that the Longwo pluton was likely to have formed under a relative high temperature and a more oxidated environment. Trace elements analyses by LA-ICP-MS technique illustrate that biotite is an important carrier of Rb, Ba, Nb, Ta, Sc, V, Cr, Co, Ni in granitic melts, but has less ability in hosting U, Th, Pb, Sr, Zr, Hf, Y, and thus the concentrations of these elements in biotite are much lower than that in the host rocks. Biotite in granitic rocks also displays quite low REE concentrations, suggesting that it is not the main mineral phase affecting the REE features of the host rocks. The mineral chemistry of biotite can be used as a powerful tool in identifying the differentiation degree of the host magmas and in evaluating the rock-forming physical-chemical conditions, and can also be employed in tracing the nature of the magma source. But in the latter aspect, other data (especially isotope compositions) should be coupled with in order to obtain more accurate information.
Tantalum and niobium mineralization is often associated with geochemically specialized granites which are characterized by enrichment in fluorine, and by the development of pervasive, postmagmatic alteration. Three major varieties of granite can be recognized:
1.
Alkali granites containing alkali pyroxenes and/or amphiboles which are characterized by high Fe, F, Nb, Zr, Rb, Sn and REE, and by low CaO, Ba, Sr and Ta/Nb. These granites occur principally in anorogenic settings, and are associated mainly with niobium mineralization.
2.
Biotite and/or muscovite granites often containing Fe-Li micas which are characterized by high F, Rb and Sn, and by low CaO, Ba, Sr and Eu. These granites occur in both anorogenic and postorogenic settings, and are associated principally with Nb-Ta(-Sn) mineralization.
3.
Lepidolite-albite granites often containing topaz which are characterized by high A12O3, F, Li, Rb, Sn, Ta and Ta/Nb, and by low Ba, Sr, Eu, Zr and REE. These granites occur principally in postorogenic settings, but also form marginal facies varieties of biotite and/or muscovite granites in both anorogenic and postorogenic settings. Lepidolite-albite granites are generally associated with Ta(-Nb-Sn) mineralization.
Many granites associated with tantalum and niobium mineralization are zoned vertically, with the finer-grained porphyritic, granophyric, pegmatitic and/or miarolitic rocks which occur in small cupolas grading downwards into medium- and coarse-grained granite. This vertical textural zonation indicates an important concentration of volatile and ore elements in the roof of the plutons from the earliest stages of crystallization.
The mineralogical and geochemical features of rare-metal granites reflect features of their source rock compositions, as well as magmatic fractionation and postmagmatic alteration processes. Trends towards increasing A12O3, Na2O, F and a variety of rare metals in the most evolved rocks may reflect crystal and/or liquid fractionation during magma evolution and/or postmagmatic alteration by interaction with fluorine-rich hydrothermal fluids.
Detailed petrographic and mineralogical study of a variety of examples is required before geochemical and isotopic modelling of these rare-metal granites can advance beyond the preliminary stage.
Hukeng granite intrusion locates in southeast limb of Wugongshan compound anticline, where developed large scale Hukeng tungsten deposit that is an important part of Wugongshan W-Cu-Bi-Mo poly-metallic metallogenic belt. Through precise LA-ICPMS U-Pb dating of zircon from Hukeng granite, the crystallization age of the intrusion was determined to be 151.6 ± 2.6Ma, corresponding to Late Jurassic era and ascribed to the first intruding event of granites in Mesozoic era in South China (164-153Ma). The formation temperature of granite magma was 676-695°C. Through the analysis of major and trace elements of Hukeng muscovite granites, they belonged to high-K calc-alkaline series and were thought to be metaluminous - weak peraluminous differentiation S-type granites. They should mainly derive from clay-enriched argillaceous rock source in upper crust and form under lithospheric extensional and thinning environment.
