Sanjiang Tethys Research Articles

In-situ geochemical and isotopic analyses of multiple pyrite generations: Insights for the giant Beiya Au polymetallic ore genesis in southwestern China

The Beiya giant Au polymetallic deposit is located in the Sanjiang Tethyan Orogen, northwestern Yunnan (SW China). The ores occur as disseminations or veinlets in granite porphyry. To unravel the ore genesis at Beiya, we analyzed the ore-related sulfide trace elements and sulfur isotopes. Py-1 is mainly euhedral-subhedral medium-/coarse-grained (size: > 100 μm), with relatively smooth surface and some pores. Py-2 is mostly coarse grained (size: 50–200 μm), with well-developed pores and inclusions. The pyrite is usually Co-Cu rich and V-Ti-Sb poor, resembling typical magmatic-hydrothermal pyrite. In addition, the Co/Ni ratios decrease from pyrite-1 (Py-1) to pyrite-2b (Py-2b), suggesting that their chemical compositions are controlled by magma fractionation. We found that Co, Ni, As, and Se occurred as solid solution in the pyrite lattice, while Pb, Bi, Ag, and Mn occurred mainly in inclusions in the pyrite. Pb and Bi display good positive correlation, suggesting that they occurred as inclusions or nano-particles in pyrite lattice. The pyrite samples have sulfur isotope value (δ34S) of 0.62–3.45 ‰ (avg. 2.35 ‰) for Py-1, 0.18–3.12 ‰ (avg. 1.85 ‰) for Py-2a, and 0.62–4.18 ‰ (avg. 2.51 ‰) for Py-2b. These characteristics may have played a vital role in the initial enrichment of Cu, Mo, Au, and ultimately formed the complex polymetallic system at Beiya.

Geology and in-situ S-Pb isotopes of the Luoboshan carbonate-hosted Pb-Zn deposit in Yunnan Province, SW China

The Luoboshan Pb-Zn deposit (0.2 Mt Pb + Zn metal reserves, @ 0.94 % Pb and 5.33 % Zn) is located in the Lanping-Simao Basin in the southern part of the Sanjiang Tethys metallogenic domain in Yunnan Province, SW China. The deposit comprises three ore bodies hosted in the lower Permian bioclastic and micritic limestone. Ore body I is stratiform (mineralization is small in scale and ores have been mined out), with average grades of 0.02–1.44 % Pb and 1.5–14.88 % Zn. Ore bodies II and III are lens-like, the average grades of them are 0.10–5.07 % Pb and 1.68–12.39 % Zn, and 0.10–1.71 % Pb and 1.14–11.79 % Zn, respectively. Two hydrothermal stages were identified in the Luoboshan deposit: Pyrite-I-sphalerite-galena-calcite (stage I) and Pyrite-II-calcite (stage II). The in-situ δ34S values of pyrite and sphalerite reveals a medium range of −8.80 ‰ to +10.1 ‰. Furthermore, Py-I and sphalerite exhibit low δ34S values from −8.80 ‰ to −3.04 ‰, but Py-II displays high δ34S values (+2.37 ‰ to +10.0 ‰). These results suggest distinct formation mechanism for reduced sulfur during different stages of Pb-Zn mineralization. A reasonable explanation is that a transition from bacterial sulfate reduction (BSR) in the early stage to thermochemical sulfate reduction (TSR) in the late stage as the likely mechanism for the formation of reduced sulfur. In addition, the in-situ Pb isotopes exhibit a narrow range (206Pb/204Pb = 18.660–18.672, 207Pb/204Pb = 15.665–15.677 and 208Pb/204Pb = 38.940–38.980), indicating that the metals originated from either a single source or a uniform mixture of different sources. Comparison of geological charateristics and isotopic geochemical data (S and Pb) indicates that the basin sedimentary strata (wall rocks) is the only and most probable source. Hence, we propose that the formation of the Luoboshan deposit involved biogenic in-situ acid generation of marine sulfates, which subsequently mixed with Pb-and-Zn-rich hydrothermal fluids, initiating TSR and resulting in Pb-Zn mineralization.

