Geochemical characteristics of the Bayan Obo giant REE–Nb–Fe deposit: Constraints on its genesis

Geochemical characteristics of different dolomites in the Bayan Obo giant REE–Nb–Fe deposit in Inner Mongolia have been studied. Intensively REE-mineralized dolomites (total REE over 800 ppm) show similar geochemical characteristics to associated carbonatite dykes, with Ba, Th, REE enrichments and Sr, Nb, Ti, Cu depletions, which is different from those of dolomites in the deposit with low REE contents (total REE less than 800 ppm). The low REE dolomites display some transitional characteristics between carbonatite dyke and sedimentary carbonate, with La depletion and Nb enrichment. This indicates that the genesis of the REE-mineralized dolomites might be related to both carbonatite magma and sedimentary carbonates. Sulfur isotope data indicates two sulfur sources, a mantle source (δ³⁴S c.a. 0‰) and seawater (δ³⁴S c.a. +25‰). It is proposed that mineralized dolomites in the Bayan Obo giant REE–Nb–Fe deposit are the product of sedimentary carbonate hydrothermally metasomatised by carbonatite magma and/or associated fluids. These dolomites formed the large-scale rare earth mineralization in the unique Bayan Obo REE–Nb–Fe deposit.     

A review of the genesis of the world class Bayan Obo Fe–REE–Nb deposits,Inner Mongolia,China: Multistage processes and outstanding questions

The Bayan Obo Fe–REE–Nb deposit is the world's largest rare earth element (REE) resource and with the increasing focus on critical metal resources has become a focus of global interest. The deposit is hosted in the Palaeoproterozoic Bayan Obo Group, mainly concentrated in the H8 dolomite marble. The ores consist of light REE enriched monazite and bastnäsite, with a wide array of other REE minerals. Niobium mineralisation is hosted primarily in aeschynite and pyrochlore, although there are a wide range of other Nb-minerals. The origin of the host dolomite and ore bodies has been a subject of intense debate. The host dolomite has been proposed to be both of sedimentary origin and an igneous carbonatite. Carbonatite dykes do occur widely in the area, and consideration of the textural, geochemical and isotopic composition of the dolomite suggests an origin via intrusion of magmatic carbonatite into meta-sedimentary marble, accompanied by metasomatism. The origin of the ore bodies is complex, indicated most strongly by an ~ 1 Ga range in radiometric age determinations. Compilation of available data suggests that the ores were originally formed around 1.3 Ga (Sm–Nd isochron ages; Th–Pb ages of zircon), close in time to the intrusion of the carbonatite dykes. The ores were subsequently subjected to several stages of deformation and hydrothermal overprint, culminating in deformation, metamorphism and fluid flow related to the Caledonian subduction of the Mongolian Plate under the North China Craton from ~ 450 to 420 Ma (Th–Pb ages of monazite). This stage resulted in the formation of the strong foliation (‘banding’) of the ore. The presence of undeformed veins with alkali mineral fills, and the overprinting of the foliation by Nb minerals suggest that secondary fluid flow events may also have contributed to the metal endowment of the deposits, as well as remobilising the original Fe and REE mineralisation. The alteration mineralogy and geochemistry of the ores are comparable to those of many REE mineralised carbonatites. Initial Nd isotope ratios at 450 Ma, however, suggest crustal sources for the metals. These conflicting lines of evidence can be reconciled if a (at least) two stage isotopic evolution is accepted for the deposits, with an original mantle-sourced, carbonatite-related metal accumulation forming around 1.3 Ga with εNd close to 0. The ore was remobilised, with associated re-equilibration of Th–Pb isotope systematics during deformation at ~ 450 Ma. A further stage of alkaline hydrothermal fluid was responsible for Nb mineralisation at this stage. The complex geological history, with multiple stages of alkaline, high field strength element-rich, metasomatic fluid flow, is probably the main reason for the exceptional metal endowment of the Bayan Obo area.     