Late Mesozoic extension tectonics was quite significant on eastern Eurasia Continent and it was expressed by extensional domal structure such as the metamorphic core complexes, syntectonic plutons, and ductile detachment fault, even graben and half graben basins. According to our field observations, laboratory work and previous research, from north to south, five extension belts have been separated: Transbaikalia-Okhotsk belt, western part of North China Block, eastern part of North China Block, south margin of North China Block and Qinling-Dabie belt and South China Block. As a largest extension tectonic at the scale of crust in the world, these entire belts have NW-SE extensional direction and this extensional structure make the middle to lower curst rocks exhumed to the surface along the detachment normal fault. Geochronological work on these ductile detachment fault indicated a very narrow period around 130 ∼ 126Ma except the eastern part of North China Block, which have a width extensional period relatively. Lithosphere delamination could be considered as the geodynamic of this large scale extension tectonic. This geodynamic model could make us to understand the time, scale, and mechanism on the topic of North China Craton destruction from the view of structure analyses.
Ongonites were defined at their type locality at Ongon Khairkhan, central Mongolia, as pristine magmatic topaz-bearing albite–quartz-keratophyres with up to 4 wt. % F and containing phenocrysts of albite, K-feldspar, quartz and rare mica and topaz hosted in a groundmass composed of the same minerals. However, detailed petrographic and SEM-EDS studies indicate that these rocks underwent considerable subsolidus exchange with deuteric fluids, as evidenced by the presence of albitic plagioclase (Ab ~100) and end-member orthoclase (Or ~100), secondary Li–Fe-rich mica (zinnwaldite) enriched in rare metals (Sn, W, Ta, Nb), pitted feldspars containing fluid inclusions, and disseminated fluorite. The ~ 120 Ma old dyke rocks, emplaced at a high structural level in the crust, are strongly peraluminous leucogranites characterized by high Al and alkalis that are also enriched in Rb, Cs, Ga and Ta, depleted in Mg, Ca, Zr, Ba, Sr and Eu, and have anomalous K/Rb, Rb/Sr, Zr/Hf and Nb/Ta ratios compared to the average continental crust. However, the suite has Nd isotopic ratios (ɛ Nd(120) ~−1) similar to those of con-temporaneous A-type granites of the Mongolian–Transbaikalian igneous province of the Central Asian Orogenic Belt. The inferred primary δ 18 O (~+6 to +7‰) and Pb isotopic values are consistent with a granitic parent magma and interaction with orthomagmatic fluids. The ongonites and constituent minerals record (1) an extensive and protracted crystal fractionation history, in part due to the presence of volatiles (particularly F) which depressed the solidus temperature of the felsic rocks and extended its duration of crystallization and (2) subsolidus exchange with fluids which includes late flux of heated meteoric water as indicated by modified whole rock δ 18 O values (+ 0.5 to +2.7‰). The interaction of the ongonites with internally derived orthomagmatic fluids is considered to result in enrichment and/or redistribution of several incompatible elements , but not to have greatly modified their original major element chemistry which indicates that this suite represents the last stages of fractionation of a highly differentiated, F-rich granitic magma. The escape of these evolved melts from the apical part of the underlying pluton is now represented by the ongonite dykes. Late-stage magmatic, water-rich fluids enriched in incompatible elements including Nb, Ta, Sn and W were responsible for the late-to post-magmatic alteration and associated W mineralization.
