East Kunlun Mountains Research Articles

Geochemical and detrital zircon U[sbnd]Pb geochronology of the qigequan formation in the yuejin 2 area: Implications for tectonics and uranium sources

The Yuejin 2 mining area lies within the Mangya region, situated on the northwestern periphery of the Qaidam Basin. Recently discovered, it harbors a medium-sized uranium deposit within the basin. Nonetheless, investigations into the sedimentary provenance and paleoclimate of the Neogene Qigequan Formation in this locale have been sparse. This study delves into a meticulous examination of petrogeochemical attributes and the detrital zircon UPb geochronology of sandstones from the Qigequan Formation within the mining area, aiming to discern their origins and tectonic context. By scrutinizing the trace and rare earth element signatures of the rocks, we discern that the lower strata of the Qigequan Formation primarily originate from felsic sedimentary sources, indicative of a continental island arc tectonic. Detrital zircon ages from the Qigequan Formation manifest primarily in three distinct periods: 169–292 Ma, 327–505 Ma, and > 670 Ma, corresponding with geological ages documented in the Altyn Tagh and East Kunlun Mountains orogenic belts. Predominantly, the Altyn Tagh orogenic belt and the Qimantag Mountains in the eastern Kunlun Mountains serve as the principal source of the Qigequan Formation. The chemical weathering index (CIA*) is 70.54, suggesting a moderate level of chemical weathering within the source region. Additionally, the angular morphology of the detrital zircon implies proximity between the source area and the basin. During the late sedimentary epoch of the Qigequan Formation, rapid uplift of the Altyn Tagh led to a notable rise in temperature within the Yuejin 2 area, transitioning from a reducing to an oxidizing environment. Surface water enriched with uranium and eroded source rocks gradually infiltrated the sand bodies of the lower Qigequan Formation. Interaction with reducing agents facilitated the enrichment and formation of ore. Paleoclimatic shifts instigated the oxidation–reduction sequence necessary for uranium mineralization. Favorable conditions, characterized by oxygen-rich paleo-water, facilitated the activation, migration, and re-enrichment of uranium. Moreover, the Himalayan movement acted as a driving force and a pivotal factor in creating the conducive conditions imperative for large-scale uranium mineralization in the area.

Effects of crustal rheology and fault strength on the formation of the Qaidam Basin: Results from 2-D mechanical modeling

The Cenozoic Qaidam Basin on the northern Tibetan Plateau is bounded by the foreland thrust belts of the East Kunlun Mountains to the south and Qilian Mountains to the north. In this basin, the maximum thickness of the accumulated sedimentary strata is ∼17 km near the geographic center of the basin. Such a basin architecture challenges the classic foreland-basin deformation model, wherein substantial sediments should be localized near the basin margins owing to thrust loads on a bending plate. Based on the available geological and geophysical constraints, we built a two-dimensional viscoelastoplastic finite element model to investigate whether the unique pattern of the Qaidam Basin was mechanically related to the rheological structure of the crust and frictional strength of the basin-bounding thrust fault. By testing reasonable bounds on the viscosity of the ductile crust and effective frictional coefficient of the thrust faults, we modeled a basin framework that fits well with that of Qaidam Basin. The model parameters include a layered rheological structure, with the viscosity of the ductile crust of the basin being two orders of magnitude greater than that of the adjacent mountain belts and an effective frictional coefficient of >0.2. Furthermore, the initial width of the basin significantly affected its deformation style. Therefore, the layered rheological structure of the crust and frictional strength of the basin-bounding thrust faults played important roles in the formation of the Qaidam Basin. Our findings suggest that the inelastic behavior of the crust affects foreland deformation, highlighting that the classic foreland basin model must be revised for some tectonic settings.

Sedimentary infill of Early-Middle Jurassic in the southeastern Tarim Basin and its constraints on the evolution of the Altyn Tagh Fault in the Northeast Tibet Plateau