Integrated U–Pb and Sm–Nd geochronology for a REE-rich carbonatite dyke at the giant Bayan Obo REE deposit,Northern China

The Bayan Obo deposit in North China contains the largest rare-earth element (REE) resources in the world, but its forming time remains controversial. Nearly one hundred carbonatite dykes occur around the Bayan Obo deposit, including dolomite, calcite and calcite–dolomite carbonatite varieties. Zircons from a REE-rich carbonatite dyke and wallrock quartz conglomerate at Bayan Obo have been analyzed for U–Pb to determine the age of the dyke. Zircon from the carbonatite dyke, analyzed by conventional isotope dilution thermal ionization mass spectrometry (ID-TIMS), yielded an upper intercept age of 1417 ± 19 Ma. This age is confirmed by SHRIMP U–Pb analysis of zircon from the same carbonatite dyke, which gave a ²⁰⁷Pb/²⁰⁶Pb weighted mean age of 1418 ± 29 Ma. In situ Nd isotope measurements of monazite collected from the carbonatite dyke gave an isochron age of 1275 ± 87 Ma. These results demonstrate that the dyke intruded ~ 1400 Ma. In view of predecessor's results, it is clarified that the REE mineralization at Bayan Obo occurred at ca. 1400 Ma, consistent with the timing of carbonatite dyke intrusion in the region. The youngest detrital zircons from the quartz conglomerate yielded a ²⁰⁷Pb/²⁰⁶Pb weighted mean age of 1941 ± 7 Ma using LA ICP-MS U–Pb method. Detrital zircons in the carbonatite dyke also gave a mean apparent age of 1932 ± 3 Ma using ID-TIMS U–Pb method and 1914 ± 14 Ma using SHRIMP U–Pb method. These ages constrain the beginning active time of the Zha'ertai–Bayan Obo rift in the northern margin of the North China Craton after ~ 1900 Ma.     

Carboniferous magmatism and mineralization in the area of the Fuxing Cu deposit,Eastern Tianshan,China: Evidence from zircon U–Pb ages,petrogeochemistry, and Sr–Nd–Hf–O isotopic compositions

The newly discovered Fuxing porphyry Cu deposit is located in the Dananhu–Tousuquan arc, adjacent to the Tuwu–Yandong Cu deposits of Eastern Tianshan, in the southern Central Asian Orogenic Belt. The Fuxing deposit is hosted by volcanic rocks (basalt and dacite) in the Early Carboniferous Qi'eshan Group and Carboniferous felsic intrusions (plagiogranite porphyry, monzogranite, and quartz diorite). New SIMS zircon U–Pb dating indicates that the plagiogranite porphyry and monzogranite emplaced at 332.1 ± 2.2 Ma and 328.4 ± 3.4 Ma, respectively. The basalts are characterized by low SiO₂ contents (47.47–54.90 wt.%), a lack of Eu anomalies, strong depletion of Na, Ta, and Ti elements but positive Sr, U, and Pb anomalies, high Y (20.8–28.2 ppm) and HREE concentrations (Yb = 2.23–3.06 ppm), and relatively low (La/Yb)_N (2.20–3.92) values; the dacite samples have high SiO₂ contents (66.13–76.93 wt.%), clearly negative Eu anomalies, high Mg^# values (36–51), and high Y (41.8–54.9 ppm) and Yb (5.76–8.98 ppm) concentrations. The basalts and dacites exhibit similar signatures as normal arc rocks, and were considered to be derived from partial melting of mantle-wedge peridotite that was previously metasomatized by slab melts. In contrast, the plagiogranite porphyry, monzogranite, and quartz diorite show the same geochemical affinity with modern adakites, which are characterized by high SiO₂ contents (67.55–79.00 wt.%), minor negative to positive Eu anomalies, strong depletion of heavy rare earth elements (Yb = 0.17–1.19 ppm) and Y (1.86–10.1 ppm), positive K, Rb, Sr, and Ba but negative Nb, Ta, Th, and Ti anomalies, and high (La/Yb)_N ratios and Mg^# values. Moreover, these adakitic felsic intrusions display relatively high positive zircon ε_Hf(t) values (+ 11.4 to + 18.3), low ⁸⁷Sr/⁸⁶Sr (0.706080–0.711239), high ¹⁴³Nd/¹⁴⁴Nd (0.512692–0.512922) ratios, and consistent zircon δ¹⁸O values (4.41‰–5.48‰), suggesting that their parental magma were most likely derived from partial melting of the subducted oceanic crust followed by mantle peridotite interaction. Based on the whole-rock geochemical and Sr–Nd–Hf–O isotopic data, as well as detailed petrographic analyses, we further suggest that the Fuxing igneous rocks and associated porphyry Cu mineralization were generated by the northward subduction of the paleo-Tianshan oceanic plate beneath the Dananhu–Tousuquan island arc during the Early Carboniferous.     

