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Magnesium isotopic compositions, along with new Sr–Nd–Pb isotopic data and elemental analyses, are reported for 12 Miocene tourmaline-bearing leucogranites, 15 Eocene two-mica granites and 40 metamorphic rocks to investigate magnesium isotopic behaviors during metamorphic processes and associated magmatism and constrain the tectonic-magmatic-metamorphic evolution of the Himalayan orogeny. The gneisses, granulites and amphibolites represent samples of the Indian lower crust and display large range in δ26Mg from −0.44‰ to −0.09‰ in mafic granulites, −0.44‰ to −0.10‰ in amphibolites, and −0.70‰ to −0.03‰ in granitic gneisses. The average Mg isotopic compositions of the granitic gneisses (−0.19 ± 0.34‰), mafic granulites (−0.22 ± 0.17‰) and amphibolites (−0.25 ± 0.24‰) are similar, indicating the limited Mg isotope fractionation during prograde metamorphism from granitic gneisses to mafic granulites and retrograde metamorphism from mafic granulites to amphibolites. The Eocene two-mica granites and Miocene leucogranites are characterized by large variations in elemental and Sr–Nd–Pb isotopic compositions. The leucogranites and two-mica granites have their corresponding (87Sr/86Sr)i varying from 0.7282 to 0.7860 and 0.7163 to 0.7191, (143Nd/144Nd)i from 0.511888 to 0.512040 and 0.511953 to 0.512076, 207Pb/204Pb from 15.7215 to 15.7891 and 15.7031 to 15.7317, 208Pb/204Pb from 38.8521 to 39.5286 and 39.2710 to 39.4035, and 206Pb/204Pb from 18.4748 to 19.0139 and 18.7834 to 18.9339. However, they have similar Mg isotopic compositions (−0.21‰ to +0.06‰ versus −0.24‰ to +0.09‰), which did not originate from fractional crystallization nor source heterogeneity. Based on hornblende/biotite/muscovite dehydration melting reaction and Mg isotopic variations in two-mica granites and leucogranites with the proceeding metamorphism, along with elemental discrimination diagrams, Eocene two-mica granites and Miocene leucogranites could be related to hornblende dehydration melting and muscovite dehydration melting, respectively. Mg isotopic compositions of Eocene two-mica granites become heavier compared to the source because of residues of isotopically light garnet in the source; while those of Miocene leucogranites become lighter because of entrainment of isotopically light garnet from the source region. Thus, a new model for crustal anatexis and Himalayan orogenesis was proposed based on the Mg isotope fractionation in the leucogranites and metamorphic rocks. This model emphasizes a successive process from Indian continental subduction to rapid exhumation of the Higher Himalayan Crystalline Series (HHCS). The former underwent high-temperature (HT) and high-pressure (HP) granulite-facies prograde metamorphism, which resulted in the hornblende dehydration melting and the formation of Eocene two-mica granites; while the latter experienced amphibolite-facies retrogression and decompression, which resulted in the muscovite dehydration melting and the formation of Miocene leucogranites.

The concentrations and isotopic geochemistry of Li are potentially useful geochemical tracers of geological processes. To fully utilize Li isotopes as geochemical tracers, it is necessary to characterize the Li isotopic compositions of the various geological reservoirs. However, the Li isotopic composition of the juvenile lower crust is currently poorly constrained. Given that lithospheric architecture of the Tibetan Plateau includes Indian upper/lower crust, Tibetan upper/lower crust and juvenile lower crust, it is necessary to determine the Li isotopic composition for each geological endmember underneath southern Tibet. Among them, the juvenile lower crust was formed directly by underplating of mantle-derived basaltic magma, which is likely to be the critical factor to control the Cu-Au mineralization in southern Tibet and is responsible for crustal thickening beneath southern Tibet. Here, we report the Li concentration and isotopic composition of the juvenile lower crust in southern Tibet. Based on whole-rock major element, trace element, and Sr–Nd–Pb isotopic data, we infer that the Yeba basalts and Gangdese gabbros were derived from partial melting of metasomatized lithospheric mantle, and have compositions similar to the juvenile lower crust. In contrast, the Dianzhong andesites and Gangdese diorites originated from partial melting of the juvenile lower crust. Therefore, these units may be considered representative of the juvenile lower crust. The juvenile lower crust has Li concentrations of 7.1–37.2 ppm (mean = 15.4 ppm), consistent with the Li concentration for the lower crust (13 ppm). Li isotopic compositions (δ7Li) vary from +0.8‰ to +6.6‰ (mean = 3.0‰), similar to values for the EMI/EMII mantle. The Li isotopic compositions of the analyzed samples were not significantly affected by alteration, metamorphism, crustal assimilation, or magmatic differentiation, and therefore represent the isotopic compositions of the juvenile lower crust. The Li systematics of the juvenile lower crust may be attributed to partial melting of subcontinental lithospheric mantle that has undergone metasomatism by Li-rich fluids derived from subducted oceanic crust and marine sediments. Our study also demonstrates near-identical Li isotopic compositions for juvenile lower crust and metasomatized lithospheric mantle, resulting from the lack of Li isotope fractionation during basalt generation and differentiation.

The Jiajika lithium (Li) deposit in Sichuan Province is the largest pegmatite-type Li deposit in China. The petrogenesis and metallogenesis of the Jiajika granitic pegmatites remain debatable. This study presents the Li isotopic compositions of two-mica granites, Li-rich and Li-poor pegmatites, and muscovites in Li-poor and Li-rich pegmatites to unravel the Li enrichment mechanism in the Jiajika deposit. The two-mica granites have a lower average δ7Li values than those of the Li-rich and Li-poor pegmatites. The Li contents of the two-mica granites are almost the same as those of the Li-poor pegmatites but are much lower than those of the Li-rich pegmatites. Muscovites in the Li-rich pegmatites have higher average Li contents and δ7Li values than those of the Li-poor pegmatites. All these results indicate the Jiajika granitic pegmatites were the final products of the extreme fractional crystallization of two-mica granitic magmas rather than direct anatectic melts, and that the Li-poor pegmatites derived from early differentiation of the two-mica granitic magmas before they evolved into Li-rich pegmatites during the late stage of magmatic differentiation. The lower average δ7Li values of the more evolved Li-rich pegmatites compared with the Li-poor pegmatites may have been caused by fluid exsolution and kinetic diffusive fractionation during melt-fluid separation. We discovered that fluid exsolution during melt-fluid separation can cause significant Li isotopic fractionation, with 7Li enriched in a H2O-poor silicate-rich melt system. Considering the crystallization ages of the two-mica granites and granitic pegmatites and other geochemical evidence (e.g., major- and trace-element compositions), the magmatic differentiation and fluid exsolution in the late stage of the granitic magma evolution together with the Li-rich surrounding strata led to the multistage enrichments in lithium, thus contributing to the formation of the Jiajika large Li deposit.

