Jiang Shaoyong

Study on lithogeochemistry of Middle Jurassic basalts from southern China represented by the Fankeng basalts from Yongding of Fujian Province

There exists an E-W trending Middle Jurassic volcanic zone in southern China. The Fankeng basalts in the Yongding basin of Fujian Province are considered to be a typical example. The Fankeng basalts have TiO2 contents in the range of 1.92%–3.21%. They are classified as high-Ti basalts. They also have higher total Fe (averaging FeO* = 11.09%). The Middle Jurassic Fankeng basalts from southwestern Fujian have obvious distinctive lithogeochemical features from early Cretaceous basalts from southeastern coast of China. They have higher HFSE, such as Th, Nb, Ta, Zr and Ti. Their element ratios related with HFSE, such as Zr/Ba, La/Nb, La/Ta, Zr/Y, Ti/Y, Ba/Nb, K/Ti and Rb/Zr are similar to those of OIB. The most samples have ɛNd(T) of −0.7–0.24, which are near chondrite. Some samples have higher ɛNd(T) of 1.87–3.55. Therefore, these basaltic magmas might be derived from depleted asthenospheric mantle. The lithogeochemical characteristics of the Fankeng basalts may be caused by interaction between asthenosphere and lithosphere at the time. The (Early-) Middle Jurassic basalts and gabbros from southeastern Hunan, southern Jiangxi and northern Guangdong provinces show similar geochemical features to those of the Fankeng basalts from the Yongding of Fujian. Occurrence of these OIB-type basalts in the area may be regarded as the petrological mark of upwelling of asthenosphere at the time. Upwelling of asthenosphere has led to tectonic extension and the formation of rifted basin in the area.

Geochemistry of pore waters from HQ-1PC of the Qiongdongnan Basin, northern South China Sea, and its implications for gas hydrate exploration

The Qiongdongnan Basin is one of the target areas for marine gas hydrate exploration in the northern margin of the South China Sea. In this study, major anion (e.g., SO4−2, Cl, Br, I), cation (e.g., Ca, Mg, K, Na), and trace element (e.g., Sr, Ba) concentrations of pore water samples collected from site HQ-1PC in the Qiongdongnan Basin were analyzed. These geochemical data suggest that the process of AOM (Anaerobic Oxidation of Methane) is dominant in sulfate-reduction zone in this site due to high upwelling iodine flux and strong microbial activities. The iodine-rich fluids, which may carry methane for the gas hydrate formation, pass through the gas hydrate stability zone, mix with brine released during gas hydrate formation, and cause the geochemical anomalies observed at site HQ-1PC. The pore water geochemical characteristics and anomalies in the Qiongdongnan Basin are quite similar to those found in other gas hydrate locations in the Shenhu area in the northern South China Sea, and a genetic link is suggested with the possibility of gas hydrate occurrence in the study area.

Types and Origin of Dolostones in Tarim Basin, Northwest China: Petrographic and Geochemical Evidence

: In the Tarim Basin of northwestern China hydrocarbon deposits have been discovered in parts of the thick strata of Cambrian dolostones. Based on petrographic study, six types of dolostone have been distinguished: Type-1, pink mud-bearing silty crystalline dolostone (PMSD); Type-2, gypsum- and salt-bearing fine crystalline dolostone (GSFD); Type-3, fine crystalline dolostone with dolomite crystals with cloudy core and clear rim (CCFD); Type-4, deep gray mud-bearing silty crystalline dolostone (GMSD); Type-5, euhedral coarse crystalline dolostone (ECD); and Type-6, xenotopic coarse crystalline dolostone (XCD). Applying petrographic and geochemical methods, the genesis of the dolostones is studied in this paper. Normally, Type-1 dolostone shows U- and Mo-depleted characteristics, reflecting a more oxidized formation environment; High δ8O and the purple color are consistent with formation of Sabkha dolostones on a supratidal flat. Types 2, 3, 4 dolostones show strata formation, similar REE patterns and 87Sr/86Sr ratios with contemporaneous limestones, suggesting a penecontemporaneous origin from seawater. Types 5 and 6 dolostones commonly occur as interbedded rocks, indicating secondary genesis after diagenesis. Type-6 dolostone has the highest order degree (OD) values (average 0.86), the lowest oxygen isotope values and positive Eu anomalies, which are consistent with previously reported hydrothermal dolostones. Differently, Type-5 shows euhedral texture, higher δ18O value, similar REE characteristic and 87Sr/86Sr ratios in comparison with contemporaneous limestones, suggesting that this type might have been dolomitized by down-transferring evaporated seawater during shallow burial stage. Dolostone fluid sources, formation environments and crystallizing dynamics are summarized and possible genetic models for the six types are proposed.