Re–Os isotopic ages of pyrite and chemical composition of magnetite from the Cihai magmatic–hydrothermal Fe deposit, NW China

The Eastern Tianshan Orogenic Belt of the Central Asian Orogenic Belt and the Beishan terrane of the Tarim Block, NW China, host numerous Fe deposits. The Cihai Fe deposit (>90 Mt at 45.6 % Fe) in the Beishan terrane is diabase-hosted and consists of the Cihai, Cinan, and Cixi ore clusters. Ore minerals are dominantly magnetite, pyrite, and pyrrhotite, with minor chalcopyrite, galena, and sphalerite. Gangue minerals include pyroxene, garnet, hornblende and minor plagioclase, biotite, chlorite, epidotite, quartz, and calcite. Pyrite from the Cihai and Cixi ore clusters has similar Re–Os isotope compositions, with ～14 to 62 ppb Re and ≤10?ppt common Os. Pyrrhotite has ～5 to 39 ppb Re and ～0.6 ppb common Os. Pyrite has a mean Re–Os model age of 262.3?±?5.6 Ma (n?=?13), in agreement with the isochron regression of ¹⁸⁷Os vs. ¹⁸⁷Re. The Re–Os age (～262 Ma) for the Cihai Fe deposit is within uncertainty in agreement with a previously reported Rb–Sr age (268?±?25 Ma) of the hosting diabase, indicating a genetic relationship between magmatism and mineralization. Magnetite from the Cihai deposit has Mg, Al, Ti, V, Cr, Co, Ni, Mn, Zn, Ga, and Sn more elevated than that of typical skarn deposits, but both V and Ti contents lower than that of magmatic Fe–Ti–V deposits. Magnetite from these two ore clusters at Cihai has slightly different trace element concentrations. Magnetite from the Cihai ore cluster has relatively constant trace element compositions. Some magnetite grains from the Cixi ore cluster have higher V, Ti, and Cr than those from the Cihai ore cluster. The compositional variations of magnetite between the ore clusters are possibly due to different formation temperatures. Combined with regional tectonic evolution of the Beishan terrane, the Re–Os age of pyrite and the composition of magnetite indicate that the Cihai Fe deposit may have derived from magmatic–hydrothermal fluids related to mafic magmatism, probably in an extensional rift environment.     

Zircon U–Pb ages of the Mianning–Dechang syenites,Sichuan Province,southwestern China: Constraints on the giant REE mineralization belt and its regional geological setting

The Himalayan Mianning–Dechang (MD) rare earth element (REE) belt in western Sichuan Province, southwestern China, is approximately 270 km long and 15 km wide, and contains total reserves of more than 3 Mt of light REEs (LREEs), comprising one giant (Maoniuping), one large (Dalucao), two small–medium-sized (Muluozhai and Lizhuang), and numerous smaller REE deposits. The belt occurs within the eastern Indo-Asian collision zone (EIACZ), where its location is controlled by large-scale strike-slip faults and tensional fissure zones. Himalayan carbonatite–syenite complexes consist predominantly of alkaline syenite stocks and carbonatite sills or dikes that host REE mineralization. Previous studies have reported inconsistent ages for alkaline magmatism syenite formation and REE mineralization. Here, we present new results of sensitive high-resolution ion micro-probe U–Pb dating of zircons from syenites from the Dalucao, Maoniuping, Lizhuang and Diaoloushan areas, the first systematic and precise age determinations for these rocks in the MD belt. The new data give concordant ages of 12.13 ± 0.19 and 11.32 ± 0.23 Ma for the Dalucao deposit, 22.81 ± 0.31 and 21.3 ± 0.4 Ma for Maoniuping, 26.77 ± 0.32 Ma for Muluozhai, and 27.41 ± 0.35 Ma for Lizhuang. These ages, which should be regarded as maximum ages for the REE mineralization in the study area, can be split into two groups, i.e. 11–12 Ma in the southern part of the MD belt and 12–27 Ma in the northern part, suggesting a progression of magmatism from north to south. These data suggest that the majority of carbonatite–syenite magmatism within the EIACZ occurred during the main stage of Himalayan metallogenesis. The ages presented in this study suggest that strike-slip shear along the MD belt was initiated at ca. 27 Ma and ended ca. 12 Ma. This timing is consistent with movements along the adjacent Ailaoshan–Red River strike-slip fault in southeastern Tibet (to the south of the MD belt) and one of the three Cenozoic strike-slip faults in eastern Tibet. Ascent of an asthenospheric mantle diapir beneath the EIACZ in the Cenozoic may have provided a thermal mechanism for the generation of magmas that formed the carbonatite–syenite complexes in the study area. Alternatitvely, the magmas may have been generated by decompression melting associated with the transition from a transpressional to a transtensional regime at 38–40 Ma. The precise age results for syenite magmatism in the study area indicate that this transition occurred prior to carbonatite–syenite magmatism and the formation of the MD REE belt, which is consistent with the regional tectonic model.     