Fault zones control the locations of many ore deposits, but the ore-forming processes in such fault zones are poorly understood. We have studied the deformation and ore textures associated with fault zones that controlled the lead–zinc mineralization of the Dongmozhazhua deposit, central Tibet, ∼100 km southwest of Yushu City. Geological mapping shows that the structural framework of the Dongmozhazhua area is defined by NW–SE-trending reverse faults and superposed folds that indicate at least two stages of deformation. The first stage is characterized by tight nearly E–W-striking folds that formed during the closure of the Jinshajiang Paleo-Tethyan Ocean in the Triassic. The second stage of deformation produced NW–SE-trending reverse faults and related structures of the Fenghuoshan–Nangqian fold-and-thrust belt associated with India–Asia collision in the late Eocene to Oligocene. Scanline surveys along the ore-controlling fault zones show an internal structure that comprises a damage zone, a breccia zone with clasts that have become rounded, and a breccia zone with lenticular clasts, and this complex architecture was formed during at least two compressional substages of deformation. The Pb–Zn mineralization in the Dongmozhazhua area occurs exclusively close to NW–SE-trending reverse fault zones. Microtextural observations reveal that mineralization occurred as veinlets and disseminated blebs in limestone clasts, and as continuous bands and cements in fractured rocks. Cataclastic sulfide grains also can be seen in the matrix of some fault zones. The types of mineralization differ with structural position. The fillings of the ore-bearing veinlets typify the products of hydraulic fracture and both types of mineralization took place concurrently with regional contraction. We consider, therefore, that the ore-bearing fluids in the Dongmozhazhua deposit were concentrated in fault zones during regional compression and that the ore minerals were precipitated during hydraulic fracturing of host rocks. Subsequent fault activity pulverized some pre-existing sulfide material into cataclastic grains in the matrix of a tectonic breccia that developed in the same faults.

The lithium isotopic compositions of adakitic rocks and K-rich volcanic rocks in southern Tibet range from + 1.3‰ to + 7.5‰ and − 4.9‰ to + 3.5‰, respectively. The subduction of the Indian Plate beneath the Lhasa Terrane means that traditional lithium isotopic compositions of various reservoirs are unable to explain the aforementioned δ7Li data. Therefore, it is necessary to determine the Li isotopic compositions of the different geological endmembers underneath southern Tibet. Here, we report the lithium isotopic composition and concentration of Indian upper and lower continental crust. On the basis of whole-rock major and trace element data and Sr–Nd–Pb isotope data, leucogranites from Luozha and Longzi are considered as representative of Indian upper crust, whereas two-mica granites from Quedang and Dala and granulites and gneisses from Nyalam are considered as representative of Indian lower crust. The Li concentration of Indian upper crust varies from 23 to 45 ppm with a mean of 34 ppm, consistent with a weighted mean Li concentration for the upper crust of 35 ± 11 ppm. In contrast, the Li abundance of Indian lower crust is estimated to be 33–84 ppm with a mean of 58 ppm, much higher than the average Li concentration for the lower crust of ~ 8 ppm because of the high modal abundance of biotite and muscovite. The Li isotopic compositions (+ 0.9‰ to + 5.6‰) of Indian upper continental crust are relatively heavy compared with the average Li isotopic composition of upper continental crust. On the other hand, the Li isotopic compositions of Indian lower continental crust (− 4.4‰ to − 0.1‰) are lighter than those of Indian upper continental crust. The isotopically heavy signature of Indian upper crust is produced by high-δ7Li fluids released from the Indian lower crust slab, whereas the lighter signature of Indian lower crust is generated by the partial melting of residual Indian lower crust slab during metamorphic dehydration of Indian lower crust.

The Sanjiang metallogenic belt includes a variety of economically important carbonate-hosted Pb-Zn deposits that share some similarities with classic Mississippi Valley-type (MVT) ore deposits but are hosted within a thrust belt rather than an orogenic foreland. This study aims to clarify the origin of mineralizing fluids responsible for this style of mineralization. Fluid inclusions trapped in ore-stage carbonate and fluorite from these deposits have salinities of ~6 to 28 wt % NaCl equiv and homogenization temperatures of 70° to 370°C that extend to much higher values than are typical of MVT deposits. The majority of ore-stage samples have fluid inclusion molar Br/Cl ratios of between seawater (1.5 × 10 −3 ) and (2.86 ± 0.04) × 10 −3 , but low-salinity fluid inclusions in late calcite have lower Br/Cl of less than (0.55 ± 0.01) × 10 −3 . In contrast, fluid inclusion molar I/Cl ratios are uniformly greater than the seawater value of ~0.8 × 10 −6 and extend from (2.1 ± 1.1) × 10 −6 to (506 ± 12) × 10 −6 . This range of Br/Cl and I/Cl values is similar to what has been reported for fluid inclusions in other MVT districts and together with the fluid salinity implies the ore-forming fluids had a dominant origin from basinal brines (e.g., sedimentary formation waters) formed by the subaerial evaporation of seawater; all the fluids were influenced by addition of organic Br and I derived from the sedimentary host rocks and some fluids were locally modified by interaction with evaporites producing low Br/Cl ratios. The fluid inclusions have 40 Ar/ 36 Ar ratios of up to 441 that are higher than the atmospheric value of 296 and typical of carbonate sedimentary rocks. The fluid inclusions have high concentrations of atmospheric 36 Ar and variable 129 Xe/ 36 Ar and 84 Kr/ 36 Ar ratios that are outside the range expected from mixing air and air-saturated water. These data are likely to reflect a complex fluid history involving acquisition of atmospheric ( 36 Ar, 84 Kr, 129 Xe) and radiogenic (e.g., 40 Ar ✼ ) noble gases trapped in sedimentary rocks and fractionation of these gases between water and hydrocarbons. The 3 He/ 4 He ratios of fluorite fluid inclusions range from a typical crustal value of 0.061 ± 0.004 to values of >0.7 Ra, indicating a minor component of mantle-derived 3 He. The fluids with the highest 3 He/ 4 He also have 4 He/ 40 Ar ✼ close to the mantle value, suggesting the 3 He could have been introduced by a volumetrically minor fluid of either magmatic or deep metamorphic origin ( 40 Ar ✼ = radiogenic 40 Ar). The new halogen and noble gas data are consistent with a model in which regional Pb-Zn mineralization formed by mixing two modified basinal brines that were transported through independent aquifers and fluid pathways to the sites of mineralization. A low-temperature brine contained organic Br, I, and H 2 S, and a high-temperature metal-rich brine (>370°C) that included a volumetrically minor magmato-metamorphic component was channeled up deeply penetrating thrust structures.