Recognition of Neogene tin mineralization in the Southeast Asian tin belt

Abstract The Southeast (SE) Asian tin belt is a major tin producer globally, with a prolonged mineralization history from the Permian to Paleogene (ca. 285–45 Ma). Tin deposits in this region are typically associated with tectonic settings that involve subduction and collision of the Paleo-, Meso-, and Neo-Tethys slabs. Ca. 40 Ma, a notable transition occurred in the tectonic regime of SE Tibet, with the Neo-Tethys subduction giving way to lateral extrusion of the Indochina block along major strike-slip faults. Previously, it was believed that this shift had brought tin mineralization to a halt. In this study, we present in situ laser ablation–inductively coupled plasma–mass spectrometry U-Pb cassiterite ages of 21–19 Ma from the Yunling tin deposit located in western Yunnan, China. Yunling produces gem-quality cassiterite that is transparent but contains low U contents, which renders usual U-Pb dating techniques unusable. To address this, a customized dating protocol involving cathodoluminescence (CL) imaging and testing of U distribution on crystallographically well-defined cassiterite crystals was applied. The study revealed two types of primary microstructures in cassiterite: volumetrically dominant concentric oscillatory growth zones and subordinate CL-dark sector zones. The U content shows a preferential enrichment in the CL-dark sector zones, typically tens of parts per million (ppm), which is two orders of magnitude greater than the U content in the concentric zone. This is significant, as the dating results (21–19 Ma) obtained through the targeted selection of CL-dark sector zones represent the youngest tin mineralization event in the SE Asian tin belt. Additionally, these results challenge the established belief that the ore-hosting Indosinian granite, dated to ca. 215 Ma, was related to Sn mineralization in the Yunling area. We suggest that emplacement of the early Neogene Sn mineralization at Yunling may be genetically connected to extensive delamination of the lithosphere in southeastern Tibet. The lithospheric delamination led to the upwelling and decompression partial melting of the asthenosphere, which provides a plausible explanation for the high temperature required for the release of Sn from biotite-dehydration melting of sedimentary protolith. The study also highlights the potential of Oligocene–Miocene-aged granites situated in the Sanjiang Tethys and adjacent areas of western Yunnan as prospective exploration targets for tin mineralization.

Geological and hydrochemical controls on water chemistry and stable isotopes of hot springs in the Three Parallel Rivers Region, southeast Tibetan Plateau: The genesis of geothermal waters

The Three Parallel Rivers Region (TPRR) is a tectonically active area in the middle segment of the Sanjiang Tethys Orogen, southeast Tibetan Plateau, characterized by many hot springs. This area is up-and-coming for producing geothermal energy, a CO2-free energy source, which will help China in reducing the effects of climate change. We report here the results of 37 geothermal springs that have been sampled to investigate the physical and chemical characteristics of the thermal water and evolution patterns. These springs are drained along three major N-S faults zones (the Lujiang Fault, the Lancangjiang Fault and the Jinshajiang-Red River Fault) to the interior of the Lanping Basin. Five hydrochemical water facies were recognized with Na-HCO3 being the primary type. Fluorine and boron that are produced through water-rock interactions are commonly enriched in these waters, and their concentrations are further controlled by secondary hydrochemical processes during water migration. The water's stable isotopes (δ18O and δD) suggest the meteoric origin of all thermal waters in the TPRR. The estimated reservoir temperatures range between 61 °C and 118 °C with the relatively hot reservoirs (> 100 °C) generally developed in major shear zones. These results indicate variable water circulation depth exceeding 3000 m, implying that the large-scale shearing displacement plays a vital role in heat acquisition. Conductive cooling and possible mixing of the thermal water with near-surface cold water occurred as the thermal water ascended along the fault systems and was ejected along the outlets of the springs. This study adds insights into hydrogeochemical constrains on evolution of water solutes over a large-scale hydrological cycle in the TPRR.

Petrogenesis of the Eocene Yulong potassic intrusion in non‐subduction setting in the Sanjiang Tethys

The giant Yulong porphyry Cu (‐Mo‐Au) deposit was formed in a post‐collisional setting in eastern Tibet, which is associated with Eocene potassic to ultrapotassic felsic intrusive rocks. The origin of the Yulong potassic intrusion remains highly disputed. Hence, we present new zircon U–Pb ages, whole‐rock geochemistry and zircon Lu‐Hf isotopes for the Yulong intrusion. Zircon U–Pb dating from the porphyritic monzogranite and porphyritic quartz monzonite yield ages of 41.1 ± 0.2, 40.8 ± 0.1 and 40.5 ± 0.2 Ma, respectively. Samples from the Yulong intrusion exhibit high K2O (4.13–4.85%), and high K2O/Na2O (1.00–1.25), [La/Yb]n (30.37–40.37) and Sr/Y (57.86–87.68) ratios, low Mg#, MgO, Ni, and Cr contents, enrichment in light rare earth elements (LREEs) and depletion of high‐field‐strength elements (HFSEs, e.g., Nb, Ta, and Ti). Zircon grains from this study have mostly positive εHf(t) values (+1.6 to +2.9), and crustal model ages (TDMC) of 0.90–1.19 Ga. These features suggest that the Yulong intrusion is derived from a thickened juvenile lower crust. During the Permian to late Triassic, Jinshajiang‐Ailaoshan Palaeo‐Tethyan oceanic subduction from the eastern margin of the North Qiangtang produced metasomatic domains within the continental lithospheric mantle and lower crust. During the late Eocene, the continuing India‐Asia continental collision may have preferentially overthickened the crust. Convective removals in the lower part of the thickened lithosphere resulted in the hot asthenosphere upwelling along the Jinshajiang‐Ailaoshan Suture, which induced the partial melting of the residual metasomatized lithospheric mantle, as well as the thickened lower crust in the Eocene, which formed the Yulong intrusion.

