Junbo Gao

Rare earth element geochemistry of Cambrian phosphorites from the Yangtze Region

Abstract Analysis on P2O5 and REE (rare earth elements) in basal Cambrian phosphorites from the Yangtze Region, showed that the phosphorites from Bailongtan in Yunnan Province, Zhijin, Jinsha, Xishui, Zunyi, Tianzhu and Tongren in Guizhou Province, Shangrao in Jiangxi Province, Jiangshan in Zhejiang Province and Nanjing in Jiangsu Province contained high REE contents in phosphorites, especially those from Tianzhu and Tongren in Guizhou Province, Shangrao in Jiangxi Province, Jiangshan in Zhejiang Province and Nanjing in Jiangsu Province. Among them, the highest REE contents reached up to 1361.59×10−6 and P2O5 contents up to 29.45%. In contrast, phosphorites from Kaiyang, Qingzhen and Zhenyuan in Guizhou Province had relatively low total REE contents and low P2O5 contents, with the lowest total REE content of 53.19×10−6, and the lowest P2O5 content of 0.07%. REE contents exhibited a positive correlation with phosphorous contents, indicating a close genetic relationship between REE and phosphorous element. The main reason was the difference of the sedimentary environments and the contents of phosphorous and REE in deep water mass formed in upwelling currents, at an upper slope facies (Slu) in reducing condition due to relatively closed sedimentary environment. It was very difficult for the upswelling phosphorous and REE to deposit, massive phosphorous deposits were hard to be formed. At a deep water ramp facies (DRa) in the ascending process of currents, phosphorus and REE underwent differentiation rather than sedimentation. Consequently, the contents of P2O5 sediments and REE were low. At a shallow water ramp facies (including shoals and tidal flats) (SRa), with active seawater circulation and phosphatic supplement, sunny weather, high nutrition and rapid growth of algae, were benefitial for the physical enrichment of carbonate sediments. Therefore, massive phosphorite deposits were easy to be formed with abundant REE minerals, and finally turned into high REE bearing phosphorous deposits.

Discovery of an abnormally high-δ 34 S barite deposit and a new understanding of global sulfur isotope variation during geological history

The evolution of the global sulfur isotope curve was plotted based on the δ34S values of evaporates resultant from oceanic evaporation. In the long period of geological history the δ34S values showed obvious peaks for three times during the process of ancient oceans’ sulfur isotope evolution, namely the Early Cambrian (+30‰), the Late Devonian (+25‰) and the Permian-Triassic transition interval (+17‰), but the causes of the abnormal rise of sulfur isotopic values during the geological period are still in question. In this paper, 18 samples collected from a large Devonian barite deposit from Zhenning County were analyzed to determine their δ34S values, revealing that the 18 samples have very high δ34S values (δ34S=41.88‰–+68.39‰), with an average close to 56.30‰, which are higher than the isotopic values of contemporary sulfates (+17‰–+25‰). A comparative analysis was conducted of the emerging of high δ34S barite deposits (from Cambrian and Devonian) and the δ34S variation curves of the ancient oceans. The results indicate that the time when the obvious peaks of δ34S values appeared and the time of massive sedimentation of high δ34S barite deposits are very close to each other, which, in our opinion, is not a coincidence. There may exist some correlations between the sulfur isotope evolution of ancient oceans during the diverse periods of geological history and the massive sedimentation of high δ34S barite deposits. Therefore, it is inferred that perhaps it was the massive sedimentation of high δ34S barites that caused the sharp rise of δ34S values in a short period of time.

Forming Mechanism Analysis of the Abnormally High δ34S Baryte Deposits: A Case Study from the Zhenning–Ziyun Large Devonian Baryte Deposits, Guizhou Province, China

ABSTRACT Stratiform baryte deposits are widespread in Cambrian and Devonian strata in China and around the world. In this article, the authors studied the sulfur isotopic features and forming mechanism of the stratiform baryte deposits occurring within the Upper Devonian cherts of the Zhenning-Ziyun county, Guizhou province, located in the Southwestern margin of the Yangtze Platform. The sulfur isotopic data from 18 baryte ore samples of the Leji section of Zhenning county are presented herein with values that range from +41.9‰ to +68.4‰ (AVG = +59.9‰). The δ34S values of the baryte ore from the Mohao section of Ziyun county are stable and show a narrow range from +41.3‰ to +47.0‰ (AVG = +44.0‰). In the Luocheng section of Ziyun county, the δ34S values of the baryte ore vary from +27.6‰ to +36.4‰ (AVG = +32.7‰). The δ34S values of all samples are higher than those of the coeval seawater sulfates (+25‰). The scanning electron microscope analysis indicates that spherical, dumbbell-shaped, clavate bacterial and bacteria-like fossils were observed, as well as the irregular schistose and framboid forms of Fe oxides that occur around the surfaces of baryte grains and inter-grains. Fe oxides take irregular schistose and framboid forms. These findings imply the significant concentration of heavy sulfur related to the seawater sulfates with the action of bacterial sulfate reduction in a closed basin. In addition, the trend of the δ34S value of baryte ores is gradually decreasing from the Leji to Mohao and Luocheng sections, which suggests that Leji area is located in the deepwater area of the restricted rift basin, and in a more closed depositional environment.

