Abstract: Geologists discovered in the 1980s that alteration halos decrease with increasing depth in many tungsten deposits of the Nanling Mountains. However, the mechanism for the formation of the alteration characteristics remains poorly understood. In this paper, the authors investigated hydrothermal flow and silica diffusion from fractures to adjacent wallrock at these tungsten deposits by using finite element based numerical experiments. The authors have found that fluid temperature and wallrock porosity exert a strong influence on silica diffusion from fractures to adjacent wallrock. Both high temperature and high porosity favor silica diffusion from fractures to adjacent wallrock and form wide alteration halos. Constant-porosity wallrock forms wider alteration halos at deeper levels, which is inconsistent with alteration characteristics of the tungsten deposits in the Nanling Mountains. Wallrock porosity that decreases with increasing depth forms alteration halos like those in those the tungsten deposits. The wall rock lithology and fracture distribution those tungsten deposits favor the formation of depth-dependent porosity and permeability. Evaluation of these two factors may help the exploration. Aqueous NaCl solutions were used in the numerical experiments. It is therefore concluded that inhomogeneous magmatic hydrothermal fluids are unnecessary in explaining the alteration characteristics at these tungsten deposits.