Magmatic Processes of Granitoids in the Hongniu-Hongshan Porphyry-Skarn Copper Deposit, Southern Yidun Terrane, China: Evidence from Mineral Geochemistry

The Hongniu-Hongshan porphyry-skarn deposit is located in the southern Yidun terrane in the Sanjiang Tethyan Metallogenic Domain (STMD). Although its metallogenesis has been well constrained in the past decade, the magmatic processes for granitoids in the Hongniu-Hongshan deposit are still poorly understood. Herein, we provide new geochemical data on magmatic minerals (plagioclase, amphibole, and clinopyroxene) in the Hongniu-Hongshan granitoids to get a better insight into these processes. The complex zoning patterns of plagioclase phenocrysts indicate magma recharge and mixing. Physiochemical estimations indicate that clinopyroxenes were crystallized in hotter (919 ± 11 °C) and more mafic (FeO: 2.8–4.6 wt.%, MgO: 0.8–1.8 wt.%) magmas in a deep chamber (18.6 ± 0.9 km) compared with the colder (819 ± 29 °C), more felsic (FeO: 0.9–2.2 wt.%, MgO: 0.3–0.6 wt.%) and shallow magma chamber (13.4 ± 1.6 km) in which amphiboles crystallized. Therefore, we suggest that magmatic minerals in the Hongniu-Hongshan granitoids were produced by multistage magmatic processes within the upper–middle crust range. In this model, the deep-seated magmas recharged into the shallow reservoir and mixed with the shallow magmas therein. The recharged hot magmas may provide heat sources and rejuvenate the shallow magma reservoirs. On this basis, we further infer that ore-forming materials could be pre-concentrated in the crustal range and mobilized by the Late Cretaceous magmatism in the southern Yidun terrane.

Geological Significance of Late Permian Magmatic Rocks in the Middle Section of the Ailaoshan Orogenic Belt, SW China: Constraints from Petrology, Geochemistry and Geochronology

The Ailaoshan orogenic belt, located in the SE margin of the Qinghai–Tibet Plateau, is an important Paleo-Tethys suture zone in the eastern margin of the Sanjiang Tethys tectonic domain. The areas of Mojiang and Zhenyuan, located in the middle part of the Ailaoshan orogenic belt, are the key parts of the Ailaoshan Paleo-Tethys Ocean closure and collision orogeny. The rhyolites outcropped in the Mojiang area, and the granite porphyries outcropped in Zhenyuan area, are systematically studied for petrology, isotope geochemistry and geochronology. The Zircon U-Pb geochronology of rhyolites and granite porphyries give weighted average ages of 253.4 ± 4.2 Ma and 253.3 ± 2.0 Ma, respectively, both of which were formed in the late Permian period. The rhyolites belong to potassic calc-alkaline to subalkaline series. The patterns of the rare earth elements (REE) show a right-inclined seagull-type distribution, and the trace elements plot is right-inclined. The granite porphyries are high potassic calc-alkaline to subalkaline. The REE patterns show a right-inclined distribution, and the trace elements plot is right-inclined, which is consistent with the typical patterns observed in the crust. The peraluminous, highly differentiated and high ASI values suggest that rhyolites and granite porphyries are S-type granites. The zircon εHf(t) of the rhyolites range from −7.22 to −0.72, and two-stage Hf zircon model ages are (TDMC) 1771–2352 Ma, indicating that the magma source area is mainly crust-derived. The zircon εHf(t) of the granite porphyries range from −0.97 to 4.08, and two-stage Hf zircon model ages are (TDMC) 1336–1795 Ma, indicating that the magma is derived from a depleted mantle source and the partial melting of ancient crustal materials. The rhyolites and granite porphyries were possibly formed in the syn-collisional tectonic setting during the late Permian, and their ages limited the time of the final closure of the Ailaoshan Ocean and the initiation of collisional orogeny.

