Experimental Study and Modeling of Water-Rock Interaction in Xiangshan Fossil Geothermal Field, South-Eastern China

Abstract

Xiangshan fossil hydrothermal system is found in a Mesozoic volcanic basin of south-eastern China. The genesis of the uranium deposits in the fossil hydrothermal system has been disputed for a long time. The experiments have been designed to simulate the water-rock interaction and the evolution of ore-forming hydrothermal fluids in Xiangshan hydrothermal system. The experiments show that oxygen isotope could easily be exchanged between the water and rock in hydrothermal system, resulting in the ‰18O decrease for rocks and ‰18O increase for water, that is, the "oxygen isotope shift". The ‰D values are relatively stable, but can apparently also vary if the rock contains a large amount of hydrogenbearing minerals such as biotite. The concentrations of U, Th for the fluids increased greatly, and uranium-enriched geothermal solution was formed after the water-rock interaction. Consequently, the experiments have proven that the ore-forming hydrothermal solution in Xiangshan Uranium Ore-field is mainly derived from the local meteoric water. EQ3/6 geochemical software package has been applied to model the formation and evolution of the hydrothermal solution, and the alteration and the uranium mineralization The project was supported by IAEA Coordinated Research Project (No.CRP-9716). The modeling reveals the content of uranium in the hydrothermal solution increased up to 0.33~0.553 mg/L after water-rock interaction, in accordance with the results of the experimental study. At the end, the F and SiO2 enriched U-bearing hydrothermal fluids could be generated by means of the water-rock interaction. U in the hydrothermal fluids remains stable in the low Eh environment, as UO2F3 -, UO2 (CO3)2 2- and other uranium complexes have very high stability.