Ages and compositions of primary and secondary allanite from the Lala Fe–Cu deposit,SW China: implications for multiple episodes of hydrothermal events

Numerous Fe–Cu deposits in southwestern China form the Kangdian Iron-Oxide Copper-Gold (IOCG) metallogenic Province. These deposits have a close association of Fe-oxides and Cu-sulfides formed at different stages, which are possibly related to multiple hydrothermal events. In this paper, U–Pb dating and chemical analyses on allanite from different stages of the Lala deposit were used to constrain timing and origin of such events. Allanite occurs as disseminated grains or patches in Fe–Cu ores and is closely associated with chalcopyrite, molybdenite, calcite and minor titanite, postdating magnetite and apatite. High-resolution backscattered electronic (BSE) imaging, electron microprobe compositions and X-ray scanning profiles demonstrate that REE-rich primary allanite was replaced by later, relatively porous and REE-poor secondary allanite. Such a replacement was promoted by interaction between primary allanite and fluid fluxes infiltrating the minerals, following an exchange scheme of REE³⁺ + Fe²⁺ → Ca²⁺ + Al³⁺. The secondary allanite has higher Fe³⁺/(Fe³⁺+Fe²⁺) ratios and U contents, indicating involvement of relatively oxidized fluids during alteration. The alteration has also produced unidentified secondary REE minerals in fractures, indicating re-deposition of some of the removed REEs. The primary and secondary allanites are dated by in situ LA-ICP-MS technique and have U–Pb ages of 1,067 ± 41 Ma and 880–850 Ma, respectively. The ~1.07 Ga primary allanite was contemporaneous with the main Mo–Cu–LREE mineralization with a molybdenite Re–Os age of ~1.08 Ga. The 880–850 Ma secondary allanite is comparable with the Ar–Ar ages (890–830 Ma) of biotite from hosting schists and undeformed sulfide veins occurring throughout the Kangdian Province, suggesting that such an event was possibly syn-deformational and represents a younger hydrothermal event. Occurrences of both primary and secondary allanites suggest that the mineralization may have involved multiple tectonothermal events including the ~1.05–1.1 Ga intra-plate and subsequent 960–740 Ma arc magmatism in the Kangdian region.     

K–Ar ages of plutonism and mineralization at the Shizhuyuan W–Sn–Bi–Mo deposit,Hunan Province,China

The Qianlishan granite complex, situated 16 km southeast of Chenzhou City, Hunan Province, China, hosts the Shizhuyuan W–Sn–Bi–Mo deposit. This complex, which intruded the Protozoic metasedimentary rocks and the Devonian clastic sedimentary and carbonate rocks, consists of mainly medium- to coarse-grained biotite granites and minor amounts of fine-grained biotite granite in addition to granite and quartz porphyry. K–Ar ages suggest three episodes of plutonism: the medium- to coarse-grained biotite granite (before 152 Ma), the fine-grained biotite granite (137 Ma), and the granite porphyry (129–131 Ma). Muscovite ages of the greisen are 145–148 Ma, suggesting that the W–Sn–Bi–Mo mineralization was related to the main, medium- to coarse-grained biotite granites. The K–Ar age of the hydrothermal vein mineralization is 92 Ma and is probably related to the porphyries.     