A combined compositional and optical spectrophotometric study of 24 biotite specimens from the granitic rocks of the Hepburn and Bishop intrusive suites of the early Proterozoic Wopmay orogen, Northwest Territories, shows that the chemical composition and the color of this mineral strongly reflect the tectonic origin of its host. The biotite quadrilateral effectively portrays the compositional trends of micas from continental-collision- and arc-related granites. -from Authors
Trace element content has been estimated for biotite and muscovite of hercynian porphyritic biotite granites and two-mica granites of Central Portugal. In the two-mica granites, biotite and muscovite together account for more than 80 % of Li, V, Cr, Zn, Nb, Sn and W, as well as significant proportions (30-75 %) of F, Cl, Co, Ga, Rb, Y, Cs and Ta. Sn and W are highly concentrated in the white mica (83 %), providing an important source for ore genesis when muscovite undergoes breakdown reactions. In the porphyritic biotite granites, biotite alone accounts for more than 75 % of Li, V, Zn, Nb and W, and between 30 to 75 % of F, Cl, Cr, Ga, Rb, Y, Sn. Cs and Nd. As expected, muscovite and biotite usually host small proportions (< 30 %) of Cu, Sr, Zr, Ba, La, Ce, Nd, Hf, Pb, Th and U, which confirms the preference of these elements for feldspars and accessory minerals in granitoid rocks. Trace element partitioning between biotite and muscovite of the two-mica granites has been evaluated, and average partition ratios (DBt/Ms ±1σ) are as follows: Zn (8.6 ± 2.5), Ni (8.4 ± 4.4), Co (5.2 ± 2.0), Cs (4.3 ± 1.3), Li (4.0 ± 2.1), Cr (3.6 ± 1.3), Nb (2.8 ± 0.6), Ta (2.5 ± 1.2), V (2.2 ± 0.4), F (1.8 ± 0.5), Rb (1.6 ± 0.3). Cl (1.4 ± 1.5), Pb (1.2 ± 0.5), Sr (0.7 ± 0.4), Sc (0.6 ± 0.6), Ba (0.6 ± 0.2), Ga (0.6 ± 0.1), Sn (0.4 ± 0.1) and W (0.4 ± 0.1).
The ash-fall and outflow sheets of the 0.7-m.y.-old Bishop Tuff represent >170 km 3of compositionally zoned rhyolitic magma emplaced during collapse of the Long Valley caldera, California. Field, mineralogic, and chemical evidence agree that tapping of the thermally and chemically zoned chamber was continuous, without interruptions sufficient to permit mixing or phase re-equilibration. Fe-Ti oxide temperatures for 68 glassy samples increase systematically with eruptive progress from 720 to 790 °C; this increase corresponds well with the stratigraphic sequence, but the temperatures in no way correspond to the degree of welding. Ubiquitous quartz, sanidine, oligoclase, biotite, ilmenite, titanomagnetite, zircon, and apatite change composition progressively with temperature. The uniformity within every sample of each mineral species (irrespective of size and whether discrete or as inclusions) is not compatible with protracted crystal settling. Euhedral allanite (ρ > 4, La + Ce > 16% by weight) occurs in all early-erupted samples (720 to 763 °C) but in none erupted later. Despite this, whole-rock La + Ce values increased threefold during the eruption. Pyrrhotite, hypersthene, and augite appear abruptly at 737 °C and occur in all later samples. These sharp isothermal interfaces indicate lack of any extensive history of crystal settling.
Whole-rock major-element gradients were modest, but many trace-element concentration gradients were very steep despite a drop of only ∼2% by weight SiO 2within the magma volume erupted. Enrichment factors (the ratio of the value in the early-erupted samples to the value in the late-erupted samples) are Ba, 0.02; Sr, <0.1; Mg, <0.1; P, 0.17; Eu, 0.12; La, 0.3; Yb, 2.35; Mn, 1.6; Sc, 1.65; Y, >2; Ta, 2.5; U, >2.5; Cs, 3.8; Nb, >5; Rb/Sr, 22; Mg/Fe, 0.1; Ce/Yb 0.2; Eu/Eu*, 0.07; Zr/Hf, 0.65; Ba/K, 0.02; and Ba/Rb, 0.01. These can neither have been established by transfer of any reasonable combination of phenocrysts nor inherited from progressive partial melting. Abundant bulk and phenocrystic data further exclude large-scale assimilation, liquid immiscibility, and contamination by underplating mafic magma.