Mississippi Valley type (MVT) Pb–Zn deposits can occur in orogenic thrust belts. However, the relationship between MVT ore-forming processes and thrusting is unclear. The 1500-km-long Sanjiang Metallogenic Belt in Tibetan Plateau is an important thrust-controlled MVT ore province with 860 Mt at 0.76–2.3% Pb, 0.3–6.1% Zn. The Zhaofayong MVT ore cluster in the Changdu area is a typical sample. The orebodies in this ore cluster are hosted in limestone, controlled by secondary faults to regional thrusts and forming along these faults. Two Pb–Zn mineralization stages in this cluster are recognized. Stage I is characterized by coarse and euhedral galena + sphalerite + calcite + fluorite + barite and Stage II by fine grained sphalerite + galena + pyrite + calcite. Sm–Nd isotopic dating of calcite forming in Stage I yields isochron ages of 41.1–38.1 Ma, suggesting the mineralization formed during extension following the first regional compression in the Changdu area. The connection between Stage I mineralization and the regional thrusting in the Changdu area can extend to the whole Sanjiang belt. Two stages of regional Pb–Zn mineralization are recognized between 65 Ma and 30 Ma and between 30 Ma and 16 Ma in the belt. The two Pb–Zn mineralization stages are consistent with those regional episodic thrusting activities and both of them immediately occurred after the episodic thrusting. An interpretation of the regional Pb–Zn mineralization is that regional compression forced the movement of hydrothermal fluids along regional thrust-nappe detachment faults and subsequent post-thrust extension caused the migration of hydrothermal fluids to the ore forming locations. The two mineralization stages in the Sanjiang Belt indicate complex processes related to India–Eurasia collision and the gradually younger mineralization ages from southeast to northwest indicate the collision follows the same direction.

During the early Ediacaran, abundant cherts (nodules and bands) were discontinuously hosted by dolostones and mudstones of the Doushantuo Formation (635–551 Ma) in the Yangtze Three Gorges area, South China. The appearance of these cherts correlates with the distribution of acanthomorphic acritarch fossils and is potentially important for the reconstruction of the biogeochemical conditions and oceanic chemistry during the early Ediacaran. However, the formation mechanisms of these cherts are still poorly understood. In this study, silicon isotopes, Ge/Si molar ratios and rare earth element (REE) + Y features are used to constrain the origin mechanism of these cherts, silica geochemical cycling and seawater redox conditions during the Early Ediacaran. Our results suggest that the studied cherts were formed by the replacement of precursor carbonate minerals, which included the dissolution of a siliceous precursor and reprecipitation of microquartz. The positive silicon isotopic composition (typically between 0.4‰ and 1.4‰) and Ge/Si molar ratios (0.04–0.95 × 10 − 6 ) indicate a major seawater source of Ge and Si in these cherts with minor detrital and negligible hydrothermal contamination. In the early Ediacaran Ocean, the dissolved silica may be derived from the river input with high δ 30 Si values. On the other hand, the output of dissolved silica was controlled by inorganic precipitation, which was subsequently fixed as microquartz in cherts during early diagenesis. During the formation of cherts, the primary REE + Y signals of seawater were preserved with little detrital influence. Meanwhile, the REE + Y patterns, obvious negative Ce anomalies and near modern seawater Y/Ho ratios indicate an oxic marine chemistry during chert formation. This oxic seawater environment may have promoted the observed diversity of Ediacaran biological systems. Finally, temporal variations related to silica sources and silicon isotopic fractionation controls in the cherts and/or ocean were also evaluated.

The early Cambrian ocean was marked by significant redox changes, but the detailed redox evolution of seawater and its relationship to biological expansion are not fully understood. Widespread chert successions are present in lower Cambrian deposits on the Yangtze Block, South China, which have great potential to record oceanic chemistry. Here, we analyze rare earth elements (REE) and Si and Fe isotopes along three regional chert sections to constrain the origin of lower Cambrian cherts and the redox conditions of the early Cambrian ocean. The REE patterns and Si isotopes illustrate that lower Cambrian cherts at the Muyang (MY; Yanjiahe Formation) and Zunyi (ZY; Niutitang Formation) locales originated from the replacement of precursor carbonates and black shales, respectively, whereas cherts at Chuanyanping (CYP; Liuchapo Formation) locale originated from direct chemical precipitation from seawater and/or hydrothermal fluids. All cherts, except for several lower CYP cherts with hydrothermal contributions, primarily preserved seawater chemical signals. The Ce/Ce* and Y/Ho ratios indicate that these cherts were deposited near a suboxic/anoxic interface, where seawater may have been ferruginous overall, while more subtle redox changes, namely, ferruginous (less reducing) and ferruginous (more reducing), were identified by Fe isotopes. Based on new data and previous results, we propose that the early Cambrian ocean on the Yangtze Block was characterized by a stratified redox framework. Before ~535 Ma in the Cambrian, oxic seawater occurred only along the inner shelf (MSC section), below which seawater was primarily ferruginous (less reducing). During ~535–526 Ma, oxic seawater gradually expanded to the outer shelf (MY section) and a metastable ferruginous (more reducing) zone dynamically existed at shelf–slope locations. Temporal and spatial comparisons revealed that the water column on the Yangtze Block may have experienced a progressive deepening of the redoxcline during the early Cambrian (~535–526 Ma). The metastable redox zone may have been an analogue of the modern oxygen minimal zone (OMZ), which is associated with biological activity and high productivity. The occurrence and spatial fluctuation of the metastable ferruginous (more reducing) zone may have been associated with the evolution of small shelly fossils from assemblage 1 (SSF1) to assemblage 3 (SSF3) and their spatial expansion from shelf to slope. Therefore, accompanied by oceanic oxygenation, the occurrence and spatial fluctuation of an OMZ-like metastable zone may have regulated the biological diversification and distribution during the early Cambrian (~535–526 Ma).

Porphyry Cu deposits (PCDs) in continental collision settings are new targets for modern mineral exploration, and the relationships among locations, tectonic environments, tonnages, and metal associations are not yet understood. The Gangdese PCD belt formed in a typical continental collision setting between the Indian and Asian plates. In the present study, new elemental and isotopic data from fertile porphyries (Cu–Mo, Cu–Mo–Pb–Zn, and Cu–W–Mo) and barren coeval and pre-ore porphyries of the Gangdese PCD belt were combined with previously published data to provide new insights into the metallogenesis of PCDs in continental collision settings. Factors that contributed to the formation of collision-related PCDs in the Lhasa terrane, Tibet, include the nature of the magmatic sources, magmatic evolution during ascent through the crust, and regional tectonics. These PCDs were not formed by partial melting of ancient lower crust. Instead, attributed to the subduction of Neo-Tethyan oceanic slab, arc magmas and slab-derived fluids resulted in the accumulation of sulfides and hydrous minerals at the base of the continental crust. Partial melting of this refertilized juvenile lower crust produced oxidized water- and Cu-rich magmas with the potential to produce large PCDs, although only under favorable regional tectonic conditions. Regional compressional stress is not conducive to the formation of large magma chambers, so the collision-related adakite-like magmas that developed during the early stages of India–Asia collision commonly lack alteration and associated mineralization. In contrast, relatively large magma chambers can develop in extensional settings within the continental collision orogen, which facilitates the formation of large and giant PCDs. Small Cu–W–Mo deposits formed during the transition between compressional and extensional settings. Thus, the regional tectonic setting and structural framework of the host orogen influenced the tonnage of PCDs. The metal associations of collision-related PCDs in the Lhasa terrane were controlled mainly by the extent of assimilation of the upper crust and the degree of fractional crystallization of the ore-forming magmas. Lead–zinc mineralization in Cu–Mo deposits is attributed to the assimilation of metapelites by fertile magmas in the upper crust, and highly fractionated fertile magmas are required to produce W mineralization.