Geochemical characteristics of the giant Nibao Carlin-type gold deposit (Guizhou, China) and their geological implications

A large gold orebody controlled by Fault F1 was newly discovered in the giant Nibao Carlin-type gold deposit from southwestern Guizhou, China. Ore or rock samples were collected and studied to reveal the relationship among various orebodies which distributed in Fault F1 and its upper and lower plates, and to figure out the possible origin of ore-forming material. Tuff or tufaceous material was considered to be basic volcanic rocks in terms of their characteristic values of Al2O3/TiO2. Positive correlation was found between Au and As, and the correlation coefficient is particularly high within the fault fracture zone of the F1. Analysis shows that As can be regarded as a preferentially indicative element in gold prospection, while minerals of hydrothermal origin such as pyrite and arsenopyrite can be used as mineralogical indicative for gold prospection. Geochemical characteristics of trace elements and rare earth elements (REE) show that mantle source materials might have been involved in gold mineralization of different ore bodies. Large-scale Emei mantle plume eruption in the Early Permian resulted in thick volcanic sedimentary rock positioned between the karst unconformity surface at the top of the Maokou Formation and the bottom of the Longtan Formation. The Yanshanian tectonic activities played a significant role in driving gold-rich fluids to pressure-release spaces such as fault fracture zone and structure-controlling alteration zone (abbreviated to Sbt, according to Chinese). Those fluids reacted with ore-hosting rocks through water–rock interactions and resulted in the formation of gold-rich arsenian pyrite and arsenopyrite. Gold mineralization might have resulted from reactions between Fe-poor and gold-rich ore-forming fluids and tuffs which was relatively rich in iron, during which Fault F1 was considered to be the key to the formation of this giant gold orebody.

Discovery and Genesis Discussion on A Early-Cambrian Large-Scale Vanadium Deposit from Eastern Guizhou, China

Recently the large scale vanadium deposit,of ZhuXi with reserves of nearly 1 tonnes: in black shales was discovered in eastern Guizhou. Sedimentology, trace element geochemistry and isotope geochemistry were applied to reveal the genesis of the ZhuXi vanadium deposit. Results show that the deposit is characterized by hydrothermal sedimentation, and the metallogenic fluid may came from the deep inflow.

GEOCHEMISTRY OF TRACE ELEMENTS OF THE QUATERNARY SEDIMENTS OF YAOSHANG FORMATION IN WEINING COUNTY, GUIZHOU PROVINCE AND ITS PALEOENVIRONMENTAL IMPLICATIONS: GEOCHEMISTRY OF TRACE ELEMENTS OF THE QUATERNARY SEDIMENTS OF YAOSHANG FORMATION IN WEINING COUNTY, GUIZHOU PROVINCE AND ITS PALEOENVIRONMENTAL IMPLICATIONS

Geochemistry of trace elements and REE of the Quaternary sediments in the Yaoshang Formation in Weining country,Guizhou,China were analyzed.Based on the previous studies about Ostracoda and pollen,we discussed the sedimentary environmental changes through the vertical variation of elements.Our results show that the 14 trace elements(eg.Li,Ba,Sr) are well correlated.Generally,∑REE is very high,and δCe and δEu are negative,except that Ce in the layers 7 and 11 are positive.LREE and HREE fractionation is obvious.And the chondrite normalized samples show a distribution pattern of LREE enrichment with gentle incline to the right.The parameter of elements ratio and REE are characterized by zonation.The content the 14 trace elements and Rb/Sr、Sr/Ba、Sr/Cu ratio indicate a relatively dry and hot climate in the layers of 2,6,8,10,12 of the Yaoshang Formation while the other layers formed undera colder and wetter climate.Sr/Cu changes between 1 and 5,indicating a humid subtropical plateau monsoon climate.Sr/Ba changing between 0.2~0.5 indicates that the Caohai sediment was deposiled in a freshwater enviroment,suggesting a very strong weathering of the parent nock of the Permian basalt which is high in REE content.The combination of Sr/Cu and Rb/Sr ratio values can be used as the indicator of climatic change.