The compositional and thermal gradients existed in the liquid prior to phenocryst precipitation and developed largely independently of crystal-liquid equilibria. Within water- and halogen-enriched high-silica roof zones of large magma chambers, chemical separations take place through the combined effects of convective circulation, internal diffusion, complexation, and wall-rock exchange to develop compositional gradients, which are linked to gradients in the structure of the melt and are controlled by the thermal and gravitational fields of the magma chamber itself. Liquid-state differentiation through processes of convection-driven thermogravitational diffusion probably requires progressive establishment of a stable density gradient in order to retard convective re-mixing of the zoned upper part of the system. These processes evidently produce differentiated tops on magma bodies that represent a wide range in initial bulk composition. The degree of enrichment within a given chamber probably reflects the repose time between eruptions, the volatile flux, and the rate of energy transfer from the mantle more than it does the bulk composition of the unerupted dominant volume. Dikes and stocks injected above such a system will be either barren of or enriched in elements susceptible to subsequent hydrothermal concentration to ore grades; enrichment or barrenness depends on the timing of emplacement relative to cycles of enrichment and eruption.
Crystal-liquid equilibria probably predominate during initial generation of the dominant magma volume as well as during its ultimate plutonic consolidation. Thermogravitationally generated, strongly differentiated capping magmas commonly erupt, but some crystallize as alaskites, leucogranites, and granite porphyries, and some may be resorbed by the dominant volume during the waning stages of a pluton’s magmatic lifetime.
Columbite–tantalite (Coltan) is the most important niobium (Nb)- and tantalum (Ta)-bearing economic mineral, commonly occurring in rare metal granite and pegmatite, alkaline granite, syenite and carbonatite. Its high U but low common Pb contents make it an ideal mineral for U–Pb isotopic dating of Nb–Ta mineralization. In order to establish a feasible coltan dating method by in situ laser-ablation (LA) ICP–MS, we determined the U–Pb ages of five coltan samples from different pegmatites and rare-metal granites in China. In order to evaluate the potential matrix effect between different minerals, a 91500 zircon was used as external standard during analyses. The results show that, compared to the recommended ages, approximately 7–15% younger ages were yielded for the analyzed coltan samples in both single spot and line raster scan analytical methods, indicating a significant matrix effect between coltan and zircon. However, by using a coltan standard from Namibia (Coltan139), the coltan sample from Dahe pegmatite (SNNT) has a weighted mean 206Pb/238U age of 363 ± 4 Ma (2σ, n = 25) and 357 ± 5 Ma (2σ, n = 20) in single spot and line raster scan analytical methods, respectively; the coltan samples from Altai No.3 pegmatite (713-79), Yichun topaz-lepidolite granite (Yi-1) and Huangshan albite granite (LS-15) have weighted mean 206Pb/238U ages of 218 ± 2 Ma (2σ, n = 20), 160 ± 1 Ma (2σ, n = 20) and 130 ± 1 Ma (2σ, n = 20), respectively, in single spot mode. These ages agree well with the previously published data, and hence support the reliability of our analytical method. Although the analyzed coltan minerals show a large variation of chemical compositions, no significant matrix effect was observed, which suggests that a coltan material should be used as an external standard for U–Pb dating of coltan by LA–ICP–MS. Using the established analytical protocol, we date the Nanping pegmatite (NP155), a main Nb–Ta deposit in China without known age, and obtain a weighted mean 206Pb/238U age of 391 ± 4 Ma (2σ, n = 20), which is considered as the best estimation of Nb–Ta mineralization time in the area.
Phase-equilibrium studies in the system FeO-Fe2O3—TiO2 permit determination of the temperature and oxygen fugacity of formation of coexisting pairs of titaniferous magnetite and
ilmenite in many rocks. Temperatures thus obtained are probably accurate to ±50°C. Temperatures indicated for most igneous
and metamorphic rocks for which data are available are generally consistent with temperatures inferred by other methods. Temperatures
for certain gabbroic rocks are too low for magmatic crystallization and probably reflect the migration of ilmenite from titaniferous
magnetite to form separate granules upon cooling.