The Altyn Tagh Fault (ATF) is the boundary between the Tarim Basin and the Tibetan Plateau, which is crucial for the tectonic evolution and uplift of the Tibetan Plateau. However, the Mesozoic tectonic evolution of the ATF remains controversial. This study reconstructed the Early-Middle Jurassic palaeogeographic pattern of the southeast depression of the Tarim Basin (SDTB) and revealed potential source-sink relationships between the SDTB and its adjacent mountains to constrain the Mesozoic tectonic evolution of the ATF based on sedimentology and provenance analyses. The results indicated that the SDTB was infilled with alluvial fan facies during the Early Jurassic and the late Early Jurassic, giving rise to the braided-river delta to lacustrine facies. During the Middle Jurassic, the lacustrine basin expanded with the wide development of lacustrine facies, and the lacustrine basin contracted and was primarily infilled with braided delta facies in the late Middle Jurassic. The Jurassic sedimentary center, the SDTB situated in front of the Altyn Tagh Mountain and East Kunlun Mountain, played a crucial role in the development of three types of hydrocarbon source rocks: charcoal mudstone, coal beds, and dark mudstone. Combined with seismic reflection data interpretations, the Early Jurassic the STDB is a rift basin that underwent a rift-depression transition in the late Early Jurassic, solidifying the characteristics of the depression basin in the Middle Jurassic. The extensional environment in the SDTB resulted from the tectonic effects of slab break-off following the closure of the Tethys Ocean and the spreading of the distal stress field of the Bangonghu–Nujiang Ocean. The analyses of gravel component and heavy mineral indicated that the Jurassic source of the basin was traced back to the ancestral Altyn Mountains, the East Kunlun Mountains, and the internal palaeo-uplift of the basin, which demonstrated that the SDTB and Western Qaidam Basin (WQB) shared a common lacustrine basin during the Jurassic, suggesting that the ATF did not initiate strike-slip movements during this period. This study significantly enhanced our understanding of the Mesozoic tectonic evolution in Northwest China, providing valuable insights into exploring Mesozoic oil-bearing basins in the northern Tibetan Plateau. Furthermore, it served as a useful reference for constraining the timing of the initial slip of similar plate boundary strike-slip faults.

A New Type of White Nephrite from Limestone Replacement along the Kunlun–Altyn Tagh Mountains: A Case from the Mida Deposit, Qiemo County, Xinjiang, China

The recently discovered Mida nephrite deposit, located in the East Kunlun Mountains, Qiemo County, Xinjiang, Northwest China, contains new types of white and greenish white nephrite formed by limestone replacement, which shows microstructures, macroscopic features and country rocks typologies that are quite different from those of the other deposits along the Kunlun–Altyn Tagh Mountains. The gemological and mineralogical characteristics of Mida nephrite are presented here. These nephrites show an ivory white color and a porcelain-like appearance, with semitranslucent-to-opaque transparency and a porcelain-to-greasy luster. Petrographic study, electron probe microanalysis (EPMA) data and scanning electron microscopy (SEM) images have indicated that the nephrite is composed of tremolite, accompanied by minor quartz, calcite and diopside. Tremolite aggregates have shown different textures, like flaky, granular, fibrous–felted, bundle, radial and metasomatic relict textures. Quartz has appeared in granular or disseminated form, dispersed in the tremolite matrix. Calcite has shown a metasomatic relict texture in the white nephrite samples. Diopside has shown euhedral grains, with some distributed with a certain geometric appearance. Based on our observations, it is suggested that the quartz in the nephrite originated from Si-rich hydrothermal fluids. We propose that the substantial size difference of mineral grains, together with uncompacted grains with inter-particle pores, are the main reasons for the internal reflection and refraction under transmitted light, which allow less transmitted light to pass through the nephrite body and generate the appearance of a semitranslucent-to-opaque transparency, ivory white color and porcelain luster. Our study has unveiled that the Mida nephrite is not typical of the two known types (D-type: dolomite-related; S-type: serpentinite-related) and is overlapped by quartz grains dispersed throughout the less compact tremolite matrix. These observations would help set it apart from the majority of nephrite jades found in the Kunlun Mountains region and provide valuable insights for enhancing comprehension of the diversity of the nephrite deposits.

Slight Mass Loss in Glaciers over the Ulugh Muztagh Mountains during the Period from 2000 to 2020

Knowledge about changes in the glacier mass balance and climate fluctuation in the East Kunlun Mountains is still incomplete and heterogeneous. To understand the changes in the glacier mass in the Ulugh Muztagh Mountains in the East Kunlun Mountains due to global warming, a time series of satellite stereo-images from the Terra Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) were derived from 2000 to 2020. Digital elevation models (DEMs) of the glaciers were generated and used to assess the changes in these glacier masses from 2000 to 2020. The results show that the surface elevation of glaciers in the Ulugh Muztagh region changed by −0.17 ± 10.74 m from 2000 to 2020, corresponding to a mass change of −0.14 ± 9.13 m w.e. The glacier mass balance increased by 0.64 ± 9.22 m w.e. in 2000–2011 and then decreased by 0.78 ± 9.04 m w.e. in 2011–2020. The annual mass balance of the glaciers was −0.0072 ± 0.46 m w.e./yr from 2000 to 2020, showing glacial stability. The equilibrium line altitude (ELA) of the glacier was 5514 m a.s.l. from 2000 to 2020. In addition, we also found that the glacier mass losses in the west and north slopes were more significant than those in the east and south slopes. There was a phenomenon of glacier surges in the Yulinchuan glacier from 2007 to 2011. Overall, the glaciers were relatively stable with respect to the total glacier thickness in the Ulugh Muztagh Mountains.