Detrital Zircon U–Pb Geochronology and Provenance of Bayan Obo Group, Northern Margin of North China Craton: New Implications for the Position of NCC in Rodinia

The paleoposition of North China Craton in Rodinia has long been in controversial. This paper mainly focuses on the U–Pb geochronological studies of detrital zircons obtained from Bayan Obo Group exposed in the Shangdu area, Inner Mongolia, aiming to provide more information for interprating this problem. Based on the acquired data, this paper comes to the following conclusions. Firstly, the depositional age of Bayan Obo Group might be from Meso– to Neoproterozoic according to the zircons U–Pb dating results. The lower succession of this group, namely Dulahala and Jianshan formations deposited between 1800 and 1650 Ma. The Halahuogete and Bilute formations deposited between 1500 and 1350 Ma. For Baiyinbaolage and Hujiertu formations, their depositional age was 1250–900 Ma. Secondly, for the provenance of Bayan Obo Group, this paper believes detrital zircons with age of 2.51–2.71 Ga and 2.00–2.48 Ga were from Guyang, Xi Ulanbulang and Zhuozi area; the Khondalite Belt provided detrital zircons with age of 1.95–1.80 Ga; zircons with age of 1.60–1.75 Ga might come from granitic rocks in Miyun Area. The magmatism after 1.60 Ga was rarely recorded in the NCC, therefore those zircons with ages younger than 1.60 Ga might come from outside of NCC. The magmatism with the same age existed in Baltic, Amazonia and Laurentia. Based on previous paleomagnetic researches, this paper proposes that NCC might receive detritus from Baltic during 1560–1350 Ma and had affinity with Laurentia and Amazonia at ～0.9 Ga in Rodinia. Baltic, Amazonia and Laurentia might be potential provenances for non–NCC detritus in Bayan Obo Group.     

Zircon U–Pb and pyrite Re–Os age constraints on pyrite mineralization in the Yinjiagou deposit,China

We report new zircon U–Pb and pyrite Re–Os geochronological studies of the Yinjiagou poly-metallic deposit, sited along the southern margin of the North China Craton (SMNCC). In this deposit, pyrite, the most important economic mineral, is intergrown/associated with Mo, Cu, Au, Pb, Zn, and Ag. Prior to our new work, the age of chalcopyrite–pyrite mineralization was known only from its spatial relationship with molybdenite mineralization and with intrusions of known ages. The U–Pb and Re–Os isotope systems provide an excellent means of dating the mineralization itself and additionally place constraints on the ore genesis and metal source. Zircons separated from the quartz–chalcopyrite–pyrite veins include both detrital and magmatic groups. The magmatic zircons confine the maximum age of chalcopyrite–pyrite mineralization to 142.0 ± 1.5 Ma. The Re–Os results yield an age of 141.1 ± 1.1 Ma, which represents the age of the chalcopyrite–pyrite mineralization quite well. The common Os contents are notably low (0.5–20.1 ppt) in all samples. In contrast, the Re contents vary considerably (3.0–199.2 ppb), most likely depending on intensive boiling, which resulted in an increase of Re within the pyrite. This study demonstrates that the main chalcopyrite–pyrite mineralization occurred late in the magmatic history and was linked to a deeper intrusion involving dominant mantle-derived materials. This mineralization event might be related to the Early Cretaceous lithospheric destruction and thinning of the SMNCC.     