The experimentally determined solubility of ilmenite in magnetite is much too small to account for most ilmeno-magnetites
by simple exsolution. Subsolidus oxidation of magnetite-ulvöspinelgSS to yield ilmenite-magnetite intergrowths has been experimentally verified and probably takes place during cooling of many
igneous and perhaps some metamorphic rocks. Oxidation at surface or hypabyssal conditions may produce metastable titanomaghemites.
In order of increasing intensity of oxidation, the following Fe—Ti oxide pairs occur in plutonic rocks: ulvöspinel-rich magnetiteSS+ilmeniteSS, ulvöspinel-poor magnetiteSS+ilmeniteSS, ulvöspinel-poor magnetiteSS+hematiteSS, hematiteSS+rutile.
The Thor Lake rare metal (Zr, Nb, REE, Ta, Be, Ga) deposits in Canada's Northwest Territories represent one of the largest resources of zirconium, niobium, and the heavy rare earth elements (HREE) in the world. Much of the potentially economic mineralization was concentrated by magmatic processes. However, there is also evidence of autometasomatic processes and remobilization of Zr and REE by hydrothermal fluids. The deposits are situated at the southern edge of the Slave province of the Canadian Shield, within the 2094 to 2185 Ma alkaline to peralkaline Blachford Lake Intrusive Complex. A layered alkaline suite dominated by aegirine nepheline syenite occurs in the center of this suite of rocks and is considered to represent the youngest phase of the complex. Much of the rare metal mineralization occurs in two subhorizontal tabular layers, which form upper and lower zones of the Nechalacho deposit (formerly the Lake zone), and in which Zr is hosted primarily by zircon, Nb primarily by ferrocolumbite and fergusonite-(Y), and HREE by fergusonite-(Y) and zircon. The LREE are present mainly in monazite-(Ce), allanite-(Ce), bastnäsite-(Ce), parisite-(Ce), and synchysite-(Ce). Much of the HREE mineralization in the lower mineralized zone occurs in secondary zircon, which forms small (10-30 μm) anhedral grains in pseudomorphs after probable eudialyte. In the upper zone, zircon is a magmatic cumulate mineral, which was replaced locally by secondary REE-bearing minerals. Element distribution maps of zircon crystals in the upper zone indicate that the HREE were mobilized from the cores and locally precipitated as fergusonite-(Y) along microfractures. The light rare earth elements (LREE) were also mobilized locally from both primary zircon and inferred primary eudialyte. The occurrence of zircon in fractures, wrapped around brecciated K-feldspar fragments, and as a secondary phase in pseudomorphs are evidence of its hydrothermal origin and/or of remobilization of primary zirconium. A model is proposed in which injection of separate pulses of miaskitic and agpaitic magma resulted in the crystallization of an upper zone rich in zircon and a lower zone rich in eudialyte. Primary eudialyte was later altered in situ to zircon-fergusonite-(Y)-bastnasite-(Ce)-parisite-(Ce)- synchysite-(Ce)-allanite-(Ce)-albitequartz-biotite-fluorite-kutnahorite- hematite-bearing pseudomorphs by an inferred fluorine-enriched magmatic hydrothermal fluid. Zirconium, niobium, and REE in both the upper and lower zones were subsequently mobilized during multiple metasomatic events, which, for the most part, served to further enrich the primary layers in REE (albitization generally dispersed REE and high field strength elements (HFSE)) and created new secondary REE-bearing phases.