Geochemical Characteristics of the Mineral Assemblages from the Niukutou Pb-Zn Skarn Deposit, East Kunlun Mountains, and Their Metallogenic Implications

The Niukutou Pb-Zn deposit is typical of skarn deposits in the Qimantagh metallogenic belt (QMB) in the East Kunlun Mountains. In this study, based on detailed petrographical observations, electron microprobe analyses (EMPAs), and laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses, we report the major and trace element compositions of the typical skarn mineral assemblages (garnet, pyroxene, ilvaite, epidote, and chlorite) in this deposit. Three hydrothermal mineralization stages with different mineral assemblages of the prograde metamorphic phase were determined, which were distributed from the inside to the outside of the ore-forming rock mass. Grt1+Px1 (Stage 1), Grt2+Px2 (Stage 2), and Px3 (Stage 3) were distinguished in the Niukutou deposit. Furthermore, the ilvaites in the retrograde metamorphic phase can be divided into three stages, namely Ilv1, Ilv2, and Ilv3. The ore-forming fluid in Stage 1 exhibited high ∑REE, U, and Nd concentrations and δEu, δCe, and LREE/HREE values, which were likely derived from a magmatic–hydrothermal source and formed at high temperatures, high fO2 values, and mildly acidic pH conditions, and probably experienced diffusive metasomatism in a closed system with low water/rock ratios. In Stages 2 and 3, the ore-forming exhibited lower ∑REE, U, and Nd concentrations and δEu, δCe, and LREE/HREE values, with high Mn content that had likely experienced infiltrative metasomatism in an open system with high water/rock ratios. From Ilv1 to Ilv3, the δEu and U contents decreased, whereas the Mn content increased, indicating that the oxygen fugacity of mineralization was in decline. The ore-forming fluid evolution of the Niukutou deposit can be characterized as follows: from Stage 1 to Stage 3, the hydrothermal fluid migrated from the deep plutons to the shallow skarn and marble; the environment altered from the high fO2 and temperature conditions to low fO2 and temperature values, and the pH and Mn contents increased. The fluids contained considerable metal ore-forming materials that were favorable for the enrichment and precipitation of the Fe content. In the retrograde metamorphic phase, with the decrease in oxygen fugacity (from Ilv1 to Ilv3), the temperature and oxygen fugacity of the theore-forming fluid environment decreased, ultimately becoming conducive to the dissolution and precipitation of Pb and Zn elements.

Multi-decadal geomorphic changes of a low-angle valley glacier in the East Kunlun Mountains: remote sensing observations and detachment hazard assessment

Abstract. Detachments of large parts of low-angle mountain glaciers in recent years have raised great attention due to their threats to lives and properties downstream. While current studies have mainly focused on post-event analysis, a few opportunities have presented themselves to assess the potential hazards of a glacier prone to detachment. Here we present a comprehensive analysis of the dynamics and runout hazard of a low-angle (∼20∘) valley glacier, close to the Qinghai–Tibet railway and highway, in the East Kunlun Mountains on the Qinghai–Tibet Plateau. The changes in morphology, terminus position, and surface elevation of the glacier between 1975 and 2021 were characterized with a stereo-image pair from the historical KH-9 spy satellite, six digital elevation models (DEMs), and 11 high-resolution images from Planet Labs. The surface flow velocities of the glacier tongue between 2009 and 2020 were also tracked based on cross-correlation of Planet images. Our observations show that the glacier snout has been progressively advancing in the past 4 decades, with a stepwise increase in advance velocity from 4.55±0.46ma-1 between 1975 and 2009 to 30.88±2.36ma-1 between 2015 and 2020. DEM differencing confirms the glacial advance, with surface thinning in the source region and thickening in the tongue. The net volume loss over the glacier tongue was about 11.21±2.66×105 m3 during 1975–2018. Image cross-correlation reveals that the surface flow velocity of the glacier tongue has been increasing in recent years, with the mean velocity below 4800 m more than tripling from 6.3±1.8ma-1 during 2009–2010 to 22.3±3.2ma-1 during 2019–2020. With a combined analysis of the geomorphic, climatic, and hydrologic conditions of the glacier, we suggest that the flow of the glacier tongue is mainly controlled by the glacier geometry, while the presence of an ice-dammed lake and a supraglacial pond implies a hydrological influence as well. Taking the whole glacier and glacier tongue as two endmember avalanche sources, we assessed the potential runout distances of these two scenarios using the angle of reach and the Voellmy–Salm avalanche model. The assessments show that the avalanche of the whole glacier would easily travel a distance that would threaten the safety of the railway. In contrast, the detachment of the glacier tongue would threaten the railway only with a small angle of reach or when employing a low-friction parameter in the Voellmy–Salm modeling.