Bangpu deposit in Tibet is a large but poorly studied Mo-rich (~ 0.089 wt.%), and Cu-poor (~ 0.32 wt.%) porphyry deposit that formed in a post-collisional tectonic setting. The deposit is located in the Gangdese porphyry copper belt (GPCB), and formed at the same time (~ 15.32 Ma) as other deposits within the belt (12 ~ 18 Ma), although it is located further to the north and has a different ore assemblage (Mo–Pb–Zn–Cu) compared to other porphyry deposits (Cu–Mo) in this belt. Two distinct mineralization events have been identified in the Bangpu deposit which are porphyry Mo–(Cu) and skarn Pb–Zn mineralization. Porphyry Mo–(Cu) mineralization in the deposit is generally associated with a mid-Miocene porphyritic monzogranite rock, whereas skarn Pb–Zn mineralization is hosted by lower Permian limestone–clastic sequences. Coprecipitated pyrite and sphalerite from the Bangpu skarn yield a Rb–Sr isochron age of 13.9 ± 0.9 Ma. In addition, the account of garnet decreases and the account of both calcite and other carbonate minerals increases with distance from the porphyritic monzogranite, suggesting that the two distinct phases of mineralization in this deposit are part of the same metallogenic event. Four main magmatic units are associated with the Bangpu deposit, namely a Paleogene biotite monzogranite, and Miocene porphyritic monzogranite, diabase, and fine-grained diorite units. These units have zircon U–Pb ages of 62.24 ± 0.32, 14.63 ± 0.25, 14.46 ± 0.38, and 13.24 ± 0.04 Ma, respectively. Zircons from porphyritic monzogranite yield e Hf (t) values of 2.2–8.7, with an average of 5.4, whereas the associated diabase has a similar e Hf (t) value averaging at 4.7. The geochemistry of the Miocene intrusions at Bangpu suggests that they were derived from different sources. The porphyritic monzogranite has relatively higher heavy rare earth element (HREE) concentrations than do other ore-bearing porphyries in the GPCB and plots closer to the amphibolite lithofacies field in Y–Zr/Sm and Y–Sm/Yb diagrams. The Bangpu diabase contains high contents of MgO (> 7.92 wt.%), FeOt (> 8.03 wt.%) but low K 2 O ( The Bangpu deposit has alteration zonation, ranging from an inner zone of biotite alteration through silicified and phyllic alteration zones to an outer propylitic alteration zone, similar to typical porphyry deposits. Some distinct differences are also present, for example, K-feldspar alteration at Bangpu is so dispersed that a distinct zone of K-feldspar alteration has not been identified. Hypogene mineralization at Bangpu is characterized by the early-stage precipitation of chalcopyrite during biotite alteration and the late-stage deposition of molybdenite during silicification. Fluid inclusion microthermometry indicates a change in ore-forming fluids from high-temperature (320 °C–550 °C) and high-salinity (17 wt.%–67.2 wt.%) fluids to low-temperature (213 °C–450 °C) and low-salinity (7.3 wt.%–11.6 wt.%) fluids. The deposit has lower δD V-SMOW (− 107.1‰ to − 185.8‰) values compared with other porphyry deposits in the GPCB, suggesting that the Bangpu deposit formed in a shallower setting and is associated with a more open system than is the case for other deposits in this belt. Sulfides at Bangpu yield δ 34 S V-CDT values of − 2.3‰ to 0.3‰, indicative of mantle-derived S implying that coeval mantle-derived mafic magma (e.g., diabase) simultaneously supplied S and Cu to the porphyry system at Bangpu. In comparison, the Pb isotopic compositions ( 206 Pb/ 204 Pb = 18.79–19.28, 207 Pb/ 204 Pb = 15.64–15.93, 208 Pb/ 204 Pb = 39.16–40.45) of sulfides show that other metals (e.g., Mo, Pb, Zn) were likely derived mainly from an ancient crustal source. Therefore, the formation of the Bangpu deposit can be explained by a two-stage model involving (1) the partial melting of an ancient lower crust triggered by invasion of asthenospheric mantle-derived mafic melts that provide heat and metal Cu and (2) the formation of the Bangpu porphyry Mo–Cu system, formed by magmatic differentiation in the overriding crust in a post-collisional setting.

A significant belt of carbonate-hosted Pb–Zn mineralization occurs in the Himalayan–Zagros collisional orogenic system. Three differing types of these Pb–Zn deposits within this belt have been identified based on variations in gangue mineral assemblages, leading to the classification of carbonate-, quartz- and fluorite-rich classes of Pb–Zn deposits. The third Pb–Zn mineralization (fluorite-rich) type is common in this orogenic system, but little research has been undertaken on it. Here, we focus on the Mohailaheng deposit, a large-sized fluorite-rich carbonate-hosted Pb–Zn deposit (> 100 Mt Pb + Zn ores with average grade of 2.18%–4.23%); the deposit is located in the Sanjiang Cenozoic thrust-fold belt, an important part of the Himalayan–Zagros collisional orogenic system and an area that formed during the early Tertiary India–Eurasia collision. The main orebodies in this deposit are stratabound and are hosted by Carboniferous limestones that are located along secondary faults associated with a regional thrust fault. The main assemblage is a sphalerite + galena + pyrite sulfide assemblage associated with a calcite + fluorite + barite + quartz + dolomite gangue assemblage. Detailed field and experimental work indicates that the deposit formed during three distinct phases of hydrothermal activity. Studies on fluid inclusion and stable isotopes of gangue minerals indicate that two dominant distinct fluids involving the deposit formation. They include (1) a low-temperature (130–140 °C), high-salinity (23–24 wt.% NaCl equivalent) basinal brine containing Na + –K + –Mg 2 + –Ca 2 + –Cl − ions and abnormally high SO 4 2 − concentrations, which probably derived from Tertiary basins in the regional district, and (2) a low- to moderate-temperature (170–180 °C) and moderate- to high-salinity (19–20 wt.% NaCl equivalent) metamorphic fluid containing Na + –K + –Mg 2 + –Cl – –SO 4 2 − ions and abnormally high F − and organic matter concentrations, that probably formed during regional metamorphism. Some evaporated seawaters and meteoric fluids were also identified in mixtures with these two dominant fluids. The Pb–Zn mineralization at Mohailaheng formed during three distinct stages, consistent with the regional tectonic history. The first stage involved the formation of favorable lithological and structural traps at Mohailaheng during regional thrusting, leading to the migration of compressed metamorphic waters at depth along a detachment zone, sequestering metals from sediments within the region. Basinal brines at the surface also began to infiltrate down along the secondary faults, dissolving gypsum from the underlying sediments. The second stage was associated with the cessation of thrusting and the onset of strike-slip movements along these thrust faults. Metamorphic fluids containing high concentrations of halogen ions, organic gases, and metals ascended into the structural traps at Mohailaheng along the reactivated thrust faults, causing fluorite, calcite, and some sulfide precipitation. Then, basinal brines rich in SO 4 2 − quickly descended into the structural traps along the reactivated faults, causing reduction of SO 4 2 − by organic matter, and producing significant amounts of H 2 S. The reduced sulfur then reacted with the metals in the fluids, causing significant sulfide precipitation. The third stage was associated with metal-depleted fluids, which only resulted in the precipitation of calcite from the diluted basinal brines. Combining these findings with research results on other fluorite-rich carbonate-hosted Pb–Zn deposits in the Himalayan–Zagros orogenic system suggests that this type of carbonate-hosted Pb–Zn deposits can also be classified as Mississippi Valley-type (MVT) deposits, and that the origin of the fluorite in these deposits may be related to multiple hydrothermal fluids involved in the mineralization evolution.