Zircon Hf-Isotopic Mapping Applied to the Metal Exploration of the Sanjiang Tethyan Orogenic Belt, Southwestern China

Zircon Hf-isotopic mapping can be regarded as a useful tool for evaluating the coupling relationship between lithospheric structure and metallic mineralization. Hence, this method shows important significance for mineral prediction. To explore this potential, the published granite zircon Hf isotope data from the Sanjiang Tethyan Orogen were systematically compiled. This study uses the Kriging weighted interpolation in the Mapgis software system to contour Hf isotopes, revealing a relation between the crustal structure and metallogenesis. The mapping results suggest that the Changning–Menglian suture zone is the boundary between ancient and juvenile crust, viz., the western terranes have ancient crust attributes, whereas the eastern terranes exhibit the properties of new juvenile crust. In addition, this study also found that the mineralization and element types in the Sanjiang Tethyan Orogen have a coupling relationship with the crustal structure. The distribution of porphyry Cu-Mo-Au deposits is mainly controlled by the new juvenile crust, whereas the magmatic-hydrothermal Sn-W and porphyry Mo-W(-Cu) deposits are closely related to the reworked ancient crust. The results of zircon Hf isotope mapping prove that the formation and spatial distribution of deposits are related to the composition and properties of the crust. Hf isotope mapping can reveal the regional metallogenic rules and explore metallogenic prediction and metallogenic potential evaluation.

Advances in research of the time scales of porphyry deposits: A case study of the Yulong porphyry Cu-Mo deposit in the eastern Tibet

精确限定多期次岩浆-热液活动的时间尺度一直是剖析斑岩矿床形成过程的热点和难点。借助矿物的高精度同位素定年、热力学数值模拟以及石英的钛扩散模型等方法，斑岩矿床中岩浆-热液活动的时间尺度已经被限定在几万年之内。本文以三江特提斯超大型玉龙斑岩铜（钼）矿床为例，重点识别含矿热液脉中普遍存在的石英，利用钛元素的扩散年代学方法，精确限定斑岩矿床中多期岩浆-热液流体活动的时间尺度。扩散模型表明玉龙斑岩矿床热液活动的时间尺度为32000~870000年，有力支持了超大型斑岩矿床可以在几万至几十万年甚至更短时间内形成的观点。此外，为避免钛扩散模型产生较大的误差，需要在精确测定石英中钛含量的基础上，结合矿床地质背景或其他实验方法合理地估测温度和压力条件。研究认为，将矿物的高精度同位素定年与元素的扩散年代学相结合，可以在更为精细的尺度上完善斑岩矿床岩浆-热液活动的时间框架。;Accurate determination of the time scales of multiple magmatic-hydrothermal pulses is a critical and difficult problem that could provide key information needed to better constrain the genesis of porphyry deposits. Recent high-resolution dating, diffusion chronometry in quartz, and numerical modeling of thermal processes have revealed that the time scales of individual ore deposition cycles may be constrained in tens of thousands of years. Taking the giant Yulong porphyry Cu-Mo deposit in Sanjiang Tethys metallogenic belt as an example, this study focuses on the ubiquitous quartz in hydrothermal veins through the titanium diffusion chronology to accurately determine time scales of multistage magma-related hydrothermal events in the porphyry deposit. Diffusion modeling for different quartz generations indicates that the time scale of hydrothermal pulses in the Yulong deposit can form in a short period of 70000 to 900000 years. These findings provide strong support that giant porphyry deposits can be formed in tens of thousands to hundreds of thousands of years or even less. Based on accurately determining titanium contents in quartz, in order to avoid tricky errors, the diffusion modeling needs to reasonably estimate the temperature and pressure combined with the geological background or other experimental methods to avoid errors. This study suggests that the combination of high precision dating and diffusion chronology models can further constrain the time frame of magmatic-hydrothermal events in porphyry deposits.

Tectonic evolution and multi–episodic metallogenesis of the Sanjiang Paleo-Tethys multi-arc-basin-terrane system, SW Tibetan Plateau

The Sanjiang Tethys region in SW China lies in the tectonic junction of the Tethys domain, and represents a key precious/base metal province in China. Based on a compilation of studies from the past decades on the formation, evolution, and metallogenesis of the tectonic units in the region, we proposed a multi-arc-basin-terrane (MABT) structural framework for the Paleo-Tethys Sanjiang region. Convergence between South China and Indochina-Pu'er blocks had led to the accretion of many island arcs and micro-continental terranes, which is different from the traditionally direct convergence between two continents. Such prolonged terrane accretion process likely formed the various metallogenetic belts that contain diverse deposit types and mineral assemblages. In this contribution, we divided the Sanjiang region into ten metallogenetic belts, according to their characteristic lithologic assemblages, structures, and interpreted tectonic settings . Subsequently, we discussed the regional tectonic-metallogenic evolution of these ten ore belts and propose their metallogenetic models. This study reveals a genetic link between the MABT system and the regional metallogenesis, which is important for future mineral exploration in this region.