Zircon U-Pb Chronology, Geological and Geochemical Characteristics of Metamorphosed Basic Rocks in the Middle Reaches of Lalingzaohuo, Qimantage Area of East Kunlun Mountains

Zircon U-Pb Chronology, Geological and Geochemical Characteristics of Metamorphosed Basic Rocks in the Middle Reaches of Lalingzaohuo, Qimantage Area of East Kunlun Mountains

Quantification of permafrost creep provides kinematic evidence for classifying a puzzling periglacial landform

AbstractMechanical processes operating on the slope surface or at depth control the dynamics of alpine landforms and hold critical information of their geomorphological characteristics, yet they often lack systematic quantification and in‐depth interpretation. This study aims to address a long‐standing issue concerning geomorphological classification from a kinematic perspective. A group of periglacial landforms consisting of several lobes were discovered in the East Kunlun Mountains of China 30 years ago but were ambiguously classified as rock glaciers and later as gelifluction deposits. Here, we use satellite Interferometric Synthetic Aperture Radar to quantitatively characterize the spatial and temporal changes of the surface movement of these landforms. We observe that: (1) its 17 lobes show a pattern of landform‐scale and uniform surface movement, especially during May to October; (2) the lobes move at a spatial mean downslope velocity of 10 to 60 cm/yr and a maximum velocity as high as 100 cm/yr in summer; (3) the landforms are nearly inactive from winter to late spring. Based on these observations, we postulate that the movement of the lobes are driven by deep‐seated permafrost creep which typically occurs in rock glaciers. The debris of Lobe No.4 is composed of both boulders and pebbles supported by fine‐grained matrix generated from the in situ weathering process. It develops a talus‐like oversteepened front around 40° and a convex transverse profile perpendicular to the creep direction, which are also characteristic features of a rock glacier. Piecing these observations together, we identify Lobe No.4 as a debris‐mantled‐slope‐connected rock glacier, with the gelifluction process occurring on the surface as small‐scale and discrete events. © 2020 John Wiley & Sons, Ltd.

Moisture sources and climate evolution during the last 30 kyr in northeastern Tibetan Plateau: Insights from groundwater isotopes (2H, 18O, 3H and 14C) and water vapour trajectories modeling

Understanding the past hydrological cycle and climate change may contribute to the sustainable utilization of water resources. Here we combined groundwater stable isotopes (2H and 18O), age dating (3H and 14C) and water vapour transport trajectories modeling to reveal the moisture source and climate evolution over the last 30 kyr in southeastern Qaidam Basin, northeast Tibetan Plateau. The compositions of δD and δ18O indicated that the groundwater was mainly recharged by precipitation and snowmelt water from the East Kunlun Mountains in the south. The groundwater stable isotopes exhibited a depleted trend from west to east across the four catchments of the basin which seemed not to alter since the late stage of the Late Pleistocene. A combination of the deuterium excess (d-excess) of groundwater and present-day moisture source modeling revealed that the moisture source had been dominated by the Westerlies without obvious impact of the summer monsoon since the last 30 kyr. The enriched δD and δ18O of groundwater during the Holocene suggested a warmer climate than the late stage of the Late Pleistocene, which seemed not to be accompanied by moisture source variations. The results of this study could be helpful for both climate change and water resource research in this region or even, over the entire Tibetan Plateau.

The Wenquan Ultramafic Rocks in North Terrane, East Kunlun Mountains, NW China – Arc Cumulates of the Paleo‐Tethys

AbstractThe Wenquan ultramafic rocks, located in the East Kunlun Orogenic belt in the northeastern part of the Qinghai‐Tibet Plateau, consist of dunite, wehrlite, olivine‐clinopyroxenite and clinopyroxenite, and exhibit cumulate textures. Olivine from dunite has high Fo (forsterite, 90–92) and NiO (0.15–0.42 wt%) contents. Cr‐spinels from all of the rocks in this suite are characterized by high Cr# (100×[Cr/(Cr+Al)], 67–91), low Mg# (100×[Mg/(Mg+Fe2+)], 17–35) and low TiO2 contents (mostly < 0.5 wt%). Clinopyroxenes display high Mg# (92–98) and low TiO2 contents (0.002–0.099 wt%), similar to those in ophiolitic cumulates. Geochemically, the Wenquan ultramafic rocks show enrichment of LILE, Sr, and Ba, and depletion of Nb and Th. Mineral chemistry and geochemistry indicate that the Wenquan cumulates were generated from a depleted mantle and likely evolved from high‐Mg basaltic magmas (Mg#=78) that underwent fractional crystallization and crustal contamination. Zircons from clinopyroxenites yield a U–Pb weighted mean age of 331 ± 2 Ma, which is nearly coeval with the formation age of the Buqingshan and A'nyemaqen Carboniferous ophiolites. The Wenquan Carboniferous ophiolites are confirmed to exist in the Central East Kunlun Fault zone, whereas previous studies have considered them to be the Proterozoic ophiolites. The Wenquan ultramafic rocks might be an arc cumulates of the Paleo‐Tethyan ocean, indicating that there were two cycles of oceanic–continental evolution along the Central East Kunlun Fault zone. Keywords Ultramafic rock, Cumulate, Ophiolite, East Kunlun Orogenic belt.