A newly discovered, shoshonitic lava-hosted Pb deposit at Nariniya in central Tibet provides an excellent example to help improve our understanding of the linkage between post-collisional potassic magmatism and ore formation in Tibet. The Pb ores exist as veins or veinlets in NWW-striking fracture zones within the potassic lava (trachyte). The veins contain quartz, galena, pyrite, and sericite (muscovite) as well as minor chalcopyrite, sphalerite, calcite, and dolomite with sericitization, pyritization, and minor silicification. The 40Ar–39Ar plateau age of the hydrothermal muscovite is 37.95 ± 0.30 Ma, which represents the Pb mineralization age. This obtained age is indistinguishable, within analytical error, from the zircon U–Pb age of 37.88 ± 0.22 Ma for potassic lava. Therefore, the ore formation can be genetically linked to potassic magmatism. Galena has similar Pb isotopic composition to magmatic feldspar from the host lava, suggesting the derivation of Pb from the magmatic system. Previous ...

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.

Narigongma is a poorly studied Mo-rich (similar to 0.06 wt.%) post-collisional porphyry Cu deposit located in southern Qinghai Province, Tibet, 400 km northwest of the Yulong porphyry Cu-Mo-Au belt. The Narigongma deposit has a similar age (43-40 Ma) to porphyry deposits in the Yulong belt, but different ore assemblages. The Narigongma deposit is associated with Eocene granodiorite and granite intrusions that were emplaced into a Permian volcanic-sedimentary rock sequence. An similar to 43.3 Ma biotite granite stock (P1 porphyry) is the earliest Eocene intrusion, and this was itself intruded by a number of smaller, similar to 43.6 Ma fine-grained granite porphyry stocks (P2 porphyry) and several post-ore quartz diorite porphyry dikes (similar to 41.7 Ma). The main Cu-Mo mineralization at Narigongma is associated with the P1 porphyry. Hydrothermal alteration surrounding the deposits is generally characterized by concentric zones that range from an inner potassic zone outward to phyllic and argillic alteration zones, and an outer propylitic zone. Hypogene mineralization at Narigongrna was characterized by early-stage precipitation of molybdenite during potassic alteration and late-stage deposition of chalcopyrite during phyllic alteration. Deposition of both the Mo and Cu mineralization stages was caused by decreasing temperature. A high degree of crystallization of the P1 porphyry occurred prior to fluid saturation that produced Mo enrichment in the residual melt due to the incompatible behavior of Mo. However, compatible Cu was sequestered by the crystallizing phases and resulted in the generation of a high-Mo/Cu magmatic-hydrothermal fluid and the final Mo +/- Cu mineralization assemblage. Zircon epsilon(Hf)(t) values of +4.1 to +7.9 are indicative of magma derivation from a depleted source. These isotopic data, coupled with other geochemical characteristics of the Narigongma porphyry, such as high SiO2 and K2O contents, low MgO contents and compatible element abundances, and highly fractionated rare earth element patterns, indicate a mixing model for the origin of the porphyry bodies. Generation of the post-collisional ore-forming porphyries occurred in two stages: (1) partial melting of metasomatized phlogopite-bearing lithospheric mantle that generated potassic to ultra-potassic mafic melts, and (2) underplating of such melts beneath thickened juvenile lower crust, which triggered partial melting of the lower crust to produce the ore-forming, high-K adakitic magma. (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Abundant bedded chert deposits occurred at the Ediacaran–Cambrian transition interval at the Yangtze Platform, South China. However, there is not a non-controversial and integrated model for the origin of these chert deposits from trace element patterns and oxygen isotopic evidence. To understand the origin and the oceanic environment of these chert deposits, we analyzed the Si isotopic composition, major and trace elements of chert samples from two stratigraphically correlated sections with a depositional age around 542 Ma. The relationship between Al2O3 content and Si isotopic values indicated three end-member sources of Si derivation. The most negative δ30Si values (−0.3‰ to −0.5‰) reflect the silicon derived from hydrothermal fluids due to intense tectonic activity. The most positive δ30Si values (up to +1.2‰) could reflect the increase of 30Si in the hydrothermal fluid by precipitation, or the signature of seawater. The medium δ30Si value (+0.2‰ to +0.7‰) could be interpreted as the mixing of the hydrothermal fluid and volcanic materials with negative δ30Si and ambient seawater with positive δ30Si. It is obvious that the injection of Si-rich hydrothermal fluid would affect the δ30Si and trace element distribution of seawater. In addition, we estimated that the δ30Si values of Ediacaran–Cambrian transitional seawater range from +2.2‰ to +3.5‰ based on assumed fractionation factors −2.3‰ < ɛ < −1.0‰ between precipitated Si in chert deposits and initial dissolved Si in seawater. According to Si isotopic composition and other geochemical evidence, we maintain that hydrothermal activity played an important role not only in the formation of bedded chert and polymetallic ore deposits, but also in the oceanic environment and concurrent evolution of life during Ediacaran–Cambrian transition interval.