Hydrothermal evolution and mineralization of the Pulang porphyry Cu-Au deposit in the Sanjiang Tethys, Southwest China: Constraints from fluid inclusions and D-O-S isotopes

Pulang is the largest porphyry Cu-Au deposit that formed in the Late Triassic and is located in the eastern Tethys. Its ore-forming fluids and mineral assemblages show some reducing characteristics. Magma and fluid are two key parameters in porphyry mineralization. Studies have shown that syn-mineralization quartz monzonite porphyries have higher oxygen fugacities, which are conducive to mineralization. However, this does not answer the question of why the deposit has reducing characteristics, which should be positively related to fluid activity. Based on detailed geologic observation, hydrothermal evolution and ore-forming process have been divided into three successive periods, namely, pre-mineralization, syn-mineralization (subdivided into three stages, including early-, inter-, and late-mineralization, corresponding to stage 1, 2, and 3 veins), and post-mineralization, in the Pulang deposit. The corresponding fluid inclusions and sulfur isotopic compositions of sulfides are studied on this basis. The results show that the salinity ranges of the stage 1 and stage 2 veins are larger, which indicates that fluid immiscibility may exist. Pyrrhotite is developed in some of the stage 2 veins, and their fluid inclusions contain CH4. However, CH4 was not found in fluid inclusions in veins that do not contain pyrrhotite. The homogenization temperatures and salinities of the fluid inclusions decrease gradually, and the hydrogen and oxygen isotopes gradually approach the meteoric water line overall, from premineralization to postmineralization. Furthermore, sulfur isotopes of pyrrhotite are the highest, indicating a possible additional source of sedimentary sulfur. That is, the ore-forming fluids were dominated by magmatic water, and meteoric water was added in the late stage as the fluid evolved. The trend of the Heishuitang fault is consistent with the extension of the main ore body in the mining area. Because the strata is affected by faults, the strata acts as a good channel for meteoric water. Meteoric water was added to the magmatic fluid after extraction and carried organic matter from the Tumugou Formation and along the cracks. The fluid was mixed, and the ore-forming elements were effectively precipitated in a reducing environment. Therefore, pyrrhotite and CH4 occur in some veins. That may be why the Pulang deposit is giant.

The genesis of Eocene granite‐related Lailishan tin deposit in western Yunnan, China: Constraints from geochronology, geochemistry, and S–Pb–H–O isotopes

The Tengchong‐Lianghe tin metallogenic district in western Yunnan is an important tin mineralization area in the Sanjiang Tethyan Metallogenic Domain from southwestern China. The Lailishan deposit is one of the largest tin deposits in this district and spatially and temporally associated with Eocene granites. LA‐MC‐ICP‐MS cassiterite U–Pb dating (49.8 ± 2.6 Ma) suggests that the tin mineralization is coeval with the Lailishan granites. The contact between the monzogranite and the syenogranite is gradual and both are weakly peraluminous, with high SiO2, Al2O3, and total alkali contents. They are characterized by negative Eu anomalies and the depletion in high‐field‐strength elements and enrichment in large‐ion lithophile elements. Compared with the monzogranite, the syenogranite is more evolved and exhibit ‘tetrad effect’. The Lailishan monzogranite was probably derived from the partial melt of the ancient crustal basement from the Tengchong Block under syn‐collisional tectonic setting. The syenogranite was cogenetic with monzogranite and experienced the process of fractional crystallization. The mineralization process is divided into three successive mineralization stages (I–III) in this study. These stages are characterized by the coarse‐grained cassiterite and high temperatures (216–337°C), light brown or colourless cassiterite and moderate temperatures (160–243°C), and dark brown cassiterite and low temperatures (116–199°C) of ore‐forming fluids, respectively. From the Stage I to later stages, both the δ18OH2O and δDSMOW values of fluids are decreasing and shift from magmatic water towards meteoric water, indicating that the ore‐forming fluids derived from granitic melts and mixed over time with meteoric water with the decrease of temperature. The δ34S values of pyrites and pyrrhotites (2.4–5.9‰) and Pb isotopic compositions for pyrites from Lailishan deposit (206Pb/204Pb = 18.687–19.402) are similar with those of Lailishan granites. We propose that the Lailishant in deposit is derived from the Lailishan granites.

Sequence and petrogenesis of the volcanic rocks from the middle Sanjiang Tethys Orogen, SW China: Implications for the Sanjiang Paleo‐Tethyan evolution