Can magmatic zircon be distinguished from hydrothermal zircon by trace element composition? The effect of mineral inclusions on zircon trace element composition

Mineral inclusions, e.g., apatite, titanite, monazite, K-feldspar, are common in magmatic zircons. Although many studies mention that light rare earth element (LREE) contents of zircons could be compromised by an analytical artefact of the accidental sampling of mineral inclusions, how and to what degree these inclusions influence analysis of zircon composition is still not well constrained. Here we report U–Pb ages and trace element abundances for zircon crystals, where apatite and K-feldspar inclusions are observed, from diorite porphyry in the Weibao deposit, East Kunlun Mountains, Northern Tibetan Plateau. Although zircon morphological and chronological evidence consistently advocates a magmatic origin without undergoing significant hydrothermal alteration, 7 of 15 analytical spots show LREE-enriched patterns and low Ce/Ce* ratios which are comparable to those for published “hydrothermal” zircon. Quantitative modelling in this study manifests that these LREE-enriched patterns and low Ce/Ce* ratios can be achieved with only 0.1 to 2 vol% contamination from sub-micrometer apatite inclusions, which in practice are hard to monitor under the LA–ICP–MS (normally with large pit diameter and depth) and conventional microscopes. Titanite, monazite, xenotime, and allanite have similar roles to apatite, and LREE contents of zircon can be significantly elevated with only 0.05 vol% contamination from these inclusions. We therefore suggest that the widely used geochemical discrimination criteria for magmatic and hydrothermal zircon, e.g., (Sm/La)N vs. La and Ce/Ce* vs. (Sm/La)N diagrams and the degree of Ce anomalies, are ambiguous since they are extremely susceptible to contamination by mineral inclusions, and that within single samples only Ce4+/Ce3+ values calculated from zircons of low LREE values probably represent the oxidation state of magmas.

Cenozoic uplift of the Qimantage Mountains, northeastern Tibet: Contraints from provenance analysis of Cenozoic sediments in Qaidam Basin

Qimantage Orogen as the western part of East Kunlun Mountains is located in the northeastern margin of the Tibet Plateau. In this contribution, detrital zircon age spectra combined with heavy mineral analysis of Cenozoic sediments from Qaidam Basin were applied to reveal the uplift history of Qimantage Mountains. Our results show that the Cenozoic staged uplift of the Qimantage Mountains exhibit eastward and northward uplift characteristics. From the middle Eocene to Quaternary, there are three major uplift stages for Qimantage Mountains: the first stage is from middle Eocene to late Eocene (~45–35.3 Ma). The range developed a topographic pattern of east‐high and west‐low at that time, and the uplift of the east part range at that time is significant perhaps linked to the Cenozoic uplift of Tibetan Plateau due to its significant crustal shortening. The second stage is from early Oligocene to late Miocene (~15–8 Ma). During this stage, the uplift is both significant in tectonic and topographic aspects, and the overall uplift occurred mainly in the middle and west part of Qimantage Mountains. The last stage is from late Miocene to Quaternary (~2.8 Ma to present), and the Qimantage Mountains experienced complete uplift. The provenance is mainly the northern Qimantage Mountains without sediments supplied from further south, and therefore, the Qaidam Basin became an intermontane basin geomorphically.

Textures and high field strength elements in hydrothermal magnetite from a skarn system: Implications for coupled dissolution-reprecipitation reactions

