No abstract

Phosphate rocks, an important ore resource in Guizhou Province, China, are mainly hosted within the Sinian Doushantuo Formation and the Cambrian Meishucun Formation. In addition, the phosphate rocks of the Cambrian Meishucun Formation are rich in biological fossils. Although numerous studies investigating the genesis of phosphate deposits have been performed, the relationship between biological activity and the formation of phosphate deposits in the lower Cambrian Meishucun Formation has not been convincingly explained. This study focuses on the biological fossil assemblage, the characteristics of phosphorus, and the relationship between biological and phosphorus enrichment of the lower Cambrian phosphorites. The primary objectives of our study are to analyze the role of organisms in the formation of phosphorites, restore the phosphorus-formation environment of the Cambrian Meishucun Formation, and construct a sedimentary model of the phosphorites in the Meishucun Formation. The results indicate that there is a significant positive correlation between biological activity and the deposition of phosphorites, that is, the higher the degree of biological enrichment and differentiation, the stronger the deposition. The geochemical analysis of several profiles in the Zhijin phosphorite block shows that the phosphorite block was deposited in an oxygen-rich environment and was affected by a high-temperature hydrothermal fluid. Upwelling ocean currents supplied abundant phosphorus and other nutrients, which provided the conditions for small shells and algae to flourish. Biochemical activity was a crucial factor in the deposition of the phosphorite.

The Saima deposit is a newly discovered niobium deposit which is located in the eastern of Liaoning Province, NE China. Its mineralization age and geochemical characteristics are firstly reported in this study. The Nb orebodies are hosted by the grey-brown to grass-green aegirine nepheline syenite. Detailed petrographical studies show that the syenite consists of orthoclase (~50%), nepheline (~30%), biotite (~15%) and minor arfvedsonite (~3%) and aegirine (~2%), with weak hydrothermal alteration dominated by silicification. In situ LA-ICP-MS zircon U-Pb dating indicates that the aegirine nepheline syenite was emplaced in the Late Triassic (229.5 ± 2.2 Ma), which is spatially, temporally and genetically related to Nb mineralization. These aegirine nepheline syenites have SiO 2 contents in the range of 55.86-63.80 wt. %, low TiO 2 contents of 0.36-0.64 wt. %, P 2 O 5 contents of 0.04-0.11 wt. % and Al 2 O 3 contents of more than 15 wt. %. They are characterized by relatively high (K 2 O + Na 2 O) values of 9.72-15.51 wt. %, K 2 O/Na 2 O ratios of 2.42-3.64 wt. % and Rittmann indexes (σ = [ω(K 2 O + Na 2 O)] 2 /[ω(SiO 2 − 43)]) of 6.84-17.10, belonging to the high-K peralkaline, metaluminous type. These syenites are enriched in large ion lithophile elements (LILEs, e.g., Cs, Rb and Ba) and light rare earth elements (LREEs) and relatively depleted in high field strength elements (HFSEs, e.g., Nb, Zr and Ti) and heavy rare earth elements (HREEs), with transitional elements showing an obvious W-shaped distribution pattern. Based on these geochronological and geochemical features, we propose that the ore-forming intrusion associated with the Nb mineralization was formed under post-collision continental-rift setting, which is consistent with the tectonic regime of post-collision between the North China Craton and Paleo-Asian oceanic plate during the age in Ma for Indosinian . Intensive magmatic and metallogenic events resulted from partial melting of lithospheric mantle occurred during the post-collisional rifting, resulting in the development of large-scale Cu-Mo mineralization and rare earth deposits in the eastern part of Liaoning Province.

Central Jilin is tectonically subordinate to the Lesser Xing'an Range–Zhangguangcai Range polymetallic metallogenic belt, an important region for Cu–Mo prospecting in NE China. Dozens of large‐scale molybdenum deposits, including Fu'anbu, Chang'anbu, Jide, and Dashihe, have been recently discovered in Central Jilin, whereas porphyry Cu or Cu–Mo deposits have not been found to date. One such example of an intracontinental porphyry Cu–Mo deposit is the Chang'anbu Cu–Mo deposit in Shulan, Jilin Province, hosted in early Yanshanian rocks. Here in this contribution, we described detailed geology based on our field observation and conducted a comparative study on the metallogenic epoch and the ore‐forming sources of the Chang'anbu Cu–Mo deposit by using zircon U–Pb dating and H–O–S–Pb stable isotopes. We propose that the Chang'anbu deposit is rare in the Lesser Xing'an Range–Zhangguangcail Range metallogenic belt and differs from other porphyry deposits that consist solely of Mo, indicating a unique mechanism of metallogenesis. Zircon U–Pb ages indicate emplacement of a granite pluton which is the main metallogenetic rock body during the Early Jurassic (182.10 ± 1.20 Ma). The pluton is spatially and temporally associated with Cu–Mo mineralization and led to large‐scale porphyry Cu–Mo mineralization during the early Yanshanian. Sulphur, Pb, H, and O isotope data suggest that magma generated by subduction of the Paleo‐Pacific oceanic crust was the main ore‐forming source of this deposit (206Pb/204Pb = 18.046–18.734; Pb207/Pb204 = 15.502–15.655; δ34S = 0.3–2‰; δDV–SMOW = −102.2–93.4‰; δ18OV–SMOW = 9.1–11.6‰). The Chang'anbu porphyry Cu–Mo deposit is representative of large‐scale polymetallic metallogenic events in Central Jilin that resulted from magmatism related to crust and mantle melting during the early Yanshanian.

The early Cambrian is a critical interval of dramatic oceanic and biochemical changes in geological history. The black shale deposits, which are rich in Mo, Ni, V, and platinum group elements (PGE), are a reflection of that interval. Among all known Cambrian black shale deposits in South China, the vanadium deposits are poorly constrained by geochronology. The newly discovered tuff layer in the Sansui Bagong vanadium deposit in Guizhou Province can provide excellent constraints on the age of vanadium deposits. In this study, we obtain a new zircon U–Pb isotopic age, which can constrain the age of the vanadium deposit. This tuff occurs in the middle part of the ore bed, and the age of the tuff layer can reflect the mineralization age of the V deposit. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is used in this study to obtain the zircon U–Pb age of the tuff and indicates that the event of volcanic activity took place at 520.9 ± 1 Ma. This age is close to the boundary between the Cambrian Terreneuvian and Series 2. This result provides a good constraint on the age of the vanadium deposits in South China and makes the vanadium deposits to be comparable with other Mo–Ni–PGE deposits. The alteration of organic carbon isotope (δ 13Corg) values can reflect changes in the marine environment and is widely used in stratigraphic correlation. The analysis of the δ 13Corg values of the ore bed in the present study reveals a positive excursion at the bottom of the deposit and a negative excursion in the V-enriched layer. The δ 13Corg values in the Bagong section are comparable to those in the Xiaotan and Longbizui sections. The ages of the Sansui vanadium deposit constrained by the U–Pb isotopic age of the tuff and the δ 13Corg values are consistent. All of the data lead us to infer that the vanadium deposit formed at approximately 521 Ma.