Simulating experiment on the hydrothermal superimposing metallogenesis of the Dongguashan strata-bound copper deposit

Series of sedimentary hydrothermal-diplogenetic copper deposits have been found scattering in the region along the middle-lower reaches of the Yangtze River, and their metallogenetic mechanism is still in hot debate. In order to reveal the ore-forming kinetics of sedimentary process and hydrothermal superimposition, and evaluate the role of sedimentary pyrite in the enrichment and precipitation of copper, a set of simulating experiments on the reaction between pyrite and CuCl2 solution were conducted. According to the physicochemical characteristics of the ore-forming fluid of the Dongguashan copper deposit, Anhui Province, 100 MPa was selected as the experimental pressure, and the experimental temperatures were set at 450, 350, 250 and 150°C, respectively. The reactions between pyrite grains isolated from the Shimenkou strata-bound pyrite deposit and the solution with 0.2 mol/L CuCl2 and 1.0 mol/L NaCl were experimentally simulated. Then, variations in surface topography and surface chemistry of the experimental pyrite grains were documented using scanning electronic microscopy (SEM), atomic force microscopy (AFM), Auger electron spectrometry (AES) and X-ray photoelectron spectroscopy (XPS), and the solution and newly formed minerals were analyzed using inductively coupled plasma (ICP-AES) and X-ray diffraction (XRD) techniques. Desulphurization of pyrite surface was observed and new copper minerals were detected. It is proposed that pyrite can act as a geochemical barrier for the enrichment and precipitation of copper from the solution under the experimental conditions. Furthermore, the ore-forming mechanism of sedimentary hydrothermal-diplogenetic copper deposits was discussed.

Origin of hydrothermal ore-forming processes in the Dapingzhang polymetallic copper deposit in the Lanping-Simao Basin, Yunnan Province

The newly discovered Dapingzhang polymetallic copper deposit in the Lanping-Simao Basin is hosted in volcanic rocks. The orebodies include an upper stratiform and lentiform massive sulfide orebody, and a lower vein type and disseminated sulfide orebody. The Rb-Sr isotopes of quartz fluid-inclusions in the vein type orebody yield an isochron age of 118±12 Ma. Fluid- inclusion analysis shows that the ore-forming fluid is of low to middle salinity (2.0–8.0 wt%NaCl), and of low to middle temperature (90~200°C). The δ34S values of pyrite and chalcopyrite minerals in this deposit concentrate at about 0‰. Our modeling and water/rock ratio calculations using available hydrogen-oxygen isotope data suggests an evolved meteoric water origin for the ore-forming fluids, instead of mixture of seawater and magmatic fluids. The Pb isotope composition indicates radiogenic Pb sources, and suggests Pb from sedimentary rocks and mantle-derived volcanic rocks in the basin both contribute to this deposit. In conclusion, the Dapingzhang deposit shares many similar geochemical characteristics with vein type copper deposits in the Lanping-Simao Basin. These data support the conclusion that the Dapingzhang polymetallic copper deposit is a hydrothermal deposit formed during Yanshanian period, not a massive sulfide deposit of submarine-exhalative-sedimentary origin as proposed by previous researchers.

Geology and geochemistry of the Furong Tin Deposit, Hunan Province, P.R. China

Recently, a giant tin deposit, the Furong deposit, has been discovered in the Qitianling granitoid, Hunan, South China. The tin mineralization occurs as disseminated crystals or veins of cassiterite. The cassiderite is found in narrow envelopes of chlorite alteration within the granite. The Qitianling granite has distinctly different petrology and mineralogy from common S-type tin granites. The oxygen and hydrogen isotope data indicate a dominant influence of surface-derived meteoric water associated with chlorite alteration. Variations in δ18O of fresh and altered granites are due to continuous isotopic exchange reaction between hydrothermal fluids and granites at variable water/rock ratios. The sulfur isotope compositions of sulfides from the tin ores indicate that both the granite and the strata supported the sulfur for mineralization. The Pb isotopic compositions of sulfides are same as those of feldspars from the granite. Thus, Pb in the ores might come from the granite. Fractional crystallization of the magma and tin deposition directly from exsolved magmatic-hydrothermal fluids may not be the major mechanism for the tin mineralization in this deposit. Instead, we suggest that interactions between fluids of meteoric origin and the granite may contribute to the release and deposition of tin.

Stable isotope composition of the Dalucao rare earth deposit in western Sichuan

Ore-forming solutions and the origin of mineralizing material in the Dalucao rare earth deposit were studied by means of H, O, C, and Si isotope analyses. The δ13C of the bastnasite range narrowly from −8.1‰ to−8.0‰, while the δD values range from −89‰ to −79‰. The quartz δ30Si of the orebody is −0.3‰, and the δD and δ180 of the quartz-fluid inclusion range from −87‰ to +6.0‰, respectively. The δD and δ180 values of the fluorite-fluid inclusion in the ore body are −99‰ to −11.6‰, respectively. The study shows that the compositions of hydrogen and oxygen isotopes in oreforming fluid of Dalucao rare earth deposit lie in the region between magmatic water and meteoric water. Therefore the ore-forming fluid was mixed from magmatic and meteoric water. The carbon isotopic features suggest that carbon of the bastnasite was from the mantle, carbon of fluorite-fluid inclusions in the ore body was from a mixture carbon source, and carbon of quartz-fluid inclusions was from the mantle.