The volcano‐sedimentary sequences from the Weixi belt provide significance for the Sanjiang Paleo‐Tethys evolution. This manuscript constrains the ages of volcano‐sedimentary sequences by LA‐ICPMS U–Pb, including bimodal volcanics (245.0–244.0 Ma) from Pantiange Formation, trachyandesite–basaltic andesite (234 ± 2.0 Ma) from Cuiyibi Formation, high silica subalkaline rhyolite (221 ± 1.4 Ma) from Waigucun Formation and Early Jurassic high silica subalkaline rhyolite (184 ± 2.2 Ma). The volcanics show various Hf isotopic composition, which includes Middle Triassic dacite (ε Hf[t] = −15.7 to −8.5, TDM2 = 2,006–1,613 Ma), Late Triassic rhyolite (ε Hf(t) = −9.8 to +1.9, TDM2 = 1,667–1,021 Ma), Late Triassic trachyandesite (ε Hf(t) = +2.6 to +5.7, TDM2 = 990–816 Ma) and Early Jurassic rhyolite (ε Hf(t) = −2.7 to +1.1, TDM2 = 1,250–1,145 Ma). The Middle–Late Triassic volcanics exhibit similar characteristics, the content of SiO2 exerts positive correlation with K2O and negative correlation with oxide of Ti, Fe, Al, Mg, Ca, Mn and P. The Hf isotopic and geochemical features indicate that the Middle Triassic basalt and rhyolite derived from mantle and crust by crystal fractionation, respectively. The absence of Lower Jurassic strata and existence of Triassic bimodal volcanic sequences suggest that the middle Sanjiang Tethys Orogen experienced collision during the Middle Triassic with local extension resulting from oblique convergence. Late Triassic–Early Jurassic rhyolite exhibit high SiO2 (77.12–77.81 wt%), K2O (4.35–6.10 wt%), Na2O (1.91–3.15 wt%) content, and a negative Eu anomaly. Together with the sedimentary sequence, the middle Sanjiang Tethys Orogen evolved into post‐collision phase during Late Triassic – Early Jurassic.

New zircon fission track analysis constraints on tectonic-mineralization epochs in the Ganzi-Litang gold belt, eastern Qinghai-Tibet plateau

The Ganzi-Litang gold belt, as a part of the Sanjiang Tethys orogen, is located between the Yidun block and Songpan-Ganzi Block/Terrane in the eastern Qinghai-Tibetan plateau and experienced oceanic slab subduction, continent-arc collision and intracontinental convergence since the Indosinian orogeny. A series of gold ore deposits have been found within the region. This paper describes the tectonic evolution and episodes of gold mineralization within this belt using zircon fission track (FT) thermochronology. A total of 15 zircon FT analysis results were obtained that can be divided into four age groups (217–213 Ma, 190–160 Ma, 155–93 Ma and 72–67 Ma) representing four distinct episodes of gold mineralization, of which the middle two episodes predominate. Eruption of the auriferous volcanic rocks that comprise the belt started at 217–213 Ma when the Ganzi-Litang Meso-Tethys ocean began to subduct. The mineralization episode at 190–160 Ma corresponds to the formation of auriferous volcanic rocks during intracontinental collisional-convergence of the Ganzi-Litang belt. Finally, the episodes at 155–93 Ma and 72–67 Ma are respectively related to the closing times of the Bangong-Nujiang and Yarlung Zangbu oceans, corresponding to tectonic quiet episodes.

A Late Cretaceous felsic magmatic suite from the Tengchong Block, western Yunnan: integrated geochemical and isotopic investigation and implications for Sn mineralization

AbstractThe Tengchong Block within the Sanjiang Tethys belt in the southeastern part of the Tibetan plateau experienced a widespread intrusion of a felsic magmatic suite of granites in its central domain during Late Cretaceous times. Here, we investigate the Guyong and Xiaolonghe plutons from this suite in terms of their petrological, geochemical, and Sr–Nd, zircon U–Pb and Lu–Hf–O isotopic features to gain insights into the evolution of the Neo-Tethys. The Guyong pluton (76 Ma) is composed of metaluminous monzogranites, and the Xiaolonghe pluton (76 Ma) is composed of metaluminous to peraluminous medium- and fine-grained syenogranite. A systematic decrease in Eu, Ba, Sr, P and Ti concentrations; a decrease in Zr/Hf and LREE/HREE ratios; and an increase in the Rb/Ba and Ta/Nb ratios from the Guyong to Xiaolonghe plutons suggest fractional crystallization of biotite, plagioclase, K-feldspar, apatite, ilmenite and titanite. They also show the characteristics of I-type granites. The negative zircon εHf(t) isotopic values (−10.04 to −5.22) and high δ18O values (6.69 to 8.58 ‰) and the negative whole-rock εNd(t) isotopic values (−9.7 to −10.1) and high initial 87Sr/86Sr ratios (0.7098–0.7099) of the Guyong monzogranite suggest that these rocks were generated by partial melting of the Precambrian basement without mantle input. The zircon εHf(t) isotopic values (−10.63 to −3.04) and δ18O values (6.54 to 8.69 ‰) of the Xiaolonghe syenogranite are similar to the features of the Guyong monzogranite, and this similarity suggests a cogenetic nature and magma derivation from the lower crust that is composed of both metasedimentary and meta-igneous rocks. The Xiaolonghe fine-grained syenogranite shows an obvious rare earth element tetrad effect and lower Nb/Ta ratios, which indicate its productive nature with respect to ore formation. In fact, we discuss that the Sn mineralization in the region was possible due to Sn being scavenged from these rocks by exsolved hydrothermal fluids. We correlate the Late Cretaceous magmatism in the central Tengchong Block with the northward subduction of the Neo-Tethys beneath the Burma–Tengchong Block.