Silicon is essential for the growth of diatoms, which utilize dissolved silicic acid in lake water and form opaline silica (SiO 2 · n H 2 O). During the uptake of dissolved silicic acid, there is a preferential incorporation of light silicon isotope ( 28 Si) into biogenic silica, resulting in the enrichment of heavy silicon isotope ( 30 Si) in dissolved silicic acid. The silicon isotope composition of diatom silica (δ 30 Si diatom ) may thus record changes in the percentage utilization of dissolved silicic acid by diatoms, which can be then related to other aspects of climate/environment. With the aim of exploring the potential of δ 30 Si diatom as an indicator of lacustrine environment, here we made the first measurements of δ 30 Si diatom in the sediment core from Lake Huguangyan, a closed crater lake in China. The result shows that δ 30 Si diatom varies from −0.6‰ to 1.1‰ and displays broad similarities to variations in contents of biogenic silica and organic carbon throughout the sediment core. δ 30 Si diatom is a reliable paleotemperature proxy in Lake Huguangyan, which is supported by good correlation between δ 30 Si diatom and available temperature records. Heavier δ 30 Si diatom indicates greater dissolved silicic acid utilization at higher temperature while lighter δ 30 Si diatom reflects decreased utilization at lower temperature. The most negative δ 30 Si diatom values in the sediment core occur between AD 1580 and 1920, which suggests AD 1580–1920 was the coldest period in Lake Huguangyan over the past 2000 years, thus providing evidence for the existence of the LIA in tropical South China. There are few means by which to reconstruct the history of temperature changes in tropical terrestrial region. δ 30 Si diatom , in this study, has proven to be a new promising paleotemperature proxy in lacustrine sediments, and may play important role in reconstructing past temperature changes at low latitude in the future. Detailed investigations on the silicon isotopes of diatoms in more lakes would be desirable in further research.

The Dongmozhazhua deposit, the largest Pb–Zn deposit in south Qinghai, China, is stratabound, carbonatehosted and associated with epigenetic dolomitization and silicification of Lower–Middle Permian—Upper Triassic limestones in the hanging walls of a Cenozoic thrust fault system. The mineralization is localized in a Cenozoic thrust-folded belt along the northeastern edge of the Tibetan plateau, which was formed due to the India–Asia plate collision during the early Tertiary. The deposit comprises 16 orebodies with variable thicknesses (1.5–26.3 m) and lengths (160–1820 m). The ores occur as dissemination, vein, and breccia cement. The main sulfide assemblage is sphalerite + galena + pyrite + marcasite ± chalcopyrite ± tetrahedrite, and gangue minerals consist mainly of calcite, dolomite, barite, and quartz. Samples of pre- to post-ore stages calcite yielded d13C and d18O values that are, respectively, similar to and lower than those yielded by the host limestones, suggesting that the calcite formed from fluids derived from carbonate dissolution. Fluid inclusions in calcite and sphalerite in the polymetallic sulfidization stage mostly comprise liquid and gas phases at room temperature, with moderate homogenization temperatures (100–140°C) and high salinities (21–28 wt% NaCl eq.). Micro-thermometric fluid inclusion data point to polysaline brines as ore-forming fluids. The dD and d18O values of ore fluids, cation compositions of fluid inclusions, and geological information suggest two main possible fluid sources, namely basinal brines and evaporated seawater. The fluid inclusion data and regional geology suggest that basinal brines derived from Tertiary basins located southeast of the Dongmozhazhua deposit migrated along deep detachment zones of the regional thrust system, leached substantial base metals from country rocks, and finally ascended along thrust faults at Dongmozhazhua. There, the base-metal-rich basinal brines mixed with bacterially-reduced H2S-bearing fluids derived from evaporated seawater preserved in the Permo–Triassic carbonate strata. The mixing of the two fluids resulted in Pb–Zn mineralization. The Dongmozhazhua Pb–Zn deposit has many characteristics that are similar to MVT Pb–Zn deposits worldwide.

In the Yangbajing area, southern Tibet, several monogenic volcanoes were conformably superimposed on the Linzizong calc-alkaline volcanic successions. According to their petrologic and geochemical characteristics, these monogenic volcanoes are composed of three rock varieties: tephritic phonolitic plugs and shoshonitic and trachytic lavas. Their geochemical systematics reveals that low-pressure evolutionary processes in the large voluminous Linzizong calc-alkaline magmas were not responsible for the generation of these potassic–ultrapotassic rocks, but the significant change in petrologic and geochemical characteristics from the Linzizong calc-alkaline to potassic–ultrapotassic magma is likely accounted for the change of metasomatic agents in the southern Tibetan lithospheric mantle source during the Paleocene to Eocene. The tephritic phonolites containing both leucite and plagioclase show primary ultrapotassic character similar to that of Mediterranean plagioleucititic magmas. Radiogenic Sr increases with SiO 2 in the xenolith-bearing trachytes strongly suggesting significant crustal assimilation in the shoshonitic magmas. The Yangbajing ultrapotassic rocks have high K 2 O and Al 2 O 3 , and show depletion of high field strength elements (HFSEs) with respect to large ion lithophile elements. In primitive mantle-normalized element diagrams, all samples are characterized by positive spikes at Th (U) and Pb with negative anomalies at Ba, Nb–Ta and Ti, reflecting the orogenic nature of the ultrapotassic rocks. They are characterized by highly radiogenic 87 Sr/ 86 Sr (i) ratios (0.7061–0.7063) and unradiogenic 143 Nd/ 144 Nd (i) (0.5125), and Pb isotopic compositions ( 206 Pb/ 204 Pb = 18.688–18.733, 207 Pb/ 204 Pb = 15.613–15.637, and 208 Pb/ 204 Pb = 38.861–38.930) similar to the global subducting sediment. Strong enrichment of incompatible trace elements and high Th fractionation from the other HFSEs (such as Nb and U) clearly indicate that the Th-enriched sedimentary component in a network veined mantle source was mainly introduced by sediment-derived melts. In addition, the ultrapotassic rocks have significant Ce (Ce/Ce* = 0.77–0.84) and Eu (Eu/Eu* = 0.72–0.75) anomalies, suggesting a subduction sediment input into the southern Tibetan lithospheric mantle source. In contrast, high U/Th (> 0.20) and Ba/Th (> 32) and low Th/La (

The Maoniuping REE deposit is a world-class deposit with 1.2 Mt of REO grading on average 2.89 wt.% REO. It is the largest in the 270-km long Mianning–Dechang REE belt and is associated with Himalayan carbonatite–alkalic complexes in the eastern Indo-Asian collisional zone, Western Sichuan Province, China. The deposit is hosted by nordmarkite stocks and carbonatite sills that display a radiometric age of 40 to 24 Ma and which intruded a Yanshanian granite pluton. The 40.3 to 27.8 Ma REE mineralization occurs as vein systems hosted in nordmarkite and carbonatite with minor altered granite and rhyolite. Four ore types are recognized based on ore texture and mineral assemblage: (1) disseminated; (2) pegmatitic; (3) brecciated; and 4) stockwork (stringer) types. Five mineralizing stages are confirmed according to vein crosscutting relationships, mineral assemblage and microthermometric results, these are: 1) carbonatite stage, 2) pegmatite stage, 3) barite–bastnaesite stage, 4) later calcite stage and 5) epigenetic oxidation stage. Varied inclusion assemblages are found in fluorite, quartz, bastnaesite, barite and calcite from stages 1 through to stage 4. The dominant inclusion types include: melt, melt–fluid, CO2-rich fluid and aqueous-rich fluid inclusions. Fluid, melt–fluid and melt inclusion studies indicate that the ore-forming fluid resulted from the unmixing of carbonatite melt and carbonatitic fluid. Initial ore-forming fluids were high-temperature (600 to 850 °C), high-pressure (> 350 MPa) and high-density supercritical orthomagmatic fluids, characterized by SO4-rich and multi-component composition (e.g. K, Na, Ca, Ba, Sr and REE). The dominant anion is not Cl, but SO4. The evolution of the ore-forming fluid is from a melt–fluid at high temperature, through a CO2-rich fluid at high to medium temperature to aqueous-rich fluid at low temperature. REE precipitation occurred from a high to medium temperature CO2-rich fluid. The main mechanism for REE precipitation was phase separation of CO2 and aqueous fluids resulting in a decrease of temperature and pressure.