The Pilbara pegmatite province in the Archean North Pilbara Craton contains at least 120 pegmatite deposits in over 27 pegmatite groups and fields, including the giant Mount Cassiterite tantalum orebody in the Wodgina pegmatite district. Generally, rare metal pegmatites in the province are hosted by mafic-ultramafic volcanic-dominated supracrustal sequences of predominantly greenschist facies, adjacent to, and rarely within, domal multiphase granitoid-gneiss complexes. The most fractionated pegmatites are hosted by tongues or inliers of greenstone belts protruding into or within the larger granitoid complexes. Most of these are within two of the six tectonostratigraphic domains of the East Pilbara granite-greenstone terrane of the North Pilbara Craton. They tend to be clustered along and within 5 km (at surface) of major faults and craton-scale lineaments that coincide with or are parallel to domain boundaries. All of the major tantalum deposits (Wodgina, Mount Cassiterite, Tabba Tabba, Strelley, Pilgangoora) are along a north-northeast-trending corridor that is within one tectonostratigraphic domain. Most rare element pegmatites are within 5 km (at surface) of their apparent parent pluton, and all are within 10 km where the pluton has been identified. Although granitic rocks of the North Pilbara Craton were emplaced over an 800 m.y. period (3600-2800 Ma), rare element class pegmatites can be tied to a number of post-tectonic plutons of a younger granite suite that were emplaced into most of the granitoid complexes and adjacent greenstone belts at 2890 to 2830 Ma. Subtle differences in the composition of these granites that have given rise to different petrogenetic pegmatite suites can be identified in processed radiometric data from airborne surveys. These parental younger granites are enriched in lithophile and volatile elements and have a highly fractionated character that permitted formation of significant pegmatite mineralization. They represent the culmination of the crustal evolution of the North Pilbara Craton and the onset of cratonization, as part of a global appearance of rare metal pegmatite mineralization post-3 Ga.
The Sn-rich Qiguling topaz rhyolite dike intrudes the Qitianling biotite granite of the Nanling Range in southern China; the granite hosts the large Furong Sn deposit. The rhyolite dike is typically peraluminous, volatile-enriched, and highly evolved. Whole-rock F and Sn concentrations attain 1.9 wt.% and 2700 ppm, respectively. The rhyolite consists of a fine-grained matrix formed by quartz, feldspar, mica and topaz, enclosing phenocrysts of quartz, feldspar and mica; it is locally crosscut by quartz veinlets. Lithium-bearing micas in both phenocrysts and the groundmass can be classified as primary zinnwaldite, “Mus-Ann” (intermediate member between annite and muscovite), and secondary Fe-rich muscovite. Topaz is present in the groundmass only; common fluorite occurs in the groundmass and also in a specific cassiterite, rutile and fluorite (Sn–Ti–F) assemblage. Cassiterite and rutile are the only Sn and Ti minerals; both cassiterite and Nb-rich rutile are commonly included in the phenocrysts. The Sn–Ti–F assemblage is pervasive, and contains spongy cassiterite in some cases; cassiterite also occurs in quartz veinlets which cut the groundmass. Electron microprobe and LA-ICP-MS compositions were used to study the magmatic and hydrothermal processes and the role of F in Sn mineralization. The presence of zinnwaldite and “Mus-Ann”, which are respectively representative of early and late mica crystallization during magma differentiation, also suggests a significant decrease in f(HF)/f(H2O) of the system. Cassiterite included in the zinnwaldite phenocrysts is suggested to have crystallized from the primary magma at high temperature. Within the Sn–Ti–F aggregates, rutile crystallized as the earliest mineral, followed by fluorite and cassiterite. Spongy cassiterite containing inclusions of the groundmass minerals indicate a low viscosity of the late fluid. The cassiterite in the quartz veinlets crystallized from low-temperature hydrothermal fluids, which possibly mixed with meteoric water. In general, cassiterite precipitated during both magmatic and hydrothermal stages, and over a range of temperatures. The original fluorine and tin enrichments, f(HF)/f(H2O) change in the residual magma, formation of Ca,Sn,F-rich immiscible fluid, decrease of the f(HF) during groundmass crystallization, and mixing of magma-derived fluids with low-saline meteoric water during the late hydrothermal stage, are all factors independently or together responsible for the Sn mineralization in the Qiguling rhyolite.
