Genesis of the Jiajika superlarge lithium deposit,Sichuan,China:constraints from He–Ar–H–O isotopes

The Jiajika granitic-and pegmatite-type lithium deposit,which is in the Songpan-Garze Orogenic Belt in western Sichuan Province,China,is the largest in Asia.Previous studies have examined the geochemistry and mineralogy of pegmatites and their parental source rocks to determine the genesis of the deposit.However,the evolution of magmatic-hydrothermal fluids has received limited attention.We analyzed He–Ar–H–O isotopes to decipher the ore-fluid nature and identify the contribution of fluids to mineralization in the late stage of crystallization differentiation.In the Jiajika ore field,two-mica granites,pegmatites(including common pegmatites and spodumene pegmatites),metasandstones,and schists are the dominant rock types exposed.Common pegmatites derived from early differentiation of the two-mica granitic magmas before they evolved into spodumene pegmatites during the late stage of the magmatic evolution.Common pegmatites have~3He/~4He ratios that vary from 0.18 to 4.68 Ra(mean1.62 Ra),and their~(40)Ar/~(36)Ar ratios range from 426.70 to 1408.06(mean 761.81);spodumene pegmatites have~3He/~4He ratios that vary from 0.18 to 2.66 Ra(mean 0.87Ra)and their~(40)Ar/~(36)Ar ratios range from 402.13 to 1907.34(mean 801.65).These data indicate that the hydrothermal fluids were shown a mixture of crust-and mantle-derived materials,and the proportion of crustderived materials in spodumene pegmatites increases significantly in the late stage of the magmatic evolution.Theδ~(18)OH_(2)O–VSMOWvalues of common pegmatites range from 6.2‰to 10.9‰,with a mean value of 8.6‰,andδDV–SMOWvalues vary from-110‰to-72‰,with a mean o f-85‰.Theδ~(18)OH_(2)O–VSMOWvalues of spodumene pegmatites range from 5.3‰to 13.2‰,with a mean of 9.1‰,andδDV–SMOWvalues vary from-115‰to-77‰,with a mean of-91‰.These data suggest that the ore-forming fluids came from primary magmatic water gradually mixing with more meteoric water in the late stage of the magmatic evolution.Based on the He–Ar–H–O and other existing data,we propose that the oreforming metals are mainly derived from the upper continental crust with a minor contribution from the mantle,and the fluid exsolution and addition of meteoric water during the formation of pegmatite contributed to the formation of the Jiajika superlarge lithium deposit.

Helium and Argon Isotopes of the Tertiary Volcanic Rocks, Shandong Peninsula: Significance for Crust Mantle Circulation

He and Ar isotopes of the Tertiary volcanic rocks from Jiyang basin and nearby Shanwang and Qixia areas in Shandong peninsula have been analyzed. The results show that all samples are characterized by an enrichment of radiogenic 4He leading to extremely low 3He/ 4He ratios (0.017 8- 3.225)×10 -6 . They also have atmospheric like 38 Ar/ 36 Ar ratios and slightly higher 40 Ar/ 36 Ar ratios (395.4-1 312.7) than air value. The enrichment of radioactive 4He is attributed to the long term radiogenic accumulation of U and Th in the mantle. The low concentration of 3He is considered to be due to 3He loss during magma eruption. The loss is more obvious in the Neogene than in the Paleogene. Low 40 Ar/ 36 Ar ratios of all volcanic rocks may imply that subducted materials with atmospheric components have been preserved in the mantle, thus indicating that crust mantle circulation occurred.

Decipher hydrocarbon generation and accumulation based on fluid inclusion and chronology:A case study from the Upper Paleozoic buried-hills in Huanghua Depression,Bohai Bay Basin

Deciphering hydrocarbon generation and accumulation stage is of significance to understand oil and gas evolution and seek exploration targets.Taking the Upper Paleozoic buried-hills in the Huanghua Depression,Bohai Bay Basin,as a case study,hydrocarbon generation environment and detailed accumulation process are revealed by fluid inclusions observations,Laser Raman spectroscopy,Fourier Infrared spectroscopy,and K-Ar isotope measurements.The results show that both oil and gas inclusion were captured in the quartz overgrowth,dissolved feldspar and calcite microfractures,showing blue to dark brown fluoresce.The grains containing oil inclusions index(GOI)of oil,oil&gas and gas being 25%,65%,and 10%and the inclusions with abundant methyl groups and short chains,both indicate high thermal maturity.One series of fluids inclusion is generally observed,evidenced by the concentrated homogenization temperature of 135-145℃ and salinity of 3%-15 w.t.%NaCl equiv,indicating one primary charging stage.The gas and gas&liquid inclusions mainly contain CH_(4),with also peaks indicating CO_(2) and N_(2.)The Carboniferous and Permian biomarkers show reducing environment with brackish water,with organic matter sources both from marine and continental.The relative content ofααα20RC_(27),ααα20RC_(28),andααα20RC_(29) exhibit source contributions both from algae and higher plants,and mainly of II2 to III kerogen.Both coal derived gas and oil associated hydrocarbons are identified from most of the buried-hills.Combining the fluid homogenization temperature and salinity,as well as the thermal evolution history,the hydrocarbon generated from the Upper Paleozoic was concentrated at the end of the Eocene(40 Ma±),while the beginning of charging is 60 Ma±.The Wumaying Buried-hill is of only coal derived gas and has potential for inner coal measure natural gas exploration.The results provide a detailed understanding of hydrocarbon accumulations in the study area,which can also be reference for improving petroleum exploration efficiency in similar basins.

