Chen Duo-fu

Hydrothermal sedimentation characteristics of the Dajiangping superlarge pyrite deposit, Yunfu, Guangdong

The Dajiangping pyrite deposit is hosted in a carbonate-clastic rock series which is characterized by a bioreef-chert suite in its middle part. Conformable lenses of various sizes constitute the orebodies which often branch transitionally into the surroundings. Syngenetic deformation fabrics can be observed in orebody No. 3. Orebody No. 4 is composed almost entirely of massive pyrite, with conspicuous hydrothermal sedimentation and bacterium-alga features. The hydrothermal sedimentation origin is also reflected by simple chemistry of the ore (predominated by quartz and pyrite), the presence of U and Th, and the REE compositions of pyrite and ankerite. The ores are rich in organic matter and the difference in organic content between the banded ores and the massive ores is mainly owing to the difference in the clastic components they contain rather than to the subsequent processes of reworking. Go/Ni ratios in the ore may reflect the temperature pattern during hydrothermal sedimentation.

Controlling factors for gas hydrate occurrence in Shenhu area on the northern slope of the South China Sea

Temperature and pressure on seafloor of the northern slope in the South China Sea are suitable for gas hydrate formation, but bottom simulation reflector (BSR), an indication of gas hydrate occurrence, only occurred in limited areas of the slope. Drillings in the BSR-distributed area (the District S) on the northern slope of the South China Sea suggested that gas hydrate only occurred at Sites SH2, SH3, and SH7 with high saturation (up to 20%–40%), and there is no hydrate at Sites SH1 and SH5 although the distance between SH1 to SH3 is only 500 m. In this paper, we investigated seafloor gradient, fault development, temperature, and pressure in the District S on the northern slope of the South China Sea to understand the possible factors controlling BSR distribution and gas hydrate occurrence. The District S is a structurally fractured continental slope zone and its seafloor gradient varied greatly. The BSR-occurred areas have an average gradient of 19.89×10−2 whereas the BSR-free zone has the average gradient of 10.57×10−2. The calculated relative structural intensities from fault densities and displacements show that the BSR-distributed areas tend to occur in the areas with a moderately high structural intensity, where faults frequently developed close to the seafloor that are possibly favored for lateral migration of gases. On the basis of temperatures and pressures at drilling sites, hydrate-occurred Sites SH2, SH3, and SH7 are located within the thermodynamically stable area for methane hydrate, and hydrate-absent Sites SH1 and SH5 are out of the thermodynamically stable area for methane hydrate formation, suggesting that both BSR and the thermodynamic stability are necessary for hydrate occurrence in the subsurface.

Influence of water flow on gas hydrate accumulation at cold vents

A cold vent is an area where methane-rich fluid seepage occurs. This seepage may alter the local temperature, salinity, and subsequent accumulation of the gas hydrate. Using a kinetic gas hydrate formation model and in situ measurement of temperature, salinity and fluid flux at the southern summit of Hydrate Ridge, we simulate the gas hydrate accumulation at three distinct fluid sites: clam, bacterial mat, and gas discharge sites. At the clam sites (pore water flux < 20 kg m−2 yr−1), pore water advection has little influence on temperature and salinity. However, the salinity and temperature are increased (peak salinity > 0.8 mol kg−1) by the formation of gas hydrate causing the base of the hydrate stability zone to move gradually from ∼115 to ∼70 meters below seafloor (mbsf). The gas hydrate saturation at the clam sites is relatively high. The water flux at the bacterial mat sites ranges from 100 to 2500 kg m−2 yr−1. The water flow suppresses the increase in salinity resulting in a salinity close to or slightly higher than that of seawater (< 0.65 mol kg−1). Heat advection by water flow increases temperature significantly, shifting the base of the hydrate stability zone to above 50 or even 3 mbsf. The gas hydrate saturation is relatively low at the bacterial mat site. At the gas discharge sites, the pore water flux could reach 1010 kg m−2 yr−1, and the temperature could reach that of the source area in 9 min. There is no gas hydrate formation at the gas discharge sites. Our simulative analysis therefore reveals that a lower pore water flux would result in lower salinity, higher temperature, and a shallower base of the hydrate stability zone. This in turn induces a lower gas hydrate formation rate, lower hydrate saturation, and eventually less gas hydrate resources.

Geochemical characteristics of volcanic rocks of the Laochang Ag polymetallic deposit, Lancang, Yunnan Province, China

In terms of major element, trace element and REE geochemical characteristics of volcanic rocks in the area studied and by making use of the TiO2-K2O-P2O5, Th-Hf-Ta and Zr-Nb-Y trianglar diagrams and the environmental discrimination diagrams of incompatible element distribution patterns, the authors have drawn some conclusions that are different from those by previous workers. It is concluded that volcanic rocks in the Laochang Ag polymetallic deposit at Lancang, Yunnan belong to continental within-plate alkali basalts, and that their geotectonic setting seems to be at the northeastern margin of East Gondwana Land.