U–Pb and Ar–Ar geochronology of the Fujiawu porphyry Cu–Mo deposit,Dexing district,Southeast China: Implications for magmatism,hydrothermal alteration,and mineralization

The Fujiawu porphyry Cu–Mo deposit is one of several porphyry Cu–Mo deposits in the Dexing district, Jiangxi Province, Southeast China. New zircon SHRIMP U–Pb data yield a weighted mean ²⁰⁶Pb/²³⁸U age of 172.0 ± 2.1 and 168.5 ± 1.4 Ma from weakly altered granodiorite porphyry and quartz diorite porphyry, respectively. Two hydrothermal biotites from granodiorite porphyry give an Ar–Ar step-heating plateau age of 169.9 ± 1.8 and 168.7 ± 1.8 Ma. Hydrothermal apatite exsolved from altered biotite yields an isotope dilution thermal ionization mass spectrometry isochron age of 164.4 ± 0.9 Ma. The apatite age is similar to the ages obtained from hydrothermal rutile (165.0 ± 1.1 and 164.8 ± 1.6 Ma) and indicates that the magmatism and hydrothermal activity in the Fujiawu deposit occurred in the Middle Jurassic. Hydrothermal fluid circulation related to multiple stages of magma emplacement resulted in Cu–Mo mineralization in the Fujiawu porphyry deposit. The zircon SHRIMP U–Pb ages and the published molybdenite Re–Os age (170.9 ± 1.5 Ma) represent the timing of magma crystallization and Mo mineralization, whereas the rutile and apatite U–Pb ages reflect the timing of Cu mineralization following quartz diorite emplacement. The data suggest slow cooling after emplacement of the quartz diorite porphyry.     

Geochemistry of magmatic and hydrothermal zircon from the highly evolved Baerzhe alkaline granite: implications for Zr–REE–Nb mineralization

The Baerzhe alkaline granite pluton hosts one of the largest rare metal (Zr, rare earth elements, and Nb) deposits in Asia. It contains a geological resource of about 100 Mt at 1.84 % ZrO₂, 0.30 % Ce₂O₃, and 0.26 % Nb₂O₅. Zirconium, rare earth elements (REE), and Nb are primarily hosted by zircon, yttroceberysite, fergusonite, ferrocolumbite, and pyrochlore. Three types of zircon can be identified in the deposit: magmatic, metamict, and hydrothermal. Primary magmatic zircon grains occur in the barren hypersolvus granite and are commonly prismatic, with oscillatory zones and abundant melt and mineral inclusions. The occurrence of aegirine and fluorite in the recrystallized melt inclusions hosted in the magmatic zircon indicates that the parental magma of the Baerzhe pluton is alkali- and F-rich. Metamict zircon grains occur in the mineralized subsolvus granite and are commonly prismatic and murky with cracks, pores, and mineral inclusions. They commonly show dissolution textures, indicating a magmatic origin with later metamictization due to deuteric hydrothermal alteration. Hydrothermal zircon grains occur in mineralized subsolvus granite and are dipyramidal with quartz inclusions, with murky CL images. They have 608 to 2,502 ppm light REE and 787 to 2,521 ppm Nb, much higher than magmatic zircon. The texture and composition of the three types of zircon indicate that they experienced remobilization and recrystallization during the transition from a magmatic to a hydrothermal system. Large amounts of Zr, REE, and Nb were enriched and precipitated during the transitional period to form the giant low-grade Baerzhe Zr–REE–Nb deposit.     

Zircon U–Pb ages,geochemistry, and Sr–Nd–Pb isotopic compositions of Middle Triassic granodiorites from the Kaimuqi area,East Kunlun,Northwest China: implications for slab breakoff