The Early Jurassic (ca. 177 Ma) Bokan Mountain granitic complex, located on southern Prince of Wales Island, southernmost Alaska, cross-cuts Paleozoic igneous and metasedimentary rocks of the Alexander terrane of the North American Cordillera and was emplaced during a rifting event. The complex is a circular body (~ 3 km in diameter) of peralkaline granitic composition that has a core of arfvedsonite granite surrounded by aegirine granite. All the rock-forming minerals typically record a two-stage growth history and aegirine and arfvedsonite were the last major phases to crystalize from the magma. The Bokan granites and related dikes have SiO2 from 72 to 78 wt. %, high iron (FeO (tot) ~ 3-4.5 wt. %) and alkali (8-10 wt.%) concentrations with high FeO(tot)/(FeO(tot)+ MgO) ratios (typically > 0.95) and the molar Al2O3/(Na2O+K2O) ratio < 1. The granitic rocks are characterized by elevated contents of rare earth elements (REE), Th, U and high field strength elements (HFSE) and low contents of Ca, Sr, Ba and Eu, typical of peralkaline granites. The granites have high positive εNd values which are indicative of a mantle signature. The parent magma is inferred to be derived from an earlier metasomatized lithospheric mantle by low degrees of partial melting and generated the Bokan granitic melt through extensive fractional crystallization. The Bokan complex hosts significant rare-metal (REE, Y, U, Th, Nb) mineralization that is related to the late-stage crystallization history of the complex which involved the overlap of emplacement of felsic dikes, including pegmatite bodies, and generation of orthomagmatic fluids. The abundances of REE, HFSE, U and Th as well as Pb and Nd isotopic values of the pluton and dikes were modified by orthomagmatic hydrothermal fluids highly enriched in the strongly incompatible trace elements, which also escaped along zones of structural weakness to generate rare-metal mineralization. The latter was deposited in two stages: the first relates to the latest stage of magma emplacement and is associated with felsic dikes that intruded along the faults and shear deformations, whereas the second stage involved ingress of hydrothermal fluids that both remobilized and enriched the initial magmatic mineralization. Mineralization is mostly composed of “new” minerals. Fluorine complexing played a role during the transportation of REE and HFSE in hydrothermal fluids and oxygen isotopes in the granites and quartz veins negate the significant incursion of an external fluid and support a dominantly orthomagmatic hydrothermal system. Many other REE-HFSE deposits hosted by peralkaline felsic rocks (nepheline syenites, peralkaline granites and peralkaline trachytes) were formed by a similar two stage process.
In the Ponte Segade area (Galicia, NW Spain) strongly differentiated granites, often associated with rare-element mineralization (Sn–Ta–Nb–Li–Be–Cs) that could be of economic interest, have recently been discovered. These granites appear in the northern sector of the Ollo de Sapo Antiform (Central Iberian Zone, Variscan Orogen). Three different muscovite-rich synkinematic and peraluminous types of leucogranite (leucogranites s.s., albite-rich leucogranites I and albite-rich leucogranites II) and two pegmatite types associated with the albite-rich leucogranites (zoned and banded) have been identified in the studied area. The geochemistry of whole rock leucogranites indicates an enrichment in incompatible elements (lithium, rubidium, beryllium, cesium and hafnium), Al2O3 and Na2O, and an impoverishment in barium, strontium, zirconium, cerium, yttrium and SiO2. Geochemical studies of zircon, muscovite, K-feldspar and tourmaline in the different types of granites and pegmatites indicate the grade of evolution of the granitic system. With differentiation of the system, the zircon is enriched in hafnium and uranium and is impoverished in zirconium. In muscovite and K-feldspar there is an increase in cesium and rubidium. The opposite behavior is observed with regards to the Mg, Fe and Ti contents. In the case of tourmaline, the increase in Li is the best indicator of the grade of evolution. By contrast, Fe and Mg decrease. The sequence of evolution of the granitic system obtained from the geochemical studies indicates that the leucogranites s.s. are the least differentiated, evolving gradually, in accordance with field relationships, to albite-rich leucogranites I. The albite-rich leucogranites II are the most evolved, but no direct relationship between them and leucogranites s.s. has been found. The banded pegmatites associated with the albite-rich leucogranites II are more differentiated than the zoned pegmatites associated with the albite-rich leucogranites I, but are the poorest in Sn, Ta and Nb.