More than 50 Sb–Au deposits and occurrences have recently been found in an E–W-striking structural–thermal dome zone, related to the South Tibetan detachment system (STDS) in the Himalayan orogen. They are mainly distributed around the thermal domes intruded by mid-Miocene leucogranite bodies, and show a metallic zoning varying from Au, Sb–Au, to Sb mineralization from the domes outwards. At least three mineralization styles are recognized, i.e., Sb-, Sb–Au-, and Au-styles of deposits, all hosted in the Mesozoic, Tethyan passive continental-margin sequences. The Shalagang-style Sb deposits, controlled by N–S-striking normal faults and E–W-striking bedding faults, are usually composed of quartz–stibnite veins with open-space filling structure. Low homogenization temperatures (135–367 °C) and low salinities (0.5–12.6 wt.% NaCl equiv.), as well as highly variable calculated δ18OH2O values (− 11.5–12.3‰) and very low δD values (− 140 to − 166‰), suggest an ore-forming fluid dominated by geothermal or meteoric water. The Mazhala-style Sb–Au deposits were controlled by small-scale bedding faults and their intersections with N–S-striking normal faults. They consist of gold-bearing quartz–stibnite veins with minor disseminated ores, and are associated with silicification, sericitization, chloritization and carbonatization. CO2-rich fluid inclusions in Au-bearing gangue quartz, calculated δ18OH2O values (5.4–11.5‰) and measured δD values (− 72 to − 119‰) for precipitation of gangue quartz suggest a mixed ore-forming fluid containing both meteoric and magmatic waters. Langkazi-style Au deposits, controlled by detachment faults and normal faults along the margin of the thermal dome, consist of Au-bearing quartz veins and disseminated ores with intensive silicification, chloritization and sericitization. Gangue quartz from Au-bearing quartz veins yielded a limited range of δ18OH2O values (1.8–8.2‰) and δD values (− 52 to − 83‰), suggesting an ore-forming fluid dominated by magmatic water. The genetic links among these deposits, the granite-intruded thermal domes and ore-controlled fault systems suggest a mid-Miocene metallogenic epoch. The spatial zonation in metallic associations, and the variation in compositions and sources of ore-forming fluids indicate that these Sb and Au deposits resulted from a post-collisional dome-centered geothermal system, driven by mid-Miocene granite magmas related to the STDS. Rb–Sr and S isotopic data indicate that the scavenging of the convective geothermal fluids through Mesozoic permeable strata play an extremely significant role in concentrating metallic Sb and minor Au, and forming Sb, Sb–Au, and Au deposits in southern Tibet.

The Himalayan Mianning–Dechang REE belt, western Sichuan, SW China, is approximately 270 km long and 15 km wide, and contains total reserves of > 3 Mt of LREE, including one giant (Maoniuping), one large (Dalucao) and a number of small-medium REE deposits (Muluozhai, Lizhuang) and occurrences. The belt is tectonically located within a continental collision zone, i.e., the eastern Indo-Asian collision zone. REE mineralization is associated with Himalayan carbonatite–alkaline complexes, which consist of carbonatitic sills or dykes and associated alkaline syenite stocks. Available dating data define a Himalayan metallogenic epoch (40–10 Ma). The magmatic–hydrothermal REE systems were controlled by Cenozoic large-scale strike-slip faulting and the resultant tensional fissure zones, developed in a transform phase from a transpressional to a transtensional regime. Alteration is characterized by fenitization, which formed fenite halos enveloping the REE orebodies. Associated REE mineralization occurs as complex vein systems consisting of various veinlets, stringer and stockwork zones, and breccia-pipe systems. The REE orebodies show various shapes from plate-like, lenticular to pipe-like in different districts. Ore types are dominated by pegmatitic, carbonatitic, breccia, and stringer (stockwork), and disseminated ores, mainly composed of barite + fluorite + aegirine-augite + calcite + bastnaesite assemblages. Bastnaesite precipitation (275–325 °C) appears in three assemblages: early clastics-bearing barite + arfvedsonite + calcite + bastnaesite; intermediate quartz + fluorite + barite + bastnaesite; and late fluorite + barite + bastnaesite assemblages. These were followed by low-temperature pyrite + galena + sphalerite assemblages. Fluid inclusion studies indicate that gangue quartz and fluorite hosts numerous melt/fluid inclusions with a large proportion of sulfate (i.e., BaSO 4 , K 2 SO 4 , and CaSO 4 ), in addition to CaCO 3 and CaF 2 , yielding abnormally high (up to 750 °C) but wide-ranging homogenization temperatures (750–125 °C). The similarity of Sr–Nd isotopic compositions for gangue minerals with those of the host carbonatite–syenite suggests that ore-forming fluids, especially the F − , HCO 3 2− , SO 4 2− components, were derived from carbonatite–syenite magmas. The O–D–C isotopic data from gangue calcite, fluorite and quartz indicate that the fluids were of orthomagmatic origin, but also involved mixing with an external fluid. On the basis of our synthesis, we propose a possible genetic model for REE mineralization, in which the hydrothermal system temporally underwent a complicated evolution from separation of high-T sulfate-bearing NaCl–KCl brine resulting in effective precipitation of REE-fluorocarbonate and sulfate, to subsequent mixing with low-temperature meteoric water precipitating minor sulfide assemblages. Spatially, this sequence of events generated a three-tier REE mineralized architecture, in which pipe-like breccia orebodies grade downwards from shallow to deep structural levels into large veinlet ore systems, and veinlet-disseminated orebodies.

