Driving Mechanisms and Their Relative Importance in Focusing Hydrothermal Fluid Flow in the Chanziping Ore District, South China

Numerical simulations were performed to examine the driving mechanisms and their relative dominance in concentrating hydrothermal ore-forming fluid flow in the Chanziping ore district, South China. A numerical scheme was employed to provide realistic strain-dependent porosity and permeability during tectonic deformation. The results indicate that the buoyancy resultant from typical geothermal gradients alone during tectonically quiescent periods could not focus ore-forming fluids to the trap area for U precipitation. The extensional tectonic deformation in the late Yanshanian-early Himalayan period caused a broad downwelling flow in the red sandstone, channeling downwards along the regional fault to mix with the basement-derived upwelling reducing fluid driven by the buoyancy force. This leads to the ore genesis in the black shale below the fracture zone. The compressive deformation in the Indo-Chinese epoch would have essentially squeezed the basinal fluid and basal brine out of the basin, and hence would have been unfavorable for the formation of the Chanziping deposit. It was also observed that tectonic deformation always overwhelms the effect of buoyancy in driving hydrothermal flow, even if an extension or shortening of as little as 1% is applied.