Subduction-modified mantle-derived Triassic high-Mg andesites in the Sanjiang Tethys, eastern Tibet

Andesites in the Yangla district of the Sanjiang Tethys, eastern Tibet, are characterized by relatively high MgO and with rich large-ion lithophile elements (LILEs), indicating similarities to typical high-Mg andesites formed in continental collisional zones. Zircon U-Pb age of 232 ± 1 Ma for these andesites shows that they occurred synchronously with the regionally widespread late Middle Triassic granitoids and bimodal volcanic rocks. Primitive mantle-normalized spidergrams for these andesites show depletion in high field-strength elements (HFSEs), with Sr-Nd-Pb isotopic compositions that are comparable to both arc- and MORB-like high-Mg rocks generated from subduction-modified sources. Modelling results suggest that an input of ~20% sediment-derived melts into mantle-derived melts could be possibly responsible for the Sr-Nd isotopic characteristic of these andesites. MORB- and arc-like compositions likely indicate a metasomatized mantle wedge modified by subduction processes during the closure of the Paleo-Tethys Ocean and subsequent continental collision. It is speculated that the late Middle to Late Triassic intra-continental re-activation associated with regional tectonic collapse and thermal relaxation triggered the melting of this previously subduction-modified mantle, leading to the formation of the high-Mg andesites in the Yangla district.

The origin of the Quemocuo carbonate-hosted Pb-Zn deposit in the Sanjiang Tethyan Belt, SW China: Constrained by Sm-Nd isochronic age and Sr-S-Pb isotope compositions

The newly discovered Quemocuo Pb-Zn deposit (7.1 Mt of sulfide ores, grading 1.65–7.17 wt% Pb and 1.13–5.7 wt% Zn) is located in the Tuotuohe area of the Sanjiang Tethyan metallogenic belt, SW China. The stratiform and veined ore body is hosted in the limestone of the Upper Permian Labuchari Formation and occur along the NE-trending fault. Four main mineralization stages are distinguished as follows: (i) pyrite + calcite + quartz, (ii) sphalerite + galena + pyrite + calcite + barite + quartz, (iii) galena + sphalerite + pyrite + calcite + barite + quartz, and (iv) calcite + dolomite. Sm-Nd dating of five calcite samples in stage II yielded an isochronic age of 34.3 ± 2.2 Ma (MSWD = 7.1), possibly reflecting the formation age of the Quemocuo Pb-Zn deposit. In situ sulfur isotopic compositions of the sulfides in stages I, II and III range from −29.5‰ to −26.5‰, −4.7‰ to +6.0‰, and +1.7‰ to +6.9‰, respectively. Such sulfur isotope signatures indicate bacterial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR) of seawater or sulfate minerals. In situ Pb isotopic ratios indicate a similar origin for the sulfides in stages I, II and III, which all are related to metamorphic basement rocks and ore-hosting sedimentary strata. Initial 87Sr/86Sr ratios of sphalerite are 0.7085–0.7117, indicating a mixed source as suggested by in situ Pb isotopes. The Quemocuo Pb-Zn deposit was proposed to be a strata-bound and normal fault-controlled epigenetic deposit, with a mixed source of basement and strata, and even volcanic rocks. Our study suggests that the formation of the Quemocuo Pb-Zn deposit is related to the Cenozoic collision event between the Indian and Eurasian plates.

Geological, sulfur isotopic, and mineral trace element constraints on the genesis of the Xiyi Pb–Zn deposit, Baoshan Block, SW China

