The Chamdo Basin is a secondary basin in the eastern part of Tibet China and is one of the most promising of petroliferous basins for new petroleum exploration. The Qamdo Basin records a complex burial history from the Mesozoic to the Cenozoic; however, the poorly constrained sedimentology of Cenozoic strata in this basin has severely obscured the overall profile and impeded further explorations of oil and gas resources. Here, we conduct whole-rock geochemical analyses of major, trace, and rare earth elements in fine-grained clastic rocks of the Paleocene Gongjue Formation, Qamdo Basin to reveal depositional environments, provenance, and tectonic setting. Petrologically, the Gongjue Formation is dominated by red fine-grained sandy mudstones/siltstones with ripple marks. The high values of the chemical index of alteration (avg. of 78.93), chemical index of weathering (avg. of 90.10), and index of compositional variability (avg. of 2.5) suggest that the basin has undergone heavy weathering. Cross-plots of La vs Th, Th vs Sc vs Zr/10, and Th vs Co vs Zr/10 reveal a continental arc tectonic setting. Paleosalinity (Sr/Ba), paleoclimate (Sr/Cu), and redox proxies (V/Cr, U/Th, and enrichment factors of Mo and U) indicate brackish to saline and oxidizing paleowater masses during deposition of the Gongjue Formation. Provenance analyses via elements and petrology reveal that sediments in the Gongjue Formation are mainly derived from intermediate-acidic rocks of the upper crust. We conclude that the first and third members are more arid climate and heavily chemically weathered than the second member. In combination with previous studies of the structural evolution of the Qamdo Basin since the Paleogene, a model is built to describe the sedimentary environment and evolution of the Qamdo Basin during transition to the Paleocene. The first and third members, i.e., the Eg1 and Eg3 members of the Gongjue Formation, are dominated by an oxidizing environment of seawater-saltwater, and the climate ranges from warm and humid to arid and hot, with relatively stable environmental changes. The Eg2 member of the Gongjue Formation is dominated by an oxidizing environment of seawater-saltwater, and the climate ranges from warm and humid to arid and hot, with more frequent environmental evolution. Our model aids in better understanding of the Paleocene climate evolution of the eastern Tibetan Plateau.
In recent years, the advancements in multi-collector inductively coupled plasma mass spectrometry (MC – ICP – MS) technology have significantly enhanced our understanding of the isotopic variations in tungsten (W) and tin (Sn) among natural samples. The application of W isotopic fractionation (δ186/184W NIST 3163) has emerged in tracing the material cycle associated with solid Earth evolution as an excellent indicator. Also, the findings of Sn isotopic fractionation (e.g. δ122/118Sn 3161a) in cassiterite have achieved initial success in revealing magmatic-hydrothermal processes. However, the mechanisms responsible for variations in W and Sn isotopes in the ore-forming processes remain to be fully addressed, and there is an urgent need to apply the W – Sn isotope systematics to geology (especially for ore deposit geology). In this contribution, the high-precision analytical methods for W and Sn isotopes, the significant variability in isotopic compositions, and the immense functionality of the tracing process are systematically summarized and reviewed. Conclusively, the prospects of the application of W and Sn isotopes in geology are listed, and it is pointed out that it is urgent to introduce W – Sn double isotope systematics into the study of ore deposits, apply the stable isotopic method for tracing the source of ore-forming materials and modelling the evolution of W and Sn isotopes in W – Sn metallogenic system. Focusing on the composition variations of W and Sn isotopes in magmatic-hydrothermal evolution is expected to explore the fractionation mechanism of those isotopes in the mineralization process from multiple perspectives, establishing the evolution model of W and Sn isotopes in the complex W – Sn metallogenic system, which can provide a fresh approach for in-depth understanding of the genesis of W – Sn mineralization, and then facilitating a new perspective for the study of large-scale W – Sn polymetallic mineralization.
Low-temperature Sb (Au–Hg) deposits in South China account for more than 50% of the world's Sb reserves, however, their genesis remains controversial. Here we report the first study that integrates U–Pb and Lu–Hf analysis by LA-(MC)-ICPMS and conventional (U–Th)/He analysis, all applied to single zircon crystals, in an attempt to constrain the origin and timing of world-class Sb (Au–Hg) deposits in Banxi (South China). Zircon separated from a quartz-stibnite ore and an altered country rock samples revealed similar U–Pb age spectra defining two major populations – Paleoproterozoic (~1900–2500 Ma) and Neoproterozoic (~770 Ma), which are characterized by variable εHf(t) values (–10.7 to 9.1 and –16.5 to 11.2, respectively) and Hf crustal model ages (TDMC) (2.48 to 3.24 Ga and 0.97 to 2.71 Ga, respectively). The U–Pb age and Hf isotopic features of the zircons are consistent with the Banxi Group in the region, indicating that the zircons involved in the low-temperature hydrothermal system were originally from the Banxi Group country rocks. Thirty-three mineralization-related zircon crystals yielded a mean (U–Th)/He age of 123.8 ± 3.8 Ma, which is interpreted to represent the timing of the latest low-temperature mineralization stage of the Banxi Sb deposit. The combined U–Pb, Lu–Hf and (U–Th)/He data suggest that Precambrian basement rocks were the major contributors to the low-temperature mineralization, and that Early Cretaceous (130–120 Ma) could be the most important ore-forming epoch for the Sb deposits in South China. This study also demonstrates the analytical feasibility of integrated U–Pb - Lu–Hf - (U–Th)/He "triple-dating", all applied to single zircon crystals. This approach reveals the full evolution of zircon, from its origin of the magmatic source, through its crystallization and low-temperature cooling. Although this study demonstrates the usefulness of this integrated approach in dating low-temperature mineralization, it has great potential for zircon provenance and other studies that may benefit from the large amount of information that can be extracted from single zircon crystals.
Multiple metallogenic types (skarn-type and vein-type) related to hypabyssal granites are found at the Huangshaping polymetallic deposit in the Nanling Range, South China. To constrain the crystallization and mineralization processes of skarn formation, three generations of magnetite and pyrrhotite from the hydrous silicate stage, oxide stage, early quartz–sulfide stage, and late quartz–sulfide stage were distinguished. The geochemical compositions of magnetite and pyrrhotite were obtained by electron probe microanalyzer (EPMA) and in-situ ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS). The results show that there may be silicate inclusions in magnetite and interaction of wall rock occurred in the mineralization process. The geochemical trends recorded in pyrrhotite show the influence of limestone during the crystallization of pyrrhotite. The re-equilibration temperatures of Po I, Po II, and Po III are 420.46, 380.45, and 341.81 °C, respectively, which suggests a continuous evolution following the high-temperature W–Sn mineralized system. The content change of Ni and V reflects a gradual decrease of oxygen fugacity from Mag I to Mag III, while the sulfur fugacity calculated from pyrrhotite gradually decreases. This continuous skarn mineralization evolution process helps us to better understand the change of metallogenic environment in the retrograde stage of the Huangshaping deposit.
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