There is increasing evidence to suggest that high field strength elements (HFSE) could be mobile to some extent in hydrothermal fluid due to the influence of halogens (e.g., fluorine and chlorine). However, in natural hydrothermal (fluid) systems, “coupled dissolution reprecipitation” (CDR) reactions at fluid-mineral interfaces that have been emphasized in the past decade may play a key role in controlling the final textures and mineral assemblages. The influences of the CDR reactions in hydrothermal systems on HFSE enrichment or depletion at the mineral scale are enigmatic. In this study, we show that enrichment of Nb and Zr can occur in magnetite on the mineral scale formed by hydrothermal fluids at medium-to-lower temperature in a skarn system. Four stages of mineralization and alteration of magnetite have been identified in the Baishiya iron skarn deposit of the East Kunlun Mountains of China. Magnetite formed in stage 1 (S1) developed obvious oscillatory zonation, whereas that formed in stages 2 (S2) and 3 (S3) shows hydrothermal alteration and metasomatic textures, and that in stage 4 (S4) developed euhedral crystals with simple zoning. Systematic variations in the trace element compositions of different magnetite grains analyzed by EMPA and LA-ICP-MS suggest that the magnetite from S1 to S3 may have formed in a metasomatic process at relatively constant temperature, whereas the magnetite from S4 formed by re-equilibrium processes at lower temperature. The magnetite from each stage can be divided into light and dark domains based on backscattered electron images. The dark domains in the magnetite from S1 and S2 have higher Nb/Ta (8.52–27.00) and Zr/Hf (18.22–52.64) ratios and silicon contents than the light domains (0.55–5.66 and 2.54–16.43, respectively). Compared with other magnetite ores, the ores from S1 and S2 are depleted of V and Ni. This depletion may be induced by increased oxygen and co-crystallized sulfide. However, these variations are unlikely to be responsible for the enrichment of Nb and Zr in magnetite at equilibrium conditions. Conversely, the dark domains of the magnetite from S1 and S2 are porous, irregular, and/or oscillatory with quartz inclusions, indicating nonequilibrium conditions. These textural features could be attributed to the CDR reactions that are ubiquitous in skarn systems. The increased silicon concentrations in magnetite due to the CDR reactions could affect the lattice parameters of the magnetite structure, leading to an overall change in the volume of magnetite ores. The reduplicative processes of volume change, dissolution, and porosity formation within magnetite are further imporved due to an incresed oxygen fugacity and co-crystallized sulfide (e.g., decreased temperature or increased sulfur fugacity) at far-from-equilibrium or local equilibrium conditions, resulting in oscillatory magnetite dark domains of S1. Ripening of the transient porosity can trap nanoscale precipitates of columbite and zircon within pores of Si-magnetite, and this precipitation could be attributed to the co-crystallized phlogopite that would incorporate fluorine from the hydrothermal fluid, and subsequently decrease the solubility of Nb and Zr in the skarn system. This scenario highlights that Nb and Zr could be scavenged and enriched into in the reaction fronts (porosity) by controlling the reaction pathway at a local scale that does not reflect the overall fluid-rock interaction history of the mineral assemblage.

Geochemical contrasts between Late Triassic ore-bearing and barren intrusions in the Weibao Cu\u2013Pb\u2013Zn deposit, East Kunlun Mountains, NW China: constraints from accessory minerals (zircon and apatite)

The Weibao copper–lead–zinc skarn deposit is located in the northern East Kunlun terrane, NW China. Igneous intrusions in this deposit consist of barren diorite porphyry (U–Pb zircon age of 232.0 ± 2.0 Ma) and ore-bearing quartz diorite and pyroxene diorite (U–Pb zircon ages of 223.3 ± 1.5 and 224.6 ± 2.9 Ma, respectively). Whole-rock major and trace element and accessory mineral (zircon and apatite) composition from these intrusions are studied to examine the different geochemical characteristics of ore-bearing and barren intrusions. Compared to the barren diorite porphyry, the ore-bearing intrusions have higher Ce4+/Ce3+ ratios of zircon and lower Mn contents of apatite, indicating higher oxidation state. Besides, apatite from the ore-bearing intrusions shows higher Cl contents and lower F/Cl ratios. These characteristics collectively suggest the higher productivity of ore-bearing quartz diorite and pyroxene diorite. When compared with ore-bearing intrusions from global porphyry Cu deposits, those from Cu–Pb–Zn skarn deposits display lower Ce4+/Ce3+ and EuN/EuN* ratios of zircon and lower Cl and higher F/Cl ratios of apatite. We conclude that these differences reflect a general geochemical feature, and that zircon and apatite composition is a sensitive tool to infer economic potential of magmas and the resulting mineralization types in intrusion-related exploration targets.