The Qimantage area of Northwest China lies in the western part of the East Kunlun Orogenic Belt, and is dominated by late Permian to Late Triassic granitoids. Among these, the Middle Triassic granitoids are mainly distributed south of the North Kunlun Fault, and consist of two main granitic assemblages: the Kaimuqi assemblage in the east and the Mositu assemblage in the west. To better constrain the Indosinian tectonic evolution of this area, we present data on the geochronology, geochemistry, and petrology of ore-bearing granodiorites from the Kaimuqi area in eastern Qimantage. The granodiorite samples have porphyritic or fine-grained textures. Laser ablation inductively coupled plasma mass spectrometry U–Pb zircon dating yields emplacement ages of 238–242 Ma, interpreted here as the result of the Middle Triassic magmatism. The granodiorites are mostly of the high-K calc-alkaline series, and are enriched in light rare earth elements, depleted in heavy rare earth elements such as Nb, Ta, P, and Ti, and have weak negative Eu (Eu/Eu*) anomalies. The Kaimuqi granodiorites have lower SiO₂ and Sr contents, and higher Na₂O/K₂O ratios than the Mositu granodiorites. They also show initial ⁸⁷Sr/⁸⁶Sr ratios of 0.712151–0.715436, εNd(t) values of ?7.4 to ?6.3, and two-stage Nd model ages of 1.53–1.61 Ga. Together with their radiogenic Pb isotopic ratios for ²⁰⁶Pb/²⁰⁴Pb_(t) (18.271–18.622), ²⁰⁷Pb/²⁰⁴Pb_(t) (15.637–15.651), and ²⁰⁸Pb/²⁰⁴Pb_(t) (38.452–37.870), these data indicate both mantle and crustal contributions to the source of the granodiorites. Field investigations show that Middle Triassic granitoids in both the Mositu and Kaimuqi assemblages contain large numbers of mafic microgranular enclaves, which supports an interpretation of mantle and crustal magmatic mixing. Based on a comparison of these results with data from coeval granites in the Mositu assemblage, we propose that the Middle Triassic granitoids in the Qimantage area were produced at ca. 240 Ma, as a result of the end of subduction and the initiation of collision during the Variscan–Indosinian orogeny. Magma mixing may be interpreted as the result of slab breakoff in a subduction zone environment, which led to fluid metasomatism and induced partial melting of an enriched lithospheric mantle, resulting in the formation of voluminous granitic magma.     

Age and Thermal History of the East Ore Section,Bayan,Obo Deposit—A Fission Track Study

Considerable attention has been paid in recent years to the study of geothermal history by using spontaneous fission tracks of 238U recorded in minerals.Apatite and zircon were used for fission track study in this paper because apatitie has been widely used as a natural geothermometer(Wang Shicheng et al., 1994) to reveal the thermal evolution of sedimentary rocks based on its low annealing temperature of fission tracks and zircon is characterized by a closing temperature above 700℃,The samples were collected from ferruginous,siliceous slate wall rock at the upper levels of the orebody and Nb-REE-Fe ores from deep tunnels.The age and thermal evolution of the orebody were discussed in terms of fission track characters and their length variations observed in the coexisting apatite and zircon in the same specimen.     

Zircon and molybdenite geochronology and geochemistry of the Kalmakyr porphyry Cu–Au deposit,Almalyk district,Uzbekistan: Implications for mineralization processes

Copper, gold and molybdenum mineralization of the Kalmakyr porphyry deposit in Uzbek Tien Shan occurs as stockworks, veinlets and disseminations in the phyllic and K-silicate alteration zones developed predominantly in a middle to late Carboniferous intrusive complex composed of monzonite and granodiorite porphyry. Zircon U–Pb dating yielded an age of 327.2 ± 5.6 Ma for the ore-hosting monzonite and an age of 313.6 ± 2.8 Ma for the ore-bearing granodiorite porphyry. Re–Os dating of seven molybdenite samples from stockwork and veinlet ores yielded model ages from 313.2 to 306.3 Ma, with two well-constrained isochron ages of 307.6 ± 2.5 Ma (five stockwork ores) and 309.1 ± 2.2 Ma (five stockwork ores and two veinlet ores), respectively. These results indicate that Cu–Au mineralization post-dated the emplacement of the monzonite, started right after the emplacement of the granodiorite porphyry, and lasted for ca. 7 Ma afterward. The geochronological and geochemical data suggest that the Kalmakyr deposit was formed in a late Carboniferous mature magmatic arc setting, probably related to the latest subduction process of the Turkestan Ocean beneath the Middle Tien Shan. The ε_Hf(t) values of zircon grains from the monzonite vary from +11 to +1.7, with an average of +5.1, and those of zircon grains from the granodiorite porphyry range from +5.7 to −1.8, with an average of +2.4. These data indicate that the magma of both monzonite and granodiorite porphyry was derived from partial melting of a thickened lower crust with input of mantle components and variable crustal contamination, and that there was more mantle contribution to the formation of the monzonite than the granodiorite porphyry. The high rhenium concentrations of molybdenite (98–899 ppm) also indicate major mantle contribution of rhenium and by inference ore metals. The relatively high Eu_N/Eu_N¹ values (average 0.68), Ce⁴⁺/Ce³ values (average 890) and Ce/Nd values (average 36.8) for zircon grains from the granodiorite porphyry than those from the monzonite (average Eu_N/Eu_N¹ = 0.33, average Ce⁴⁺/Ce³ = 624, average Ce/Nd = 3.9) suggest that the magma for the syn-mineralization granodiorite porphyry has higher oxygen fugacity than that for the pre-mineralization monzonite. Based on these data, it is proposed that while the monzonite was emplaced, the oxygen fugacity and volatile contents in the magma were relatively low, and ore metals might disperse in the intrusive rock, whereas when the granodiorite porphyry was emplaced, the oxygen fugacity and volatile contents in the magma were increased, favoring copper and gold enrichment in the magmatic fluids. The Kalmakyr deposit formed from a long-lived magmatic-hydrothermal system connected with fertile magmatic sources in relation to the subduction of the Turkestan Ocean beneath the Middle Tien Shan.     