The recently discovered Xiyi Pb–Zn deposit in Yunnan Province, SW China, contains 1.2 Mt of Pb and Zn metal. It is located within the Baoshan Block, in the Sanjiang Tethys orogenic belt and is hosted along fractures within carbonates of the Lower Carboniferous Xiangshan Formation. The deposit is hosted by veins within NE–SW trending strike-slip and steeply dipping NW–SE trending normal fault zones. The formation of the deposit involved sedimentary, hydrothermal, and supergene oxidation stages. The hydrothermal stage associated with mineralization include three sub-stages (i) a steeply-dipping quartz–calcite-rich sphalerite sub-stage; (ii) a polymetallic sulfide sub-stage; and (iii) a vein-host calcite–pyrite sub-stage. The deposit is enriched in Pb, Zn, Sb, Cd, Ag, and U, and depleted in Ni, Se, Ti, and Co. The ores have total rare-earth element (REE) concentrations of 17.4–186 ppm and are enriched in the light REE (LREE) with (La/Yb)N ratios of 3.75–9.23 and strongly positive Eu (Eu/Eu* = 1.42–4.60) and slightly negative Ce (Ce/Ce* = 0.70–0.88) anomalies. The positive Eu anomalies and LREE enrichment suggest that the ore-forming fluids were exsolved from a concealed intrusion during magmatic degassing. The S isotopic compositions of sulfides and sulfates indicate that S within the deposit was derived from both magmatic sources and barite within the hosting carbonates via thermochemical sulfate reduction. In-situ LA–ICP–MS analysis indicates that sphalerite within the deposit is texturally and chemically variable, with elevated Fe, Mn, Cd, Cu, Ag, Sb, Sn, Pb, and Tl contents. In comparison, galena within the deposit is enriched in Sb, Ag, Cd, Sn, and Tl, and pyrite has elevated Ag, As, Sb, and Pb contents. The trace element compositions of sphalerite and galena indicate that the Xiyi deposit may be the product of a distal hydrothermal event although the direct genetic relationship is not clear. The fracture-hosted nature, mineral assemblages, geochemistry, and mineral S isotopic and trace-element compositions of the Xiyi Pb–Zn deposit indicate that it is an epigenetic carbonate-hosted Pb–Zn deposit. This study also indicates that sphalerite is a more useful mineral than galena in terms of using mineral trace-element geochemistry to discriminate different deposit types.

Chemical composition, genesis and exploration implication of garnet from the Hongshan Cu-Mo skarn deposit, SW China

In this study, we present new mineral chemical data on the garnet from the Hongshan Cu-Mo deposit, one of the largest skarn deposits in the Sanjiang Tethyan tectonic domain, in SW China. The aims are to unravel the formation mechanism of the oscillatory-zoned garnet at Hongshan, and to explore possible applications using garnet trace element to distinguish different types of skarn deposits.Two generations of garnets, i.e., poorly-zoned garnet I (Grt I) and the well-zoned garnet II (Grt II), are identified from the endoskarn and exoskarn, at Hongshan based on field and petrographic observations. EPMA and LA-ICP-MS analyses show that Grt I contains a narrow compositional range (Adr40.07-61.25 Grs36.65-57.08), and chondrite-normalized HREE-enriched and LREE-depleted patterns with minor positive Eu anomalies. In contrast, Grt II shows a wide compositional range (Adr27.78-67.53 Grs30.67-70.92 to almost pure andradite Adr94.41-99.99 Grs0.00-4.21) with interlayered Al-rich and Fe-rich zones. In Grt II, its Al-rich zone is chemically similar to Grt I, whereas its Fe-rich zone exhibits chondrite-normalized LREE-enriched and HREE-depleted patterns with distinct positive Eu anomalies. The positive REE3+ vs. Mg correlations, and the poor REE3+ vs. Y and Ca correlations, altogether indicate that the REE3+ incorporated into garnet was co-influenced by the menzerite-type substitution mechanism (Ca2+-1VIIIREE3++1VIIIAl3+-1VIMg2++1VI), fluid chemistry, and the physicochemical conditions of precipitation. The higher U contents and HREE-enriched patterns of Grt I suggest that it was formed under a relatively reducing and near-neutral pH condition. Varying U contents and REE patterns across different oscillatory zones indicate that Grt II was probably formed under alternating physicochemical fluctuations led by repeating crack-seal cycles in the hydrothermal system. Besides, chemical data compilation suggests that the garnet from different types of skarn deposits has different compositions, and can be used as mineral prospecting tools. We suggested that discrimination plots, such as δEu vs. V and δCe vs. U, can distinguish Cu skarn deposit from other skarn deposit types, whilst the U vs. W plot can discriminate between Cu, W, W-Sn and W-Mo skarn deposits.

Late Triassic-Cenozoic Thermochronology in the Southern Sanjiang Tethys, SW China, New Insights from Zircon Fission Track Analysis

The Sanjiang Tethys orogenic belt is located in the southeast side of the Qinghai-Tibet Plateau. It has undergone the opening and closing movements in different periods of Tethys oceans, complex accretive orogeny and strong mineralization from Paleozoic to Mesozoic. Using zircon fission track (ZFT) thermochronology, this study reveals the Sanjiang Tethys has experienced multi-stage tectonic activities during the Late Triassic-Cenozoic. The 15 ZFT ages with their decomposition components obtained from Sanjiang Tethysian region range from 212 to 19 Ma, which not only shows 6 age groups of 212, 179–172, 156–133, 121–96, 84–70 and 50–19 Ma, but also constrains the age limit of the tectonothermal events. These age groups recorded the Paleo-Tethys main and branches ocean opening/closure time. The age-elevation plot indicates the Sanjiang region had differential uplifting and exhumation and fast uplifting times of ca. 133, 116 and 80 Ma, coinciding with the age groups mentioned above. These results show new geochronological evidences and viewpoints.