Sources of fluids and metals and evolution models of skarn deposits in the Qimantagh metallogenic belt: A case study from the Weibao deposit, East Kunlun Mountains, northern Tibetan Plateau

The Weibao Cu-Pb-Zn deposit is typical of skarn deposits in the Qimantagh metallogenic belt (QMB), East Kunlun Mountains, northern Tibetan Plateau. It comprises three ore blocks from west to east, known as Weixi, Main and Weidong. Mineralization in the Weibao deposit is intimately related to Late Triassic intrusions occurring at Weixi and Weidong, and orebodies are predominantly hosted by the Langyashan Formation (marine carbonate rocks), and to a lesser extent the Devonian volcanic rocks. Skarns from Weixi and Weidong are characterized by a high garnet/pyroxene ratio and diopside- and andradite-rich composition of pyroxenes and garnets, indicating a proximal, oxidized type. In contrast, skarn mineralogy of Main indicates a slightly reduced condition, typical of Pb-Zn skarn deposits. At least five hydrothermal mineralization stages can be identified and the microthermometric study indicates a general trend of cooling and dilution of the magmatic-hydrothermal fluids. Significant precipitation of Cu-Fe sulfides commenced from the fluid with the temperature of 340–448 °C and the salinity of 2.1–15.0 wt% NaCl equiv. Pb-Zn sulfides however mainly precipitated when the temperatures declined to <370 °C and the salinity declined to <7.6 wt% NaCl equiv. Carbon, hydrogen and oxygen isotopic composition of the fluids suggests a predominantly igneous source for the initial fluids, which were modified outward by fluid-rock interaction and mixing with meteoric waters. Both sulfur and lead isotope composition of sulfides supports a mixed sulfur and metal reservoir consisting of Triassic intrusive rocks and wall rocks. Compared to early skarn-forming and late quartz-carbonate stages, two ore-forming stages show clear evidence of fluid boiling and fluid-rock interaction. Combined with microthermometric data of ore-forming stages, it can be deduced that fluid cooling, boiling and fluid-rock interaction were responsible for the significant metal precipitation. The Weibao deposit shows many similarities with skarn deposits in the QMB, and its genetic model therefore can be extrapolated to other skarn deposits in this region.

Middle Devonian volcanic rocks in the Weibao Cu–Pb–Zn deposit, East Kunlun Mountains, NW China: Zircon chronology and tectonic implications

The Weibao copper–lead–zinc deposit, located in the eastern part of the Qimantagh area, East Kunlun Orogenic Belt (EKOB), consists of three skarn ore blocks known as Weixi, Main and Weidong from west to east. The mineralization within the Weibao Cu–Pb–Zn deposit is hosted by the Mesoproterozoic Langyashan Formation. In this study, we describe for the first time basaltic lavas that intruded into this host sequence and chronological, isotopic, major and trace element data of these volcanic rocks are presented here to constrain their eruption age as well as the tectonic setting. Two basaltic lava samples yield sensitive, high-resolution ion-microprobe (SHRIMP) U–Pb zircon ages of 393.0±5.5Ma–392.0±5.0Ma, indicating that volcanic rocks in the Weibao deposit erupted in Middle Devonian. The majority of the volcanic rocks have compositions characterized by high potassium, light rare earth element (LREE)-enriched patterns in chondrite-normalized rare earth elements (REE) diagrams, and evident enrichment of Rb, Ba and K and depletion of Th, U, Nb and Ta contents in primitive mantle-normalized patterns, although the degrees of enrichment and depletion are variable. These characteristics of major and trace element data highlight a hornblende-dominated fractionation during ascent of magmas. The εHf(T) values of zircons are relatively scattered and slightly enriched, ranging from −2.6 to +7.5. Modelling the features of the major, trace and isotopic element data indicates a hybrid origin involving combined depleted mantle (and hence asthenospheric mantle) and melts and/or fluids inherited from an early subduction event. Besides, these geochronological and geochemical data, together with previously published data in the EKOB, suggest that the Weibao basaltic lavas formed in a post-collisional setting, and that the Qimantagh area underwent strong interactions between mantle and crust in Early Paleozoic–Middle Devonian.

Chronology for terraces of the Nalinggele River in the north Qinghai-Tibet Plateau and implications for salt lake resource formation in the Qaidam Basin

Abstract The Nalinggele River, located in the northern Qinghai-Tibetan Plateau, is the largest river originating from the northern slope of the eastern Kunlun Mountains and flowing into the enclosed Qaidam Basin. Controversy exists whether the formation of its terraces reflects the history of the plateau uplift, or climate changes, or both. The salt lakes in the enclosed Qaidam Basin fed by this river bear an unusual world-class enrichment of lithium and boron elements, which are critical resources for regional economy. The concentration of lithium is one of the highest found in brine resource in the world, and sourced from the upstream water of the river. The chronology of the river terraces is essential to understand the plateau uplift history, the regional climate change, the evolution of the terminal salt lakes, and the process of enrichment of the valuable elements. However, the age constraint for the terraces of the rivers in the east Kunlun Mountains is still limited and debated, and no dating work has been reported yet for terraces of the Nalinggele River. Five terraces were identified in this river. In this study, the terraces were dated using quartz optically stimulated luminescence (OSL) dating. The results indicate that the OSL ages (∼7.5 ka, ∼9.9 ka, ∼11.3 ka, and ∼13.1 ka) of the four terraces (T1, T2, T3, and T4) are consistent with geomorphic relationship, and that they formed since ∼13.1 ka and were probably triggered by climatic change rather than by the plateau uplift. The sediment of T5 is too coarse to collect OSL samples and not dated. Based on our dating we confirm the previous view that the lithium concentration in the terminal salt lakes should have initiated since 15–13 ka. We further propose that the formation of salt lakes and the enrichment of their resources in the Qaidam Basin should have occurred mainly in interglacial periods (e.g. the Holocene), a warm and humid period, not as previously thought to be in the glacial periods, a dry and cold period, and that during the glacial periods the basin was mainly under wind erosion.