Carbon and Oxygen Isotopic Characteristics of REE—Fluorocarbonate Minerals and Their Genetic Implications,Bayan Obo Deposit,Inner Mongloia,China

REE－fluorocarbonates as major REE minerals in the Bayan Obo deposit,the largest REE deposit in the world,were analyzed for their stable isotopic compositions,The δ^13 C and δ^18 O values of huanghoite,cebaite and bastnaesite from late-stage veins vary in the ranges of 7.8--4.0‰ and 6.7-9.4‰,respectively,These data are relatively similar to those of bastnaesites from banded ores:δ^13C-5.6--5.2‰ andδ^18O3.6-5.5‰.The REE fluorocarbonates from both late-staege veins and banded ores are characterized by lower δ^13 C and δ^18O values,especially the δ^18O values of bastnaesites from banded ores.Compared with them,the disseminated bastnaesits the dolomite-type ores possess rather highδ^13 C and δ^18O values,i.e.,-2.1-0.4‰ and 8.6-12.9‰ respectively.The high values are typical of the sedimentary host dolomite rocks as well as of the dolomite-type-ores.The carbon and oxygen isotopic characteristics of REE fluorocarbonate minerals provide new evidence for the hypothesis on the origin of Bayan Obo deposit-epigenetic hydrothermal metasomatism.     

U–Pb dating of zircon and cassiterite from the Early Cretaceous Jiaojiguan iron-tin polymetallic deposit,implications for magmatism and metallogeny of the Tengchong area,western Yunnan,China

The newly discovered Jiaojiguan deposit, a medium-scale skarn iron-tin polymetallic deposit on the Sino-Burma boundary of Yunnan Province (SW China), is spatially associated with the biotite monzonitic granite. Here, we report new in situ zircon LA-MC-ICP-MS U–Pb ages, trace element and Hf isotope data from the granite, and U–Pb dating ages of cassiterite from the ore bodies. In this study, we obtain a weighted mean ²⁰⁶Pb/²³⁸U age of 124.1 ± 1.4 Ma for the zircon and a ²⁰⁷Pb/²⁰⁶Pb-²³⁸U/²⁰⁶Pb intercept age of 123.8 ± 2.2 Ma for the cassiterite. The granite crystallized during the Early Cretaceous, with zircons exhibiting εHf(t) values from ?5.8 to ?0.6 and two-stage Hf model ages (T_DM2) of 1.21–1.54 Ga. The close temporal and spatial links between pluton emplacement and ore-forming events suggest that magmatic-hydrothermal events were the key factors that triggered the genesis of the iron-tin polymetallic deposits in the area. Regional geochronological data show that tin mineralization took place three times during the Cretaceous–Palaeogene in the Tengchong block due to re-melting of the underlying supposed Proterozoic (1.5 ± 0.5 Ga) Sn-rich strata/materials. Compared with those in the Bangong–Nujiang metallogenic belt (BNMB), we propose that the Cretaceous iron-tin polymetallic mineralization events in Tengchong–Baoshan closely resemble those of the Bangong–Nujiang belt in northern Tibet, both of which have experienced similar tectono-magmatic-metallogenic histories since the Mesozoic.