The Dazi volcanic basin is located in the eastern segment of the Gangdese magmatic belt, south Tibetan collision zone. The compositions of Dazi Eocene (40–38 Ma) volcanic rocks range from picrite through basalt to basaltic andesite with low-K tholeiitic affinities. The primitive samples are characterized by high-MgO (6.34–14.35 wt.%) and compatible trace elements (Cr = 131–844 ppm, Ni = 45.2–268 ppm). The contents of K 2 O (0.02–0.80 wt.%) and incompatible trace elements are relatively low, and showing depletion of high field strength elements (HFSE). Their 87 Sr/ 86 Sr i ratios are between 0.7043 and 0.7061 and e Nd ( t ) values vary from + 5.24 to + 3.58. The ratios of 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb are 18.347–18.678, 15.583–15.631 and 38.398–38.854, respectively. The geochemical and isotopic characteristics indicate that the magmas originated from an asthenospheric mantle source which experienced contamination by variable subduction components. The occurrences of the most primitive rocks at south Tibet, represented by high-MgO rocks in the Dazi area, suggest an elevated geothermal gradient for the south Tibetan mantle sources during the Indo–Asia collision. The identification of the MgO-rich rocks and the delineation of their hot asthenospheric mantle source will provide new evidence for a rollback and then breakoff of the subducting Neo-Tethyan slab during the early stage of the India–Asia collision. Combining with the geochronologic and geochemical results of syn-volcanic mafic dykes in the LVS and the abundant mafic microgranular enclaves (MMEs) in the Gangdese granitoids, and the relatively low volume of mafic rocks and their association with volumetrically significant felsic lavas indicates that the underplating of mantle-derived magma under dominantly compressive stress regime was existing in this area during the Eocene.

Lithium isotopic compositions of fluid inclusions and hosted gangue quartz from a giant volcanogenic massive sulfide deposit in China provide robust evidence for inputting of magmatic fluids into a Triassic submarine hydrothermal system. The δ7Li results vary from +4.5‰ to +13.8‰ for fluid inclusions and from +6.7‰ to +21.0‰ for the hosted gangue quartz(9 gangue quartz samples containing primary fluid inclusions). These data confirm the temperature-dependent Li isotopic fractionation between hydrothermal quartz and fluid (i.e., Δδ7Liquartz-fluid = –8.9382 × (1000/T) + 22.22(R2 = 0.98; 175 °C–340 °C)), which suggests that the fluid inclusions are in equilibrium with their hosted quartz, thus allowing to determine the composition of the fluids by using δ7Liquartz data. Accordingly, we estimate that the ore-forming fluids have a δ7Li range from −0.7‰ to +18.4‰ at temperatures of 175–340 °C. This δ7Li range, together with Li–O modeling , suggest that magmatic fluid played a significant role in the ore formation. This study demonstrates that Li isotope can be effectively used to trace magmatic fluids in a seafloor hydrothermal system and has the potential to monitor fluid mixing and ore-forming process.

Carbonatite-associated rare-earth-element (REE) deposits are the most significant source of the world’s REEs; however, their genesis remains unclear. Here, we present new Sr-Nd-Pb and C-O isotopic data for Cenozoic carbonatite-hosted giant REE deposits in southwest China. These REE deposits are located along the western margin of the Yangtze Craton that experienced Proterozoic lithospheric accretion and controlled by Cenozoic strike-slip faults related to Indo-Asian continental collision. The Cenozoic carbonatites were emplaced as stocks or dykes with associated syenites and tend to be extremely enriched in Ba, Sr and REEs and have high 87Sr/86Sr ratios (>0.7055). These carbonatites were likely formed by melting of the sub-continental lithospheric mantle (SCLM), which had been previously metasomatized by high-flux REE- and CO2-rich fluids derived from subducted marine sediments. The fertility of these carbonatites depends on the release of REEs from recycled marine sediments and on the intensity of metasomatic REE refertilization of the SCLM. We suggest that cratonic edges, particularly along ancient convergent margins, possess the optimal configuration for generating giant REE deposits; therefore, areas of metamorphic basement bounded or cut by translithospheric faults along cratonic edges have a high potential for such deposits.

Major and trace element compositions, and Sr–Nd–Pb and O–C isotope data for Cenozoic carbonatites (WSC) in western Sichuan, east Tibet, China are presented in this paper. The WSC occur in the eastern Indo-Asian collision zone (EIACZ), and occur as sills or dykes in the contemporaneous syenitic intrusions, which form a 270-km long, NS-trending belt of carbonatite–alkalic complexes, controlled by strike-slip faults that accommodated and adjusted to the collision strain. These carbonatite–alkalic complexes with Cenozoic potassic felsic rocks and calc-alkaline (shoshonitic) lamprophyres form a semi-continuous potassic igneous province with magmatism peaking at ∼35 Ma in the EIACZ. The WSC are characterized by low SiO2 ( The WSC are extremely enriched in LILE (Sr, Ba) and light REE, but relatively depleted in high-field strength elements (Nb, Ta, P, Zr, Hf, Ti), suggesting a metasomatized mantle source. Their δ18OV-SMOW (6.4∼10.5‰) and δ13CV-PDB (− 3.9 to − 8.5‰) values are similar to those of primary, mantle-derived carbonatites, also suggesting a mantle genesis. However, they have extremely low aNd(t) of − 3.2 to − 18.7 and relative high (87Sr / 86Sr)i of 0.706020∼0.707923, as well as a wide range of 207Pb / 204Pb (15.362 ∼ 15.679) and 208Pb / 204Pb ratios (38.083∼39.202), which distinguish them from most carbonatites around the world. Their Sr–Nd, Sr–Pb and Nd–Pb isotopic signatures indicate that some carbonatites underwent Sr–Nd–Pb contamination by crustal materials, but the least-contaminated carbonatites were derived from a transitional source between EMI and EMII components. The TDM and modeling on Sr–Nd isotopic compositions indicate that the formation of this source probably was related to recycling of oceanic crust and pelagic/terrigenous sediments with various mass ratios. Partial melting of the mantle source was most likely triggered by a Cenozoic asthenospheric mantle diapir related to Indian–Asian continent collision at 65–45 Ma. Rising and emplacement of carbonatitic magmas with coeval potassium-rich magmas took place in the tectonic regime of the transition from transpression to transtension at Eocene/Oligocene boundary in the EIACZ.

The Muluozhai REE deposit, located about 60 km to the southwest of the Mianning county, Sichuan Province, is the third largest light REE deposit in Sichuan. The Muluozhai REE deposit is tectonically located on the northern Jinpingshan Mountains, a Cenozoic intracontinental orogenic belt in southwestern China. The authors analyzed the essential ores from two tunnels for their carbon, hydrogen, oxygen, and sulfur isotopic compositions of the mineralizing fluid. The δ13CV-PDB values of fluid from fluid inclusions of the quartz and fluorite vary from −2.5 to −9.0 per mil, which show characteristics of mantle-derived carbon. The δDV-SMOW values of fluid from fluid inclusions of the calcite, quartz, fluorite, and bastnaesite range from −63 to −87 per mil, which are characteristics of mantle-derived hydrogen. The δ34SV-CDT values of barite and galena vary in a narrow range of +0.1 to +2.2 per mil and −8.6 to −9.3 per mil, respectively, showing the isotopic characteristics of mantle-derived sulfur. The δ13CV-PDB values and the δ18OV-SMOW values of calcite range from −6.6 to −6.8 per mil and from +8.4 to +9.1 per mil, respectively, which are fallen into the range of the “primary carbonatites”, showing the carbon and oxygen in the ores of the Muluozhai ore veins were mainly derived from depths. The stable isotopic data suggest a mantle source for the rare earth elements mineralization and a dynamic process involving mantle materials and tectonics.