AFT dating constrains the Cenozoic uplift of the Qimen Tagh Mountains, Northeast Tibetan Plateau, comparison with LA-ICPMS Zircon U–Pb ages

The Tibetan Plateau (TP) is the highest plateau in the world, which has been the focus of Cenozoic geological studies. The Northeast Tibetan Plateau (NETP) is a key location to decipher the Cenozoic evolution history of the TP. Understanding the building of the Qimen Tagh Mountains located in NETP will help to constrain the development of the northern boundary of the main TP, test the existence of a Paleo-Qaidam Basin and test the eastward growth model of the TP. In this study, granite samples from the Qimen Tagh Mountains were dated by LA-ICPMS and apatite fission track (AFT). The LA-ICPMS zircon U–Pb ages give two magmatic events around ~405 and ~255Ma from two different sites. AFT modeling shows that the initial uplift took place at ~40–30Ma in these mountains, which should be controlled by the Altyn Tagh Fault. Compiling previously low-temperature thermochronometry results, it reveals that the initial Cenozoic uplift of the northern boundary of the TP (Qimen Tagh and East Kunlun mountains), soon after the India–Eurasia collision in the southern TP, has divided the Paleo-Qaidam Basin into several sub-basins. The approximate NE–E growth process occurred along the lithospheric Altyn Tagh and Kunlun faults. The current basin and range morphology of the NETP took place around ~8Ma.

Geology, geochronology and geochemistry of the Saishitang Cu deposit, East Kunlun Mountains, NW China: Constraints on ore genesis and tectonic setting

The Saishitang Cu deposit, located on the eastern area of the Eastern Kunlun Orogenic Belt (EKOB), and on the northern margin of the Tibetan Plateau, is one of the most important copper deposits in Qinghai Province, China. Skarn and Cu orebodies mainly occur as stratoids or lenses along the contact zone between the quartz diorite and Lower to Middle Triassic metamorphosed tuff and marble. In this study, we use new zircon U–Pb ages, molybdenite Re–Os ages, granitoid geochemical compositions, and zircon Hf isotope data to constrain the ore genesis and the tectonic setting of the Saishitang Cu deposit. The quartz diorite is composed of inner medium- to fine-grained quartz diorite phase and an outer porphyritic quartz diorite phase. U–Pb dating of zircons by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) yield ages of 222.7±2.3Ma and 222.6±2.4Ma for the inner and outer phase of quartz diorite, respectively. Molybdenite separated from ore-bearing quartz veins yields a Re–Os isochron age of 223.4±1.5Ma, indicating a close temporal link between magmatism and mineralization. Geochemically, the quartz diorite shows high contents of SiO2 (59.63–68.21wt.%), Al2O3 (15.57–18.13wt.%) and Na2O (3.07–3.56wt.%), with A/CNK values between 0.90 and 1.01 and Mg# between 46 and 52, classified as the calc-alkaline I-type granite. The trace element patterns are characterized by high LREE (LREE/HREE=7.36–13.03), weak negative Eu anomalies (δEu=0.69–0.86), relative enrichment of Rb, Th, K, and Pb, and depletion of Nb, Ta, P, and Ti. The quartz diorite has variable εHf(t) values (−2.5 to 4.87) and Mesoproterozoic two-stage model ages (997 to 1387Ma). Based on petrological and geochemical features, we infer that the quartz diorite is derived from the partial melting of thickened lower crust with an additional input of mantle components. According to the geological, geochemical, and mineralogical features, we propose that the Saishitang Cu deposit is a skarn deposit with minor porphyry-type mineralization and the porphyry–skarn mineral system is related to the crust–mantle interaction. Based on the tectonic evolution of the Paleo-Tethys Ocean during the Late Triassic, we infer that the intrusions and associated Cu mineralization were related to regional extension of the EKOB in a post-collision environment. The results also show that most porphyry–skarn Cu deposits in the EKOB have similar mineralization ages (240–220Ma) and the Late Triassic granitoids in the EKOB present significant potential for the prospecting of porphyry–skarn Cu deposits.