Geochemical characteristics of noble gases in natural gas and their application in tracing natural gas migration in the middle part of the western Sichuan Depression, China

Noble gases in natural gas, from Xiaoquan, Xinchang, Hexingchang and Fenggu gas reservoirs in the middle part of the western Sichuan Depression, China, were analysed. Results show that the volume content of crustal noble gases accounts for 97.9% to 99.7% of the total noble gas content, indicating that the noble gases in the study area are very largely derived from the crust. Moreover, the 40Ar time-accumulating effect of source rocks is used to determine the complex relationship between gases and source rocks in this area, and the results agree well with that from analysis of source rock light hydrocarbons. Due to the short migration distance, the separation of 4He and 40Ar is not significant in Xujiahe natural gas and Lower and Middle Jurassic natural gas, so it is difficult to trace natural gas migration. However, this separation characteristic of 4He and 40Ar in Middle and Upper Jurassic natural gas is significant, which indicates that natural gas migration was from the Middle Jurassic to Upper Jurassic formations. In addition, the variation trends of 3He/4He ratio and δ13C1 value indicates that natural gas migration is from the Xujiahe formation to the Jurassic layer in the study area.

He–Ar Isotopic Tracing of Pyrite from Ore-forming Fluids of the Sanshandao Au Deposit, Jiaodong Area

The Sanshandao Au deposit is located in the famous Sanshandao metallogenic belt,Jiaodong area.To date,accumulative Au resources of 1000 t have been identified from the belt.Sanshandao is a world-class gold deposit with Au mineralization hosted in Early Cretaceous Guojialing-type granites.Thus,studies on the genesis and ore-forming element sources of the Sanshandao Au deposit are crucial.He and Ar isotopic analyses of fluid inclusions from pyrite(the carrier of Au)indicate that the fluid inclusions have 3 He/4 He=0.043–0.21 Ra with an average of 0.096 Ra and 40 Ar/36 Ar=488–664 with an average of 570.8.These values represent the initial He and Ar isotopic compositions of ore-forming fluids for trapped fluid inclusions.The comparison of H–O isotopic characteristics combined with deposit geology and wall rock alteration reveals that the ore-forming fluids of the Sanshandao Au deposit show mixed crust–mantle origin characteristics,and they mainly comprise crust-derived fluid mixed with minor mantle-derived fluid and meteoric water during the uprising process.The ore-forming elements were generally sourced from pre-Cambrian meta-basement rocks formed by Mesozoic reactivation and mixed with minor shallow crustal and mantle components.

Helium and argon isotope geochemistry of the Tibetan Qulong porphyry Cu-Mo deposit,China

The Qulong porphyry Cu-Mo deposit,generated in the Miocene post-collisional extension environment of the Gangdese Copper(Molybdenum)Metallogenic Belt,is one of the largest porphyry Cu deposits in China.This study reports the noble gas isotopic compositions of volatiles released from fluid inclusion reserved in pyrite from the Qulong deposit.3He/4 He and 40Ar/36Ar ratios range from 0.54 to 1.015 Ra and 300-359,respectively.Concentrations of 4 He and 40Ar range from 1.77 to 2.62×10^(-8)cm^(3)STP and 1.7-34×10^(-8)cm^(3)STP,respectively.The isotopic composition of noble gases indicates that the ore-forming fluids of the Qulong Cu-Mo deposit were a mixture of fluid containing mantle component,which is exsolved from the porphyry magma,and crustal fluid characterized by atmospheric Ar and crustal radiogenic He.Theδ34S values of pyrite and molybdenite range from-0.52‰to 0.31‰,with an average of-0.12‰,indicating a magmatic origin.More mantle components were involved in the Cu-Mo deposit than in the Mo-Cu deposit in the Qulong-Jiama ore-district.

^(40)Ar/^(39)Ar ISOTOPIC DATING THE BIOTITES FROM THE IGNEOUS AND METAMORPHIC ROCKS OF THE ZHONGSHAN STATION AREA

The Zhongshan Station of China is located in the Larsemann Hills, East Antarctica. Low pressure granulite facies gneisses together with late granites are outcroped in the region. Three biotite samples from a garnet segregation, a syenogranite and a granite-pegmetite were measured with 40Ar/39Ar incremental heating technique. Biotites from the garnet segregation give an 40Ar/39Ar plateau age of 504±1Ma. Biotites from the syenogranite yield an 40Ar/39Ar plateau age of 494 ±1 Ma. Biotites from the granite-pegmatite give an 40 Ar/39 Ar plateay age of 486±1Ma. They verify 500 Ma thermal event called 'Pan African event' by previous K-Ar and Rb-Sr data. They are cooling ages of the biotites when the paleogeotherm of the area droped to the K-Ar closure temperature for biotite.